There shall be no increase in the rate of storm water discharge from any activity covered by this Title Six than would have occurred from the land prior to the activity.
(a) Method of Computation. Peak discharge and runoff shall be computed using the soil-cover complex method as set forth in the latest edition of "Urban Hydrology for Small Watersheds", Technical Release No. 55 as published by SCS, or by any other method approved by the York City Engineer.
(b) Rainfall Data. For the purposes of this Title Six, the following rainfall depths shall be used for design:
Recurrence Interval (Years) | 24-Hour Rainfall Depth (Inches) |
2 | 3.1 |
5 | 4.1 |
10 | 4.9 |
25 | 5.5 |
50 | 6.2 |
(c) Design Storms.
(1) All storm water retention/detention facilities shall be designed on the basis of providing adequate control for all storm frequencies of a 24-hour duration up to and including a 50-year design storm.
A. Outflow (Qo) determination. The maximum permitted storm water discharge (Qo) in cubic feet per second, from any site shall not exceed the calculated discharge from the site at pre-development ground cover and soil conditions for all design storms specified below. The maximum permitted storm water discharge (Qo) shall be calculated using the SCS method for twenty-four hour rainfalls having recurrence intervals of 2, 5, 10, 25 and 50 years.
B. Existing runoff volume (VR1). Existing runoff volume (VR1), in inches, shall be determined using the SCS method at pre- development conditions for the 50-year rainfall depth listed in subsection (b) hereof.
C. Future runoff volume (VR2). The future runoff volume (VR2), in inches, shall be determined at post-development conditions (including any future expansion) for the 50-year rainfall depth listed in subsection (b) hereof.
D. Minimum required detention storage (VS). The minimum required detention storage (VS), in cubic feet, shall be determined from the following formula:
VS (cubic feet) = (VR2 - VR1) x (site area in acres) x 4175
(2) Storm sewer systems within a development shall be designed to accommodate the peak rate of runoff from a 10-year design storm.
(3) Where Federal or State laws impose a greater design frequency, they shall prevail.
(4) Emergency spillways or overflow structures shall be designed to release a 100 year design storm computed at post-development conditions.
(5) For land development in the Willis Run watershed, subsection (c)(l) hereof is superseded in that peak discharge, runoff and storage requirements shall be determined as follows:
A. Outflow (Qo) determination. The maximum permitted storm water discharge (Qo), in cubic feet per second, from any site shall not exceed the calculated discharge from the site at pre-development ground cover and soil conditions. The maximum permitted storm water discharge (Qo) shall be calculated using the Soil Conservation Service method for a 3.74 inch rainfall per twenty-four hour period.
B. Existing runoff volume (VR1). Existing runoff volume (VR1) in inches, shall be determined using the Soil Conservation Service method at pre-development conditions for a 3.74 inch rainfall per twenty-four hour period.
C. Future runoff volume (VR2). The future runoff volume (VR2), in inches, shall be determined using the Soil Conservation Service method at post-development conditions (including any future expansion) for a 3.74 inch rainfall per twenty-four hour period.
D. Minimum required detention storage (VS1). The minimum required detention storage (VS1), in cubic feet, shall be determined from the following formula:
VS1 (cubic feet) = (VR2 - VR1) x (site area in acres) x 4175.
(d) Release Rate. The peak release of storm water from retention/detention facilities shall not be greater than the pre-development peak discharge rate. The City of York Engineer may require a reduced peak discharge rate should downstream conditions warrant such action.
(e) Maintenance of Natural Drainageways. All natural streams, channels, swales, drainage systems and/or areas of surface water concentration shall be maintained in their existing condition unless an alteration is approved by the City of York Engineer. All encroachment activities shall comply with the requirements of Chapter 105 (Water Obstructions and Encroachments) of Title 25, Rules and Regulations of the Pennsylvania Department of Environmental Resources.
(f) Methods of Storm Water Runoff Detention Control.
(1) The following is a listing of detention and control methods which may be utilized in storm water management systems, if appropriate.
A. Detention basins.
B. Retention basins.
C. Roof-top storage.
D. Parking lot and street ponding.
E. Seepage pits, seepage trenches or other infiltration structures.
F. Porous pavement and concrete lattice block surfaces.
G. Grassed channels and vegetated strips.
H. Cisterns and underground reservoirs.
I. Routing flow over grass.
J. Decreased impervious area coverage.
(2) The use of other control methods which meet the criteria in this article shall be permitted when approved by the City of York Engineer. Various combinations of methods shall be tailored to suit the particular requirements of the type of development and the topographic features of the project area. (Ord. 13-1981 §1. Passed 5-20-81.)