Environmental Engineering Reference
In-Depth Information
15
Postdevelopment uncontrolled
10
Postdevelopment w/detention
5
Predevelopment
0
10
20
Time interval (hr)
30
40
Figure 1-23
Detention basins hold the increased runoff but do not reduce the volume,
which subsequently floods downstream.
1
Hydrograph with detention
3
2
4
5
Predevelopment hydrograph
Basin 5
Basin 3-4
Basin 2
Basin 1
Time
Figure 1-24
Multiple hydrographs in a watershed with detention basins. The compound
hydrograph is actually greater than it was predevelopment.
volume. Figure 1-24 illustrates the effect of multiple detention structures within
a common drainage basin, with the net result an actual increase in the flow rate
downstream, because of the delayed timing in releases from multiple structures.
It is obvious that detention-based designs will never be capable of controlling
or mitigating the volume increase of rainfall runoff produced by land develop-
ment. However, we now have countless thousands of these rate control structures
situated across the suburban landscape. One of the current issues in stormwater
management is how to modify these structures to improve their function beyond
simple rate control, as discussed in a later chapter.
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