4.2
All conveyance systems shall be designed, at a minimum, to convey a peak design flow rate resulting from a 100‑year frequency storm event, with the following exceptions:
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Other governing authorities may require that the design of some structures be based on a larger storm event.
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Some runoff treatment BMPs are designed to function primarily under low flow conditions. Unless higher flows are diverted from these runoff treatment BMPs per Vol II–4.8.1 Flow Splitters, they shall also be designed to have sufficient conveyance capacity for 100‑year storm flow rates.
For all existing and proposed conveyance systems receiving drainage from a contributing area of 25 acres or less and having a time of concentration of 100 minutes or less, the Rational Method may be used. For all other conditions, either the Santa Barbara Urban Hydrograph Model (SBUH), Western Washington Hydrology Model (WWHM), or MGSFlood shall be used. For public road projects, design flows may be determined using the hydrologic modeling procedures in the WSDOT Highway Runoff Manual or the procedures outlined in this section.
4.2.1 Rational Method
The traditional Rational Method, as described in most engineering manuals, is preferred by Kitsap County for designing systems serving smaller contributing basins primarily because it tends to provide higher runoff flow rates than hydrograph methods do, resulting in a more conservative design with a built-in factor of safety. Modeling guidance for the Rational Method is provided in Appendix F.
4.2.2 Santa Barbara Urban Hydrograph Method
Calculations shall be conducted in accordance with the instructions found in Volume III of the Ecology Manual. Modeling shall be performed using a 10-minute time step.
4.2.3 Western Washington Hydrology Model (WWHM)
The Western Washington Hydrology Model (WWHM) (latest approved version) is an approved continuous simulation model. See Volume II, Chapter 5 and Volume III, Chapter 2 of the Ecology Manual for a description of how to use WWHM. Modeling shall be performed using a 15-minute time step.
4.2.4 MGSFlood
MGSFlood is also an approved continuous simulation model. As of the publication of this manual, MGSFlood Version 4.49 had limited approval status and was not approved by Ecology for modeling bioretention. Refer to Ecology’s Approval Status of Continuous Simulation Models web page for updates related to the approval status of MGSFlood. Consult with the MGSFlood user manual for a complete description of how to use the model. Modeling shall be performed using a 15-minute time step.