design considerations - springfield, mo

EXTENDED WET DETENTION BASIN

Design Considerations

Conduct water balance to assure the permanent pool can be maintained and adequate flushing is availableMinimize algae growth– Depth of permanent pool ≥ 6 feet– Optional aeration

Adequate basin lining to ensure it is watertight

EXTENDED WET DETENTION BASIN

Typical Profile

EXTENDED WET DETENTION BASIN

Permanent Pool Residence Time

For large ponds, if the residence time of the permanent pool is > 24 hours during the 2-year 24-hour storm event, then the surcharge WQCV is not required.

Residence Time, t (hrs)Permanent Pool Volume, Vp (ft3)Avg. inflow for the 2-yr 24-hr storm event, Q2-yr,avg (cfs)

Example:Vp = 20 ac-ft (871,200 ft3) Q2yr, avg = 10 cfst = 24.2 hrs, surcharge WQCV not required

3600,2 ⋅=

− avgyr

p

QV

t

EXTENDED WET DETENTION BASIN

Basin Volume and Geometry

Calculate surcharge WQCV (ft3)Min Permanent Pool Volume, Vp (ft3)

Min Depth of Permanent Pool = 6 ftOptional Littoral Zone – 18” deep, 10’ wideBasin L:W Ratio (between 1:1 and 4:1)Side slopes ≥ 4:1 H:V (ft/ft)(encourage safety bench)

WQCVVp ⋅= 2.1

EXTENDED WET DETENTION BASIN

Typical Profile

EXTENDED WET DETENTION BASIN

Residence Time and WQCV

1. Surcharge WQCV and Minimum Permanent Pool Volume

A) Contributing Watershed Area Area = 46.50 acres

B) Water Quality Capture Volume (WQCV) WQCV = 85,030 cubic feet Calculate WQCV as described in Section 4.2 of WQ Chapter

C) Minimum Permanent Pool Volume: Vol = WQCV * 1.2 Min. Vp = 102,036 cubic feet

2. Check Residence Time of Permanent Pool

A) Design Permanent Pool Volume, Vp Design Vp = 102,366 cubic feet

B) Average Inflow Rate to Pond during 2-year Storm Event, Q2yr, avg Q2yr, avg 7.00 cfs (Calculated using appropriate hydrologic analysis method in Ch. 5, Runoff)

C) Residence Time, t t = 4.1 hoursNOTE: Permanent Pool Residence Time is less than 24 hours. WQCV Surcharge is Required.

Previous Example

EXTENDED WET DETENTION BASIN

L:W Ratio = 3.0Z = 4 ft/ft

Dp = 6.0 ftPreliminary Basin Geometry

3. Preliminary Basin Geometry (assumes a trapezoidal basin)

A) Basin Length to Width Ratio (L:W), should be between 2:1 and 4:1 L:W = 3.0

B) Basin Side Slopes, Z (Horizontal:Vertical), should be 4:1 or flatter Z = 4.0 ft/ft

C) Permanent Pool Depth, Dp (Min = 6 feet) Dp = 6.00 feet

D) Basin Bottom Width, W W = feet

E) Basin Bottom Length, L L = feet

F) Calculated Permanent Pool Volume (may be slightly larger than required) Calc. Vp = cubic feet

G) Calculated WQCV Depth, DWQCV DW QCV = feet

H) Calculated WQCV Calc. WQCV = cubic feet

3. Preliminary Basin Geometry (assumes a trapezoidal basin)

A) Basin Length to Width Ratio (L:W), should be between 2:1 and 4:1 L:W = 3.0

B) Basin Side Slopes, Z (Horizontal:Vertical), should be 4:1 or flatter Z = 4.0 ft/ft

C) Permanent Pool Depth, Dp (Min = 6 feet) Dp = 6.00 feet

D) Basin Bottom Width, W W = 59.00 feet

E) Basin Bottom Length, L L = 177.00 feet

F) Calculated Permanent Pool Volume (may be slightly larger than required) Calc. Vp = 103,554 cubic feet

G) Calculated WQCV Depth, DWQCV DW QCV = 3.0 feet

H) Calculated WQCV Calc. WQCV = 85,041 cubic feet

3. Preliminary Basin Geometry (assumes a trapezoidal basin)

A) Basin Length to Width Ratio (L:W), should be between 2:1 and 4:1 L:W = 3.0

B) Basin Side Slopes, Z (Horizontal:Vertical), should be 4:1 or flatter Z = 4.0 ft/ft

C) Permanent Pool Depth, Dp (Min = 6 feet) Dp = 6.00 feet

D) Basin Bottom Width, W W = 59.00 feet

E) Basin Bottom Length, L L = 177.00 feet

F) Calculated Permanent Pool Volume (may be slightly larger than required) Calc. Vp = 103,554 cubic feet

G) Calculated WQCV Depth, DWQCV DW QCV = feet

H) Calculated WQCV Calc. WQCV = cubic feet

Add WQCV on top of Permanent Pool

EXTENDED WET DETENTION BASIN

Perforated Plate (Iterative Solution)

D) For a Perforated Plate Select: i) Perforation Diameter, dperforation (Min = 0.5", Max = 4.0") dperforat ion = 3.5 inches ii) Number of Holes per Row, nholes per row (Min = 1, Max = 8) nholes per row = 3 iii) Row Spacing, Rs (Min varies based on dperforat ion ,Max = 12") Rs = 8 inches

E) Results for Perforated Plate i) Number of Rows, nrows nrows = ii) Outlet Area Per Row, Ao Ao = square inches iii) Total Outlet Area, Aot Aot = square inches iv) Drain Time for WQCV (should fall between 12 and 15 hours) Drain Time = hours

D) For a Perforated Plate Select: i) Perforation Diameter, dperforation (Min = 0.5", Max = 4.0") dperforat ion = 3.5 inches ii) Number of Holes per Row, nholes per row (Min = 1, Max = 8) nholes per row = 3 iii) Row Spacing, Rs (Min varies based on dperforat ion ,Max = 12") Rs = 8 inches

E) Results for Perforated Plate i) Number of Rows, nrows nrows = 5 ii) Outlet Area Per Row, Ao Ao = 28.86 square inches iii) Total Outlet Area, Aot Aot = 144.32 square inches iv) Drain Time for WQCV (should fall between 12 and 15 hours) Drain Time = 15.0 hours

Drain WQCV in 12 to 15 hours

EXTENDED WET DETENTION BASIN

Trash Rack

Prevents clogging of perforationsShould not interfere with hydraulic capacityMinimum open area of trash rack is a function of perforation diameter and the total orifice opening area (orifice area per row * number of rows)Extend trash rack 24” below micro-poolMicro-pool ≥ 5% WQCV

EXTENDED WET DETENTION BASIN

rowsoot nAA ⋅=

Trash RackOpen Area

Aot = Total Outlet AreaAo = Outlet Area per Rownrows = Number of RowsAt = Trash Rack Open Area

⎟⎠⎞⎜

⎝⎛⋅=

ot

tott A

AAA 5.0

EXTENDED WET DETENTION BASIN D = 3.5 in

Aot = 144 in2

From FigureTrash Rack Sizing At / Aot = 50

6. Trash Rack for Perforated Plate

A) Needed Open Area: At = 0.5 * (Figure WQ-7 Value) * Aot At = 3,600 square inches (Factor of 0.5 accounts for variable innundation of the outlet perforations)

B) Height of Trash Rack: HTR (Min Height = Dwqcv + 24 inches = 60 inches.) HTR = inches

C) Width of Concrete Opening: Wconc = (At / R) / HT R Wconc = inches Effective open area, R = 0.6 for wire screens, R = 0.71 for aluminum bar grates

D) Width of Trash Rack Screen, WT R (Minimum Width = Wconc + 6") WTR = inches

E) Type of Trash RackStainless Steel #93 VEE Wire S.S. #93 VEE Wire (Johnson Screens)Aluminum Bar Grate Aluminum Bar Grate (Klemp KRP)

F) Open Space between: S.S. #93 VEE Wires #93 VEE Wire Slot OpeningAluminum Bearing Bars (Vertical Alignment) Bearing Bar Spacing

G) Spacing of Support Rods (O.C.) On Center Spacing

H) Type and Size of:Support Rods for S.S. #93 VEE Wire ScreenBearing Bars for Aluminum Bar Grate

6. Trash Rack for Perforated Plate

A) Needed Open Area: At = 0.5 * (Figure WQ-7 Value) * Aot At = 3,600 square inches (Factor of 0.5 accounts for variable innundation of the outlet perforations)

B) Height of Trash Rack: HTR (Min Height = Dwqcv + 24 inches = 60 inches.) HTR = 60 inches

C) Width of Concrete Opening: Wconc = (At / R) / HT R Wconc = 85 inches Effective open area, R = 0.6 for wire screens, R = 0.71 for aluminum bar grates

D) Width of Trash Rack Screen, WT R (Minimum Width = Wconc + 6") WTR = inches

E) Type of Trash RackStainless Steel #93 VEE Wire S.S. #93 VEE Wire (Johnson Screens)Aluminum Bar Grate Aluminum Bar Grate (Klemp KRP)

F) Open Space between: S.S. #93 VEE Wires #93 VEE Wire Slot OpeningAluminum Bearing Bars (Vertical Alignment) Bearing Bar Spacing

G) Spacing of Support Rods (O.C.) On Center Spacing

H) Type and Size of:Support Rods for S.S. #93 VEE Wire ScreenBearing Bars for Aluminum Bar Grate

6. Trash Rack for Perforated Plate

A) Needed Open Area: At = 0.5 * (Figure WQ-7 Value) * Aot At = 3,600 square inches (Factor of 0.5 accounts for variable innundation of the outlet perforations)

B) Height of Trash Rack: HTR (Min Height = Dwqcv + 24 inches = 60 inches.) HTR = 60 inches

C) Width of Concrete Opening: Wconc = (At / R) / HT R Wconc = 85 inches Effective open area, R = 0.6 for wire screens, R = 0.71 for aluminum bar grates

D) Width of Trash Rack Screen, WT R (Minimum Width = Wconc + 6") WTR = 96 inches

E) Type of Trash RackStainless Steel #93 VEE Wire S.S. #93 VEE Wire (Johnson Screens)Aluminum Bar Grate X Aluminum Bar Grate (Klemp KRP)

F) Open Space between: S.S. #93 VEE Wires #93 VEE Wire Slot OpeningAluminum Bearing Bars (Vertical Alignment) 1 3/16" Bearing Bar Spacing

G) Spacing of Support Rods (O.C.) 2" On Center Spacing

H) Type and Size of:Support Rods for S.S. #93 VEE Wire ScreenBearing Bars for Aluminum Bar Grate 1-1/4" x 3/16" rectangular bar

EXTENDED WET DETENTION BASIN

Additional Criteria

Use water tolerant species above permanent pool due to periodic inundationFor littoral bench, use 4 to 6” organic topsoil layer vegetated with aquatic speciesForebay for permanent pool volume > 20,000 ft3– Forebay volume = 5 to 10% of WQCV

EXTENDED WET DETENTION BASIN

Forebay Volume

7. Pre-sedimentation Forebay Basin - Enter design values

A) Volume (5% to 10% of WQCV from 1B) 6,000 cubic feet (5% - 10% of Design Volume equals 4252 to 8503 cubic feet.)

B) Gravel Filter Forebay Outlet yes yes/no (May be incorporated into Littoral Bench)

C) Solid Driving Surface on Bottom and Sides of Forebay yes yes/no

STRUCTURAL BMPs

Constructed Wetland Basin

CONSTRUCTED WETLAND BASIN

Description

Shallow extended wet detention basinRequires perennial base flowVegetated with aquatic plant species(e.g. rushes, willows, sedges, cattails and reeds)

Provides:– Natural aesthetic qualities– Wildlife habitat– Erosion control– Pollutant removal– Flood control storage

Outlet structure sized to drainWQCV in 22 to 28 hours