[Ord. 1-1975, 2/24/1975, § 400; as amended by Ord. 4-1986, 6/2/1986, § 2; and by Ord. 6-1992, 10/5/1992, § 2]
1. 
In addition to the requirements of § 9-302, and where deemed necessary by the Township Engineer and the Thornbury Township Planning Commission, or the Board of Supervisors, the applicant shall submit with the plan a detailed drainage study prepared by a registered professional engineer qualified in hydrology in the Commonwealth. This study shall include:
A. 
A plan of the property showing the location of all present and proposed ditches, streams, pipes and other drainage structures, and proposed cuts and/or fills. In addition to showing present elevations and dimensions, and location and extent of all proposed grading and/or drainage, the plan shall clearly indicate all woodlands, buildings, parking areas, and driveways. Further, the plan shall indicate the present and proposed sources, storage, and disposition of water being channeled through or across the premises, together with elevations, gradients, and maximum flow rates. The plan shall describe the work to be performed and disposition of cut and/or fill, the materials to be used and the manner or method of performance including provisions for protecting and maintaining existing drainage facilities whether on public or private property. The applicant shall also supply the supporting data for the plan as developed by the Engineer.
B. 
Calculations to determine runoff, which shall be based on the Soil-Cover Complex Method, a description of which is available from the USDA Soil Conservation Service and outlined in the "Erosion and Sediment Control Handbook, Delaware County, Pennsylvania, 1973," as amended.
(1) 
The design criteria for storm sewer piping on inlet systems within a subdivision being developed shall be designed for a twenty-five-year frequency storm; culverts across roadways shall be designed for a fifty-year frequency storm: open watercourses or swales shall be designed for the one-hundred-year frequency storm as prescribed in accordance with the following:
(a) 
The coefficient of runoff used for all areas upstream of any drainage structure shall be computed on the basis of existing land use and the projected land use described and shown on the Thornbury Township Comprehensive Plan, as amended, and adjacent municipalities' comprehensive plans, where applicable.
(2) 
The following provisions apply to the carrying and disposal of stormwater runoff:
(a) 
All drainage facilities shall be designed to carry surface water in such a manner as to prevent erosion or overflow.
(b) 
The applicant shall agree to the granting and recording of easements covering the installation and maintenance of drainage facilities.
(c) 
The rate of stormwater runoff shall be no greater during and after construction for two-, ten-, twenty-five-, fifty- and one-hundred-year frequency storms, when the development is completed, than that which existed before the development began; appropriate measurements or calculations shall be provided to verify such provisions.
C. 
A soils investigation report, if load-bearing fill is proposed, which shall consist of test borings, laboratory testings and engineering analysis to correlate surface and sub-surface conditions with the proposed grading plan. The results of the investigation shall be presented in a report by a registered professional soils engineer which shall include: data regarding the nature, distribution and supporting ability of existing soils and rocks on the site, conclusions and recommendations to insure stable soil conditions and ground water control as applicable. The Township of Thornbury may require such supplemental reports and data as is deemed necessary by the Township Engineer. Recommendations included in such reports and approved by the Township Engineer shall be incorporated in the plan or specifications. In addition:
(1) 
Fills toeing out on natural slopes steeper than four horizontal to one vertical shall not be made unless a report is received which is deemed acceptable by the Township Engineer and approved by the Board of Supervisors. The report shall be made by a registered professional soils engineer certifying that he has investigated the property, made soil tests and that in his opinion such steeper slopes will safely support the proposed fill.
(2) 
Natural and/or existing slopes exceeding five horizontal to one vertical shall be benched or continuously stepped into competent materials prior to placing all classes of fill.