A. 
The purpose of the stormwater management requirements is to protect downstream structures and property from flooding resulting from increased stormwater runoff created by new development.
B. 
The applicant shall demonstrate by appropriate computations that postdevelopment runoff discharges do not exceed existing site runoff discharges for a five-year, ten-year and twenty-five-year precipitation event of twenty-four-hour duration. Use the following rainfall for the specific precipitation event.
Precipitation Event
Rainfall (P)
(inches)
5 year, 24 hour
3.4
10 year, 24 hour
4.2
25 year, 24 hour
4.8
A. 
Perform stormwater management computations by the method specified for the specific site size.
Site Size
(acres)
Computation Method
Less than 5
Rational
5 and larger
NRCS Method
B. 
Use attached methodology to perform stormwater management computations.
Stormwater management requirements are waived when the computed increase in runoff for a twenty-five-year, twenty-four-hour event is less than 0.200 CFS. Provide calculations documenting runoff increase.
A. 
Design structure to safely store runoff in excess of existing runoff resulting from the proposed development based on the rainfall from a twenty-five-year precipitation event of twenty-four-hour duration.
B. 
Applicants may consider the stage discharge of water from the structure in determining required storage. This can be accomplished by acceptable storm hydrograph routing procedures.
C. 
Store no water on a roof. Store no more than five inches of water on parking areas. Store no more than 10 feet of water in stormwater basins measured from the pond bottom to the crest of the emergency spillway. Storage must be open and accessible for visual inspection. Fence required with two feet or more of water storage depth. Fence shall be four feet height minimum.
A. 
Compute discharge rates for existing conditions and postdevelopment conditions for five-year, ten-year and twenty-five-year precipitation events with a duration of 24 hours. Design discharge for postdevelopment conditions not to exceed existing conditions discharge for the same year precipitation event. Design discharges to safely pass existing downstream drainage structures or provide for the upgrade of the structure at the applicant's expense.
B. 
A multistaged discharge structure may be required to comply with this criteria. Minimum discharge conduit size is 1.25 inches in diameter or 1.23 square inches. Provide positive flow with no mechanical assistance (pump, syphon, etc.) for discharge facilities. Provide complete dewatering of the storage facility. Slope bottom of basins to outlet structure at a 1% minimum slope to assist dewatering of storage facility.
Equip ponds with emergency spillway. Design emergency spillway to pass 100% of the one-hundred-year, twenty-four-hour storm design flow plus one foot of freeboard. Use 5.7 inches of rainfall (P) for the one-hundred-year, twenty-four-hour precipitation event. Provide erosion protection compatible with computed discharge velocities. Spillways must be constructed of concrete or gabion baskets. Barrel riser is not considered a spillway.
A. 
Provide earth detention facility basin with cut or fill slopes no steeper than two horizontal to one vertical. The slope requirement may be waived when steeper slopes are documented to be stable by a geotechnical analysis. Place all fill in compacted lifts. Where fill height exceeds five feet above existing ground, perform compaction testing during construction and provide the Borough of Westmont with documentation of fill compaction to a density of 95% of the Standard Proctor (ASTM D698).
B. 
Provide vegetation cover for all earth detention facilities.
A. 
Maintenance of the stormwater management facility is the responsibility of the owner. Maintain the structure to be fully functional at all times. Remove sediment when 10% of pond water storage volume is lost. Vegetation within the facility shall be maintained and mowed so as to prevent vegetation from exceeding 10 inches in height.
B. 
The Borough of Westmont maintains the right to access and inspect stormwater facilities. Elimination, alteration or improper maintenance will result in a notice served to the owner by the Borough of Westmont. Should the owner not comply with the notice, the Borough of Westmont may cause the work to be done and lien all costs against the property.
A. 
Ownership: provide detention facility and access roadway property fee simple to the Borough of Westmont.
B. 
Access: provide all-weather roadway to facility and provision for construction equipment access to bottom of basins.
C. 
Fence: protect open basins with a perimeter fence, acceptable to the Borough, of durable maintenance-free materials with locking vehicle gate (twelve-foot width).
D. 
Vegetation: seed basin bottom, slopes and embankment crest with an appropriate lawn seed mix that permits mowing.