Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 19

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 18
18 Figure 4.1. Example of the SRICOS Method for constant velocity and uniform soil. order to investigate the influence of the difference between depth versus time curve proceeds from Point B on Figure 4.2c the two velocity histories or hydrographs on the depth of until Point C after a time t2. The z versus t curve for the scour at a bridge pier, the case of a sequence of two different sequence of Floods 1 and 2, follows path OA on the curve for yet constant velocity floods scouring a uniform soil was first Flood 1 then switches to BC on the curve for Flood 2. This is considered (Figure 4.2). Flood 1 has a velocity v1 and lasts a shown as curve OAC on Figure 4.2d. time t1 while the subsequent Flood 2 has a larger velocity v2 A set of two experiments was conducted to investigate and lasts a time t2. The scour depth z versus time t curve for this reasoning. For these experiments, a pipe with a diame- Flood 1 is described by: ter of 25 mm was placed in the middle of a flume. The pipe was pushed through a deposit of clay 150 mm thick that was t made by placing prepared blocks of clay side by side in a tight z = ( 4.4) 1 t arrangement. The properties of the clay are listed in Table 4.1. + i1 z zmax 1 The water depth was 400 mm and the mean flow velocity was v1 = 0.3 m/s in Flood 1 and v2 = 0.4 m/s for Flood 2 (Fig- For Flood 2, the z versus t curve is: ure 4.3a). The first of the two experiments consisted of set- ting the velocity equal to v2 for 100 hours and recording the t z versus t curve (Figure 4.3c). The second of the two experi- z = ( 4.5) ments consisted of setting the velocity equal to v1 for 115 hours 1 t + (Figure 4.3b) and then switching to v2 for 100 hours (Fig- i 2 z zmax 2 ure 4.3d). Also shown on Figure 4.3d is the prediction of the After a time t1, Flood 1 creates a scour depth z 1 given by portion of the z versus t curve under the velocity v2 accord- Equation 4.4 (Point A on Figure 4.2b). This depth z 1 would ing to the procedure described in Figure 4.3. As can be seen, have been created in a shorter time t* by Flood 2 because v2 the prediction is very reasonable. is larger than v1 (Point B on Figure 4.2c). This time t* can be found by setting Equation 4.4 with z =z1 and t = t1 equal to 4.3 BIG FLOOD FOLLOWED BY SMALL FLOOD Equation 4.6 with z =z 1 and t = t*. AND GENERAL CASE t1 Flood 1 has a velocity v1 and lasts t1 (Figure 4.4a). It is fol- t* = ( 4.6) zi 2 + t1 z i 2 1 - 1 lowed by Flood 2, which has a velocity v2 smaller than v1 and i1 z zmax 1 zmax 2 lasts t2. The scour depth z versus time t curve is given by Equation 4.4 for Flood 1 and by Equation 4.5 for Flood 2. When Flood 2 starts, even though the scour depth z 1 was due After a time t1, Flood 1 creates a scour depth z 1. This depth z 1 to Flood 1 over a time t1, the situation is identical to having had is compared with z max 2; if z 1 is larger than z max 2 then when Flood 2 for a time t*. Therefore, when Flood 2 starts, the scour Flood 2 starts, the scour hole is already larger than it can be