Minimizing Roadway Embankment Damage from Flooding (2016)

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72 CHAPTER TEN COUNTERMEASURES, MAINTENANCE, AND REPAIR • Douglas and Krolak (2008), Highways in the Coastal Environment, HEC-25, Vol. 1 • Douglas, Webb, and Kilgore (2014), Highways in Coastal Environment: Assessing Extreme Events, HEC-25, Vol. 2 • Clopper (1989), Hydraulic Stability of Articulated Concrete Block Revetment Systems During Overtopping Flow, FHWA-RD-89-199 • Lagasse, Clopper, Zevenbergen, and Girard (2007), Countermeasures to Protect Bridge Piers from Scour, NCHRP Report 593 • Lagasse, Clopper, Zvenbergen, and Ruff (2006), NCHRP Report 568: Riprap Design Criteria, Recommended Specifications, and Quality Control • Fay, Akin, and Shi (2012), Cost-Effective and Sustainable Road Slope Stabilization and Erosion Control. OVERTOPPING When overtopping is anticipated, mitigation measures or countermeasures, or a combination of both, could be adopted. Mitigation measures include adding culverts to prevent overtopping, raising the level of the embankment crest, and adopting fuse plugs. Countermeasures include protecting the downstream slope to minimize the potential of back erosion and, ultimately, the failure of the embank- ment using vegetation, riprap and geotextile, precast articu- lated concrete block matrices, gabions, and concrete lining of the slopes. This section presents commonly used protection mea- sures against overtopping. Clopper and Chen (1988) give further recommendations about the design and construction aspects of each system. Important considerations include selecting the extent of protection, ensuring that the slopes can carry the weight of the protection system, and preventing the erosion of soil particles through the protection system. Vegetation Based on the survey results, various seed mixes are used in roadway embankments based on weather and moisture con- ditions. The seed mixes are developed by relevant states, and although they are not designed for flooding resistance, they INTRODUCTION This chapter presents a compilation of the countermeasures, maintenance, and repair solutions adopted in current practice to minimize roadway embankment damage from flooding. All the failure modes specified in chapter two are addressed herein with the corresponding countermeasures based on the DOT engineers’ feedback through the case examples, survey responses, and interviews. In the case of constructing a new roadway embankment, a key design element is selecting an adequate roadway location. This would be achieved through understanding the site characteristics and constraints (e.g., geology, geotechnical characteristics, stream hydraulic char- acteristics and meandering potential, upstream and down- stream conditions, construction activities, and stormwater management facilities). In case of an old problematic site, it is essential to understand the key issue leading to recurring or aggravated damage. Possible issues include an increase in the severity of flooding events and stream instability issues caused by changes in upstream or downstream conditions (including man-made activities). Through highlighting the main issues, an adequate design approach would be adopted. Such approaches would include one or a combination of relo- cating the embankment, stabilizing the stream, or designing for failure modes as discussed herein. The following references can be visited for relevant information: • Clopper and Chen, (1988), Minimizing Embankment Damage During Overtopping Flow, FHWA-RD-88-181 • Richardson, Simons, and Lagasse (2001), River Engineering for Highway Encroachments, Highway in the River Environments, HDS-6, FHWA NHI 01-004 • Brown and Clyde (1989), Design of Riprap Revetment, HEC-11, FHWA-IP-89-016 • Kilgore and Cotton (2005), Design of Roadside Channels with Flexible Linings, HEC-15, 3rd ed., FHWA-NHI-05-114 • Lagasse, Zevenbergen, Spitz, and Arneson (2012), Stream Stability Highway Structures, HEC-20, 4th ed., FHWA-HIF-12-004 • Lagasse et al. (2009a), Bridge Scour and Stream Instability Countermeasures, HEC-23, Vol. 1 • Lagasse et al. (2009b), Bridge Scour and Stream Instability Countermeasures, HEC-23, Vol. 2

73 can help significantly in slowing down erosion as explained in “Overtopping of Embankments” in chapter five. To play an effective role in resisting erosion, however, the result- ing vegetation has to be constantly maintained to satisfy the minimum requirements listed under “Overtopping of Embankments.” Table 20 presents maximum permissible velocities and shear stresses for vegetative linings. Figure 109 presents a comparison between recommended limiting values for erosion resistance of plain and reinforced grass. TABLE 20 MAXIMUM PERMISSIBLE VELOCITIES AND SHEAR STRESSES FOR VEGETATIVE LININGS Cover (1) Slope range (%) (2) Permissible Velocity, fps (m/s) Erosion-resistant soils (3) Easily eroded soils (4) Bermunda grass 0–5 8 (2.4) 6 (1.8) 5–10 7 (2.1) 5 (1.5) >10 6 (1.8) 4 (1.2) Buffalo grass, Kentucky Bluegrass, Smooth 0–5 7 (2.1) 5 (1.5) Brome, Blue Grama 5–10 6 (1.8) 4 (1.2) >10 5 (1.5) 3 (0.9) Grass mixture 0–5a 5 (1.5) 4 (1.2) 5–10a 4 (1.2) 3 (0.9) Lespedeza Sericea, Weeping Love Grass, Ischaemum (yellow bluestem), Kudzu, Alfalfa, Crabgrass 0–5b 3.5 (1.1) 2.5 (0.8) Annual–used on mild slopes or as temporary protection until perma- nent covers are estab- lished, common lespe- deza sudan grass 0.5c 3.5 (1.1) 2.5 (0.8) a Do not use on slopes steeper than 10%. b Do not use on slopes steeper than 5%, except for side slopes in a combination channel. c Use on slopes steeper than 5% is not recommended. Additional references include NCHRP Report 430 (Fay et al. 2012), which refers to abundant literature relevant to the topic. Riprap and Geotextile Riprap has been successfully used for erosion protection in both coastal and riverine environments. Guidance for siz- ing riprap is available in a number of references including Brown and Clyde (1989), Richardson et al. (2001), Kilgore and Cotton (2005), Douglas and Krolak (2008), and Lagasse et al. (2006a, 2009a and b, 2012), and a typical riprap instal- lation is shown in Figure 110. It is important to place a filter between the soil and the riprap layer once applied on slopes, to protect the soil par- ticles from erosion. Without a filter, the soil under the riprap may continue to erode through the large voids in the riprap. In the end, the riprap may not move away, but may simply move significantly as the underlying soil erodes away. The filter may be a sand filter or a geosynthetic filter. Relevant design guidelines can be found in Heibaum (2004) for sand filters and in Koerner (2012) for geosynthetic filters. FIGURE 110 Typical riprap installation (after Brown and Clyde 1989). Gabions (Wire-Enclosed Rock) According to Powledge et al. (1989), geotextiles perform well with vegetation and when placed beneath any other form of sur- face protection. Gabions are wire baskets filled with rock; they have been used successfully for protection against erosion. Gabi- ons are relatively expensive and are susceptible to wire failure from corrosion (Brown and Clyde 1989; Powledge et al. 1989). Precast Articulated Concrete Block Blankets Precast articulated concrete mats consist of precast blocks tied together to form a mat. Two types are commonly used today: the open block and the closed block. The open blocks have open cells that allow for grass growth, which further increases ero- sion resistance. These systems could withstand high-velocity flows up to 26 ft/s (7.9 m/s) on a cohesive subsoil without failure (Powledge et al. 1989). A typical section is shown in Figure 111. FIGURE 111 Section of a typical precast concrete blocks system (after Brown and Clyde 1989). Concrete Lining For concrete lining of the slopes, it is important to consider the possibility of a “blow-out” failure. Such failure is illustrated

74 in Figure 112. This is the case of a possible blow-out failure in RM 335 in Real County, Texas. The blow-out could likely be the result of the hydrostatic pressure in the embankment. FIGURE 112 Failure of concrete lining (RM-335, Real County, Courtesy of TxDOT). THROUGH-SEEPAGE To alleviate the hydraulic pressures developing within the embankment and possibly leading to erosion on the down- stream slope, a pervious toe can be integrated into the sys- tem, as shown in Figure 83. If both through-seepage and underseepage are anticipated, a combined solution of a pervious toe and a partially penetrating toe trench can be adopted (Figure 84). Other horizontal and inclined drainage options are presented in Figure 85. UNDERSEEPAGE-SEEPAGE A number of measures are available to improve the drainage conditions within the embankment. Such measures include cutoff walls placed beneath the embankment, riverside blan- kets, downstream seepage berms (Figure 86), pervious toe trenches (Figure 87), and pressure relief wells. WAVE-EROSION The design procedures to protect embankments against wave action generally involve installing adequate revetments. The introduction to chapter three provides an example of the use of a combined solution in coastal environment that includes the protection of the seaward slope using sheet piles, soldier piles, articulate concrete blocks, and miscellaneous asphalt and performance turf. The installation takes place on the eroding slope as a result of wave action. Relevant guidelines include Douglas and Krolak (2008), USACE (2012), and Jones et al. (2005). As shown in the chapter three section “Damage Due to Overtopping and Wave Action of Riverine Highways, Min- nesota,” paving the slopes is an option in certain cases. It is important to take measures to avoid rapid deterioration of the pavement and to extend the pavement sufficiently down the slope. SOFTENING BY SATURATION To overcome softening resulting from saturation, a soil that has a modulus insensitive to an increase in water content can be used. LATERAL SLIDING Lateral sliding can be minimized by using a keyway between the embankment and the natural soils. Using a flatter slope also improves the situation. CULVERTS HDS-5 discusses essential aspects of culvert design and provides additional references in this regard. Important considerations are relevant to erosion at inlet and outlet, aggradation, degradation and debris control, site-specific concerns, and structural considerations. Erosion control measures at the inlets include slope pav- ing, channel paving, headwalls, wing walls, and cutoff walls. Protection against scour at the outlets can vary from instal- lation of end walls to simple riprap placement to installation of energy dissipation devices. Relevant information can be found in HEC-14. Aggradation- and degradation-related problems are included in references such as Highways in the River Envi- ronment (1975). As for debris control measures that can be adopted for the culvert to function as designed, relevant dis- cussion can be found in HEC-9. HDS-5 further discusses site-related modifications including the location and orienta- tion of barrels, the case of multiple barrels, and the selection of culvert shape and materials. Culverts could also face structural problems. For this purpose, proper selection of the bedding and fill are essential to resist the applied forces. Pipe bedding is a very impor- tant consideration and can vary from state to state. Culvert buckling under normally applied traffic loads could occur in dry conditions, while during flooding such problems as barrel floatation could be faced. A number of practical mea- sures are listed in HDS-5. If such measures fail, anchoring the culvert might be a potential solution. In addition, the cli- mate conditions can be considered in the design process. For

75 instance, granular fill is often used to reduce frost heave and potentially improve compaction. PAVEMENTS A number of practices were extracted from the survey results that would decrease the impact of flooding on pavements: • Using rockfill for the subbase to avoid scour/erosion (Arkansas Practice) • Using black base, graded aggregate base, and underd- rain (Florida) • Allowing for a freeboard of 2 ft at design flood for a culvert (Idaho) • Using recycled asphalt pavement, geogrid with vegeta- tion, riprap, or cap clay on the slope, and paving the slope (Minnesota) • Armoring the slope (Nevada) • Keeping the subbase above design flood level (Tennessee). STREAM STABILIZATION If stream stabilization is adopted to minimize roadway embankment damage, a number of stabilization methods could be used as shown in the chapter three sections “MD- 24 Deer Creek Stream Stabilization, Maryland” and “Kim- sey Run Project, West Virginia.” Such methods include armoring the bank with riprap or implicated stone wall, or using tree trunks to strengthen the bank. Using in-stream structures such as cross vanes and rock vanes in the channel would contribute to decreasing the velocity of the water in the stream and divert it away from the embankment. SUMMARY This chapter presented the different countermeasures and mitigation solutions for each of the failure modes identified in chapter two, which are generally adopted in current prac- tice. This information is based on the successful and failed case examples (chapter two) and the survey (chapter eight). The selection of an adequate measure is site-dependent. In other words, what works at one site does not necessarily work at another site.