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i <br /> I Subsurface haploration, Geologic Hazard, and <br /> pre(iminary Georechnica!Engineering Repon <br /> Sho-Isle Ski!!s Center Addition �o�gi�y�ards and Midga/fons <br /> �oen, Washingron <br /> � <br /> turbidity reading is 25 NTU or lower. if the monitored turbidity exceeds 250 NTU, the results <br /> � must be reported to Ecology wit6in 24 hours and corrective action taken. Daily turbidiry <br /> i monitoring is continued until the corrective action lowers the turbidity to below 25 NTU. <br /> ' In ordez to meet the current 'r.cology requirements, a pxoperly develeped, constructed, and <br /> maintained erosion conuol plan consistent with the City of Everett standards and best <br /> management erosion control practices will be required for this project. AESI is available to <br /> assist the project civil engineer in developing site-specific eros.on conuol pians. Based on past <br /> experience, it will be necessary to make adjustments'and pto���e sucicess of theSTESC plan <br /> TESC plan in order to optimize its effectiveness. U_tunately, lementation and <br /> depends on a proactive approach to project planniag and contractor imp <br /> maintenance. <br /> The erosion hazard of the site soils is low. The most effective erosion contro] measure round <br /> maintenance of adequate ground cover. Maintaining cover measures atop disrurbed g <br /> provides che greatest reduction to che potential generation of turbid runoff and sediment <br /> transport• During �e local wet season (Ocfober 1" through Mazch 31"), exposed soil should I <br /> not remain uncovered for more than 2 days unielastic sheetinly �aw mulchcrushed �rockvor l, <br /> measures can include erosion control matting, P g � <br /> recycled concrete, or marure hydroseed. ,� <br /> Flow-control measures are also essential for wllecting and con:rolling the site runoff. Flow � <br /> paths across slopes should be kept to less than 50 feet in order to ieduce the erosion and <br /> sediment transport potential of ccncentrated flow. Ditch/swale spacing will need to be <br /> shortened with increasing slope gradient. Ditches and swales that exceed a gradient of about <br /> '7 to 10 percent, depending on their flow length, should have properly consuucted check dams <br /> installed to reduce the flow velocity of the runoff and Leduce the erosion poteutial within ihe <br /> ditch. Flow paths tbat ue required to be conscructed on gradients between 10 to IS percent <br /> should be placed in a riprap-lined swale with the riprap properly sized for the flow conditions. <br /> Flow paths constructed on siope gradients steeper than 15 percent should be placed in a Fipe <br /> slope drain. AESI is available to assist the project civil engineer in developing a suitable <br /> erosion control plan with proper flow control. <br /> Some fine-grained surface soils are the result of natnral weatheting processes that have broken <br /> down parent rnaterials into their miner�l components. These tnineral components r.an have an <br /> inhe,ent electrical charge. Ele.cuically chazged mi��.eral fines will attract oppositely chazged <br /> particles and can combine, (flocculate) to fotm larger particles that wi(1 settle out of suspension. <br /> The sediments produc;� during the receni glaciation of Puget Sound aze, however, most <br /> �ommoniy the suspended soils that ue cattied by site stotm water. The fine-grained frac[ion <br /> of the glacially derived soil is referred to as "rock flour," which is primarily a silt-sized <br /> particle with no electrica] charge. These particles, once suspended in water, may have settling <br /> times in periods of monihs, not hours. <br /> cnr�i.'c 71✓r <br /> _ "'___ ___• _-_' <br /> � A.\.11/1.1A%L�%tw���iil v':.:�: ' <br /> June 11, 2008 ••--- page 7 <br /> F�/�-FE(W(RI3A5-Prolea+1200801131F�1 R'P <br />