Detention Storage from a Hydrograph
Mediumstormwaterdetentionhydrograph
Runoff from a development produces a triangular inflow hydrograph with a peak of 6 m3/s and a total base time of 90 minutes. The downstream permit allows a constant release of 2 m3/s. The detention storage required is most nearly:
- A3,600 m³
- B7,200 m³
- C10,800 m³
- D16,200 m³
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Answer: B. 7,200 m³
Required storage is the area between the inflow hydrograph and the allowable outflow line. By similar triangles, inflow exceeds 2 m³/s for a width of tb(1 − Qa/Qp) = 90(1 − 2/6) = 60 minutes = 3,600 s. That region is itself a triangle of height (6 − 2) = 4 m³/s, so storage = ½(3,600)(4) ≈ 7,200 m³. The value 16,200 m³ is the total inflow volume ½(5,400)(6), which ignores the fact that the basin is releasing throughout the storm.
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