2021-08-09-minutes-public-works-study-session

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hoss... rQ KliTFTA3 COU!V'F'Y KIZ`TZTAB Co DEPARTMENT OF PUBLIC WORKS TO: Flood Control Zone District Advisory Committee COPY: Arden Thomas, Water Resources Manager Candie Leader, Administrative Assistant DATE: February 02, 2021 FROM: Mark R. Cook, Director RE: Lick Creek Model Peak Flows Recently, the Committee heard from Lick Creek residents regarding their concerns for increased flows resulting from Public Works Lick Creek Fish Barrier Removal project. This memorandum provides technical data intended to assist the Committee to better understanding resident concerns and to factually represent the County's design decisions. I am attaching Chapter 4 (EXHIBIT 1) of the final design report utilized by the Department of Fish and Wildlife in granting the Hydraulic Project Approval for the project. The project was qualified as a Fish Habitat Enhancement Project under RCW 77.55.181 (EXHIBIT 2). Early in the project formulation stage, the County presented our desire realigning Lick Creek directly to the Teanaway River removing any element of creek flooding to area residents. The small amount of resulting bank erosion was offered to be mitigated by the Federal Highway Administration. The community vehemently objected to removing the creek from the front of their properties. The properties are in a channel migration zone of the Teanaway River and recurrently experience various levels of flooding from Lick Creek. The properties have flooded in the past and will likely flood in the future. In the absence of stream gage data for the area, our consultant modeled area hydrology and then utilized USGS Region 2 Regression Equations that were updated in 2016. The previous 100 -year flow rate utilized for Lick Creek is 287 cubic feet per second (CFS). The revised 100 - year flow rate used in our report is estimated at being 268 CFS (slightly less than the historic value). Watershed wildland fire impact from the Jolly Mountain fire was considered utilizing aerial imagery from 2017. USGS "StreamStats" predicts canopy cover at approximately 53%. Our model incorporating canopy loss from fire utilizes approximately 36%. Less canopy — more predictive runoff. Our resulting forecast of the 100 -year Lick Creed design flow rate is 448 CFS. This is not a "new flow rate" as represented by some of the area residents. Recognizing the bridge has a design life of 50 years or more, we elected to size the structure for the 448 CFS. It is important to note that forest canopy re-establishes rapidly, and we are now nearly four years beyond the Jolly Mountain Fire and forest canopy continues to recover and reduce runoff. In 2019, Public Works elected to collect survey data of downstream properties at significant expense which delayed the project one year. This effort was done to accurately assess any adverse impact to downstream property owners. The resulting digital topographic model was 411 N. Ruby, Suite #1 TEL (509) 962-7523 Ellensburg, WA 98926 FAX (509) 962-7663