12 2026 02 12 Crestmont CC Stormwater Management Report

ined in the table below. Table 15 – POA-1 Peak Discharge Summary – Current Storm Events POA-1 Design Storm Event Existing Peak Allowable Peak Proposed Peak Discharge (CFS) Discharge (CFS) Discharge (CFS) 2-Year 13.48 12.57 11.32 10-Year 23.47 22.52 19.76 100-Year 44.56 42.92 38.87 15

Table 16 – POA-1 Peak Discharge Summary – Future Storm Events POA-1 Design Storm Event Existing Peak Allowable Peak Proposed Peak Discharge (CFS) Discharge (CFS) Discharge (CFS) 2-Year 16.83 15.52 14.07 10-Year 29.21 27.85 24.77 100-Year 58.39 56.05 53.61 Table 17 – POA-2 Peak Discharge Summary – Current Storm Events POA-2 Design Storm Event Existing Peak Allowable Peak Proposed Peak Discharge (CFS) Discharge (CFS) Discharge (CFS) 2-Year 9.63 7.23 5.67 10-Year 17.05 14.95 10.31 100-Year 32.74 29.56 23.96 Table 18 – POA-2 Peak Discharge Summary – Future Storm Events POA-2 Design Storm Event Existing Peak Allowable Peak Proposed Peak Discharge (CFS) Discharge (CFS) Discharge (CFS) 2-Year 11.97 9.00 7.27 10-Year 21.02 18.44 14.02 100-Year 42.47 38.36 31.63 3.2 Stormwater Quality Design 3.2.1 Design Criteria The project site is subject to water quality treatment requirements as the redevelopment results in an increase in regulated motor vehicle surface coverage. In accordance with N.J.A.C. 7:8- 5.5.(a), major developments are subject to providing water quality treatment when the development results in an increase of one-quarter acre or more of regulated motor vehicle surface. The proposed redevelopment results in approximately 0.85± acres of new regulated motor vehicle surface within the limit of disturbance. Refer to Figure 8 – Required Water Quality Treatment Map for the total area of

edevelopment results in approximately 0.85± acres of new regulated motor vehicle surface within the limit of disturbance. Refer to Figure 8 – Required Water Quality Treatment Map for the total area of regulated motor vehicle surface that is subject to treatment 16

requirements. 3.2.2 Design Methodology The proposed stormwater quality BMPs that have been chosen for the project design consist of small-scale bioretention basins and a porous asphalt pavement system. Small-Scale Bioretention Basin 2 (SSBB-2) has been designed in accordance with Chapter 9.7 of the BMP Manual to achieve 80% TSS removal, meaning that it has been designed with a 24-inch deep soil bed planted site-tolerant meadow mix grasses, and can store the entire water quality design storm event runoff volume below the first orifice opening of the outlet control structure with the water surface elevations not exceeding 12 inches. Small-Scale Bioretention Basin 1 (SSBB-1) has been designed to achieve 90% TSS removal, meaning that it has been designed with a 24-inch deep soil bed planted with a terrestrial forested community, and can store the entire water quality design storm event runoff volume below the first orifice opening of the outlet control structure with the water surface elevations not exceeding 12 inches. Both bioretention basins have been design with underdrains due to the relatively poor-draining in-situ soils, as well as to minimize any potential infiltration in areas of potential karst formations. Refer to the supporting calculations in Appendix E of this report. The proposed porous pavement system (PAPS-1) has been designed to achieve 80% TSS removal. The system consists of a porous asphalt surface course, a choker course that will filter pollutants, and an underlying stone storage bed subbase. The stone storage bed layer of the porous pavement system has been adequately sized to route the 100-year design storm event; therefore, there is more than adequate capacity to store the water quality storm event runoff volume, thus meeting the criteria to achieve 80% TSS removal. The porous asphalt pavement system meets all the design criteria set forth in Chapter 9.6 of the BMP Manual. Refer to the supporting calculations in Appendix E of this report. 3.2.3 Design Summary Approximately 1.09± acres of the total 1.48± acres of proposed regulated motor vehicle surface in the area of redevelopment will be treated by a combination of small-scale bioretention basins and the porous asphalt pavement system. As part of the proposed stormwater management design, approximately 0.77± acres of existing regulated motor vehicle surface, located outside of the

ntion basins and the porous asphalt pavement system. As part of the proposed stormwater management design, approximately 0.77± acres of existing regulated motor vehicle surface, located outside of the proposed limit of disturbance area, will be captured and conveyed to the proposed BMPs for water quality treatment. The proposed TSS removal rate for the project has been calculated to be 68.2%, which is greater than the 67.2% that is required. Refer to Figure 9 – Proposed Water Quality Treatment Map for a map indicating which portions of regulated motor vehicle surface are being treated at what TSS removal rate based upon the proposed BMPs that are 17

treating those respective areas. 3.2.4 Trash and Waste Catch basins are proposed to have Type N Eco curb pieces to prevent trash and floatables from entering the proposed conveyance systems. The BMP outlet control structures will also be fixed with trash racks and rock screens to prevent debris and floatables from entering the downstream conveyance systems. 3.3 Groundwater Recharge 3.3.1 Design Criteria Per N.J.A.C. 7:8-5.4.(b).1.i, one hundred percent (100%) of the average annual pre-construction groundwater recharge volume for the disturbed area of the project site must be maintained in the post-construction condition. A geotechnical investigation was performed, with laboratory permeability testing results indicating that site soils within the proposed development area can be reclassified to HSG D. Hydrologic soil group D soils result in zero annual groundwater recharge. Therefore, there is no groundwater recharge occurring on the project site within the proposed redevelopment area. 3.3.2 Design Methodology The groundwater recharge volume analysis is performed using the NJDEP New Jersey Groundwater Recharge Spreadsheet (NJGRS), which is based on the data and computational procedures outlined in New Jersey Geological Survey Report GSR-32. The spreadsheet compares the pre-construction and post-construction land cover types within the property’s native underlying soil classifications to develop the total annual recharge volume across the project site in both the pre-construction and post-construction conditions. The post-development annual groundwater recharge deficit for the project site is zero due to all underlying soils within the proposed disturbance area being reclassified to HSG D. 3.3.3 BMP Design All proposed BMP’s have been designed to be under-drained due to poorly draining in-situ soils, and to minimize the potential for infiltration within potential underlying karst features. 18

3.3.4 Design Summary No groundwater recharge is proposed in the post-construction condition, which mirrors existing conditions in the area of redevelopment due to the presence of HSG D soils. 3.4 Non-Structural Stormwater Management Strategies Nonstructural strategies were analyzed and implemented to the maximum extent practical for this project. As per NJAC 7:8-5.3(b), there are nine nonstructural strategies: 1. Protect areas that provide water quality or are susceptible to erosion; 2. Minimize, break up and/or disconnect impervious surfaces; 3. Maximize protection of natural drainage features and vegetation; 4. Minimize decrease in time of concentration; 5. Minimize land disturbance, clearing and grading; 6. Minimize soil compaction; 7. Provide low maintenance vegetation; 8. Provide vegetated conveyance systems; and, 9. Provide pollutant source controls. Retaining walls are proposed within the redevelopment area in order to maximize the protection of existing steep slopes, trees, and underlying vegetation. Drainage swales are provided as a vegetated conveyance system in order to minimize the decrease in time of concentration. Native and drought tolerant plant species are proposed to the maximum extent feasible in order to provide low maintenance vegetation and stabilization. Areas susceptible to erosion are proposed to be lined with vegetated turf reinforcement matting, and outfalls are proposed to be stabilized with riprap aprons. Soil compaction within proposed vegetated areas will be minimized by providing decompaction measures as required by the soil conservation district. 3.5 Stormwater Conveyance Design 3.5.1 Design Criteria The on-site subsurface collection and conveyance system is designed to convey the 25-year design storm event. The proposed drainage swales were also analyzed for capacity and stability. The proposed drainage swale design is governed by the standards outlined within the NJDOT Roadway Design Manual and the Standards for Soil Erosion and Sediment Control in New Jersey. The NJDOT 19

channel design standards require utilizing the 10-year design storm and providing a minimum of 1 foot of freeboard, while the Soil Erosion standards require utilizing the 25-year design storm and providing a minimum of 0.3 feet of freeboard. Both storm events and freeboard requirements were calculated in order to determine which standard governs. The proposed off-site subsurface collection and conveyance system within the County Route 611 right-of-way is designed to convey the 25-year design storm event. Inlet spread calculations must be performed for all inlets which may have an effect on the County Road right-of-way. The inlet spread calculations analyze the spread of flow across roadways and driveways for the 10- year design storm event utilizing the minimum 10-minute duration rainfall intensity. The inlet spread calculations are performed utilizing the criteria set forth within the NJDOT Roadway Design Manual. 3.5.2 Design Methodology The conveyance system was analyzed using the rational method for estimating runoff for the 25- year design storm event. The project site was divided into subareas based upon topography to determine the contributing runoff to each individual inlet or roof drain. Weighted runoff coefficients were calculated based upon the land cover type within each delineated sub-area. A runoff coefficient (C) was selected in accordance with Table 10-4: Recommended Coefficient of Runoff Values for Various Selected Land Uses from Section 10 of the 2015 New Jersey Department of Transportation (NJDOT) Roadway Design Manual. Values of time of concentration were chosen based on land cover and slope of the flow path from the hydraulically most distant point in the subarea to the appropriate inlet. Unless otherwise specified, the minimum time of concentration used for each inlet is 10 minutes per Section 10.3.5.C.1.e of the NJDOT Roadway Design Manual. Rainfall intensities were taken from the National Oceanic and Atmospheric Administration (NOAA) Precipitation Frequency Data Server for the project site; refer to Appendix D. The proposed conveyance systems have been analyzed utilizing a starting tailwater elevation corresponding to the maximum design water elevation for the corresponding design storm event for the respective BMP that each conveyance network discharges into. The outflow from the proposed detention BMPs is inputted as a known

esign water elevation for the corresponding design storm event for the respective BMP that each conveyance network discharges into. The outflow from the proposed detention BMPs is inputted as a known discharge rate from the outlet control structure for the purpose of analyzing the capacity of the proposed conveyance system. In order to account for the time difference between the peak flow discharged from the detention BMPs and the peak flow discharged from the undetained areas of the site, the quantity control analysis performed within the Hydrology Studio software was analyzed to determine the time difference. The outflow from the detention BMP that corresponded to the time of the peak flow from the undetained watersheds was utilized as the known discharge rate for the outlet 20

control structures within the conveyance system analysis. Detailed design calculations for the stormwater conveyance system are included in Appendix D of this report. 4.0 SOIL EROSION AND SEDIMENT CONTROL Soil erosion and sediment control measures have been designed and located within the project site to minimize the amount of sediment carried by stormwater runoff, both during and after construction of the project. The SESC design was completed in accordance with the New Jersey Standards for Soil Erosion and Sediment Control. 5.0 STORMWATER MAINTENANCE PLAN The stormwater management systems for the proposed development area are intended to collect, convey, and detain the stormwater runoff. Regular maintenance procedures are required to verify the consistent and proper operation of the stormwater management facilities and prevent problems and malfunctions. The maintenance program provides the stormwater maintenance procedures for the site, which can be found under separate cover. 6.0 CONCLUSION The stormwater management systems have been designed so that the post-construction peak runoff rates meet the required peak rate reductions for the 2-, 10-, and 100-year design storm events for each point of analysis. The proposed small-scale bioretention basins and porous asphalt pavement system have been designed in accordance with the BMP Manual in order to achieve the required quantity control for the 2-, 10-, and 100-year design storm events, while simultaneously providing 80% or 90% TSS removal for water quality treatment. The proposed stormwater conveyance systems have been designed to safely and effectively convey the runoff generated from the 25-year design storm event. Therefore, the engineering design of the stormwater management systems has been performed in accordance with and meets the regulations specified under the Township of West Orange Code of Ordinances, NJDOT, and the NJDEP stormwater rules. 7.0 REFERENCES 1. Hydrology Studio. Hydrology Studio 2026 v 3.0.0.39. 2. Hydrology Studio. Stormwater Studio 2026 v 3.0.0.40. 3. Hydrology Studio. Studio Express 2026 v 1.0.0.20. 21