Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 66


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 65
65 3 rate in the part of the approach channel limited by two lines rep- resenting the extensions of the banks of the contracted channel. Test 10 2.5 The approach recommended here, however, is to replace the nominal velocity V1B1/B2 used in Equations 7.9 and 7.10 by 2 the velocity VHec obtained by using a program like HEC-RAS. With this approach, the complex geometry of the approach y = 0.4621x + 1 channel can be handled by the program, and a more repre- Kq 1.5 R2 = 0.9923 sentative velocity can be used. The problem is to find the 1 proper relationship between the HEC-RAS calculated veloc- ity, VHec, and the nominal velocity V1B1/B2. This was done by 0.5 conducting a series of HEC-RAS analyses to simulate the flow condition in the flume tests, obtaining the resulting 0 velocity VHec, and correlating it to V1B1/B2. The velocities are 0 1 2 3 4 listed in Tables 7.1 and 7.2 and the correlation graph is shown 1/Tan ( ) in Figure 7.22. Regression analysis gave the following rela- Figure 7.21. Transition angle effect on the location of the tionship between the two variables: maximum scour depth. B1 VHec = 1.14 V1 (7.16) B2 Bridge contractions are often short, and the abutments work like a thin wall blocking the flow. The uniform con- In this figure, the point for Test 1 is still outstanding traction scour depth cannot develop under these conditions. because of the severe backwater effect during that test, which Instead, two back contraction scour holes behind the con- exhibited an extreme contraction case ((B2/B1) = 0.25). tracted section can develop (Figure 7.5). Further, as shown Now, it is possible to rewrite Equations 7.9 and 7.10 in Figure 7.9, when the contraction length is between 6.76B2 using VHec. and 0.5B2, the maximum depth of contraction scour Zmax and For maximum contraction scour the location of that depth Xmax are unaffected by the length of the contracted channel. If the long contraction channel of c 12 Test 2 is chosen as the reference case, the correction factors Zmax 1.49VHec KL for contraction length are = 1.9 - (7.17) H1 gH1 gnH11 3 K L Zmax = 1.0 For uniform contraction scour K L Zunif = void (7.15) c 12 K L Xmax = 1.0 Z unif 1.57VHec = 1.41 - (7.18) H1 gH1 gnH11 3 Test 14 has the shortest contraction length with L/B1 = 0.125. The maximum scour depth is 1.79 times the scour depth in Test 2, and Xmax is also multiplied at the same time. 150 This situation is very like the "thin wall" pier scour. There- fore, when L/B2 is smaller than 0.25, it is necessary to increase the predicted Zmax and Xmax values. This case has not been 120 y = 1.1352x evaluated in this research. R2 = 0.8173 VHec (cm/s) 90 7.9 SRICOS-EFA METHOD USING HEC-RAS GENERATED VELOCITY Test 1 60 Equations 7.9 and 7.10, which have been proposed to calcu- late contraction scour depths, have a shortcoming: they are developed from tests in rectangular channels. For channels with 30 irregular cross sections, which is the case in most real situa- 30 60 90 120 150 tions, some researchers have recommended the use of the flow V1(B1/B2)(cm/s) rate ratio Q1/Q2 instead of the contraction ratio B1/B2 to account for the flow contraction (e.g., Sturm et al., 1997). Here, Q1 is Figure 7.22. Relationship between the nominal velocity the total flow rate in the approach channel and Q2 is the flow and the HEC-RAS calculated velocity.