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Soos Creek Watershed Modeling & Analysis to Address Bioassessment Impairments for the Soos Creek Fine Sediment Total Maximum Daily Load Study

Publication number Date Published Date Revised
24-03-003January 2024February 2024
VIEW NOW Soos Creek Watershed Modeling & Analysis to Address Bioassessment Impairments for the Soos Creek Fine Sediment Total Maximum Daily Load Study (Number of pages: 165) (Publication Size: 46459KB)

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Author(s) T. Mohamedali
Description The Soos Creek watershed includes Big Soos, Soosette, Little Soos, Jenkins, and Covington Creeks. The watershed covers an area of about 66 square miles in the Puget Lowlands of King County. It has been the focus of several monitoring and modeling studies. Monitoring data show these creeks are impaired for bioassessment/aquatic health based on Benthic Index of Biotic Integrity (B-IBI) scores. Excess fine sediment is one of three identified stressors, in addition to flow alteration and physical habitat degradation, contributing to these impairments. This report presents the technical analysis conducted to address these impairments and to inform a fine sediment Total Maximum Daily Load (TMDL) study.

Washington State Department of Ecology (Ecology) extended and recalibrated an existing HSPF (Hydrologic Simulation Program — Fortran) model of the watershed to simulate flow and sediment for 2001 – 2015. The recalibration process resulted in adequate model performance watershed-wide based on comparisons between simulated and observed flow and sediment.

The calibrated model was used to compare flow and sediment loads between existing/baseline and forested conditions and to calculate the sediment loading capacity (the amount of sediment the creeks can receive and still meet water quality standards). This loading capacity varies with flow since flow is one of the main drivers of sediment pollutant loads.

Statistical analysis between B-IBI scores and other available data demonstrated that while B-IBI scores are highly variable, they are responsive to flashy flows, fine sediment, the extent of impervious and forest cover in the watershed, and other local factors. We recommend the TMDL study take a holistic approach to addressing B-IBI impairments, including establishing load and wasteload allocations to reduce stormwater and its impacts and meet the targeted loading capacity.
NOTES Erratum for revised version (February 2024): 1) Figure 4 (pg. 23): corrected typo in the figure legend. 2) Figure 26 (pg. 85): corrected typo in title and y-axis label of bottom plot. 3) Figure 49 (pg. 118): Added label for red line. 4) A Typo in Appendix A (pg. 138) has been corrected. Original sentence: “KCWD #111/Lake Meridian WD Records for Wells 6 and 7 were received via public records request on June 9, 2017 from KCWD #111.” Corrected version: Changed “7” to “9”. 5) Figures D4–D6 (Appendix D, pg. 160–162): added figure legends.
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Keywords watershed model, Soos Creek, total suspended solids, fine sediment, TMDL, Total Maximum Daily Load, stormwater, TSS, B-IBI, high pulse count, Big Soos, Covington, Jenkins, Little Soos, Soos Creek Watershed, Soosette, flow alteration, aquatic health, flashy flows, impervious cover, Benthic Index of Biotic Integrity, HSPF
WEB PAGE Soos Creek watershed fine sediment TMDL
WATERSHED Water Resource Inventory Area 09 Duwamish-Green

QAPP Addendum: Soos Creek Bioassessment TMDL Modeling and Analysis