module 3 of 4 stormwater management design review

Stormwater Management Design ReviewNJ DEP Division of Watershed Protectionand Restoration

October 12, 2021

Module 3 of 4

Agenda(cont’d. on next slide)

Module 3No. Subject Duration

1 How to Review a Project 1 (xx minutes)Goal 1: Learn to identify which design and performance standards must be

met on siteGoal 2: Learn how to review for compliance with the GI, stormwater runoff

quantity and water quality, and groundwater recharge standardsGoal 3: Learn how to review BMPs for compliance with the design criteria in

the BMP Manual

2 How to Review a Project 2 (xx minutes)Goal 1: Learn how to review BMPs for compliance with the design criteria

in the BMP Manual

Goal 2: Calculate Detention Time

Goal 3: Learn how to review the safety requirements

Goal 4: MTD Design

Goal 5: Groundwater Recharge Spreadsheet

Agenda

Module 3 (cont’d.)No. Subject Duration

3 Groundwater Mounding (xx minutes)Goal 1: Learn the requirements under N.J.A.C. 7:8Goal 2: Understand the concepts of groundwater moundingGoal 3: Learn about the methods of calculationGoal 4: Review examples of groundwater mounding analyses

- Quiz 3 and Review (xx minutes)

REVIEWA PROJECT 1

HOW TO

Minesh Patel and Anthony RobalikNJDEP Division of Watershed Protection and Restoration

SWMDR Training Module 3October 12, 2021

Presentation Goals

• Learn to identify the design and performance standards that must be met onsite

• Learn how to review applications for compliance with the green infrastructure, water quantity, water quality, and groundwater recharge standards

• Learn how to review BMPs for compliance with the design criteria in the BMP Manual

Presentation Layout

Review Checklist: Pp. 14 – 18 of Chapter 3.4 in the Tier A Guidance Document at

https://www.nj.gov/dep/dwq/pdf/Tier_A/Tier_A_Chapter_3-4.pdf

Municipal Guidance Document:

STORMWATER MANAGEMENT REPORT

Checking Application

• Pre- and post-construction site plans

• Pre- and post-construction grading plans

• Stormwater report with calculations

• Details of BMPs

• Maintenance plan

The applicant needs to submit, at a minimum…

Reading the Report

• Existing conditions

• Proposed conditions

• Subdrainage areas and offsite discharge point(s)

• Disturbance and change in regulated impervious surface and change in regulated motor vehicle surface for each subdrainage area

• Soil survey and testing information

Site description

Reading the Report

• What’s required

o BMP design details

o Hydraulic and hydrologic calculations

o Groundwater recharge information

o Groundwater mounding analysis if infiltration is proposed

Design and Performance Standards

Reading the Report

• Time of concentration – calculated, not assumed (see Chapter 5)

• Routing calculations for stormwater runoff quality and the 2-, 10- and 100-year storm events

• Hydrographs

• Groundwater recharge spreadsheet

• Groundwater mounding analysis if infiltration is proposed

Hydraulic and Hydrologic Calculations

Determining Applicable Design & Performance Standards

1. Has the project disturbed one acre or more since February 2, 2004?

2. Has the project increased regulated impervious coverage by ¼ acre or more since February 2, 2004?

3. Has the project increased regulated motor vehicle surface by ¼ acre or more since March 2, 2021?

4. Is there a combination of 2 and 3 above that totals an area of ¼ acre or more?

5. What is the SCO’s definition of major development?

Is the project a major development?

• Residential Site Improvement Standards:

o 1 acre or more of disturbance (N.J.A.C. 5:21)

• Stormwater Control Ordinance:

o N.J.A.C. 7:8 Definition of Major Development

o More stringent standard adopted by municipality

• NJDEP Division of Land Use Regulation:

o 1 acre or more of disturbance

o ¼ acre or more increase in impervious coverage

“Major Development” definitions:

• Watershed

• Category 1

• Flooding problems

• Water quality impairments or TMDLs

• Is the site in a regulated area?

o Flood Hazard Area

o CAFRA

o Freshwater Wetlands

o Highlands

• Is the site in the Pinelands?

Where does it discharge to?

• Green Infrastructure

• Stormwater runoff quantity control

• Stormwater runoff quality

• Groundwater recharge

Design and Performance Standards for Major Developments:

When may variances or waivers

granted?

Variances – N.J.A.C. 7:8-4.6

• There may be instances where site constraints for using green infrastructure is technically impracticable to meet design and performance standards of a major development project.

• The rule allows variances to be granted at the local level to provide mitigation options for those instances when it is technically impracticable to meet standards.

7:8-4.6 Variance from the design and performance standards for stormwater management measures

Variances - N.J.A.C. 7:8-4.6

• Technically impracticable to meet any one or more of the design and performance standards on site; and

• The proposed design achieves maximum compliance with the design and performance standard

o Technical impracticable exists only when the standard can not be met for engineering, environmental, or safety reasons

Approval of a variance applies to the individual

drainage area and design and performance standard

Municipality may approve a variance if

the Applicant demonstrates:

Variances – MitigationN.J.A.C. 7:8-4.6(a)3

Mitigation Requirements:

• Selected from municipal mitigation plan or proposed by applicant, provided it meets the criteria within the municipal mitigation plan

• Be approved no later than preliminary or final site plan approval of the major development

• Be located in the same HUC 14 as the portion of the major development that was granted the variance

• Be constructed prior to or concurrent with the major development

• Comply with the green infrastructure standards at N.J.A.C. 7:8-5.3

• Applicant or entity assuming maintenance responsibility for the associated major development shall be responsible for maintenance, with a written agreement submitted to the review agency

Approved variance must be submitted to county review

agency and DEP within 30 days of approval

If variance is from green infrastructure standards

• Mitigation project must provide green infrastructure BMPs to manage an equivalent or greater area and amount of impervious surface than the area of major development granted the variance

• Vegetative filter strips and grass swales excluded as mitigation measures

• GI BMPs used for mitigation must be sized to manage the Water Quality Design Storm (at a minimum)

• GI BMPs used for mitigation are subject to the specified drainage area limitations

If variance is from groundwater recharge standards

• Mitigation project must equal or exceed GW recharge deficit

If variance is stormwater runoff quality standards

• Mitigation project must provide sufficient TSS removal to equal or

exceed the deficit resulting from the variance

If variance is stormwater runoff quantity standards

• Mitigation project must provide equivalent peak flow rate

attenuation upstream and discharge to the same water course

• Construction of underground utility line, if revegetated upon completion

• Construction of aboveground utility line, if existing conditions are maintained to MEP

• Construction of public pedestrian access, if made of permeable materials and no greater than 14 ft. wide

Linear development exemptions exist for…

• Enlargement of existing public roadway or railroad, construction or enlargement of public pedestrian access, if applicant demonstrates:

o Public need for project that can’t be met another way

o D&P Standards met to maximum extent practicable

o Meeting the standards would require condemning existing in-use structures

o Applicant does not have and cannot get rights to other lands for mitigation

Waivers from strict compliance with N.J.A.C. 7:8-5.3 exist for…

• Stormwater Runoff Quantity, if in Tidal Flood Hazard Area and flooding will not be increased

• Stormwater Runoff Quality, if site has NJPDES permit with specific effluent limit

• Groundwater Recharge, for previously developed areas in urban redevelopment area

When Design and Performance Standards do not apply…

• “Previously developed portions of areas:

o Delineated on State Plan Policy Map as:

• Planning Area 1

• Designated Centers, Cores, or Nodes

o CAFRA Centers, Cores, or Nodes

o Urban Enterprise Zones

o Urban Coordinated Council Empowerment Neighborhoods

Urban Redevelopment Area

• Mitigation Plan

• Construction of utility lines, public pedestrian access

• Enlargement of public roadway, railroad, public pedestrian access

• Urban Redevelopment Area

• NJPDES Permits w/ discharge limits

• Tidal Flood Hazard Area

Situations that may be eligible for variances/waivers (summary):

INFRASTRUCTURE

GI in the NJ BMP Manual

Chapter 9: GI BMPs

Green Infrastructure StandardsN.J.A.C. 7:8-5.3(b)

Best Management Practice Maximum Contributory Drainage Area

Dry Wells 1 acre

GI Manufactured Treatment

Device (MTDs)

2.5 acres

Pervious Paving Systems Area of additional inflow cannot exceed

3 times the area occupied by the BMP

Small-scale Bioretention Systems 2.5 acres

Small-scale Infiltration Basins 2.5 acres

Small-scale Sand Filters 2.5 acres

Contributory Drainage Area Limitation (N.J.A.C. 7:8-5.3(b))

GI in the NJ BMP Manual

Chapter 10: GI BMPs for Stormwater Runoff Quantity

• May only be used for Groundwater Recharge and/or Stormwater

Runoff Quality, when applicable, with a Waiver or Variance from

N.J.A.C. 7:8-5.3

Non-GI in the NJ BMP Manual

Chapter 11: Non-GI BMPs

• May only be used for Groundwater Recharge, Stormwater Runoff

Quantity and/or Stormwater Runoff Quality, when applicable, with

a Waiver or Variance from N.J.A.C. 7:8-5.3

STORMWATER RUNOFF

QUANTITY

Stormwater Runoff Quantity

• Protects against flooding

• Can be met in three different ways

• Must be met for each point of analysis

The water quantity standard…

• Allows reviewer to identify where stormwater flows

• Required to determine drainage areas and time of concentration

Topography

• Site plans must show:

o Existing and proposed contours

o Point of analysis

o Tc flow path

o Drainage area to each point of analysis

Topography

• For sites with multiple drainage areas that don’t converge or have very different cover:

o Each DA should be calculated separately

o Each DA should have separate Tc calculation

o Each DA should meet water quantity standard

Topography

• Reviewer needs to verify:

o All drainage areas are delineated correctly

o Verify that proper time of concentration flow path is used

o Verify that time of concentration is calculated properly

Topography

• Look for depressions on existing site

• Water ponds in natural depressions

• Ignoring depression storage will overestimate existing discharge volumes and peak flows

Topography

• Should be clearly marked on plans

• Should be specific: forest, grass, bare soil, impervious coverage

• Include any existing BMPs

Land Cover

• Reviewer needs to verify:

o What’s on site plan matches actual conditions

o Land cover used in the calculations is the most pervious cover that has existed in past 5 years

Land Cover

• Soils on-site must be determined

• Required information: HSG and soil type

• Should be submitted with report

• Best way to find soil info is NRCS Web Soil Survey:o https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htmo Google “Web Soil Survey”

• Sometimes cannot be found in soil survey

• If information not available, two ways to find HSG:o Default hydrologic soil groups

o Soil testing

• Information on both methods in Chapter 12

• Poor soil information is one of the most common review issues encountered

• Soil information can make or break a stormwater management design

• Often the best practice to review soil testing first

✓ Identify if the roughness sheet flow coefficient for woods is 0.4 or less.

✓ Determine if the time of concentration in preconstruction condition is calculated appropriately. It is important that existing times of concentration are not underestimated.

Stormwater Management Review Checklist

• Required for both Rational/Modified Rational Methods and NRCS Method

• Calculated in accordance with the NEH and also Chapter 5 of the BMP Manual

Time of Concentration

• Review sheet flow, shallow concentrated flow, and channel flow inputs

o Maximum allowable Manning’s roughness coefficient is 0.4 in NJ

• Pre-construction Tc should always be proven by calculation

• Post-construction Tc can no longer be assumed as an allowable minimum for the NRCS method

Tc summary…

✓ Identify if the runoff from impervious and pervious surfaces are calculated separately and not with a weighted average of the CN numbers for impervious surface and pervious surface.

✓ Determine if the curve number selection makes sense.

✓ If rational method is used, determine if the selected runoff coefficient makes sense and the source of the runoff coefficient is provided

✓ Review the applicability of the selected modelling method

o Rational Method for peak flows & Modified Rational Method for hydrographs

o NRCS Method

Acceptable Methods of Calculations

• Used to calculate amount of runoff caused by given precipitation

• Curve numbers – NRCS Method

• Runoff Coefficients – Rational/Modified Rational

Curve Numbers/Runoff Coefficients

• Stormwater report and plans should clearly show area for each different land cover and HSG

• Both must always be based on “good” hydrologic condition

Area (sf) Land Cover Soil TypeHydrologic

Soil Group

Number

38,173 Pavement Bucks silt loam B 98

24,689 Lawn Bucks silt loam B 61

14,787 Lawn Lehigh silt loam C 74

9,950 Forest Dunellen sandy

2,654 Pavement Lehigh silt loam C 98

• Curve numbers are available in NEH, Part 630, Chapter 9 Hydrologic Soil-Cover Complexes

• Runoff coefficient is available in Chapter 5

• Reviewer is responsible for ensuring that the designer’s chosen value is reasonable

• Required to route pervious and impervious areas separately

• Never use a weighted average of pervious and impervious areaso Underestimates flow rates and volumes

• Subdrainage areas to a discharge point (point of analysis)o Peak flow rates from each subdrainage area cannot be

arithmetically added. Hydrographs of subdrainage areas shall be combined and the peak flow rate of the combined hydrograph shall be used at the discharge point

Runoff Routing

✓ Identify if any existing depression areas are included in the model.

• Depressions and high permeability soils clearly provide stormwater management benefits

• Not possible to accurately calculate time of concentration

• Depression storage should be modeled as an existing basin in pre-development conditions

Dealing With Depression Storage

• If depression area is in middle of site:

o Model depression area

o Calculate time of concentration from depression outlet to point of interest

o Lag depression outflow hydrograph by calculated time of concentration

o Calculate time of concentration for remaining area and route it separately

• Determine time of concentration to depression area

• Calculate stage-storage data for depression area

• Model outlet from depression area as a weir

• Route contributory drainage area to depression area

Stormwater Quantity Control BMPs

✓ Determine if the rainfall depth and intensity for 2-, 10-and 100-year storms are selected properly from NOAA’s partial duration data or NRCS’s county rainfall summary data.

✓ Determine if the appropriate rainfall distribution is used in the calculations.

• For NRCS Method:o 2-, 10- and 100-year, 24-hour storms (NOAA Precipitation

Frequency Data Server)o Water Quality Design Storm (BMP Manual Chapter 5)

• For Rational/Modified Rational Method:o 2-, 10- and 100-year, IDF Curves (NOAA Precipitation

Frequency Data Server)o Water Quality Design Storm Intensity-Duration Curve

(BMP Manual Chapter 5)

Rainfall Depth and Intensity

• NRCS developed other distributions for NJ in 2012:

o NOAA_C

o NOAA_D

• NJ BMP Manual requires use of these rainfall distributions when designing BMPs

• NOAA_C and NOAA_D already built into many stormwater modeling programs

Rainfall Distribution (NRCS Method only)

• Applicant must show critical storage volume calculation

• Detention basin design must be based on storm event that results in critical storage volume

Design Storm Event (Modified Rational Method)

✓ Identify if the DelMarVa unit hydrograph is used, if so, determine if the site meets the criteria for its use –coastal plain, flat topography, permeable soils, low imperviousness.

• SCS Unit Hydrograph may be acceptable throughout the entire state, but must be used outside coastal plain area

• DelMarVa Hydrograph is acceptable in the coastal plain area only

• Must use the same hydrograph in both pre and post conditions

Unit Hydrograph (NRCS Method only)

Review compliance with the standards

Option 1Demonstrate that at no point does the post-development hydrograph exceed the pre-development hydrograph for the 2-, 10- and 100-year storms (N.J.A.C. 7:8-5.6)

• Applicant must submit pre- and post-development hydrographs

• Total runoff volume must be equal or lower in post-development as compared to pre-development

Review compliance with option 1

Reduce the post-development peak flows to 50, 75, and 80% of pre-development peak flows for 2-, 10-, and 100-year storms (N.J.A.C. 7:8-5.6)

• Stormwater BMPs with detention function to provide storage volume and reduce the peak flow rate may be required

• Applicant must submit routing calculations

• Most commonly used option to meet stormwater runoff quantity control standard

Option 3:

Stormwater Quantity Control BMPs

• Various GI BMPs can be designed to provide detention function:

BMPs with detention component providing storage and reducing peak flow rates

Chapter 9 BMPs:

o Cisterns

o Green roofs

o Pervious paving systems

o Small-scale bioretention

systems

o Small-scale infiltration basins

o Small-scale sand filters

Chapter 10 BMPs:

o Bioretention systems

o Infiltration basins

o Sand filters

o Standard constructed

wetlands

o GI Wet ponds

• The report shall provide routing calculations of all design storms for every subdrainage area

o Check if pervious surface and impervious surface are separately calculated

o Check NOAA rainfall distribution

o Check rainfall depth

o Check Tc

Option 3:

• The report shall provide routing calculations of the stormwater BMP for all design storms

o Check if exfiltration is used

o Check if the discharge from the orifice is within the reduced flow rate

o Check if the smallest orifice is 2.5” or larger

Option 3:

• Exfiltration may be used in small-scale GI BMP routing calculations for 2, 10, 100-year storms

o See Chapter 5 for conditions to use exfiltration in the routings

• Allowing exfiltration in the routing calculation does not necessarily mean infiltration of the entire 2, 10, 100-year storms.

Option 3:

o Check groundwater mounding analysis

o Check if discharge from the outlet structure will increase if mounding is higher than the bottom of BMP

Option 3:

o Small-scale GI BMPs in Table 5-1 and GI BMPs in Table 5-2 may be used to address the stormwater runoff quantity requirement

• Detention components can be incorporated to provide detention function which reduce peak flow rates.

• Detention components must be situated in the same location as small-scale GI BMP or GI BMP

o Detention space above the maximum water level of the water quality design storm, or

o Below the surface component of the BMP

Use of Detention Component in GI BMPs

STORMWATER RUNOFF QUALITY

Stormwater Runoff Quality

• Remove TSS by 80%

• Remove nutrients to maximum extent feasible

• Applies at net increase of ¼ acre or more of

regulated motor vehicle surface

• Removal rates apply to each on-site drainage area,

unless the runoff converges on-site

• Pursuant to FHACA Rules, runoff from WQDS

discharged within a 300-foot riparian zone must

reduce TSS by 95%

• Regulated Motor Vehicle Surface:

o Any net increase in motor vehicle surface

o Existing motor vehicle surface that is currently receiving

water quality treatment, where the water quality

treatment will be modified or removed.

• Pervious paving areas

o Often used to replace traditional pavement

o Considered regulated motor vehicle surface toward

the ¼ acre increase

o If designed properly, pervious paving systems can

provide required TSS removal

• Rooftop runoff

o Not considered significant source of TSS

o Can be infiltrated using dry well

o Can be significant source of nutrients

o May require pretreatment – See design criteria in BMP

Manual

• Design storm based on historic data

• Nonlinear rainfall distribution resulting in 1.25 inches of precipitation in 2 hours

• Relatively common and intense storm

• Designed to capture frequent storms that cause significant stormwater runoff pollution

Water Quality Design Storm

Quality

Design

Quality

Design

• Water quality criteria met through implementation of Chapter 9 GI BMPs:

o Cisternso Drywellso Grass swaleso Green roofso GI MTDso Pervious paving systemso Small-scale bioretention systemso Small-scale infiltration basinso Small-scale sand filterso Vegetative filter strips

• NJ BMP Manual contains design criteria for the various Chapter 9 GI BMPs

BMPs for Meeting Water Quality

% TSS and Nutrient Removal Rates for BMPs

BMP TSS RemovalRate

Phosphorus Removal Rate*

Nitrogen Removal Rate*

Bioretention Systems 80-90% 60% 30%

Standard Constructed Wetlands 90% 50% 30%

Extended Detention Basins 40-60% 20% 20%

Infiltration Basins 80% 60% 50%

Manufactured Treatment Devices 50% or 80% N/A N/A

Pervious Paving Systems 80% 60% 50%

Sand Filters 80% 50% 35%

Vegetative Filter Strips 60-80% 30% 30%

Wet Ponds 50-90% 50% 30%

Grass Swales ≤ 50% N/A N/A

Subsurface Gravel Wetlands 90% N/A 90%

• Not all BMPs meet 80% TSS removal

• Can use multiple BMPs in series to achieve reduction

• For two BMPs in series: 𝑅 = 𝐴 + 𝐵 −𝐴𝑥𝐵

Water Quality BMPs in Series

• When using BMPs in series, arrange BMPs from upstream to downstream in:1. Ascending order of TSS removal rate2. Ascending order of nutrient removal rate3. By relative ease of sediment and debris removal

• Do not use two of same type of BMPs in series

Vegetative Filter Strip (60%)

Small-scale Infiltration Basin (80%)

Total % TSS Removal, 𝑅 = 60 + 80 – [(60*80)/100] = 92%

Water Quality BMPs in Series

• Required on all major developments

• Shall be addressed to maximum extent feasible

• Some BMPs have adopted nutrient removals

• Nutrient removal often best performed through source control

Nutrient Removal

• Stormwater Management rules require:

o Design BMPs in accordance with BMP Manual or

o Alternative designs if design engineer provides documentation demonstrating capability of alternative removal rates and methods

• Any approved alternatives must be submitted to the Department

Water Quality BMPs

• Ensure water quality BMPs chosen are adequate to achieve required TSS removal rate

• Ensure small-scale GI BMPs from Table 5-1 are used unless a waiver or variance from NJAC 7:8-5.3 is obtained

• Ensure runoff from all drainage areas requiring water quality treatment is being collected and treated

• Ensure BMP design meets criteria under applicable BMP Manual subchapter

• Ensure a deed notice of the stormwater BMP is submitted for approval and recorded in county clerk's office prior to the commencement of construction

Reviewing for Water Quality Compliance

• BMPs must each drain within 72 hours of storm

o BMPs installed subject to pedestrian traffic, drain time is 24 hours

• In general, most GI BMPs require at least 2 ft separation from seasonal high water table

o Underdrain GI BMP needs 1 ft separation

o GI MTD is subject to manufacturer’s specification

Water Quality BMPs – General Design Criteria

• Commonly used BMPs:

o Small-scale infiltration basins

o Small-scale bioretention systems

o GI Manufactured treatment devices

o Grass swales

o Pervious paving systems

Water Quality BMPs

• Designed to infiltrate runoff into subsoil

• 80% TSS removal rate

• Can also be used to meet the water quantity and groundwater recharge requirements

Small-scale Infiltration Basins

• Must be designed to infiltrate Water Quality Design Storm volume

• Must meet infiltration permeability testing requirements in accordance with Chapter 12

• Maximum depth from WQDS of 24 inches

Small-scale Infiltration Basins –Design Criteria

• Must include a 6 inch, AASHTO M-6 or ASTM C-33 sand layer

• Bottom of basin must be level and un-compacted

• Subsurface infiltration basins require 80% TSS pretreatment

• Design hydraulic conductivity of most restrictive soil horizon below basin bottom should be between 0.5 –10 inches/hour

Small-scale Infiltration Basins –Design Criteria

• Designed for water quantity control but no outlet structure

• Underdrains or liners

• Topsoil or vegetation

• Standing water 72 hours after the precipitation stops

Small-scale Infiltration Basins –You Should Not See …

101Courtesy of NJDOT

• Thick soil bed and dense vegetation to enhance pollutant removal

• TSS removal rate based on types of plants and soil bed thickness

Small-scale Bioretention Systems

• Vary in size from small rain gardens to basins

• Can be designed to infiltrate or to be underdrained

Small-scale Bioretention Systems

• Must include a soil bed at 18 - 24 inches in depth

• Bioretention mix must consist of following, by volume:

o 85 - 95% sand (< 25% fine or very fine sand)

o No more than 15% silt and clay

o 2 - 5% clay

• Bioretention mix must be amended with 3 - 7% organics, by weight

Small-scale Bioretention Systems –Design Criteria

• Maximum water quality depth of 12 in. for flat-bottomed systems

• Maximum water quality depth of 18 in. at down-gradient end of a bioretention swale

• Maximum bottom slope of 10% for bioretention swale

• Minimum density of vegetation of 85%

Small-scale Bioretention Systems –Design Criteria

• Applies to any BMP designed to infiltrate runoff

o Small-scale infiltration basin

o Dry well

o Small-scale bioretention system (w/o underdrain)

o Small-scale sand filter (w/o underdrain)

Infiltration Criteria

• Soil permeability must be tested

• Must apply a factor of 2 to any tested permeability

𝐷𝑒𝑠𝑖𝑔𝑛 𝑃𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑡𝑦 =𝐹𝑖𝑒𝑙𝑑 𝑃𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑡𝑦

• Minimum design permeability of 0.5 inches/hour, maximum of 10 inches/hour

• Must have at least 2 feet of separation from SHWT

• Depth to SHWT must be proven via soil testing

• Must assess groundwater mounding impacts

• Protection of infiltration area from compaction and sedimentation during construction

• Post-construction soil testing to verify as-built conditions are sufficient to allow infiltration

Infiltration BMPs – What you should see

More Information:

Stormwater Management UnitBureau of Flood Hazard and Stormwater Engineering

Watershed Engineering ElementDivision of Watershed Protection and Restoration

401 East State Street

PO Box 420, Mail Code 401-2B

Trenton, NJ 08625-420

Tel: 609-633-7021

www.njstormwater.org

Minesh Patelminesh.patel@dep.nj.gov

Anthony Robalikanthony.robalik@dep.nj.gov

REVIEWA PROJECT 2

HOW TO

Changi WuNJDEP Division of Watershed Protection and Restoration

SWMDR Training Module 3October 12, 2021

Presentation Goals

Goal 1: Learn how to review BMPs for compliance

with the design criteria in the BMP ManualGoal 2: Calculate Detention Time

Goal 3: Learn how to review the safety requirements

Goal 4: MTD DesignGoal 5: Groundwater Recharge Spreadsheet

Example: Small-Scale Bioretention System

• Step 1: Calculate the runoff volume and peak flow rate for water quality design storm by using NRCS methodo 939 cf and 0.74 cfs

• Step 2: Sizing the pretreatmento In this case, a MTD is used for pretreatment by following the

MTD certification letter

• Step 3: Run the routing calculation with exfiltrationo Infiltration area is initially set up as 600 sf

o Side slope is 3:1 (H:V)

o The size of surface areas and the dimensions of the basin at different heights can be calculated accordingly based on the slope

o The exfiltration rate is half of the tested saturated hydraulic conductivity of the most restrictive soil layer as described in Chapter 12• 1 in/hr is used for exfiltration rate

• Areas on the side slope cannot be used for exfiltration

• The discharge rate through the infiltration area shall be calculated:o 1 in/hr x 600 sf x 1/12 in/ft x 3,600 hr/sec = 0.01 cfs

• Step 3: Run the routing calculation with exfiltration (Continued)

• Adjust the height of the first orifice to result in a water depth that only allows for infiltration of the water quality design storm

o 0.98 ft

• The runoff volume discharged through exfiltration is 939 cfs

• The drain time is the discharged volume through exfiltration divided by the exfiltration rate

o 18.78 hrs

• Step 4: Check the Separation from SHWT

o Lowest point of the basin is 3 ft below the existing ground level

o SHWT is 8 feet below the existing ground level

o SHWT is 5 feet from the lowest point of the basin

• Step 5: Groundwater Mounding Analysis

o 1 in/hr for recharge rate as the initial input

o Horizontal hydraulic conductivity rate is 1 times the recharge rate as the project is outside the coastal plain

o Use the drain time calculated in step 3 as the duration of infiltration

o Use half of the basin dimensions calculated in step 3 as input for x and y

• The mounding height is 2.861 ft from the water table

• SHWT is 5 feet below the lowest point of the basin

• The mounding will reach to 2.139 feet below the lowest point of the basin

Water Quality

Extended Detention Basins

• Extended detention basins cannot be used for groundwater recharge

• TSS removal rate based on detention time

• Detention Time: time between when the maximum storage volume is achieved to when only 10% of the maximum volume remains

• Use Water Quality Design Storm for TSS removal rate calculation

Water Quality

Courtesy of NJDOT

Extended Detention Basins – Design Example

Water Quality

• Maximum storage: 0.832 acre-ft at 2.10 hrs

• 10% of maximum storage: 0.0832 acre-ft

Extended Detention Basins –

Design Example

Water Quality

Water QualityWater Quality

Water Quality

• Maximum storage: 2.10 hr• Detention time ends at 17.85 hr• Detention time = 17.85 - 2.10 = 15.75 hr• TSS removal rate ≈ 47%

Wet Ponds

• Wet ponds (GI)o designed to maintain at least a 10-foot wide area of native

vegetation along at least 50 percent of the shoreline and to include a stormwater runoff retention component designed to capture stormwater runoff for beneficial reuse, such as irrigation

o used for water quantityo used for water quality only when a Waiver or Variance from

N.J.A.C. 7:8-5.3 is obtainedo BMP Chapter 10.5

• Wet Ponds (Non-GI)o used for water quantity and quality only when

a Waiver or Variance from N.J.A.C. 7:8-5.3 is obtainedo BMP Chapter 11.6

Two Different BMPs

Wet Ponds

• Minimum 0.25 acre permanent pool surface area

• Minimum ratio of the permanent pool volume to the Water Quality Design Storm volume is 1:1

• Minimum drainage area of 20 acres

• Requires very low permeability soils or an impermeable liner

Design Criteria

Wet Ponds

TSS Removal Rate

• Without extended detention time

o TSS removal rate is based on the ratio of volume in the permanent pool to the volume of the Water Quality Design Storm volume

o Ratio is from 1:1 to 3:1 for TSS removal rate from 50% to 80%

• Additional TSS removal up to 90% can be provided using extended detention

o Ratio of permanent pool volume to Water Quality Design Storm runoff volume (1:1 to 3:1) and

o Detention time (12 to 24 hours)

Water Quality

SAFETY REQUIREMENTS

Safety Requirement

Escape Provision NJAC 7:8-6.2

2.5 ft below

permanent water

1 to 1.5 ft above

permanent water

• Safety Ledges

o Required for basins having a permanent pool of water >2.5 ft

o Requires two steps

• Each step 4-6 ft wide

• 1st step 2.5 ft below permanent water level

• 2nd step 1 to 1.5 ft above permanent water level

MANUFACTURED TREATMENT DEVICES (MTDs) DESIGN

MTD Design

Green Infrastructure Guidance for MTDs (April 23, 2020)

For an MTD to be considered “green infrastructure” in accordance with the March 2, 2020 amendments to the Stormwater Management rules at N.J.A.C. 7:8, the MTD must meet the GI definition noted at amended N.J.A.C. 7:8-1.2.Specifically, the MTD shall (1) infiltrate into the subsoil; and/or (2) treat stormwater runoff through filtration by vegetation or soil.

MTD Design

• Must be verified by NJCAT and certified by NJDEP

• Devices that are not currently certified by NJDEP cannot be used to meet the design and performance standards

• Sizing calculation in the certification letter

• http://www.njstormwater.org/treatment.html

• Maximum contributory drainage area of a GI MTD is 2.5 acres

MTD Design

MTD Design must follow the design criteria and use only models listed in the Certification

• The maximum treatment flow rate (MTFR) for the manufactured treatment device (MTD) is calculated using the New Jersey Water Quality Design Storm (1.25 inches in 2 hrs) in N.J.A.C. 7:8-5.5. …

• The stormwater treatment unit shall be installed using the same configuration reviewed by NJCAT, and sized in accordance with the criteria specified in the certification

• Sizing configuration by the maximum treatment flow rate (MTFR) and maximum inflow impervious area, whichever results in the highest minimum configuration

MTD Design

MTD Design must follow the design criteria and use only models listed in the Certification

Sample language in the Certification Letter settingthe

design criteria:

In order for an GI MTD to meet the definition of GI, the GI MTD

must treat stormwater runoff through filtration by vegetation. To

this end, consistent with the vegetative cover requirement for

bioretention systems, the minimum vegetative cover in an GI

MTD is 85% in order to qualify as GI under the Stormwater

Management rules at N.J.A.C. 7:8. The vegetative cover should

be determined based on the expected coverage of the

proposed plantings when matured. Plant death or damage shall

require replanting to maintain this 85% coverage requirement if

the system is installed as GI.

MTD Design

Sample language in the Certification Letter setting the

design criteria:

Any MTD with bio-filtration media that would be placed "below

ground" as a vault without any vegetation can be considered

GI (for NJ purposes) only if the device infiltrates the entire Water

Quality Design Storm into the subsoil. Further, the below ground

device (vault) would need to meet the NJDEP Stormwater BMP

Manual conditions of having the soil below the MTD meet the

minimum tested infiltration rate of one inch per hour, have at

least two feet of separation from the seasonal high water table, and infiltrate into the subsoil.

MTD Design

• MTDs are commonly sized using the rational method

• Q = ciA

• Many people mistakenly think the intensity of the Water Quality Design storm is the average of the 1.25 inches of rain in two hours

o 1.25 inches / 2 hours = 0.625 inches/hour– This is not correct!

Design Criteria

Water Quality

Design Criteria

• Tc must be calculated in accordance with Chapter 15, in Part 630 of the NEH.

• For Tc = 10 minutes, i = 3.2 inches/hr

o Using 0.625 inches/hr underestimates flow by 80.5%

Water Quality

• BMPs should be designed to meet criteria in the BMP Manual

• Multiple BMPs can be used in series when necessary

• Infiltration BMPs require detailed soil permeability testing

• Detention BMPs must calculate the detention time to determine TSS removal rate

• Use intensity-Duration-Frequency curve in Figure 5-16 when using Rational Method to calculate peak flow rate

In summary…

GROUNDWATER RECHARGE

Groundwater Recharge

• Demonstrate that 100% of the pre-development groundwater recharge is maintained or

• Infiltrate the difference in the 2-year storm runoff volume

Rule Requirements

• Pros:o Simple calculation

• Cons:o Requires larger infiltration structure,

o Greater risk of failure,

o Larger groundwater mound

Infiltrate 2-year storm difference:

• Use NRCS Method

• Show pre- and post-construction 2-year runoff volumes

• Follow all other assumptions of NRCS Method

o Curve numbers and time of concentration match quantity and quality analyses

The calculation of the 2-year storm difference should…

• Requires use of New Jersey Groundwater Recharge Spreadsheet

• Need to review:

o Pre-development vs. post-development land use inputs

o BMP designs

Maintain 100% of pre-development groundwater recharge:

• Inputs (Orange cells):o Municipality

o Pre- and post-development land covers

o Pre- and post-development soils

o Acres

• Output:o Post-development annual recharge deficit

(Annual Recharge Worksheet)

Groundwater Recharge Spreadsheet

• Soils and land covers must match site plans

• Soils determined by soil survey or Chapter 12 defaults

• Pre- and post-development areas should match

(Annual Recharge Worksheet)

• Parameter from Annual Recharge Worksheeto Vdef: Post-development deficit recharge

• Default from page 1 or other inputo Aimp: Post-D Impervious area (Targeted

Impervious area) contributing to recharge calculation• Default value is the total impervious surface from the first

page• Can be partial impervious surface runoff to recharge BMP

• Automatically populated outputo Annul BMP recharge volume

(BMP Calculation)

• segBMP = the post-developed site segment in which the proposed recharge BMP will be locatedo Used to specify where on the site the recharge BMP will be

located

o Can be set to zero if this is still undetermined or if BMPs will be placed over multiple segments

o Porous pavement, segment shall be the impervious area of the post-development

Parameter from Annual Recharge Worksheet

• ABMP = BMP Area (the footprint of a BMP, excluding sloped areas)

• dBMP = BMP effective depth (maximum equivalent water depth that can be achieved in the BMP before overflow begins)

o If the BMP is filled with stone, the effective depth is the actual depth x the porosity

o For sloped BMP like bioretention swale, use averaged dBMP but adjust ABMP to keep that ABMP x dBMP should always equal the storage volume

BMP Design Inputs

• dBMPu = Vertical distance from the vegetated ground surface to the maximum water level of the BMPo Positive if the maximum level is below the ground

surface and negative if above the ground surface

• dEXC = Vertical distance from the vegetated ground surface to the bottom of the BMP

BMP Design Inputs

BMP Design

Groundwater Recharge Example

GL EL.=15 Berm EL.=16

• dBMPu = 15 (GL)– 13.36 (orifice)

= 1.64 ft (19.7 inches)

• dEXC = 15 – 12 = 3 ft (36 inches)

BMP Design

• The BMP effective depth: 1.36 ft (13.36 – 12 ft)

• Volume below first outlet: 59,400 ft3

BMP Design

• dBMP = 1.36 ft

• ABMP = ?o Remember: BMP area times effective depth must equal

volume

o 𝐴𝐵𝑀𝑃 =𝑉

𝑑𝐵𝑀𝑃=

59,400

1.36=43,676 ft2

BMP Design

• ABMP = 43,676 ft2 (the same)

• dBMP = 1.36 ft (the same)

• dBMPu = -1.36 feet (16.3 inches)

• dEXC = 0 inches

What if this BMP had been designed as a bioretention system?

• Recharge of stormwater with high pollutant loading or industrial stormwater exposed to source material is prohibited

• All recharge BMPs must meet the infiltration criteria and structural design criteria in Chapters 9, 10, 11

• All BMPs designed to infiltrate must consider adverse hydraulic impact on the groundwater table

Rule Requirements

Water Quality

Summary

• Highlights of BMPso Small-Scale Bioretention System

o Extended detention basins

o Wet ponds

• Soil Testing – Appendix E/ Chapter 12

• Groundwater Recharge

More Information:

Tel: 609-633-7021

changi wuChangi.wu@dep.nj.gov

MOUNDINGREQUIREMENTS, CALCULATIONS

and EXAMPLES

GROUNDWATER

Changi Wu/ Lisa Schaefer

NJDEP Division of Watershed Protection

and Restoration

SWMDR Training Module 3

October 12, 2021

Introduction

• Requirements Under N.J.A.C. 7:8

• Concepts

• Method of Calculation

• Examples

Groundwater Mounding

• N.J.A.C. 7:8-5.4(a)2.iv, “The design engineer shall assess the hydraulic impact on the groundwater table and design the site so as to avoid adverse hydraulic impacts. “

• Localized increase in the height of the groundwater table as a result of infiltration

• Caused by concentrating large amount of recharge into one area

• Can cause basin failure or damage to nearby structures

Background

https://pubs.usgs.gov/sir/2010/5102/support/sir2010-5102.pdf4

METHOD of

CALCULATION

• Hantush equation developed in 1967 to calculate mounding beneath infiltration basin

• Easiest way to use the Hantush method is a spreadsheet developed by USGS

• Can only calculate the maximum height of mounding

Calculation

https://www.njstormwater.org/bmp_manual/

Chapter 13

Download Hantush Excel Spreadsheet

The Hantush Spreadsheet

• Recharge rate (in/hr)

• Specific yield (dimensionless)

• Horizontal hydraulic conductivity (in/hr)

• Basin dimensions (feet)

• Duration of infiltration (hours)

• Initial thickness of saturated zone (ft)

Input Parameters

• Recharge rate (in/hr)

o Design soil permeability rate (vertical saturated soil hydraulic conductivity)

• Horizontal hydraulic conductivity

o Rarely a tested parameter

o In the coastal plain: 5 𝑥 recharge rate

o Outside the coastal plain: 1 𝑥 recharge rate

Input Parameters

• Specific yield

o Specific yield governs how much water the unsaturated zone can store when recharged runoff reaches the water table

o The volume of water that will drain from the soil, as a result of gravity, divided by the total volume of the soil

o Default value 0.15 and up to a maximum value 0.2, with supporting soil test results

Input Parameters

• Basin dimensions (assumes rectangular and vertical sides)

o Input as ½ length in x-direction and ½ length in y-direction of the footprint of a BMP

• Circular shape BMP use the radius of the circular basin as both x and y.

• Irregular shape BMP, convert the shape to a rectangular shape that has same depth of stormwater runoff to be infiltrated and is best fitted to the original shape.

• If a BMP is designed with sloped sides, use the bottom footprint as the length and the width of the BMP and use the total volume of the runoff to be infiltrated divided by the area of the bottom footprint to calculate the duration of infiltration period 11

Input Parameters

• Initial thickness of saturated zone (ft)

o Unless proven by a field test showing the thickness of saturated zone beneath the proposed BMP, the default value is 10 ft.

o Note that the thickness of saturated zone used in the Hantush spreadsheet is not the thickness of a large regional aquifer. It is the distance from the Seasonal High Water Table (SHWT) to the first hydraulically restrictive layer.

Input Parameters

• Duration of infiltration (hours)

o Time for infiltrating the runoff volume through the BMP

o Using the recharge rate in the input

o Maximum value is 72 hr

Input Parameters

• Maximum mounding height decreases

o when following inputs increase:

• Horizontal hydraulic conductivity increases

• Specific yield increases

• Initial thickness of saturated zone increases

How the Input Values Affect the Results

• When the mounding height is higher than the bottom of the BMP, the recharge rate is impacted because of the reduced hydraulic gradient

o Use a smaller recharge and longer duration of infiltration

o The recharge rate times the duration of infiltration and the area of the footprint shall be equal to the volume infiltrated

Interpreting the Results

𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑟𝑢𝑛𝑜𝑓𝑓 𝑡𝑜 𝑏𝑒 𝑖𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒𝑑 𝑐𝑓 =

Duration of 𝑥 𝐼𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑥 𝑅𝑒𝑐ℎ𝑎𝑟𝑔𝑒

infiltration period (hr) 𝑎𝑟𝑒𝑎 𝑠𝑓 𝑟𝑎𝑡𝑒 𝑖𝑛/ℎ𝑟

• When result shows that mounding height reaches the bottom of the infiltration BMP o decrease recharge rate & longer infiltration period, or

o design a larger but shallower basin

Interpreting the Results

GROUNDWATER

MOUNDING

EXAMPLES

Introduction

• Demonstrate simple analysis

• Demonstrate calculating impacts to nearby structures

Examples

1. No adverse hydraulic impact to groundwater

2. Adverse hydraulic impact to groundwater and adjustments required

Scenarios:

No Adverse Hydraulic Impact to Groundwater Scenario

• A major development located in the coastal plain proposes an

infiltration basin, whose bottom footprint measures 50 ft by 50 ft, to

infiltrate 5,000 cf of runoff generated by the Water Quality Design

Storm (WQDS).

o The maximum depth of ponding water is designed to be 2 ft

o Soil testing was performed in accordance with Chapter 12

o The soil testing report shows that the Seasonal High Water Table

(SHWT) is 7.5 ft below the 6 inch thick bottom sand layer of the

o The tested soil permeability rate of the most restrictive soil horizon

below the basin is 4 in/hr

o No nearby underground structures are present

Step #1: Calculate the duration of infiltration period

Duration of infiltration period, t (hours)

= 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑟𝑢𝑛𝑜𝑓𝑓 𝑡𝑜 𝑏𝑒 𝑖𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒𝑑 𝑐𝑓 𝑥 12 𝑖𝑛/𝑓𝑡

𝐼𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑎𝑟𝑒𝑎 𝑠𝑓 𝑥 𝑅𝑒𝑐ℎ𝑎𝑟𝑔𝑒 𝑟𝑎𝑡𝑒 𝑖𝑛/ℎ𝑟

Infiltration area

= 50 ft 𝑥 50 ft = 2,500 sf

Recharge Rate = ½ Tested Infiltration rate

= 0.5 𝑥 4 in/hr = 2 in/hr

Therefore, t

= 5,000 𝑐𝑓 𝑥 12 𝑖𝑛/𝑓𝑡

2,500 𝑠𝑓 𝑥 2 𝑖𝑛/ℎ𝑟= 12 hr

Step #2: Prepare the inputs for the spreadsheet

Parameter Value

Recharge rate (R) 2 in/hr (4 ft/day)

Specific yield (Sy) 0.15

Horizontal hydraulic conductivity (Kh) 10 in/hr (20 ft/day)

½ length of basin (x direction) 25 ft

½ length of basin (y direction) 25 ft

Duration of infiltration period (t) 12 hr (0.5 days)

Initial thickness of saturated zone (hi(0)) 10 ft

Step #2: Input Section of the Hantush Spreadsheet

Step #2: Results Section of the Hantush Spreadsheet

How the Hantush Spreadsheet Represents an Adverse Hydraulic Impact

Results Section of the Hantush Spreadsheet

Drywell Scenario

• A site is located outside the coastal plain. A 7 ft high by 5 ft diameter drywell is proposed to store and infiltrate 137.4 cf of roof runoff from a single-family home for the Water Quality Design Storm.

o The house has a basement floor located just above the groundwater table.

o The edge of the drywell is 10 ft away from the house. o Soil testing was performed in accordance with Chapter 12o The soil testing report shows that SHWT is 2 ft below the bottom of

the drywello The tested soil permeability rate of the most restrictive soil horizon

below the basin is 4 in/hro The duration of infiltration period, t =

= 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑟𝑢𝑛𝑜𝑓𝑓 𝑡𝑜 𝑏𝑒 𝑖𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒𝑑 𝑐𝑓 𝑥 12 𝑖𝑛/𝑓𝑡

𝐼𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑎𝑟𝑒𝑎 𝑠𝑓 𝑥 𝑅𝑒𝑐ℎ𝑎𝑟𝑔𝑒 𝑟𝑎𝑡𝑒 𝑖𝑛/ℎ𝑟

= 137.4 𝑐𝑓 𝑥 12 𝑖𝑛/𝑓𝑡

19.62 𝑠𝑓 𝑥 2 𝑖𝑛/ℎ𝑟= 42 hr

Drywell Scenario

Step #1: Prepare the inputs for the spreadsheet

Parameter Value

Recharge rate (R) 2 in/hr (4 ft/day)

Specific yield (Sy) 0.15

Horizontal hydraulic conductivity (Kh) 2 in/hr (4 ft/day)

½ length of basin (x direction) 2.5 ft

½ length of basin (y direction) 2.5 ft

Duration of infiltration period (t) 42 hr

Initial thickness of saturated zone (hi(0)) 10 ft

Drywell scenario

Centerline of the drywell

Drywell Scenario

Any Questions?

Tel: 609-633-7021

Changi WuChang.i.wu@dep.nj.gov

Lisa Schaeferlisa.schaefer@dep.nj.gov

module 3 of 4 stormwater management design review

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