SW4220601_Stormwater Narrative_20221221

Havenbrooke Subdivision Stormwater Narrative
Havenbrooke Subdivision is a 71 lot single family development on a 74.97 acre parcel
in Northeast Forsyth County. The subdivision is being developed as low density since
the percent of impervious coverage is 10.34% which is well below the State's
requirement of being less than 24% BUA, and the density is 0.947 units per acre which
is far less than the maximum 2 units per acre allowed for low density. In trying to
adhere to the low -density requirement of the State the subdivision is being developed
with ditch section roads using 3:1 side slopes with a 2' bottom width with ditch liner
and sodded with dense fescue. The pipe system for the development is limited to
cross pipes with rip rap aprons on the downstream side to keep the outlet velocity at
a non -erosive rate. To meet the State's low -density requirements per 15A NCAC
02H.1003 (2) the following design requirements were used for the low -density project
and are listed and described below:
(a) DENSTIY THRESHOLDS:
The density for this project is less than 2 dwelling units per acre. The impervious
percentage for the net 74.19 acre tract (74.97 ac. less surface water area) is 10.34%
(7.67 acres impervious).
(b) DISPERSED FLOWS:
Stormwater flows for this project will be mostly conveyed using ditch section flows
except where cross pipes are needed to convey stormwater across the NCDOT
designed streets. The outflows from these culverts will use rip rap aprons to reduce
velocities to be non -erosive. Calculations for all aprons have been provided in the
calculation booklet.
(c) VEGETATED CONVEYANCES:
Vegetated conveyances have been designed with side slopes no steeper than 3:1 with
a 2' bottom width to carry the runoff in roadside swales, curb outlets, and dispersed
flows to aid in treating the runoff. Also at the end of all pipe outlets a vegetated
conveyance has been designed to treat the runoff prior to outfalling into the natural
buffer areas. Calculations for the conveyances are contained within.
(d) CURB OUTLET SYSTEMS
Even though curb and gutter are not being used within the development, outlet
systems have been provided in suitable locations on all streets to convey stormwater
to vegetated conveyance systems to provide additional treatment from roadside
runoff. At the end of all curb outlets vegetated conveyances were designed using a
3:1 slope trapezoidal ditch with a minimum length of 100', a 2' bottom width, and
slopes 5% or less. These ditches are to be and sodded with tall fescue. These areas are
depicted on sheets C8.0 Et 8.1 with design information and details.
15A NCAC 2H.1003(4) VEGETATED SETBACKS:
Buffers have been provided on all existing intermittent and perennial streams within
the development. These have been denoted and shown on the plans.
15A NCAC 2H.1003(7) DEED RESTRICTIONS AND PROTECTIVE COVENANTS:
The permittee shall record deed restrictions and protective covenants prior to the
issuance of a certificate of occupancy to ensure that projects will be maintained in
perpetuity consistent with the plans and specifications approved by the permitting
authority.