20131070 Ver 1_Staff Comments_20131118

Strickland, Bev
From: Devane, Boyd
Sent: Monday, November 18, 2013 5:15 PM
To: Fox, Tim; Cranford, Chuck
Cc: Strickland, Bev
Subject: Revised version of comments for Elk Creek project 1331070
Attachments: ElkCreekBannerElk.docx
Attached are my revised comments. Let me know if you have questions or suggestions.

Your 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.
a. It seems like the proposal is to design a detention basin over a sand filter. Although this
is an acceptable approach, you might consider some modifications. You might want
some detention first and follow that will a sand filter.
b. Boyd Devane of our Raleigh Office talked to you on November 18th about the issue of
peak control. He explained that your efforts to include peak control calculations in the
design had led to confusion about the design. However, he explained that the Division
of Water Resources only required the system to provide water quality treatment for the
1.0" design storm. However, he later learned that the Town of Banner Elk requires
some peak flow attenuation. Therefore, you will need to reconsider your design to
include the water quality treatment for the first inch of runoff and some peak flow
control as required by the Town.
c. Since the sand filter is the major treatment mechanism, you should use the Sand Filter
Supplement 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. The size of the sand filter is not 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 -ch11
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 gravel. The drawing shows a 150 -300
sand layer which is much smaller than the washed stone layer which is 40 -80mm
g. 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.
h. In the revised version, please include all calculations as shown in Chapter 11 of the DWR
Stormwater Manual.
i. 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
Boyd will be happy to work with you to develop a workable stormwater solution for this site. He
can be reached at boyd.devane @ncdenr.gov or at 919 - 807 -6373.