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21
Figure 4.5. Scour of a two-layer soil (hard layer over soft layer).
is contained in Layer 1 and does not reach Layer 2. If, how- identical to having had Layer 2 scoured over an equivalent
ever, the scour depth reaches z 1 (Point A on Figure 4.5a), time t* (Point B on Figure 4.6b). Therefore, when Layer 2
Layer 2 starts to be eroded. In this case, even though the scour starts being eroded, the scour depth versus time curve pro-
depth z 1 was due to the scour of Layer 1 over a time t1, at that ceeds from Point B to Point C on Figure 4.6b. The combined
time the situation is identical to having had Layer 2 scoured curve for the two-layer system is OAC on Figure 4.6c.
over a time t* (Point B on Figure 4.5b). Therefore, when Layer In the general case, there may be a series of soil layers with
2 starts being eroded, the scour depth versus time curve pro- different erosion functions. The computations proceed by
ceeds from Point B to Point C on Figure 4.5b. The combined stepping forward in time. The time steps are t long, the
curve for the two-layer system is OAC on Figure 4.5c. velocity is the one for the corresponding flood event, and the
erosion function (z versus t) is the one for the soil layer cor-
responding to the current scour depth (bottom of the scour
4.5 SOFT SOIL LAYER OVER HARD SOIL hole). When t is such that the scour depth proceeds to a new
LAYER AND GENERAL CASE
soil layer, the computations follow the process described in
Layer 1 is soft and z 1 thick. Layer 2 underlies Layer 1 and Figures 4.5 or 4.6 depending on the case. The same SRICOS
is harder than Layer 1. The scour depth z versus time t curve program mentioned for the velocity hydrograph also handles
for Layer 1 is given by Equation 4.4 (Figure 4.6a), and the z these calculations. The output of the program is the scour
versus t curve for Layer 2 is given by Equation 4.5 (Figure depth versus time curve for the multilayered soil system and
4.6b). If z 1 is larger than the maximum depth of scour in for the complete velocity hydrograph.
Layer 1, z max 1, then the scour process is contained in Layer 1
and does not reach Layer 2. If, however, the scour depth 4.6 EQUIVALENT TIME
reaches z 1 (Point A on Figure 4.6a), Layer 2 starts to erode.
In this case, even though the scour depth z 1 was due to the The computer program SRICOS is required to predict the
scour of Layer 1 over a time t1, at that time the situation is scour depth versus time curve as explained in the preceding
