Storm Drainage and Flood Control
Storm Drainage and Flood Control
Storm drainage systems collect and convey runoff to receiving waters. Candidates must understand hydrology (Rational Method, TR-55), hydraulics (open channel flow, culvert design, inlet capacity), detention/retention, floodplain management, and stream stability. Key regulations include FEMA floodplain ordinances and local drainage criteria.
Traps include: confusing detention (temporary storage, slow release) with retention (no release, infiltration/evaporation). Another trap is culvert design: inlet control vs. outlet control; the headwater elevation must be checked for both, and the higher headwater governs. Also, inlet spacing must ensure spread does not exceed allowable limits (e.g., half a lane width). Time of concentration is critical for Rational Method; it is the sum of sheet flow, shallow concentrated flow, and channel flow times. A common error is to use the wrong rainfall intensity for the computed tc.
Worked example: A 20-acre urban area has a runoff coefficient C=0.7. The 25-year rainfall intensity for the time of concentration (20 min) is 3.5 in/hr. Using the Rational Method, Q = C i A = 0.7 × 3.5 × 20 = 49 cfs. If a detention basin is required to limit outflow to pre-development Q (C=0.2, same i), pre-Q = 0.2 × 3.5 × 20 = 14 cfs. The basin must store the difference in runoff volumes during the storm. Use a hydrograph method (e.g., Modified Rational) for sizing.
Rule of thumb: For culvert design, minimum diameter is 18 inches; check both inlet and outlet control. For detention, a common rule is to provide storage for the 2-year and 10-year storms, and safe passage for the 100-year.