District
|
Subareas
|
Design Storm Postdevelopment
|
Design Storm Predevelopment
| |
---|---|---|---|---|
A
|
2-year
|
1-year
| ||
5-year
|
5-year
| |||
10-year
|
10-year
| |||
25-year
|
25-year
| |||
100-year
|
100-year
| |||
B
|
2-year
|
1-year
| ||
5-year
|
2-year
| |||
10-year
|
5-year
| |||
25-year
|
10-year
| |||
100-year
|
100-year
| |||
C
|
(See below)
|
(See below)
|
Explanation of District C, Provisional Direct Discharge District. Development sites which can discharge directly to the Conococheague Creek main channel or major tributaries or indirectly to the main channel through an existing stormwater drainage system (i.e., storm sewer or tributary) may do so without control of postdevelopment peak rate of runoff. Sites in District C will still have to comply with the criteria for groundwater recharge (§ 166-14); water quality (§ 166-15); and streambank erosion (§ 166-16). If the postdevelopment runoff is intended to be conveyed by an existing stormwater drainage system to the main channel, assurance must be provided that such system has adequate capacity to convey the increased peak flows or will be provided with improvements to furnish the required capacity. When adequate capacity of downstream system does not exist and will not be provided through improvements, the postdevelopment peak rate of runoff must be controlled to the predevelopment peak rate as required in District A provisions (i.e., ten-year postdevelopment flows to 10 predevelopment flows) for the specified design storms.
|
Table 1
| |||
---|---|---|---|
Acceptable Computation Methodologies For
Stormwater Management Plans
| |||
Method
|
Method Developed By
|
Applicability
| |
TR-20 (or commercial computer package based
on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer
model is desirable or necessary.
| |
TR-55 (or commercial computer package based
on TR-55)
|
USDA NRCS
|
Applicable for land development plans within
limitations described in TR-55.
| |
HEC-1/HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of full hydrologic computer
model is desirable or necessary.
| |
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer
model is desirable or necessary; simpler than TR-20 or HEC-1.
| |
Rational Method (or commercial computer package
based on Rational Method)
|
Emil Kuichling (1889)
|
For sites less than 200 acres, or as approved
by the municipality and/or Municipal Engineer.
| |
Other methods
|
Varies
|
Other computation methodologies approved by
the municipality and/or Municipal Engineer.
|
Rev = [(S) (Rv) (A)]/12
| ||
Where:
| ||
Rev = Recharge volume
(acre-feet)
| ||
S = Soil specific recharge factor (inches)
| ||
Rv = Volumetric runoff
coefficient
| ||
A = Site area contributing to the recharge facility
(acres)
| ||
And:
| ||
Rv = 0.05 + 0.009 (I)
| ||
Where:
| ||
I = percent impervious area
| ||
And:
| ||
S may be obtained based upon hydrologic soil
group based upon the table below recommended as general values by
DEP. More site specific data may be utilized to determine recharge
if justified by the design engineer and approved by the Municipal
Engineer.
|
Hydrologic Soil Group
|
Soil Specific Recharge Factor (S)
| |
---|---|---|
A
|
0.38 inches
| |
B
|
0.25 inches
| |
C
|
0.13 inches
| |
D
|
0.06 inches
|
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.
|
"_________________________________________________________,
certify that the proposed detention basin (circle one) is/is not underlain
by limestone."
|
WQv
|
=
|
[(P)(Rv)(A)]/12
| ||
Where:
| ||||
WQv
|
=
|
Water quality volume
| ||
P
|
=
|
Rainfall amount [(90% of events producing this
rainfall (Appendix B, Table B-5)]
| ||
A
|
=
|
Area in acres
| ||
Rv
|
=
|
0.05 + 0.009(I) where I is the impervious surface
ratio
|