WQv = [P x Rv x
A]/12
| |||
Where:
| |||
1.
|
P
|
=
|
Rainfall depth in inches, using first flush rainfall amount,
1.5 inches
|
2.
|
A
|
=
|
Project area in acres
|
3.
|
Rv, volume runoff coefficient = (0.05
+ 0.009 x I]) where I is the impervious surface percentage (impervious
area ÷ total project area) x 100%
|
To accomplish the above, the applicant shall submit original
and innovative designs for review. Such designs may achieve the water
quality objectives through a combination of BMPs.
|
Stormwater Credit
|
Description
|
---|---|
Natural area conservation
|
Conservation of natural areas, such as forest, wetlands, or
other sensitive areas, in a protected easement thereby retaining their
predevelopment hydrologic and water quality characteristics. Using
this credit, a designer may subtract conservation areas from the total
site area when computing the required water quality volume.
|
Vegetated roof
|
Credit may be given for water quality and volume benefits for
vegetated roof covers where vegetation is grown on, and completely
covers, an otherwise flat or pitched roof (less than or equal to thirty-degree
slope). Green roof, if adequately designed, may be counted as lawn
for runoff calculations.
|
Disconnection of rooftop runoff
|
Credit may be given when rooftop runoff is disconnected and
then directed over a pervious area where it may either infiltrate
into the soil or filter over it. Credit is typically obtained by grading
the site to promote overland flow or by providing bioretention on
single-family residential lots. If a rooftop area is adequately disconnected,
the impervious area may be deducted from the total impervious cover.
|
Disconnection of non-rooftop runoff
|
Credit may be given for practices that disconnected surface
impervious cover by directing it to pervious areas where it may either
infiltrate into the soil or filter over it. As with rooftop runoff,
if area is adequately disconnected, the impervious area may be deducted
from the total impervious cover.
|
Stream buffer credit
|
Credit may be given when a stream buffer effectively treats
stormwater runoff. Effective treatment constitutes capturing runoff
from pervious and impervious areas adjacent to the buffer and treating
runoff through overland flow across a grass or forested area. Areas
treated in this manner may be deducted from total site area.
|
Grass channel/open section roads
|
Credit may be given when open grass channels are used to reduce
the volume of runoff and pollutants during smaller storms. If designed
according to appropriate criteria, these channels may meet water quality
criteria for certain types of residential development. Curbs may be
eliminated where adequately designed swales or bioretention facilities
are located in or adjacent to parking areas.
|
Environmentally sensitive rural development
|
Credit may be given when a group of environmental site design
techniques are applied to low density or rural residential development.
This credit eliminates the need for structural practices to address
water quality volume.
|
Removal of additional impervious areas
|
Credit may be given when existing impervious areas are removed
and restored to lawn or other landscape areas representing a net reduction
in impervious cover. The stormwater volume reduction resulting from
the restoration may be counted toward water quality and recharge volume
requirements.
|
For design and applicability of nonstructural BMPs refer to Chapter 5 of the "Pennsylvania Stormwater Best Management Practices Manual," December 2006 or latest edition. For the nonstructural BMPs proposed, the applicant shall utilize and submit appropriate checklists included in Chapter 8, Section 8.8, of said manual to demonstrate that the BMPs are applicable to the project an to determine the amount of volume or peak rate credit applicable.
|
Rev = [S x Rv x
A]/12
| |||
Where:
| |||
Rev
|
=
|
Recharge Volume (acre-feet)
| |
A
|
=
|
Site area contributing to the recharge facility (acres)
| |
Rv
|
=
|
Volumetric runoff coefficient = 0.05 + 0.009 x I where I is
the impervious surface percentage (impervious area ÷ total
project area) x 100%
| |
S
|
=
|
Soil specific recharge factor (inches) based on the hydrologic
soil group as provided in the table below:
|
Hydrologic Soil Group
|
Recharge Factor, S
|
---|---|
A
|
0.38
|
B
|
0.26
|
C
|
0.14
|
D
|
0.07
|
If more than one hydrologic soil group (HSG) is present at a
site, a composite recharge volume shall be computed based upon the
proportion of total site area within each HSG.
|
Neshaminy and Little Neshaminy Creek
|
Release Rate
|
---|---|
2-year storm event, post development
|
1-year storm event, predevelopment
|
5-year storm event, post development
|
2-year storm event, predevelopment
|
10-year storm event, post development
|
5-year storm event, predevelopment
|
25-year storm event, post development
|
10-year storm event, predevelopment
|
50-year storm event, post development
|
25-year storm event, predevelopment
|
100-year storm event, post development
|
50-year storm event, predevelopment
|
Pennypack Creek
|
Release Rate
|
---|---|
2-year storm event, post development
|
1-year storm, predevelopment
|
5-year storm event, post development
|
5-year storm event, predevelopment
|
10-year storm event, post development
|
10-year storm event, predevelopment
|
25-year storm event, post development
|
25-year storm event, predevelopment
|
50-year storm event, post development
|
50-year storm event, predevelopment
|
100-year storm event, post development
|
100-year storm event, predevelopment
|
Precipitation Frequency
(inches)
| |
---|---|
Design Storm
(years)
|
24 Hours
|
1
|
2.94
|
2
|
3.54
|
5
|
4.45
|
10
|
5.20
|
25
|
6.29
|
50
|
7.21
|
100
|
8.21
|
Precipitation Intensity (inches/hour)
| ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
Design Storm
(years)
|
5 min.
|
10 min.
|
15 min.
|
30 min.
|
60 min.
|
120 min.
|
3 hrs.
|
6 hrs.
|
12 hrs.
|
24 hrs.
|
1
|
4.45
|
3.55
|
2.96
|
2.03
|
1.27
|
0.76
|
0.56
|
0.35
|
0.21
|
0.12
|
2
|
5.29
|
4.24
|
3.55
|
2.45
|
1.54
|
0.92
|
0.67
|
0.42
|
0.26
|
0.15
|
5
|
6.23
|
4.99
|
4.21
|
2.99
|
1.92
|
1.15
|
0.85
|
0.53
|
0.32
|
0.19
|
10
|
6.91
|
5.53
|
4.66
|
3.37
|
2.20
|
1.33
|
0.98
|
0.61
|
0.38
|
0.22
|
25
|
7.72
|
6.15
|
5.20
|
3.85
|
2.56
|
1.57
|
1.16
|
0.73
|
0.46
|
0.26
|
50
|
8.29
|
6.61
|
5.58
|
4.20
|
2.84
|
1.76
|
1.30
|
0.83
|
0.53
|
0.30
|
100
|
8.86
|
7.04
|
5.93
|
4.54
|
3.13
|
1.95
|
1.45
|
0.94
|
0.60
|
0.34
|
NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/
|