Hydraulic & Hydrologic Engineering Stormwater Design of … · 2012. 11. 22. · BAE Stormwater Engineering GroupBAE Stormwater Engineering Group “We Bring Engineering to Life”

Stormwater Stormwater EngineeringEngineeringHydraulic & Hydrologic Hydraulic & Hydrologic

Design of Stormwater Design of Stormwater WetlandsWetlands

Jon Hathaway, EIExtension Associate

NCSU Bio. And Ag. Engineering

““We Bring Engineering to LifeWe Bring Engineering to Life””BAE Stormwater Engineering GroupBAE Stormwater Engineering Group

6 Step Process6 Step Process1. Watershed Analysis (Runoff Volume

and Peak Flow)2. Determine H20 Storage Volume3. Calculate Surface Area4. Size Drawdown Orifice5. Size Overflow Weir6. Diagram Internal Features

Additional TopicsAdditional Topics1. Perch or Excavate to Water Table?2. Bypass or Flow-Through?3. Forebay Design

Watershed AnalysisWatershed Analysis(Determine Treatment Volume)(Determine Treatment Volume)

Watershed AnalysisWatershed Analysis

Determine VolumeDetermine Volume * Fact Finding *

Watershed AreaWatershed Land Use(s)Soil Type(s) within WatershedSoil Hydrologic Group

» Use Land Use Map & County Soil Survey » Verify soils data in the field

Determine VolumeDetermine Volume * Calculate Runoff from Curve Number * (SCS – Curve Number Method)

P = PrecipitationCN = Curve NumberS = Storage Component

Determined from CNR/O = Runoff

Determine VolumeDetermine Volume * Describing Land Use *

Curve Numbers - Relate Land Use and Soil Hydrologic GroupRange from 30 to 100Higher Number = More Runoff

» USDA-NRCS Tables

Curve NumbersCurve Numbers

Land Use A B C D Paved Parking Lots; Roofs 98 98 98 98 Commercial & Bus. Distr. 89 92 94 95 Townhouses 77 85 90 92 Residential Lot (1/2 AC) 54 70 80 85 Residential Lot (1 AC) 51 68 79 84 Open Space: grass > 75% 39 61 74 80

Soil Group

Curve NumbersCurve NumbersKeep Separate or Composite?

NCSU suggests Discrete Method for Urban Watersheds

Base Decision upon Soil Type Differences within watershed & Land Use

May Use Composite for Watersheds with low Imperviousness (< 10%)

Is Land Use Separation Distinct or Mixed

“Connected” Imperviousness

Curve NumbersCurve NumbersIf Discrete:Calculate runoff from impervious areas separately – determine composite for rest of watershed and calculate runoff(Can calculate for each land use in watershed if desired)If Composite:

CNcomp = %Wa ∗ CNa + %Wb∗ CNb + ...

Determine VolumeDetermine Volume

Decide Design Storm Calculate Runoff from Impervious and Pervious land uses Use Watershed Area to Find…

Total Runoff

Determine VolumeDetermine Volume * Decide Design Storm *

Varies from 0.5” to 1.5”Retrofit versus New Development

Depends upon Built upon area, wetnessP = 1.0 in. “Old” Typical

Can be sized based on Water Quality Storm (earlier presentation)

Determine VolumeDetermine Volume * Calculate Runoff from Curve Number * (SCS – Curve Number Method)

P = PrecipitationCN = Curve NumberS = Storage ComponentR/O = Runoff

Determine VolumeDetermine Volume * Calculate Runoff from Curve Number *

S = 1000 ÷ CN – 10

R/O = (P - 0.2S)2 ÷ (P + 0.8S)

Determine VolumeDetermine VolumeSimple Method ?

Imp% = Percent Imperviousness in WatershedP = Depth of Rainfall (in)RO = Depth of Runoff (in)V = Volume of Runoff (ft3)A = Area of Watershed (ft2)

V = ((Imp%*0.9) + 0.05) * P * A

Determine VolumeDetermine Volume

10,000

20,000

30,000

40,000

50,000

60,000

70,000

Precipitation Depth (in)

CompositeCN

Discrete

Simple

Determine VolumeDetermine Volume * Example * Given:

Total Watershed Area = 35 AC20 AC is school (soil group B)

15 AC is fair condition grass (soil group C)

Find:Volume of Water to be Treated if P=1.5”

Determine VolumeDetermine Volume * Example *Runoff Per Land Use/Soil Type:

School CN = 88S = 1000/88 - 10 = 1.36

Field CN = 79S = 1000/79 - 10 = 2.66

Determine VolumeDetermine Volume * Example *Runoff Per Land Use/Soil Type:

School R/O =(1.5-0.2*1.36)2 ÷ (1.5+0.8*1.36)= 0.58 in

Field R/O =(1.5-0.2*2.66)2 ÷ (1.5+0.8*1.36)= 0.26 in

Determine VolumeDetermine Volume * Example *Runoff Per Land Use/Soil Type:Volume of School R/O =0.58 in * 20 AC = 11.6 ac-in

Volume of Field R/O =0.26 in * 15 AC = 3.9 ac-in

Total R/O = Volume to Treat = 15.5 ac-inOr (1.3 ac-ft)

Watershed AnalysisWatershed Analysis(Determine Peak Flow)(Determine Peak Flow)

Peak Flow during Large StormsPeak Flow during Large StormsOn large systems or high risk, detailed modeling would be appropriate

Need to make sure that outlet can pass the peak flow from a large storm

Use Rational MethodMay not be accurate, be sure to include factor of safetyWidely used despite downfalls

What is Peak Flow ?What is Peak Flow ?Largest amount of flow experienced during a given storm

12/16/03 12/17/03 12/18/03 12/19/03

Rational MethodRational MethodBasic Equation:

Qp = C * I * A

Where: Qp = Peak flow (cfs)C = Rational Coefficient (composite)

I = Rainfall Intensity (in / hr)A = Watershed Area (acres)

Rational CoefficientRational CoefficientSimilar to Curve Number – Variable with land useComposite for given watershedThe higher the rational coefficient the higher the peak flow

Rainfall IntensityRainfall IntensityVaries depending on storm size Typically duration used is either 6 hour (safe estimate) or 24 hourSee Handout (State of NC Erosion and Sediment Control Planning and Design Manual)

From Our Earlier ExampleFrom Our Earlier ExampleIntensity (Greensboro):

Choose 2 year event – 6 hour = ~0.36 in/hr

Rational Coefficient:School = 0.6Field = 0.15

Composite = (0.6 * 20 acres) + (0.15 * 15 acres) 35 acres

Composite = 0.41

From Our Earlier ExampleFrom Our Earlier ExampleQp = C * I * A

Qp = 0.41 * 0.36 in/hr * 35 acres

Qp = ~ 5.2 cfs

Calculate Surface AreaCalculate Surface Area

Calculate Surface AreaCalculate Surface AreaFunction of Volume and Allowable Depth

Capture Volume Previously Determined

Allowable Depth of Water Varies from 6”to 18”

* 12” suggested *

Wetland VolumesWetland Volumes

Normal Pool

First Flush Volume

Peak Mitigation (or extra) Volume

Wetland Cross Section – Definition of Storage Depths

Calculate Surface AreaCalculate Surface AreaS/A = Surface Area RequiredVolume = Total Volume CapturedDepth = Depth of water over normal pool (Depth of Storage Volume)

S/A = Volume ÷ Depth

Calculate Surface AreaCalculate Surface Area * Earlier Example * Given:

Volume 15.5 ac-inDepth = 9 in

Find:Surface Area of Wetland

Calculate Surface AreaCalculate Surface Area * Example * Find Surface Area

S/A = 15.5 ac-in ÷ 9 in S/A = 1.7 AC

Calculate Surface AreaCalculate Surface AreaRemember !!

Account for some additional surface area if you are bypassing larger storms

Size OutletSize Outlet

Retaining StormwaterRetaining Stormwater How Long Can Water stay in

wetland (above normal pool)?• Plant Tolerance• Rainfall Event Frequency

– Mean time between events = 3 days

– 2-4 days reasonable

Retaining StormwaterRetaining Stormwater Common Devices Used to Retain

Stormwater :Orifice (draw down)Weir (overflow)

(Flashboard Riser ?)

Retaining StormwaterRetaining Stormwater Weir Equation

Q = Flow over WeirCw = Weir Coefficient (Commonly - 3.0)

L = Length of WeirH = Height of Water

upstream of weir

Retaining StormwaterRetaining Stormwater * Devices Used to Retain Stormwater *

Weir Equation

Q = Cw * L * H 3/2

Flow over WeirFlow over WeirH 2/3 H

Retaining StormwaterRetaining Stormwater Orifice Equation Q = Flow through Orifice N = Number of Orifices (holes) Cd = Coefficient of Discharge (Commonly 0.6)

A = Area of Opening g = Gravity (32.2 ft/s2) H = Water Height over Orifice Centerline

Retaining StormwaterRetaining Stormwater* Device Used to Retain Stormwater *Orifice Equation

Q = N * (Cd * A * (2*g*H)1/2)

Retaining StormwaterRetaining Stormwater How Long to Empty Wetland of Treatment

Volume ?

Quick & Dirty Method:

1. Calculate Flow rate at 1/3 Full Height 2. Divide this Flow rate into Total Volume

Retaining StormwaterRetaining Stormwater* Earlier Example *

Given:Wetland Storage (above Normal Pool) = 56,000 cubic feet (15.5 ac-in)Size of drawdown orifice = 5 inchesHeight of Water over orifice = 12 in

Find:Amount of Time to Release Storage

Retaining StormwaterRetaining Stormwater Time to Release Storage: 1. Flow over weir when 1/3 full

Q = N * (Cd * A * (2*g*H)1/2)

2*g*H = 2 * 32.2 ft/s2 * 0.33 ft = 21.3

Q = 1 * (0.6 * 0.14 ft2 * (21.3)1/2) = 0.39 cfs

Retaining StormwaterRetaining Stormwater Time to Release Treatment Volume: 2. Time to Release Water

Time = Vol / Discharge Rate Time = 56,000 cf / 0.39 cfs Time = 91,900 s = 40 hours

Retaining StormwaterRetaining StormwaterBypass larger storms – What Size ??

Designer choice based on riskCost of failure

Safety of individuals living in areaMonetary cost

Must be able to pass peak flow for chosen stormWatershed Peak Flow = Peak Wetland Outflow

Retaining StormwaterRetaining Stormwater* Earlier Example *

Overflow Weir Sizing

2 year – Peak Flow = 5.2 cfs

Will allow 6 inches over weir during event(Infrequent event, should not impact vegetation)

Retaining StormwaterRetaining StormwaterWater will leave through:

drawdown orificeoverflow weir

The outflow from these two devices combined should be 5.2 cfs

Retaining StormwaterRetaining StormwaterFlow through Orifice

Total depth over orifice is 18 inches

Q = N * (Cd * A * (2*g*H)1/2)

2*g*H = 2 * 32.2 ft/s2 * 0.67 ft = 43.1

Q = 1 * (0.6 * 0.14 ft2 * (43.1)1/2)Q = 0.55 cfs

Retaining StormwaterRetaining StormwaterNeed to route 5.2 cfs - have 0.55 cfs routed

through orifice - Weir must handle 4.65 cfs

Q = Cw * L * H 3/2

L = Q / (Cw * H3/2)

Retaining StormwaterRetaining StormwaterL = Q / (Cw * H3/2)

L = 4.65 cfs ÷ (3.0 * (0.5 ft)3/2)

L = 4.38 ft (should include factor of safety)

L = 4.38 * 1.2 = 5.26

L = 5.3 ft

Diagram Internal FeaturesDiagram Internal Features

Stormwater Wetland FeaturesStormwater Wetland Features

Forebay

Outlet

Shallow Water

DeepPools

Upland “Shallow”Land

Revised Revised PondingPonding DepthsDepths

Deep Pool

Shallow Water

Shallow Land

18-36” 0-12”1-6”

How much of Each?How much of Each?

10-20%

40-60%

30-40%

Perch or Excavate to Perch or Excavate to Water Table ?Water Table ?

Perch or Excavate to W.T.?Perch or Excavate to W.T.?

Perch or Excavate to W.T.?Perch or Excavate to W.T.?Determine Soil Type at W/L Site

What is Seasonally Low Water Table? > 4’ - You choose< 4’ - Excavate to 6” below W. T.

Highly dependent on Soil ConditionsDependent on Desired Cost of Construction

Clay Liner

Perch or Excavate to W.T.?Perch or Excavate to W.T.?

Determine Soil Type at W/L SiteWhat is Permeability?

0.06 - 0.2 in/hr, Compact Necessary> 0.2 in/hr, Import Clay & Compact

Want K < 0.01 in/hrWant K < 0.01 in/hr

Perch or Excavate to W.T.?Perch or Excavate to W.T.? * Summary *

If Seasonally Low Water Table at or near Surface -Excavate. If not, Perch.

Perch or Excavate to W.T. ?Perch or Excavate to W.T. ?

Bypass or FlowBypass or Flow--Through ?Through ?

Water BypassWater Bypass

Flow Through Main Body?

Bypass Large Storms?

Bypass or Not?Bypass or Not?If Actual Surface Area > 0.80 of Required Design Surface Area, you do NOT need to bypass flow

Flow SplitterFlow Splitter

Bypass Weir

Normal Pool

2-Yr Storm Pool

Drawdown Orifice

“Emergency” Spillway

What Size Storm?What Size Storm? It depends…

None. All storms through Wetland

“Extreme”Event. 10-25 YR

“Moderate”Event. 2-YR

All Storms > First Flush Event Bypass

Wetland

ForebayForebay DesignDesign

ForebayForebay Design Design More Information on the Way !!!

Questions ?Questions ?

Hydraulic & Hydrologic Engineering Stormwater Design of … · 2012. 11. 22. · BAE Stormwater Engineering GroupBAE Stormwater Engineering Group “We Bring Engineering to Life”

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