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Home My WebLink About20131070 Ver 1_Staff Comments_20131118 (2763)Strickland, Bev From: Devane, Boyd Sent: Tuesday, November 19, 2013 9:20 AM To: Fox, Tim; Cranford, Chuck Cc: Strickland, Bev Subject: One more revision to Elk Creek Comments #13 -1070 Attachments: ElkCreekBannerElk.docx I realized last night that I had written my comments as if the letter would have been sent to the engineer David Ramsey but that you would be sending it to the project contact. I also thought of another issue I forgot to mention. Hopefully, this will be my last version. Please let me know when you send the letter out. I told the engineer that I would send him a copy of my comments. thanks 1 The proposal of dry retention with filtration is an acceptable way to treat the stormwater at this site. However, there has been some confusion in our understanding of the project design and how it will function. Some of the issues that need clarification or revision are: a. It seems like the proposal is to design a detention basin over a sand filter. Although this is an acceptable approach, your project would benefit from some modifications. You might want some detention first and follow that will a sand filter or divert the design storm to a filter and then use detention. b. Boyd Devane of our Raleigh Office talked to the consulting engineer, David Ramsey, on November 18th about the issue of peak control. He explained that his inclusion of peak control calculations in the design had led to confusion about the design of the system. Mr. Devane noted that the Division of Water Resources only required the system to provide water quality treatment for the 1.0" design storm. However, Mr. Devane later learned that the Town of Banner Elk does require some peak flow attenuation. Therefore, the design will need to reconsider the 85% water quality treatment for the first inch of runoff and must include some peak flow control as required by the Town. We do not believe that the design as proposed will achieve both the water quality and peak control requirements. c. Since the sand filter is the major treatment mechanism, the Sand Filter Supplement should be used rather than the Dry Extended Basin supplement. Also, the Sand Filter O &M Agreement is more appropriate than using the Dry Basin O &M form. d. We could not find the dimensions of the sand filter in the information provided. The sand filter must be designed consistent with the requirements of Chapter 11 of the Division of Water Resources (previously DWQ) Stormwater BMP Manual. http://Portal.ncdenr.org/web/wq/WS/su/bmp-chll e. The plan did not seem to include any information on the volume of detention storage or the need for any pretreatment or forebay. f. The depiction of the sand filter on the "Sediment Basin Cross section" drawing has some unclear elements such as the depth of the sand or of the gravel. The drawing shows a 150 -300mm sand layer being much smaller than the washed stone layer at 40 -80mm. Please correct the drawing. g. On the drawing with the "Detention /Sediment Basin" information, we do not believe the riser with the gravel- surrounded intake base will work properly. It seems from the drawing that some of the stormwater could short - circuit the sand filter and enter the openings of the aggregate at the base of the riser. Please re- evaluate that design. It also seems like the underdrain should be connected to the outlet. h. There was not a large -sized version of the Sediment Basin Crosssection provided. However the large -sized drawing of the "Detention /Sediment Basin" was stacked into the Sediment Basin Plan drawing. Parts of the drawing were clipped from view. i. In the revised version, please include all calculations as shown in Chapter 11 of the DWR Stormwater Manual. j. There are other methods of treating the stormwater that may work on your project. Since there are trout waters downstream of this site, we would strongly recommend consideration of permeable pavement or another infiltration device that might minimize the volume of runoff and the elevation of temperature from the runoff. Recent changes in Chapter 18 of our BMP Stormwater Manual allow use of pavements throughout the state and in many situations, this can be a cost - effective method of treatment. See http: / /portal.ncdenr.org /web /wq /ws /su /bmp -ch18 Mr. Devane will be happy to work with your consultants to develop a workable stormwater solution for this site. He can be reached at boyd.devane @ncdenr.gov or at 919 - 807 -6373.