20131070 Ver 1_Staff Comments_20131118 (2762)

Strickland, Bev
From:
Devane, Boyd
Sent:
Monday, November 18, 2013 3:42 PM
To:
Fox, Tim; Cranford, Chuck
Cc:
Strickland, Bev
Subject:
Elk Creek Project #131070
Attachments:
ElkCreekBannerElk.docx

Your proposal of dry retention with filtration is an acceptable way to treat the stormwater.
However, it is difficult to determine from the information presented how the system has been
designed 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.
b. Some of our concern is probably caused by the confusion about whether your project
needs to provide peak flow control. There will be some peak flow control inherent to
the design but not to the extent that you proposed. Although our Supplement Form
does have options for peak control matching, those requirements are not necessary for
projects in this basin. We understand how this point causes confusion. You are only
required to treat the 1.0" design storm as is discussed in Chapter 11 and Chapter 3.
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. No 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 pavements or another infiltration device that might reduce
the elevation of temperature from the runoff. Use of permeable pavement can be a
cost - effective method of treatment while providing superior protection for temperature
rises in the sensitive trout waters.