Borough of Lansdale, PA Stormwater Management

The measurement of impervious areas shall include all of the impervious areas in the total proposed development even if development is to take place in stages.

For development taking place in stages, the entire development plan must be used in determining conformance with this chapter.

For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter.

Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated, or otherwise altered without written notification of the adjacent property owner(s) from the developer. Such stormwater flows shall be subject to the requirements of this chapter.

All regulated activities shall include such measures as necessary to:

Protect health, safety, and property;

Meet the water quality goals of this chapter by implementing measures to:

Minimize disturbance to floodplains, wetlands, and wooded areas.

Create, maintain, repair or extend riparian buffers.

(c)

Avoid erosive flow conditions in natural flow pathways.

(d)

Minimize thermal impacts to waters of this commonwealth.

(e)

Disconnect impervious surfaces (i.e., disconnected impervious areas, DIAs) by directing runoff to pervious areas, wherever possible.

To the maximum extent practicable, incorporate the techniques for low impact development practices (e.g., protecting existing trees, reducing area of impervious surface, cluster development, and protecting open space) described in the Pennsylvania Stormwater Best Management Practices Manual, Pennsylvania Department of Environmental Protection (PADEP), No. 363-0300-002 (2006).

Infiltration BMPs should be spread out, made as shallow as practicable, and located to maximize the use of natural on-site infiltration features while still meeting the other requirements of this chapter.

The design of all facilities over karst shall include an evaluation of measures to minimize the risk of adverse effects.

Storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 and not more than 72 hours from the end of the design storm.

The design storm volumes to be used in the analysis of peak rates of discharge should be obtained from the Precipitation Frequency Atlas of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Springs, Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.

For all regulated activities, SWM BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Storm Water Management Act.[2]

[2]

Editor's Note: See 35 P.S. § 691.1 et seq., and 32 P.S. § 680.1 et seq., respectively.

Various BMPs and their design standards are listed in the Pennsylvania Stormwater Best Management Practices Manual (PA BMP Manual).

§ 338-10 Permit requirements by other governmental entities.

Approvals issued and actions taken under this chapter do not relieve the applicant of the responsibility to secure required permits or approvals for activities regulated by any other code, law, regulation or ordinance.

§ 338-11 Volume control.

Volume controls will mitigate increased runoff impacts, protect stream channel morphology, maintain groundwater recharge, and contribute to water quality improvements. Stormwater runoff volume control methods are based on the net change in runoff volume for the two-year storm event.

Volume controls shall be implemented using the design storm method in Subsection B(1) or the simplified method in Subsection B(2) below. For regulated activities equal to or less than one acre, this chapter establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations of the procedures associated with each methodology, and other factors. All regulated activities greater than one acre must use the design storm method.

Design storm method (any regulated activity). This method requires detailed modeling based on site conditions. For modeling assumptions, refer to § 338-13A.

Post-development total runoff should not be increased from predevelopment total runoff for all storms equal to or less than the two-year, twenty-four-hour duration precipitation.

The following applies in order to estimate the increased volume of runoff for the two-year, twenty-four-hour duration precipitation event:

To calculate the runoff volume (cubic feet) for existing site conditions (predevelopment) and for the proposed developed site conditions (post-development), it is recommended to use the Soil Cover Complex Method as shown below. Table B-3 in Appendix B of this chapter is available to guide a qualified professional and/or an applicant to calculate the stormwater runoff volume. The calculated volume shall be either reused, evapotranspired, or infiltrated through structural or nonstructural means.


Step 1: Runoff (inches) = Q = (P - 0.2S)² / (P + 0.8S)
	Where:
		P	=	2-year rainfall (inches)
		S	=	(1,000/CN) - 10, the potential maximum retention (including initial abstraction, IA)
Step 2: Runoff volume (cubic feet) = Q x Area x 1/12
	Where:
		Q	=	Runoff (inches)
		Area	=	SWM area (square feet)

Simplified method (regulated activities less than or equal to one acre):

Stormwater facilities shall capture the runoff volume from at least the first two inches of runoff from all new impervious surfaces.

Volume (cubic feet) = (2 inches runoff / 12 inches) * impervious surface (square feet)

At least the first inch of runoff volume from the new impervious surfaces shall be permanently removed from the runoff flow, i.e., it shall not be released into the surface waters of the commonwealth. The calculated volume shall be either reused, evapotranspired, or infiltrated through structural or nonstructural means.

Volume (cubic feet) = (1 inch runoff / 12 inches) * impervious surface (square feet)

(c)

Infiltration facilities should be designed to accommodate the first 0.5 inch of the permanently removed runoff.

(d)

No more than one inch of runoff volume from impervious surfaces shall be released from the site. The release time must be over 24 to 72 hours.

Stormwater control measures. The applicant must demonstrate how the required volume is controlled through stormwater best management practices (BMPs), which shall provide the means necessary to capture, reuse, evaporate, transpire or infiltrate the total runoff volume.

If natural resources exist on the site and a SWM site plan submission is required for the regulated activity, the applicant shall determine and display the total acreage of protected area where no disturbance is proposed on the plan. The acreage of the protected area should be subtracted from the total site area and not included in the stormwater management site area acreage used in determining the volume controls.

Stormwater Management Site Area =

{Total Site Area (for both pre- and post-development conditions) - Protected Area}

See Table B-2 in Appendix B of this chapter for guidance to assess the total protected area. For additional reference, see Chapter 5, Section 5.4.1, of the PA BMP Manual.

Calculate the volume controls provided through nonstructural BMPs. Table B-5 in Appendix B of this chapter is recommended as guidance.

(c)

Volume controls provided through nonstructural BMPs should be subtracted from the required volume to determine the necessary structural BMPs.

Required

Volume Control (cubic feet)

Nonstructural

Volume Control (cubic feet)

Structural

Volume Requirement (cubic feet)

(d)

Calculate the volume controls provided through structural BMPs. Table B-6 in Appendix B of this chapter is recommended as guidance. See PA BMP Manual, Chapter 6, for description of the BMPs.

(e)

Infiltration BMPs intended to receive runoff from developed areas shall be selected based on the suitability of soils and site conditions (see Table B-6 in Appendix B of this chapter for a list of infiltration BMPs). Infiltration BMPs shall be constructed on soils that have the following characteristics:

[1]

A minimum soil depth of 24 inches between the bottom of the infiltration BMPs and the top of bedrock or seasonally high water table.

[2]

An infiltration rate sufficient to accept the additional stormwater load and dewater completely as determined by field tests. A minimum of 0.2 inches/hour (in/hr) should be utilized, and for acceptable rates a safety factor of 50% should be applied for design purposes (e.g., for soil which measured 0.4 in/hr, the BMP design should use 0.2 in/hr to insure safe infiltration rates after construction).

[3]

All open-air infiltration facilities shall be designed to completely infiltrate runoff volume within three days (72 hours) from the start of the design storm.

(f)

Soils. A soils evaluation of the project site shall be required to determine the suitability of infiltration facilities. All regulated activities are required to perform a detailed soils evaluation by a qualified design professional, which at minimum address soil permeability, depth to bedrock, and subgrade stability. The general process for designing the infiltration BMP shall be:

[1]

Analyze hydrologic soil groups as well as natural and man-made features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration may not be ruled out without conducting these tests.

[2]

Provide field tests, such as double ring infiltrometer or hydraulic conductivity tests (at the level of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Percolation tests are not recommended for design purposes.

[3]

Design the infiltration structure based on field determined capacity at the level of the proposed infiltration surface and based on the safety factor of 50%.

[4]

If on-lot infiltration structures are proposed, it must be demonstrated to the Borough that the soils are conducive to infiltrate on the lots identified.

[5]

An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the Borough.

§ 338-12 Stormwater peak rate control, management districts for Neshaminy Creek Watershed.

Peak rate controls for large storms, up to the one-hundred-year event, is essential in order to protect against immediate downstream erosion and flooding. The following peak rate controls have been determined through hydrologic modeling of the Neshaminy Creek Watershed.

Standards for managing runoff from each subarea in the Neshaminy Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year design storms are shown in Table 304.1. Development sites located in each of the management districts must control proposed development condition runoff rates to existing condition runoff rates for the design storms in accordance with Table 304.1 below.

Table 304.1 Peak Rate Runoff Control Standards by Stormwater Management Districts in the Neshaminy Creek Watershed
District	Design Storm Post-Development (Proposed Conditions)	Design Storm Predevelopment (Existing Conditions)
A	2-year	1-year
	5-year	5-year
	10-year	10-year
	25-year	25-year
	50-year	50-year
	100-year	100-year
B	2-year	1-year
	5-year	2-year
	10-year	5-year
	25-year	10-year
	50-year	25-year
	100-year	50-year

General. Proposed condition rates of runoff from any regulated activity shall not exceed the peak release rates of runoff from existing conditions for the design storms specified on the Stormwater Management District Map (see Appendix D of this chapter). As shown on this map, portions of the Borough are located in both District A and B.

District boundaries. The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries as part of the SWM site plan.

Sites located in more than one district. For a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall meet the management district criteria for the district in which the discharge is located.

Off-site areas. When calculating the allowable peak runoff rates, developers do not have to account for runoff draining into the subject development site from an off-site area. On-site drainage facilities shall be designed to safely convey off-site flows through the development site.

Site areas. The stormwater management site area is the only area subject to the management district criteria. Nonimpacted areas or nonregulated activities bypassing the stormwater management facilities would not be subject to the management district criteria.

Alternate criteria for redevelopment sites. For redevelopment sites, one of the following minimum design parameters shall be accomplished, whichever is most appropriate for the given site conditions as determined by the Borough:

Meet the full requirements specified by Table 304.1 and § 338-12A through F; or

Reduce the total impervious surface on the site by at least 20% based upon a comparison of existing impervious surface to proposed impervious surface.

§ 338-13 Stormwater peak rate control for areas not in Neshaminy Creek Watershed.

Areas in the Borough covered by an alternative PADEP-approved Act 167 Stormwater Management Plan (i.e., Wissahickon or Skippack Creek Watersheds) must control proposed development condition runoff rates to existing condition runoff rates for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year design storms as required by the release rate map and control standards in the applicable, approved plan.

For any areas in the Borough not included in a PADEP-approved Act 167 Stormwater Management Plan, not shown on a release rate map or without control standards, the post-development discharge rates shall not exceed the predevelopment discharge rates.

§ 338-14 Calculation methodology.

The following criteria shall be used for runoff calculations:

For development sites not considered redevelopment, the ground cover used to determine the existing conditions runoff volume and flow rate shall be as follows:

Wooded sites shall use a ground cover of "woods in good condition." A site is classified as wooded if a continuous canopy of trees exists over 1/4 acre.

The undeveloped portion of the site, including agriculture, bare earth, and fallow ground, shall be considered as "meadow in good condition," unless the natural ground cover generates a lower curve number (CN) or Rational "C" value (i.e., woods) as listed in Tables B-4 or B-7 in Appendix B of this chapter.

For development and redevelopment sites, the ground cover used to determine the existing conditions runoff volume and flow rate for the developed portion of the site shall be based upon actual land cover conditions. If the developed site contains impervious surfaces, 20% of the impervious surface area shall be considered meadow in the model for existing conditions.

Stormwater runoff peak discharges from all development sites with a drainage area equal to or greater than 200 acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 305.1 summarizes acceptable computation methods. The method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site. The Borough may allow the use of the Rational Method (Q = CIA) to estimate peak discharges from drainage areas that contain less than 200 acres.

	Where:
		Q = Peak flow rate, cubic feet per second (cfs)
		C = Runoff coefficient, dependent on land use/cover
		I = Design rainfall intensity, inches per hour
		A = Drainage area, acres

All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms according to the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding to the Doylestown rain gage, seen in Table B-1 in Appendix B of this chapter. The SCS Type II rainfall curve from NOAA is found on Figure B-1 in Appendix B of this chapter. This data may also be directly retrieved from the NOAA Atlas 14 website: https://hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html. If a hydrologic computer model, such as PSRM or HEC-1/HEC-HMS, is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours.

Table 306.1 Acceptable Computation Methodologies for Stormwater Management Plans
Method	Method Developed By	Applicability
TR-20 (or commercial computer 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/HEC-HMS	U.S. 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 200 acres, or as approved by the Borough and/or Municipal Engineer
Other methods	Varies	Other computation methodologies approved by the Borough and/or Municipal Engineer

All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from NOAA Atlas 14, Volume 2, Version 2.1. 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 the Manning Equation.

Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be based on Table B-4 in Appendix B of this chapter.

Runoff coefficients (C) for both existing and proposed conditions for use in the Rational Method shall be consistent with Table B-7 in Appendix B of this chapter.

Runoff from proposed sites graded to the subsoil will not have the same runoff conditions as the site under existing conditions because of soil compaction, even after topsoiling or seeding. The proposed condition "CN" or "C" shall increase by 5% to better reflect proposed soil conditions.

The Manning Equation is preferred for one-dimensional, gradually varied, open channel flow. In other cases, appropriate, applicable methods should be applied; however, early coordination with the Borough is necessary.

Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using the generally accepted hydraulic analysis technique or method of the Borough.

The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using the Storage Indication Method. For drainage areas greater than 200 acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The Borough may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.

§ 338-15 Other requirements.

Hot spots.

The use of infiltration BMPs is prohibited on hot spot land use areas unless specifically approved by the PADEP. Examples of hot spots are listed in Appendix G of this chapter.

Stormwater runoff from hot spot land uses shall be pretreated. In no case may the same BMP be employed consecutively to meet this requirement. Guidance regarding acceptable methods of pretreatment is located in Appendix G of this chapter.

West Nile Guidance requirements. All wet basin designs shall incorporate biologic controls consistent with the West Nile Guidance found in Appendix H of this chapter.

§ 338-16 Erosion and sediment control during regulated earth disturbance activities.

[Added 12-16-2015 by Ord. No. 2015-1881]

Additional erosion and sediment control standards and criteria are recommended to be applied where infiltration BMPs are proposed. They shall include the following:

These areas shall be protected from sedimentation and compaction during the construction phase.

BMPs shall not be constructed, nor the areas receive runoff, until the entire drainage areas tributary to the infiltration BMPs have achieved final stabilization.

§ 338-17 Nonstructural project design to minimize stormwater impacts.

[Added 12-16-2015 by Ord. No. 2015-1881]

The design of all regulated activities should include the following to minimize stormwater impacts:

The applicant should find practicable alternatives to the surface discharge of stormwater, the creation of impervious surfaces, and the degradation of waters of the commonwealth and must maintain, as much as possible, the natural hydrologic regime of the site.

An alternative is practicable if a determination is made by the Borough that it is available and capable of implementation after taking into consideration existing technology and logistics in light of overall project purposes and other municipal requirements.

§ 338-18 Groundwater recharge requirements.

[Added 12-16-2015 by Ord. No. 2015-1881]

Infiltration best management practices (BMPs) may be required by the Borough if they can be reasonably implemented based upon site conditions and will not be required if the site qualifies for an exemption from the infiltration requirements of this chapter.

Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soils and site conditions and shall be constructed on soils that have the following characteristics:

A minimum soil depth of 24 inches between the bottoms of the infiltration BMPs and bedrock or other limiting zones.

An infiltration rate sufficient to accept the additional stormwater load and dewater completely as determined by field tests conducted by the applicant's qualified professional.

All open-air infiltration facilities shall be designed to completely infiltrate the recharge (infiltration) volume (Re_v) within three days (72 hours) from the end of the design storm.

All subsurface and contained facilities such as capture-and-reuse systems must have storage available equivalent to the water volume control amount within three days (72 hours) from the end of the design storm.

(4)

Pretreatment shall be provided prior to infiltration.

(5)

The size of the infiltration facility shall be based upon the following volume criteria:

Where practicable and appropriate, the recharge volume shall be infiltrated on site. The recharge volume shall be equal to one inch of runoff (I) over all proposed impervious surfaces.

The Re_v required shall be computed as:

Re_v = (1/12) * (I)

Where:
	Re_v	=	Recharge volume (cubic feet)
	I	=	Impervious area within the limits of earth disturbance (square feet)
	An asterisk (*) in equations denotes multiplication.

Soils. A detailed soils evaluation of the project site shall be developed by the applicant to determine the suitability of infiltration facilities. The evaluation shall be performed by a qualified professional, and, at a minimum, address soil permeability, depth to bedrock, and subgrade stability. The general process for designing an infiltration BMP shall be:

Analyze hydrologic soil groups as well as natural and man-made features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration must be implemented if these tests are not completed.

Perform field tests, such as double ring infiltrometer or hydraulic conductivity tests (at the level of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Percolation tests are not accepted for design purposes.

Design the infiltration structure for the required recharge volume (Re_v) based on field tests at the elevation of the proposed infiltration surface.

(4)

If on-lot infiltration structures are proposed by the applicant's qualified professional, the applicant must demonstrate to the Borough that the soils are conducive to infiltrate on the lots identified.

(5)

The applicant must install an impermeable liner in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the Borough.

§ 338-19 Stream bank erosion requirements (channel protection).

[Added 12-16-2015 by Ord. No. 2015-1881]

If a perennial or intermittent stream passes through the site, the applicant shall create a riparian buffer extending a minimum of 50 feet to either side of the top of bank of the channel. The buffer area shall be established and maintained in an undisturbed state. This buffer area may be maintained as a meadow with minimal mowing of the grassed area, or as a forested buffer, being planted with appropriate native vegetation (refer to Appendix B of the BMP Manual for plant lists). If the applicable rear or side yard setback is less than 50 feet, the buffer width may be reduced to 25% of the setback to a minimum of 10 feet. If an existing buffer is legally prescribed (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this chapter, the existing buffer shall be maintained. The Borough may allow a smaller buffer width of not less than 10 feet based upon site-specific conditions. This does not include lakes or wetlands.

The Borough may require the following stream bank erosion/channel protection requirements if they can be reasonably implemented based upon site conditions:

In addition to the control of water quality volume (in order to minimize the impact of stormwater runoff on downstream stream bank erosion), the primary requirement is to design a BMP to detain the proposed conditions two-year, twenty-four-hour storm event to the existing conditions one-year flow using the SCS Type II distribution. Additionally, provisions shall be made (such as adding a small orifice at the bottom of the outlet structure or a sand filter) so that the proposed conditions one-year, twenty-four-hour storm event takes a minimum of 24 hours to drain from the facility from a point when the maximum volume of water from the one-year, twenty-four-hour storm event is captured (i.e., the maximum water surface elevation is achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the water volume control orifice is at the invert of the facility).

The minimum orifice size in the outlet structure of the BMP shall be three inches in diameter, where possible, and a trash rack shall be installed to prevent clogging. On sites with small drainage areas contributing to this BMP that do not provide enough runoff volume to allow a twenty-four-hour attenuation with the three-inch orifice, the calculations shall be submitted showing this condition. Orifice sizes less than three inches can be utilized, provided that the design will prevent clogging of the intake. It is recommended that the design, to accommodate maintenance, include a replaceable and/or porous media filter cartridge.

§ 338-20 Stormwater peak rate control for Wissahickon Watershed management districts.

[Added 12-16-2015 by Ord. No. 2015-1881]

Peak rate requirements.

The Wissahickon Creek Watershed has been divided into stormwater management districts as shown on the attached Management District Map. The District A peak rate requirements specified in Table 312.1 below shall be implemented in the Borough in addition to all other applicable requirements.

Standards for managing peak rates of runoff from subareas of the Borough located in the Wissahickon Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year storm events are shown in Table 312.1. Development sites located within the management district must control proposed condition runoff rates to existing condition runoff rates for the design storms in accordance with Table 312.1.

Table 312.1 Peak Rate Control Standards by Stormwater Management District A In The Wissahickon Creek Watershed
District	Proposed Condition Design Storm	Existing Condition Design Storm
A	2-year	Reduce to	1-year
	5-year		5-year
	10-year		10-year
	25-year		25-year
	50-year		50-year
	100-year		100-year

Projects that are required to obtain a NPDES permit for stormwater discharges associated with construction activities are required to show no increase in peaks from existing conditions.

(4)

When adequate capacity in the downstream system does not exist and will not be provided through improvements, the proposed conditions peak rate of runoff must be controlled to the predevelopment conditions peak rate provisions for the specified design storms. The predevelopment condition for new development is the existing condition.

District boundaries. The boundaries of the stormwater management districts are shown on an Official Map that is available for inspection at the Montgomery County Planning offices. A copy of the Official Map at a reduced scale is attached to this chapter. 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 (or most accurate data required) provided as part of the drainage plan.

Sites located in more than one district. For a proposed development site located within two or more stormwater management district, the peak discharge rate from any subarea shall meet the management district criteria in which the discharge is located.

Off-site areas. Off-site areas that drain through a proposed development site are not subject to release rate 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.

Site areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area utilizing stormwater management measures shall be subject to the management district criteria. In other words, unimpacted areas bypassing the stormwater management facilities would not be subject to the management district criteria.

Meet the full requirements specified by Table 312.1 and § 338-20A through F; or