Borough of Conshohocken, PA Part 3 STORMWATER MANAGEMENT STANDARDS

§ 19-301 General Requirements.

[Ord. No. 10-2022, 7/20/2022]

1. For all regulated activities, unless preparation of an SWM site plan is specifically exempted in § 19-302:

A. Preparation and implementation of an approved SWM site plan is required.

B. No regulated activities shall commence until the municipality issues written approval of an SWM site plan, which demonstrates compliance with the requirements of this chapter.

2. SWM site plans approved by the municipality, in accordance with § 19-406, shall be on site throughout the duration of the regulated activity.

3. The municipality may, after consultation with DEP, approve measures for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do not conflict with, state law including, but not limited to, the Clean Streams Law.[1]

[1] Editor's Note: See 35 P.S. § 691.1 et seq.

4. Erosion and Sediment Controls.

A. For all regulated earth disturbance activities, erosion and sediment control BMPs shall be designed, implemented, operated, and maintained during the regulated earth disturbance activities (e.g., during construction) to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law.[2] Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (E&S Manual³),[3] No. 363-2134-008, as amended and updated.

[2] Editor's Note: See 35 P.S. § 691.1 et seq.

[3] Editor's Note: See Part 9, References.

B. No regulated earth disturbance activities within the Borough shall commence until approval by the Borough of an erosion and sediment control plan for construction activities. Evidence of any necessary permit(s) for regulated earth disturbance activities from the DEP or Montgomery County Conservation District must be provided to the Borough prior to commencement of the regulated earth disturbance activities.

C. The DEP has regulations that require an erosion and sediment control plan for any earth disturbance activity of 5,000 square feet or more, under 25 Pa. Code § 102.4(b). The applicant is required to meet these regulations. In addition, under 25 Pa. Code Chapter 92,[4] a DEP "NPDES Construction Activities" permit is required for any earth disturbance one acre or more with a point source discharge to surface waters or the Borough's storm sewer system, or five acres or more regardless of the planned runoff. This includes earth disturbance on any portion of, part of or during any stage of, a larger common plan of development.

[4] Editor's Note: Chapter 92 of Title 25 of the Pennsylvania Code was reserved 10-8-2010, effective 10-9-2010. It was replaced by Chapter 92a, National Pollutant Discharge Elimination System (NPDES) Permitting, Monitoring and Compliance.

D. A copy of the erosion and sediment control plan and any required permit from the DEP or Montgomery County Conservation District shall be available at the project site at all times.

5. Impervious Areas.

A. 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.

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

C. For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter; except that the volume controls in § 19-303 and the peak rate controls of § 19-304 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.

D. The date of the municipal adoption of this chapter shall be the starting point from which to consider tracts as "parent tracts" in which future subdivisions and respective impervious area computations shall be cumulatively considered.

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

7. All regulated activities shall include such measures as necessary to:

A. Protect health, safety, and property.

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

(1) Minimize disturbance to floodplains, wetlands, and wooded areas.

(2) Maintain or extend riparian buffers.

(3) Avoid erosive flow conditions in natural flow pathways.

(4) Minimize thermal, physical, chemical, and biological impacts to waters of this commonwealth.

(5) Disconnect impervious surfaces by directing runoff to pervious areas, wherever possible.

(6) Protect and maintain existing uses (e.g., drinking water use; cold water fishery use) and maintain the level of water quality necessary to protect those uses in all streams, and to protect and maintain water quality in "special protection" streams, as required by statewide regulations at 25 Pa. Code Chapter 93.

C. Incorporate methods described in the Pennsylvania Stormwater Best Management Practices Manual (BMP Manual⁴).[5]

[5] Editor's Note: See Part 9, References.

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

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

10. Normally dry, open top, storage facilities shall completely drain both the volume control and rate control capacities over a period of time not less than 24 hours and not more than 72 hours from the end of the design storm.

11. The design storm volumes to be used in the analysis of peak rates of discharge shall be obtained from the latest version of the Precipitation-Frequency Atlas of the United States, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring Maryland. NOAA's Atlas 14⁵[6] can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.

[6] Editor's Note: See Part 9, References.

12. 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,[7] and the Storm Water Management Act.[8]

[7] Editor's Note: See 35 P.S. § 691.1 et seq.

[8] Editor's Note: See 32 P.S. § 680.1 et seq.

13. All SWM BMPs used to meet the requirements of this chapter shall be in accordance with the BMP Manual⁴[9] and § 22-410, Subsection 5. The more stringent requirements shall apply.

[9] Editor's Note: See Part 9, References.

§ 19-302 Exemptions.

[Ord. No. 10-2022, 7/20/2022]

1. Regulated activities that result in 600 square feet or less of additional impervious area may be exempt from the requirements in § 19-303, § 19-304, and Part 4 of this chapter. Regulated activities that result in 601 square feet to 1,000 square feet of additional impervious area may use the Simplified Approach to Stormwater Management for Small Projects included in Appendix B[1] to be exempted from the requirements in § 19-303, § 19-304, and Part 4 of this chapter and § 22-410, Subsections 3 and 5, of Chapter 22, Subdivision and Land Development. The first 1,000 square feet of additional impervious will not be exempted from projects which exceed 1,000 square feet in cumulative additional impervious except as permitted in § 19-302, Subsections 2 and 3. All Applicants seeking an exemption shall submit documentation as deemed necessary by the Borough of Conshohocken to determine compliance with the exemption criteria.

[1] Editor's Note: Said appendix is included as an attachment to this chapter.

2. Agricultural activity is exempt from the SWM site plan preparation requirements of this chapter provided the activities are performed according to the requirements of 25 Pa. Code Chapter 102.

3. Forest management and timber operations are exempt from the SWM site plan preparation requirements of this chapter provided the activities are performed according to the requirements of 25 Pa. Code Chapter 102.

4. Exemptions from any provisions of this chapter shall not relieve the applicant from the requirements in § 19-301, Subsections 4 through 11, from implementing such measures as are necessary to protect health, safety, and property, or from the responsibility to secure required permits or approvals for activities regulated by any other applicable code, rule, act or ordinance.

5. The municipality may deny or revoke any exemption pursuant to this section at any time for any project that the municipality believes may pose a threat to public health and safety or the environment.

6. Any and all exemptions shall be at the discretion of the municipality, as recommended by the Municipal Engineer, upon review of site conditions, topography, soils, and other factors as desired.

§ 19-303 Volume Controls.

[Ord. No. 10-2022, 7/20/2022]

1. 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.

2. The green infrastructure and low-impact development practices provided in the BMP Manual⁴[1] shall be utilized for all regulated activities wherever possible.

[1] Editor's Note: See Part 9, References.

3. Water volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activity areas equal or less than one acre that do not require hydrologic routing to design the stormwater facilities, this chapter establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology and other factors. All regulated activities greater than one acre and those that require hydrologic routing to design the stormwater storage facilities must use the Design Storm Method.

A. The Design Storm Method (CG-1 in the BMP Manual⁴[2]) is applicable to any size of regulated activity. This method requires detailed modeling based on site conditions.

(1) Do not increase the postdevelopment total runoff volume for all storms equal to or less than the two-year twenty-four-hour duration precipitation.

(2) For modeling purposes:

(a) Existing (predevelopment) nonforested pervious areas must be considered meadow in good condition.

(b) Twenty percent of existing impervious area, when present, shall be considered meadow in good condition in the model for existing conditions.

(c) Runoff volume must be calculated for each land use type and soil. The use of a weighted CN value for volume calculations is not acceptable.

(3) The calculated volume shall be either reused, evapotranspired, or infiltrated through structural or nonstructural means.

(4) To calculate the runoff volume (cubic feet) for existing site conditions (predevelopment) and for the proposed developed site conditions (postdevelopment), use the Soil Cover Complex Method:

Soil Cover Complex Method:
Step 1: Runoff (in) = Q = (P-0.2 S)2/(P + 0.8S)
Where:
P	=	Two-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	=	Stormwater management area (square feet)

[2] Editor's Note: See Part 9, References.

B. The Simplified Method (CG-2 in the BMP Manual⁴[3]) provided below is independent of site conditions and should be used if the Design Storm Method is not followed. This method is not applicable to regulated activities greater than one acre or for projects that require design of stormwater storage facilities. For new impervious surfaces:

(1) Stormwater facilities shall capture 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)

(2) At least the first one inch of runoff from new impervious surfaces shall be permanently removed from the runoff flow, i.e., it shall not be released into the surface waters of this commonwealth. Runoff removal options include reuse, evaporation, transpiration, and infiltration.

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

(3) Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases where soils are suitable for infiltration based on the criteria of § 22-410, Subsection 5C(1), at least the first 0.5 inch of the permanently removed runoff shall be infiltrated.

[3] Editor's Note: See Part 9, References.

C. The applicant shall demonstrate how the required volume is controlled through SWM BMPs, which shall provide the means necessary to capture, reuse, evaporate, transpire or infiltrate the required volume.

§ 19-304 Rate Controls.

[Ord. No. 10-2022, 7/20/2022]

1. Postdevelopment discharge rates shall not exceed the predevelopment discharge rates for the one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year, twenty-four-hour storm events. If it is shown that the peak rates of discharge indicated by the postdevelopment analysis are less than or equal to the peak rates of discharge indicated by the predevelopment analysis for one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year, twenty-four-hour storms for each point of interest, then the requirements of this section have been met. Otherwise, the applicant shall provide additional controls as necessary to satisfy the peak rate of discharge requirement.

2. Stormwater runoff peak discharges from all drainage areas greater than one acre shall be calculated using the NRCS Soil-Cover Complex Method. The Borough may allow the use of the Rational or Dekalb Rational Method (Q=CIA) to estimate peak discharges from drainage areas that contain one acre or less, with the support of the Borough Engineer. The method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site.

A. All calculations using the NRCS Soil-Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms obtained from the latest version of the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding to the Conshohocken station for the precipitation depth data using the upper bound of the 90% confidence interval for the various return period storms. If a hydrologic computer model is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. This data may also be directly retrieved from the NOAA Atlas 14 website: http://hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html.

(1) Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil-Cover Complex Method shall be based on Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended from time to time by NRCS).

B. All calculations using the Rational Methods shall use rainfall intensities consistent with appropriate times-of-concentration for overland flow and return periods obtained from the latest version of the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding to the Conshohocken station for the precipitation intensity using the upper bound of the 90% confidence interval for the various return period storms. If a hydrologic computer model is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. This data may also be directly retrieved from the NOAA Atlas 14 website: http://hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html.

(1) Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be consistent with Table 1 in Appendix A.[1]

[1] Editor's Note: Said appendix is included as an attachment to this chapter.

(2) 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 from time to time by NRCS). Roughness coefficients shall be consistent with Table 2 in Appendix A.[2]

[2] Editor's Note: Said appendix is included as an attachment to this chapter.

C. The Borough has the authority to require that computed existing runoff rates be reconciled with field observations and conditions.

D. The design of any SWM BMP intended to meet the rate control requirements shall be verified by routing the design storm hydrographs through the proposed facility.