A.
Stormwater pipe sizes. Diameter on public easements and rights-of-way:
15 inches minimum (use the Rational Method to determine the pipe diameter);
Q = CIA, where Q = maximum expected discharge in cubic feet per second,
C = runoff factor expressed as a percent of the total water falling
on an area, I = the rainfall for the time of concentration of the
drainage area in inches per hour for a given storm frequency, and
A = the drainage area expressed in acres.
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Pipe Material:
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Concrete pipe is preferred. PVC may be used, but must be laid
in bedding of 12 inches of sand below and above the pipe.
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B.
Design criteria. The design and installation shall be based on adequate
survey and investigations.
(1)
Compliance with laws and regulations. Design, installation and/or
construction shall comply with state and local laws, ordinances, rules
and regulations.
(a)
In order to control pollution of public waters by soil sediment
from accelerated stream channel erosion and floodplain erosion caused
by accelerated stormwater runoff from developed area, the peak rate
of runoff from an area after development may be no greater than the
peak rate of runoff from the same area before development for all
twenty-four-hour storms from one- to 100-year frequency. Design and
development to match the peak rate of runoff for the one-, two-, five-,
ten-, twenty-five-, fifty- and 100-year storm may be considered adequate
to meet the rule.
(b)
If the volume of the runoff from an area after the development
will be greater than the volume of runoff from the same area before
development, it shall be compensated by reducing the peak rate of
runoff from the critical storm and all more frequent storms occurring
on the development area to the peak rate of runoff from a two-year
frequency, twenty-four-hour storm occurring on the same area under
predevelopment conditions. Storms of less frequent occurrence (longer
return periods) than the critical storm, up to the 100-year storm,
shall have peak runoff rates no greater than the peak runoff rates
from equivalent size storms under predevelopment conditions.
(c)
The critical storm for a specific development area is determined
as follows:
[1]
Determine the total volume of runoff from a two-year frequency,
twenty-four-hour storm occurring in the development area before and
after development using one of the following methods:
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Soil Cover Complex Method (TR 55)
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USGS Bulletin 13 Method
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Rational Method
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Corps of Engineers Method
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PSU III Method
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[2]
From the volumes in Subsection B(1)(a) of this rule, determine the percent of increase in volume of runoff due to development and using this percentage, select the twenty-four-hour critical storm from this table:
[a]
If the percentage of increase in volume of runoff
is:
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Equal to or Greater Than
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And Less Than
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Twenty-Four-Hour "Critical Storm" for Discharge Limitation Will
Be:
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|---|---|---|
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0
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10
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1 year
|
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10
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20
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2 year
|
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20
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50
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5 year
|
|
50
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100
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10 year
|
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100
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250
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25 year
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250
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500
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50 year
|
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500
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—
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100 year
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Example:
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Discharge before development = 35 cfs
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Discharge after development = 74 cfs
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Design year = (74/35 - 1) = 1.114
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Percentage increase = 1.114 x 100 = 1.114% design for 25 years.
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[b]
Methods for controlling increases in stormwater
runoff peaks and volumes may include, but not be limited to:
[i]
Retarding flow velocities by increasing friction.
[ii]
Grading and use of grade control structures to
provide a level of control in flow path and storm gradients.
[iii]
Induced infiltration of increased stormwater
runoff into the soil, where practical.
[iv]
Provisions for detention and retention.
[c]
Stormwater runoff and design calculations shall
be prepared by a qualified Pennsylvania registered professional engineer.
C.
Rainfall frequency data.
(1)
Available from United States Department of Commerce, National Weather
Service and Pennsylvania Department of Environmental Protection, Research
Publication No. 70.
(2)
Commonwealth of Pennsylvania, Department of Environmental Protection,
Rainfall Duration Frequency Table for Pennsylvania.
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Office of Resources Management
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Bureau of Dams and Waterway Management
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Division of Stormwater Management
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PO Box 2357
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Harrisburg, Pennsylvania 17120
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February 1983
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(3)
The design storm frequencies for the Stowe Township watershed that
shall be used for all analyses are as follows:
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Design Storm
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Rainfall Depth
(inches) 24 hour
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Rainfall Depth
(Inches) 1 hour
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|---|---|---|
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2-year
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2.6
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1.25
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5-year
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3.3
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1.60
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10-year
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3.8
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2.00
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25-year
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4.4
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2.40
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50-year
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4.7
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2.75
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100-year
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5.0
|
3.10
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D.
Maintenance of natural drainageways. All natural streams, channels,
swales, drainage systems and/or areas of surface water concentration
shall be maintained in their existing condition unless an alteration
is approved by the Township. All encroachment activities shall comply
with the requirements of Chapter 105 (Dam Safety and Water Management)
of Title 25, Environmental Protection, of the Pennsylvania Department
of Environmental Protection.[1]
[1]
Editor's Note: See 25 Pa. Code Chapter 105.
E.
Methods of stormwater runoff detention and control.
(1)
The following is a listing of detention and control methods which
may be utilized in stormwater management systems, if appropriate.
The choice of control techniques is not limited to the ones appearing
on this list.
(a)
Detention basins.
(b)
Roof-top storage.
(c)
Parking lot and street ponding.
(d)
Seepage pits, seepage trenches or other infiltration structures.
(e)
Porous pavements and concrete lattice block surfaces.
(f)
Grassed channels and vegetated strips.
(g)
Cisterns and underground reservoirs.
(h)
Routed flow over grass.
(i)
Decreased impervious area coverage.
(2)
The use of other control methods which meet the criteria in this
section will be permitted when approved by the Township Engineer.
Various combinations of methods should be tailored to suit the particular
requirements of the type of development and the topographic features
of the project area.
A.
In order to implement the provisions of the county stormwater management
plan, the Township is hereby divided into one stormwater management
district which shall be designated as "Stowe Township."
B.
When a project or land-disturbance activity is isolated in more than
one stormwater management district, stormwater may not be transferred
from a district with stricter stormwater management criteria to a
district with less strict criteria, unless the need for such a transfer
is identified in the county stormwater management plan, the regional
water quality management plan or the state water plan.
A.
Stormwater rate and quality.
(1)
Stowe Township district. There shall be no increase in the rate of
stormwater discharge from any activity covered by this chapter that
would have occurred from the land prior to the activity using the
prior condition. The peak release rate of stormwater from detention
facilities shall be no greater after development than prior to development
activities.
(2)
Erosion and sedimentation. All land-disturbance activities shall
be conducted in such a way as to minimize accelerated erosion and
resulting sedimentation. Measures to control erosion and sedimentation
shall at a minimum meet the standards of the Conservation District
and Chapter 102 (Erosion and Sediment Control) of Title 25, Environmental
Protection, of the Pennsylvania Department of Environmental Protection.[1]
[1]
Editor's Note: See 25 Pa. Code Chapter 102.
