All stormwater drainage system designs, plans and/or construction
shall comply with all of the elements of the Martins/Jacoby Act 167
Watershed Plan (and those plan elements are herein incorporated by
reference in this chapter as those they more fully appeared at large
herein) and be in conformance with any and all stormwater and drainage
studies and plans of the Township of Upper Mount Bethel and/or the
County of Northampton, and/or the Pennsylvania Department of Transportation
and/or the Pennsylvania Department of Environmental Protection.
A. Method of computation.
(1) Peak discharge and runoff shall be computed using the soil-cover
complex method as set forth in the latest edition of "Urban Hydrology
for Small Watersheds Technical Release No. 55" as published by NRCS
or the rational method or other methods as approved by the Township
Engineer. If the rational formula is utilized, the design parameters
shall be obtained from the following data:
(a) Value of Coefficient "C" (per Act 167). Coefficient "C" used for
calculation of runoff shall be used based on the anticipated ultimate
use of the land.
B. Time of concentration. The time of concentration shall be the time
which is takes to reach the location of the drainage facility to be
designed and shall include overland flow time plus gutter flow time
plus time of flow through pipes, culverts or natural streams.
(1) Overland flow. Overland flow time shall be computed using a generally
accepted chart or formula.
(2) Velocity of flow in open channels.
(a)
The velocity of flow in open channels and in closed drains not
under pressure shall be determined by Manning's equation.
Q
|
=
|
Flow in cubic feet per second
|
n
|
=
|
Coefficient of roughness
|
A
|
=
|
Cross-section areas of flow
|
R
|
=
|
Hydraulic radius
|
S
|
=
|
Slope in feet per foot
|
(b)
The coefficient of roughness, "n," shall be as follows unless
otherwise approved by the Township Engineer.
Manning "n" Values by Typical Reach Description
|
---|
Reach Description
|
Manning "n"
|
---|
Natural stream, clean, straight, no rifts or pools
|
0.030
|
Natural stream, clean, winding, some pools and shoals
|
0.040
|
Natural stream, winding, pools, shoals, stony with some weeds
|
0.050
|
Natural stream, sluggish with deep pools and weeds
|
0.070
|
Natural stream or swale, very weedy or with timber underbrush
|
0.100
|
Concrete pipe, culvert or channel
|
0.012
|
Corrugated metal pipe
|
0.012 to 0.027*
|
HDPE or PVC pipe
|
0.011 to 0.017*
|
*
|
Depending upon type and diameter.
|
C. Permissible stream velocities in open channels. The permissible stream
velocities to be used in open channel are as follows:
Material Allowable
|
Velocity (fps)
|
---|
Well-established grass on good soil:
|
Short pliant bladed grass
|
5 to 6
|
Bunch grass, soil exposed
|
2 to 3
|
Stiff stemmed grass
|
2 to 3
|
Earth without vegetation:
|
Fine sand or slit
|
1 to 2
|
Ordinary firm loam
|
2 to 3
|
Staff clay
|
3 to 5
|
Clay and gravel
|
4 to 5
|
Coarse gravel
|
4 to 5
|
Soft shale
|
5 to 6
|
Other:
|
Bituminous or cement stabilized channels
|
6
|
Paved channels
|
10 to 15
|
D. Methods of stormwater runoff detention, retention and control.
(1) The following is a listing of detention, retention and stormwater
control methods [best management practices (BMPs)] 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. Additional
information on BMPs and the use of combined methods are included in
the Pennsylvania Handbook of Best Management Practices for Developing
Area (PA Assoc. of Conservation Districts, PADEP and CH2M Hill, 1998).
(a)
Detention basins or retention facilities.
(c)
Parking lot and street pounding.
(d)
Seepage pits, seepage trenches, porous pipes or other infiltration
structures.
(e)
Porous pavement and concrete lattice block surfaces.
(f)
Grassed channels and vegetated strips.
(g)
Cisterns and underground reservoirs.
(h)
Routed flow over grass (flow spreaders).
(i)
Decreased impervious area coverage.
(2) A combination of various methods may be used and tailored to meet
the particular requirements of the type of development and the topographic
features of the project area.
E. Other methods. The use of other control methods, which meet the criteria
in the above sections, will be permitted when approved by the Township
Engineer.
Right of entry. Duly authorized representatives of the municipality
may enter at reasonable times upon any property within the municipality
to investigate or ascertain the condition of the subject property
in regard to any aspect regulated by this chapter.