Government Canal Stormwater Analysis Study

Client: City of Pacific, Washington

The City of Pacific contracted Clear Creek Solutions as the stormwater modeling subconsultant to find a pump solution to alleviate flooding in and around the Government Canal.

The canal is a tributary to the White River in southern King County, Washington.

WWHM-SWMM was used to model pumping alternatives.

Study Area

The study area was located in the City of Pacific, Washington, between the Union Pacific Railroad tracks on the west and the BNSF Railway (C Street) on the east.

The study area included 1022 acres of highly industrialized areas in the northern portion and 127 acres of wetlands and residential areas in the southern portion near the White River.

Process and Solution

To prevent canal water from flooding the residential areas along Government Canal, King County plans to construct a new pump station.

Clear Creek Solutions used WWHM-SWMM (HSPF hydrology linked to SWMM hydraulics) to model the Government Canal drainage area, the canal flow, and the pump options at the confluence with the White River.

WWHM-SWMM simulated the stormwater runoff from the study area for both current and proposed alternative conditions for the period of October 1948 through April 2018.

Clear Creek Solutions used WWHM-SWMM HSPF continuous simulation hydrology to compute runoff.

The January 9, 1990, flood event was selected for the design event because it is between the 50-year and 100-year flood and it is a high-volume event.

The combination of high peak and high volume make this event ideal for analyzing the performance of the proposed pump station.

Clear Creek Solutions used WWHM-SWMM HSPF hydrology and SWMM5 hydraulics to model five pump station pumping options.

For each pump option the HSPF portion of the model was run for the time period of 1948 to 2018.

The SWMM hydraulic portion of the model was run for a shorter three-year time period. This three-year period included multiple flood events ranging from a 2-year flood to greater than the 50-year flood. The largest event during this three-year period occurred on January 9, 1990.

Results

WWHM-SWMM model analysis showed that to keep the Government Canal flood depth from exceeding a flood depth of 4.57 feet that three 21.5 cfs pumps are required.

If the flood depth is allowed to reach 6 feet then a single 21.5 cfs pump is required.

The City of Pacific and King County will use this information to decide on the appropriate pump station design for Government Canal.

The project was completed on time and within budget using WWHM-SWMM’s HSPF continuous simulation hydrology and SWMM5’s dynamic wave hydraulics to accurately and efficiently model the study area.

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