TM 5-820-3/AFM 88-5, Chap. 3
j. The Saint Anthony Falls (SAF) stilling basin
diameter of the incoming pipe. The required well
diameter can be determined from the equation in
shown in figure 5-19 is a hydraulic jump energy
figure 5-17. The top of the well should be located
dissipator. To function satisfactorily this basin must
at the elevation of the invert of a stable channel or
have sufficient tailwater to cause a hydraulic jump
drainage basin. The area adjacent to the well may
to form. Design equations for determining the
be protected by riprap or paving. Energy
dimensions of the structure in terms of the square
dissipation is accomplished without the necessity of
of the Froude number of flow entering the
maintaining a specified tailwater depth in the
dissipator are shown in this figure. Figure 5-20 is a
vicinity of the outlet. Use of the stilling well is not
design chart based on these equations. The width of
recommended with Q/Do5/2 greater than 10.
basin required for good energy dissipation can be
i. The US Bureau of Reclamation (USBR)
calculated from the equation in figure 5-19. Tests
impact energy dissipator shown in figure 5-18 is an
used to develop this equation were limited to basin
efficient stilling device even with deficient tail-
widths of three times the diameter of the outlet.
water. Energy dissipation is accomplished by the
But, other model tests indicate that this equation
impact of the entering jet on the vertically hanging
also applies to ratios greater than the maximum
baffle and by the eddies that are formed following
shown in figure 5-19. However, outlet portal
impact on the baffle. Excessive tailwater causes
velocities exceeding 60 feet per second are not
flow over the top of the baffle and should be
recommended for design containing chute blocks.
avoided. The basin width required for good energy
Parallel basin sidewalls are recommended for best
dissipation for a given storm drain diameter and
performance. Transition sidewalls from the outlet
discharge can be calculated from the information in
to the basin should not flare more than 1 on 8.
figure 5-18. The other dimensions of energy
dissipator are a function of the basin width as
shown in figure 5-18. This basin can be used with
Q/Do5/2 ratios up to 21.
5-22
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