Drainage Design Manual - the City of San Luis Obispo

9.4.2 Allowable OutflowThe allowable outflow from a basin is determined by the size of the existing drainage facilitiesdownstream. If the environmental impacts and proportional social costs for expanding thesefacilities are high, a large basin may be required to reduce outflows to acceptable levelsdownstream.The amount of outflow can be regulated with a variable opening gate or a fixed outfall structure.The City or County prefers the latter. Such a structure must include an outfall pipe of a size andlength that will give positive control on the outfall head. A typical fixed outfall structureconsists of a principal spillway or a series of pipes of graduated size, and an emergency spillway.The principal outlet regulates the design discharge from the watershed above at a water level inthe basin that does not exceed a certain maximum elevation. The emergency spillway is used torelieve the basin of extra runoff, which could threaten the integrity of the basin. In some cases, itis used in conjunction with the primary spillway. If flow through the emergency spillway isoccurring, the combined flow of both the primary and emergency spillways must not exceed thecapacity of downstream pipes and waterways.The interaction between basin size, inflow and outflow can be modeled using the San LuisObispo SLO/Zone 9 HEC-HMS hydrology model.9.4.3 Freeboard RequirementsRegardless of the size of the watershed upstream of a basin, the 100-year storm must be routedthrough the basin and the basin sized so that the maximum water surface elevation (due to a 100-year storm) is 600 mm (2 ft) below the lowest point along the top of the basin. At a minimumthe basin must contain the 10-year flow without release to emergency spillway. If flows over theemergency spillway do occur, provisions must be made or be in place that will convey suchflows safely.9.4.4 Basin Dewater ProvisionsThe basin and its outfall must be sized so that approximately 85% of the total stormwaterstorage, excluding sediment storage in the basin can be recovered within twenty-four hours ofthe peak inflow.9.4.5 Secondary UsesBasin size also varies if secondary use is desired. If the basin is to double as a passive, natureviewingand permanent pond area, the depth of the permanent pond should not be counted as aflow storage area. Also, if the basin is to be used as an active play field, the basin bottom or aportion of it, should be raised to avoid constant inundation. Additionally, the basin side slopesshould be flattened to allow turfing and mowing and use by the public.SLO Creek Drainage Design Manual 104 February 2003