SW3220305_Stormwater Narrative_20220325

309 E Morehead St Ste 160
Charlotte, NC 28202
T: 704.870.3670
January 31, 2022
Mr. Jim Farkas
Environmental Engineer
DEMLR Stormwater Program
EBow:nAND . 512 N Salisbury St, 6th floor
ell Raleigh, NC 27604 156673
Subject: WSACC - Rocky River Wastewater Treatment Plant (RRRWWTP) Expansion DB
Phase 3
Stormwater Management Memo
Dear Jim Farkas:
The RRRWWTP is being expanded to increase its capacity in two phases, Phase 3 and
Phase 4. Phase 3 is being designed and the stormwater improvements for it are
described in this memo. Phase 4 will be covered in a future design. The Phase 3
expansion will increase the impervious area of the site and add conveyances to transport
stormwater through the site.
The RRRWWTP parcel area is 241.46 acres with 13.2 acres of existing impervious area.
The proposed impervious area from the Phase 3 expansion is 1.5 acres which will put
RRRWWTP at a total impervious area of 14.7 acres. Taking into account the 6.6 acres of
open surface water area on the site, this yields a proposed percent impervious of 6.24%
which classifies the project as low density according to NCDEQ Stormwater rule 1017 (4).
The existing stormwater management includes stormwater networks and one stormwater
control measure (SCM) in the north portion of the site. The stormwater networks convey
water to the fringes of the property where they drain to Rocky River through vegetated
swales. One stormwater network drains to the SCM in the north portion of the site. This
SCM is composed of two permanent sedimentation basins that discharge to Rocky River.
There will be several additions and modifications to the stormwater management of the
site. The changes include the rerouting of an existing stormwater network, the addition of
a culvert, the addition of a French drain and storm drain combination, the adjustment of
grades near the gravity thickeners, and the addition of open top structures.
The first change is the partial rerouting of an existing stormwater network from the
proposed Aeration Basin No. 6 being constructed on top of it. The portion of the network
that is under the proposed basin will be replaced with a trench drain at the edge of the
aeration basin structure. The trench drain will connect to the existing network and
discharge to the vegetated Swale at the north of the site. A small portion of the drainage
area that went to the existing network in front of HPOAS Basin No. 5 will not be able to be
reconnected to the rerouted system due to topography and the use of the trench drain.
Since the area of this portion is small and the site is grassy, the network in this area will
be removed and a vegetated Swale will be added to route flow around the HPOAS Basin
No. 5. Infiltration will treat most of the runoff from this small drainage area and the rest
will flow to the existing stormwater networks downstream. As a final note, the drainage
area to the rerouted network will have less impervious area after the expansion is
complete due to the aeration basins having open tops to collect rainfall.
03-Strm-Management

Mr. Jim Farkas
NC Department of Environmental Quality
Page 2
The second change is the addition of a culvert since the equalization (EQ) tank access
road crosses an existing vegetated swale. The grades to construct the road will be raised,
and ditches will be added on the high side of the road to direct water to the culvert, under
the road, and back to the existing vegetated swale.
The third change is the use of a French drain behind the proposed maintenance canopy.
Due to the grades around the maintenance canopy, drainage will be caught behind the
structure with minimal ways to drain. To remediate this, a French drain will be installed at
the back of the structure to collect drainage. The French drain will connect to a storm
drain that will convey water under the parking area and connect to the culvert under the
EQ tank access road. A ditch above the French drain will be installed to drain emergency
flows to the east to prevent the maintenance canopy from flooding.
The fourth change is the grade adjustments for the installation of the gravity thickeners
and Temporary Sludge Pump Station Number 2 (TSPS No. 2). Grades will be adjusted to
direct flow around the structures to prevent ponding of water and instead drain to
existing stormwater networks. A swale will be added behind TSPS No. 2 to convey
stormwater around the structure to an existing stormwater network.
The fifth change is the addition of open top structures throughout the site. The open top
structures will reduce the runoff leaving the site since the intercepted flow will be
directed through the wastewater treatment process. The open top structures include the
EQ tank, Aeration Basin No. 6, and several vaults. The total area of these open top
structures is approximately 1.1 acres.
Sincerely,
Brown and Caldwell
;•`��:oFessio•�:�
Kim Devendorf, PE
Charlotte, NC Office 65
'•'• FNG I NEE••'
02/04/22 1:13 PM