2022-02-23_PERMIT FILE - M2021059 (13)

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To: Mr. Pctcr Date: Fcbruar�,' 21 2022 Pa-e 21 Comment 4 Please explain why there is no outlet protection for flood flows at the lower end of Cell 1. Include a discussion of velocities estimated with the 2D HEC-RAS model. Response 4 During development of our erosion protection plan, we assumed that established grassy vegetation will conservatively tolerate shear stresses of up to 2.1 lb/ft2. This shear tolerance was referenced from the Washington State Aquatic Habitat Guidelines Program, Integrated Streambank Protection Guidelines, 2003,and the wetland seed mix specified in Attachment 4 most closely corresponds to the Class B vegetation referenced in the Integrated Streambank Protection Guidelines. Therefore,all shear stresses below 2.1 lb/ft2 required grassy revegetation but did not require hard armor. As seen in the revised 2D model, shear stresses at the east end of Cell 1 are below 2.1 lb/ft2 for all river flow rates below 9,000 cfs including when Cell 1 is discharging to the river. Comment 5 Please explain why no inlet protection is needed in areas other than the control points for each cell (shown on page 6 of the Inlet Protection Calculations). For example,for Cell I there is an area 250-300 feet east of the southwest corner where velocities in RAS Mapper (9,000 cfs) are greater than 13 feet per second. Response 5 Thank you for pointing out this deficiency. In order to reduce the potential for erosion caused by water entering the pits, we've made the following changes (Attachment 6): • Reduced the slopes of the pit walls above the waterline from 3:1 to 5:1 in the following general areas: — Cell 1 —West and south sides — Cell 2 —West, north and east sides — Cell 3 —West and north sides • Expanded the extent of the "pit inlet weirs" at the crests of the inlets to cover the following areas: — Cell 1 —Southwest, and southern sides — Cell 2—Northwest, north and northeastern sides — Cell 3 —Northwest, north and northeastern sides • Added hardened inlet protection so that the following areas are now covered: 20220209_!Oa5_DBMS_Adequacy_Review Response_MASTE R.docx TELESTO