2022 08 Drainage Report

Vil. RUNOFF RATES The following is a comparison of the existing and proposed conditions runoff rates: Existing and Proposed Conditions Peak Runoff Rates Results Si otal NIAC 7:8 EXISTING Bate ALLOWABLE | PROPOSED RUNOFF RATE REQUIRED RUNOFF BUNGEE REDUCTION RATE RATE 2 Year 2.33 cfs 50% LIT cfs 1.17 cfs 10 Year 5.58 cfs 25% 4.19 cfs 2.49 cfs 25 Year 8.06 of Meet or reduce 8.06 cfs 3.46 cfs existing 100 Year 12.84 cfs 20% 10.27 cfs 7.51 cfs VOI. WATER QUALITY As per NJAC 7:8-5.5 for Stormwater Runoff Quality Standards, stormwater runoff quality standards are applicable when a major development results in an increase of one-quarter acre or more of regulated motor vehicle surface. The subject development results in an increase of only approximately 0.14 acres of vehicular traveled surface and is therefore not subject to stormwater quality requirements. However, nonetheless each of the four (4) proposed small-scale bioretention basins with underdrains does infiltrate the full water quality storm through the bioretention media soil bed to achieve a TSS removal rate of 80% and would satisfy the water quality requirements aspect of NJAC 7:8 if it were required. IX. GROUNDWATER RECHARGE As pteviously noted, on-site review of the tract was initially performed by Dynamic Engineering Consultants, PC to verify existing site conditions and land cover characteristics. Furthermore, Dynamic Earth, LLC performed test pits within the site to establish the seasonal high-water table and soil permeability rates. Due to the shallow water table depth and low permeability rates observed, as per Chapter 12 of the New Jersey Best Practices Manual, the soil shall be considered a D rated soil due to the characteristics observed via soils testing. Therefore, as per NJAC 7:8-5.4 Groundwater Recharge Standards, groundwater recharge requirements are not applicable to the proposed development. SHI Builders August 2022 DEC# 4266-99-001 -6-

X. STANDARDS FOR OFF-SITE STABILITY This project has been designed to satisfy the Off-Site Stability standards set forth in the Standards for Soil Erosion and Sediment Control in New Jersey. Conditions Where Practices Applies For purposes of analysis, two areas of concern shall be addressed: (1) at the point of discharge and (2) downstream of the discharge point (which may require a watershed-based analysis). Technical criteria for demonstrating off-site stability include consideration of proximity to a defined waterway, site topography (slope), soil texture, vegetative cover and other factors. Where the potential for erosive forces from stormwater runoff exceeds the threshold of acceptability as defined below, the plan shall provide for the construction of a stabilized channel, installation of a conduit to a stable condition or other types of hydraulic improvements to the channel. Additional design criteria or restrictions from the New Jersey Department of Environmental Protection or the New Jersey Pinelands Commission may also apply. Compliance with this Standard will be provided for both areas of concern: (1) at the point of discharge and (2) downstream of the discharge point. Design Criteria This Standard involves two areas of analysis: (1) at the point(s) of storm sewer discharge and (2) beyond the site boundaries, typically a receiving channel or waterway. Stability documentation for each area is outlined in detail below. Generally, the analyses involve the manipulation of peak rates of discharge for the 2 and 10 year, 24-hour storm events such that peak flow rate values and velocities are below established thresholds. Discharges shall be located in areas with low gradient topography and covered in perennial erosion resistant vegetation as noted in table 21-1 below. The proposed discharges have been designed to be located in areas with low gradient topography to the greatest extent feasible due to the exceptional topographic and physical features uniquely affecting the property, and will covered in perennial erosion resistant vegetation as noted in table 21-1. Special consideration shall be given to the use of infiltration for peak flow modifications as follows: POINT OF DISCHARGE STABILITY ANALYSIS When infiltration practices are proposed, an alternate analysis (failure analysis) must be provided which ignores infiltration (no dead

odifications as follows: POINT OF DISCHARGE STABILITY ANALYSIS When infiltration practices are proposed, an alternate analysis (failure analysis) must be provided which ignores infiltration (no dead storage volume available, no static or dynamic infiltration loss rates in the routing calculations, etc.) and demonstrates that no erosion will occur at the point of discharge if infiltration fails to function. Flow rates based solely upon basin inlet and outlet hydraulics must be used in comparison to Table 21-1 (below) to document a stable outlet. No infiltration is proposed as part of the proposed development to reduce flow rates. SHI Builders August 2022 DEC# 4266-99-001 -7-

Downstream (Off-Site) Stability Analysis Infiltration may be used to meet peak flow reduction requirements (outlined below) for the purposes of documenting stability of the downstream receiving channel, provided that the complete loss of infiltration function does not result in an increase in peak flow values above the predevelopment levels. No infiltration is proposed as part of the proposed development to reduce flow rates. Point of Discharge - Methods for Achieving Stability 1. No well-defined waterway below the point of discharge: There is no well-defined waterway below the point of discharge; therefore, this method will be used. Stability can be achieved by one of the following alternatives: A. Retain pre-existing runoff characteristics. Do no increase the amount and rate of runoff for the development and do not concentrate flows, Due to the increase in impervious surfaces for the proposed development, this alternative is not feasible; therefore, alternative “b” will be utilized as described below. B, Where there is no well-defined channel, no sandy condition, no trees or brush to substantially concentrate the flows and it can be reasonably assumed that the flow will disperse over a broad area. The combinations of slopes and soils in table 21-1 and the following criteria are considered stable for flows of 10 cfs or less for a 25 year, 24 hr design storm, SHI Builders August 2022 DEC# 4266-99-001 -8-

Table 21-1 Non-Erosive V. ies for Point Discharges Maximem Stable Slope for Point Discharges for Various Soils Soil Type Perennial, Natural Vegetation Maximum Slope (%) Sands 18 Sandy loam 2.0 Silt loam, loam 25 Sandy clay loam 3.5 Clay loam. 5.0 Graded loam to gravel 5.0 Discharge from each proposed stormwater basin will disperse over a broad area. It should be noted that existing downstream surface is vegetated an no sandy surface conditions were observed. The existing vegetation will not substantially concentrate flows. Stability Criteria (in conjunction with table 21-1) i The maximum discharge rate shall be 10 cfs or less for the twenty-five (25) year storm. The discharge rate for the proposed outfall does not exceed 10 cfs for the 25-year storm. ti, Multiple outlets may be utilized to reduce individual outlet flow rates to levels below the thresholds noted above. Outlets should be spaced no closer than 50 ft horizontally to avoid re-mixing of flows. Not applicable as the outlets are below the threshold noted above. iii, Flow over the outlet area shall be less than 0.5 cfs/ft. Designers shall not design excessive widths which will cause flows to concentrate. The proposed flows do not exceed 0.5 cfs/ft. As noted above, the peak 25-year peak flow from any of the four (4) proposed basins is 0.68 CFS. As such, the flow over the outlet area is 0.18 cfs/ft. SHI Builders August 2022 DEC# 4266-99-001 -9-

wv. Conduit outlet protection shall be provided in accordance with that Standard and may include: flat aprons, preformed scour holes, impact basins, stilling wells, plunge pools, etc. Level spreaders are not an acceptable design. The velocities existing the proposed flared end sections are within the allowable velocity in Table 12-1 of 5.0 fps as detailed below. The highest proposed velocity rate is 4.80 fps. = WW, VI E ¥ SOIL TEXTURE ALLOWABLE VELOCITY (fi/sec.) Sand 18 Sandy loam 25 Silt loam (also high lime clay), loam 3.0 Sandy clay loam 35 Ta 4.0 (Clay. fine gravel, loam to gravel 5.0 obbles: 3.3 Shale (non-weathered) 60 —— ——— v Topography shows broad uniform outlet area where flows will not concentrate. The topography downstream of the point of discharge is broad and generally uniform and flows will not concentrate. vi, Discharge locations shall contain perennial, erosion resistant vegetation. The discharge location contains perennial, erosion resistant vegetation. vii, Peak discharge velocities (in the last pipe section) do not exceed 2 fps for the 10-year storm. The maximum peak 10-year flow from any of the four (4) proposed basins is 0.20 CFS. As shown in the enclosed stormwater conveyance calculations, the velocity in the last pipe section is 0.13 fps. vil. The maximum length of slope below the outlet(s) is 100 feet. The proposed basin discharge is tributary to the downstream area and therefore this criterion is satisfied. Downstream of the Point of Dischar; ESite Stability Analysis, In addition to ensuring erosion does not occur at the point of discharge, areas downstream and beyond the immediate area of site development may be damaged due to erosive forces resulting from extended duration of hydrograph peak flows. An unintended consequence of the practice of detaining and slowly releasing stormwater is the ability for peak flows to be sustained Sor longer periods of time, offering an opportunity for upstream discharges to coincide with project site discharges. The resulting combined discharge may be equal to or even exceed that of the pre-development condition. To limit the potential for such an occurrence the designer may choose either of two approaches for downstream stability protection: 1. Examine hydraulic characteristics of the receiving stream channel considering upstream discharge in combination with site discharge to

ther of two approaches for downstream stability protection: 1. Examine hydraulic characteristics of the receiving stream channel considering upstream discharge in combination with site discharge to assess channel stability. The scope and scale of the analysis shall be appropriate to the scale of SHI Builders August 2022 DEC# 4266-99-001 -10-

the project and the post development peak discharge rate and volume. Of particular concern are hydraulic control points, (culverts, bridges, etc.), bends in streams and sudden changes in channel cross sections downstream of the discharge point. The following may be utilized to assess stability: The proposed design utilizes approach 2 as described below. 2. In lieu of performing a comprehensive watershed analysis, design a detention facility that reduces peak flows to the following levels. Infiltration may be used to meet these criteria: 2-year storm — 50% of the predevelopment peak 10-year storm — 75% of the predevelopment peak Reductions in peak flows are to be compared to pre-developed drainage area points of discharge in the event that drainage is re-directed in the post developed condition. Reductions are only required of the developed or modified portions of the project site. As documented in the subsections above, the proposed peak flows meet the above reductions, thereby achieving off-site stability. XI. CONCLUSION The proposed development has been designed with provisions for the safe and efficient control of stormwater runoff in a manner that will not adversely impact the existing drainage patterns, adjacent roadways, or adjacent parcels. In addition, the proposed development satisfies the runoff quantity, quality and groundwater recharge requirements set forth by the Township of West Orange Land Use Ordinance and NJAC 7:8 through the use of the proposed aboveground small-scale bioretention basins with underdrains. With this stated, it is evident that the proposed development will not have a negative impact on the existing drainage conditions, water quality or groundwater recharge on-site or within the vicinity of the subject site. SHI Builders August 2022 DEC# 4266-99-001 -ll-

APPENDIX

NRCS WEB SOIL SURVEY