Hydraulic & Hydrologic Stormwater Engineering Design of Stormwater Wetlands Jon Hathaway, EI Extension Associate NCSU Bio. And Ag. Engineering
6 Step Process 1. Watershed Analysis (Runoff Volume and Peak Flow) 2. Determine H 2 0 Storage Volume 3. Calculate Surface Area 4. Size Drawdown Orifice 5. Size Overflow Weir 6. Diagram Internal Features
Additional Topics 1. Perch or Excavate to Water Table? 2. Bypass or Flow-Through? 3. Forebay Design
Watershed Analysis (Determine Treatment Volume)
Watershed Analysis
Determine Volume * Fact Finding * Watershed Area Watershed Land Use(s) Soil Type(s) within Watershed Soil Hydrologic Group» Use Land Use Map & County Soil Survey» Verify soils data in the field
Determine Volume * Calculate Runoff from Curve Number * (SCS Curve Number Method) P = Precipitation CN = Curve Number S = Storage Component Determined from CN R/O = Runoff
Determine Volume * Describing Land Use * Curve Numbers - Relate Land Use and Soil Hydrologic Group Range from 30 to 100 Higher Number = More Runoff» USDA-NRCS Tables
Curve Numbers Soil Group 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
Curve Numbers Keep 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 Numbers If 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: CN comp = %W a CN a + %W b CN b +...
Determine Volume Decide Design Storm Calculate Runoff from Impervious and Pervious land uses Use Watershed Area to Find Total Runoff
Determine Volume * Decide Design Storm * Varies from 0.5 to 1.5 Retrofit versus New Development Depends upon Built upon area, wetness P = 1.0 in. Old Typical Can be sized based on Water Quality Storm (earlier presentation)
Determine Volume * Calculate Runoff from Curve Number * (SCS Curve Number Method) P = Precipitation CN = Curve Number S = Storage Component R/O = Runoff
Determine Volume * Calculate Runoff from Curve Number * S = 1000 CN 10 R/O = (P - 0.2S) 2 (P + 0.8S)
Determine Volume Simple Method? Imp% = Percent Imperviousness in Watershed P = Depth of Rainfall (in) RO = Depth of Runoff (in) V = Volume of Runoff (ft 3 ) A = Area of Watershed (ft 2 ) V = ((Imp%*0.9) + 0.05) * P * A
Determine Volume 70,000 Runoff Volume (ft3) 60,000 50,000 40,000 30,000 20,000 10,000 Composite CN Discrete Simple 0 1 Precipitation Depth (in)
Determine Volume * Example * Given: Total Watershed Area = 35 AC 20 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 Volume * Example * Runoff Per Land Use/Soil Type: School CN = 88 S = 1000/88-10 = 1.36 Field CN = 79 S = 1000/79-10 = 2.66
Determine 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 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-in Or (1.3 ac-ft)
Watershed Analysis (Determine Peak Flow)
Peak Flow during Large Storms On 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 Method May not be accurate, be sure to include factor of safety Widely used despite downfalls
What is Peak Flow? Largest amount of flow experienced during a given storm 0.2 0.18 0.16 dishcarge (cfs) 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 12/16/03 12/17/03 12/18/03 12/19/03
Rational Method Basic Equation: Q p = C * I * A Where: Q p = Peak flow (cfs) C = Rational Coefficient (composite) I = Rainfall Intensity (in / hr) A = Watershed Area (acres)
Rational Coefficient Similar to Curve Number Variable with land use Composite for given watershed The higher the rational coefficient the higher the peak flow
Rainfall Intensity Varies depending on storm size Typically duration used is either 6 hour (safe estimate) or 24 hour See Handout (State of NC Erosion and Sediment Control Planning and Design Manual)
From Our Earlier Example Intensity (Greensboro): Choose 2 year event 6 hour = ~0.36 in/hr Rational Coefficient: School = 0.6 Field = 0.15 Composite = (0.6 * 20 acres) + (0.15 * 15 acres) 35 acres Composite = 0.41
From Our Earlier Example Q p = C * I * A Q p = 0.41 * 0.36 in/hr * 35 acres Q p = ~ 5.2 cfs
Calculate Surface Area
Calculate Surface Area
Calculate Surface Area Function of Volume and Allowable Depth Capture Volume Previously Determined Allowable Depth of Water Varies from 6 to 18 * 12 suggested *
Wetland Volumes Wetland Cross Section Definition of Storage Depths Peak Mitigation (or extra) Volume Normal Pool First Flush Volume
Calculate Surface Area S/A = Surface Area Required Volume = Total Volume Captured Depth = Depth of water over normal pool (Depth of Storage Volume) S/A = Volume Depth
Calculate Surface Area * Earlier Example * Given: Volume 15.5 ac-in Depth = 9 in Find: Surface Area of Wetland
Calculate Surface Area * Example * Find Surface Area S/A = 15.5 ac-in 9 in S/A = 1.7 AC
Calculate Surface Area Remember!! Account for some additional surface area if you are bypassing larger storms
Size Outlet
Retaining 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 Stormwater Common Devices Used to Retain Stormwater : Orifice (draw down) Weir (overflow) (Flashboard Riser?)
Retaining Stormwater Weir Equation Q = Flow over Weir C w = Weir Coefficient (Commonly - 3.0) L = Length of Weir H = Height of Water upstream of weir
Retaining Stormwater * Devices Used to Retain Stormwater * Weir Equation Q = C w * L * H 3/2
Flow over Weir H 2/3 H Weir
Retaining Stormwater Orifice Equation Q = Flow through Orifice N = Number of Orifices (holes) C d = Coefficient of Discharge (Commonly 0.6) A = Area of Opening g = Gravity (32.2 ft/s 2 ) H = Water Height over Orifice Centerline
Retaining Stormwater * Device Used to Retain Stormwater * Orifice Equation Q = N * (Cd * A * (2*g*H) 1/2 )
Retaining 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 Stormwater * Earlier Example * Given: Wetland Storage (above Normal Pool) = 56,000 cubic feet (15.5 ac-in) Size of drawdown orifice = 5 inches Height of Water over orifice = 12 in Find: Amount of Time to Release Storage
Retaining 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 ft 2 * (21.3) 1/2 ) = 0.39 cfs
Retaining 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 Stormwater Bypass larger storms What Size?? Designer choice based on risk Cost of failure Safety of individuals living in area Monetary cost Must be able to pass peak flow for chosen storm Watershed Peak Flow = Peak Wetland Outflow
Retaining 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 Stormwater Water will leave through: drawdown orifice overflow weir The outflow from these two devices combined should be 5.2 cfs
Retaining Stormwater Flow 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/s 2 * 0.67 ft = 43.1 Q = 1 * (0.6 * 0.14 ft2 * (43.1) 1/2 ) Q = 0.55 cfs
Retaining Stormwater Need to route 5.2 cfs - have 0.55 cfs routed through orifice - Weir must handle 4.65 cfs Q = C w * L * H 3/2 Or L = Q / (C w * H 3/2 )
Retaining Stormwater L = Q / (C w * H 3/2 ) 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 Features
Stormwater Wetland Features Shallow Water Upland Shallow Land Outlet Forebay Deep Pools
Revised Ponding Depths 18-36 1-6 0-12 Deep Shallow Shallow Pool Water Land
How much of Each? 10-20% 30-40% 40-60%
Perch or Excavate to Water Table?
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 Conditions Dependent on Desired Cost of Construction Clay Liner
Perch or Excavate to W.T.? Determine Soil Type at W/L Site What is Permeability? 0.06-0.2 in/hr, Compact Necessary > 0.2 in/hr, Import Clay & Compact Want K < 0.01 in/hr
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.?
Bypass or Flow-Through?
Water Bypass Bypass Large Storms? Flow Through Main Body?
Bypass or Not? If Actual Surface Area > 0.80 of Required Design Surface Area, you do NOT need to bypass flow
Flow Splitter Emergency Spillway 2-Yr Storm Pool Bypass Weir Normal Pool Drawdown Orifice
What Size Storm? None. All storms through Wetland It depends Moderate Event. 2-YR Storm Extreme Event. 10-25 YR Storm All Storms > First Flush Event Bypass Wetland
Forebay Design
Forebay Design
Forebay Design More Information on the Way!!!
Questions?