A.
Preamble. The Department
recognizes that same developments cause unreasonable increases in stormwater
runoff by decreasing the infiltrative capacity of the soils on a development
site. The Department also recognizes that increases in stormwater runoff cause
increased danger of flooding, the pollution of surface water bodies, and the
depletion of groundwater resources.
B.
Scope of Review. In
determining whether the proposed development will have an unreasonable effect
on runoff/infiltration relationships, the Department shall consider all
relevant evidence to that effect, such as evidence that:
(1) A stormwater management system will
infiltrate, detain, or retain water falling on the site during a storm of an
intensity equal to a twenty-five year, twenty-four hour storm such that the
rate of flow of stormwater from the development does not exceed the rate of
outflow of stormwater from the site prior to the undertaking of the
development.
(a) Developments which convey
stormwater directly into the ocean (excluding estuarine tidewaters) exclusively
in manmade piped or open drainage systems are exempt from the requirements of
this subsection.
(2) The
physical, biological, and chemical properties of the receiving waters will not
be unreasonably degraded by the stormwater runoff from the development
site.
(3) The peak discharge of the
receiving waters will not be increased as the result of the stormwater runoff
from the development site for storms up to a level of intensity of a
twenty-five year, twenty-four hour storm.
C.
Submissions. Applications for
approval of proposed developments shall include evidence that affirmatively
demonstrates that there will be no unreasonable effect on runoff/infiltration
relationships, including information such as the following, when appropriate:
(1) Evidence that the proposed stormwater
management system has been designed by a professional engineer or other person
duly qualified to undertake the design. The designer of the system will
evaluate the effectiveness of various stormwater methods and develop and make
available for review the hydraulic computations based on accepted engineering
practices to demonstrate that the standards established under subsection B,
above, will be met.
(2) Evidence
that the stormwater management system will take into consideration the upstream
runoff which must pass over or through the development site. The system will be
designed to pass upstream flows generated by a twenty-five year frequency
through the proposed development without overloading the system or flooding
areas not specifically planned for such flooding.
(3) Evidence that the design of piped or open
channel systems will be based on a ten year flow frequency without overloading
or flooding beyond channel limits. In addition, the areas expected to be
flooded by runoff of a twenty-five year frequency will be designated, and no
structures will be planned within such area.
(4) Evidence that, where permanent
embankment-type storage or retention basins are planned, the basins will be
designed in accordance with good engineering practice, such as outlined in the
Soil Conservation Service Engineering Field Manual or other appropriate
references.
(5) Evidence that
rights-of-way or easements will be designated for all components of the
stormwater management system lying outside of established street
lines.
(6) Evidence that the
developer will maintain all components of the stormwater management system
until it is formally accepted by the municipality or a quasi-municipal
district, or is placed under the jurisdiction of a legally created association
that will be responsible for the maintenance of the system. The charter of such
an association must be acceptable to the Department.
(7) Evidence that the stormwater management
system will be fully coordinated with project site plans, including
consideration of street patterns, pedestrian ways, open space, building siting,
parking areas, recreational facilities, and other utilities, especially
sanitary wastewater disposal facilities.
(8) When the construction of a development is
to occur in phases, the planning of the stormwater management system should
encompass the entire site which may ultimately be developed, and not limited to
an initial or limited phases of the development.
NOTE: The following references may be of
assistance to a developer in making the necessary computations and in designing
the stormwater management system:
"Urban Hydrology for Small Watersheds", Technical Release
No. 55, USDA, Soil Conservation Service, University of Maine, Orono,
Maine.
"Water Resources Protection Measures in Land Development - A
Handbook", Tourbier and West macott, University of Delaware Water Resources
Center, Newark, Delaware.
D.
Terms and Conditions. The
Department may, as a term or condition of approval, establish any reasonable
requirement to ensure that there will be no unreasonable effect on
runoff/infiltration relationships.
