New development increases impervious surfaces, thereby altering the natural drainage features, increasing peak discharge rates and volumes and reducing groundwater recharge which maintains wetlands and baseflow in streams. Development also increases the concentrations and types of pollutants such as nutrients, solids, metals, salts, pathogens, pesticides and hydrocarbons.
Emergency repairs to existing roads, including repaving, and its drainage system within the existing layout of the roadway, are exempt from these standards.
The following is required for all work and projects involving land which is one acre or more, and for all projects which include roadways, subdivisions for single family homes, parking plans, industrial and commercial development.
A. 
All projects must be designed and constructed to meet the volume and rate of the flow of water from the 100-year frequency storm event
B. 
No new stormwater outfalls may discharge untreated stormwater directly into or cause erosion in, wetlands or waters of the Commonwealth.
C. 
Systems must be designed so that the post-development peak discharge rates do not exceed the pre-development peak discharge rates.
D. 
Loss of recharge to groundwater should be minimized through use of infiltration measures to the maximum extent practicable, approximating the predevelopment recharge.
E. 
For new development, systems must remove 80% of the average annual post-development load of Total Suspended Solids (TSS). Using Best Management Practices (BMPS) listed in DEP's Stormwater Management Technical handbook creates a rebuttable presumption that the 80% standard is met.
F. 
Erosion and sedimentation controls must be used during construction and work on the land.
G. 
Any analysis must be accompanied by a plan or plans of the total tributary area indicating catchment and sub-catchment boundaries, hydrologic soils classifications and cover types for both pre- and post-development conditions. If off-site areas are included in the analysis and detailed topography is not available, they may be indicated with site boundaries on a copy of the appropriate USGS quadrangle sheet.
H. 
Primary catchment delineation should be based on specific point or area of discharge from the area of analysis.
I. 
Although catchment and sub-catchment boundaries within the area of analysis may be realigned due to development, the total area and perimeter of the analysis must be the same for pre- and post-development. Any modifications of catchment boundaries which is to be accomplished by regrading in order to satisfy the attenuation requirement must be supported by proposed grading plans or other appropriate documentation.
J. 
The determination of catchment time of concentration must include an accurate estimate and evaluation of the various forms of sheet flow, shallow concentrated and open channel. Assumptions of sheet flow distances in excess of 300 feet should be substantiated.
K. 
Natural or man-made ponding areas along the flow route can substantially impact peak flow rates and lag time. A culvert or bridge can delay flow and reduce peak rates if there is a significant storage area available behind it. In such cases detailed storage routing procedures should be used.
L. 
Storm drainage conduits are not always installed coincident with surface flow paths. In some cases conduits may actually cross surface drainage divides. Drain inlets and conduits typically lack sufficient hydraulic capacity to handle major rainfall events. Therefore, the drainage analyses and resultant stormwater management system design must account for all runoff in excess of storm drain capacity.
M. 
The determination of storm runoff by any methodology cannot be considered exact. The hydraulic capacity of any system component, however accurately determined, may be compromised by blockage. Therefore, stormwater storage facilities proposed for the attenuation of peak flow rates should incorporate adequate freeboard and/or overflow capacity to minimize risk of failure or damage in the event that outlet capacity is reduced or if actual inflow exceeds design flows.
N. 
The Commission requires that the design of attenuation facilities take into account the potential development of the entire tributary watershed including off site areas. Even though facilities may be provided to attenuate the rate of discharge from upslope development, it will still cause an increase in the volume of runoff.
O. 
Since some runoff can be expected from all but the most permeable soils during significant rainfall, storage basin discharge due to infiltration during the storm normally cannot be substantiated. If such infiltration losses are to be assumed in design, they must be supported by appropriate data including existing and proposed soil profile and permeability evaluation of the involved surface and subsoils.
P. 
In order to facilitate the review, all calculations, hydrographs, details and similar supporting data should be included with the submittal. The submittal should also include a tabular comparison of pre- and post-development catchment areas, cover complex numbers (CN), time of concentration (TC) and maximum runoff rates.
Q. 
All supporting data, calculations and plans should prove that the design would accommodate the unimpeded and unobstructed flow of water during the 100-year frequency storm. This information should also include two cross-sections of the streambed, i.e. 20 feet each side (upstream and downstream). If the cross sections are the same, a typical cross section only may be submitted.
R. 
All stormwater systems must include information on operation and maintenance.