On all properties of one acre or more which are proposed for subdivision or land development under the terms of Title Seven or any development which results in 3,000 square feet or more of new impervious surface, on-site storm water detention shall be required where post-development discharges exceed pre-development discharges and where detrimental effects may be caused downstream or on adjacent properties. All plans and computations required to evaluate the need for on-site storm water retention shall be prepared by a registered professional engineer with expertise in storm water management and shall comply with the following requirements:
(a) Design Criteria. The procedure for determining projected run-off from the site shall be as follows:
(1) Compute the pre-development discharges for the two, ten and one hundred year storms using the U.S. Soil Conservation Service (SCS) Technical Release No. 55 (TR-55), January, 1975. The computations should assume actual existing soil and land use conditions on the site, using the watershed existing land use map and the SCS Soils Classification Map for the watershed. A site plan showing the hydrological soil group and land uses, as described in Table 2.2 of TR-55, should be included in the applicant's submission.
(2) Compute the post-development discharges for the two, ten and one hundred year storms using TR-55. The computations should include all reductions proposed for on-site storm water management techniques.
(3) Compare post-development discharges to pre-development discharges. If post-development discharges are greater than pre-development discharges, on-site retention is required. The size of the retention facility shall be determined for the excess discharges for the two, ten and one hundred year design storms so that none of the three design storm flows exceed the pre-development storm flows.
(b) Off-Site Storage. Off-site storage may be substituted for required on-site retention, provided that:
(1) Proper legal arrangements are executed;
(2) The off-site facility is approved by the Engineer; and
(3) No problems are created between the development site and the off-site storage facility.
(c) No Harm Option. Where the applicant proposes to increase the pre-development maximum rate of runoff for the site, then the applicant is required to demonstrate that the proposed storm water management system will not result in any harm or adverse effects to downstream areas.
All plans and computations must be prepared and certified by a registered professional engineer with expertise in storm water management. The procedure for utilizing the no-harm option, and the information to be included, is as follows:
(1) Determine discharge control values at all "points of interest" between the proposed development site and the base of the watershed.
(2) Using the SCS TR-55 hydrograph method and the existing watershed land use and soils maps that may be obtained from Allegheny County, compute the two, ten and one hundred year storm pre-development discharge values at the point of discharge from the site and at all points of interest between the site and the base of watershed. The Engineer may identify additional points of interest where there are existing obstructions or known problems, or other points may be identified during the course of the plan review process or applicant's own engineering evaluations.
(3) Compare the pre-development discharge values computed by the applicant to the discharge values shown in available watershed studies or independent studies. If the applicant's values are plus or minus twenty percent (20%) of the comparison values, the applicant's values are acceptable and may be used for all further evaluations and comparisons. This computation establishes, for the applicant, the discharge values downstream that must not be exceeded after development.
(4) Using the SCS TR-55 method, compute the post-development discharges for the two, ten and one hundred year storms for the same locations used above. The calculations of the post-development discharges should include the proposed on and off-site storm water management controls proposed by the applicant.
When this analysis indicates that downstream improvements are feasible, the developer may make downstream improvements in lieu of providing on-site detention with the approval of Council, affected landowners and the Department of Environmental Resources (DER), if applicable.
(5) When the computed post-development discharges for the two, ten, and one hundred year storms at all points of interest do not exceed the computed pre-development discharges at the same points, then the applicant shall have demonstrated, within reasonable limits, that no harm or adverse effects will occur downstream.
(Ord. 2748. Passed 11-17-86.)