Any landowner and any person engaged in the alteration or development of land which may affect stormwater runoff characteristics shall implement such measures as are reasonably necessary to prevent injury to health, safety, or other property. Such measures shall include such actions as are required:
A. 
To assure that the maximum rate of stormwater runoff is no greater after development than prior to development activities; or
B. 
To manage the quantity, velocity, and direction of resulting stormwater runoff in a manner which otherwise adequately protects health and property from possible injury.
For purposes of stormwater management, each subarea of the Ridley Creek watershed is assigned a release rate percentage, as defined by this chapter and shown on the Ridley Creek Watershed Release Rate Map (Plate No. 1) available in the municipal offices. (This percentage is applicable to any particular site in that subarea.) The postdevelopment peak stormwater runoff rate discharging from the outfalls of a development site cannot exceed the subarea release rate percentage in order to comply with the Ridley Creek Watershed Plan. The following procedure should be followed in applying the release rate percentage.
A. 
Computations.
(1) 
Compute predevelopment and postdevelopment runoff hydrographs and peak discharges for the two-, ten-, twenty-five-, and one-hundred-year storms using the U.S. Soil Conservation Service's (SCS) Soil-Cover Complex Method. The twenty-four-hour total runoff depths for these return periods for the Ridley Creek watershed shall be:
Return Period
Depth
(inches)
2-year
2.92
10-year
4.68
25-year
5.54
100-year
6.85
(2) 
The computations should assume actual existing soil and land use conditions on the site as of the date of adoption of the watershed plan or its most recent update, using the existing land use map used in the development of the watershed plan (and subsequent updates) and the SCS Soil Classification Map for the watershed. The computations for postdevelopment discharges should include all reduction for proposed on-site infiltration techniques.
B. 
Compare postdevelopment discharges to the predevelopment discharges. If the postdevelopment rate is greater, on-site storage is required. Off-site storage may be substituted, provided that:
(1) 
Proper legal arrangements (easements, perpetual covenants, etc.) are made;
(2) 
No problems are created between the development site and the off-site storage location; and
(3) 
It is approved by the municipal engineer.
C. 
If on-site storage is required, the size of the facility(s) shall be determined by applying the release rate percentage to the postdevelopment discharges for the two-, ten-, and twenty-five-year design storms. Provisions shall also be made for safely passing the postdevelopment one-hundred-year runoff flows without damaging (i.e., impairing the continued function of) these systems. The storage area shall be designed in conformity with the provisions of this chapter.
D. 
The proposed plan and computations must be prepared by a registered professional engineer with expertise in stormwater management.
This section applies only in subareas that are immediately adjacent to Ridley Creek.
A. 
Development sites in these subareas may discharge total stormwater runoff flows through outfalls directly into Ridley Creek.
B. 
Stormwater outfalls must be constructed so as to prevent erosion and scour of the Ridley Creek channel. Under these conditions, postdevelopment peak runoff rates may exceed predevelopment peak runoff rates.
C. 
Stormwater management plans for sites proposing to use direct discharge must be approved by the engineer for the municipality in which the development is to take place.
If an applicant proposes to exceed the specified release rate percentage for a subarea, one of the following evaluations must be completed.
A. 
If the stormwater runoff flow from the development is proposed to be directed into an existing or proposed stormwater conveyance channel (i.e., closed storm sewers and concrete lined or rip-rap protected channels), the postdevelopment discharge may exceed the prescribed release rate percentage. The applicant must demonstrate sufficient capacity in the proposed conveyance channel, and the proposed system must be approved by the municipal engineer.
B. 
An applicant may demonstrate through acceptable engineering analysis and design that an increase in the postdevelopment discharge rate will not result in injury or damage to persons or property downstream from the development site.
(1) 
The evaluation of downstream impacts must show that at any point in time, the flow rates on the existing conditions runoff hydrograph at the outlet of the subarea(s) in which the development site is located shall not be increased by more than 5% for storm discharges resulting from future conditions runoff (with stormwater management provisions) from the two-, ten-, and twenty-five-year rainfall events for the particular site. Existing conditions runoff hydrographs for all applicable subareas shall be those used in the analyses performed for the development of the Ridley Creek Watershed Plan or its most recent update (available at the Delaware County Planning Department).
(2) 
The municipal engineer may identify points of interest downstream of the site where there are existing obstructions or known problems or other points, and the applicant shall be required to demonstrate no adverse impact as a result of exceeding the prescribed release rate percentage(s) for the subarea(s).
(3) 
All plans and computations must be prepared and certified by a registered professional engineer with expertise in stormwater management. These must be submitted to the municipal engineer for review and approval.