Pier and Contraction Scour in Cohesive Soils (2004)

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85 CHAPTER 10 VERIFICATION OF THE SRICOS-EFA METHOD 10.1 BACKGROUND The SRICOS-EFA Method for complex pier and con- traction scour in cohesive soils was developed on the basis of flume tests for the maximum scour depth equations and numerical simulations for the maximum initial shear stress equations. As with any new method, there is a need to verify the method against other measurements. These measure- ments should preferably be full-scale case histories. For this project, the case histories had to satisfy the following site requirements: 1. Channel contraction exists; 2. A bridge with piers in the water exists; 3. A gage station exists giving the hydrograph over a period of time t; 4. The soil is cohesive; 5. The site can be accessed with a drill rig; and 6. The riverbed cross section was documented at the beginning and at the end of the same period of time t as the hydrograph. A survey of U.S. DOTs was conducted and many sites were collected. Upon further review, it was found that none of the sites had the requirements necessary for evaluating the method. Since this avenue could not be pursued, it was decided to look in the literature for existing data associated with the topic of complex pier and contraction scour. The fol- lowing databases were found: 1. Mueller (1996) for complex pier scour, 2. Froehlich (1988) for complex pier scour, and 3. Gill (1981) for contraction scour. These databases were created primarily for cohesionless soils, but it was felt that it would be useful to compare the SRICOS-EFA Method to cohesionless soils measure- ments. The following gives a brief description of the data- bases and shows the comparisons between measured and predicted scour depth. Note that since the data pertains to cohesionless soils, the comparison is limited to evaluating the equations for the maximum scour depth Zmax(complex pier and contraction). 10.2 MUELLER (1996) DATABASE: PIER SCOUR The Mueller Database was obtained from report FHWARD-95-184, “Channel Scour at Bridges in the United States.” More than 380 pier scour measurements were col- lected at 56 bridge sites in Alaska, Arkansas, Colorado, Delaware, Georgia, Illinois, Indiana, Louisiana, Maryland, Mississippi, Montana, New York, Ohio, and Virginia. Figure 10.1 shows the comparison between the complex pier scour depth calculated by the SRICOS-EFA Method and the mea- surements in the database. The equation used was the SRI- COS equation for the maximum pier scour depth. Figure 10.2 shows the predictions by the HEC-18 equation compared to the measurements for the same database. Both SRICOS and HEC 18 appear to be conservative; there is less scatter in the SRICOS predictions. In order to investigate the influence of D50 on the match between SRICOS-EFA predictions and measurements, the database was divided in three D50 categories. Figures 10.3 and 10.4 show the results. No obvious trends are evident. 10.3 FROEHLICH (1988) DATABASE: PIER SCOUR The Froehlich Database was obtained from an ASCE report, “Analysis of Onsite Measurements of Scour at Piers.” In the Froehlich Database, there are 79 pier scour measurement points, 50 cases for round-nosed pier, 9 cases for square-nosed pier, and 20 cases for sharp-nosed pier. Figure 10.5 shows the comparison between the complex pier scour depth calculated by the SRICOS-EFA Method and the measurements in the database. The equation used was the SRICOS equation for the maximum pier scour depth. Figure 10.6 shows the HEC-18 equation compared with the same database. With this data- base, HEC-18 appears to be more conservative than SRICOS. 10.4 GILL (1981) DATABASE: CONTRACTION SCOUR The Gill Database was obtained from the Journal of the Hydraulic Division of the American Society of Civil Engi- neers (ASCE) in an article entitled “Bed Erosion in Rectangu-

lar Long Contraction.” Gill (1981) ran some contraction tests on sand in the laboratory. The experiments were conducted in a rectangular steel channel that was 11.4 m in length, 0.76 m in width and 0.46 m in depth. There were two sizes of con- tracted sections in the channel. In the first series of experi- ments, the effective length of the contraction was 1.83 m, excluding the upstream (inlet) and downstream (outlet) transi- tions, each 0.46 m long. In the second series of experiments, the effective length of the contraction was 2.44 m with transi- tions each 0.46 m long. The width of the contracted section was 0.5 m. Two types of nearly uniform sand were used in the experiments. The average size of the coarse sand, D50, was 86 K Factors Approach (Mueller Database) 0 2 4 6 8 10 0 2 4 6 8 10 Measured Scour Depth (m) Pr ed ic te d Sc o u r D ep th (m ) HEC-18 Method (Mueller Database) 0 2 4 6 8 10 0 2 4 6 8 10 Measured Scour Depth (m) Pr ed ic te d Sc o u r D ep th (m ) 0 2 4 6 8 0 2 4 6 8 Measured Scour Depth (m) Pr ed ic te d Sc o u r D ep th (m ) D50 (0.075mm~4.75 mm) 0 2 4 6 8 0 2 4 6 8 Measured Scour Depth (m) Pr ed ic te d Sc o u r D ep th (m ) D50 (4.75mm~75mm 0 2 4 6 8 0 2 4 6 8 Measured Scour Depth (m) Pr ed ic te d Sc o u r D ep th (m ) D50 (75mm~300mm) Figure 10.1. SRICOS-EFA predictions against Mueller (1996) Database. Figure 10.2. HEC-18 predictions against Mueller (1996) Database. Figure 10.3. SRICOS-EFA predictions versus Mueller Database for various ranges of D50.

87 0 2 4 6 8 10 0 2 4 6 8 10 Measured Scour Depth (m) Pr ed ic te d Sc o u r D ep th (m ) D50 (0.075mm~4.75 mm) 0 2 4 6 8 10 0 2 4 6 8 10 Measured Scour Depth (m) Pr ed ic te d Sc o u r D ep th (m ) D50 (4.75mm~75mm) 0 2 4 6 8 0 2 4 6 8 Measured Scour Depth (m) Pr ed ic te d Sc o u r D ep th (m ) D50 (75mm~300mm) K Factors Approach (Froehlich Database) 0 1 10 100 0.1 1 10 100 Measured Scour Depth (m) Pr ed ic te d Sc ou r D ep th (m ) HEC-18 Method (Froehlich Database) 0 1 10 100 0.1 1 10 100 Measured Scour Depth (m) Pr ed ic te d Sc ou r D ep th (m ) Figure 10.4. HEC-18 predictions versus Mueller Database for various ranges of D50. Figure 10.5. SRICOS-EFA predictions against Froehlich (1988) Database. Figure 10.6. HEC-18 predictions against Froehlich (1988) Database. 1.53 mm; D50 of the fine sand was 0.92 mm. The angle of tran- sition at the contraction was approximately 15 degrees. The scour depth was obtained by averaging several depth readings taken along the centerline of the channel. Accord- ing to the location of the measurements, the scour depth mea- sured by Gill was the uniform scour depth in this study. Therefore, the Gill (1981) Database was used to verify the uniform contraction scour equation Zunif, not Zmax. The SRICOS-EFA Method calls for a value of the critical veloc- ity Vc measured in the EFA. Since this data was not available in Gill’s database, the expression recommended in HEC-18 was used.

where Vc is the critical velocity of the bed material, m/s; y is the water depth in the upstream flow, m; and D50 is the parti- cle corresponding to 50% passing by weight, m. Figure 10.7 shows the comparison between the uniform contraction scour depth calculated by the SRICOS-EFA Method and the mea- surements in the database. Figure 10.8 shows the HEC-18 equation compared with the same database. As shown, the SRICOS Method is reasonably good, but the HEC-18 Method is severely under predicting. V y Dc = 6 19 10 1 1 6 50 1 3. ( . ) 88 10.5 REMARKS While it would have been preferable to find a number of full-scale case histories, at least the comparison to databases obtained primarily on cohesionless soils gave an idea of how the SRICOS-EFA Method compares to the HEC-18 Method. Note that the comparison is only based on the maximum depth of scour values and does not involve the scour rate, which is the major difference between the current HEC-18 Method and the SRICOS-EFA Method. Overall, it was found that the perfor- mance of the SRICOS-EFA Method is similar to the HEC-18 Method except for the contraction scour depth where the SRICOS-EFA Method is much closer to the measurements. SRICOS-EFA Method (Gill Database) 0 5 10 15 20 25 30 35 40 45 50 55 0 5 10 15 20 25 30 35 40 45 50 55 Measured Scour Depth (mm) Pr ed ic te d Sc o u r D ep th (m m ) HEC-18 Method (Gill Database) 0 5 10 15 20 25 30 35 40 45 50 55 0 5 10 15 20 25 30 35 40 45 50 55 Measured Scour Depth (mm) Pr ed ic te d Sc o u r D ep th (m m ) Figure 10.8. HEC-18 Method against Gill (1981) Database. Figure 10.7. SRICOS-EFA Method against Gill (1981) Database.