20121074 Ver 1_Infiltration Pond Sediment Forebay (AL)_20130314

Strickland, Bev
From: Lucas, Annette
Sent: Thursday, March 14, 2013 9:25 AM
To: Neal Tucker
Cc: Moran, Mike (MMoran @Ashleyfurniture.com); Ben Healy; Christian Youngman; Matthews,
Matt; Karoly, Cyndi; Wakild, Chuck; Bush, Ted; Homewood, Sue; Basinger, Corey; Randall,
Mike
Subject: RE: 12 -160 Infiltration Pond Sediment Forebay - Ashley Furniture
ME
I left you a phone message yesterday morning — not sure if you received it.
I wanted to let you know that your ideas sound reasonable to me. The 20% sizing for the forebay will work well I
believe. I also think that it is wise that you are bypassing the storms exceeding 1" to avoid sediment resuspension. In
addition to your ideas, I wanted to offer that it would be reasonable to discharge the roof water to the main treatment
are of the pond if that was helpful /feasible.
Thanks,
Annette
Annette M. Lucas, PE
Environmental Engineer
NCDENR I DWQ I Wetlands and Stormwater Branch
1650 Mail Service Center, Raleigh, NC 27699 -1650 (N111a.i.l.)
512 N. Salisbury St, Raleigh, NC 27604 1 9th Floor (Location &, Parcels)
Phone: [phone] Fax: [phone] Email: l nnetLe.1 -u as'(rLL(- Uicnr.��t-ry
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From: Neal Tucker [mailto:NTucker @stimmelpa.com]
Sent: Tuesday, March 12, 2013 3:22 PM
To: Lucas, Annette
Cc: Moran, Mike (M Mora n @Ashleyfurniture.com); Ben Healy; Christian Youngman
Subject: 12 -160 Infiltration Pond Sediment Forebay - Ashley Furniture
Annette,
I wanted to follow -up with you on our telephone conversation regarding the size of the sediment forebay and TSS yield
estimate.

I've done some research on sediment yield and the information available is pretty limited as it applies to our situation —
i.e. sediment yield from gravel parking areas, or densely developed areas. I first looked at the erosion control manual. It
outlines the minimum required sediment storage volume for sediment basins for construction sites, but does not speak
specifically to soil loss rates (inches /year), that would be useful for estimating cleanout frequencies for a given sediment
storage volume. Next, I looked briefly at the Revised Universal Soil Loss Equation, but that appears to be more
applicable to the natural landscape and agricultural lands and much less applicable to developed site conditions such as
we normally deal with. I then came back to the Manual and the sediment storage requirement for wet detention
ponds. Whereas the Manual does not provide guidance for sedimentation rates, it does require the one foot of
sediment storage in the bottom, regardless of the permanent pool depth. This makes it difficult to back calculate a
sedimentation rate. Then I looked at a wet pond with permanent pool depth of 3 feet that would be required to serve
the site, and calculated the storage depth in the bottom 1 foot of the pond. This equates to roughly %" of sediment load
per acre, or twice the 1/8" per acre required for sediment basin design.
We have agreed that the roof will not contribute to the TSS loading . The roof area accounts for 32.5% of the drainage
area. I would argue that TSS loading from the asphalt and concrete pavements will be negligible and will be limited to
what is 'tracked on' for concrete, and 'tracked on' plus 'surface wear' for asphalt. In both cases very negligible. Asphalt
and concrete pavements account for 37.0% of the drainage area. There will be some TSS loading from grassed areas,
but much less than the 1/8" required for sediment basin design. The grassed areas account for 21.6% of the drainage
area. The majority of the TSS loading will come from the gravel paving areas. I have found nothing useful for estimating
TSS loading from this type surface. Maybe this is a good future research project for Bill's stormwater group! The gravel
paving areas account for 8.9 % of the drainage area.
Given the lack of data available for estimating the TSS yield from this watershed, I propose to treat sediment storage in
the infiltration pond similar to what would be required for a wet detention pond but with a minor difference. Given the
rapid infiltration rate in the pond (weighted average of 16.8 -19.2 inches per hour), I believe the vast majority of the
TSS will be contained in the forebay, with little to none getting to the main body of the pond. I propose allocating 20%
of the pond area as the forebay. The pond is currently sized for a 3 foot pool depth, which should yield a infiltration
time of 1.8 to 2.1 hours. If we assume as sediment builds that the forebay 'blinds' and no longer infiltrates, the
remaining 80% of the pond will impound to 3.75 feet (assuming no infiltration during the storm), and the resulting
infiltration time increases to 2.3 to 2.7 hours.
As we discussed previously, the design will bypass storm events larger than 1" to a spillway upstream of the pond, so
any concern for re- suspension of sediment and flushing into the main body of the pond should be minimal.
Let me know what you think of this approach. We're getting in the final detailing stages and should complete the design
soon. If there are aspects of this that we need to discuss, please call me on my cell (336) 817 -2524.
Thanks
Neal Tucker, PE
Director of Civil Engineering
ntucker a.stimmelpa. com
Stimmel Associates, P.A.
601 N. Trade St. Suite 200
Winston - Salem, NC 27101
v: 336.723.1067 ext.104
f: 336.723.1069
c: 336.817.2524
www.stimmelpa.com
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