SW3220402_Stormwater Narrative_20220627

VILLAGE AT GRANITE QUARRY - PHASE 2
BMP STORMWATER DESIGN NARRATIVE
DPR No. 17031
Rev 2-18-2021, 5-7-2021, 2-04-2022, 3-15-2022
The enclosed package shows the results of the stormwater BMP design calculations for Phase 2
of Village at Granite community. The development is in Granite Quarry, Rowan County, North
Carolina, at 3040 Faith Road. The net site area is approximately +/-108 acres and the existing
conditions consist of covered buildings with pavement, open space and woods. This phase of
the project's proposed development will be a 132 lot single-family residential homes.
This phase has two (2) proposed stormwater BMP facilities for the site, a wet pond and a sand
filter. All were designed per the latest NCDENR Stormwater design manual. Topographic maps
and field observations were used to delineate the watershed basins and hydraulic path feeding
into the study conveyance system. With this information, hydraulic lengths, time of
concentrations (TOC), and basin slopes were determined for the pre -developed conditions. TOC
of 5 minutes was used for all post -developed conditions. The total impervious areas of the
development were calculated. The composite curve numbers (CN) were calculated based on the
hydrologic soil types and groups from the U.S. Department of Agriculture, SCS method for the
pre -developed and post -developed conditions.
Surface areas for the wet pond was sized to handle the first inch of rainfall based on total
impervious area of the watershed using the Schueler simple method and both basins were sized
to provide for channel protection using the 1-year, 24-hr storm. Modeling results indicate that
2-year and 10-year (24-hr storm events, using SCS Method) release peak flow rates for the post -
development detention routed conditions are less than that of pre -developed conditions.
Precast concrete outlet structures are designed with 6" thick top slabs. The weir heights vary to
allow the 100-year storm to pass below the bottom of the top slab. This insures that these
outlets function as weirs and do not transition into orifices.
Refer to the detailed maps, calculations, results, and details as shown in this package and in the
civil engineering subdivision plans.
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Table 1: DRAINAGE AREAS:
Basin No.
Drainage Area
Pre -Developed
(+/- Ac.)
Drainage Area
Post -Developed
(+/- Ac.)
Phase 2 BMP #7
5.5
22.71
Phase 2 BMP #8
3.0
6.3
Phase 2 Temp BMP #9
2.5
-----
Phase 2 Temp BMP #10
5.0
-----
Phase 2 Temp. BMP #11
7.0
-----
Phase 2 Temp BMP #12
4.5
-----
Phase 2 Undetained Areas*
8.5
5.5
Total
36.0
34.51
TABLE 2: PRE -DEVELOPED DISCHARGES:
1- Year
(cfs)
2 - Year
(cfs)
10 - Year
(cfs)
25 - Year
(cfs)
50-Year
(cfs)
100-Year
(cfs)
Phase 2 BMP #7
0.279
0.920
4.164
6.821
9.137
11.64
Phase 2 BMP #8
1.303
2.219
5.116
7.047
8.646
10.32
Phase 2 Temp BMP #9
0.938
1.599
3.709
5.118
6.282
7.496
Phase 2 Temp BMP #10
0.135
0.503
2.725
4.646
6.391
8.289
Phase 2 Temp. BMP #11
3.283
5.963
14.58
20.52
25.50
30.71
Phase 2 Temp BMP #12
1.076
2.050
5.468
7.831
9.811
11.89
Phase 2 Undetained Areas*
3.189
6.917
19.34
27.95
35.18
42.80
Total
7.036
14.31
40.44
59.40
75.57
92.87
TABLE 3: POST -DEVELOPED DISCHARGES:
1- Year
(cfs)
2 - Year
(cfs)
10 - Year
(cfs)
25 - Year
(cfs)
50 - Year
(cfs)
100 - Year
(cfs)
Phase 2 BMP #7
41.73
58.01
105.41
134.88
158.40
182.32
Phase 2 BMP #8
12.19
16.78
30.12
38.33
44.87
51.51
Phase 2 Undetained Areas*
1.147
3.133
10.50
15.73
20.18
24.90
Total
54.95
77.92
145.32
188.27
222.83
258.21
TABLE 4: ROUTED -DEVELOPED DISCHARGES:
1- Year
(cfs)
2 - Year
(cfs)
10 - Year
(cfs)
25 - Year
(cfs)
50 - Year
(cfs)
100 - Year
(cfs)
Phase 2 BMP #7
0.000
0.000
2.357
3.927
10.85
25.47
Phase 2 BMP #8
0.121
0.281
1.744
8.056
20.04
31.52
Phase 2 Undetained Areas*
1.147
3.133
10.50
15.73
20.18
24.90
Total
1.195
3.233
10.67
17.28
35.07
59.72
TABLE 5: ROUTED -DEVELOPED DISCHARGES EVELATION:
1 - Year
2 - Year
10 - Year
1 25 - Year
1 50 - Year
1 100 -Year
Phase 2 BMP #7
724.45'
725.42'
726.39'
727.09'
727.44'
727.74'
Phase 2 BMP #8
733.00'
734.15'
734.95'
735.38'
735.68'
735.91'
FEMA Elevation BMP #7
724.50'
724.50'
724.50'
725.00'
725.50'
726.00'
FEMA Elevation BMP #8
726.30'
726.30'
726.30'
726.80'
727.30'
727.80'
*Undetained areas include areas that are disturbed during construction but covered with grass in
the final condition.
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