EXECUTIVE SUMMARY

The Upper Yellow Breeches Watershed Stormwater Management Plan has been prepared in accordance with the content and format requirements of Pennsylvania’s Stormwater Management Act (i.e., Act 167) of 1978. Act 167 requires counties to prepare and adopt a stormwater management plan for each designated watershed in that particular county. The preparation of stormwater management plans at the watershed-level provides for the incorporation of sound engineering standards and criteria into local codes and ordinances to manage runoff from new development sites in a coordinated, watershed-wide approach. The goal of the Upper Yellow Breeches Watershed Stormwater Management Plan is to provide the watershed municipalities with technically sound and administratively consistent standards and criteria for the effective management of stormwater runoff from new development sites. To meet this goal, the Cumberland County Planning Commission, working in cooperation with the watershed municipalities through a Watershed Plan Advisory Committee (WPAC), developed a model stormwater management ordinance for municipal adoption that is specific to the hydrologic and hydraulic characteristics of the Upper Yellow Breeches Watershed. The technical provisions that are set forth in this model ordinance were derived from the Upper Yellow Breeches Watershed Stormwater Runoff Computer Model, which was developed for the plan. This hydrologic computer model calculates peak stormwater flow rates for various points of interest and can be used to estimate the stormwater implications of various development scenarios throughout the Upper Yellow Breeches Watershed. Detailed information related to the development, calibration, and use of this hydrologic computer model is included in Chapter IV of the plan. Chapter III of the plan provides detailed information related to the physical characteristics of the Upper Yellow Breeches Watershed. The following watershed characteristics are documented in the plan:

• location and drainage area; • existing and future land use; • topography and stream bed profile; • soils and geology; • groundwater and surface water hydrology; • wetlands and floodplains; • significant hydrologic obstructions and drainage problem areas; and • existing stormwater management facilities.

ES−1 Most of these watershed characteristics were also compiled within a Geographic Information System (GIS) database for future reference. Where appropriate, this information is presented graphically in the plan. As previously mentioned, Chapter IV of the plan contains detailed information related to the development, calibration, and use of the Upper Yellow Breeches Watershed Stormwater Runoff Model. This chapter provides the technical basis for the development of the watershed-based standards and criteria for the control of stormwater runoff. In developing these standards and criteria, the initial model run results were compared to existing flow data for calibration purposes. This data comparison indicated that the use of standardized composite runoff curve numbers calculated for each subwatershed overestimated the peak stormwater runoff rates actually realized in the watershed. This overestimation of peak stormwater runoff rates was attributed to the watershed’s limestone geology and extensive colluvium and associated high infiltration rates throughout much of the watershed. As such, adjustments to the composite runoff curve numbers in the form of uniform reductions must be incorporated into the stormwater runoff calculations for proposed development sites to more accurately depict existing conditions. Chapter IV contains detailed information related to the necessary curve number reductions. The standards and criteria that were developed for the management of stormwater runoff in the Upper Yellow Breeches Watershed take into consideration both quantity and quality controls. These standards and criteria, which are documented in Chapter V, include stormwater management provisions for the following:

• protection of water quality; • maintaining groundwater recharge; • maintaining groundwater recharge in areas underlain by limestone geology; • preservation of natural drainage features and depressions; • avoiding adverse impacts on downstream properties; • prevention of stream bank erosion; and • control of over bank flooding and extreme flood events.

A brief summary of each of these standards and criteria is provided below.

PROTECTION OF WATER QUALITY

The objective is to capture and treat 90% of the average annual stormwater runoff volume.

This volume of stormwater runoff is identified and calculated as the Water Quality Volume (WQv).

Treatment of the WQv should be provided at all new development sites.

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MAINTAINING GROUNDWATER RECHARGE

In order to maintain pre-development hydrologic conditions on a development site, opportunities must be provided for the infiltration of rainfall directly into the ground and to pond runoff on the ground surface from which it is ultimately evaporated or infiltrated. A site recharge volume requirement is proposed to mimic pre-development hydrologic conditions. By mimicking pre-development groundwater recharge rates, groundwater levels are not lowered thereby maintaining stream flow during dry weather.

MAINTAINING GROUNDWATER RECHARGE IN LIMESTONE AREAS

Stormwater management facilities for the recharge of groundwater in limestone bedrock areas must provide for infiltration opportunities distributed over a very large area. Examples include filter strips, large bioretention areas, and pervious pavement. Stormwater management facilities that create concentrated sources of infiltration, such as infiltration trenches or dry wells, shall not be used in limestone bedrock areas.

PRESERVATION OF NATURAL DRAINAGE FEATURES AND DEPRESSIONS

For all land development plans, the elimination of natural drainageways or depressions shall be accepted only if the developer cannot reasonably comply with this standard because of a property’s configuration and/or topography.

AVOIDING ADVERSE IMPACTS ON DOWNSTREAM PROPERTIES

In many areas of the Upper Yellow Breeches Watershed, large land areas drain to low areas that have either an indirect or no means for the discharge of stormwater runoff to a perennial stream. Subsequently, all stormwater runoff that is discharged from a development site must be conveyed to a perennial stream in a manner that has no adverse impacts on downstream properties located between the development site and the perennial stream.

ES−3 PREVENTION OF STREAMBANK EROSION

In order to minimize the occurrence of critical erosive velocities in natural stream channels, stormwater runoff from new development sites must be gradually released. To attain this objective, stormwater runoff from new development sites, for the 1-year, 24-hour storm event (2.4 inches) must be released over a minimum period of 24 hours.

CONTROL OF OVERBANK FLOODING AND EXTREME FLOOD EVENTS

The objective is to prevent an increase in the frequency and magnitude of “out-of-bank” nuisance flooding generated by new development and to maintain the boundaries of the regulated 100-year floodplain of the Yellow Breeches Creek. To meet this objective, the post-development peak discharge of stormwater runoff shall not exceed the pre-development peak discharge of stormwater runoff. Projects that are proposed to not meet this standard must demonstrate proof of “no harm” at all points downstream from the site.

Various types of structural and non-structural stormwater best management practices (BMPs) that can be implemented on a site to meet these standards and criteria are presented in Chapter VI. From a stormwater management perspective, the primary benefit provided by development site planning with non-structural practices is that less stormwater runoff is generated, thereby reducing the required size for structural stormwater management facilities and the cost to construct and maintain the facilities. In addition, the use of non-structural practices also serves to reduce the quantity of pollutants that are conveyed in stormwater runoff. Six types of non-structural stormwater BMPs are presented for use in the Upper Yellow Breeches Watershed. They include the following:

• Natural Area Conservation; • Disconnection of Rooftop Runoff; • Disconnection of Impervious Surface Runoff; • Stream Buffers; • Grass Channels; and • Environmentally Sensitive Development Credit.

For the purpose of promoting the use of these non-structural stormwater BMPs in the planning of development sites, “structural facility size reduction (SFSR) credits” are included, which may reduce

ES−4 the size of structural stormwater management facilities required to meet the standards and criteria established for the Upper Yellow Breeches Watershed. Five types of structural BMPs are also presented for use in the Upper Yellow Breeches Watershed. These BMPs have been found to be effective in stormwater quality control and/or the infiltration of stormwater runoff. They include the following:

• Stormwater Ponds; • Stormwater Wetlands; • Stormwater Infiltration Facilities; • Stormwater Filtering Facilities; and • Stormwater Swale Sytems.

The plan also includes a BMP Selection Matrix, which can aid in the selection of the appropriate stormwater BMPs for any new development site. The remaining plan chapters present the Upper Yellow Breeches Watershed Model Stormwater Management Ordinance and the administrative procedures associated with plan adoption and implementation.

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