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


Typical Profile


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


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


Typical Profile


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


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





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





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


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


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


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



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



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


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


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

design considerations - springfield, mo

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