The Board of Supervisors of the Township finds that:

A. Inadequate management of accelerated stormwater runoff resulting from development throughout a watershed increases flood flows and velocities, contributes to erosion and sedimentation, overtaxes the carrying capacity of existing streams and storm sewers, greatly increases the cost of public facilities to convey and manage stormwater, undermines floodplain management and flood reduction efforts in upstream and downstream communities, reduces groundwater recharge, and threatens public health and safety.

B. A comprehensive program of stormwater management, including reasonable regulation of development and activities causing accelerated erosion, is fundamental to the public health, safety, welfare, and the protection of the people of the Township and all of the people of the Commonwealth, their resources, and the environment.

(Ord. 4-2004, 6/2/2004; §101)

The purpose of this Chapter is to:

A. Promote health, safety, and welfare within the Township.

B. Implement the requirements of the Chester Creek and Ridley Creek Stormwater Management Plan.

C. Implement the requirements of the National Pollutant Discharge Elimination System Phase II (NPDES II); by minimizing the damages described in §19-101.A through provisions designed to:

(1) Manage accelerated runoff, erosion, and sedimentation problems at their source by regulating activities that cause these problems.

(2) Utilize and preserve the existing natural drainage systems.

(3) Encourage recharge of groundwater where appropriate and prevent degradation of groundwater quality.

(4) Maintain existing flows and quality of streams and watercourses in the Township and the Commonwealth.

(5) Preserve and restore the flood-carrying capacity of streams.

(6) Provide proper maintenance of all permanent stormwater management facilities that are constructed in the Township.

(7) Provide performance standards and design criteria for watershed-wide stormwater management and planning.

(Ord. 4-2004, 6/2/2004, §101)

The Township is empowered to regulate land use activities that affect runoff by the authority of the Act of October 4, 1978, 32 P.S., P.L. 864 (Act 167), §680.1 et seq., as amended, the "Storm Water Management Act," the Pennsylvania Municipalities Planning Code, Act 247, as amended, and the applicable Municipal Code, as amended.

(Ord. 4-2004, 6/2/2004, §103)

1. This Chapter shall apply to those areas of the Township that are located within the Chester Creek watershed or the Ridley Creek watershed, as delineated in the Chester Creek Stormwater Management Plan, or in the Ridley Creek Stormwater Management Plan which are hereby adopted as part of this Chapter.

2. This Chapter shall only apply to permanent stormwater management facilities constructed as part of any of the regulated activities listed in this Section. Stormwater management and erosion and sedimentation control during construction activities are specifically not regulated by this Chapter but shall continue to be regulated under existing laws and ordinances.

3. This Chapter contains only the stormwater management performance standards and design criteria that are necessary or desirable from a watershed-wide perspective. Local stormwater management design criteria (e.g., inlet spacing, inlet type, collection system design and details, outlet structure design, etc.) shall continue to be regulated by the applicable Township ordinances or at the Township Engineer's discretion.

4. The following activities are defined as "regulated activities" and shall be regulated by this Chapter:

A. Land development and/or redevelopment.

B. Subdivision.

C. Construction of new or additional impervious or semi-pervious surfaces (driveways, parking lots, etc.).

D. Construction of new buildings or additions to existing buildings.

E. Diversion or piping of any natural or man-made stream channel.

F. Installation of stormwater management facilities or appurtenances thereto

G. Placement of fill material.

(Ord. 4-2004, 6/2/2004, §104)

Approvals issued pursuant to this Chapter do not relieve the applicant of the responsibility to comply with or to secure required permits or approvals for activities regulated by any other codes, rules, statutes, or ordinances.

(Ord. 4-2004, 6/2/2004, §107)

For the purposes of this Chapter, certain terms and words used herein shall be interpreted as follows:

A. Words used in the present tense include the future tense; the singular number includes the plural, and the plural number includes the singular; words of masculine gender include feminine gender, and words of feminine gender include masculine gender.

B. The words "includes" or "including" shall not limit the term to the specific example but are intended to extend its meaning to all other instances of like kind and character.

C. The word "person" includes an individual, firm, association, organization, partnership, trust, company, corporation, or any other similar entity.

D. The words "shall" and "must" are mandatory; the words "may" and "should" are permissive.

E. The words "used" or "occupied" include the words "intended, designed, maintained, or arranged to be used, occupied, or maintained."

ACCELERATED EROSION - the removal of the surface of the land through the combined action of man's activity and the natural processes at a rate greater than would occur because of the natural process alone.

ACCESSORY STRUCTURE - a structure detached from a principal building located on the same lot and customarily incidental and subordinate to the principal building or use.

AGRICULTURAL ACTIVITIES - the work of producing crops and raising livestock including tillage, plowing, disking, harrowing, pasturing, and installation of conservation measures. Construction of new buildings or impervious area is not considered an agricultural activity.

ALTERATION - as applied to land, a change in topography as a result of the moving of soil and rock from one location or position to another; also, the changing of surface conditions by causing the surface to be more or less impervious; land disturbance.

APPLICANT - a landowner or developer who has submitted a drainage plan or filed an application for approval to engage in any regulated activities as defined in § 19-104 of this Chapter.

AS-BUILT DRAWINGS - a set of engineering or site drawings that delineates the specific permitted stormwater management facility as actually constructed.

BMP (BEST MANAGEMENT PRACTICE) - stormwater structures, facilities, and techniques to maintain or improve the water quality of surface runoff. Pennsylvania Handbook of Best Management Practices for Developing Areas, Spring 1998.

BUFFER - see "riparian buffer."

CHANNEL EROSION - the widening, deepening, and headward cutting of small channels and waterways due to erosion caused by moderate to large floods.

CISTERN - an undergTound reservoir or tank for storing rainwater.

COMBINED SEWERS - a sewerage system that carries both sanitary sewage and stormwater runoff.

CONSERVATION DISTRICT - the Delaware and Chester County Conservation Districts as appropriate for the individual municipality within the Chester Creek or Ridley Creek watershed.

CULVERT - a structure with appurtenant works that carries a stream under or through an embankment or fill.

DAM - an artificial barrier, together with its appurtenant works, constructed for the purpose of impounding or storing water or another fluid or semifluid, or a refuse bank, fill, or structure for highway, railroad, or other purposes which does or may impound water or another fluid or semi-fluid.

DEED RESTRICTION - see "restrictive covenant."

DESIGN STORM - the magnitude and temporal distribution of precipitation from a storm event measured in probability of occurrence (e.g., a 5-year storm) and duration (e.g., 24 hours), used in the design and evaluation of stormwater management systems.

DESIGNEE - the agent of the Delaware County Planning Department and/or agent of the Board of Supervisors involved with the administration, review, or enforcement of any provisions of this Chapter by contract or memorandum of understanding.

DETENTION BASIN - an impoundment structure designed to manage stormwater runoff by temporarily storing the runoff and releasing it at a predetermined rate.

DETENTION DISTRICT - those subareas in which some type of detention is required to meet the plan requirements and the goals of Act 167.

DEVELOPER - a person, partnership, association, corporation, or other entity, or any responsible person therein or agent thereof, that undertakes any regulated activity of this Chapter.

DEVELOPMENT - see "land development."

DEVELOPMENT SITE - the specific tract of land for which a regulated activity is proposed.

DISCHARGE EASEMENT - the grant of a property right to allow runoff in excess of the previous quantity and/or rate of flow.

DOWNSLOPE PROPERTY LINE - that portion of the property line of the lot, tract, or parcels of land being developed located such that all overland or pipe flow from the site would be directed towards it.

DRAINAGE CONVEYANCE FACILITY - a stormwater management facility designed to transmit stormwater runoff, including streams, channels, swales, pipes, conduits, culverts, storm sewers, etc.

DRAINAGE EASEMENT - a right granted by a landowner to a grantee allowing the use of private land for stormwater management purposes.

DRAINAGE PERMIT - a permit issued by the Township after the drainage plan has been approved. Said permit is issued prior to or with the final Township approval.

DRAINAGE PLAN - the documentation of the stormwater management system, if any, to be used for a given development site, the contents of which are established in §19-304.

EARTH DISTURBANCE - any activity including, but not limited to, construction, mining, timber harvesting, and grubbing which alters, disturbs, and exposes the existing land surface.

EASEMENT - a right-of-way granted, but not dedicated, for limited use of private land for a public or quasi-public purpose (e.g., utility lines) and within which the owner of the property shall not erect any permanent structures.

EPHEMERAL STREAMS - streams that carry only surface runoff and are dry except during precipitation events. The groundwater table is generally located below the bottom of ephemeral streams.

EROSION - the movement of soil particles by the action of water, wind, ice, or other natural forces.

EROSION AND SEDIMENT POLLUTION CONTROL PLAN - a plan that is designed to minimize accelerated erosion and sedimentation. Said plan must be submitted to and approved by the Delaware County Conservation Districts of the appropriate municipality before construction can proceed.

EXISTING CONDITIONS - the initial condition of a project site prior to the proposed construction. If the initial condition of the site is undeveloped land, the land use shall be considered as "meadow" on "B" soils unless the natural land cover is proven to generate lower curve numbers or Rational "C" value, such as forested lands.

FLOOD - a general but temporary condition of partial or complete inunda- tion of normally dry land areas from the overflow of streams, rivers, and other waters of this Commonwealth.

FLOODPLAIN - any land area susceptible to inundation by water from any natural source or delineated by applicable Department of Housing and Urban Development, Federal Insurance Administration, Flood Hazard Boundary Map as being a special flood hazard area.

FLOODWAY - the channel of the watercourse and those portions of the adjoining floodplains that are reasonably required to carry and discharge the 100-year frequency flood. Unless otherwise specified, the boundary of the floodway is as indicated on maps and flood insurance studies provided by the Federal Emergency Management Agency (FEMA). In an area where no FEMA maps or studies have defined the boundary of the 100-year frequency floodway, it is assumed - absent evidence to the contrary - that the floodway extends from the stream to 50 feet from the top of the bank of the stream.

FOREST MANAGEMENT/TIMBER OPERATIONS - planning and activities necessary for the management of forestland. These include timber inventory and preparation of forest management plans, silvicultural treatment, cutting budgets, logging road design and construction, timber harvesting, site preparation, and reforestation.

FREEBOARD - a vertical distance between the elevation of the design high water and the top of a dam, levee, tank, basin, or diversion ridge. The space is required as a safety margin in a pond or basin.

GRADE - a slope, usually of a road, channel, or natural ground, specified in percent and shown on plans as specified herein. To grade, to finish the surface of a roadbed, top of embankment, or bottom of excavation.

GRASSED WATERWAY - a natural or constructed waterway, usually broad and shallow, covered with erosion-resistant grasses, used to conduct surface water from cropland.

GROUNDWATER RECHARGE - replenishment of existing natural underground water supplies.

IMPERVIOUS SURFACE - a surface that has been compacted or covered with material to the extent that it is highly resistant to infiltration by water including, but not limited to, conventional impervious surfaces such as paved streets, roofs, compacted stone, and sidewalks. In addition, the following shall be considered impervious surfaces when used by motor vehicles: graveled areas, paver blocks, bricks, and cobblestone.

IMPOUNDMENT - a retention or detention basin designed to retain stormwater runoff and release it at a controlled rate.

INFILTRATION STRUCTURES - a structure designed to direct runoff into the ground (e.g., French drains, seepage pit, and seepage trench).

INLET - a surface connection to a closed drain. A structure at the diversion end of a conduit. The upstream end of any structure through which water may flow.

INTERMITTENT STREAMS - streams which flow only during wet seasons. The groundwater table generally is at or above the bottom of intermittent streams during wet seasons but drops below the stream bottom during dry seasons. Stream flow in intermittent streams is primarily due to precipitation but does have some groundwater contribution during wet seasons.

LAND DEVELOPMENT - (i) the improvement of one lot or two or more contiguous lots, tracts, or parcels of land for any purpose involving (a) a group of two or more residential or nonresidential buildings, whether proposed initially or cumulatively, or a single nonresidential building on a lot or lots regardless of the number of occupants or tenure, or (b) the division or allocation of land or space, whether initially or cumulatively, between or among two or more existing or prospective occupants by means of, or for the purpose of, streets, common areas, leaseholds, condominiums, building groups, or other features; (ii) any subdivision of land; (iii) development in accordance with §503(1.1) of the Pennsylvania Municipalities Planning Code.

LAND/EARTH DISTURBANCE - any activity involving grading, tilling, digging, or filling of ground or stripping of vegetation or any other activity that causes an alteration to the natural condition of the land.

MAIN STEM (MAIN CHANNEL) - any stream segment or other runoff conveyance facility used as a reach in the Chester Creek or Ridley Creek hydrologic model.

MANNING EQUATION IN (MANNING FORMULA) - a method for calculation of velocity of flow (e.g., feet per second) and flow rate (e.g., cubic feet per second) in open channels based upon channel shape, roughness, depth of flow, and slope. "Open channels" may include closed conduits so long as the flow is not under pressure.

MUNICIPALITY - Thornbury Township, Delaware County, Pennsylvania.

NONPOINT SOURCE POLLUTION - pollution that enters a watery body from diffuse origins in the watershed and does not result from discernible, confined, or discrete conveyances.

NRCS - Natural Resource Conservation Service (previously the Soil Conservation Service (SCS)).

OPEN CHANNEL - a drainage element in which stormwater flows with an open surface. Open channels include, but shall not be limited to, natural and man-made drainageways, swales, streams, ditches, canals, and pipes flowing partly full.

OUTFALL - point where water flows from a conduit, stream, or drain.

OUTLET - points of water disposal from a stream, river, lake, tidewater, or artificial drain.

PARKING LOT STORAGE - involves the use of impervious parking areas as temporary impoundments with controlled release rates during rainstorms.

PEAK DISCHARGE - the maximum rate of stormwater runoff from a specific storm event.

PENN STATE RUNOFF MODEL (CALIBRATED) - a computer-based hydrologic modeling technique.

PERENNIAL STREAMS - streams that flow year round. Perennial streams derive their flow from both groundwater and runoff, and the groundwater table never drops below the streambed.

PIPE - a culvert, closed conduit, or similar structure (including appurte- nances) that conveys stormwater.

PLANNING COMMISSION - the Planning Commission of Thornbury Township.

PMF - PROBABLE MAXIMUM FLOOD - the flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in any area. The PMF is derived from the probable maximum precipitation (PMP) as determined based on data obtained from the National Oceanographic and Atmospheric Administration (NOAA).

RATIONAL FORMULA - a rainfall-runoff relation used to estimate peak flow.

REDEVELOPMENT - reconstruction of an existing improved, developed property, as of the date of adoption of this Chapter.

REGULATED ACTIVITIES - actions or proposed actions that have an impact on stormwater runoff and that are specified in §19-104 of this Chapter.

RELEASE RATE - the percentage of pre-development peak rate of runoff from a site or subarea to which the post-development peak rate of runoff must be reduced to protect downstream areas.

RESTRICTIVE COVENANT - a restriction on the use of land usually set forth in the deed. Restrictive covenants (a.k.a. deed restrictions) usually run with the land and are binding upon subsequent owners of the property.

RETENTION BASIN - an impoundment in which stormwater is stored and not released during the storm event. Stored water may be released from the basin at some time after the end of the storm.

RETURN PERIOD - the average interval, in years, within which a storm event of a given magnitude can be expected to recur. For example, the 25-year return period rainfall would be expected to recur on the average once every 25 years.

RIPARIAN BUFFER - a vegetative strip paralleling the banks of a perennial or intermittent stream or other water body (including wetlands and ponds). The buffer shall contain appropriate native vegetation throughout its width with the exception of a minimum 5-foot wide strip of land maintained in meadow grass or forbs at its outer boundary. See also Appendix C, Riparian Buffer Technical Reference Guide.

RISER - a vertical pipe extending from the bottom of a pond that is used to control the discharge rate from the pond for a specified design storm.

ROOFTOP DETENTION - temporary ponding and gradual release of stormwater falling directly onto flat roof surfaces by incorporating controlled-flow roof drains into building designs.

RUNOFF - any part of precipitation that flows over the land surface.

SEDIMENT BASIN - a barrier, dam, or retention or detention basin located and designed to retain rock, sand, gravel, silt, or other material transported by water.

SEDIMENT POLLUTION - the placement, discharge, or any other introduction of sediment into the waters of the Commonwealth occurring from the failure to design, construct, implement, or maintain control measures and control facilities in accordance with the requirements of this Chapter.

SEDIMENTATION - the process by which matter is accumulated or deposited by the movement of water.

SEEPAGE PIT/SEEPAGE TRENCH - an area of excavated earth filled with loose stone or similar coarse material into which surface water is directed for infiltration into the ground.

SHEET FLOW - runoff that flows over the ground surface as a thin, even layer, not concentrated in a channel.

SOIL-COVER COMPLEX METHOD - a method of runoff computation developed by the NRCS that is based on relating soil type and land use/cover to a runoff parameter called curve number (CN).

SOIL GROUP, HYDROLOGIC - a classification of soils by SCS into four runoff potential groups. The groups range from A soils, which are very permeable and produce little runoff, to D soils, which are not very permeable and produce much more runoff.

SPILLWAY - a depression in the embankment of a pond or basin that is used to pass the peak discharge which is greater than the maximum design storm controlled by the pond.

STORAGE INDICATION METHOD - reservoir routing procedure based on solution of the continuity equation (inflow minus outflow equals the change in storage) with outflow defined as a function of storage volume and depth.

STORM FREQUENCY - the number of times that a given storm "event" occurs or is exceeded on the average in a stated period of years. See "return period."

STORM SEWER - a system of pipes and/or open channels that conveys intercepted runoff and stormwater from other sources but excludes domestic sewage and industrial wastes.

STORMWATER - the total amount of precipitation reaching the ground surface.

STORMWATER MANAGEMENT FACILITY - any structure, natural or man-made, that, due to its condition, design, or construction, conveys, stores, or otherwise affects stormwater runoff. Typical stormwater management facilities include, but are not limited to, detention and retention basins, open channels, storm sewers, pipes, and infiltration structures.

STORMWATER MANAGEMENT PLAN - the plan for managing stormwater runoff in the Chester Creek and Ridley Creek watersheds adopted by Delaware and Chester Counties as required by the Act of October 4, 1978, P.L. 864, (Act 167), and known as the "Act 167 Stormwater Management Plan, Chester Creek Watershed" and the "Act 167 Stormwater Management Plan for Ridley Creek Watershed."

STORMWATER MANAGEMENT SITE PLAN - the plan prepared by the applicant or his representative indicating how stormwater runoff will be managed at the particular site of interest according to this Chapter.

STREAM ENCLOSURE - a bridge, culvert, or other structure in excess of 100 feet in length upstream to downstream which encloses a regulated water of this Commonwealth.

SUBAREA - the smallest drainage unit of a watershed for which stormwater management criteria have been established in the Stormwater Management Plan.

SUBDIVISION - the division or re-division of a lot, tract, or parcel of land by any means into two or more lots, tracts, parcels, or other divisions of land including changes in existing lot lines for the purpose, whether immediate or future, of lease, transfer of ownership, or building or lot development; provided, however, that the subdivision by lease of land for agricultural purposes into parcels of more than 10 acres not involving any new street or easement of access or any residential dwellings shall be exempt.

SWALE - a low-lying stretch of land that gathers or carries surface water runoff.

TIMBER OPERATIONS - see "forest management."

TIME OF CONCENTRATION (TC) - the time for surface runoff to travel from the hydraulically most distant point of the watershed to a point of interest within the watershed. This time is the combined total of overland flow time and flow time in pipes or channels, if any.

TR-20 - the computer-based hydrologic modeling technique adapted to the Chester Creek watershed for the Act 167 plan. The model has been "calibrated" to reflect actual recorded flow values by adjusting key model input parameters.

TR-55 - s method for determining runoff volumes and rates developed by NRCS.

WATERCOURSE - a channel or conveyance of surface water having defined bed and banks, whether natural or artificial, with perennial or intermittent flow.

WATERS OF THE COMMONWEALTH - any and all rivers, streams, creeks, rivulets, ditches, watercourses, storm sewers, lakes, dammed water, wetlands, ponds, springs, and all other bodies or channels of conveyance of surface and underground water, or parts thereof, whether natural or artificial, within or on the boundaries of this Commonwealth.

WETLAND - those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions, including swamps, marshes, bogs, and similar areas.

(Ord. 4-2004, 6/2/2004, §201)