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Stormwater Technical Memorandum PertZ 2707 Colby Avenue, Suite 900, Everett, WA 98201 1 P 425.252.7700 I F 425.339.6018 Infiltration Rate: 0.1 in/hr. Due to the small size of the project a full geotechnical investigation is not warranted. Recent projects in the vicinity of the site have identified this rate as a conservative rate for the Alderwood type soils found in the area. A further safety factor of 50% is applied to account for inconsistencies in the soils and other long term degradation of the infiltration capacity. A previously completed geotechnical report included a boring near where the portables will be placed. See attached excerpt. The boring indicates that the undisturbed, very dense till soil layer begins between 3 and 5 feet below the ground surface. No groundwater was encountered in the borings, but some mottling was observed. Overflow: The facility is designed to act similar to the existing condition and minimize construction impacts. Although the facility is designed to infiltrate all runoff from the new roof areas there may be conditions which will exceed the capacity of the facility due to either exceptionally intense rainfall or failure of the facility. In the event that the facility does exceed its design depth water will flow out of the top of the facility and disperse into the adjacent grass area on the downhill side. The adjacent ground slopes gently to the north and east as shown in the attached Figure 1. Overflow from the more southerly portable structures will be partially intercepted by the facilities below the more northern structures. The majority of the overflow which leaves the area around the portables will flow overland to an existing bioswale located near the property's northern boundary. Construction Stormwater Pollution Prevention The project will involve only minimal ground disturbance and has a low risk of erosion. Only the ground immediately beneath the portables will be disturbed in order to remove vegetation and topsoil. Exposed soils will be covered with a layer of stormwater facility gravel within a few days of exposure, if not the same day. In addition, the construction will be limited to summer months as the portable structures must be installed during summer vacation when students are not present, and must be completed and ready for use for the next school year. There is therefore a lower risk of a significant rainfall event occurring while the ground is exposed. The project will expose much less than one acre of land, so erosion control measures must comply with the small parcel criteria listed in the Stormwater Manual. Table 1 summarizes the projects approach to compliance with the five elements associated with small parcel construction stormwater control. Table 1. Summary of • • • Approach Criteria Project Approach Provision of Construction The work area is adjacent to a paved surface which the Access contractor will use for access. No additional access protection is warranted. Stabilize Denuded Areas Only the areas immediately below the portables will be disturbed. These areas will be stabilized with crushed aggregate surfacing Protection of Adjacent Construction will occur within the site and well away Properties from any adjacent properties. Maintenance of Erosion and No BMPs are proposed, so no maintenance other than Sediment Control BMPs good housekeeping is required. Implementation of Other If erosion problems do occur during the construction of BMPs as Determined by the the project, they will be addressed as appropriate in City of Everett. coordination with the City inspector. Page 2 File location: XaEverett Public Schools\Projects1201S0040 - Silver Lake Elementary Portables\Design\Drainage\Silver Lake Elementary Drainage Memorandum - Everett.docx