Client: City of Olympia Public Works, Olympia, WA

Clear Creek Solutions constructed a WWHM/SWMM model of the City of Olympia drainage systems impacted by a rise in sea levels and associated Puget Sound high tides.

City GIS data was used to create SWMM models of the City of Olympia drainages that may be impacted by a rise in sea levels. These SWMM models were first created in SWMM5. Subsequently, they were imported into XPSWMM, a more complex and slower model, for refinements and analysis. This approach allowed continuous analysis of the drainage system using the EXTRAN module of SWMM. The model included as much detail about the drainage network as existed in the City GIS database.

The precipitation time series included data through December 10, 2007 was used to calibrate the models to recent events. All known or observed water surface elevations were used in an effort to calibrate the model performance as best as possible. While flow and stage data simply does not exist for these drainage systems, this approach was sufficient to analyze current and future scenarios.

Current and potential future tide scenarios were identified and added to the models. The models were then run to identify potential drainage problem areas. Potential implications to the LOTT Alliance wastewater treatment facility and combined sewer overflows were addressed. The potential flooding recurrence interval and potential contributions to the combined system were computed.

Clear Creek Solutions provided analysis and results for five scenarios. Each scenario was analyzed for the following:

a) Flooding volume and duration for each of three different tidal conditions at the identified problem areas. The analysis defined the magnitude of the potential problems.
b) Scenarios addressed implications to the LOTT facility. City and LOTT staff helped to define the significance of impacts to LOTT.
c) Volume of flow into the combined system and interval of contribution.