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Countermeasures to Protect Bridge Abutments from Scour (2007)

Chapter: Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments

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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
×
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Suggested Citation:"Chapter 7 - Lab Results II: Aprons at Wing-Wall Abutments." National Academies of Sciences, Engineering, and Medicine. 2007. Countermeasures to Protect Bridge Abutments from Scour. Washington, DC: The National Academies Press. doi: 10.17226/17620.
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79 7.1 Introduction The findings of the DOT survey presented in Chapter 4, the literature review presented in Chapter 5, and the prelim- inary experiments presented in Chapter 6 indicate that suit- ably positioned aprons of riprap, cable-tied blocks, or geobags hold promise as an effective scour countermeasure for wing-wall abutments. The present chapter investigates such aprons in further detail. Observations and data were obtained from a series of laboratory experiments. In partic- ular, the experiments focused on the performance of riprap, cable-tied blocks, and geobags placed as an apron around pile-supported wing-wall abutments retaining erodible embankments. Furthermore, the aprons were subject to live-bed flow con- ditions in which the channel bed was in a dune regime. It is of interest to note that existing guidelines for abutment apron design are based on laboratory experiments performed exclu- sively in clear-water scour and do not account for the dis- lodging effects of bed forms passing around or over an apron. In this respect, the findings from the experiments reported here are novel. The capacity of dunes to destabilize riprap, cable-tied blocks, or geobags around the edges of an apron poses a severe test of the stability of an armor apron formed from riprap, cable-tied blocks, or geobags. The present exper- iments showed that the passage of dunes may readily dislodge portions of a protective apron if the edges are not protected. This finding concurs with similar findings from efforts at armoring beds around piers (e.g., Chiew, 2000). A further novel aspect of the experiments is the finding regarding the importance of protecting the embankment region beneath and immediately behind the pile cap of wing- wall abutments supported by piles. Heretofore, little diagnos- tic attention has been given to the manner whereby the earthfill embankment immediately behind a wing-wall abut- ment may erode when a scour hole develops. Besides observations on the performance of cable-tied blocks and small geobags as riprap alternatives, the findings from the experiments include an evaluation of geobags used as a substitute for filter-cloth underlay to riprap. The findings also include a summary of the maximum scour depths asso- ciated with the baseline scour conditions, the use of a riprap or cable-tied blocks apron, and selections from the geobag arrangements tested. Design guidelines are given and include using current riprap configurations for sizing and placing geobags. Two sets of experiments were conducted: • Use of riprap and cable-tied blocks for protection of an abutment sited near the bank of a narrow channel. The aim of the experiment was to investigate the use of riprap and cable-tied blocks as wing-wall abutment scour coun- termeasures. Both riprap and cable-tied block aprons were placed around abutments to protect them from scour, which could potentially undermine the abutments if no protection were provided. A series of experiments were conducted with live-bed conditions. Flow depth, flow velocity, and apron extent were systematically varied for both protection types to determine the minimum required apron extent to sufficiently protect the abutment from scour. Different riprap sizes and apron burial depths were also investigated. • The use of geobags and riprap for protection of an abut- ment sited near the bank of a narrow channel. The aim of this study was to determine whether an apron of geobags in addition to riprap, or without riprap, could be an effec- tive countermeasure for wing-wall abutments. As geobags are relatively easy to transport and place, they hold prom- ise as a potentially useful temporary armor material for use when riprap is not immediately available. Therefore, there is interest to determine if and how geobags may function in minimizing scour. C H A P T E R 7 Lab Results II: Aprons at Wing-Wall Abutments

7.2 Experiments on Aprons of Riprap or Cable-Tied Blocks This section describes the experiments conducted to deter- mine the performance behavior of an apron of riprap or cable-tied blocks placed around a wing-wall abutment under live-bed conditions. The experiments were completed at the University of Auckland, New Zealand. 7.2.1 Experiment Layout A 1.5-m wide, 1.2-m deep, and 45-m long recirculating flume was used to conduct the experiments. The flume is sup- ported on two universal beams that are centrally pivoted so that the slope of the flume can be adjusted by electrically driven screw-jack supports at either end of the flume. The flume’s channel consists of an inlet section, a 35-m long chan- nel, and an outlet section. A false floor 0.4 m high was placed along the length of the channel section, with a 3-m long recess located 26 m downstream of the inlet section. Figure 7-1 shows the flume looking in the downstream direction, and Figure 7-2 shows a longitudinal cross section of the flume. Water enters the flume at the base of the inlet section, and the sediment slurry enters the flume at the top of the inlet sec- tion. As the flow enters the inlet section, it passes through a wave skimmer that suppresses surface wave formation. The floor of the channel section slopes up to the false floor height 0.4 m above the channel floor. The water and entrained sediment flow from the inlet section down the channel sec- tion of the flume and into the outlet section. The sediment entrained in the flow collects in the sand trap and is pumped back to the inlet section of the flume as a sand slurry using a 30-kW pump and a 100-mm diameter pipeline. The sedi- ment-free water passes over the sediment trap and is pumped back to the inlet section of the flume through 250-mm and 300-mm diameter pipelines using 22-kW and 45-kW pumps, respectively. The pumps are controlled by variable-speed con- trollers that regulate the flow rate in the flume. The flume is filled from the laboratory reservoir via an inflow pipe located at the back of the outlet section of the flume and is drained by an outlet valve in the bottom of the outlet section. During experiments, the water levels in the flume are controlled by an overflow pipe in the outlet section of the flume. Model Channel and Abutment A fixed floodplain 0.4 m wide was constructed along the length of the rectangular channel section of the flume, as shown in Figure 7-3. The floodplain was constructed from concrete blocks 240 mm high. The main channel bank was constructed from sheet metal glued to the concrete blocks on the floodplain and riveted to the false floor of the flume, with a side slope of 1:1 (H:V). The sheet metal lining the main channel bank was painted and sprinkled with sand to simulate the roughness of the sediment bed. Figure 7-4 shows the construction of the floodplain in the flume. The main channel and the recess were filled with bed sediment to a depth of 140 mm above the false floor level. This sedi- ment allowed sufficient depth for the equilibrium bed forms to fully develop in the main channel along the length of the flume. In order to generate appropriate flow velocities on the floodplain, it was necessary to increase the roughness. Rows of five evenly spaced 35 × 35 × 30 mm roughness blocks were glued onto the floodplain with a row spacing of 400 mm along the entire floodplain (Figure 7-3). A 600-mm wide Perspex wing-wall abutment model with a 45-degree flare angle was used for this study. Detailed dimensions of the wing-wall abutment are given in Figure 7-5. The abutment model protruded 150 mm into the main channel from the edge of the floodplain. The top of the abutment was placed 75 mm above the top of the floodplain. The abutment extended to the bottom of the recess in the flume and was fixed to the floor of the flume. The fixed main channel bank was extended down under the same angle (1:1) to the bottom of the recess and fitted around the abutment model. The edge of the sheet metal forming the main channel bank around the abutment was sealed onto the abutment. A concrete embankment 75 mm high, 400 mm long, and 600 mm wide at the crest, with side slopes of 2:1 (H:V) was cast on top of the floodplain behind the abutment model. Fig- ure 7-6 shows a photo of the embankment and the wing-wall abutment, with the sediment in the main channel leveled 100 mm below the top of the floodplain. 80 Figure 7-1. The 1.5-m wide flume used for experi- ments on riprap and cable-tied blocks.

Figure 7-2. Dimensions of the 1.5-m wide flume. Figure 7-3. Layout of the channel and the wing-wall abutment.

Bed Sediment Uniform coarse sand was used as the bed material for all the experiments. A sieve analysis was carried out for the sand (Van Ballegooy, 2005). The sediment properties are summa- rized in Table 7-1. The geometric standard deviation g was calculated from g  (d84/d16)0.5. The bed material was considered to be uni- form because g  1.5. Using the Shields entrainment func- tion, the critical shear velocity for the sediment was determined. Model Riprap Stone Four different riprap sizes were used for the scour coun- termeasure experiments. The riprap properties are summa- rized in Table 7-2 and illustrated in Figure 7-7. Riprap type R2 was painted yellow, R3 was painted orange, and R4 was painted green. The reason for painting the riprap was to allow for better visibility in the flow and for clearer photographs of each experiment. The paint also facilitated retrieving the riprap stones at the conclusion of each test. The critical shear velocity for each riprap size was determined in the same way as for the bed sediment. Model Cable-Tied Blocks Conventionally, cable-tied blocks are constructed from precast concrete and are joined together using stainless steel 82 Figure 7-4. Floodplain construction in the 1.5-m wide flume. Figure 7-5. Wing-wall abutment dimensions. Figure 7-6. Wing-wall abutment and approach embankment setup in the 1.5-m wide flume, with the sand in the main channel leveled. Description d16 (mm) d84 (mm) d50 (mm) σg Ss u*c (ms-1) Filter sand 0.62 1.04 0.82 1.30 2.65 0.020 Table 7-1. Bed sediment data. Description d16(mm) d84 (mm) d50 (mm) σg Ss u*c (ms-1) R1 20 21 18 1.08 2.65 0.137 R2 28 32 25 1.13 2.65 0.162 R3 40 43 38 1.06 2.65 0.193 R4 61 66 56 1.09 2.65 0.239 Table 7-2. Riprap properties.

or synthetic cables to form a mattress. By adopting the McCorquodale et al. (1993) and Parker et al. (1998) recom- mendations for cable-tied block design, the required block height to satisfy the stability criteria was determined using (7-1) Where:   weight per unit area of the block mattress as a whole, acb  0.1, cb  block density,   fluid density, and V  approach velocity. The minimum required block height, Hb, can be calculated from (7-2) Where pm is the volume fraction pore space within the mattress. Assuming a block density of cb  2,400 kg/m−3 and a vol- ume fraction pore space of pm  0.15, the minimum required height of the block was determined to be 2 to 13 mm (depending on the flow velocity). A block height of 10 mm was used. This ensured that the cable-tied block mats would remain stable for all test conditions. The blocks had the shape of a truncated square pyramid similar to the shape of cable-tied blocks used in practice. The dimensions for the three different blocks are given in Figure 7-8. The blocks used (see Figure 7-9) were dimen- sionally identical to those used in the Cheung (2002) experiment. The blocks were made from concrete using molds. A 1:5:1 water to sand to cement ratio was used to H g pb cb m = −   ( )1     = − a Vcb cb cb 2 yield a saturated block density of cb = 2,080 kg/m−3. The mixture was left to set in the mold for at least 24 hours before the concrete blocks were carefully removed. The edges were smoothed using a fine grit sand paper. The blocks were glued with a two-part epoxy glue to a porous shade cloth with a grid size of 5 mm to form a mat- tress. Attaching the blocks to the shade cloth simulated the linking together of actual blocks with cables. A 1-mm gap was left between each concrete block. For each of the experiments, sections of block matting were tied together to form the required apron extent at the abutments. Model Geotextile Most of the experiments were carried out with a geotextile placed underneath the countermeasure apron to eliminate the winnowing of sand from between the riprap stones or cable-tied blocks. The geotextile used for testing was flexible enough to ensure that the riprap or cable-tied blocks would be in contact with the bed at all times. The geotextile used was a commercial nonwoven geotextile identical to the geotextile used in the Cheung (2002) experi- ment. The properties of the geotextile are given in Table 7-3. Approach-Flow Distribution Uniform flow was established along the flume for four dif- ferent flow velocities (approximately V/Vc  1.1, 1.4, 1.8, and 2.1) and two different flow depths (ym  100 and 170 mm). With live-bed conditions, bed forms develop in the flume. As the bed forms developed, the slope of the flume was increased to maintain a uniform flow. When the bed forms were fully developed, the flow velocity upstream of the abutment was measured using the particle-tracking velocimetry (PTV) technique, and the bed profiles were measured using the acoustic depth sounder. Similitude between laboratory experiments and field scale was satisfied by the use of the aforementioned u*/u*c 83 Figure 7-7. The four riprap sizes used. Figure 7-8. Dimensions (mm) of the blocks used for the cable-tied block mats. Description Hb Lb Lt ρcb p B1 B2 B3 10 20 30 25 32 38 20 25 32 2080 2080 2080 0.17 0.19 0.18

ratio, of which values above 1.0 represent a condition called “live-bed scour.” This condition is extreme for scouring of armouring units such as riprap, cable-tied blocks, and geobags because the bed forms travelling past the armour- ing units can dislodge individual units and therefore cause failure. Figure 7-10 shows the surface velocity distributions across the flume for the four flow velocities at two different flow depths. The major grid lines on the vertical axis of the graphs are multiples of the critical velocity for sediment entrain- ment. The lower graph shows the velocity distributions for the flow depth ym  100 mm. These velocity distributions fin- ish at the edge of the main channel bank because there is no overbank flow at this flow depth. The upper graph shows the velocity distributions for the flow depth ym  170 mm. These velocities are highest in the main channel and are lowest in the floodplain (yf  70 mm). The minor anomaly that the surface velocity using the PTV technique for 1.8Vc is lower in the floodplain than for 1.5Vc is not considered significant because the flows on the floodplain were relatively low and the prime interest was in the live-bed conditions in the main channel. 84 Figure 7-9. Cable-tied block mats. Properties Geotextile Information Name T500S Puncture Strength (kN) 500 Elongation Strength (%) 60 Trapezoid Tear (kN) 100 Apparent Opening Size (mm) 0.35 Permeability (lm-2s-1) 130 Table 7-3. Characteristic properties of the geotextile material used. (a) mold for making the concrete blocks (b) block types used in the experiment (c) radial sections of the cable-tied block mat for the experiments reported in Chapter 8 (d) rectangular sections of cable-tied block mat for the experiments discussed herein

Bed Forms Bed forms can be seen in Figure 7-11. After the velocity dis- tributions were measured, the flow was stopped and 10-m long bed profiles were measured longitudinally in the flume at 100-mm spacings. Average and maximum bed-form heights λH, lengths λL, and trough depths λD were determined for each flow condition from the bed profiles and are sum- marized in Table 7-4. Layout of Riprap and Cable-Tied Block Aprons The aprons of riprap stones were carefully placed to a thickness of 2d50 (equivalent to two riprap layers) at different elevations relative to the initial bed level, termed the average bed level. The placement level of the apron, db, is defined as the distance between the average bed level and the bottom of the apron (Figure 7-12). For the riprap type R3, three placement levels were used with the riprap aprons: db  2d50 (80 mm) with the top of the apron flush with the aver- age bed level, db  1d50 (40 mm) with one riprap layer placed above the average bed level, and db  0 with both layers placed on top of the average bed level. For the other riprap types (R1, R2, and R4), the placement levels were db  1d50 (20, 27, and 60 mm, respectively) with the top of the first riprap layer buried flush with the average bed level and the other riprap layer placed on top of the average bed level. The cable-tied block mat was placed on the surface of the average bed level and was attached to the abutment face. For the experiments at higher velocities, larger blocks were glued to the leading edge of the cable-tied block mat to prevent fail- ure of the mat from uplift, which could result in overturning of the mat. The riprap and cable-tied block aprons were 1.35 m long in all cases. The 0.5-m upstream extension was selected as a result of preliminary experiments, which demonstrated that the 0.5-m extension was adequate to ensure that undermining of the leading edge of the aprons did not extend as far as the upstream corner of the abutment (i.e., the extent of undermining was always less than 0.5 m). At the downstream end, the longitudinal extent of under- mining of the apron was negligible. Therefore, a 0.25-m downstream extension was selected to give a reasonable mar- gin of protection downstream of the abutment. A filter was placed beneath the aprons to prevent winnowing of the bed 85 Figure 7-10. Velocity distributions across the 1.5-m wide flume for the four different flow velocities and two different flow depths. Figure 7-11. Bed-form parameter definition diagram. Flow Parameters λ H λ L λ D ym V/Vc Mean Max. Mean Max. Mean Max. 1.1 0.048 0.073 1.221 2.140 0.031 0.060 1.4 0.054 0.086 1.022 1.904 0.032 0.081 1.8 0.051 0.080 0.984 1.736 0.033 0.066 0.100 2.2 0.056 0.088 1.005 1.957 0.033 0.074 1.1 0.064 0.110 1.142 2.151 0.043 0.088 1.5 0.080 0.138 1.295 2.172 0.059 0.106 1.8 0.085 0.132 1.184 2.191 0.041 0.085 0.170 2.1 0.078 0.135 1.389 2.521 0.048 0.099 Table 7-4. Bed-form characteristics (m).

sediment from between the riprap stones and the cable-tied blocks. The experiments were conducted with two flow depths, ym  100 mm (bank-full) and ym  170 mm (maximum flood level without overtopping the embankment). At each flow depth, four different flow velocities were used. For the bank- full condition, the experiments were conducted with average main channel velocity ratios of V/Vc  1.1, 1.4, 1.8, and 2.2. Likewise, for the maximum flood-level condition, the veloci- ties tested were V/Vc  1.1, 1.5, 1.8, and 2.1. Each run com- menced from a flatbed, as shown in Figure 7-6. Experimental durations ranged from 8 hours to 72 hours, based on a requirement that at least 50 bed forms would propagate past the abutment during the run. At the end of each experiment, any sand covering the apron was carefully removed and the settlement of the apron was measured. The depth to which the outer edge of the apron had settled was taken as the maximum scour depth at that point, where settlement depth is defined as the distance from the average bed level to the top of the apron after settlement. Two cases of settlement were observed depending on the extent of undermining of the apron. These are identified in Figure 7-13 and discussed below. The settlement of the apron at the abutment face ds1, the settlement of the outer edge of the apron ds2, the horizontal distance from the abutment face to the outer edge of the apron 2, and the horizontal distance to the point where the apron was undermined Wmin were measured at both the upstream and downstream corners of the abutment.Accuracy of measurements was 5 mm. 7.2.2 Experimental Results Summary of Results The measurements from the experiments are summarized in Table 7-5. Two cases of settlement were observed depending on the extent of undermining of the apron. These are identified in Figure 7-13. For Case I, no settlement occurred at the abut- ment face, while for Case II the entire apron was subject to settlement. For Case I, ds1 was taken to be negative when the top of the apron was above the average bed level (nd50 > db) and vice versa. For Case II, Wmin  0 because the entire apron was subject to settlement. General Trends Figures 7-14 and 7-15 show the results for two series of riprap experiments (ym  100 mm and ym  170 mm, respec- tively), and Figures 7-16 and 7-17 show the results for the equivalent series of cable-tied block experiments (ym  100 mm and ym  170 mm, respectively). For the two riprap experimental sets, R3 riprap was used for the aprons and was placed at a burial depth db  80 mm (nd50). For the two cable- tied block experiments, R1 riprap was used for the aprons and placed at a burial depth db  0. In Figures 7-14 through 7-17, apron width increases across the page from W  100 mm to W  400 mm, while the flow velocity increases down the page from V/Vc  1.1 to V/Vc  2.1 (or 2.2 for the experiments with ym  100 mm). The results for four riprap experimental series with differ- ent apron widths (W  100 mm to W  400 mm) and vary- ing apron burial depths are shown in Figures 7-18 and 7-19. For these experimental sets, R3 riprap was used for the aprons, which were tested at a flow depth of ym  170 mm. Apron burial depth increases down the page from db  0 to db  80 mm (nd50), while flow velocity increases across the page from V/Vc  1.1 to V/Vc  2.1. Riprap size was varied in a final experimental series, as shown in Figure 7-20. R1 riprap was used for d50  20, 86 Figure 7-12. General layout of riprap apron.

R2 riprap was used for d50 = 27, R3 riprap was used for d50 = 40, and R4 riprap was used for d50 = 60. For these experi- ments, a 200-mm wide apron was used, placed at a burial depth db = 1d50, such that one riprap layer was buried flush with the average bed level and the other riprap layer was placed on top. The aprons were tested with a flow depth of ym = 170 mm. Riprap size increases across the page from d50 = 20 mm to d50 = 60 mm, while flow velocity increases down the page from V/Vc = 1.1 to V/Vc = 2.6. Riprap type R4 was tested at a high flow velocity (V/Vc = 2.6) to deter- mine the flow velocity at which shear failure of the riprap would occur. Figures 7-14 to 7-20 depict the systematic variations in scour formation at the wing-wall abutment as a conse- quence of variations in ym, V/Vc, W, db, and d50. The data from Table 7-5 are scattered due to the variability in bed- form height and consequent apron settlement, but a trend of increased settlement of the outer edge of the apron with both increasing flow depth and increasing flow intensity is apparent. Also, the corresponding scour depth at the upstream end of the apron was typically larger than that at the downstream end because the bed forms were larger at the upstream end. The data also show that lowering the placement level of the apron did not affect the settlement depth at the outer edge of the apron, although the apron remained more intact (i.e., Wmin was larger for lower apron placement levels), because less material was undermined from the apron. Apron type (riprap or cable-tied blocks) and increasing apron width both had little effect on the settlement depth. 7.2.3 Experimental Observations Riprap and Cable-Tied Block Apron Behavior The movement of bed forms through the bridge section undermined the outer edges of the protective apron,as illustrated in Figure 7-21. Whenever bed-form troughs propagated past the apron,the bed was lowered locally and the apron would be under- mined and settle if the bed lowering was more than had occurred previously during the experiment. The edge of the apron was more susceptible to undermining, thereby causing deeper settle- ment of the apron at its outer edge. This settling process contin- ued as subsequent bed forms with deeper troughs propagated past the apron,thereby further undermining the toe of the apron. Subsequent bed forms with shallower troughs propagated over the apron without causing further settlement. The outer edge of the apron typically settled to the level of the deepest bed form that propagated past the abutment during the particular run. After a sufficient number of bed forms had passed, the edge of the apron was assumed to have settled to its equilibrium position. The cable-tied block aprons, consisting of interconnected blocks and being attached to the abutment face,were constrained to settle downward when undermined. The more loose nature of riprap aprons resulted in apron stones rolling down into the bed- form troughs and the apron spreading as erosion took place. Comparison of Riprap Observations with Lauchlan (1999) The three clear-water failure mechanisms for riprap aprons identified by Chiew (1995) were confirmed in the 87 Figure 7-13. Definition diagram of the apron settlement measurements.

88 Upstream Downstream ym (m) V/Vc W (m) d50 (m) db (m) ds1 (m) ds2 (m) Wmin (m) α 2 (m) ds1 (m) ds2 (m) Wmin (m) α2 (m) Riprap Protection 0.100 1.1 0.100 0.040 0.080 0.000 0.080 0.050 0.200 0.000 0.060 0.070 0.165 0.100 1.4 0.100 0.040 0.080 0.005 0.085 0.000 0.200 0.000 0.065 0.070 0.165 0.100 1.8 0.100 0.040 0.080 0.010 0.095 - 0.230 0.000 0.075 0.050 0.200 0.100 2.2 0.100 0.040 0.080 0.040 0.110 - 0.240 0.000 0.105 0.000 0.240 0.170 1.1 0.100 0.040 0.080 0.000 0.105 0.040 0.300 0.000 0.085 0.060 0.250 0.170 1.5 0.100 0.040 0.080 0.000 0.105 0.000 0.230 0.000 0.085 0.060 0.230 0.170 1.8 0.100 0.040 0.080 0.000 0.135 0.000 0.260 0.000 0.105 0.040 0.250 0.170 2.1 0.100 0.040 0.080 0.105 0.175 - 0.325 0.000 0.145 0.040 0.305 0.100 1.1 0.200 0.040 0.080 0.000 0.075 0.100 0.300 0.000 0.055 0.150 0.270 0.100 1.4 0.200 0.040 0.080 0.000 0.085 0.080 0.310 0.000 0.055 0.150 0.280 0.100 1.8 0.200 0.040 0.080 0.000 0.120 0.060 0.320 0.000 0.065 0.150 0.300 0.100 2.2 0.200 0.040 0.080 0.000 0.125 0.040 0.340 0.000 0.085 0.150 0.300 0.170 1.1 0.200 0.040 0.080 0.000 0.103 0.100 0.300 0.000 0.100 0.130 0.320 0.170 1.5 0.200 0.040 0.080 0.000 0.130 0.080 0.350 0.000 0.100 0.130 0.320 0.170 1.8 0.200 0.040 0.080 0.000 0.140 0.060 0.360 0.000 0.095 0.130 0.345 0.170 2.1 0.200 0.040 0.080 0.025 0.145 - 0.360 0.000 0.115 0.100 0.360 0.100 1.1 0.300 0.040 0.080 0.000 0.200 0.200 0.380 0.000 0.045 0.250 0.360 0.100 1.4 0.300 0.040 0.080 0.000 0.200 0.200 0.390 0.000 0.065 0.250 0.370 0.100 1.8 0.300 0.040 0.080 0.000 0.180 0.180 0.415 0.000 0.065 0.230 0.380 0.100 2.2 0.300 0.040 0.080 0.030 0.150 0.150 0.470 0.000 0.075 0.220 0.410 0.170 1.1 0.300 0.040 0.080 0.000 0.200 0.200 0.400 0.000 0.075 0.230 0.390 0.170 1.5 0.300 0.040 0.080 0.000 0.180 0.180 0.405 0.000 0.075 0.230 0.380 0.170 1.8 0.300 0.040 0.080 0.000 0.150 0.150 0.410 0.000 0.105 0.200 0.390 0.170 2.1 0.300 0.040 0.080 0.000 0.100 0.100 0.470 0.000 0.115 0.150 0.390 0.170 1.1 0.400 0.040 0.080 0.000 0.085 0.330 0.480 0.000 0.350 0.500 0.075 0.170 1.5 0.400 0.040 0.080 0.000 0.105 0.320 0.500 0.000 0.330 0.520 0.075 0.170 1.8 0.400 0.040 0.080 0.000 0.130 0.280 0.550 0.000 0.300 0.540 0.095 0.170 2.1 0.400 0.040 0.080 0.000 0.150 0.210 0.580 0.000 0.280 0.610 0.135 0.170 1.1 0.100 0.040 0.040 0.005 0.130 - 0.300 -0.035 0.115 0.040 0.280 0.170 1.5 0.100 0.040 0.040 0.020 0.135 - 0.305 -0.035 0.115 0.040 0.285 0.170 1.8 0.100 0.040 0.040 0.070 0.160 - 0.305 0.005 0.115 - 0.285 0.170 2.1 0.100 0.040 0.040 - 0.190 - 0.350 0.025 0.165 - 0.360 0.170 1.1 0.200 0.040 0.040 -0.035 0.120 0.090 0.385 -0.035 0.090 0.100 0.355 0.170 1.5 0.200 0.040 0.040 -0.035 0.125 0.050 0.385 -0.035 0.090 0.100 0.355 0.170 1.8 0.200 0.040 0.040 -0.035 0.130 0.050 0.360 -0.035 0.110 0.090 0.370 0.170 2.1 0.200 0.040 0.040 0.075 0.165 - 0.450 -0.035 0.165 0.050 0.430 0.170 1.1 0.300 0.040 0.040 -0.035 0.105 0.150 0.400 -0.035 0.075 0.200 0.400 0.170 1.5 0.300 0.040 0.040 -0.035 0.130 0.150 0.470 -0.035 0.120 0.200 0.460 0.170 1.8 0.300 0.040 0.040 -0.035 0.135 0.150 0.470 -0.035 0.130 0.200 0.460 0.170 2.1 0.300 0.040 0.040 0.045 0.165 - 0.530 -0.035 0.145 0.150 0.515 0.170 1.1 0.400 0.040 0.040 -0.035 0.120 0.270 0.560 -0.035 0.090 0.320 0.560 0.170 1.5 0.400 0.040 0.040 -0.035 0.130 0.240 0.560 -0.035 0.105 0.310 0.580 0.170 1.8 0.400 0.040 0.040 -0.035 0.155 0.220 0.590 -0.035 0.115 0.280 0.580 0.170 2.1 0.400 0.040 0.040 -0.035 0.175 0.120 0.650 -0.035 0.155 0.200 0.640 0.170 1.1 0.100 0.040 0.000 0.005 0.115 - 0.360 -0.075 0.095 0.040 0.350 0.170 1.5 0.100 0.040 0.000 0.045 0.145 - 0.360 -0.075 0.105 0.040 0.350 0.170 1.8 0.100 0.040 0.000 0.075 0.165 - 0.360 -0.005 0.145 - 0.430 0.170 2.1 0.100 0.040 0.000 - 0.000 - - - 0.000 - - 0.170 1.1 0.200 0.040 0.000 -0.075 0.130 0.060 0.430 -0.075 0.120 0.090 0.420 0.170 1.5 0.200 0.040 0.000 -0.075 0.140 0.040 0.430 -0.075 0.125 0.090 0.430 0.170 1.8 0.200 0.040 0.000 -0.025 0.140 - 0.440 -0.075 0.125 0.060 0.440 0.170 2.1 0.200 0.040 0.000 0.035 0.155 - 0.450 -0.075 0.145 0.000 0.460 Table 7-5. Apron settlement measurements.

89 0.170 1.1 0.300 0.040 0.000 -0.075 0.115 0.150 0.490 -0.075 0.095 0.200 0.470 0.170 1.5 0.300 0.040 0.000 -0.075 0.125 0.150 0.490 -0.075 0.100 0.200 0.480 0.170 1.8 0.300 0.040 0.000 -0.075 0.135 0.090 0.510 -0.075 0.115 0.150 0.490 0.170 2.1 0.300 0.040 0.000 0.045 0.145 - 0.510 -0.075 0.135 0.150 0.530 0.170 1.1 0.200 0.020 0.020 -0.020 0.130 0.110 0.410 -0.020 0.105 0.150 0.360 0.170 1.5 0.200 0.020 0.020 0.065 0.180 - 0.420 -0.020 0.165 0.120 0.460 0.170 1.8 0.200 0.020 0.020 - - - - - - - - 0.170 2.1 0.200 0.020 0.020 - - - - - - - - 0.170 1.1 0.200 0.027 0.027 -0.025 0.105 0.110 0.340 -0.025 0.100 0.145 0.340 0.170 1.5 0.200 0.027 0.027 -0.025 0.140 0.100 0.400 -0.025 0.105 0.120 0.340 0.170 1.8 0.200 0.027 0.027 -0.025 0.140 0.060 0.400 -0.025 0.120 0.100 0.380 0.170 2.1 0.200 0.027 0.027 0.085 0.170 - 0.470 -0.025 0.155 0.070 0.480 0.170 1.1 0.200 0.061 0.061 -0.060 0.105 0.110 0.390 -0.060 0.075 0.120 0.300 0.170 1.5 0.200 0.061 0.061 -0.060 0.115 0.070 0.400 -0.060 0.085 0.120 0.310 0.170 1.8 0.200 0.061 0.061 -0.060 0.130 0.060 0.410 -0.060 0.085 0.060 0.320 0.170 2.1 0.200 0.061 0.061 -0.015 0.155 - 0.440 -0.060 0.115 0.050 0.340 Cable-Tied Block Protection 0.100 1.1 0.100 - 0.000 -0.005 0.065 - 0.071 -0.005 0.060 - 0.076 0.100 1.4 0.100 - 0.000 0.020 0.090 - 0.071 0.015 0.080 - 0.076 0.100 1.8 0.100 - 0.000 - - - - - - - - 0.100 2.2 0.100 - 0.000 - - - - - - - - 0.170 1.1 0.100 - 0.000 0.075 0.145 - 0.071 0.065 0.135 - 0.071 0.170 1.5 0.100 - 0.000 - - - - - - - - 0.170 1.8 0.100 - 0.000 - - - - - - - - 0.170 2.1 0.100 - 0.000 - - - - - - - - 0.100 1.1 0.150 - 0.000 -0.005 0.065 - 0.133 -0.005 0.060 0.000 0.137 0.100 1.4 0.150 - 0.000 0.000 0.085 - 0.124 -0.005 0.075 0.000 0.130 0.100 1.8 0.150 - 0.000 0.000 0.085 - 0.124 -0.005 0.075 - 0.127 0.100 2.2 0.150 - 0.000 - - - - - - - - Upstream Downstream ym (m) V/Vc W (m) d50 (m) db (m) ds1 (m) ds2 (m) Wmin (m) α 2 (m) ds1 (m) ds2 (m) Wmin (m) α2 (m) 0.170 1.1 0.150 - 0.000 0.005 0.110 - 0.107 0.000 0.085 0.000 0.124 0.170 1.5 0.150 - 0.000 0.035 0.120 - 0.124 0.000 0.105 0.000 0.107 0.170 1.8 0.150 - 0.000 - - - - - - - - 0.170 2.1 0.150 - 0.000 - - - - - - - - 0.100 1.1 0.200 - 0.000 -0.005 0.070 0.100 0.171 0.100 0.176 0.100 0.176 0.100 1.4 0.200 - 0.000 -0.005 0.080 0.100 0.160 0.075 0.179 0.075 0.179 0.100 1.8 0.200 - 0.000 0.010 0.085 0.100 0.153 0.100 0.166 0.100 0.166 0.100 2.2 0.200 - 0.000 0.015 0.085 - - 0.075 0.167 0.075 0.167 0.170 1.1 0.200 - 0.000 -0.005 0.105 0.050 0.157 0.075 0.175 0.075 0.175 0.170 1.5 0.200 - 0.000 0.005 0.135 0.025 0.136 0.025 0.147 0.025 0.147 0.170 1.8 0.200 - 0.000 0.000 0.135 0.025 0.136 0.075 0.175 0.075 0.175 0.170 2.1 0.200 - 0.000 Over-turned 0.000 0.120 0.000 0.120 0.100 1.1 0.300 - 0.000 -0.005 0.070 0.200 0.271 0.200 0.276 0.200 0.276 0.100 1.4 0.300 - 0.000 -0.005 0.070 0.200 0.271 0.175 0.279 0.175 0.279 0.100 1.8 0.300 - 0.000 0.005 0.150 0.125 0.215 0.175 0.267 0.175 0.267 0.100 2.2 0.300 - 0.000 0.010 0.155 0.100 0.226 0.150 0.262 0.150 0.262 0.170 1.1 0.300 - 0.000 -0.005 0.070 0.175 0.279 0.225 0.276 0.225 0.276 0.170 1.5 0.300 - 0.000 -0.005 0.125 0.100 0.256 0.150 0.280 0.150 0.280 0.170 1.8 0.300 - 0.000 0.005 0.130 0.125 0.242 0.175 0.262 0.175 0.262 0.170 2.1 0.300 - 0.000 0.035 0.185 - - 0.100 0.243 0.100 0.243 0.100 1.1 0.400 - 0.000 -0.005 0.070 0.300 0.371 0.300 0.376 0.300 0.376 0.100 1.4 0.400 - 0.000 -0.005 0.075 0.300 0.366 0.300 0.371 0.300 0.371 0.100 1.8 0.400 - 0.000 -0.005 0.095 0.275 0.356 0.275 0.367 0.275 0.367 0.100 2.2 0.400 - 0.000 0.005 0.150 0.200 0.332 0.200 0.352 0.200 0.352 0.170 1.1 0.400 - 0.000 -0.005 0.065 0.300 0.376 0.300 0.384 0.300 0.384 0.170 1.5 0.400 - 0.000 -0.005 0.090 0.250 0.370 0.300 0.376 0.300 0.376 0.170 1.8 0.400 - 0.000 -0.005 0.135 0.200 0.348 0.275 0.367 0.275 0.367 0.170 2.1 0.400 - 0.000 -0.005 0.195 - - 0.250 0.325 0.250 0.325 Table 7-5. (Continued).

the filter layer could become exposed if riprap shear failure or excessive apron settlement occurred—that is, for the experiment, d50  27 mm and V/Vc  2.1 shown in Figure 7-20 and the experiment db  0 and V/Vc  1.8 shown in Figure 7-18. Lauchlan (1999) concluded that increasing the riprap apron layer thickness increases the scour protection by reduc- ing the winnowing of bed sediment from underneath the riprap blanket. This conclusion could not be made in the cur- rent wing-wall abutment scour countermeasure study because all of the experiments were run with a filter placed underneath the riprap apron, thereby preventing winnowing from occurring. However, the preliminary experiments showed that increasing the riprap layer thickness increased the scour protection by reducing the extent of the apron destabilized by the propagation of bed forms. Lauchlan (1999) identified the apron placement level rel- ative to the average bed level as the most important param- eter affecting the performance of the riprap apron. Because bed-form destabilization is the prominent failure mecha- nism under mobile-bed conditions, and because less mate- 90 Figure 7-14. Performance of riprap apron protection at wing-wall abutments under live-bed conditions (db  2d50  80 mm, and ym  100 mm). present study. Shear, winnowing, and edge failure were all found to occur in both the clear-water experiments with spill-through abutments and the live-bed experiments with wing-wall abutments. Additionally, under mobile bed condi- tions, Lauchlan (1999) identified destabilization of the riprap through the propagation of bed forms as another fail- ure mechanism for riprap aprons. Both in the present wing- wall abutment scour countermeasure study and in the pier scour countermeasure study by Lauchlan (1999), destabi- lization of the riprap through the propagation of bed forms was observed to be the dominant failure mode of riprap aprons. Shear failure of the riprap occurred when the flow velocity exceeded the critical velocity of the riprap stones (Figure 7-20). Winnowing failure was the dominant failure mode at the upstream corner of the abutment for the preliminary exper- iment that was run without filter fabric placed underneath the apron. Lauchlan (1999) observed that sediment beneath the riprap apron cannot be winnowed through the riprap layers when a filter is used, also preventing the riprap from subsiding into the bed. However, under high flow velocities,

rial is undermined from the apron by the troughs of the propagating bed forms, increasing the burial depth increases the stability of the apron. The current experiments show that as the burial depth is increased, apron width can be reduced to afford similar levels of scour protection at the abutment. Lauchlan (1999) also observed that, with increased burial depths, the riprap stones are less susceptible to shear failure, but this effect was not investigated in this experiment. 7.2.4 Discussion Comparison of the Experimental Results with the Clear-Water Spill-Through Abutment Study For the spill-through abutment clear-water scour counter- measure experiments, settlement (or scour) depth reduced with increased apron width (for the case where e  Bf), because the local scour hole was deflected farther from the abutment. Similar trends would also be expected for live-bed conditions, but were not observed in the present study. Similitude between laboratory experiments and field scale was satisfied by the use of the aforementioned u*/u*c ratio, of which a value of just below 1.0 represents a condition called “clear-water scour.” This condition is extreme for scouring because the velocity is as high as possible without the movement of the channel bed, which causes infilling of the sediment hole. The maximum equilibrium scour depth as a function of the mean flow velocity occurs at threshold conditions and decreases slightly thereafter with increasing velocity (Melville and Coleman, 2000). The two preliminary experiments that were run just below threshold conditions with no protection gave the maximum equilibrium local scour depth at the abut- ment for the two flow depths used. For both experiments, the deepest point of scour occurred at the upstream corner of the abutment face, with maximum scour depths of 80 mm for the 100-mm flow depth and 100 mm for the 170-mm flow depth. The preliminary experiments were repeated with a 200-mm wide apron in place, for which the maximum scour depth was significantly reduced, consistent with the results in the clear-water spill-through abutment study. 91 Figure 7-15. Performance of riprap apron protection at wing-wall abutments under live-bed conditions (db  2d50  80 mm, and ym  170 mm).

Contrary to expectations, the measured scour depths for clear-water conditions are considerably smaller than the measured scour depths at the outer edge of the apron for live- bed conditions. It is apparent that the troughs of the bed forms in the live-bed experimental work were very much deeper than the local scour at the abutment; the former, therefore, dominated the maximum scour depth ds2. Bed Forms Section 7.2.3 outlines how the bed forms affect the coun- termeasure aprons, and the previous paragraph shows that the scour at the abutment is governed by the largest bed forms that propagate past the abutment (for the wing-wall abut- ment configuration tested). Van Rijn (1984) and Yalin (1992) both presented methods to predict the average equilibrium bed form heights and lengths, but it is the maximum equilib- rium bed form height that dominates the scour at the abut- ment. Ashley (1990) compared bed form height-to-length ratios for 1,500 subaqueous bed forms and developed an expression for the maximum bed form height λH-max: (7-3) H L ave− −=max . .0 16 0 84 Where λL-ave is the average bed form length. Figure 7-22 com- pares the measured λH-max values from Table 7-4 with the pre- dicted λH-max values from Equation 7-3 for the eight different flow conditions run in the flume. There is a good correlation between the measured and predicted λH-max values, confirm- ing that Equation 7-3 developed is a good estimate for pre- dicting λH-max, where λL-ave is estimated from the expression developed by Yalin (1992). Riprap Stability For the experimental series shown in Figure 7-20, the flow velocity was systematically increased for the different riprap sizes until shear failure of the riprap occurred. Shear failure occurs where the flow dislodges the riprap stones from the apron and carries them downstream.The experiments for which shear fail- ure occurred can be seen in Figure 7-20 at the bottom of each of the columns. The dislodged riprap stones that were carried downstream by the flow are circled for each of these experi- ments. Table 7-6 summarizes the upstream and bridge section surface flow velocities V2-surf, and the bridge section depth- averaged flow velocities V2-ave, for each of the experiments shown in Figure 7-20. The bridge section velocities were obtained from 92 Figure 7-16. Performance of cable-tied block apron protection at wing-wall abutments under live-bed conditions (db  0, and ym  100 mm).

the PTV flow field measurements and acoustic Doppler velocimeter measurements of the vertical velocity distribution. Figure 7-23 shows the riprap size normalized with the flow depth as a function of the Froude number at the bridge sec- tion Fr2, as well as the riprap sizing equation from Lagasse et al. (2001) for vertical-wall abutments. The experimental data for the stable riprap are depicted by solid black symbols and lie above the curve. Likewise, the experimental data for the unstable riprap (experiments where shear failure occurred) are depicted by hollow symbols and lie below the curve. The comparison in Figure 7-23 confirms that either the Lagasse et al. (2001) equation or the Pagan-Ortiz (1991) equation, with appropriate factors of safety, is suitable for predicting riprap stone sizes that are resistant to shear failure at wing-wall abutments. As noted above, riprap size selection is appropriately based on stability against shear and edge fail- ure, although consideration of the possibility of winnowing or bed form undermining is also important in design. Cable-Tied Block Stability Cable-tied block aprons are subject to two observed flow- induced failure modes, as described by Parker et al. (1998). The failure modes are overturning and roll-up of the leading edge of a cable-tied block mat, which can occur in the absence of sufficient anchoring or toeing in of the leading edge, and uplift of the inner mat, which occurs at higher flow velocities when the leading edge is sufficiently anchored. The cable-tied block aprons used in the countermeasure experiments were sized so that the blocks were large enough for the mat to resist uplift failure at the highest flow velocity condition. In the pre- liminary cable-tied block experiments, overturning of the lead- ing edge of the mat was observed to occur for the two highest flow velocity conditions. To prevent the leading edge of the cable-tied block mat from overturning at the higher flow veloc- ities, type B3 blocks were fixed onto the front of the mat to anchor the leading edge. Van Ballegooy (2005) investigated the stability against overturning of the leading edge of cable-tied block mats, rec- ommending the following expression: (7-4) Where: C  critical dimensionless shear stress, Pb  protrusion of the blocks above bed level, and L  horizontal dimension of the blocks. C bP L = + − ⎛⎝⎜ ⎞⎠⎟0 002 0 06 12. . exp 93 Figure 7-17. Performance of cable-tied block apron protection at wing-wall abutments under live-bed conditions (db  0, and ym  170 mm).

W = 100 mm W = 200 mm (7-5) Where: Scb  specific gravity of the blocks and n  Manning coefficient. H Y S Fr n Y b cb = −( ) 158 1 2 2 0 33. 94 Figure 7-18. Performance of riprap apron protection at wing-wall abutments under live-bed conditions (d50  40 mm, and ym  170 mm). Van Ballegooy showed that this equation was conservative when applied to a cable-tied block mat buried with its surface flush with the surrounding sediment bed—that is, for zero pro- trusion (Pb  0). For this condition, C  0.062 and Equation 7-4 can be expressed in the following form using the Manning equation for flow resistance:

W = 300 mm W = 400 mm Equation 7-5 provides a simple means of estimating block size to resist failure due to overturning and roll-up of the leading edge. In use of Equation 7-5, care needs to be taken to ensure that the leading edge of the mat remains buried. Data Analysis As is apparent from the data in Table 7-5, apron settlement and corresponding scour depths at the upstream end of the apron were typically larger than equivalent values at the 95 Figure 7-19. Performance of riprap apron protection at wing-wall abutments under live-bed conditions (d50  40 mm, and ym  170 mm).

downstream end because the bed forms were larger at the upstream end. At the downstream end of the abutment, the flow was fully contracted in the main channel. Conse- quently, the velocity increased, which caused the crests of the bed forms to wash out and the troughs to fill in because of a limitation of sediment supply. The measured scour depths are shown in Figure 7-24 as a functions of approach-flow inten- sity V/Vc. The data are scattered due to the variability in bed form height and consequent apron settlement, but a trend of increased settlement with both increasing flow depth and increasing flow intensity is apparent. Since bed form height increases with both flow depth and flow velocity, the measured scour depths were plotted as a function of the maximum bed form height λH-max.Again, the data are scattered, but a trend of increased settlement with increasing λH-max is apparent. Most of the settlement depths for the upstream corner of the abut- ment are less than 1.2λH-max, and for the downstream corner of the abutment they are equal to about 1.0λH-max. Figure 7-25 shows the scour depth as a function of the maximum bed-form height. 96 Figure 7-20. Performance of riprap apron protection at wing-wall abutments under live-bed conditions (db  1d50, W  200 mm, and ym  170 mm). Figure 7-21. Riprap movement in response to bed-form propagation.

ym (m) V/Vc V2-surf (ms-1) V2-ave (ms-1) Fr2 d50 (m) d50/ym Riprap Stability 0.170 1.1 0.50 0.43 0.33 0.020 0.12 Stable 0.170 1.5 0.71 0.62 0.48 0.020 0.12 Shear Failure 0.170 1.1 0.50 0.43 0.33 0.027 0.16 Stable 0.170 1.5 0.71 0.62 0.48 0.027 0.16 Stable 0.170 1.8 0.79 0.69 0.53 0.027 0.16 Shear Failure 0.170 2.1 0.96 0.84 0.65 0.027 0.16 Shear Failure 0.100 1.1 0.49 0.43 0.43 0.040 0.40 Stable 0.100 1.4 0.59 0.51 0.51 0.040 0.40 Stable 0.100 1.8 0.78 0.64 0.64 0.040 0.40 Stable 0.100 2.2 1.01 0.87 0.88 0.040 0.40 Shear Failure 0.170 1.1 0.50 0.43 0.33 0.040 0.24 Stable 0.170 1.5 0.71 0.62 0.48 0.040 0.24 Stable 0.170 1.8 0.79 0.69 0.53 0.040 0.24 Stable 0.170 2.1 0.96 0.84 0.65 0.040 0.24 Shear Failure 0.170 1.1 0.50 0.43 0.33 0.060 0.35 Stable 0.170 1.5 0.71 0.62 0.48 0.060 0.35 Stable 0.170 1.8 0.79 0.69 0.53 0.060 0.35 Stable 0.170 2.1 0.96 0.84 0.65 0.060 0.35 Stable 0.170 2.6 1.10 0.96 0.74 0.060 0.35 Shear Failure Because the apron settlement ds2 is a function of the maxi- mum bed form height λH-max, an approximate envelope approximation for the settlement depth is given by the following equation: (7-6) Where C4  1.2 and 1.0 for the upstream and downstream abutment corner positions, respectively, and λL-ave can be approximated from Yalin (1992). Figure 7-26 shows the meas- ured apron settlements plotted against the predicted apron settlements using Equation 7-6. From the figure, it can be seen that Equation 7-6 envelops the bed form induced settle- ments reasonably well. d C Cs H L ave2 4 4 0 840 16≈ = − − max . . The angle β (see Figure 7-13) was calculated for each experiment from the measured values of ds1, ds2, Wmin, and 2 using the following expression: (7-7) The values of for the wing-wall abutment scour experi- ments are shown in Figure 7-27, with average values of 25 degrees for the riprap aprons and 40 degrees for the cable-tied block aprons at both the upstream and downstream corners of the abutment. The different values of for the riprap and cable-tied block protection can be directly related to the  = − − ⎛ ⎝⎜ ⎞ ⎠⎟ −tan 1 2 1 2 d d W s s min 97 Figure 7-22. Comparison of measured maximum bed-form heights with predicted maximum bed-form heights using Equation 7-3. Table 7-6. Flow parameters at the wing-wall bridge section.

manner in which the aprons settled. Because the cables pre- vented the cable-tied block aprons from increasing in width, sand was eroded from beneath the apron, and the outer edge of the apron folded down, retaining the sand at an angle larger than the repose angle of the sand. When a riprap apron was undermined, the apron increased in width as the riprap stones rolled forward and down into the bottom of the troughs. The slope of the undermined section of the riprap apron was typ- ically slightly less than the repose angle of the sand. Figure 7-28 is a schematic diagram showing the settlement of an idealized riprap apron. The thickness of the apron is nd50, where n is the number of layers of riprap. For cable-tied block mats, n  1 and nd50 is taken equal to the block height H. For the case when Wmin 0, ds1 and db are related as follows: (7-8) Where ds1 is taken to be negative when the top of the apron is above the average bed level (nd50 db) and vice versa. Figure 7-28 shows the settlement and spreading of five riprap stones. From the experimental work, the portion of apron that settled (W-Wmin) was observed to increase with increasing ds2 and decrease with increasing db, as shown in Figure 7-29. The spread length (slope length) Ls of the apron after settlement can be given by the following equation: (7-9)L C d d n d n s s b = − + −( )⎡⎣ ⎤⎦5 2 501 sin d d nds b1 50= − 98 Figure 7-24. Scour depth as a function of flow intensity for both the upstream and downstream corners of the abutment. Figure 7-23. Normalized riprap size for apron protec- tion at the bridge abutments as a function of the Froude number of the flow in the contracted bridge section.

99 Figure 7-25. Scour depth at the abutment as a function of the maximum bed-form height for both the upstream and down- stream corners of the abutment. Figure 7-26. Comparison of the predicted scour depths using Equation 7-6 with the measured scour depth for both the upstream and downstream corners of the abutment.

Where the coefficient C5 varies for the upstream and down- stream locations depending on the direction of movement of the undermined riprap stones. At the upstream corner of the wing- wall abutment, the riprap stones moved both laterally away from the abutment and upstream when rolling into the scour regions. The distance Ls for the upstream corner was aligned 45 degrees to the abutment face, and C5 = . At the downstream corner of the wing-wall abutment, the riprap stones rolled laterally away from the abutment into the scour region. The distance Ls for the downstream corner was aligned perpendicular to the abutment face, and C5 = 1. A value of C5 = 1 applied to cable-tied block aprons, irrespective of the location. An expression for Wmin can be derived as follows: (7-10) Where the coefficient C6 represents the proportion of Ls that was covered by riprap stones. Figure 7-30 shows a plot of the experimental data for riprap aprons in a rearranged form of Equation 7-10. The figure shows that a value of C6 = 0.8 is appropriate. For riprap, C5 = 1 at the downstream corner of the riprap layer, C5 = at the upstream corner, C6 = 0.8, and = 252 W W C L W C C d d n d n s s b min = − = − − + −( )⎡⎣ ⎤⎦ 6 5 6 2 501 sin 2 degrees. The limiting condition for design is when Wmin = 0, when Equation 7-10 reduces to the following: (7-11) Where the coefficient C1 = 1.68 and 1.19 at the upstream and downstream corners of the riprap layer, respectively. For cable-tied block aprons, C6 = 1 because the cables in the mat prevented spreading. Hence, Equation 7-11 reduces to (7-12) The limiting condition for design is when Wmin = 0. With = 40 degrees for cable-tied blocks, Equation 7-12 reduces to: (7-13) Equations 7-12 and 7-13 both imply that Wmin increases linearly with W, which is in agreement with the experimental data from Table 7-5. The equations also imply that Wmin decreases with increasing ds2 and increases with increasing db. The reduced minimum apron width for increased scour depth is a consequence of larger bed forms propagating through the bridge section as a result of deeper flows and higher flow velocities. Regarding placement level, the deeper the apron is buried below the average bed level, the smaller the volume of the material undermined from the apron dur- ing bed form propagation (consistent with the experimental study of Korkut, 2004). When the apron is buried below the expected scour depth (db > ds), the apron cannot be under- mined, so there is no apron loss (W = Wmin). When the pre- dicted values of Wmin (from Equations 7-12 and 7-13) are less than zero, settlement occurs at the abutment face. Figure 7-31 compares the measured Wmin data with the predicted Wmin values using Equations 7-12 and 7-13. Figure 7-31 demonstrates a reasonable agreement between the measured and predicted Wmin values, even though there is a lot of scatter in the measured values of Wmin. The protec- tive aprons settled in response to the propagation of bed forms through the bridge section. The bed forms are inher- ently variable in size and shape, and, as a result, the associ- ated apron settlement is also inherently variable. This is the reason for the scatter in measured values of Wmin shown in Figure 7-31. Equations 7-12 and 7-13 have a similar structure with the exception of the factor C5C6/nsin , which takes values of 1.34 and 0.95 for riprap protection (n = 2) at the upstream and downstream corners, respectively, and 1.56 for cable-tied block protection. At the upstream corner of the abutment, Wmin is slightly less for cable-tied block aprons than for riprap aprons, while Wmin values are considerably larger for riprap aprons at the downstream corner. The implication is W d ds b= −1 55. ( ) W W d ds b min = − −[ ]2 sin W C d d ds b= − +1 2 50( ) 100 Figure 7-27. Angle of apron settlement as a function of flow intensity for both the upstream and down- stream corners of the abutment. Figure 7-28. Apron settlement due to undermining of an idealized riprap apron.

101 Figure 7-29. Portion of apron that was undermined (W-Wmin) as a function of the scour depth for both the upstream and downstream corners of the abutment. Figure 7-30. Portion of apron that was undermined (W-Wmin) as a function of the riprap spread distance for both the upstream and downstream corners of the abutment.

that cable-tied block aprons need to be wider than riprap aprons (with two riprap layers) to afford the same level of protection at wing-wall abutments. Conversely, for the case where only one layer of riprap is used in an apron, such an apron will need to be considerably wider than a cable-tied block apron to afford the same level of protection at the abutment. For Case I in Figure 7-13, where Wmin 0, an expression for 2 can be developed (as shown in Figure 7-30) as follows: (7-14) 2 2 501 = + − + −( )⎡⎣ ⎤⎦W d d n ds bmin tan By substituting Equation 7-12 into 7-14, a simplified expression can be developed for 2 as follows: (7-15) For cable-tied block protection, C5, C6, and n all equal 1, so Equation 7-15 reduces to the following: (7-16) Figure 7-32 compares the measured 2 data with the pre- dicted 2 values using Equations 7-15 and 7-16. Equations 7-15  2 2 1 = + −⎛ ⎝⎜ ⎞ ⎠⎟ −[ ]W d ds b cos sin  2 5 6 2 501= + −⎛ ⎝⎜ ⎞ ⎠⎟ − + −( )⎡⎣W n C C n d d n ds b cos sin ⎤⎦ 102 Figure 7-31. Comparison of the predicted values of Wmin using Equations 7-12 and 7-13 with the measured values of Wmin for both riprap and cable-tied block protection. Figure 7-32. Comparison of the predicted values of 2 using Equations 7-15 and 7-16 with the measured values of 2, for both riprap and cable-tied block protection.

and 7-16 tend to underpredict the α2 values slightly, but overall there is a good agreement between the measured and predicted 2 values.Underprediction of the 2 values is conservative,how- ever, because the troughs of the bed forms are predicted to pass closer to the abutment face. Equations 7-15 and 7-16 have a similar structure with the exception of the factor (ncos – C5C6)/nsin , which takes values of 0.81 and 1.19 for riprap protection (n  2) at the upstream and downstream corners, respectively, and −0.36 for cable-tied block protection. This shows that α2 exceeds W for riprap aprons after settlement. The stones from the riprap apron tend to settle and move away from the abutment face, deflecting the troughs of the bed forms farther away from the abutment. Conversely, 2 is slightly less than W for cable-tied block aprons after settlement, allowing the troughs of the bed forms to pass closer to the abutment face. For the case where ds1 0 (Case II in Figure 7-13), insuffi- cient protection has been placed around the abutment (Wmin  0). In this situation, a riprap apron, which settles at the abutment face due to insufficient extent, will still afford some protection because the scour depth at the abutment face is typically less than what it would be if no protection had been provided. Conversely, a cable-tied block apron with insuffi- cient apron width offers minimal protection and typically induces deeper scour at the abutment face because the apron, being attached to the abutment face, creates a larger obstruc- tion to the flow. 7.3 Experiments on Aprons of Geobags and Riprap The program of experiments consisted of the following sets of investigations: 1. Diagnostic experiments on geobag stability at solid abut- ment; 2. Baseline scour condition at the single, pile-supported abutment (without an apron); 3. Scour performance of riprap aprons at a single, pile- supported abutment; 4. Scour performance of geobag aprons at a single, pile- supported abutment; and 5. Scour performance of an armor apron across the entire bed of a short bridge. By virtue of the inherent trial-and-error nature of the study—to determine how well an apron worked—consider- ably more experiments were conducted than are presented herein. Many of the experiments were ended without the scour having reached an equilibrium state; if a test apron failed, there was little point in continuing the experiment. Korkut (2004) and Morales (2006) provide full documenta- tion of the experiments. 7.3.1 Experiment Layout Flume experiments were conducted using the same flume as that used for the preliminary experiments described in Chapter 6. The flume was 27.4 m long, 0.91 m wide, and 0.45 m deep. The flume recirculated sediment. The flume was fit- ted with a 200-mm deep bed of sand. Figure 7-33 shows the sand bed along the flume. Approach-Flow Conditions The experiments were conducted under live-bed flow con- ditions, with u*/u*c  1.5 (here u* is the shear velocity, and u*c is the critical value of the shear velocity associated with bed particle movement). The main hydraulic parameters for the flume flow were the following: mean velocity, V0  0.55 m/s; and flow depth, y0  100 mm. The sediment parameters were the following: median particle size, d50  0.45 mm; standard deviation of sediment size, g  1.4; specific gravity of parti- cles  2.4; and, the angle of sediment repose,   30 degrees. The average height of the dunes moving along the flume bed was 34 mm. Similitude between laboratory experiments and field scale was satisfied by the use of the aforementioned u*/u*c ratio, of which a value above 1.0 represents a condi- tion called “live-bed scour.” This condition is extreme for scouring of armouring units such as geobags because the high velocity can dislodge a geobag and therefore constitute failure. 103 Figure 7-33. Flume with sand bed in dune regime.

Wing-Wall Configurations The bulk of the experiments were conducted using a sin- gle wing-wall abutment that replicated, at a scale correspon- ding to about 1:40, the width of abutments typical of two-lane roads in the United States; the road width is about 12 m (40 ft). Three variations on this abutment form were used: 1. A solid wing-wall abutment whose walls extended the full bed depth. Such an abutment in practice would be founded on sheet piles. The abutment was made from Plexiglas to facilitate observation of geobag behavior and scour development. Figure 7-34 shows this test abutment. These experiments were useful for obtaining bed-level observations concerning the manner whereby the flow field around a wing-wall abutment could entrain geobags and at times erode sand from around geobags. 2. A wing-wall abutment on a pile cap supported on two rows of circular piles. Figure 7-35 shows the dimensions of this model abutment, which was used for the final per- formance testing of riprap and geobag aprons. 3. A pair of opposing wing-wall abutments, representing the layout of a small short-span bridge. Figure 7-36 illustrates this layout, which was used for determining the performance of an apron extending between two opposing abutments. Depending on the presence and the arrangement of an apron at an abutment, the maximum depth of scour could occur at any of three locations, as indicated in Figure 7-37: in 104 Figure 7-34. Plexiglas abutment used for viewing geobag stability. Figure 7-35. Model dimensions of a 45-degree wing-wall abut- ment of the pile-supported form commonly used in the United States.

Fr KS n ≤0.8 1.02 2.0 >0.8 0.69 0.1 front of the abutment itself (A), in front of loose apron pro- tection (B), and downstream of an apron formed of armor elements linked like a mattress (C). Table 7-5 lists the selec- tion of scour experiments reported herein. Riprap Sizing The diameter of riprap stone used was estimated using the relationship proposed by Richardson and Davis (1995): (7-17) Where: DR  equivalent riprap diameter, Y  flow depth in the bridge section, KS  shape factor associated with abutment shape (wing- wall or spill-through), D Y K S FrR s S n = −( )1 Fr = U/(gY)0.5 = Froude number for the mean flow in the bridge (con- tracted) section, and SS = specific gravity of riprap stone. For wing-wall abutments, values of KS and n are given in Table 7-7 (Richardson and Davis, 1995). For the flow conditions used in the experiments using SS = 1.4 and determined for the available stone, Equation 7-17 gives DR = 22 mm, which is about the same as the required thickness of geobag estimated using Equation 7-15. The riprap stone that was selected for the flume tests was sieved so that DR 22 mm, with a shape factor (major axis/minor axis) of about 2.1. Geobag Sizing The simulated, large geobags were sized in accordance with a design method proposed by Pilarczyk (2000). The method estimates a geobag thickness, DB. The aerial extent of a geobag should exceed DB and otherwise can be sized for handling ease or to fit a site. The general form of Pilarczyk’s relationship for geobag thickness is as follows: (7-18) Where: SSB = specific gravity of the geobag; V = depth-averaged mean velocity; g = gravity acceleration; = stability parameter; C = critical value of the Shields parameter for particle (geobag) entrainment; KT = turbulence factor; Kh = depth parameter; and Ksl = slope parameter, expressed as: (7-19) Where  is the angle of the boundary on which the geobag is placed, and C is the angle of repose of the sediment form- ing the boundary. For the experiments,  and were 26.7 degrees and 30 degrees, respectively. Pilarczyk (2000), who gives the background to Equation 7-19, recommends for geobags that , C, and KT be 0.75, 0.05, and 2.0, respectively. Ksl = − ⎡ ⎣⎢ ⎤ ⎦⎥1 2 2 sin sin  D S K K K V gn SB C T h sl = −( ) 0 035 1 2 2 . 105 Figure 7-36. Layout of two opposing wing-wall abutments. Figure 7-37. Locations of deepest scour: A, no protec- tive apron; B, loose protective apron that partially fails; C, protective apron tied to abutment. Table 7-7. Values of Ks and n for Equation 7-17 (Richardson and Davis, 1995).

The depth parameter Kh is defined as a function of water depth y and equivalent roughness ks. Pilarczyk suggests using ks  Dn. However, since Dn is unknown initially, the measured thickness of the geobag sample was used as a trial value. The required thickness of the geobags, Dn, was calculated as 22 mm using a bulk-specific gravity of the model geobags meas- ured to be 1.46. Accordingly, the model geobags used in the experiments were selected to be 22 mm thick and 95 × 55 mm in plan area. 7.3.2 Procedure In general terms, the experimental procedure was similar to that used for the experiments at the University of Auckland.The bed sediment was placed as a 0.2-m thick layer along the whole length of the flume. A trial-and-error procedure was used to adjust the flow through the flume so as to obtain uniform flow 0.1 m deep and an average velocity (discharge/flow area) that was 1.5 times greater than the critical velocity for incipient motion of the bed sediment. Achieving this flow condition required adjustment of flume slope once the appropriate dis- charge had been set.Water depth in the flume was controlled by means of a tailgate. The tailgate was adjusted to set the water depth, yet still enable sediment to pass into the tail box. Once the flow condition had been set, the flow was left running through the flume for about 2 days so that the bed forms (i.e., dunes) over the bed would attain nominally steady dimensions. The average height and length of the dunes was fairly constant along the flume, though some scatter in magnitudes occurred. Once the bed forms were fully developed, the flume slope was adjusted so as to en- sure that the requisite flow depth occurred for the discharge over the dune bed. To check for overall uniformity of flow depth, water surface elevations were measured at 10 posi- tions along the flume. Additionally, an acoustic Doppler velocimeter (ADV) was used to measure the velocity profiles to check with the estimated velocity based on flow depth and discharge. With the flow condition determined and the flume slope set, the flume was drained and the test abutment placed in the flume. The bed at the abutment location was levelled when the abutment was placed, but the bed elsewhere in the flume remained in the dune regime condition, thereby enabling the flow to quickly establish itself when the experiment began. The flume was filled to the prescribed depth, the flume’s pump started, and the experiment then begun. Slight adjust- ment of water level occurred early in the experiment to ensure that the prescribed average water depth prevailed along the flume. The experiments varied in their duration. Tests in which the apron arrangement failed were stopped soon after the failure occurred.Typically, the scour reached an equilibrium condition shortly after apron failure, owing mainly to the live-bed condi- tion of the experiment.Tests in which the apron remained intact were run for 2 days, over which time the bed conditions were monitored periodically.At the end of each experiment, the loca- tion and depth of maximum scour were recorded. 7.3.3 Results: Solid-Wall Abutments Described in this section are experiments conducted to meas- ure the performance of the geobags as a scour countermeasure. The experiments progressed from a baseline condition that was used as a reference for the subsequent experiments toward the solution.The effectiveness of each countermeasure was assessed in terms of the reduction of the scour depth that occurred when the abutment was not protected by a countermeasure. The geometry of the wing-wall abutment used throughout the experiments is given in Figure 7-38. Baseline Scour This experiment produced a reference, baseline scour depth for use in evaluating the effect of the countermeasure applied to reduce the scour depth. The baseline experiment was conducted without a countermeasure. The maximum scour depth occurred at the upstream corner of the abutment and was 162 mm. As mentioned above, the dunes moving along the flume had a height of 34 mm. Geobags on Bed Surface In this experiment, an apron of loose geobags was placed on the bed surface around the abutment. The geobags were 55 mm wide, 95 mm long, and 14 mm thick. Figure 7-39 shows the apron. 106 Figure 7-38. Maximum scour depth for baseline scour.

The geobag apron failed to withstand the flow. Some geobags became embedded in the sediment, and some were rolled away from the abutment. The apron began failing along the abutment’s streamwise face. Bed sediment was winnowed from around the edges of the geobags and the face of the abutment. Consequently, scour still began around the abut- ment’s sides and beneath the geobags. As scour progressed, some geobags became exposed to higher flow velocity and subsequently became unstable. Eventually, the flow forces caused some of these geobags to turn over and move away from the abutment. Gradually, the apron disintegrated. Figures 7-40 through 7-44 depict this failure process. The maximum scour depth, measured at the upstream corner of the abutment, was 156 mm, only a 4-percent reduction of the baseline scour depth (Figure 7-44). Geobags on Filter Cloth For this experiment, a geotextile underlay fabric, acting as a geofilter cloth, was used in combination with geobags. Figure 7-45 shows the layout of the geobags and filter cloth. The gap between the filter cloth and abutment was sealed in order to minimize the sediment winnowing. Figure 7-45 shows the apron at the start of the experiment. During this experiment, however, sediment was eroded from around the perimeter of the apron. This erosion under- mined the apron’s upstream row of geobags and gradually led to the apron’s complete failure. Geobags slid into the scour hole that formed around the apron. The collapse of the front row of geobags exposed the latter rows and filter cloth. Water flow lifted the filter cloth and caused most of the remaining geobags to slide into the scour hole. Subsequently, a highly 107 Figure 7-39. Layout of loose geobags placed as an apron. Figure 7-40. Scour begins at the interface of the geobag apron and the abutment. Figure 7-41. Flow entrains a geobag at abutment face. Figure 7-42. Flow lifts a geobag from the apron.

turbulent flow formed around the abutment and jumble of the filter cloth and geobags and resulted in a scour hole that was deeper than the baseline scour hole. The failure process is shown in Figures 7-46 through 7-49. Apron of Tied Geobags on Filter Cloth In this experiment, the geobags were tied to each other along their longitudinal axis, which extended around the perimeter of the abutment. The geobags were tied to prevent them from being entrained by flow or sliding into the scour hole and to enable them to form overall a more flexible apron around the abutment. This apron arrangement remained stable during much of the experiment, but eventually it too failed, owing to the same 108 Figure 7-43. The geobag is swept from the apron, which then begins to break up. Figure 7-44. An apron of geobags is placed on a geofilter cloth, which was fixed to the abutment. Figure 7-45. Apron of geobags on a geofilter cloth. Figure 7-46. Sediment erosion from around and beneath the apron; geobags on geofilter. Figure 7-47. Side view of the final scour hole; geobags on geofilter.

scour process described above. The resulting scour hole is shown in Figure 7-50. It is evident that the upstream corner of the front row of the tied geobags slid into the scour hole. Since the geobags were tied to each other, the movement at the corner led the rear bags to slide toward the direction of the scour hole. Once the rear bags were displaced, the filter cloth along the upstream face of the abutment began to lift and expose the underlying sediment. Subsequently, the two other rows of the geobags forming the apron slid apart. This same experiment was repeated with the geobags tied in all side directions (i.e., geobags were tied to geobags all around). The intent was to prevent the front row of geobags from sliding into the scour hole developing around the geobag system. However, again the apron eventually failed, though taking longer to do so. Apron undermining started at its upstream corner, then progressed in the manner described previously. Figure 7-50 shows a view after the experiment. Apron of Layered Geobags An apron was formed of geobags placed in three vertical and two horizontal rows; it was toed into the bed around the abutment. No geofilter cloth was laid below the geobags. Instead, the lower layer of geobags, together with a shingled, overlapping placement of geobags, served as a geofilter as well as increased apron bulk. The toe of geobags was used in an effort to prevent undermining of the apron’s upstream edge. Although the toe protected the apron from being undermined along its upstream side, the apron still failed because of the scour hole formed around the apron’s lower perimeter. Once the scour hole formed, the geobags slid into the hole simultaneously. The system failed after an hour. Geobag Apron Sloped into the Bed The aim of this experiment was to test the performance of an apron of geobags placed on a slope into the bed around the abutment. Geobags were placed at a slope of 1:2 (V:H) with a toe that consisted of three rows of geobags stacked vertically around the two rows of geobags, as can be seen in Figure 7-51. The bottom elevation was just below the average trough ele- vation of dunes moving along the flume. The geobags were placed in an overlapping manner like roofing shingles such that the overlap of bags prevented sediment from being win- nowed through gaps between adjoining geobags. 109 Figure 7-48. Front view of the scour hole; geobags on geofilter. Figure 7-49. The apron failed once the geobags were rolled into the scour hole that formed around the edge of the apron; circumferentially tied geobags were on a geofilter. Figure 7-50. Scour hole formed after the experiment; fully tied geobags were on a geofilter.

This apron proved to be very effective. It completely pre- vented the scour from occurring at the abutment. However, scour still developed around the geobag system, especially immediately downstream of the apron. The maximum depth of that scour was 85 mm—that is, 48 percent of the maximum depth when the scour hole occurred at the abutment. Consid- ering the average dune height as 34 mm, it can be said that the scour depth was decreased considerably. Figures 7-52 and 7-53 show the final state of the bed around the abutment. 7.3.4 Results: Pile-Supported Abutments Next, experiments were performed to evaluate scour at pile- supported abutments, these being common in many rivers. Ini- tially, a baseline case was tested without any countermeasure, followed by riprap and then by geobags. The experiments are described herewith and summarized in Table 7-8. Baseline Scour Condition The initial experiments were conducted to observe scour development and to measure scour depth at the pile- supported abutment when the abutment was not protected with an apron of any form, and to determine a baseline depth against which geobag performance could be compared. As no prior study has tested wing-wall abutments, it is useful to include here a brief description of the scour process. Figure 7-54 shows the consequent scour form that devel- oped at the abutment with the pile cap at the level of the main-channel bed. The maximum scour depth, dSmax, occurred at the leading corner of the abutment (point A in Figure 7-37), where the flow contraction was greatest and wake eddies were generated. The maximum scour depth extended 146 mm below the average bed level of the flume (about 5.2 m at full scale). The baseline experiments revealed two important mecha- nisms whereby the wing-wall abutment could eventually fail. One mechanism was the mass failure of the embankment that occurred once the scour hole had deepened to the extent that the embankment’s earthfill lost its geotechnical stability. The second mechanism had not been reported heretofore, largely because it is difficult to observe. It occurred as follows. As the scour deepened to below the pile cap and exposed the piles, the embankment’s earthfill was eroded out from beneath the pile cap. Gradually, a cavity developed within the embank- ment, undermining the embankment immediately behind the abutment. This development is depicted in Figure 7-55. 110 Figure 7-52. A view after the experiment, including a double layer of tied geobags with no geofilter. Figure 7-51. Sloped apron formed of overlapping geobags, with the base of apron toe at the average trough level of dunes. Figure 7-53. The sloped apron of tied geobags worked well. Scour did not occur at the abutment, but was shifted out from it.

Eventually, scour deepening caused the embankment side slopes to become unstable and to slide into the scour hole, where sediment had been removed by the flow. As the embankment collapsed, the flow passed around the exposed abutment. 111 noitpircseD tnemirepxE 1: Baseline Scour This experiment was conducted to produce reference baseline scour depth that can be used to determine the scour- reducing influence of a geobag apron. 2: Embankment Protected with Geobags The side slopes of erodible embankment behind pile-supported abutment were protected with geobags. No geobag apron. 3: Geobag Protection Under the Pile Cap Geobags were placed under the pile cap in addition to the side slopes to prevent winnowing. 4: Testing Performance of Riprap I This experiment tested performance of riprap to protect pile-supported wing- wall abutment with erodible embankment. 5: Testing Performance of Riprap II This experiment repeated Experiment 4, but with rigid embankment. 6: Protection of Apron and Embankment This experiment placed geobags in a manner replicating the riprap configuration found to be commonly used for Iowa DOT bridges. 7: Apron with Geobags also under the Pile Cap This experiment repeated Experiment 6, but with geobags placed under the pile cap as a filter. 8: Partially Tied Geobag Apron Only the geobags at the upper and the lower layers of the apron were tied together. 9: Geobag Mattress In addition to the two rows of the apron, the geobags at the half downstream part of the upper layer of the apron toe were tied together. 10: Fully Tied Apron of Geobags The entire apron of geobags was tied together. 11: Steep Embankment Slope Performance of the geobag system used in the previous experiment was tested for a steeper embankment side slope. 12: Pile Cap Lowered, No Geobags The pile cap was placed deeper in the bed. Table 7-8. List of representative principal experi- ments (Korkut 2004 and Morales 2006 document full list of exploratory experiments). Figure 7-54. Scour development at the abutment and embankment when unprotected. (a) Before scour (b) Scour exposes piles (c) Cavity forms behind the pile cap Figure 7-55. As scour exposes piles (shown in a and b above), embankment soil may be sucked under the pile cap, forming a cavity behind the pile cap (shown in c).

riprap apron sliding into the scour hole forming around the apron. As the riprap apron slid, it exposed the pile cap so that embankment sediment was winnowed from beneath the pile cap. The embankment then failed due to winnowing of sedi- ment from beneath the pile cap. Additionally, the deepening scour hole caused the embankment side slope to become unstable. The manner of riprap apron failure essentially was the same as that reported by Chiew (2000) and Parker et al. (1989) for riprap aprons placed around model bridge piers. It is evident from the experiment that riprap aprons placed locally around an abutment may not work in dune-bed channels unless the apron toe extends deep enough to be below the trough eleva- tion of dunes moving through the bridge opening. Abutment with Geobag Apron A series of trial-and-error experiments was conducted to determine whether and how scour would be prevented by one or more large geobags fitted as an apron around the perime- ter of the test wing-wall abutment. All of these experiments essentially showed that, for large geobags to be effective at preventing scour depth at an abutment, the geobags must be tied together so as to form a more or less continuous apron, and the apron itself should be tied to the abutment. Other- wise, the geobags would slide away from the abutment, expose the abutment footing, and cause scour of sediment from beneath the footing. However, even though an apron of tied geobags eliminated scour at the abutment, it caused the location of deepest scour to shift downstream of the abut- ment. The maximum scour depth dSmax was 110 mm, which was about equal to the flow depth. Figure 7-58 illustrates a typical result from the experiments. The location of maxi- mum scour depth moved from location A to C in Figure 7-37. It was found that, as additional geobags were placed around the abutment, the scour shifted farther downstream of the abutment. Therefore, to be effective, the geobags have 112 A countermeasure-related observation from these experi- ments is that the embankment’s earthfill beneath and behind the pile cap must be protected. Two options for doing this are to place armor material immediately behind the pile cap and to place the pile cap at a lower elevation. These options were tested in the experiments. The experiment with riprap placed immediately behind the pile cap produced a deeper scour (165 mm at the leading corner), but the embankment did not fail. The experiment with the pile cap lowered showed that a lower pile cap resulted in a still larger maximum scour depth of 182 mm, at the abutment’s upstream corner. Although this scour depth exceeded the baseline scour depth (Experiment 3 in Table 7-8), scour could not progress substantially lower than about the pile-cap base. Use of scour protection imme- diately behind the pile cap, or use of a lowered pile cap, there- fore enables a wing-wall abutment and its approach embankment to better withstand scour. Abutment with Riprap Apron A series of experiments were conducted to determine how scour develops when an apron of riprap stone is placed around the abutment, an example of which is shown in Figure 7-56. The riprap apron consisted of a layer of riprap about two stones thick, with a toe three to four stones thick. The riprap stones simulated were scaled down to uniform-sized riprap of d50  22 mm. Figure 7-57a illustrates the initial arrangement of the riprap apron used in the flume experiments, and Figure 7-57b shows the resultant scour hole, whose maximum depth occurred at the upstream corner of the abutment and was 85 mm. The experiments showed that edge failure of the riprap apron led to apron failure and to scour progression beneath and around the abutment, including the abutment pile cap. The failure started at the apron’s upstream edge, where accelerated flow and the passage of dunes destabilized the apron’s riprap toe, and resulted in large parts of the entire Figure 7-56. Example of an actual apron at a wing-wall abutment.

113 (a) Before scour (b) After scour Figure 7-57. The failure of a riprap apron due to scour (Experiment 4, Table 7-8). The location of deep- est scour is indicated. to extend out a substantial distance from the abutment. The present experiments and the preliminary experiments indi- cate that apron width should exceed road width. This finding indicates that, for single- or double-span bridges, the apron formed of geobags (or riprap) should extend across the full width of the bridge waterway. Also, if the additional geobags were not secured to the abutment, they would slide into the scour hole. In sizing the bag thickness using Equations 7-17 through 7-19, it should be kept in mind that the slope angle may increase substantially as scour develops and that bag thickness should be based on the anticipated slope associated with scour hole formation. Because an advantage of geobags is that they can be formed and placed by hand, especially in circumstances where an immediate temporary countermeasure is needed, series of experiments were conducted to determine how an apron of relatively small geobags would perform as an alternative to a riprap apron. The experiments involved an apron of geobags placed in a design layout essentially the same as for the riprap apron described above. The apron consisted of a layer of geobags, two bags thick, with a toe three to four bags thick. The apron generally conformed with the layout of the riprap apron shown in Figure 7-56. Early experiments revealed that, though the geobags that were loosely placed reduced scour depth, they might not fully protect the abutment pile cap. The experiments showed that an apron of suitably posi- tioned and connected geobags (acting like cable-tied blocks), such as one generally conforming to the apron in Figure 7-56, can reduce the scour depth at an abutment. However, as with a riprap apron, scour may occur at position B on the perime- ter of the geobag apron if the geobags are loosely placed or at position C downstream from the apron if the geobags are tied together as a mattress. Once edge scour occurs, at either posi- tion B or position C, the edge geobags (as with riprap) are dis- lodged into the scour hole. (a) Before scour (b) After scour Figure 7-58. Scour failure of an apron formed of loosely placed small geobags. The deepest scour is indicated with the arrow.

that the protection (geobag or riprap) must extend as a mat across essentially the full opening of a bridge waterway. 7.3.5 Mat Across Bridge Waterway The foregoing findings with aprons show that the bridge waterway should be fully lined with a large protective apron, or mat, that essentially links the aprons extending from each abutment. Further experiments investigating the extent and layout that are required for an effective mat led to the mat lay- out design guide presented in Figure 7-60, which reflects the following recommendations: • The mat should extend upstream and downstream of the abutment by a distance of minimally one bridge width to ensure that the waterway bed is protected from the accelerat- ing flow through the waterway. Mats providing this extent of waterway coverage were able to prevent scour in the waterway. • The mat should be sloped. The bottom of the slope should coincide approximately with the trough elevation of bed sediment dunes passing through the bridge waterway.At this bottom elevation, dunes do not cause the upstream or downstream edges of the mat to be undermined and fall apart. An additional advantage of the sloped mat is that it enables low flows to concentrate at the center of the water- way; this is an advantage for fish passage. Additionally, the center slope minimizes flow blockage through the waterway. • The mat should have a toe and a heel, each of which are three geobag- or riprap-stone-thicknesses deep below the mat. 114 Figure 7-59. Maximum scour depths for experiments described in Table 7-8; depths of scour at locations A (), B (), and C () in Figure 7-37. When a full mat (double layer) was placed across the channel, scour depth was zero. The experiments showed that the edge failure is the princi- pal factor that results in the failure of the geobag apron, just as it is for the riprap apron. It was found that such failure can be eliminated or substantially reduced by fully linking the geobags to form a flexible apron, by then sloping the apron into the bed, and by forming a suitably deep toe of geobags (as for riprap) around the apron’s perimeter. Also, it was found that geobag size did not affect the performance of a tied apron of geobags for the experiment conditions tested. Of greater importance was that the geobag elements be linked to form a flexible apron of sufficient coverage around the abutment. Scour at the abutment itself was eliminated when geobags or riprap were placed under the pile cap to prevent the win- nowing erosion of riverbank and embankment soil through the exposed region beneath the pile cap (Experiment 7 in Table 7-8). Though the abutment itself was protected, the bed scoured downstream of the geobag apron, with scour depths exceeding the maximum scour depth at the abutment for the baseline case. Summary of Scour Data The scour depth results associated with the experiments are plotted in Figure 7-59, which shows how appropriate geobag use may reduce scour depth at the abutment (dsA), but with the consequence of shifting scour to positions B (dsB) and C (dsC) (positions are indicated in Figure 7-37). In shifting the scour location, geobag use may not eliminate scour at a bridge. As is shown in the ensuing discussion, it quickly becomes evident

115 Figure 7-60. Recommended minimum extent of mat formed from geobags or riprap for single-span bridges. An early series of tests with a single layer of geobags not tied to each other resulted in local failure of the mat near the abut- ments, owing to the winnowing of sand from between the geobags. The same result occurred with a single layer of riprap. An illustrative geobag mat failure is given in Figure 7-61. If repeated with a double layer of geobags, or a double layer of riprap, minimal winnowing of sand occurred, and no scour developed at the bridge opening. For example, the perform- ance of a mat formed from a double layer of riprap is shown in Figures 7-62. The mat remained in tact and inhibited scour at the either abutment. Geobags as Filter Under Riprap Mat The flume tests showed that, for geobags to serve as an effective form of filter cloth beneath a single layer of riprap, and for the riprap to be stable, it was necessary for the geobags to be placed slightly below the local level of the channel bed. For this arrangement, the riprap remained stable. Otherwise, when the geobags were placed on top of the channel bed or the riprap was placed on the geobags, (a) Setup (b) After test, showing failure of mat owing to winnowing of sand from between bags Figure 7-61. Performance of a geobag mat (single- bag thick) with riprap toe and heel. the riprap was exposed such that the riprap stone was less stable than when the geobags were placed below level. A larger size of riprap stone would be needed in this situation. 7.4 Summary of Results from Riprap, Cable-Tied Blocks, and Geobags Local scour in the general vicinity of an abutment can- not be eliminated completely by an apron of riprap or geobags. An apron shifts the scour region away from an abutment. The experiments show that an apron can prevent scour from developing at the abutment itself, but that

116 significant scour can occur readily near the downstream edge of the apron. A possible concern in using an apron is to ensure that shifting of scour does not imperil a nearby pier or portion riverbank. Moreover, if the scour is likely to extend to an adjacent pier, then the abutment and pier countermeasure apron should be placed so as to protect both elements of a bridge. The experiments show that it is necessary to protect the fol- lowing regions of the river bed and banks near an abutment: • The river bed at the abutment pile cap, • The riverbank immediately upstream of the abutment and a short distance downstream of the abutment, • The side slopes of embankment immediately behind the abutment (standard stub for a wing-wall abutment or spill- through abutment), and • The area beneath and immediately behind the pile cap. For use of riprap or cable-tied block alone, the following conclusions emerged from this study: • For the range of experimental investigation in this study, the scour at wing-wall abutments in live-bed conditions is directly related to the level of the deepest bed form trough that propagates past the abutment, which is predictable using existing expressions, together with any localized scour that may occur. • Stones on the outer edge of riprap aprons tend to settle and move away from the abutment, pushing the troughs of the bed forms farther away from the abutment. Conversely, cable-tied block mats remain intact during settlement. The outer edge of the apron settles vertically, allowing the troughs of the bed forms to pass closer to the abutment face than for an equivalent riprap apron. • Equations 7-10 and 7-11 allow prediction of the mini- mum apron width remaining horizontal after erosion. Equation 7-12 allows prediction of the horizontal distance between the abutment face and the point of deepest scour. These predictions, along with prediction of apron settle- ment, facilitate assessment of the stability of an abutment structure. With regard to the specific use of geobags for wing-wall abutments, the following conclusions can be drawn: • Geobags are a promising alternative to riprap for use as a bridge abutment scour countermeasure. • It is necessary to connect the geobags placed as an apron around an abutment. The initiation of the failure of geobag apron shown in Figure 7-58 was due to the failure of an individual geobag placed in front of the abutment. • The apron should have a perimeter toe whose lower level approximately coincides with the average elevation of (a) Before flow (b) During flow (c) After flow Figure 7-62. Performance of riprap mat (double- stone layer) for a single-span bridge. Mat layout is as given in Figure 7-60.

dunes moving through the channel in the vicinity of the bridge. • The geobags should be placed in a shingled manner, whereby adjoining geobags overlie joints between underlying geobags. • It is necessary to place geobags (or riprap) immediately under the pile cap in order to prevent the winnowing of embankment sediment from beneath the pile cap. • Geobags may serve as a useful alternative to a geotextile fil- ter cloth placed beneath a riprap apron because geobags are more readily placed than is an underlay cloth for blocking the winnowing of sediment from between bed-armor ele- ments like riprap stone. The geobags, though, should be placed somewhat below bed level so as not to increase riprap exposure to flow. 117

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TRB's National Cooperative Highway Research Program (NCHRP) Report 587: Countermeasures to Protect Bridge Abutments from Scour examines selection criteria and guidelines for the design and construction of countermeasures to protect bridge abutments and approach embankments from scour damage. The report explores two common forms of bridge abutments--wing-wall (vertical face with angled walls into the bank) and spill-through (angled face).

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