Wake County Stormwater Workshop Guidance on the New Stormwater Ordinance and Design Manual August 29, 2006.

Wake County Stormwater Wake County Stormwater WorkshopWorkshop

Guidance on the New Guidance on the New Stormwater Ordinance and Stormwater Ordinance and

Design ManualDesign Manual

August 29, 2006August 29, 2006

Wake County Stormwater Presentation August 2006

IntroductionIntroduction

►Wake County met with a group of Wake County met with a group of stakeholders and formed a committee stakeholders and formed a committee to create a new stormwater ordinanceto create a new stormwater ordinance

►After a year long process, of After a year long process, of education, discussion, and debate - education, discussion, and debate - the committee came to an agreement.the committee came to an agreement.



►Decisions of the Committee:Decisions of the Committee: Runoff Runoff volumevolume management is the goal. (not management is the goal. (not

peak discharge management)peak discharge management) Runoff volume management will be achieved by Runoff volume management will be achieved by

managing the curve number.managing the curve number. Target curve numbers are set and proposed Target curve numbers are set and proposed

developments should match the target or developments should match the target or manage the excess runoff for a 3-inch manage the excess runoff for a 3-inch precipitation depth.precipitation depth.

Excess runoff will be infiltrated or stored and Excess runoff will be infiltrated or stored and released over a period of 2 to 5 days.released over a period of 2 to 5 days.



► Target Curve Target Curve NumbersNumbers

Zoning District

Maximum Composite Curve Number, By Soil Group

A B C D

R-80W and R-80 37 60 73 79

R-40W and R-40 41 62 75 80

R-30, R-20, R-15, R-10, R-5, Residential Highway, General Business and Office and Institutional

43 63 76 81



►Goals of Ordinance:Goals of Ordinance: Protect streams and other resources.Protect streams and other resources. Encourage low impact design approaches.Encourage low impact design approaches. Allow flexibility for designer.Allow flexibility for designer. Meet the Neuse Rules.Meet the Neuse Rules.


Design ManualDesign Manual

► Intent:Intent: Provide procedure to meet the ordinance.Provide procedure to meet the ordinance.

►Curve Number Method (TR-55)Curve Number Method (TR-55)►SCS Runoff Equation (TR-55)SCS Runoff Equation (TR-55)►Storage/Infiltration DevicesStorage/Infiltration Devices

Discuss design approaches.Discuss design approaches.►Conventional designConventional design►Low impact designLow impact design►Hybrid designHybrid design


Curve NumbersCurve Numbers

► Integers ranging from 30 to 98.Integers ranging from 30 to 98.►Based on Land-use and Hydrologic Soil Based on Land-use and Hydrologic Soil

Group.Group.►Hydrologic Soil Group is determined by Hydrologic Soil Group is determined by

soil series.soil series.►Curve numbers represent the amount Curve numbers represent the amount

of runoff expected from a specific of runoff expected from a specific area. A high curve number represents area. A high curve number represents more runoff than a low curve number.more runoff than a low curve number.



Curve NumbersCurve Numbers

Surface Cover TypeCurve Numbers for Hydrologic Soil Group

A B C D

Woods 30 55 70 77

Open Space (Lawns, Pasture, etc) 39 61 74 80

Impervious 98 98 98 98

Surface Cover TypeCurve Numbers for Hydrologic Soil Group

A B C D

Open Space (Grass portion of R/W) 80 80 80 80

Impervious (Paved portion of R/W) 98 98 98 98


ProcedureProcedure

► Curve Number MethodCurve Number Method Calculate Target Curve Calculate Target Curve

Number for Proposed Number for Proposed Site based on Zoning and Site based on Zoning and Soil CompositionSoil Composition

Calculate Proposed Site Calculate Proposed Site Curve Number based on Curve Number based on Soil Composition and Soil Composition and proposed land use dataproposed land use data

Zoning District

Maximum Composite Curve Number, By

Soil Group

A B C D

R-80W and R-80 37 60 73 79

R-40W and R-40 41 62 75 80

R-30, R-20, R-15, R-10, R-5, Residential Highway, General Business and Office and Institutional

43 63 76 81


ExampleExample

►Calculate Target Curve NumberCalculate Target Curve Number Proposed Site zoned as R-40WProposed Site zoned as R-40W 75% B Soils, 25% C Soils75% B Soils, 25% C Soils

(0.75*62) + (0.25*75) = 65.25(0.75*62) + (0.25*75) = 65.25

Target Curve Number for site is 65.Target Curve Number for site is 65.


ExampleExample

► Calculate Proposed Curve NumberCalculate Proposed Curve Number 10-acre Site10-acre Site

►Proposed Woods, HSG B (CN=55) = 4 acresProposed Woods, HSG B (CN=55) = 4 acres►Proposed Open Space, HSG B (CN= 61) = 1.5 acresProposed Open Space, HSG B (CN= 61) = 1.5 acres►Proposed Open Space, HSG C (CN=74) = 2 acresProposed Open Space, HSG C (CN=74) = 2 acres►Proposed Impervious (CN=98) = 1.5 acresProposed Impervious (CN=98) = 1.5 acres►Proposed Impervious in R/W (CN=98) = 0.5 acresProposed Impervious in R/W (CN=98) = 0.5 acres►Proposed Open Space in R/W (CN=80) = 0.5 acresProposed Open Space in R/W (CN=80) = 0.5 acres

(55*4)+(61*2)+(74*2)+(98*2)+(80*0.5) = 695.5(55*4)+(61*2)+(74*2)+(98*2)+(80*0.5) = 695.5

695.5/10 = 69.55695.5/10 = 69.55 Proposed Curve Number is 70.Proposed Curve Number is 70.


ProcedureProcedure

►Calculate Runoff Volume to be stored or Calculate Runoff Volume to be stored or infiltrated using SCS Runoff Equation.infiltrated using SCS Runoff Equation.

►SCS Runoff Equation (TR-55):SCS Runoff Equation (TR-55):

Q* = (P – 0.2S)Q* = (P – 0.2S)22

(P + 0.8S)(P + 0.8S)Where:Where:

Q* = runoff in inchesQ* = runoff in inches

P = Precipitation Depth in inches (3 inches)P = Precipitation Depth in inches (3 inches)

S = Potential maximum retention in inchesS = Potential maximum retention in inches


ProcedureProcedure

► In order to calculate Q* - first calculate S.In order to calculate Q* - first calculate S.

S = (1000/CN) – 10S = (1000/CN) – 10

►Q* must be calculated for the Target and Q* must be calculated for the Target and Proposed curve numbers and then Proposed curve numbers and then multiplied by the site acreage to determine multiplied by the site acreage to determine the total runoff volume for each CNthe total runoff volume for each CN

► The difference between the two volumes The difference between the two volumes must be handled by stormwater devicesmust be handled by stormwater devices


ExampleExample

►Calculate Runoff Volume.Calculate Runoff Volume. Site acreage = 10 acresSite acreage = 10 acres Target CN = 65Target CN = 65 Proposed CN = 70Proposed CN = 70

SStargettarget = (1000/65)-10 = 5.38 inches = (1000/65)-10 = 5.38 inches

Q*Q*targettarget = (3+ (0.2*5.38)) = (3+ (0.2*5.38))22/(3+(0.8*5.38)) = 0.51 inches/(3+(0.8*5.38)) = 0.51 inches

SSproposedproposed = (1000/69)-10 = 4.49 inches = (1000/69)-10 = 4.49 inches

Q*Q*proposed proposed = (3+ (0.2*4.29))= (3+ (0.2*4.29))22/(3+(0.8*4.29)) = 0.71 inches/(3+(0.8*4.29)) = 0.71 inches

Q*Q*proposed proposed - Q*- Q*targettarget = 0.71 – 0.51 = 0.20 inches = 0.71 – 0.51 = 0.20 inches0.20 inches * 10 acres = 2.0 acre-inches = 0.20 inches * 10 acres = 2.0 acre-inches = 7,405 ft7,405 ft33


ProcedureProcedure

►Neuse Rules dictate that any development Neuse Rules dictate that any development of 15% impervious or greater must not of 15% impervious or greater must not cause an increase in the post-development cause an increase in the post-development 1-year, 24-hour storm from the pre-1-year, 24-hour storm from the pre-development conditions.development conditions.

►Wake County has adopted the Neuse Rules Wake County has adopted the Neuse Rules for the Cape Fear River Basin as well.for the Cape Fear River Basin as well.

► Therefore, sites of 15% impervious or more Therefore, sites of 15% impervious or more must meet both the Wake County must meet both the Wake County Stormwater Ordinance and the Neuse RulesStormwater Ordinance and the Neuse Rules


ProcedureProcedure

►For these sites – the peak flow must be For these sites – the peak flow must be calculated using the SCS method.calculated using the SCS method.

►Time of concentrations for pre- and Time of concentrations for pre- and post-development must be calculated post-development must be calculated using methodology in TR-55. It is also using methodology in TR-55. It is also included in the design manual.included in the design manual.

►Type II SCS curve with a 3-inch total Type II SCS curve with a 3-inch total depth should be used.depth should be used.


Design ApproachesDesign Approaches

► Three approaches can be used for volume Three approaches can be used for volume management and peak flow management, if management and peak flow management, if required:required: Conventional approachConventional approach

►This is the typical approach to stormwater on a This is the typical approach to stormwater on a subdivision and usually incorporates a wet or dry pond.subdivision and usually incorporates a wet or dry pond.

Low impact design approachLow impact design approach►This includes trying to mimic the natural hydrologic This includes trying to mimic the natural hydrologic

conditions by allowing for as much infiltration and conditions by allowing for as much infiltration and transpiration as possible.transpiration as possible.

Hybrid of the two approachesHybrid of the two approaches


Low Impact DevelopmentLow Impact Development

►Minimize runoff by:Minimize runoff by: Maximizing wooded areas by preserving Maximizing wooded areas by preserving

woods on lots and in common open space.woods on lots and in common open space. Minimizing impervious areas by using Minimizing impervious areas by using

narrow roads, reducing on lot impervious, narrow roads, reducing on lot impervious, and/or replacing with pervious pavers.and/or replacing with pervious pavers.

Maximizing opportunity for infiltration and Maximizing opportunity for infiltration and transpiration by using open ditch section transpiration by using open ditch section roads instead of curb and gutter and roads instead of curb and gutter and disconnecting impervious areas. disconnecting impervious areas.


Low Impact DevelopmentLow Impact Development


Land Use PracticesLand Use Practices

► Preservation of Wooded Areas and Preservation of Wooded Areas and ReforestationReforestation Important tool used to lower volume of runoff by Important tool used to lower volume of runoff by

lowering the proposed curve number.lowering the proposed curve number. Due to the locations of houses, driveways, and septic Due to the locations of houses, driveways, and septic

fields being largely unknown at the construction fields being largely unknown at the construction drawing stage – it may not be practical to depict a drawing stage – it may not be practical to depict a boundary on lots of preserved wooded areas at the boundary on lots of preserved wooded areas at the preliminary plan or construction drawing level.preliminary plan or construction drawing level.

Therefore – the County has allowed a set assumption Therefore – the County has allowed a set assumption on lots to be made without designation of a on lots to be made without designation of a boundary. boundary.

Preserved wooded areas and reforested areas can be Preserved wooded areas and reforested areas can be set aside in common open space as well and does set aside in common open space as well and does not have a set assumption as lots do.not have a set assumption as lots do.



► Preservation of Preservation of Wooded Areas Wooded Areas and Reforestationand Reforestation Use maximum Use maximum

assumption of assumption of woods on lots.woods on lots.

If exceeding If exceeding assumption – assumption – boundary for all boundary for all preserved woods preserved woods on lots must be on lots must be provided.provided.

Zoning DistrictMaximum Assumption

of Woods on Lots without Designation

R-80W and R-80 30%

R-40W and R-40 20%

R-30 15%

R-20, R-15, R-10, R-5, Residential Highway, General Business and Office and Institutional

0%



►Disconnected Impervious AreasDisconnected Impervious Areas TR-55 describes as impervious areas in which TR-55 describes as impervious areas in which

the runoff is spread over a pervious area as the runoff is spread over a pervious area as sheet flow.sheet flow.

TR-55 provides an equation in order to reduce TR-55 provides an equation in order to reduce the composite curve number to reflect the the composite curve number to reflect the reduction in runoff for impervious surfaces reduction in runoff for impervious surfaces that has the opportunity to infiltrate. that has the opportunity to infiltrate.

The opposite of this would be impervious The opposite of this would be impervious surfaces that are connected by pipe systems surfaces that are connected by pipe systems or do not have adequate opportunity to or do not have adequate opportunity to infiltrate.infiltrate.



►Disconnected ImperviousDisconnected Impervious TR-55 Equation:TR-55 Equation:

CNCNcc = CN = CNpp + [(P + [(Pimpimp/100)*(98-CN/100)*(98-CNpp)*(1-(0.5R))])*(1-(0.5R))]

Where:Where:

CNCNcc = Composite Curve Number = Composite Curve Number

CNCNpp = Pervious Runoff Curve Number = Pervious Runoff Curve Number

PPimpimp = Percent Impervious = Percent Impervious

R = Ratio of Disconnected Impervious Area to Total R = Ratio of Disconnected Impervious Area to Total Impervious AreaImpervious Area



►Disconnected Impervious SurfaceDisconnected Impervious Surface How is “opportunity to infiltrate” quantified?How is “opportunity to infiltrate” quantified? Wake County:Wake County:

►RooftopRooftop Minimum of 50 ft in direction of flow across pervious Minimum of 50 ft in direction of flow across pervious

surface before channelizationsurface before channelization Maximum of 500 sq. ft. of rooftop to downspoutMaximum of 500 sq. ft. of rooftop to downspout Splashblock or equivalent area of gravel dissipaterSplashblock or equivalent area of gravel dissipater

►RoadwayRoadway Minimum flow length across pervious surface before Minimum flow length across pervious surface before

channelization is equal to flow path across impervious channelization is equal to flow path across impervious surfacesurface

Maximum flow path across impervious surface is 75 ftMaximum flow path across impervious surface is 75 ft Maximum slope of pervious surface receiving flow is 5%Maximum slope of pervious surface receiving flow is 5% Pervious pavers and pavement is automatically Pervious pavers and pavement is automatically

disconnected.disconnected.



►Disconnected Impervious SurfaceDisconnected Impervious Surface As with preserved wooded areas, a As with preserved wooded areas, a

maximum assumption can be used for the maximum assumption can be used for the amount of impervious surface that is amount of impervious surface that is disconnected.disconnected.

This is due to the fact that house and This is due to the fact that house and driveway locations, areas to each driveway locations, areas to each downspout, and grades are largely downspout, and grades are largely unknown on the lots at the preliminary unknown on the lots at the preliminary plan and construction drawing level. plan and construction drawing level.



►Disconnected Disconnected Impervious SurfaceImpervious Surface With this With this

assumption, an R-80 assumption, an R-80 subdivision would subdivision would apply an R=0.5 to apply an R=0.5 to the TR-55 equationthe TR-55 equation

If exceeding If exceeding assumption, assumption, disconnection must disconnection must be shown on plans.be shown on plans.

Zoning District

Maximum Assumption of Disconnection

without Designation

R-80W, R-80, R-40W, R40, R-30

50%

R-20, R-15, R-10, R-5, Residential Highway, General Business and Office and Institutional

25%


Integrated Management Integrated Management Practices (IMPs)Practices (IMPs)

► Low impact development’s coined phrase Low impact development’s coined phrase for microscale and distributed management for microscale and distributed management techniques.techniques.

► Typically used to manage stormwater at the Typically used to manage stormwater at the source. source.

► Therefore – are most commonly used on lots Therefore – are most commonly used on lots for residential uses. for residential uses.


Integrated Management Integrated Management Practices (IMPs)Practices (IMPs)

NCSU-BAE


Best Management Practices Best Management Practices (BMPs)(BMPs)

NCSU-BAE


Example SiteExample Site►R-40WR-40W►53.25 53.25

acresacres►100% B 100% B

SoilsSoils►Cluster Cluster

S/DS/D►44 Lots44 Lots


ConclusionConclusion

www.wakegov.com/environment/water/www.wakegov.com/environment/water/stormwater.htmstormwater.htm

Questions?Questions?

Wake County Stormwater Workshop Guidance on the New Stormwater Ordinance and Design Manual August 29, 2006.

Documents

proposed curve number

low curve number

proposed site curve

high curve number

soil group abcd r

introduction wake county

curve numbers integers

curve number method