Drainage Design Manual - the City of San Luis Obispo

Table 4-9Example Rational Method Runoff Rate ComputationRecurrence Area C T c AnnualPrecipi 15 min i 30 min i Tc C a QInterval (hectares) (min) (mm) (mm/hr) (mm/hr) (mm/hr) (m 3 /s)2-year 32.73 0.55 16.6 500 36 25 35 1 1.7510-year 32.73 0.55 16.6 500 61 41 59 1 2.95100-year 32.73 0.55 16.6 500 86 58 83 1.25 5.194.1.5 Use of SLO/Zone 9 Hydrology Model for Catchment Areas Greater than 40 Hectares(100 acres)A detailed numerical computer hydrology model was developed as part of the SLO WaterwayManagement Plan (the SLO/Zone 9 hydrology model). This model uses the Corps of Engineersrunoff hydrograph method HEC-HMS that converts precipitation excess to basin runoff andchannel discharge.The SLO/Zone 9 model shall be used for all hydrologic calculations in the SLO Watershed fordrainage facilities with more than 40 hectares (100 acres) of contributing watershed area. Inaddition, the SLO/ Zone 9 model shall be used in all City and County required or sponsored AreaPlans, Specific Plans, Master Plans including Drainage Master Plans and Development MasterPlans.The SLO/Zone 9 hydrology model shall be used as a design aid for calculating flow in openchannels with catchments over 40 hectares (100-acres) and in the design of all hydraulic anddrainage structures in such catchments including:Open channelsBridgesCulverts associated with such open channelsFlood controlStormwater detention basinsA copy of the latest /updated version of the SLO/Zone 9 model, including model documentationcan be obtained from the City Engineering Department for the cost in time and materials toreproduce the model, as determined by the City Engineer. The project proponent or designengineer will be responsible for making changes and revisions to the model, consistent with themodeling approach, and with the approval of the City and/or County Public Works Director.Procedures for updating the HEC-HMS model should be consistent with those outlined in theHydrology and Hydraulics section of the San Luis Obispo Waterway Management Plan.Generally, there should be no need to develop new precipitation data; as precipitation alreadydeveloped for the Zone 9 model characterizes the whole basin for a design event. The modelparameters likely requiring modification includes sub-basin delineation and sub-basin parametersSLO Creek Drainage Design Manual 34 February 2003