10901 27TH AVE SE Geotech Report 2025-07-29

' Proposed James Monroe Elementary School Replacement Snhstzface Exploration, Geologic Hazards, and Geotechnical Engineering Report Everett, Washington Geologic Hazards and Mitigations Therefore, the flow length within a temporary sediment control trap or pond has virtually no effect on the water quality of the discharge, since sediment will not settle out of suspension in ' 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 sediment traps and ponds are necessary to control the release rate of the runoff and to provide ' a catchment for sand-sized and larger soil particles, but are very ineffective at reducing the turbidity of the runoff. ' To mitigate the erosion hazards and potential for off-site sediment transport, we recommend the following: I 1. The 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 ' ground-cover measures, access roads, and staging areas in the project bid to give the selected contractor a workable site. The selected contractor needs to be prepared to implement and maintain the required measures to reduce the amount of exposed ' ground. A site maintenance plan should be in place in the event storm water turbidity measurements are greater than the Ecology standards. ' 2. All TESC measures for a given area to be graded or otherwise worked should be installed prior to any activity within that area. The recommended sequence of construction within a given area would be to install sediment traps and/or ponds and ' establish perimeter flow control prior to starting mass grading. 3. During the wetter months of the year, or when large storm events are predicted during ' the summer months, each work area should be stabilized so that if showers occur, the work area can receive the rainfall without excessive erosion or sediment transport. The required measures for an area to be "buttoned-up" will depend on the time of year and ' the duration the area will be left un-worked. During the winter months, areas that are to be left un-worked for more than 2 days should be mulched or covered with plastic. During the summer months, stabilization will usually consist of seal-rolling the ' subgrade. Such measures will aid in the contractor's ability to get back into a work area after a storm event. The stabilization process also includes establishing temporary storm water conveyance channels through work areas to route runoff to the approved ' treatment facilities. 4. All disturbed areas should be revegetated as soon as possible. If it is outside of the growing season, the disturbed areas should be covered with mulch, as recommended in the erosion control plan. Straw mulch provides the most cost-effective cover measure and can be made wind-resistant with the application of a tackifier after it is placed. ' October.5, 2009 ASSOCIATED EARTH SCIENCES, INC. tfWG/Id-KE090312A3-Projects 1200903I21KEIWP Page 9