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

OCR for page 85
86 K Factors Approach (Mueller Database) 10 8 Predicted Scour Depth (m) 8 Predicted Scour Depth (m) 6 6 4 4 2 2 0 D50 (0.075mm~4.75 mm) 0 2 4 6 8 10 Measured Scour Depth (m) 0 Figure 10.1. SRICOS-EFA predictions against Mueller 0 2 4 6 8 (1996) Database. Measured Scour Depth (m) 8 lar Long Contraction." Gill (1981) ran some contraction tests on sand in the laboratory. The experiments were conducted in Predicted Scour Depth (m) 6 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- 4 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, 2 the effective length of the contraction was 2.44 m with transi- tions each 0.46 m long. The width of the contracted section D50 (4.75mm~75mm was 0.5 m. Two types of nearly uniform sand were used in the 0 experiments. The average size of the coarse sand, D50, was 0 2 4 6 8 Measured Scour Depth (m) HEC-18 Method 8 (Mueller Database) 10 Predicted Scour Depth (m) 6 Predicted Scour Depth (m) 8 4 6 4 2 D50 (75mm~300mm) 2 0 0 2 4 6 8 0 0 2 4 6 8 10 Measured Scour Depth (m) Measured Scour Depth (m) Figure 10.3. SRICOS-EFA predictions versus Mueller Database for various ranges of D50. Figure 10.2. HEC-18 predictions against Mueller (1996) Database.

OCR for page 85
87 K Factors Approach (Froehlich Database) 10 100 Predicted Scour Depth (m) 8 Predicted Scour Depth (m) 10 6 4 1 2 D50 (0.075mm~4.75 mm) 0 0 0.1 1 10 100 Measured Scour Depth (m) 0 2 4 6 8 10 Measured Scour Depth (m) Figure 10.5. SRICOS-EFA predictions against Froehlich (1988) Database. 10 8 Predicted Scour Depth (m) 1.53 mm; D50 of the fine sand was 0.92 mm. The angle of tran- sition at the contraction was approximately 15 degrees. 6 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- 4 sured by Gill was the uniform scour depth in this study. Therefore, the Gill (1981) Database was used to verify the 2 uniform contraction scour equation Zunif, not Zmax. The D50 (4.75mm~75mm) SRICOS-EFA Method calls for a value of the critical veloc- 0 ity Vc measured in the EFA. Since this data was not available in Gill's database, the expression recommended in HEC-18 0 2 4 6 8 10 was used. Measured Scour Depth (m) 8 HEC-18 Method (Froehlich Database) Predicted Scour Depth (m) 6 100 Predicted Scour Depth (m) 4 10 2 D50 (75mm~300mm) 1 0 0 2 4 6 8 Measured Scour Depth (m) 0 Figure 10.4. HEC-18 predictions versus Mueller 0.1 1 10 100 Measured Scour Depth (m) Database for various ranges of D50. Figure 10.6. HEC-18 predictions against Froehlich (1988) Database.