Township of Tobyhanna, PA Stormwater Management

The boundaries of the stormwater management districts are shown on an official map which is available for inspections at the municipal office. A copy of the official map at a reduced scale in included in the Tobyhanna Creek Watershed Stormwater Management Plan, Volume I, Plate I.[1]

Editor's Note: Said plate is on file in the Township offices.

The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours provided as part of the drainage plan.

Sites located in more than one district. For a proposed development site located within two or more districts, all flows generated on or conveyed to a given district shall meet the applicable release rate for that district. The peak discharge rate from any subarea shall be the predevelopment peak discharge for that subarea multiplied by the applicable release rate. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea.

Off-site areas. Off-site areas which drain through a proposed development site are not subject to the districts criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site. Only those flows generated from the development site shall comply with the release rate for the applicable district.

Impact areas. Where the area to be impacted by a proposed development activity differs significantly from the development site, only the proposed impact area shall be subject to the release rate criteria.

No Harm option.

The developer has the option of using a less restrictive runoff control (including no detention) if the developer can prove that no harm would be caused by discharging at a higher runoff rate than that specified by the plan. The no-harm option is used when a developer can prove that the postdevelopment hydrographs can match predevelopment hydrographs, or if it can be proved that the postdevelopment conditions will not cause increases in peaks at all points downstream. Proof of no harm would have to be shown based upon the following downstream impact evaluation which shall include a downstream hydraulic capacity analysis in accordance with § 124-78G to determine if adequate hydraulic capacity exists. The land developer shall submit to the municipality this evaluation of the impacts due to increased downstream stormwater flows in the watershed.

(a)

The downstream impact evaluation shall include hydrologic and hydraulic calculations necessary to determine the impact of hydrograph timing modifications due to the proposed development upon any dam, highway, structure, natural point of restricted streamflow or stream channel section.

(b)

The downstream capacity evaluation shall continue downstream until the increase in flow diminishes due to additional flow from tributaries and/or stream attenuation.

(c)

The peak flow values to be used for downstream areas for the design return period storms (two-, ten-, twenty-five- and one-hundred-year) shall be the values from the calibrated Penn State Runoff Model for the Tobyhanna Creek Watershed. These flow values would be supplied to the developer by the Monroe Conservation District upon request.

(d)

Developer-proposed runoff controls which would generate increased peak flow rates at storm drainage problem areas shown in Figure III-7, Volume II,[2] would, by definition, be preluded from successful attempts to prove no-harm, except in conjunction with proposed capacity improvements for the problem areas consistent with § 124-78H.

[2]

Editor's Note: See the definition of "Stormwater Management Plan."

(e)

A financial distress shall not constitute grounds for granting a no-harm exemption.

Capacity improvements may be provided as necessary to implement the no-harm option which proposes specific capacity improvements to provide that a less stringent discharge control would not create any harm downstream.

Any no harm justifications shall be submitted by the developer as part of the drainage plan submission per Article XXII of this Part 3.

Downstream hydraulic capacity analysis. Any downstream capacity hydraulic analysis conducted in accordance with this article shall use the following criteria for determining adequacy for accepting increased peak flow rates:

Channels or swales must be able to convey the increased runoff associated with a two-year return period event within their banks at velocities consistent with protection of the channels from erosion. Acceptable velocities shall be based upon criteria included in the DEP Erosion and Sediment Pollution Control Program Manual (April 1990).

Channels or swales must be able to convey the increased twenty-five-year return period runoff without creating any hazard to persons or property.

Culverts, bridges, storm sewers or any other facilities which must pass or convey flows from the tributary area must have sufficient capacity to pass or convey the increased flows associated with the twenty-five-year return period runoff event, except for facilities located within a floodplain which must be capable of passing or conveying the one-hundred-year return period runoff. Any facilities which constitute stream enclosures per DEP Chapter 105 regulations shall be designed to convey the one-hundred-year return period runoff.

(4)

Any available capacity in the downstream conveyance system as documented by a developer may be used by the developers only in proportion to his development site acreage relative to the total upstream undeveloped acreage from the identified capacity (i.e., if his site is 10% of the upstream undeveloped acreage, he may use up to 10% of the documented downstream available capacity).

Capacity improvements. In certain instances, primarily within the provisional direct discharge areas, local drainage conditions may dictate more stringent levels of runoff control than those based upon protection of the entire watershed. In these instances, if the developer could prove that it would be feasible to provide capacity improvements to relieve the capacity deficiency in the local drainage network, (i.e., downstream) then the capacity improvements could be provided by the developer in lieu of runoff controls on the development site. Peak flow calculations are to be performed assuming that the contributing drainage area is in the existing condition and then assuming that the contributing drainage area is developed per the Tobyhanna Creek Watershed stormwater management current zoning and using the specified runoff controls. Any capacity improvements would be designed using the larger of the above peak flows and the capacity criteria specified in § 124-78G of this article. All new development in the entire subarea(s) within which the proposed development site is located shall be assumed to implement the release rate of the management district in which it is located.

If capacity improvements are proposed and the downstream conveyance system is situated in another municipality, the land developer shall inform the affected municipality of the downstream hydraulic capacity analysis and shall provide a copy of the drainage plan to that municipality containing the proposed capacity improvements for its review.

When any downstream capacity improvements are proposed to occur in an other municipality, the other municipality, at its discretion, may request the municipality in which the development occurs to incorporate its comments into the subdivision plan. Upon receipt of such a request, the municipality in which the land development will occur shall furnish a written response to the other municipality within 30 days of the receipt of the request stating its decision.

The granting of any approval does not relieve the applicant from obtaining any permits or approvals from the municipality where the capacity improvements will occur as they relate to the design installation or construction of the capacity improvements.

Regional detention alternatives. For certain areas within the study area, it may be more cost-effective to provide one control facility for more than one development site than to provide an individual control facility for each development site. The initiative and funding for any regional runoff control alternatives are the responsibility of prospective developers. The design of any regional control basins must incorporate future development of the entire upstream watershed. The peak outflow of a regional basin would be determined on a case-by-case basis using the hydrologic model of the watershed consistent with protection of the downstream watershed areas. Hydrologic model refers to the calibrated version of the Penn State Runoff Model as developed for the stormwater management plan.

§ 124-79 Design criteria for stormwater management facilities.

Any stormwater management facility located or discharging to state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation.

Any stormwater management facility (i.e., detention basin) designed to store runoff and requiring a berm or earthen embankment required or regulated by this article shall be designed to provide an emergency spillway to handle flow rates up to the one-hundred-year postdevelopment conditions. Emergency spillways shall be constructed on undisturbed ground and shall not be constructed on embankment fill. The height of embankment must be designed to provide a minimum 1.0 foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year postdevelopment inflow. Should any stormwater management facility require a dam safety permit under PADEP Chapter 105, the facility shall be designed in accordance with Chapter 105 which may require facilities to pass storms larger than the one-hundred-year event.

Any facilities that constitute water obstructions or encroachments (e.g., culverts, bridges, outfalls, or stream enclosures), and any work involving wetlands, shall be designed in accordance with Chapter 105 regulation(s) (as amended) and may require a permit from PADEP. Any other drainage conveyance facility that is not regulated by Chapter 105 must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of the roadway. Roadway crossings located within floodplains must be able to convey runoff from a one-hundred-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of roadway. Any facility that constitutes a dam as defined in PADEP Chapter 105 regulations may require a dam safety permit.

Any drainage conveyance facility and/or channel that is not regulated by Chapter 105 must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. Roadway crossings located within floodplains must be able to convey runoff from a one-hundred-year design storm.

Storm sewers must be able to convey postdevelopment runoff from a twenty-five-year design storm without surcharging inlets, unless designed for detention or parking lot ponding purposes.

Adequate erosion protection shall be provided along all open channels and at all points of discharge.

The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The municipality shall reserve the right to disapprove any design that would result in the occurrence or continuation of an adverse hydrologic or hydraulic condition within the watershed.

§ 124-80 Water quality requirements.

The following are the minimum water quality criteria established to meet the objective of the plan. Because the standard for water quality may result in a fairly small outlet orifice in detention facilities, the Municipal Engineer shall require a minimum standard such as a three-inch diameter orifice.

In addition to the performance standards and design criteria requirements of Article XXI of this article, the land developer shall comply with the following water quality requirements of this article unless otherwise exempted by the provisions of this article. In addition to the water quantity requirements of this article, the land developer shall control for water quality as follows:

Residential areas.

(a)

Stormwater management facilities shall be provided to detain the one-year, twenty-four-hour design storm using the SCS Type II distribution or applicable methedology. Provisions shall be made so that the one-year storm takes a minimum of 24 hours to drain from the facility from a point where the maximum volume of water captured from the facility for the one-year storm is achieved (i.e., the maximum water surface elevation achieved in the facility). On a smaller site such as where the time-of-concentration is 15 minutes or less, the criteria shall apply to the appropriate calculation methodology, i.e., the modified rational method.

(b)

Release of this water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility). The design of the facility shall consider and minimize the chances of clogging and sedimentation potential.

Commercial/industrial areas. Commercial and industrial sites shall detain the first 1/2 inch of runoff for a twenty-four-hour period.

As an alternative to the requirements of Subsection A(1) and (2), the water quality objectives may be achieved through a combination of BMP's including, but not limited to, infiltration structures, detention/retention basins, vegetated filter strips and buffers or any combination of BMP's listed in Appendix 4[1] of the Tobyhanna Creek Watershed Stormwater Management Plan, Volume II. The combination of BMP's shall be designed according to the requirements listed under § 124-80D, BMP selection criteria, and in consultation with the Municipal Engineer.

Editor's Note: Said appendix is on file in the Township offices.

To accomplish Subsection A above, the land developer may submit original and innovative designs to the Municipal Engineer for review and approval.

In selecting the appropriate BMPs or combinations thereof, the land developer shall consider the following at a given development site.

Total contributing drainage area.

Permeability and infiltration rate of the site soils (hydrologic soil group).

Slope and depth to bedrock (soils).

(4)

Seasonal high-water table.

(5)

Proximity to building foundations and well heads.

(6)

Erodibiity of soils.

(7)

Land availability and configuration of the topography.

The following additional factors SHOULD be considered when evaluating the suitability of the EMP's used to control water quality at a given development site:

Peak discharge and required volume control.

Streambank erosion.

Efficiency of the BMP's to mitigate potential water quality problems.

(4)

The volume of runoff that will be effectively treated.

(5)

The nature of the pollutant being removed.

(6)

Maintenance requirements.

(7)

Creation/protection of aquatic and wildlife habitat.

(8)

Recreational value.

(9)

Enhancement of aesthetic and property value.

§ 124-81 Calculation methodology.

Stormwater runoff from all development sites shall be calculated using either the rational method or a soil-cover-complex methodology.

Any stormwater runoff calculations involving drainage areas greater than 200 acres, including on- and off-site areas, shall use a generally accepted calculation technique that is based on the NRCS soil-cover-complex-method. Table 1[1] summarizes acceptable computation methods. It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular development site. The municipality may approve the use of the rational method to estimate peak discharges from drainage areas that contain less than 200 acres.

Editor's Note: Said table is located at the end of this section.

All calculations consistent with the requirements of this article using the soil-cover-complex method shall use the appropriate design rainfall depths for the various return period storms presented in Table B-1 in Appendix B of this article.[2] If a hydrologic computer model such as PSRM or HEC-1 is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The NRCS S-curve shown in Figure B-1, Appendix B of this article shall be used for the rainfall distribution.

[2]

Editor's Note: Said appendix is on file in the Township offices.

For the purposes of predevelopment flow rate determination, undeveloped land shall be considered as meadow conditions, unless the natural ground cover generates a lower curve number or rational C value (i.e., forest).

All calculations using the rational method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from the design storm curves from PA Department of Transportation Design Rainfall Curves (1986) (Figure B-2), Appendix B. Times-of-concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS). Times-of-concentration for channel and pipe flow shall be computed using Manning's equation.

Runoff curve numbers (CN) for both existing and proposed conditions to be used in the soil-cover-complex method shall be obtained from Table B-2 in Appendix B of this article.

Runoff coefficients (c) for both existing and proposed conditions for use in the rational method shall be obtained from Table B-3 in Appendix B of this article.

Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations and to determine the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with Table B-4 in Appendix B of the article.[3] Outlet structures for stormwater management facilities shall be designed to meet the performance standards in § 124-78 of this article using any generally accepted hydraulic analysis technique or method.

[3]

Editor's Note: Said appendix is on file in the Township offices.

The design of any stormwater detention facilities intended to meet the performance standards of this article in § 124-78 of this article shall be verified by routing the design storm hydrograph through these facilities using the storage-indication method. For drainage areas greater than 20 acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The municipality may approve the use of any generally accepted full hydrograph approximation technique which shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.

The municipality has the authority to require that computed existing runoff rates be reconciled with field observations and conditions. If the designer can substantiate through actual physical calibration that more appropriate runoff and time-of-concentration values should be utilized at a particular site, then appropriate variations may be made upon review and recommendations of the Municipal Engineer. Calibration shall require detailed gauge and rainfall data for the particular site in question. Calibration should not be misrepresented to mean a best guess as to what might happen based on experience, as is oftentimes the case.

TABLE 1 Acceptable Computation Methodologies for Stormwater Management Plans
Method	Method Developed By	Applicability
TR-20 (or commercial package based on TR-20)	USDA NRCS	Applicable where use of full hydrology computer model is desirable or necessary
TR-55 (or commercial computer package based on TR-55)	USDA NRCS	Applicable for land development plans within limitations described in TR-55
HEC-1	US Army Corps of Engineers	Applicable where use of full hydrologic computer model is desirable or necessary
PSRM	Penn State University	Applicable where use of a hydrologic computer model is desirable or necessary; simpler than TR-20 or HEC-1
Rational method (or commercial computer package based on rational method)	Emil Kuichling (1889)	For sites less than 20 acres, or as approved by the municipality and Municipal Engineer
Other methods	Varies	Other computation methodologies approved by the municipality and Municipal Engineer

§ 124-82 Erosion and sedimentation requirements.

Whenever earth is proposed to be disturbed, such activity must be in conformance with Chapter 102 regulations as amended and in and the standards and specifications of the appropriate municipal government.

Additional erosion and sedimentation control design standards and criteria that must be applied where infiltration BMPs are proposed include the following:

Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase so as to maintain their maximum infiltration capacity.

Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has been final stabilized in accordance with the DEP Erosion and Sediment Pollution control Program Manual (April, 1990). Exceptions to this requirement are infiltration basins designed to accept construction phase sediment. Design criteria for such basins are found in Appendix 3.[1]