New FLOOD HAZARD AREA INDIVIDUAL PERMIT ENGINEER’S REPORT Bypass... · 2018. 6. 1. · HEC-RAS backwater analysis for this tributary has been performed. This tributary will be spanned

553 Becket t Road, Suite 408Logan Tow nship, NJ 08085Tel: 856.467.3001 Fax: 856.467.3005

FLOOD HAZARD AREA INDIVIDUAL PERMIT

ENGINEER’S REPORT

FOR

U.S. ROUTE 322 BYPASS

FUTURE BYPASS RIGHT-OF-WAY BEING PORTIONS OF VARIOUS LOTS AND BLOCKS

TOWNSHIP OF HARRISON GLOUCESTER COUNTY, NEW JERSEY

PREPARED BY:

MASER CONSULTING P.A.

553 BECKETT ROAD, SUITE 408 LOGAN TOWNSHIP, NEW JERSEY 08085

__________________________ JAMES F. BIEGEN, P.E.

NEW JERSEY PROFESSIONAL ENGINEER LICENSE NO. 34322

MARCH 9, 2009 REVISED JULY 6, 2009

MC PROJECT NO. 07000608G

TABLE OF CONTENTS

PAGE

SITE DESCRIPTION 1 TRIBUTARY CHARACTERISTICS AND STORM FLOW RATES 3 HEC-RAS BACKWATER ANALYSIS 14 TRIBUTARY ROADWAY CROSSINGS 18 RIPARIAN ZONES 29 LOW IMPACT DEVELOPMENT SUMMARY 31 STORMWATER MANAGEMENT METHODOLOGY AND DESIGN 32 GROUNDWATER RECHARGE 44 WATER QUALITY 45 STORM SEWER DESIGN 48 SOIL EROSION AND SEDIMENT CONTROL MEASURES 49 OPERATION AND MAINTENANCE 50 SUMMARY CONCLUSION 51

APPENDICES

APPENDIX A PROJECT LOCATION MAP (U.S.G.S.) PROJECT SOILS MAP TAX MAP

APPENDIX B WETLAND LETTER OF INTERPRETATION

APPENDIX C NATURAL HERITAGE DATABASE DATA REQUEST RESPONSE

APPENDIX D PREDEVELOPMENT DRAINAGE ANAYLSIS

APPENDIX E PEAK STORMWATER FLOW REDUCTION CALUCLATIONS

APPENDIX F POST DEVELOPMENT DRAINAGE ANALYSIS

APPENDIX G HEC RAS BACKWATER ANALYSIS TRIBUTARY “A”

APPENDIX H HEC RAS BACKWATER ANALYSIS TRIBUTARY “B”

APPENDIX I HEC RAS BACKWATER ANALYSIS TRIBUTARY “C”

APPENDIX J HEC RAS BACKWATER ANALYSIS TRIBUTARY “D”

APPENDIX K HEC RAS BACKWATER ANALYSIS TRIBUTARY “E”

APPENDIX L GROUNDWATER RECHARGE CALCULATIONS

APPENDIX M N.J.D.E.P. NON-STRUCTURAL STRATEGIES POINT SYSTEM WORKSHEET

APPENDIX N STORM SEWER DESIGN COMPUTATIONS CONDUIT OUTLET PROTECTION CALCULATIONS RAINFALL INTENSITY CURVE TABLE BASIN RATING TABLE CALCULATIONS

APPENDIX O SOIL TEST PIT LOGS

FLOOD HAZARD AREA INDIVIDUAL PERMIT ENGINEER’S REPORT Page 1 U.S. ROUTE 322 BYPASS JULY 6, 2009 TOWNSHIP OF HARRISON, GLOUCESTER COUTY, NEW JERSEY 07000608G

SITE DESCRIPTION

Gloucester County proposes to construct a two lane roadway to bypass the existing roadway

network in the vicinity of Mullica Hill to alleviate existing traffic congestion. The route of the

bypass is approximately 1.3 miles in total length. The route of the future roadway will cross five (5)

waters of the State of New Jersey. These waters have associated riparian zones and floodplains

which a Flood Hazard Verification Application has been submitted for approval by the NJDEP.

This report has been prepared in support of the N.J.D.E.P. Flood Hazard Individual Permit

Application. This report and supporting documentation utilizing the Flood Hazard Verification

Study prepared by Maser Consulting will demonstrate compliance with stormwater management

and flood hazard regulations for the proposed roadway improvements. This report will provide the

design for the stormwater management facilities; document the water quality and groundwater

recharge designs, include a discussion on low impact development and discuss the impacts of the

roadway construction on the existing floodplains and riparian zones. The information contained

within this report has been indicated on the plans entitled “Flood Hazard Area Individual Permit

Plan for US Route 322 Bypass” sheets 1 through 38 of 38, dated February 27, 2009 as revised. This

report has two primary components, impacts to the Flood Hazard Areas and Riparian Zones, and

documentation of Stormwater Management for the actual proposed roadway improvement.

This report was prepared following currently accepted engineering standards, including the

Standards for Soil Erosion and Sediment Control in New Jersey and N.J.D.E.P. Flood Hazard Area

Control Rules (N.J.A.C. 7:13).

As determined by the N.J.D.E.P., freshwater wetlands or other regulated environmental features are

present in the course of the future right-of-way pursuant to the Freshwater Wetlands Letter of

Interpretation/Line Verification; file number 0808-08-0002.1-FWW080001 dated November 6,

2008.

The U.S. Department of Agriculture and the Soil Conservation Service (SCS), in cooperation with

the New Jersey Agricultural Experiment Station, has prepared the Soil Survey of Gloucester

County, New Jersey. The U.S. Department of Agriculture and the Natural Resources Conservation

Service also prepared a National Cooperative Soil Survey – Web Soil Survey. These surveys


contain data regarding soils and shallow subsurface conditions throughout the County and Country

respectively. This information is useful at the planning level to draw general conclusions about the

suitability of a site for certain land uses.

According to both the published County Soil Survey and Web Soil Survey, the soils types

underlying the site and contributory drainage area include the following:

Soil Label Soil Name H.S.G.

AvsC Aura-Sassafras Loamy Sands B BEXAS Berryland and Mullica Soils, Occasionally Flooded B/D BumA Buddtown-Deptford Complex B BuuB Buddtown-Urban Land Complex B DocB Downer Loamy Sand B FapA Fallsington Loam B/D

FmhAt Fluvaquents, Loamy, Frequently Flooded D FrkA/FrkB Freehold Sandy Loam B

FrrB Freehold-Urban Land Complex B JdrA Jade Run Fine Sandy Loam B

MaoB/MaoC2 Marlton Sandy Loam C MauB Marlton-Urban Land Complex C

SabC/SabD Sassafras Loamy Sand B SacC Sassafras Sandy Loam B SapB Sassafras-Urban Land Complex B

UdauB Udorthents-Urban Land Complex B WeeB/WeeD Westphalia Fine Sandy Loam B WehB/WehC Westphalia-Urban Land Complex B

WoeA Woodstown Sandy Loam C WokA Woodstown-Glassboro Complex C WooB Woodstown-Urban Land Complex C


Contained within portions of each contributory drainage areas are underlying acid producing soil

geology. According to the published N.J.D.E.P. I-MAP Geology data, the following geological

deposits underlay the study areas are:

Geological Symbol Geological Name Possible Acid Producing

Tkl Lower Member Kirkwood Formation Possible Acid Producing Deposit

Tvt Vincetown Formation Non-Acid Producing Deposit

Tht Hornerstown Formation Non-Acid Producing Deposit

Kns Navesink Formation Possible Acid Producing Deposit

Kml Mt. Laurel Formation Non-Acid Producing Deposit

TRIBUTARY CHARACTERISTICS AND STORM FLOW RATES

The future roadway improvement consisting of 1.3 miles of new two (2) lane roadway and

associated right-of-way is located in the Township of Harrison, Gloucester County, New Jersey.

This proposed right-of-way and properties which the proposed right-of-way is being carved out of is

considered the project site. The future right-of way will be constructed to the north of Mullica Hill

through residential, woodlands and agricultural areas. The future right-of-way will cross within

wetland areas and cross five (5) existing tributaries. As previously documented in the Flood

Verification Report, each tributary has been named as follows:

Tributary “A” An easterly flowing tributary located to the north of Mullica Hill discharging into the Jefferson Branch of Raccoon Creek (Tributary “B”). At the location of the future right-of-way, the contributory drainage area based on USGS mapping is 79 acres. In accordance with the Flood Hazard Regulations, this tributary at this location will have an associated riparian buffer and a regulated floodplain. A HEC-RAS backwater analysis for this tributary has been performed. This tributary will be spanned by a three (3) sided box culvert with a buried foundation extending the width of the channel, backfilled with natural materials. The HEC-RAS calculations have been completed to demonstrate that there is no increase in peak water surface elevations.


Tributary “B” A southerly flowing tributary located to the northeast of Mullica Hill discharging into Raccoon Creek near the center of Mullica Hill. The tributary is also known as the Jefferson Branch of Raccoon Creek. At this report’s point of analysis, which is located at a culvert crossing downstream of the proposed right-of-way, the contributory drainage area is 442 acres. In accordance with the Flood Hazard Regulations, this tributary will have an associated riparian buffer and a regulated floodplain. This tributary will be spanned by a precast arch. HEC-RAS calculations have been completed to demonstrate that there is no change in the existing stream elevations.

Tributary “C” A westerly flowing tributary located to the west of Walters Road in

the middle of an existing agricultural field discharging into Raccoon Creek to the north of the Mullica Hill Pond. The tributary is unnamed. At this report’s point of analysis, which is located at the crossing of an abandoned railroad grade, the contributory drainage area is 91 acres. In accordance with the Flood Hazard Regulations, the tributary will have an associated riparian buffer and a regulated floodplain. This tributary will be spanned by a three (3) sided box culvert with a buried foundation extending the width of the channel, backfilled with natural materials. Proposed HEC-RAS calculations have been completed to demonstrate that there is no change in the existing stream elevations.

Tributary “D” A westerly flowing tributary located to the west of Walters Road and

to the immediate north of the abandoned railroad grade discharging into Tributary “C”. The tributary is unnamed. At this report’s point of analysis, which is located at the confluence with Tributary “C”, the contributory drainage area is 93 acres. In accordance with the Flood Hazard Regulations, the tributary will have an associated riparian buffer and a regulated floodplain. The roadway will fill an area of the existing floodplain. This report will demonstrate that there is no increase in the existing elevation of the floodplain by a routing analysis. Proposed HEC-RAS calculations have been completed to demonstrate that there is no change in the existing offsite stream elevations.


Tributary “E” A westerly flowing tributary and agricultural ditch located to the north of the Clems Run Road intersection ultimately discharging to the Raccoon Creek to the north of the Mullica Hill Pond. The tributary is unnamed. At this report’s point of analysis, which is located at the culvert crossing of the existing Route 322, the contributory drainage area is 64 acres. In accordance with the Flood Hazard Regulations, the tributary will have an associated riparian buffer and a regulated floodplain. The crossing will be accomplished by removing the existing 24” culvert and replacing the structure with an elliptical culvert with half the section backfilled with natural materials. This report will demonstrate that the possibility of existing flooding on the existing roadway has been reduced or eliminated by providing a hydraulically efficient culvert. This report will demonstrate by a flood routing in concert with the stormwater management mitigation that there are reductions in the peak flow rates for storm events studied.

Tributary “F” An unnamed southerly flowing tributary crossing Swedesboro Road

(Rt. 322) approximately 500 feet to the west of the road improvements. An existing stormwater collection system discharges to the tributary. This tributary does not require a flood study since no improvements are proposed at the crossing. Runoff from the intersection of Mullica Hill Road and Bridgeton Pike are contributory to the drainage area. Existing drainage patterns will remain unchanged.


For reference, the following is a diagram of the tributary and road network surrounding the study

area:

The existing tributaries all drain to the Raccoon Creek and Birch Creek watershed which ultimately

drains to the Delaware River. The site is identified on State mapping as being located within

Planning Area PA-3. Within the HUC-14 watershed, there are currently no Category 1 waters as

defined by NJ I-map.

It should be noted that prior Stream Encroachment permits have been issued by N.J.D.E.P. for

Tributary “B”, Tributary “C” and Tributary “E” as part of previous residential subdivision

applications. The prior permits established the associated floodplain for these tributaries based

upon prior regulations. Tributary “B” and “C” have floodplains studies approved under N.J.D.E.P.

Permit Number 0808-04-04-0014.2 FHA 0600021 and Tributary “E” has been studied under

NJDEP Permit Number 0808-05-0012.2 FHA 06002. These reports have been superseded by the

Flood Verification Study report and the prior permits superseded by this Flood Hazard Individual

Permit Report.


The drainage areas used to analyze and calculate the peak rates of runoff for the study area were

established based upon the hydrologic limit of disturbance of the existing and proposed topography.

The following is a listing of the drainage areas used in this report and a general description of the

location:

OVERALL TRIBUTARY DRAINAGE AREAS

A-1 A portion of the contributory drainage area of Tributary “A” to the future

right-of-way crossing. This area is a mixture of developed residential areas and undeveloped woodlands. Field reconnaissance has established the western boundary of this drainage area is the centerline of NJSH Route 45.

A-2 The remaining contributory drainage area of Tributary “A” to the confluence

of Tributary “A” with Tributary “B”. B-3 A headwater portion of the contributory drainage area of Tributary “B”.

This area is primarily located north of Breakneck Road and consists of baseball and soccer fields.

B-4 A headwater portion of the contributory drainage area of Tributary “B”. This

area is primarily located between Colson Lane, Breakneck Road and Cedar Road. This area is a mixture of developed areas including Clearview High School, residential areas, agricultural areas and woodlands.

B-5 The contributory drainage area to the lower reaches of Tributary “B” to the

confluence with Tributary “A”. This area is primarily located to the south of Colson Lane. This area primarily contains agricultural areas and woodlands with some residential areas.

B-6 The contributory drainage area to the lower reaches of Tributary “B” from

the confluence with Tributary “A” to the point of analysis. This area is primarily located to the south of Colson Lane. This area primarily contains agricultural areas and woodlands with some residential areas.

C-7 The total contributory drainage area to Tributary “C”. This area is located to

the north of the abandoned railroad grade and crosses beyond Walters Road. The area primarily contains agricultural areas and woodlands.

D-8 The total contributory drainage area to Tributary “D”. This area is located to

the north of the abandoned railroad grade and crosses beyond Walters Road. The area primarily contains agricultural areas, woodlands and a portion of the newly constructed soccer complex.

E-9 The contributory drainage area to Tributary “E” at the existing US Route 322

roadway crossing. This area is located to the north of Mullica Hill Road (US Route 322) and to the east of Walters Road. The area primarily contains agricultural areas, woodlands with some residential areas. In the eastern


section of this drainage area, approximately 15 acres of this drainage area based on field reconnaissance may not contribute flow and is redirected by an established residential development to discharge to drainage area D-8. However, to be conservative and consistent with NJDEP practice and advice, the drainage area has been determined based upon the USGS map.

E-10 The total remaining drainage area to Tributary “E” downstream of the

existing US Route 322 roadway crossing to Tributary “E” station 5+00. This area is generally located to the south of Mullica Hill Road (US Route 322). The area primarily contains agricultural areas, woodlands with some residential areas.

F-11 The contributory drainage area to existing stormwater collection facilities

located to the west of NJSH Route 45. This area primarily consists of existing impervious surfaces from the existing Route 322 (Swedesboro Road), a restaurant parking lot and a portion of a shopping center parking lot. This area was not included in the Flood Hazard Verification Report since the area does not influence on of the studied tributaries, however, the stormwater runoff from the area eventually discharges to Raccoon Creek. This area drains to Tributary F. No stormwater basins are proposed in this drainage area as there has been a limited increase in impervious surfaces. The overall analysis demonstrates that mitigation in this area is not required.

G-12 A contributory drainage area located directly to the south of the proposed

bypass roadway which contributes runoff directly to the Raccoon Creek point of analysis. This area has been included within the overall model as the farmed field portion of the drainage area and will be converted to grass and meadow.


TRIBUTARIES

Tributary “B” Basin The former railroad grade (currently occupied by an unimproved extension

of Chatham Lane) creates an obstruction along Tributary “B” during storm events thereby creating a ponding area. The existing culverts are essential an outlet control structure. The existing tributary base flow is obstructed by the culvert. The railroad grade is significantly elevated from the valley and does not appear to become over-toped. The tailwater created from the ponding area will be utilized as the tailwater elevation for the HEC-RAS analysis.

Tributary “D” Basin The former railroad grade creates an obstruction along Tributary “D” during

storm events thereby creating a ponding area. The existing culverts are essential an outlet control structure. The existing tributary base flow is unobstructed by the culvert. At this location, the railroad grade may be overtopped by the 100-year storm event. The tailwater created from the ponding area will be utilized as the tailwater elevation for the HEC-RAS analysis.

Tributary “E” Basin A flood routing which included the ponding effect of the existing roadway

was not previously included in the Flood Verification Study. As a result of the roadway construction, it is necessary to document the existing and proposed flow rates due to the impact of the proposed culvert being installed in the area.


The following is a network diagram for the hydrology model utilized for the establishment of the peak flow rates for the various drainage areas and tributaries: FUTURE US ROUTE 322 BYPASS TRIBUTARY HYDROLOGY NETWORK DIAGRAM MC PROJECT No. 07-000608G


A stormwater model has been completed that establishes the peak regulatory flow rates for

floodplain management and use within a HEC-RAS backwater analysis model. To meet the

technical standards established by the NJDEP Flood Hazard Area Control Act Rules N.J.A.C. 7:13,

the stormwater model is based upon the peak flow from a 1% annual chance storm event plus a

safety factor of 25% of the peak flow rates. To evaluate the proposed contributory drainage area for

compliance with the above standards, the Soil Conservation Service unit hydrograph method and

the Pondpack version 10.0 hydrologic/hydraulic modeling software from Bentley Systems, Inc. was

utilized. The software utilized is a computer model based upon USDA Technical Release 55 Urban

Hydrology for Small Watersheds.

As required by the local soil conservation district, this contributory drainage has been modeled

using the Delmarva Unit Peak Factor unitless hydrograph. The contributory drainage areas have

been modeled based upon the existing conditions obtained from aerial photography. Prior permit

applications associated with Tributaries “B”, “C” and “E” based peak flow rates on future build out

conditions and current municipal zoning. This approach to modeling within the regulation has been

abandoned in favor of estimating the peak flow rates on existing conditions plus a factor of safety of

25%.

Existing peak stormwater flows were calculated in the prior verification report based upon the

current design storms for Gloucester County. The following table establishes the contributory

drainage area, CN number, time of concentration, peak flow rate for the 1% annual chance storm for

each contributory drainage area, which remain the same as the information provided in the

verification report (See Appendix for calculations.):


PEAK FLOW SUMMARY TABLE PER DRAINAGE AREA

USED FOR FLOOD HAZARD DETERMINATION

Drainage Area

Area (Acres)

CN TC

(min) Peak Flow (c.f.s)

A‐1 78.8 76 34 185 c.f.s.

A‐2 24.8 70 31 54 c.f.s. B‐3 38.9 67 30 79 c.f.s. B‐4 190.9 72 59 289 c.f.s B‐5 96.7 71 49 161 c.f.s B‐6 38.2 68 32 77 c.f.s C‐7 91.1 71 46 159 c.f.s D‐8 92.7 67 46 148 c.f.s E‐9 63.6 65 79 65 c.f.s

E‐10 10.8 70 32 23 c.f.s

The following table establishes the peak flow rate for the 1% annual chance storm for each studied

tributary and the 25% safety factor that has been used to complete the HEC-RAS backwater

analysis. All stormwater peak flow rates have been computed utilizing TR-55 methodology (See

Appendix for calculations.):

PEAK FLOW SUMMARY TABLE FOR FLOOD HAZARD ELEVATION FLOW RATE AND

FLOOD HAZARD AREA DESIGN FLOW RATE USED FOR HEC-RAS

Tributary Tributary Station

HEC‐RAS River Sections

Future Road Station Crossing

Peak Flow per Flood

Verification Report (c.f.s.)

Peak Flow + 25% (c.f.s.)

Tributary “A” 30+00 RS 11 to RS 6 25+90 185 c.f.s. 231 c.f.s. Tributary “A” 23+00 RS 6 to RS 1 n/a 238 c.f.s. 298 c.f.s. Tributary “B” 37+50 RS 31 to RS 12 36+35 487 c.f.s. 609 c.f.s. Tributary “B” 21+00 RS 12 to RS 1 n/a 758 c.f.s. 947 c.f.s. Tributary “C” 25+00 RS 26 to RS 4 53+50 159 c.f.s. 199 c.f.s. Tributary “C” 11+50 RS 4 to RS 1 n/a 206 c.f.s. 258 c.f.s. Tributary “D” 17+00 RS 13 to RS 1 63+50 148 c.f.s. 185 c.f.s. Tributary “E” 17+50 RS 22 to RS 7 77+75 65 c.f.s. 82 c.f.s. Tributary “E” 12+50 RS 7 to RS 1 n/a 82 c.f.s. 103 c.f.s.


There is a proposed modification to the floodplain storage for Tributaries “B”, “D” and “E”. The

following three tables provide the existing and proposed peak flow rates and static flood elevations

at the controlling structures.

TRIBUTARY “B” EXISTING AND PROPOSED PEAK FLOW AND STATIC ELEVATIONS

Storm Event Existing Peak

Flow Proposed Peak

Flow Existing Flood Elevation

Proposed Flood

Elevation

WQ Storm 13 c.f.s. 13 c.f.s. 30.0 ft. 30.0 ft. 2 Yr Storm 91 c.f.s. 90 c.f.s. 38.8 ft. 38.7 ft. 10 Yr Storm 119 c.f.s. 119 c.f.s. 45.2 ft. 45.1 ft. 25 Yr Storm 130 c.f.s. 129 c.f.s. 48.1 ft. 48.1 ft. 100 Yr Storm 144 c.f.s. 144 c.f.s. 52.4 ft. 52.5 ft.

TRIBUTARY “D” EXISTING AND PROPOSED PEAK FLOW AND STATIC ELEVATIONS


Flow Proposed Peak


Proposed Flood

Elevation

WQ Storm 1 c.f.s. 1 c.f.s. 63.8 ft. 63.9 ft. 2 Yr Storm 16 c.f.s. 15 c.f.s. 67.7 ft. 67.1 ft. 10 Yr Storm 26 c.f.s. 34 c.f.s. 72.2 ft. 72.4 ft. 25 Yr Storm 49 c.f.s. 51 c.f.s. 72.5 ft. 72.6 ft. 100 Yr Storm 117 c.f.s. 114 c.f.s. 72.8 ft. 72.9 ft.

TRIBUTARY “E” EXISTING AND PROPOSED PEAK FLOW AND STATIC ELEVATIONS


Flow Proposed Peak


Proposed Flood

Elevation

WQ Storm 0.2 c.f.s. 0.2 c.f.s. 77.7 ft. 77.2 ft. 2 Yr Storm 8 c.f.s. 8 c.f.s. 79.0 ft. 77.4 ft. 10 Yr Storm 22 c.f.s. 22 c.f.s. 80.6 ft. 77.7 ft. 25 Yr Storm 37 c.f.s. 35 c.f.s. 81.1 ft. 78.0 ft. 100 Yr Storm 67 c.f.s. 63 c.f.s. 81.2 ft. 78.6 ft.


As shown above, there are no appreciable changes to the flow and water surface elevations for

Tributary “B”, “D” and “E”. For clarification, within Tributary “E” there appears to be a change in

the peak flood elevations, which was an anticipated result of the calculations. The HEC-RAS

Backwater Analysis study shows that for Tributary “E”, the water surface elevations in the studied

cross-sections remain within regulatory limits. The decrease in the flood elevation is due to the

relocation of the storage area, providing a larger more efficient culvert and due to the adjustment in

the contributory drainage area. In the vicinity of Tributary “E”, in order to obtain the peak flow

reductions, a portion of the drainage area has been redirected into Tributary “D”. The cross-section

summary table provided in the HEC-RAS Backwater Analysis discussion provides the final

justification for this apparent increase in elevations. The preceding tables demonstrate that the fill

proposed within the flood plain has no significant effect, thereby supporting the hardship waiver for

fill in a flood plain.

HEC-RAS BACKWATER ANALYSIS

The Flood Hazard Verification Report established the flood plain elevations for Tributaries “A”,

“B”, “C”, “D” and “E” in accordance with N.J.A.C. 7:13-3.6, the flood hazard verification has been

prepared utilizing NJDEP Method 6. A proposed HEC-RAS Backwater Analysis has been prepared

for Tributaries “A”, “B”, “C”, “D” and “E” since new culverts and structures are proposed. The 1%

annual chance flood establishing the floodplain and the 125% flow establishing the flood hazard

area have been shown on the Flood Hazard Individual Permit Plan.

The limits of the flood hazard delineation are generally shown on the plans 500 feet upstream and

500 feet downstream of the proposed roadway crossing. The backwater calculations utilized

additional cross-section beyond the range of the required study to establish for the model tailwater

boundary conditions. In addition, interpolated cross-section has been generated by the HEC-RAS

software based upon the known cross-sections to resolve the energy equations. The backwater

analysis was computed based upon a steady flow simulation.

Various Manning’s Roughness coefficients have been utilized in preparing the stream modeling

which have been assigned based upon the land cover the reach may traverse. In general, the

following Manning coefficients were used for the tributary channels:


Clean, straight, full stage, no rift or deep pools 0.030

Clean, winding, some pools and shoals 0.040

Clean, winding, some pools and shoals, but some

Weeds and stones 0.045

Sluggish reaches, weedy, deep pools 0.070

Very weedy reaches 0.100

Outside of the main channel areas, the following Manning’s Roughness coefficients have been used

for the floodplain areas:

Heavy stand of timber, a few down trees,

Little undergrowth, flood stage below branches 0.100

Heavy stand of timber, a few down trees,

Little undergrowth, flood stage reaching branches 0.120

Scattered brush, heavy weeds 0.050

Pasture, no brush, short grass 0.030

Cultivated areas, mature row crops 0.035

Cultivated areas, mature field crops 0.040

The plans include a delineation of the flood hazard area on the studied tributaries. In addition, the

plan denotes the location of site color photographs which have been provided in the supplemental

report entitled “Flood Hazard Verification Color Photograph Supplement” dated December 12,

2008. Metes and Bounds have been provided on the plans in the area of the future right-of-way for

the flood hazard area.


The following tables summarize the flood elevation at the cross-sections in the vicinity of the

proposed roadway crossings to demonstrate that the proposed floodplain elevations remain

substantially similar to the existing floodplain elevations. The computed flood elevations computed

with HEC-RAS where compiled based upon the existing design flow plus 25% and therefore do not

represent the reduced peak flow rates computed in the post-development analysis.

TRIBUTARY “A” EXISTING AND PROPOSED FLOOD ELEVATIONS

Tributary Cross‐Section

Existing Flood

Elevation

Proposed Flood

Elevation

Net Change in Flood

Elevation

A‐1 52.35 ft. 52.35 ft. 0.00 ft. A‐2 52.36 ft. 52.36 ft. 0.00 ft. A‐3 52.38 ft. 52.38 ft. 0.00 ft. A‐4 52.41 ft. 52.41 ft. 0.00 ft. A‐5 52.44 ft. 52.44 ft. 0.00 ft. A‐6 52.64 ft. 52.64 ft. 0.00 ft. A‐7 59.72 ft. 59.39 ft. ‐0.33 ft. A‐8 61.52 ft. 61.52 ft. 0.00 ft. A‐9 62.41 ft. 64.21 ft. 0.00 ft. A‐10 65.38 ft. 65.38 ft. 0.00 ft. A‐11 66.57 ft. 66.57 ft. 0.00 ft.

TRIBUTARY “B” EXISTING AND PROPOSED FLOOD ELEVATIONS


Existing Flood

Elevation

Proposed Flood

Elevation

Net Change in Flood

Elevation

B‐16 52.43 ft. 52.43 ft. 0.00 ft. B‐17 52.44 ft. 52.44 ft. 0.00 ft. B‐18 52.44 ft. 52.42 ft. ‐0.02 ft. B‐20 52.46 ft. 52.50 ft. 0.04 ft. B‐21 52.50 ft. 52.56 ft. 0.06 ft. B‐22 52.54 ft. 52.60 ft. 0.06 ft. B‐23 52.57 ft. 52.63 ft. 0.06 ft. B‐24 52.63 ft. 52.69 ft. 0.06 ft. B‐25 52.69 ft. 52.75 ft. 0.06 ft. B‐26 53.04 ft. 53.08 ft. 0.04 ft.


TRIBUTARY “C” EXISTING AND PROPOSED FLOOD ELEVATIONS


Existing Flood

Elevation

Proposed Flood

Elevation

Net Change in Flood

Elevation

C‐5 61.73 ft. 61.73 ft. 0.00 ft. C‐6 61.75 ft. 61.75 ft. 0.00 ft. C‐10 65.23 ft. 64.80 ft. ‐0.43 ft. C‐11 65.89 ft. 65.65 ft. ‐0.24 ft. C‐12 66.91 ft. 66.90 ft. ‐0.01 ft. C‐13 67.82 ft. 67.42 ft. ‐0.40 ft.

TRIBUTARY “D” EXISTING AND PROPOSED FLOOD ELEVATIONS


Existing Flood

Elevation

Proposed Flood

Elevation

Net Change in Flood

Elevation

D‐1 72.90 ft. 73.00 ft. 0.10 ft. D‐2 72.90 ft. 73.00 ft. 0.10 ft. D‐3 72.91 ft. 73.01 ft. 0.10 ft. D‐9 78.04 ft. 78.08 ft. 0.04 ft. D‐10 78.04 ft. 78.08 ft. 0.04 ft. D‐11 78.04 ft. 78.08 ft. 0.04 ft.

TRIBUTARY “E” EXISTING AND PROPOSED FLOOD ELEVATIONS


Existing Flood

Elevation

Proposed Flood

Elevation

Net Change in Flood

Elevation

E‐5 77.20 ft. 77.19 ft. ‐0.01 ft. E‐6 77.77 ft. 77.79 ft. 0.02 ft. E‐7 79.47 ft. 79.42 ft. ‐0.05 ft. E‐12 81.54 ft. 80.96 ft. ‐0.58 ft. E‐13 81.69 ft. 81.52 ft. ‐0.17 ft. E‐14 81.88 ft. 81.82 ft. ‐0.06 ft. E‐15 82.12 ft. 82.11 ft. ‐0.01 ft.


As shown in the tables above, the flood hazard elevations satisfy N.J.A.C. 7:13-11.7(c) since there

is less than 0.2 feet of impact the flood hazard water surface elevation for the proposed roadway

crossings.

In the existing conditions, the calculations in the verification report suggested that Walter’s Road

would be overtopped by the design flood. The design plans indicate that the existing pavement on

Walter’s Road will be removed and re-forested (for riparian zone compensation); however, the road

embankment will not be removed, so that existing flow conditions will remain unchanged. The

overtopping water will be conveyed to Tributary “D’s” channel by a proposed culvert as noted on

the plans. The existing culvert will be lengthened beneath the proposed roadway. The proposed

roadway will not be overtopped by the design storm.

TRIBUTARY ROADWAY CROSSINGS

There are a total of five (5) tributary crossings required for the Mullica Hill By-pass. The following

table demonstrates the proposed freeboard at each tributary crossing:

ROADWAY FREEBOARD

Roadway Station

Tributary Proposed Flood

Elevation

Proposed Roadway Elevation

Roadway Freeboard

25+50 Tributary “A” 59.72 ft. >62.00 ft. > 2.28 ft. 36+50 Tributary “B” 52.50 ft. >65.00 ft. >12.50 ft. 53+50 Tributary “C” 64.80 ft. >70.50 ft. >5.7 ft. 63+00 Tributary “D” 73.00 ft. >75.00 ft. >2.0 ft. 77+50 Tributary “E” 80.96 ft. >82.50 ft. >1.29 ft.

Pursuant to N.J.A.C. 7:13-11.6, the proposed roadway will be constructed with at least one (1) foot

of freeboard between the proposed flood hazard elevation and lowest elevation of the roadway. It

should be noted, that in existing conditions, a portion of Walter’s Road and Mullica Hill Road by

Clems Run Road were predicted to overtop during the flood hazard design storm. The existing

condition on Mullica Hill has been eliminated by providing additional culvert capacity.


As referenced in N.J.A.C. 7:13-11.7, each of the tributaries has a classification which is utilized to

determine the preference of the proposed crossings. The Flood Hazard Design Manual offers the

following general definitions of classifications:

Class A waters include the highest quality waters, such as Category One waters and waters containing certain fishery resources, within which aquatic biota require the highest level of protection. Class B waters include a set of waters that have less ecological value and, therefore, do not warrant the same level of protection as Class A waters. Lastly, Class C waters are those of low ecological value, such as those which contain no fishery resources or which are wholly manmade, and which, therefore, do not warrant as much protection as other regulated waters.

Using the classifications, the tributaries can be classified as follows:

Tributary “A” The tributary is a Class “B” water that has a channel width of about

five (5) feet. A pre-cast three-sided culvert with cast in-place footings is proposed. The top width of the culvert is eight (8) feet and the effective height of the culvert is three (3) feet, thereby leaving two (2) feet of fill at the bottom of the culvert.

.


Tributary “B” The tributary is a Class “B” water that has a channel width of about

10 feet. A precast concrete arch with cast in-place footings is proposed to span the floodway. The bottom width of the arch is approximately 66 feet.


Tributary “C” The tributary is a Class “B” water that has a channel width of about

10 feet. A pre-cast three-sided culvert with cast in-place footings is proposed. The top width of the culvert is ten (10) feet and the effective height of the culvert is four (4) feet, thereby leaving two (2) feet of fill at the bottom of the culvert.


Tributary “D” The tributary is a Class “C” water that has an existing 30 inch

concrete pipe forming the primary channel beneath Walter’s Road. This existing culvert will be extended to convey the stormwater beneath the proposed bypass. The existing pipe cannot be removed, due to a Green Acres restriction, the channel enhanced without increase flood elevations downstream of the crossing. Therefore, due to the limiting criteria, the tributary at the Walter’s Road location cannot be classified as Class “B”. The existing culvert’s upstream headwall extends onto lands funded by Green Acres. The existing culvert will be extended to provide conveyance beneath the proposed bypass for low-flow conditions. Since Walter’s Road is predicted to be overtopped during the 100-year storm event, a secondary side-relief culvert has been provided to convey the excess stormwater peak flow. Backfilled culvert are not proposed at this crossing.


Tributary “E” The tributary is a Class “C” water that has a channel width of less

than five (5) feet. A 58 inch by 91 inch elliptical concrete pipe is proposed for the crossing. Half of the elliptical concrete pipe will be backfilled with natural materials.

Tributary “F” No study of tributary “F” is required. The crossing located to the west of Mullica Hill is only included in this report as a point of analysis. No improvement to the roadway in the vicinity of Tributary “F” is proposed at this time.

The Flood Hazard Verification plan provides the construction details for the proposed crossings as

summarized in the above sketches.

A backfilled elliptical concrete pipe has been provided for tributary crossing “E” as the tributary can

be classified as a Class “C” water. Tributary “E” is a Class “C” water in the vicinity of the existing

roadway crossing and within the contributory agricultural ditch since it does not contain a fishery

resource, there are no category one waters within the watershed, the ditch cannot support trout or

anadromous species, and in general the drainage way is a manmade feature. Specifically, the

tributary is manmade since the feature is centered along a straight property line between two farm

fields. The tributary conveys surface runoff from the agricultural fields and does not contain a

water, nor does the feature appear to convey groundwater. The classification of Tributary “E” as a

Class “C” water is appropriate since the tributary is not documented on the USGS or County Soil

Survey. The following aerial photographs document the progression of the agricultural ditch

supporting the Class “C” conclusion:


Tributary “E” Aerial Photograph circa 1930. As shown in the above photograph, the wetlands and

ditch are not observable although there appears to be a drainage pattern that follows the existing

property line. The entire area is an agricultural field on both sides of the appears to be the same

crop.


ditch are not observable. The entire area is an agricultural field.



ditch are not observable. The entire area is remains an agricultural field.

Tributary “E” Aerial Photograph circa 1990. As shown in the above photograph, the existing

conditions of the field indicate a changing drainage pattern in the field. The location of the ditch is

now appearing on the photograph as it was created as a result of agricultural activities.


Tributary “E” Aerial Photograph circa 2007. As shown in the above photograph from Google

Earth imagery, the existing conditions of the field have significantly changed from 1980. The

agricultural ditch which became evident in 1990 has continued to regress into a tree line feature.

Utilizing the aerial photograph, it can be concluded that the feature of Tributary “E” has developed

as a result of agricultural practices and is a manmade ditch and was not occurring prior to 1990.

Due to the Class “C” classification, the proposed elliptical pipe backfilled with natural materials is

appropriate since there is insufficient cover for the construction of a three-sided culvert.

In order to provide the necessary crossings of the tributaries mentioned above and the wetlands

areas, retaining walls exceeding four (4) feet in height are required. The plans indicate that at the

time of construction, the contractor shall provide for review structure calculations signed and sealed

by a New Jersey engineer structural calculations for the retaining walls which include a

consideration for the design flood impacts. In order to minimize impacts to the environmentally

sensitive areas, a soldier pile wall retaining wall has been proposed. This type of wall will eliminate

the need for spread footings.

The majority of the road crossings have been designed as three-sided culverts (arches) that maintain

a natural backfilled stream channel. The following summary is a summary of the proposed


velocities of the tributary flow as obtained from the HEC-RAS model. Since it is anticipated that in

some tributaries, the 100-year storm will be slow moving due to ponding effects, the 2, 10, 25 and

100 year velocities have been studied:

TRIBUTARY “A” CROSSING VELOCITIES

Storm Event Peak

Tributary Flow

Proposed Flood

Elevation

Approximate Flood Velocity

2 Year 23 c.f.s. 62.1 ft. 2.6 f.p.s. 10 Year 58 c.f.s. 62.7 ft. 3.4 f.p.s. 25 Year 87 c.f.s. 63.0 ft. 3.8 f.p.s. 100 Year 150 c.f.s. 63.6 ft. 4.2 f.p.s.

TRIBUTARY “B” CROSSING VELOCITIES

Storm Event Peak

Tributary Flow

Proposed Flood

Elevation



TRIBUTARY “C” CROSSING VELOCITIES

Storm Event Peak

Tributary Flow

Proposed Flood

Elevation




TRIBUTARY “E” CROSSING VELOCITIES

Storm Event Peak

Tributary Flow

Proposed Flood

Elevation



A velocity study of Tributary “D” is not required since the flow will be contained within a concrete

culvert and the outlets will be protected by conduit outlet protection. Placement is of rip-rap is

acceptable within tributary “D” as there is existing rip-rap located in the channel providing stability.

According to the Standards for Soil Erosion and Sediment Control, channels are considered stable

when the velocities are less than the following as determined by general soil classifications.

ALLOWABLE VELOCITY FOR VARIOUS SOIL TEXTURES

Tributary Crossing

Underlying Soil Type

SCD 10‐Yr Maximum Velocity

HEC‐RAS 10 Yr Velocity

Tributary “A” Loam 3.6 f.p.s. 3.4 f.p.s. Tributary “B” Loam 3.6 f.p.s. 2.2 f.p.s. Tributary “C” Sandy Loam 2.1 f.p.s. 5.5 f.p.s. Tributary “E” Sandy Loam 2.1 f.p.s. 2.7 f.p.s.

As shown above, the velocity for the 10-year storm for tributary “C” and “E” exceeds the SCD

guideline for maximum velocity at the culvert crossing; therefore the crossing requires further study.

Utilizing the existing HEC-RAS flood models, the following table compares existing and proposed

tributary “C” velocities in the area of the crossing:


TRIBUTARY “C” EXISTING VERSUS PROPOSED VELOCITIES

Storm Event Peak

Tributary Flow

Existing Flood Velocity (XS C‐7)


2 Year 23 c.f.s. 4.0 f.p.s. 3.9 f.p.s. 10 Year 58 c.f.s. 5.7 f.p.s. 5.5 f.p.s. 25 Year 87 c.f.s. 6.4 f.p.s. 6.3 f.p.s. 100 Year 150 c.f.s. 7.3 f.p.s. 7.6 f.p.s.

TRIBUTARY “E” EXISTING VERSUS PROPOSED VELOCITIES

Storm Event Peak

Tributary Flow

Existing Flood Velocity (XS E‐7)


2 Year 8 c.f.s. 2.2 f.p.s. 2.1 f.p.s. 10 Year 22 c.f.s. 2.7 f.p.s. 2.7 f.p.s. 25 Year 35 c.f.s. 3.1 f.p.s. 3.0 f.p.s. 100 Year 64 c.f.s. 2.8 f.p.s. 3.0 f.p.s.

As shown within the above table, the existing and proposed tributary velocities in the vicinity of the

proposed Tributary “C” and Tributary “E” culvert crossings remain significantly unchanged. As

shown above, the existing velocities exceed the general soil stability guidelines for the channel

stability. The proposed culvert will not further detriment the existing stream channel conditions by

increasing the tributary velocities, therefore, no addition soil stability measures are required or

proposed for mitigation.

RIPARIAN ZONES

In accordance with N.J.A.C. 7:13-4.1, the riparian zones have been established within 500 feet of

the proposed future roadway crossings and were included as part of the Flood Hazard Verification

Application. Such riparian zones have been noted on the Flood Hazard Individual Permit Plans.

The Individual Permit plans have been prepared to document the proposed disturbances to the

various riparian zones. The proposed roadway improvement will require riparian zone mitigation


since the maximum riparian zone disturbances found in N.J.A.C. 7:13-10.2 Table C will be

exceeded requiring a hardship request pursuant to N.J.A.C. 7:13-9.8. Pursuant to N.J.A.C. 7:10-

10.2(s) and (t), the riparian zones exceeding the maximum permissible requirement will be

mitigated at a 2:1 riparian zone compensation. The plans indicate the location of the proposed

riparian zone mitigation for the project.

The individual permit plans provide mitigation planting notes which require the compensation areas

to be replanted with indigenous, non-invasive vegetation (specifically trees) of equal or greater

density as the cleared, cut, or removed vegetation. The mitigation areas will be deed restricted

against future development that could remove the planted vegetation within the mitigated riparian

zone. The riparian zone mitigation areas will be monitored for a period of a three (3) growing

seasons to verify the vegetation has established.

The following is a summary of the riparian zone disturbances with the difference between the

maximum disturbance area and proposed disturbance and the requirement for the compensation:

RIPARIAN ZONE DISTURBANCES

Riparian Zone Disturbance Location

Riparian Zone

Disturbance (square feet)

Riparian Zone Allowable Disturbance (square feet)

Mitigation Disturbance (square feet)

Required Riparian Zone Compensation (square feet)

Disturbance #1 15,480 s.f. 50 ft. 5,000 s.f. 10,480 s.f. 20,960 s.f. Disturbance #2 14,730 s.f. 150 ft. 6,000 s.f. 8,730 s.f. 17,460 s.f. Disturbance #3 12,120 s.f. 50 ft. 5,000 s.f. 7,120 s.f. 14,240 s.f. Disturbance #4 31,125 s.f. 150 ft. 15,000 s.f. 16,125 s.f. 32,250 s.f. Disturbance #5 40,995 s.f. 150 ft. 15,000 s.f. 25,995 s.f. 51,990 s.f. Disturbance #6 31,915 s.f. 150 ft. 15,000 s.f. 16,915 s.f. 33,830 s.f. Disturbance #7 13,590 s.f. 150 ft. 7,500 s.f. 6,090 s.f. 12,180 s.f.

As discussed in the riparian zone disturbance table, there is a total riparian zone compensation of

182,910 s.f. or 4.2 acres.


RIPARIAN ZONE COMPENSATION

Riparian Zone Compensation

Location

Compensation Area

(square feet)

Compensation #1 61,130 s.f. Compensation #2 79,975 s.f. Compensation #3 10,220 s.f. Compensation #4 6,915 s.f. * 2 =

13,830 s.f. Compensation #5 16,060 s.f.

It should be noted that existing paved areas where riparian zone mitigation is proposed by replacing

the pavement with forest plantings counts twice in the compensation calculations. As shown in the

riparian zone compensation table, a total of 183,215 s.f. (4.2 acres) of riparian zone restoration has

been provided thereby exceeding the requirements compensation requirements and satisfying

N.J.A.C. 7:13-10.2(s) and (t).

LOW IMPACT DEVELOPMENT SUMMARY

According to the NJDEP Stormwater Management Rules at N.J.A.C. 7:8, the groundwater recharge,

stormwater quality, and stormwater quantity standards established by the Rules for major land

development projects must be met by incorporating nine specific nonstructural stormwater

management strategies into the project’s design to the maximum extent practicable.

The NJDEP utilizes the New Jersey Nonstructural Stormwater Management Strategies Point

System (NSPS) as a tool to assist engineers, planners and regulators that the site incorporates the

strategies to the maximum extent feasible. As stated in the NSPS User’s Guide, if the NSPS proves

that sufficient nonstructural stormwater management measures have been utilized, then no further

proof is required. However, if the NSPS fails to demonstrate compliance, the project is not

automatically disapproved. The Nonstructural Stormwater Management Strategies Point System,

Low Impact Development Checklist (see appendix) demonstrates compliance with nonstructural

stormwater management strategies. It should be noted that the NSPS User Guide indicates that the

NSPS worksheet is not intended for use on a roadway project and the Standard LID Checklist

should be utilized. In this case, the NSPS worksheet quantifies in a mathematical comparison the


LID techniques proposed for the road improvement, whereas the LID Checklist is not sufficient in

providing a mathematical calculation, and therefore has been omitted from this report. As shown on

the plans, the road improvements incorporate grass lined swales in lieu of drainage pipes to provide

significant stormwater conveyance thereby satisfying the intent of low impact development.

The site is located in Planning Area 3. As shown on the worksheet, the site is in compliance with

the NJDEP Regulations. No additional measures of compliance are required. The analysis has

included the area of disturbance, areas proposed for riparian zone compensation and areas dedicated

to open spaces. The drainage area maps indicate the areas considered in the calculation which total

107 acres. A summary of the existing site points, proposed site points, required site points ratio and

ratio of proposed to existing site points is provided in the table below.

Low Impact Development Summary Table:

Points Analysis Results Total Existing Site Points 266 Total Proposed Site Points 342 Required Site Points Ratio 100% Ratio of Proposed to Existing Site Points 129%

Proposed Nonstructural Measures are Adequate As shown above, the proposed improvement satisfies NJAC 7:8-5.3 by providing sufficient non-

structural measures such as lawn area and grass lined drainage swales. In summary, there is a total

of 6,155 linear feet of grass lined swales, 7,563 linear feet of stormwater piped conveyance for a

total conveyance system of 13,718 linear feet. The existing farm fields consisting of a soybean

cover have been considered in the post development analysis to be lawn and open space.

STORMWATER MANAGEMENT METHODOLOGY AND DESIGN

A stormwater management system is proposed to mitigate the additional runoff that will be

generated from the proposed development. The stormwater management proposal is to provide

stormwater attenuation for the entire development using a storm sewer system to convey runoff into

the proposed bio-retention basins, drainage swales through vegetative filter strips and some small

infiltration basins. The drainage analysis has been completed by utilizing the stormwater study

previously prepared for the flood hazard verification.


MULLICA HILL BYPASS

STORMWATER MANAGEMENT NETWORK DIAGRAM

EXISTING CONDITIONS



MULLICA HILL BYPASS

STORMWATER MANAGEMENT NETWORK DIAGRAM

PROPOSED CONDITIONS


NOTE: The above storm diagram does not mimic the natural tributary layout and has been formatted to fit this page. Refer to the pre-development models for a representative diagram of the tributary network.


The stormwater runoff resulting from the proposed development must be managed in accordance

with the Township of Harrison’s Land Use Ordinance, Gloucester County Land Development

Resolution, the Soil Erosion and Sediment Control Standards for the State of New Jersey,

N.J.D.E.P.’s Best Management Practices and N.J.D.E.P. Flood Hazard Regulations. Peak

stormwater flow reductions are required for this site development.

To meet the standards, the stormwater system must be designed so that the post-development peak

runoff rate for the two-year storm event is 50%; ten-year storm is 75% and one hundred year storm

is 80% of the pre-development peak runoff rate. To evaluate the proposed site for compliance with

the above standards, the Soil Conservation Service unit hydrograph method and the Pondpack v8.0

hydrologic/hydraulic modeling software from Haestad Methods was utilized.

In order to mitigate the effects of the increase in the impervious surfaces within the overall

watershed, the stormwater management facilities are designed as bio-retention basins and

infiltration basins. Soil test pits revealed a thick clay horizon within the right-of-way which creates

a perched water table. This condition limits the types of stormwater facilities described in the BMP.

As a result, bio-retention basins have been proposed to provide stormwater attenuation and water

quality treatment. Due to a shallow seasonal high water table and underlying low permeable soils,

the bottom of the bio-retention basins will be lined with an impermeable liner to prevent the seepage

of shallow groundwater into the bio-retention basins. The separation between the seasonal high

ground water and the bottom of the bio-retention basins is achieved by the use of a 2 foot thick clay,

thereby providing the minimum separation has been provided. The liner will not interfere with the

operation of the bio-retention basins and the 90% TSS removal rate of the basins remains

achievable. There will not be an impact to the groundwater as a result of the installation of the

liners. The groundwater will continue to flow around the basins. Dewatering is not proposed. The

bio-retention basins will not be considered for groundwater recharge mitigation.

The required stormwater management peak flow reductions have been provided as required by the

regulations. The design also utilizes the change in land cover from farm fields to grass and forested

areas.


As a result of the preservation of a large tract of land, the existing farm fields will be converted into

an open space lawn area. The change in cover effectively lowers the runoff coefficient of the

existing farm fields. This change in cover has been considered in the overall drainage analysis and

considered for peak flow reductions, ground water recharge and low impact development.

As required by the local soil conservation district, this site has been modeled using the Delmarva

Unit Peak Factor unit less hydrograph. Existing peak stormwater flows were calculated based upon

the current design storms for Gloucester County. These existing design storms were then used to

calculate the required proposed peak flow reductions. This report will demonstrate that the

proposed peak flow rates discharged to the tributary network is now less than existing peak flow

rates.

In order to compute the regulatory peak flow reductions, the existing stormwater model was

abridged to only compute the stormwater runoff from the disturbed areas as a result of the roadway

construction. By using this technique, the drainage analysis for the tributary network can be utilized

for roadways post-development calculations thereby providing an overall watershed drainage

analysis. The stormwater model for the regulator peak flow reductions has been included within the

appendix of this report and such disturbance areas indicated on the existing drainage area maps.

The following tables demonstrate that the attenuation provided is sufficient to meet or exceed the

above criteria at the discharge locations. Peak flows provided below have been grouped into three

(3) tributary groups. Tributaries “A” and “B” join at the Chatham Lane culverts. Tributaries “C”

and “D” join immediately to the south of the old railroad grade and Tributary “E” flows towards the

Mullica Hill Pond. The following summary tables provide calculations assuming no infiltration.

All stormwater peak flow rates have been computed utilizing TR-55 methodology (See Appendix

for calculations.)


EXISTING STORMWATER OUTFLOW TO TRIBUTARY “A” AND “B” ASSUMING NO

INFILTRATION WITHIN THE BASINS (Model Node Reference- Trib. “B” Pond IN)

STORM (YEAR)

EXISTING PEAK FLOW

EXISTING ONSITE PEAK

FLOW SUBJECT TO

FLOW REDUCTION

REGULATED REDUCTION

REQUIRED REDUCTION

MAXIMUM POST DEVELOPMENT

PEAK FLOW

WQ 13.0 c.f.s. N/A N/A N/A 13.0 c.f.s. 2 YR 121.7 c.f.s. 2.6 c.f.s. 50% 1.3 c.f.s. 120.4 c.f.s. 10 YR 307.0 c.f.s. 6.5 c.f.s. 75% 1.6 c.f.s. 305.4 c.f.s. 100 YR 759.6 c.f.s. 16.2 c.f.s. 80% 3.2 c.f.s. 756.4 c.f.s.

PROPOSED PEAK FLOW SUMMARY FOR TRIBUTARY “A” AND “B” ASSUMING NO INFILTRATION WITHIN THE BASINS (Model Node Reference- Trib. “B” Pond IN)

STORM (YEAR)


PEAK FLOW

PROPOSED POST DEVELOPMENT

PEAK FLOW WQ 13.0 c.f.s. 13.4 c.f.s. 2 YR 120.4 c.f.s. 120.0 c.f.s. 10 YR 305.4 c.f.s. 301.7 c.f.s. 100 YR 756.4 c.f.s. 743.3 c.f.s.

EXISTING STORMWATER OUTFLOW TO TRIBUTARY “C” AND “D” ASSUMING NO INFILTRATION WITHIN THE BASINS (Model Node Reference- Trib. “C, D”)

STORM (YEAR)

EXISTING PEAK FLOW


FLOW SUBJECT TO

FLOW REDUCTION

REGULATED REDUCTION

REQUIRED REDUCTION


PEAK FLOW



PROPOSED PEAK FLOW SUMMARY FOR TRIBUTARY “C” AND “D” ASSUMING NO INFILTRATION WITHIN THE BASINS (Model Node Reference- Trib. “C, D” )

STORM (YEAR)


PEAK FLOW



EXISTING STORMWATER OUTFLOW TO TRIBUTARY “E” ASSUMING NO INFILTRATION WITHIN THE BASINS (Model Node Reference- Trib. “E” IN)

STORM (YEAR)

EXISTING PEAK FLOW


FLOW SUBJECT TO

FLOW REDUCTION

REGULATED REDUCTION

REQUIRED REDUCTION


PEAK FLOW


PROPOSED PEAK FLOW SUMMARY FOR TRIBUTARY “E” ASSUMING NO INFILTRATION WITHIN THE BASINS (Model Node Reference- Trib. “E” IN)

STORM (YEAR)


PEAK FLOW



The overall stormwater management system complies with the State requirements for the 2, 10 and

100-year frequency storms when infiltration is not considered in the design. As shown above, the

peak discharges from the site has been reduced to less than the required maximum peak flow

discharges, with the exception of the water quality storm, which is not a regulated storm event for


peak flow reductions. The stormwater management system complies with the Municipal, County

and State stormwater requirements for all the drainage areas and storm frequencies analyzed in this

report.

The following tables demonstrate that the attenuation provided is sufficient to meet or exceed the

above criteria when reviewing the overall watershed the discharge locations. The following

summary tables provide calculations assuming no infiltration although there will be some

infiltration as a result of providing water quality swales and infiltration basins. All stormwater peak

flow rates have been computed utilizing TR-55 methodology (See Appendix for calculations.)

EXISTING STORMWATER OUTFLOW SUMMARY TO RACCOON CREEK NO INFILTRATION WITHIN THE BASINS (Model Node Reference- Raccoon Creek)

STORM (YEAR)

EXISTING PEAK FLOW


FLOW SUBJECT TO

FLOW REDUCTION

REGULATED REDUCTION

REQUIRED REDUCTION


PEAK FLOW



PROPOSED PEAK FLOW SUMMARY TO RACCOON CREEK NO INFILTRATION WITHIN THE BASINS (Model Node Reference- Raccoon Creek)

STORM (YEAR)


PEAK FLOW



As shown above, the overall watershed will be provided with a reduction in peak flow rates as a

result of the construction of the roadway and mitigating stormwater basins and swales. It should be

noted that the existing and peak flow calculations for drainage area F-11 have not been summarized

in a separate table. The F-11 drainage area is contributory to the final point of analysis (Raccoon

Creek), and there is an overall peak flow reduction for the entire project.

The emergency spillway for the bio-retention basins has been designed to pass the peak 100-year

basin inflow. Spillways have been provided for basins that pass the routed 100 year flow plus 50%.

The embankments required for the bio-retention basins are not classified as dams.

The following table demonstrates that the bio-retention basins and swales will be dry within 60

hours after the onset of the 100-year storm event therefore satisfying the maximum drain time

requirement as indicated within Best Management Practices of 72 hours. The extended detention

basins will detain stormwater for a longer period of time, which is necessary to achieve water

quality.

STORMWATER DRAIN TIME SUMMARY (BMP METHOD)

BIO-RETENTION BASIN STATION 16+00

BASIN

PEAK VOLUME (AC-FT)

TIME OF PEAK VOLUME (HRS)

TIME TO DRAINED BASIN

(HRS)

DRAIN TIME (HRS)

MAXIMUM WSE DEPTH

WQ 0.071 AC-FT 1.9 HOURS 7.0 HOURS 5.1 HOURS 0.15 FT 2 YR 0.207 AC-FT 15.2 HOURS 31.9 HOURS 16.7 HOURS 0.44 FT 10 YR 0.504 AC-FT 16.80 HOURS 46.2 HOURS 29.4 HOURS 1.04 FT 25 YR 0.759 AC-FT 17.60 HOURS 56.3 HOURS 38.7 HOURS 1.53 FT 100 YR 1.319 AC-FT 19.6 HOURS 75.0 HOURS 55.4 HOURS 2.54 FT


STORMWATER DRAIN TIME SUMMARY (BMP METHOD) EXTENDED DETENTION BASIN STATION 57+00

BASIN

PEAK

VOLUME (AC-FT)



(HRS)

DRAIN TIME (HRS)

MAXIMUM WSE DEPTH


STORMWATER DRAIN TIME SUMMARY (BMP METHOD) EXTENDED DETENTION BASIN STATION 58+00

BASIN

PEAK

VOLUME (AC-FT)



(HRS)

DRAIN TIME (HRS)

MAXIMUM WSE DEPTH


STORMWATER DRAIN TIME SUMMARY (BMP METHOD) INFILTRATION BASIN STATION 66+00

BASIN

PEAK

VOLUME (AC-FT)



(HRS)

DRAIN TIME (HRS)

MAXIMUM WSE DEPTH

WQ 0.024 AC-FT 2.0 HOURS 11.2 HOURS 9.2 HOURS 1.1 FT 2 YR 0.203 AC-FT 15.6 HOURS 49.2 HOURS 33.6 HOURS 2.5 FT 10 YR 0.240 AC-FT 12.8 HOURS 51.2 HOURS 38.4 HOURS 2.7 FT 25 YR 0.263AC-FT 12.5 HOURS 51.4 HOURS 38.9 HOURS 2.8 FT 100 YR 0.300 AC-FT 12.4 HOURS 51.6 HOURS 39.2 HOURS 2.9 FT



BASIN

PEAK

VOLUME (AC-FT)



(HRS)

DRAIN TIME (HRS)

MAXIMUM WSE DEPTH

WQ 0.041 AC-FT 1.3 HOURS 16.8 HOURS 15.5 HOURS 1.6 FT 2 YR 0.042 AC-FT 12.3 HOURS 36.7 HOURS 24.4 HOURS 1.6 FT 10 YR 0.045 AC-FT 12.4 HOURS 38.7 HOURS 26.3 HOURS 1.6FT 25 YR 0.047 AC-FT 12.3 HOURS 38.9 HOURS 26.6 HOURS 1.7 FT 100 YR 0.051 AC-FT 12.3 HOURS 39.1 HOURS 26.8 HOURS 1.7 FT

STORMWATER DRAIN TIME SUMMARY (BMP METHOD) BIO-RETENTION BASIN STATION 70+00

BASIN

PEAK

VOLUME (AC-FT)



(HRS)

DRAIN TIME (HRS)

MAXIMUM WSE DEPTH



BASIN

PEAK

VOLUME (AC-FT)



(HRS)

DRAIN TIME (HRS)

MAXIMUM WSE DEPTH

WQ 0.098 AC-FT 1.3 HOURS 26.9 HOURS 25.6 HOURS 1.5 FT 2 YR 0.099 AC-FT 12.3 HOURS 48.7 HOURS 36.4 HOURS 1.5 FT 10 YR 0.104 AC-FT 12.3 HOURS 49.4 HOURS 37.1 HOURS 1.5FT 25 YR 0.107 AC-FT 12.3 HOURS 49.6 HOURS 37.3 HOURS 1.6 FT 100 YR 0.113 AC-FT 12.4 HOURS 50.0 HOURS 37.6 HOURS 1.6 FT



BASIN

PEAK

VOLUME (AC-FT)



(HRS)

DRAIN TIME (HRS)

MAXIMUM WSE DEPTH


The bio-retention basins and infiltration basins have been designed so that the water quality water

surface elevation does not exceed six (6) inches and within the infiltration basins two (2) feet. The

maximum depth for the water quality storm per the BMP design guideline is one (1) foot. The bio-

retention basin will be backfilled with a material that will achieve 4 inches per hour and have under-

drains that convey the stormwater to the outlet control structure. Orifices have been provided on

some of the bio-retention basin under-drains to provide additional stormwater attenuation which

assist in achieving the regulatory peak flow reductions.

The stormwater features have been design so that the existing flooding conditions within the

tributary network do not impact the storage capacity of the proposed mitigation features. The

following table provides a summary of the basin storage areas with a comparison to the flood hazard

elevation at the discharge location:

STORMWATER BASIN ELEVATION COMPARISON TO FLOOD HAZARD ELEVATION

BASIN

BASIN ELEVATION

DOWNSTREAM TRIBUTARY

FLOOD HAZARD

ELEVATION FREEBOARD

BIORETENTION BASIN 16+00 83.00 FT TRIBUTARY “A” 52.91 FT 30.09 FT

BIORETENTION BASIN 57+00 65.00 FT TRIBUTARY “C” 61.78 FT 3.22 FT

INFILTRATION BASIN 66+00 77.00 FT TRIBUTARY “D” 73.00 FT 4.00 FT

INFILTRATION BASIN 67+00 78.00 FT TRIBUTARY “D” 73.00 FT 5.00 FT

BIORETENTION 77.00 FT TRIBUTARY “D” 73.00 FT 4.00 FT


BASIN 70+00

As shown in the above table, the storage capacity of the stormwater bio-retention basins is not

impacted by the flood hazard within the downstream tributary. Therefore, no further study of the

impacts of the tributary floodwater on the functioning of the basin is required.

GROUNDWATER RECHARGE

To achieve the N.J.D.E.P groundwater recharge requirements for the development, one of two

criteria must be met provided recharge only if recharge is permitted under N.J.A.C. 7:8-5.4. Either

one hundred percent of the site’s average annual pre-developed groundwater recharge volume must

be maintained after development, or one hundred percent of the difference between the site’s pre-

and post-development 2-year runoff volume must be infiltrated.

For this project, the site’s average annual pre-developed groundwater recharge volume has been

maintained by considering the drainage in land cover of the proposed open space areas and the use

of some small infiltration basins. The existing farm fields will be converted into open space with

grass cover. The limits of the groundwater recharge analysis are noted on the drainage area maps.

The New Jersey Groundwater Recharge Spreadsheet Version 2.0 was utilized to compute the annual

groundwater recharge. The following table compares the existing versus post development

groundwater recharge volumes. The analysis has included the areas proposed for riparian zone

compensation.

STORMWATER GROUNDWATER SUMMARY Total Site Pre-Developed Annual Recharge 1,652,817 cubic feet Site Post-Development Recharge 1,538,619 cubic feet Basin 66+00 27,015 cubic feet Basin 67+00 48,230 cubic feet Basin 77+00 112,057 cubic feet Basin 80+00 24,952 cubic feet Total Site Post Development Annual Recharge 1,750,873 cubic feet Groundwater Recharge Surplus 98,056 cubic feet

In summary, groundwater recharge will be fully satisfied by the conversion of the agricultural

areas to lawn and open space and the use of the small infiltration basins. Additional mitigation is


not required. The bio-retention stormwater basins have not been included in the calculations for

groundwater recharge since the basins will be clayed lined to prevent the infiltration of

groundwater into the bottom of the basins.

WATER QUALITY To achieve the N.J.D.E.P water quality requirements, a minimum Total Suspended Solids (T.S.S)

removal rate of 80 percent is required. In accordance with the New Jersey Stormwater Best

Management Practices (BMP) Manual, the standard recognizes a T.S.S. removal rate of 90% for

bio-retention basins, 80% for infiltration basins and 70% for forested vegetative filter strips.

The water quality analysis has been completed based upon an overall analysis of the project. In

certain areas, runoff from the roadway will be directly discharged to the existing tributary network.

This is an acceptable practice since the bio-retention basins will provide water quality exceeding the

State’s requirement and the overall weighted TSS analysis demonstrates sufficient water quality has

been provided. The proposed swales near the Tributary “A” and “B” wetlands have been design to

by-pass the road inlets and convey the water quality storm to the inlet near the final discharge

location. Although, the BMP design manual does not provide credit for these swales, such swales

will provide additional water quality which will have a positive effect on the project’s overall water

quality treatment. To further supplement water quality, a portion of Walter’s Road will discharge

by sheet flow (no curbs) prior to collection in the storm system, therefore obtaining 70% TSS

removal through a 35 foot wide reforested vegetative filter strips graded at 1% slope. A similar

filter strip and swale system is proposed in other portions of the project to provide additional water

quality mitigation and satisfy low impact development.

Stormwater Management Basin 57+00 is proposed as an extended detention basin, whereas water

quality treatment is obtained by retaining stormwater volume within the basin after an extended

period of time. The following is a summary of the water quality treatment for Basin 57+00:


WATER QUALITY STORMWATER DRAIN TIME SUMMARY EXTENDENDED DETENTION BASIN STATION 57+00

BASIN

PEAK

VOLUME (AC-FT)


TIME TO 10% VOLUME

REMAINING (HRS)

DRAIN TIME (HRS)

WATER QUALITY

TREATMENT

WQ 0.158 AC-FT 2.6 HOURS 25.9 HOURS 23.3 HOURS 55%

As noted below, the use of vegetative filter strips prior to discharge into Basin 57+00 will provide

initial treatment of 70% TSS removal for a combined treatment of 86%. It should be noted that

Basin 58+00 does not maintain sufficient volume to provide documented water quality treatment,

therefore the discharge from Basin 58+00 is directed into Basin 57+00 to provide the necessary

treatment at 86%. Filter strips are provided prior to discharge into Basin 58+00. The overall water

quality analysis does not include the portion of the project discharging to Tributary “E”. As

demonstrated below, the stormwater design in the area of tributary “E” relies on the redistribution of

existing drainages areas to the Tributary “D” watershed. As a result, there is a net decrease in

impervious surfaces to Tributary “E” thereby providing a default improvement in water quality

were a TSS removal analysis is not required.


The following is a summary of the water quality calculation for tributaries “A”, “B”, “C” and “D”:

Water Quality Location

Contributory Drainage Area

Initial Water Quality Treatment

Secondary Water Quality

Treatment

Total Water Quality

Treatment

Water Quality Treatment

Area 16+00 4.2 acres 90% 0% 90% Bio-retention

Area 25+50 0.3 acres 0% 0% 0% Drainage Swales

Area 44+00 1.3 acres 70% 0% 70%

Vegetative Filter Strip and

Drainage Swales

Area 57+00 (Walters Road) 0.9 acres 70% 55% 86%*


Extended Detention

Area 57+00 5.1 acres 70% 55% 86%


Extended Detention

Walters Road 0.1 acres 70% 0% 70% Vegetative Filter Strip

Area 66+00 (Filter Strip) 0.6 acres 70% 80% 94%


Infiltration Area 66+00 5.7 acres 80% 0% 80% Infiltration

Area 66+00 0.3 acres 0% 0% 0% Existing Impervious Area 67+00 1.1 acres 80% 0% 80% Infiltration Area 70+00 1.7 acres 90% 0% 90% Bioretention

Area 77+00 3.2 acres 80% 90% 98% Infiltration and Bioretention Summary 24.5 acres - - 84% -

*Example Calculation (70%+55%)-(70%*55%/100%) = 86%

Utilizing the formulas provided in Best Management Practices, the overall water quality treatment is

84% TSS removal. Water Quality for individual stream tributaries is realized at 81% for Tributary

“A” and “B”; 86% for Tributary “C” and 84% for Tributary “D”.

Water quality for Tributary “E” can be demonstrated by the removal of contributory impervious

surfaces. In the existing condition, there is a total of 1.4 acres of impervious surfaces contributory


to Tributary “E”. In the proposed conditions, there is a total of 1.0 acres of impervious surfaces

contributory to Tributary “E”. Therefore, there has been a loss of 0.4 acres of impervious surfaces

which results in an improvement to existing water quality within the Tributary. Due to the addition

of a small infiltration basin to supplement groundwater recharge, 0.34 acres of impervious surface

in Tributary “E” area achieves 80% TSS, therefore when considering the treated area, there is a

resultant loss of 0.7 acres.

As demonstrated above, the loss of impervious surfaces, the proposed bio-retention basins, extended

detentio

Documents

New FLOOD HAZARD AREA INDIVIDUAL PERMIT ENGINEER’S REPORT Bypass... · 2018. 6. 1. · HEC-RAS backwater analysis for this tributary has been performed. This tributary will be spanned