9001 AIRPORT RD SNO ISLE SKILL CTR EXPANSION 2009 Geotech Report 2018-01-02 MF Import

i I Subsurjace Fxploration, Geobgic Hau+rd, and Sho-/sle Skil/s CenterAddidon PreUminary Gectechnical Er+gineering Report Eventt. Washington GeoJogic Harords and Mitigations ITherefore, the flow length within a temporary sediment control trap or pond has virtually no effect on the water qualiry of the discharge since it is not going to settle out of suspension in I the time it takes to flow from one end of the pond to the other. Reduction of turbidity from a construction site is almost entirely a function of cover measures and flow control. Temporary � sed'unent Vaps and ponds aze necessary to control the release rate of the runoff and w provide � a catchment for sand-sized and lazger soil particles, but aze very ineffective at reducing the twbidity of the runoff. ISilt fencing should be utilized as buffer protection and not as a flow-control measure. Silt fencing is meant to be placed pazallel with topographic contours to prevent sediment-laden I runoff from leaving a work azea or entering a sensitive area. Silt feaces should not be placed to cross contour lines without having sepazate flow control in front of the silt fence. A swale/berm combination should be conswcted to provide flow control rather than let tt►e � runoff build up behind tt►e silt fence and utilize the silt fence as the flow-control measure. Runoff flowing in front of a silt fence will cause additional erosion and usually will cause a failure of the silt fence. Improperly installed silt fencing 6as the potential to cause a much I lazger erosion hazard than if the silt fence was not installed at all. The use of silt fencing should be limited to protect sensitive azeas, and swales should be used to provide flow control. ( 7.1 Erosion Hazard MitiAation � To mitigate the erosion hazards and potential for off-site sediment transport, we would recommend the following: � 1. 1'he winter performance of a site is dependent on a well-conceived plan for control of site erosion and storm water runoff. It is easier to keep the soil on the ground than to remove it from storm water. The owner and the design team should include adequate I ground-cover measures, access roads, and staging azeas in che project bid to give the selected contractor a workable site. The selected conuactor needs to be prepazed to implement and maintain the required measures to reduce the amount of exposed I ground. A site maintenance plan should be in place in the event storm water turbidiry measurements are greater than the Ecology standazds. � 2. All TESC measures for a given azea to be graded or otherwise worked should be instalied prior to any activiry within an azea other than installing the TESC features or timber hazvesting. The re�ommended sequence of construction witLin a given azea � after timber hazvesting would be to install sediment traps and/or ponds and establish perimeter flow control prior to starting mass grading. I3. During the wetter months of the year, or when lazge storm events are predicted during the summer months, each work azea should be stabilized so that if showers occur, the Iwork area can receive the rainfali without excessive erosion or sediment uansport. The ��Q�� Z� ASSOCIA7ED E1RTN SCI6NCES,/NC. � EG/Id-EE090113A5-Pro1�t12Q'N02131EE1WP �e g ,'