A.
All regulated activities in the Borough of Chambersburg which do not fall under the exemption criteria shown in § 251-20 shall submit a drainage plan consistent with the Conococheague Creek Watershed Stormwater Management Plan to the Borough for review. This criteria shall apply to the total proposed development even if development is to take place in stages. Impervious cover shall include, but not be limited to, any roof, parking or driveway areas and any new streets and sidewalks. Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious for the purposes of comparison to the exemption criteria.
B.
Stormwater drainage systems shall be provided in order
to permit unimpeded flow along natural watercourses, except as modified
by stormwater management facilities or open channels consistent with
this chapter.
C.
The existing points of concentrated drainage that
discharge onto adjacent property shall not be altered without permission
of the affected property owner(s) and shall be subject to any applicable
discharge criteria specified in this chapter.
D.
Areas of existing diffused drainage discharge shall
be subject to any applicable discharge criteria in the general direction
of existing discharge, whether proposed to be concentrated or maintained
as diffused drainage areas, except as otherwise provided by this chapter.
If diffused drainage discharge is proposed to be concentrated and
discharged onto adjacent property, the developer must document that
adequate downstream conveyance facilities exist to safely transport
the concentrated discharge, or otherwise prove that no erosion, sedimentation,
flooding or other harm will result from the concentrated discharge.
E.
Where a development site is traversed by watercourses,
drainage easements shall be provided conforming to the line of such
watercourses. The terms of the easement shall prohibit excavation,
the placing of fill or structures and any alterations that may adversely
affect the flow of stormwater within any portion of the easement.
Also, maintenance, including mowing of vegetation within the easement,
shall be required, except as approved by the appropriate governing
authority.
F.
When it can be shown that, due to topographic conditions,
natural drainageways on the site cannot adequately provide for drainage,
open channels may be constructed conforming substantially to the line
and grade of such natural drainageways. Work within natural drainageways
shall be subject to approval by PaDEP through the joint permit application
process, or, where deemed appropriate by PaDEP, through the general
permit process.
G.
Any stormwater management facilities regulated by
this chapter that would be located in or adjacent to waters of the
commonwealth or wetlands shall be subject to approval by PaDEP through
the joint permit application process, or, where deemed appropriate
by PaDEP, the general permit process. When there is a question whether
wetlands may be involved, it is the responsibility of the developer
or his agent to show that the land in question cannot be classified
as wetlands, otherwise approval to work in the area must be obtained
from PaDEP.
H.
Any stormwater management facilities regulated by
this chapter that would be located on state highway rights-of-way
shall be subject to approval by the Pennsylvania Department of Transportation
(PENNDOT).
I.
Minimization of impervious surfaces and infiltration
of runoff through seepage beds, infiltration trenches, etc., are encouraged
strongly, where soil conditions permit, to reduce the size or eliminate
the need for detention facilities.
J.
Roof drains must not be connected to streets, sanitary
or storm sewers or roadside ditches to promote overland flow and infiltration/
percolation of stormwater where advantageous to do so. When it is
more advantageous to connect directly to streets or storm sewers,
then it shall be permitted on a case-by-case basis by the municipality.
K.
Special requirements for watersheds draining to high-quality
(HQ) and exceptional value (EV) waters. The temperature and quality
of water and streams that have been declared as exceptional value
and high quality are to be maintained as defined in Chapter 93, Water
Quality Standards, Title 25 of Pennsylvania Department of Environmental
Protection Rules and Regulations. Temperature-sensitive BMP's and
stormwater conveyance systems are to be used and designed with storage
pool areas and supply outflow channels and should be shaded with trees.
This will require modification of berms for permanent ponds and the
relaxation of restrictions on planting vegetation within the facilities,
provided that capacity for volumes and rate control is maintained.
At a minimum, the southern half on pond shorelines shall be planted
with shade or canopy trees within 10 feet of the pond shoreline. In
conjunction with this requirement, the maximum slope allowed on the
berm area to be planted is 10 to one. A long-term maintenance schedule
and management plan for the thermal control BMP's is to be established
and recorded for all development sites.
[Amended 7-14-2014 by Ord. No. 2014-04]
C.
Standards for managing runoff are shown below. Development sites
located in the Borough of Chambersburg must control postdevelopment
runoff rates to predevelopment runoff rates for the design storms
as follows:
|
Design Storm Postdevelopment
|
Design Storm Predevelopment
| |
|---|---|---|
|
Two-year
|
One-year
| |
|
Five-year
|
Five-year
| |
|
Ten-year
|
Ten-year
| |
|
Twenty-five-year
|
Twenty-five-year
| |
|
Fifty-year
|
Fifty-year
| |
|
One-hundred-year
|
One-hundred-year
|
A.
General. Postdevelopment peak rates of runoff from
any regulated activity shall meet the peak release rates of runoff
prior to development for the design storms specified on the Stormwater
Management District Watershed Map (Appendix D)[1] and § 251-11, of the Ordinance.
[1]
Editor's Note: Appendix D is on file in the
Borough offices.
B.
Off-site areas. Off-site areas that drain through
a proposed development site are not subject to release rate criteria
when determining allowable peak runoff rates. However, on-site drainage
facilities shall be designed to safely convey off-site flows through
the development site.
C.
Site areas. Where the site area to be impacted by
a proposed development activity differs significantly from the total
site area, only the proposed impact area utilizing stormwater management
measures shall be subject to the management district criteria. In
other words, unimpacted areas bypassing the stormwater management
facilities would not be subject to the management district criteria.
D.
No Harm option. For any proposed development site not located in a provisional direct discharge district, the developer has the option of using a less restrictive runoff control (including no detention) if the developer can prove that no harm would be caused by discharging at a higher runoff rate than that specified by the plan. The no harm option is used when a developer can prove that the postdevelopment hydrographs can match predevelopment hydrographs, or if it can be proved that the postdevelopment conditions will not cause increases in peaks at all points downstream. Proof of no harm would have to be shown based upon the following downstream impact evaluation which shall include a downstream hydraulic capacity analysis consistent with § 251-12G to determine if adequate hydraulic capacity exists. The land developer shall submit to the municipality this evaluation of the impacts due to increased downstream stormwater flows in the watershed.
(1)
The downstream impact evaluation shall include hydrologic
and hydraulic calculations necessary to determine the impact of hydrograph
timing modifications due to the proposed development upon a dam, highway,
structure, natural point of restricted streamflow or any stream channel
section, established with the concurrence of the municipality.
(2)
The evaluation shall continue downstream until the
increase in flow diminishes due to additional flow from tributaries
and/or stream attenuation.
(3)
The peak flow values to be used for downstream areas
for the design return period storms (two-, five-, ten-, twenty-five-,
fifty- and one-hundred-year) shall be the values from the calibrated
model for the Conococheague Creek Watershed. These flow values can
be obtained from the watershed plan.
(4)
Developer-proposed runoff controls which would generate increased peak flow rates at storm drainage problem areas would, by definition, be precluded from successful attempts to prove no-harm, except in conjunction with proposed capacity improvements for the problem areas consistent with § 251-12F.
(5)
A financial distress shall not constitute grounds
for granting a no harm exemption.
(6)
Capacity improvements may be provided as necessary
to implement the no harm option which proposes specific capacity improvements
to provide that a less stringent discharge control would not create
any harm downstream.
E.
Downstream hydraulic capacity analysis. Any downstream
capacity hydraulic analysis conducted in accordance with this chapter
shall use the following criteria for determining adequacy for accepting
increased peak flow rates:
(1)
Natural or man-made channels or swales must be able
to convey the increased runoff associated with a two-year return period
event within their banks at velocities consistent with protection
of the channels from erosion. Acceptable velocities shall be based
upon criteria included in the DEP Erosion and Sediment Pollution Control
Program Manual.
(2)
Natural or man-made channels or swales must be able
to convey increased twenty-five-year return period runoff without
creating any hazard to persons or property.
(3)
Culverts, bridges, storm sewers or any other facilities
which must pass or convey flows from the tributary area must be designed
in accordance with DEP Chapter 105 regulations (if applicable) and,
at minimum, pass the increased twenty-five-year return period runoff.
F.
Regional detention alternatives. For certain areas
within the study area, it may be more cost effective to provide one
control facility for more than one development site than to provide
an individual control facility for each development site. The initiative
and funding for any regional runoff control alternatives are the responsibility
of prospective developers. This initiative and funding requirement
can be satisfied through written agreement with the Borough of Chambersburg
("Borough") or the Chambersburg Area Municipal Authority ("CAMA")
for the provision of shared detention alternatives in exchange for
the payment of user rates. The design of any regional control basins
must incorporate reasonable development of the entire upstream watershed.
The peak outflow of a regional basin would be determined on a case-by-case
basis using the hydrologic model of the watershed consistent with
protection of the downstream watershed areas. "Hydrologic model" refers
to the calibrated model as developed for the stormwater management
plan.
[Amended 11-14-2022 by Ord. No. 2022-13]
G.
Hardship option. The development of the plan and its standards and criteria was designed to maintain existing peak flows throughout the Conococheague Creek Watershed as the watershed becomes developed. There may be certain instances, however, where the standards and criteria established are too restrictive for a particular landowner or developer. The existing drainage network in some areas may be capable of safely transporting slight increases in flows without causing a problem or increasing flows elsewhere. If a developer or homeowner may not be able to possibly meet the stormwater standards due to lot conditions or if conformance would become a hardship to an owner, the hardship option may be applied. The landowner would have to plead his/her case to the Borough with the final determination made by the Borough in accord with the provisions of Article I, § 251-8 of this chapter. Any landowners pleading the hardship option will assume all liabilities that may arise due to exercising this option.
A.
Any stormwater facility located on state highway rights-of-way
shall be subject to approval by the Pennsylvania Department of Transportation
(PENNDOT).
B.
Any stormwater management facility (i.e., detention
basin) designed to store runoff and requiring a berm or earthen embankment
required or regulated by this chapter shall be designed to provide
an emergency spillway to handle flow up to and including the one-hundred-year
postdevelopment conditions. The height of embankment must be set as
to provide a minimum 1.0 foot of freeboard above the maximum pool
elevation computed when the facility functions for the one-hundred-year
postdevelopment inflow. Should any stormwater management facility
require a dam safety permit under PaDEP Chapter 105, the facility
shall be designed in accordance with Chapter 105 and meet the regulations
of Chapter 105 concerning dam safety which may be required to pass
storms larger than one-hundred-year event.
C.
Any facilities that constitute water obstructions
(e.g., culverts, bridges, outfalls or stream enclosures), and any
work involving wetlands as directed in PaDEP Chapter 105 regulations
(as amended or replaced from time to time by PaDEP), shall be designed
in accordance with Chapter 105 and will require a permit from PaDEP.
Any other drainage conveyance facility that does not fall under Chapter
105 regulations must be able to convey, without damage to the drainage
structure or roadway, runoff from the twenty-five-year design storm
with a minimum 1.0 foot of freeboard measured below the lowest point
along the top of the roadway. Roadway crossings located within designated
floodplain areas must be able to convey runoff from a one-hundred-year
design storm with a minimum 1.0 foot of freeboard measured below the
lowest point along the top of roadway. Any facility that constitutes
a dam as defined in PaDEP chapter 105 regulations may require a permit
under dam safety regulations. Any facility located within a PENNDOT
right of way must meet PENNDOT minimum design standards and permit
submission requirements.
D.
Any drainage conveyance facility and/or channel that
does not fall under Chapter 105 regulations must be able to convey,
without damage to the drainage structure or roadway, runoff from the
ten-year design storm. Conveyance facilities to or exiting from stormwater
management facilities (i.e., detention basins) shall be designed to
convey the design flow to or from that structure. Roadway crossings
located within designated floodplain areas must be able to convey
runoff from a one-hundred-year design storm. Any facility located
within a PENNDOT right-of-way must meet PENNDOT minimum design standards
and permit submission requirements.
E.
Storm sewers must be able to convey postdevelopment
runoff from a twenty-five-year design storm without surcharging inlets,
where appropriate.
F.
Adequate erosion protection shall be provided along
all open channels and at all points of discharge.
G.
The design of all stormwater management facilities
shall incorporate sound engineering principles and practices. The
Borough shall reserve the right to disapprove any design that would
result in the occupancy or continuation of an adverse hydrologic or
hydraulic condition within the watershed.
Stormwater runoff from all development sites
shall be calculated using either the rational method or a soil-cover
complex methodology.
A.
Any stormwater runoff calculations shall use generally
accepted calculation technique that is based on the NRCS soil-cover
complex method. Table 305-1 summarizes acceptable computation methods.[1] It is assumed that all methods will be selected by the
design professional based on the individual limitations and suitability
of each method for a particular site. The municipality may allow the
use of the rational method to estimate peak discharges from drainage
areas that contain less than 200 acres. The soil complex method is
recommended for drainage areas greater than 200 acres.
[1]
Editor's Note: Table 305-1 is included at the end of this chapter.
B.
All calculations consistent with this chapter using
the soil-cover complex method shall use the appropriate design rainfall
depths for the various return period storms according to the region
for which they are located as presented in Table B-1 in Appendix B
of this chapter.[2] If a hydrologic computer model such as HEC-HMS or HEC-1
is used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The alternating block method to develop design
hyetographs described in Figure B-1, Appendix B of this chapter shall
be used for the rainfall distribution.[3]
[2]
Editor's Note: Table B-1 is included at the end of this chapter.
[3]
Editor's Note: Figure B-1 is included at the end of this chapter.
C.
For the purposes of predevelopment flow rate determination,
undeveloped land shall be considered as "meadow" in good condition,
unless the natural ground cover generates a lower curve number or
rational "C" value (i.e., forest), as listed in Table B-2 or B-3 in
Appendix B of this document.[4]
[4]
Editor's Note: Tables B-2 and B-3 are included
at the end of this chapter.
D.
All calculations using the rational method shall use rainfall intensities
consistent with appropriate times of concentration for overland flow
and return periods from the design storm curves from Pennsylvania
Department of Transportation Design Rainfall Curves (1986) (Figures
B-3 to B-4). Times of concentration for overland flow shall be calculated
using the methodology presented in Chapter 3 of Urban Hydrology for
Small Watersheds, NRCS, TR-55 (as amended or replaced from time to
time by NRCS). Times of concentration for channel and pipe flow shall
be computed using Manning's equation. Overland flow path lengths may
not exceed 100 linear feet.
[Amended 7-14-2014 by Ord. No. 2014-04]
E.
Runoff curve numbers (CN) for both existing and proposed
conditions to be used in the soil-cover complex method shall be obtained
from Table B-2 in Appendix B of this chapter.
F.
Runoff coefficients (c) for both existing and proposed
conditions for use in the rational method shall be obtained from Table
B-3 in Appendix B of this chapter.
G.
Where uniform flow is anticipated, the Manning equation
shall be used for hydraulic computations and to determine the capacity
of open channels, pipes and storm sewers. Values for Manning's roughness
coefficient (n) shall be consistent with Table B-4 in Appendix B of
the Ordinance.[5] Outlet structures for stormwater management facilities
shall be designed to meet the performance standards of this chapter
using any generally accepted hydraulic analysis technique or method.
[5]
Editor's Note: Table B-4 is included at the end of this chapter.
H.
The design of any stormwater detention facilities
intended to meet the performance standards of this chapter shall be
verified by routing the design storm hydrograph through these facilities
using the storage-indication method. For drainage areas greater than
200 acres in size, the design storm hydrograph shall be computed using
a calculation method that produces a full hydrograph. The Borough
may approve the use of any generally accepted full hydrograph approximation
technique that shall use a total runoff volume that is consistent
with the volume from a method that produces a full hydrograph.
|
TABLE 305-1
| |||
|---|---|---|---|
|
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
|
United States 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 Borough and/or Borough Engineer
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by
the Borough and/or Borough Engineer
| |
A.
Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, Erosion Control, and in accordance with the Franklin County Conservation District.
B.
Additional erosion and sedimentation control design
standards and criteria that must be or are recommended to be applied
where infiltration BMPs are proposed shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected
from sedimentation and compaction during the construction phase so
as to maintain their maximum infiltration capacity.
(2)
Infiltration BMPs shall not be constructed nor receive
runoff until the entire contributory drainage area to the infiltration
BMP has received final stabilization.
A.
The ability to retain and maximize the groundwater
recharge capacity of the area being developed is encouraged strongly.
Design of the infiltration/recharge stormwater management facilities
shall give consideration to providing groundwater recharge to compensate
for the reduction in the percolation that occurs when the ground surface
is paved and roofed over. These measures are encouraged, particularly
in hydrologic soil groups A and B and should be utilized wherever
feasible. Soils used for the construction of basins shall have low-erodibility
factors ("K" factors).
B.
Infiltration BMPs shall meet the following minimum
requirements:
(1)
Infiltration BMPs intended to receive runoff from
developed areas shall be selected based on suitability of soils and
site conditions and shall be constructed on soils that have the following
characteristics:
(a)
A minimum depth of 24 inches between the bottom of the facility
and the seasonal high water table and/or bedrock (limiting zones)
is required for areas of non-hot spot runoff (parking lots, access
roads, truck bays, and similar). In areas of hot spot runoff, a minimum
depth of 48 inches between the bottom of the facility and the seasonal
high water table and/or bedrock (limiting zones) is required.
[Amended 7-14-2014 by Ord. No. 2014-04]
(b)
An infiltration and/or percolation rate sufficient
to accept the additional stormwater load and drain completely as determined
by field tests conducted by the owner's professional designer or as
otherwise approved by the Municipal Engineer.
(c)
Infiltration BMPs receiving only roof runoff
may be placed in soils having a minimum depth of 24 inches between
the bottom of the facility and the limiting zone.
(d)
The size of the recharge facility shall be based
upon the following equation (DEP):
|
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 Borough engineer.
| ||||
|
Hydrologic Soil Group
|
Soil Specific Recharge Factor (S)
(inches)
| |
|---|---|---|
|
A
|
0.38 inches
| |
|
B
|
0.25 inches
| |
|
C
|
0.13 inches
| |
|
D
|
0.06 inches
|
(2)
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.
(a)
The recharge volume provided at the site shall
be directed to the most permeable HSG available, if feasible.
(b)
The recharge facility shall be capable of completely infiltrating
the recharge volume within three days (72 hours).
[Amended 7-14-2014 by Ord. No. 2014-04]
(c)
Infiltration areas should be located a minimum
of 10 feet from a building foundation.
C.
A detailed soils evaluation of the project site may
be required to determine the suitability of recharge facilities. The
evaluation shall be performed by a qualified design professional,
and at a minimum, address soil permeability, depth to bedrock, susceptibility
to sinkhole formation and subgrade stability. The general process
for designing the infiltration BMP shall be:
(1)
Analyze hydrologic soil groups as well as natural
and man-made features within the watershed to determine general areas
of suitability for infiltration practices.
(2)
Provide field test to determine appropriate percolation
rate and/or hydraulic conductivity.
(3)
Design infiltration structure for required storm volume
based on field determined capacity at the level of the proposed infiltration
surface.
D.
Extreme caution shall be exercised where infiltration
is proposed in geologically susceptible areas such as strip mine or
limestone areas. Extreme caution shall also be exercised where salt
or chloride would be a pollutant since soils do little to filter this
pollutant and it may contaminate the groundwater. It is also extremely
important that the qualified design professional evaluate the possibility
of groundwater contamination from the proposed infiltration/recharge
facility and recommend a hydrogeologic justification study be performed
if necessary. Whenever a basin will be located in an area underlain
by limestone, a geological evaluation of the proposed location shall
be conducted to determine susceptibility to sinkhole formations. The
design of all facilities over limestone formations shall include measures
to prevent groundwater contamination and, where necessary, sinkhole
formation. The infiltration requirement in the high-quality/exceptional
waters shall be subject to the Department's Chapter 93 and Antidegradation
Regulations. The Borough may require the installation of an impermeable
liner in detention basins. A detailed hydrogeologic investigation
may be required by the municipality. It shall be the developer's responsibility
to verify if the site is underlain by limestone. The following note
shall be attached to all drainage plans and signed and sealed by the
developers engineer/surveyor/geologist:
|
", certify that the proposed detention
basin (circle one) is/is not underlain by limestone."
|
E.
The Borough may require the developer to provide safeguards
against groundwater contamination for uses which may cause groundwater
contamination should there be a mishap or spill.
F.
Where pervious pavement is permitted for parking lots,
recreational facilities, nondedicated streets or other areas, pavement
construction specifications and maintenance schedules shall be noted
on the plan.
G.
Recharge/infiltration facilities may be used in conjunction
with other innovative or traditional BMPs, stormwater control facilities
and nonstructural stormwater management alternatives.
A.
In addition to the performance standards and design criteria requirements of Article III of this chapter, the land developer shall comply with the following water quality requirements of this article unless otherwise exempted by provisions of this chapter.
(1)
Developed areas will provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The recharge volume computed under § 251-16 may be a component of the water quality volume. If the recharge volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than recharge/infiltration BMP's.
(2)
The water quality volume (WQv) is the storage capacity needed to treat stormwater runoff produced
by "P" inch of rainfall (90% rule) from the developed areas of the
site (For "P" values, see Appendix B, Table B-5).[1] The following calculation formula is used to determine
the storage volume, WQv, in acre-feet of storage:
|
WQv
|
=
|
[(P)(Rv)(A)]/12
| |
|
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
|
[1]
Editor's Note: Table B-5 is included at the end of this chapter.
B.
WQv shall be designed as part of a stormwater management
facility which incorporates water quality BMP's as a primary benefit
of using that facility, in accordance with design specifications contained
in Pennsylvania Handbook of Best Management Practices for Developing
Areas. The following factors shall be considered when evaluating the
suitability of BMPs used to control water quality at a given development
site:
C.
The following additional factors should be considered
when evaluating the suitability of BMPs used to control water quality
at a given development site:
(1)
Peak discharge and required volume control.
(2)
Streambank erosion.
(3)
Efficiency of the BMPs to mitigate potential water
quality problems.
(4)
The volume of runoff that will be effectively treated.
(5)
The nature of the pollutant being removed.
(6)
Maintenance requirements.
(7)
Creation/protection of aquatic and wildlife habitat.
(8)
Recreational value.
(9)
Enhancement of aesthetic and property value.
Applying the water quality criteria in § 251-17 above will also help the streambank erosion problem, detain the two-year postdevelopment storm to the one-year predevelopment storm and detaining the one-year postdevelopment storm a minimum of 24 hours.
