4702 W GLENHAVEN DR 2020-06-16 - Laserfiche WebLink

Oskar Property Deck Limited CAO,Everett,WA Materials Testing&Consulting,Inc. July 17,2019 19B182 vegetation outside of the planned building footprint should be avoided, and is prohibited in proximity to or on the slope face. Marginal upland areas disturbed during construction should be revegetated at the end of construction with appropriate plantings to limit future erosion. The extent of alteration to existing vegetation outside of the project area, if any, is not known. We recommend a goal of low impact or vegetative enhancement to these areas be applied. Following construction and for long-term site care, maintaining existing vegetation and installing additional beneficial ground plantings as needed on upland areas as well as near the slope crest is encouraged, assuming installation is done in a manner that minimizes slope face disturbance and erosional hazard in the long term. Adding brushy vegetation to the site pad and upper slope will increase erosional and hydrologic resistance, and assist in retaining loose cover soils along the crest area to limit potential for crest retreat. General recommendations for erosion control are provided in the section below, including typical beneficial native plantings well suited for sloping topography and root structure development. However, major landscaping alterations should not be undertaken without the involvement of a qualified professional. In general, MTC recommends avoiding altering the slopeward area by terracing or similar landscaping unless adequately designed as part of a comprehensive improvement which also takes into account surface stability and retainment of loose cover soils. In the event that significant surface alterations are proposed that are outside of the current known scope of improvements, MTC should be contacted to review and consult on the geotechnical feasibility and implementation of landscape improvement plans in consideration of critical area development. Drainage Controls and Outfalls Permanent drainage controls are required for the existing property due to the direct association of the project site to the critical area slope, and the potential for increased surface erosion and instability to occur on the slope face without suitable controls in place. This requirement also applies to the proposed deck structure and any modifications to the house roof drainage as a result of the deck construction. We recommend any new or reconstructed/rerouted downspouts relating to the deck replacement project be appropriately controlled and directed so as not to pose a risk to the stability of the bordering slope. The full extent of drainage controls in place for the existing structure was not reviewed by MTC as part of this focused project scope. However, generally speaking the dispersion or infiltration of stormwater in proximity to the slope crest is not recommended, as increased runoff or localized storm water inundation near the critical area slope will be detrimental to long-term erosional and global slope stability. No downslope natural drain courses suitable for stormwater release were observed on the site, and downslope tightlining for release past the slope base is assumed to be inviable due to property boundary restrictions. We recommend new downspouts, if utilized, should be directed to an existing site stormwater control system for disposal, assuming an existing system is present. 9 ay