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HomeMy WebLinkAbout701153016 Stormwater Mgmt ~ .... STRATUM GROUP 1451 Grant Street, Bellingham, W A 98225 Phone (360) 714-9409 July 31, 2005 Michael Havens Re: Clearing aDd Storm Water Impacts Tax Parcel 701153016, Toandos Road Jefferson County, Washington roL.l"OS - ~ Dear Mr. Havens: Clearing of trees from the site as described by you in your email to me will have no impact on storm water. Most of the site is underlain by sand and gravel (advance glacial outwash and/or transition beds). These soils are very well draining with very rapid permeability. Clearing of trees from areas underlain by these well-drained soils will not lead to any increase in storm water flow on the surface. The ravine on the north side of the property and the ravine south of the property are dry ravines with no indication of flowing water at any time. These features appear to be remnant outwasb ravines and no longer transmit water even dwing extreme rainfall events. The following recommendations are made to ensure that the clearing will not have any negative impacts to storm water or to slope stability. The first recommendation is the same as the recommendation I made in my initial report. The other recommendations are based on my understanding of the geology of the site and slopes on the site. Ground cover vegetation within the ISO-foot landslide buffer should not be disturbed. Trees within the buffer may be thinned or limbed for view purposes. However, if the trees are ~ trees should be left on at least 30-foot centers. The mature trees on the slope are hydrologically mature and as such will greatly reduce the risk of the upper slope from becoming saturated. The trees also playa significant role in landslide deposit stability. That is large wood debris greatly slows the rate at which erosion of a landslide debris pile at the base of the bluff in the event that another landslide takes place. No clearing of trees should take place within the pipe stem portion of the east end of the property as this area is underlain by glacial till and clearing in this area may lead to an increase in storm water flow. Clearing in this area may be appropriate subject to further on-site review. No clearing of tress should take place on the steep slope between the access road and the southeast comer of the property without additional review as to the potential stonn water impacts. If this area is underlain by till (I did not look specifically at this slope), increased run off down the slope could lead to erosion and slope stability problems. July 31,2005 Tax Parce1701153016, Toandos Road, Jefferson County, WA CleariRg and Stonn Water Impacts The clearing of between 3 and 5 acres with the above areas excluded will not in my opinion increase the risk of another large slope failure on the shoreline bluff. While water plays a role in the failures on the bluff, the main drivers of the bluff slope failures have been erosion of the toe of the slope and saturation of the steep slope itself. Though the slide is very large, it is mot a deep-seated failure driven by excess ground water recharge. The size of the failures appears large simply because of the very high bluff slope. I will forward this letter directly to Gregg Ballard for his review along with your email. I will follow up with you after I return from vacation. We appreciate the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Stratum Group k)~~ Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist Stratum Group FiIc:7.3I.OS 2 Page 1 of2 Dan McShane Sent: Subject: "Havens, Michael LCDR CUSNC N43" <michael.havenS@me,navy.mil> "Dan McShane" <Dan@McShane.us> <kimninnemann@yahoo.com>; "Havens, Michael LCDR CUSNC N43" <michael.havenS@me.navy,mil>;"S Havens (E-mail)..<jazette626@msn.com> Sunday, July 31, 200512:07 AM RE: Toandos Property From: To: Cc: Dan, Thank you so much for getting back to me, We're hoping you can assist us with our current action on the property that you reviewed last year, Bottom Line: We need an addendum to the Geotech that addresses the impact of clearing 3-5 acres of timber and the associated storm water management. Also, what you think that would cost. Desired closure: 7 Aug is best case if possible Details: We currently have a permit request in process with Jefferson county to clear up to 5 acres of the property to be used for; replanting trees (evergreens, hardwoods, orchard, ornamental), building site including "residential" landscaping area, and an appropriate safety zone around the future building site, While the report you generated last year addresses the bluff and appropriate management of it. the county does not feel it addresses fully enough the management of stromwater associated with clearing as much area as we are proposing, As such, they have requested a "general storm water management statement" (he even mentioned "one page") that addresses how the potential increase in run off will be managed on the property and whether their will be any threat to destabilization, I have verbally described to the county (Greg Ballard) that we intend to leave all the trees in the ravine to the north, several groves of trees along the ridge leading to the ravine (to assist in run off absorption) and that the ravine to the south is not our property anyway and also serves as an excellent area for infiltration and evapor- transportation, The contour of each ravine also allows some pooling, assisting even more with reduction in run- off, Please let us know as soon as you can 1) What your initial thoughts/questions are about what I've laid out above 2) Whether you believe you can address the county's concerns and if so, if you'll need to visit the property again and 3) an estimate of how much you believe your additional support will run us, Thanks for your help again Dan. Greg expressed his confidence in the work you generate and that he thought everything at the property was supportable, In the interest of completeness though. he wanted the geotech report to more fully cover the intended work at the property. Michael and Sabrina Havens -----original Message--- From: Dan McShane [mailto:Dan@McShane.us] Sent: Friday, July 29, 2005 3: 13 AM To: Havens, Michael LCDR OJSNC N43 SUbject: Toandos Property Michael Here is my email, 7/31/2005