PAF 198127 Spec Section 5 of 5.pdf

4.0 Design Examples

This chapter provides design examples based on hypothetical case studies located in Palau.

Step-by-step design guidance for the following Best Management Practices (BMPs) are presented within four case studies:

4.1 Case Study #1: Medium-Density Residential Site

• Dry Swale

• Dry Detention Basin

4.2 Case Study #2: Commercial Site

• Bioretention with Grass Channel Pretreatment

• Bioretention with Filter Strip Pretreatment

• Sand Filter

4.3 Case Study #3: Commercial Site

• Infiltration Trench with Oil/Grit Separator Pretreatment

4.4 Case Study #4: Single-Family Residential Site

• Cistern and Drywell

• Rain Garden

• Permeable Pavers

• Swale

It should be noted that the case studies presented in this chapter are hypothetical even though they may be based upon actual development sites. Names and site characteristics have been revised as necessary to provide simplified and effective design examples. The design examples are for illustration purposes only for a particular type of stormwater practice, and do not necessarily include a comprehensive stormwater management design for the entire site, nor do they necessarily include examples of all the criteria required for the site. Sample calculations for all of the Unified Stormwater Sizing Criteria are presented in Volume I, Section 2.2.4. It should also be noted that for simplicity, the case studies presented often provide computed values or particular constraints that the designer would normally need to determine or research for an actual site design.

Palau Stormwater Management Manual Volume II-Chapter 4

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4.1 Case Study #1: Medium Density Residential Site

This case study represents a medium-density residential site located in Melekeok on the island of Babeldaob. Mountain View Estates (see Figure 4.1) is a hypothetical subdivision consisting of approximately 23 ¼-acre lots, with 2,000 linear feet of 20-ft wide residential roads. The total site consists of 20 acres with 10 acres of preserved open space. The disturbed portion of the site consists of 10 acres, 25% impervious cover. The site is located in the volcanic uplands of Babeldaob. On-site soils as determined by the NRCS soil maps are Aimeliik-Palau, classified as HSG “B”. Two stormwater practice design examples are presented for this case study, consisting of a dry swale and a dry detention basin. The data for the site is summarized in Table 4.1.

Table 4.1 Base Data for Mountain View Estates Hydrologic Data

Location: Melekeok, Babeldaob Pre Post

Drainage Area Area A

Site Area: 10 acres CN Value 61 78 Impervious Area: 2.5 acres tc 0.26 0.23

Soils Type: Aimeliik-Palau, “B”

Figure 4.1 Medium Density Residential Site Plan

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4.1.1 Dry Swale Design Example

This example focuses on the design of a dry swale for the Mountain View Estates residential subdivision (see Figure 4.1). This step-by-step example shows how the water quality volume requirements for this portion of the site will be met by the dry swale. In general, the primary function of dry swales is to provide water quality treatment and convey larger storms in a non-erosive condition. For this example, the post-development, 1-year and 2-year peak discharges are computed to check for non-erosive flows and to ensure that the channel has capacity to convey the 2-year event. The channel protection and peak control requirements for this catchment area will be met after discharge from the swale by use of a dry detention basin, which will be presented as a separate design example in Section 4.1.2.

a) b)

WASHED LIMESTONE

AGGREGATE DIAPHRAGM

1.2’

2.4’2.4’

Figure 4.2 Plan View (a) and Cross-Section (b) of Dry Swale

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Better Site Design (BSD) measures (see Chapter 3) were incorporated at this new development site to the maximum extent practicable. For example, impervious surfaces were reduced by decreasing the width of the streets and driveways. The rooftop runoff was directed into cisterns/drywells. In addition, lots were clustered such that 10 acres of open space could be preserved around the river. These techniques help improve the value of the development and decrease the site’s impact on the water resources. Also, by utilizing BSD techniques and decreasing the overall runoff from the site, the developer would save money by having lower stormwater volume requirements (thus, smaller BMPs to build) and less infrastructure to construct.

Step 1: Compute preliminary runoff control volumes.

Table 4.2 provides a summary of the volume requirements that were determined in Volume I, Section 2.2.4 (Sample Calculations).

Table 4.2 Summary of General Storage Requirements for Mountain View Estates

Symbol Category Volume Required (ac-ft) Notes

Rev Recharge Volume 0.056

WQv Water Quality Volume 0.25 Includes Rev

Cpv Stream Protection 1.1 Average ED release rate is 0.55 cfs over 24 hours

Qp-25 Overbank Control 1.43

In order to meet the Rev at this site, the runoff from the 23 rooftops will be directed into cisterns that overflow into drywells. The design of these practices are not included in this example, but are discussed in more detail in Section 4.4. Since rooftop runoff does not need to be treated (Volume I, Section 2.2.2.1), it can discharge directly into the ground via drywells, with the volume counting towards both recharge and water quality requirements. Thus, the WQv required to be treated by the dry swales is reduced, as follows:

Average house area at site = 1,000 ft2 Number of houses = 23 Net impervious area reduction = (23) (1,000 ft2) / (43,560 ft2/ac) = 0.53 acres New impervious area = 2.5 – 0.53 = 1.97 acres; new I = (1.97 ac / 10 ac) = 0.2

Revised WQv = [(P) (I) (A)] / 12 = [(1.2in) (0.2) (10 ac) (1ft/12in) = 0.2 ac-ft = 8,712 ft3

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Step 2: Determine if the development site and conditions are appropriate for the use of a dry swale.

The proposed road grade in the vicinity of the dry swale is approximately 3.0%. Soils are modestly to well drained. There is sufficient area to accommodate the dry swale within the roadside right of way.

Step 3: Confirm local design criteria and applicability.

There are no additional requirements for this site.

This dry swale design will have an “open bottom” (i.e., no impermeable liner (Figure 4.2)) to allow for some groundwater recharge, but there is also an underdrain to convey the water that does not infiltrate downstream to a dry detention basin where the channel protection and overbank flood requirements will be met (see Section 4.1.2). The underdrain will ultimately discharge into the sediment forebay via the riprap outlet shown in Figure 4.3.

Step 4: Determine size of dry swale area.

The dry swale treats the WQv by filtering it through a filter bed. The following equation (also used for filtering BMPs) is used to size the surface area of the dry swale, given an average depth of 3 inches.

Af = (WQv) (df) / [ (k) (hf + df) (tf)]

Where: Af = surface area of filter bed (ft2) WQv = Water quality volume (ft3) df = filter bed depth (ft) (recommended 2.5 ft from Volume I, Figure 3.13) k = filter media coefficient of permeability (ft/day) (use same media as bioretention) hf = average height of water above filter bed (ft) tf = design filter bed drain time (days) (2 days is recommended)

Af = (8,712 ft3) (2.5 ft) / [(1 ft/day) (0.25 ft + 2.5 ft) (2 days)] (With k = 1 ft/day, hf = 0.25 ft, tf = 2 days)

Af = 3,960 ft2

Step 5: Size a channel and compute the required length to store the water quality volume.

Provide bottom width, depth, length, and slope necessary to store WQv with an average of 3 inches of ponding.

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Assume a trapezoidal channel. Per the site plan, there is approximately 2,000 ft available for the swale along both sides of the road, not including driveways where there will be culverts connecting the swale. The outlet of the swale will be at the proposed dry detention basin described in Section 4.1.2. The existing uphill invert is 408.5 ft, and the infiltration basin invert is 391.0 ft.

Slope = (408.5 ft -391.0 ft)/1000 ft (based on one side of the road only) = 0.0199 = 1.75% Minimum slope 1.0% and maximum slope 4.0%, OK Side slopes = 2:1 (given the space constraints of this development, use 2:1 slopes)

For a surface area of Af = 3,960 ft2 and a total L = 2,000 ft, the WQv width needs to be (3,960 ft2 / 2,000 ft) = 1.98 ft. A minimum bottom width of 2 ft is required for dry swales according to Volume I, Section 3.2.4.5d; thus, the bottom width will be rounded up to 2.0 ft.

Step 6: Check the velocity of the 1-year storm and the hydraulic capacity of the 2-year storm.

• Develop Site Hydrologic Input Parameters and Perform Preliminary Hydrologic Calculations

(see Table 4.3).

Note: For this design example, the 1-year storm is used to check the channel geometry for non-erosive conditions, and the 2-year storm is used to check the conveyance capacity of the channel.

Any hydrologic model using SCS procedures, such as TR-20, HEC-HMS, or HEC-1, can be used to perform preliminary hydrologic calculations. In this example, TR-55 was used to compute these values (see Table 4.3). Note, the resulting design geometry for the dry swales is conservative because it is sized based on the flows at the downstream end of each subwatershed;

thus, the depth and width of the dry swale in the upstream areas may be reduced.

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Table 4.3 Mountain View Estates Post-Developed - TR-55 Output for 1-, 2-, and 25-year storm events

PEAK DISCHARGE SUMMARY

JOB: Mountain View Estates

DRAINAGE AREA NAME: POST DEVELOPMENT

GROUP CN from AREA

COVER DESCRIPTION SOIL NAME A,B,C,D? TABLE 2-2 (In acres)

Grass Aimeliik-Palau B 61 7.50 Ac.

Impervious (roads, roofs, driveways) Aimeliik-Palau B 98 2.50 Ac.

AREA SUBTOTALS: 10.00 Ac.

Time of Concentration Surface Cover Manning 'n' Flow Length Slope

2-Yr 24 Hr Rainfall = 7.2 In Pipe Diameter Avg Velocity Tt (Hrs)

Sheet Flow dense grass 'n'=0.24 50 Ft. 2.00%

0.09 Hrs

Shallow Flow Short Grass Pasture 50 Ft. 2.00%

7.00 F.P.S. 0.01 Hrs.

Channel Flow 'n'=0.130 2000 Ft. 2.00%

0.26 Hrs.

Total Area in Acres = 10.00 Ac. Total Sheet Total Shallow Total Channel

Weighted CN = 70 Flow= Flow= Flow =

Time Of Concentration = 0.36 Hrs. 0.09 Hrs. 0.01 Hrs. 0.26 Hrs.

RAINFALL TYPE IA

Precipitation Runoff Qp, PEAK TOTAL STORM

STORM (P) inches (Q) DISCHARGE Volumes

1 Year 5.8 In. 2.7 In. 5.5 CFS 96,195 Cu. Ft.

2 Year 7.2 In. 3.79 In. 8.3 CFS 137,577 Cu. Ft.

25 Year 12.4 In. 8.4 In. 20 CFS 305,646 Cu. Ft.

Check to ensure non-erosive velocity for 1-year storm:

Roadway slope = 2.0%, check velocity and depth for the following parameters:

Q1-year = 5.5 cfs Bottom width = 2 ft Side slopes = 2:1 Longitudinal slope = 1.75% Manning’s roughness coefficient (n) = 0.03

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Using Manning’s equation for a trapezoid channel:

Q = (a) (v) = (a)[1.49/n (R)2/3 (S)1/2]; a = bd + zdd; R=a / (b+2d√(zz+1)) where v = velocity (ft/s) n = Manning’s roughness coefficient, R = hydraulic radius (ft) a = cross sectional area (ft2) S = channel longitudinal slope b = bottom width (ft) d = depth of water (ft) z = side slope (z:1)

Solve for depth and velocity.

Results:

a = (2.0 * d) + 2(d * d) = 2.0d + 2d2 R = (2.0d + 2d2) / (2.0 + 2d√(5)) = (2.0d + 2d2) / (2.0 + 4.47d) Q = (2.0d + 2d2) [1.49/.03 * ((2.0d + 2d2) / (2.0 + 4.47d)) 2/3 * 0.01751/2 ]

Depth Due to complexity of equation, solve for d by trial and error. First, choose a depth that seems reasonable and solve for Q. Repeat until the solution for Q is equal to Q1-year (5.5 cfs).

Spreadsheets can be set up to help streamline this process, or an open channel model can be used.

Trial 1. Try a depth of 0.5 ft.

a = 2.0(0.5) + 2(0.5)2 = 1.5 ft2 R = 1.5 ft2 / (2.0 + 4.47(0.5)) = 0.35 ft Q = 1.5 ft2 [1.49/.03 * (0.35 ft) 2/3 * 0.01751/2 ] = 4.93 cfs < Q1-year , so try a larger depth.

Trial 2. Try a depth of 0.55 ft.

a = 2.0(0.55) + 2(0.55)2 = 1.71 ft2 R = 1.71 ft2 / (2.0 + 4.47(0.55)) = 0.38 ft Q = 1.71 ft2 [1.49/.03 * (0.38 ft) 2/3 * 0.01751/2 ] = 5.9 cfs > Q1-year , so try a smaller depth.

Trial 3. Try a depth of 0.53 ft a = 2.0(0.2) + 2(0.53)2 = 1.62 ft2 R = 1.62 ft2 / (2.0 + 4.47(0.53)) = 0.37 ft Q = 1.62 ft2 [1.49/.03 * (0.37 ft) 2/3 * 0.01751/2 ] = 5.5 cfs = Q1-year Thus, depth = 0.53 ft to accommodate this storm.

Velocity v = Q/a v = 5.5 cfs / 1.62 ft2 = 3.4 ft/s (v is less than 5.0 ft/s, OK)

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Check to ensure adequate capacity for 2-year storm:

From hydrology information, the 2-year flow is 8.3 cfs. Compute depth and provide an additional 0.5 ft of freeboard above 2-year flow.

Roadway slope = 2.0%, depth for the following parameters:

Q2-year = 8.3 cfs Bottom width = 2.0 ft Side slopes = 2:1 Longitudinal slope = 1.75% Manning’s Coeff. = 0.03

Solve for depth using Manning’s equation and trial and error method shown above. Using a depth of 0.55 ft in Trial 1 above gave a solution of Q = 5.9 cfs; thus, we know the depth for the 2-year storm is more than 0.5 ft.

Try a depth of 0.66 ft:

a = 2.0(0.66) + 2(0.66)2 = 2.19 ft2 R = 2.19 ft2 / (2.0 + 4.47(0.66)) = 0.44 ft Q = 2.19 ft2 [1.49/.03 * (0.44 ft) 2/3 * 0.01751/2 ] = 8.36 cfs = Q2-year

Results:

Depth = 0.66 ft. Provide 0.66 ft plus additional 0.5 ft of freeboard. Total swale depth = 0.66 ft +

0.5 ft = 1.2 ft for a total top width of 6.8 ft.

Step 7: Design pretreatment.

Pretreat with grassed side slopes and washed, rounded limestone aggregate diaphragm (curtain drain) for the sheet flow and sediment forebays for the concentrated inflow points (i.e., a pipe).

Step 8: Choose vegetation for the channel.

Choose vegetation based on factors such as resistance to erosion, resistance to drought and inundation, cost, aesthetics, maintenance, etc.

Based on the project slope range (0-5%), and 1-year velocity equal to approximately 3.4 ft/s, choose appropriate grass for channel (moist to well drained soils, higher permissible velocities, and good establishment rate). See local NRCS office for guidance.

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4.1.2 Dry Detention Basin Design Example

This example focuses on the design of a dry detention basin for the Mountain View Estates residential subdivision (see Figure 4.1). This step-by-step example shows how the channel protection and peak control requirements for this portion of the site will be met by the dry detention basin. A dry detention basin was chosen because the site is dominated by volcanic geology, as shown on the geologic map (Volume I, Figure 2.2) and verified by geotechnical investigation. Thus, an infiltration basin for quantity control would not be possible.

Groundwater was found at an elevation of 379 ft. The overflow from the basin will discharge into an adjacent freshwater river. The water quality volume for this catchment area will be met prior to discharge to the basin by use of a dry swale, which is presented as a separate design example in Section 4.1.1.

Figure 4.3 Plan View of Dry Detention Basin

Figure 4.4 Cross-Section of Dry Detention Basin

DRY DETENTION BASIN

OUTLET STRUCTURE

388’

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Step 1: Compute the Cpv and the Qp-25 Volume using the Unified Stormwater Sizing Criteria

The basin will be sized to manage the Cpv and the Qp-25 volume with 1 ft of freeboard, with discharges directed to the river via an outlet pipe. The overflow from storms greater than the 25-year storm will be discharged to the river by means of an emergency spillway, where it can be shown that the overflow will not cause damage to neighboring houses or structures.

Table 4.2 provides a summary of the volume requirements that were determined in Volume I, Section 2.2.4 (Sample Calculations). The pre- and post-development runoff characteristics that were determined with a TR-55 computer model are shown in Tables 4.5 and 4.6 below.

Table 4.4 Summary of Storage Requirements for the Dry Detention Basin

Symbol Category Volume Required (ac-ft) Notes

Cpv Stream Protection 1.1 Average ED release rate is 0.55 cfs over 24 hours

Qp-25 Overbank Control 1.43

Step 2: Determine basin location and preliminary geometry. Conduct basin grading.

This step involves initially grading the basin (establishing contours) and determining the elevation-storage relationship for the basin. Storage must be provided for the 1-year storm, and 25-year storm. An elevation-storage table and curve is prepared using the average area method for computing volumes. A 2-ft berm is included around the edge of the basin. See Figure 4.3 for pond grading and Figure 4.5 for Elevation-Storage Data.

Step 3: Size pretreatment.

Even though the dry swale provides treatment, use a sediment forebay at the outfall of the dry swale for additional sediment/trash removal from large storm events. Size forebay using the equation in Volume I, Section 3.2.4.4c:

As = 0.066 (WQv) = 0.066 * (8,712 ft3 * 0.25)

As = 144 ft2

Use a minimum 3 ft depth with a length to width ratio of 1.5:1. Sediment forebay dimensions should be approximately 8.5 ft x 13 ft with a depth of 3 ft.

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Table 4.5 Mountain View Estates Pre-Developed - TR-55 Output

PEAK DISCHARGE SUMMARY

JOB: Mountain View Estates

DRAINAGE AREA NAME: PRE DEVELOPMENT

GROUP CN from AREA

COVER DESCRIPTION SOIL NAME A,B,C,D? TABLE 2-2 (In acres)

Wood Aimeliik-Palau B 55 10.00 Ac.

AREA SUBTOTALS: 10.00 Ac.

Time of Concentration Surface Cover Manning 'n' Flow Length Slope

2-Yr 24 Hr Rainfall = 7.2 In Avg Velocity Tt (Hrs)

Sheet Flow Woods: Dense Underbrush 'n'=0.80 50 Ft. 4.00%

0.18 Hrs

Shallow Flow Forest w/Heavy Litter 1800 Ft. 3.00%

2.50 F.P.S. 1.16 Hrs.

Total Area in Acres = 10.00 Ac. Total Sheet Total Shallow

Weighted CN = 55 Flow= Flow=

Time Of Concentration = 1.34 Hrs. 0.18 Hrs. 1.16 Hrs.

RAINFALL TYPE IA

Precipitation Runoff Qp, PEAK TOTAL STORM

STORM (P) inches (Q) DISCHARGE Volumes

1 Year 5.8 In. 1.40 In. 1.2 CFS 50,820 Cu. Ft.

2 Year 7.2 In. 2.25 In. 2.4 CFS 81,675 Cu. Ft.

25 Year 12.4 In. 6.1 In. 8.5 CFS 222,156 Cu. Ft.

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Table 4.6 Mountain View Estates Post-Developed - TR-55 Output

PEAK DISCHARGE SUMMARY

JOB: Mountain View Estates

DRAINAGE AREA NAME: POST DEVELOPMENT

GROUP CN from AREA

COVER DESCRIPTION SOIL NAME A,B,C,D? TABLE 2-2 (In acres)

Grass Aimeliik-Palau B 61 7.50 Ac.

Impervious (roads, roofs, driveways) Aimeliik-Palau B 98 2.50 Ac.

AREA SUBTOTALS: 10.00 Ac.

Time of Concentration Surface Cover Manning 'n' Flow Length Slope

2-Yr 24 Hr Rainfall = 7.2 In Pipe Diameter Avg Velocity Tt (Hrs)

Sheet Flow dense grass 'n'=0.24 50 Ft. 2.00%

0.09 Hrs

Shallow Flow Short Grass Pasture 50 Ft. 2.00%

7.00 F.P.S. 0.01 Hrs.

Channel Flow 'n'=0.130 2000 Ft. 2.00%

0.26 Hrs.

Total Area in Acres = 10.00 Ac. Total Sheet Total Shallow Total Channel

Weighted CN = 70 Flow= Flow= Flow =

Time Of Concentration = 0.36 Hrs. 0.09 Hrs. 0.01 Hrs. 0.26 Hrs.

RAINFALL TYPE IA

Precipitation Runoff Qp, PEAK TOTAL STORM

STORM (P) inches (Q) DISCHARGE Volumes

1 Year 5.8 In. 2.7 In. 5.5 CFS 96,195 Cu. Ft.

2 Year 7.2 In. 3.79 In. 8.3 CFS 137,577 Cu. Ft.

25 Year 12.4 In. 8.4 In. 20 CFS 305,646 Cu. Ft.

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Figure 4.5 Estimated Storage-Elevation Table/Curve for Basin Design

Elevation Area Average

Area Depth Volume Cumulative Cumulative Elevation ft ft^2 ft^2 ft ft^3 Volume Volume ft ft^3 ac-ft

381.0 0 381.0

382.0 987 494 1 494 494 0.01 382.0

383.0 2012 1500 1 1500 1993 0.05 383.0

384.0 6999 4506 1 4506 6499 0.15 384.0

385.0 13668 10334 1 10334 16832 0.39 385.0

386.0 54226 33947 1 33947 50779 1.17 386.0

387.0 64071 59149 1 59149 109928 2.52 387.0

388.0 74325 69198 1 69198 179126 4.11 388.0

389.0 88872 81599 1 81599 260724 5.99 389.0

Step 4: Compute release rate for Cpv control and establish Cpv elevation.

Set the Cpv pool elevation.

• Required Cpv storage = 1.1 ac-ft (see Table 4.4).

• From the elevation-storage table, read elevation 386 ft.

• Set Cpv wsel = 386.0 ft

Size Cpv orifice.

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• Size to release average of 0.55 cfs.

• Set invert of orifice at wsel = 382.0 ft

• Head = (386.0 ft – 382.0 ft)/2 = 2.0 ft

Note: use average head

Use orifice equation to compute cross-sectional area and diameter.

• Q = CA(2gh)0.5, where: C = discharge coefficient (0.6) A = cross-section area of orifice (D2/4*п) g = acceleration due to gravity (32.2 ft/s2) h = head, height above the center of the orifice (use h = 2.0 ft)

• A = 0.55 cfs / [(0.6)((2)(32.2 ft/s2)(2.0ft))0.5]

• A = 0.081 ft2, A =πD2 / 4;

• D = 0.32 ft = maximum allowed orifice diameter 3.85 in

• Use 8in pipe with 3in orifice plate to achieve equivalent diameter

Compute the stage-discharge equation for the 3in dia. Cpv orifice

• QCpv = CA(2gh)0.5 = (0.6) (0.05 ft2) [((2) (32.2ft/s2))0.5] (h0.5),

• QCpv = (0.24) (h0.5), where: h = wsel – 382.13 ft Note: head is measured from the WSEL to the center line of the orifice.

Step 5: Calculate Qp-25 (25-year storm) release rate and water surface elevation.

In order to calculate the 25-year release rate and water surface elevation, the designer must set up a stage-storage-discharge relationship for the control structure for the Cpv orifice plus the 25-year storm.

Develop basic data and information

• The 25-year pre-developed peak discharge = 8.5 cfs

• The post developed inflow = 20 cfs

• From previous estimate Qp-25 = 1.43 ac-ft. Adding 15% to account for ED storage (per recommendations in Section 9.3) yields a preliminary volume of 1.65 ac-ft.

• From elevation-storage table (Figure 4.5), read elevation 386.5 ft.

Size 25-year overflow riser (modeled as a weir) to release 8.5 cfs at a water surface elevation of

386.5 ft.

At wsel 386.5 ft:

• Cpv orifice releases 0.55 cfs, therefore

• Allowable Qp-25 = 8.5 cfs – 0.55 = 7.9 cfs.

• Set rim (weir crest) elevation at wsel = 386.0 ft (this is max Cpv elevation)

• Max head = (386.5 ft – 386.0 ft) = 0.5 ft

• Use weir equation to compute circumference → Q = CLh3/2 where C = discharge coefficient

(use the maximum 3.1 for broad-crested weirs), L = weir length (ft), and h = head or height of water above the weir (ft).

• L = 7.9 cfs / (3.1) (0.53/2) = 7.2 ft circumference

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• Diameter = 7.2 ft / π = 2.3ft

Use one (1) 2.0 ft diameter riser structure for 25-year release.

• Q25 = (3.1) (6.28) h3/2, Q25 = (19.5) h3/2, where h = wsel – 386.0 ft

Size barrel to release approximately 8.5 cfs at elevation 386.5 ft using an approved pipe sizing method (e.g., culvert-sizing software, NRCS pipe sizing equation). Pipe sizing calculations determine the flow capacity of a given pipe at different flow conditions based on factors such as diameter, slope, length, pipe material (i.e., roughness), upstream and downstream conditions, etc.

In this case, a culvert-sizing software was used to determine that an 18-in diameter concrete pipe, 0.7% slope, 300 feet long with free discharge into the downstream river would be sufficient to use for this outlet.

As a final check, you can route the 25-year post-developed condition inflow through the pond using computer software (e.g., HydroCAD, TR-55). This type of model provides a convenient way to route an inflow hydrograph from the contributing drainage area during a given design storm event into a stormwater storage practice and produces an outflow hydrograph based on the entered outlet configuration (see Table 4.8 – TR-55 Results).

• The hydrologic/hydraulic model computes 25-year wsel at 386.5 ft with discharge = 7.96 cfs

< 8.5 cfs, OK.

Since the 25-year wsel is at 386.5 ft, set the emergency spillway invert at elevation 388.0 ft (this allows for more than a foot of freeboard above the 25-yr wsel) and design according to spillway criteria in Section 6.1. This basin is considered an excavated pond rather than an embankment pond. “Token” or emergency spillways (those placed above the water elevation of the largest managed storm) must be a minimum 8 ft wide, 1 ft deep, with 2:1 side slopes.

Table 4.7 provides a summary of the storage, stage, and discharge relationships, and Figure 4.6 illustrates the profile of the outlet control structure, determined for this design example.

Table 4.7 Summary of Controls Provided for Basin Example

Control Element

Type/Size of Control

Storage Provided

Elevation

Discharge

Units Acre-feet ft cfs Channel Protection (Cpv)

8in pipe with 3in orifice plate 1.1 386.0 0.55

Overbank Protection (Qp-25)

One 2-ft diameter riser structure, 18in barrel.

1.43

386.5

8.0

Emergency Spillway (storms>Qp-25)

Grass channel 8ft bottom width, 1ft deep, 2:1 side slopes

4.11

388.0

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Table 4.8 Sample TR-55 Results from HydroCAD

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Figure 4.6 Profile of Outlet Control Structure (not to scale)

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4.2 Case Study #2: Commercial Site

This case study represents a portion of a commercial site located on the island of Koror. The North Beach Shopping Center (see Figure 4.7) is a hypothetical commercial development which includes a restaurant and a bank, as well as other shops, businesses and green space. The total site area is 10 acres, with 65% impervious cover.

The following examples show how to manage the stormwater from the three small subcatchment areas associated with the restaurant and the bank that have a total site area of approximately 1.43 acres. The three catchment areas each drain to separate stormwater treatment practices, ultimately discharging to coastal waters in a dock area. Catchment A drains to the SW portion of the site and is comprised of 0.78 acres with 90% impervious cover; Catchment B drains to the SE portion of the site and is comprised of 0.46 acres with 91% impervious cover; and Catchment C drains to the middle portion of the site and is comprised of 0.19 acres with 89% impervious cover. While the impervious percentage of each subcatchment is greater than 70%, the site as a whole still meets the post-construction criteria of less than 70% impervious cover for developments over 1/2 acre. On-site soils as determined by the NRCS soil maps are classified as “Udorthents-Urban,” and a HSG of “D” was verified at the site by a test pit.

Three stormwater practice design examples are presented for this case study, consisting of a bioretention system with a grass channel for pretreatment, a bioretention system with a grass filter strip for pretreatment, and a perimeter sand filter. This case study is a good example of how to effectively, and more aesthetically, treat stormwater close to the source instead of the traditional technique of using a single large practice to handle the runoff from the entire 10-acre development.

Table 4.9 Base Data for North Beach Shopping Center

Location: Koror Catchment Area B Catchment Area A Site Area: 0.46 acres

Site Area: 0.78 acres Impervious Area: 0.42 acres Impervious Area: 0.7 acres Catchment Area C

Site Area: 0.19 acres Soils Type: Udorthents-Urban, “D” as verified by test pit at site Impervious Area: 0.17 acres

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Figure 4.7 North Beach Shopping Center - Commercial Site Plan

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4.2.1 Bioretention Design Example with Grass Channel Pretreatment This example focuses on the design of a bioretention system associated with catchment area “A” of the North Beach Shopping Center commercial development (see Figure 4.7), which drains to the SW portion of the site and is comprised of 0.78 acres with 90% impervious cover. This step-by-step example shows how the recharge volume and water quality volume requirements for this portion of the site will be met by the bioretention system. In addition, this example will show how to design a grass channel for pretreatment. In general, the primary function of bioretention facilities is to provide water quality treatment and not large storm attenuation. As such, flows in excess of the water quality volume are typically routed to a downstream facility for large storm attenuation. For this example, the channel protection and peak control requirements are not required because the site ultimately discharges to a tidally-influenced body of water (see Volume I, Section 2.2.2.3).

Figure 4.8 Plan View of Bioretention with Grass Swale

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Figure 4.9 Typical Sections of a Bioretention Facility and a Grass Swale

Step 1: Compute Recharge Volume (Rev) and the Water Quality Volume (WQv) using the

Unified Stormwater Sizing Criteria

Recharge, Rev

The site is located in the volcanic region (Volume I, Figure 2.1), soil HSG D, so use F = 0.06 in (see Volume I, Table 2.2).

Rev = [(F) (I) (A)] / 12 = [(0.06 in) (0.9) (0.78 ac)] (1ft/12in) = 0.0035 ac-ft

CROSS SECTION

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Water Quality Volume, WQv

This site discharges to a dock area, which is water quality classification B (see Volume I, Table 2.6) and falls under the 80% capture rule.

WQv = [(P) (I) (A)] / 12 = [(0.7 in) (0.9) (0.78 ac) (1ft/12in) = 0.041 ac-ft

The WQv is greater than Rev – thus, no additional volume must be recharged other than the WQv.

Bioretention will be designed without an impermeable liner (as an exfilter) to allow for infiltration. Total WQv to be treated by the bioretention is: WQv = 0.041 ac-ft = 1,786 ft3.

bioretention area.

Site Specific Data:

Existing ground elevation at practice location is 10.0 ft, mean sea level. Soil boring observations reveal that the seasonally high water table is at 2.0 ft and underlying soil is urban fill HSG “D”.

There are no additional local criteria that must be met for this design.

Step 4: Determine size of bioretention filter area.

Use sizing equation and values provided in Volume I, Section 3.2.4.4d:

Af = (WQv) (df) / [(k) (hf + df) (tf)] df = filter bed depth (ft) (use 4 ft per Volume I, 1.5 ft min. allowed with designer justification as long as 0.75 WQv criteria is still met) k = coefficient of permeability of filter media (ft/day) (see Volume I, Section 3.2.4.4d for appropriate values) hf = average height of water above filter bed (ft) (max 6in allowed, so ave = 0.25ft) tf = design filter bed drain time (days) (48 hours is recommended)

Af = (1,786 ft3) (4 ft) / [(1 ft /day) (0.25ft + 4 ft) (2 days)] (With df = 4 ft, k = 1.0 ft /day, hf = 0.25ft, tf = 2 days)

Af = 841 ft2

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Step 5: Set design elevations and dimensions.

Assume a roughly 2:1 rectangular shape. Given a filter area requirement of 841 ft2, say facility is roughly 20ft by 40ft. See Figure 4.8. Set top of facility at 10.0 ft, with the berm at 11.0 ft.

The facility is 5 ft deep from rim to bottom of planting soil, which will allow 3 ft of clearance above the seasonally high water table. See Figure 4.9 for a typical section of the facility.

Step 6: Design conveyance to facility.

Stormwater treatment practices can be either on or off-line. On-line facilities are generally sized to receive, but not necessarily treat, larger storms. Off-line facilities are designed to receive a more or less exact flow rate through a weir, channel, manhole “flow splitter”, etc. The facility in this example is situated to receive only the WQv via a flow splitter in a grass channel.

To design a flow splitter for this offline bioretention area, refer to Section 9.3 for guidance onthe modified curve number (CN) technique for water quality peak flow (WQf) calculations.

Using the WQv, a modified CN is computed utilizing the following equation from Section 9.3:

CN = 1000 / [10 + 5P +10Q - 10(Q² + 1.25 QP)½]

Where P = rainfall, in inches (use 0.7 inches in this case – see Step 1) Q = runoff volume, in watershed inches (equal to WQv ÷ area) Q = (0.041 ac-ft) (12 inches/ft) / (0.78 acres) = 0.63 inches

CN = 1000 / [10 + 5(0.7 in) +10 (0.63 in) - 10((0.63 in)² + 1.25 (0.63 in)(0.7 in))½]

CN = 1000 / [10 + 3.5 + 6.3 – 9.74] = 99.4

The time of concentration (tc) is calculated as 2.3 min, but use a minimum of 5 min (0.08 hr).

Read initial abstraction (Ia) from TR-55 Table 4.1 or calculate Ia = 200/CN - 2 = 0.012

Compute Ia/P = 0.012 / 0.7 in = 0.017

Approximate the unit peak discharge (qu) from TR-55 Exhibit 4-IA for appropriate tc.

qu = 180 csm/in

Using the WQv, compute the peak discharge (WQf) WQf = qu * A * Q where WQf = the peak discharge for the water quality event, in cfs qu = the unit peak discharge, in cfs/mi²/inch (180 csm/in) A = drainage area, in square miles (0.00122 sq miles)

Q = runoff volume, in watershed inches (equal to WQv ÷ area) (0.63 inches)

WQf = 180 * 0.00122 * 0.63 = 0.14 cfs

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Use the orifice equation to size pipe to bioretention area. WQf = CA(2gh)1/2 where: C = discharge coefficient (0.6) A = cross-section area of orifice (D2/4*п) g = acceleration due to gravity (32.2 ft/s2) h = head, height above the center of the orifice (assume 1 ft)

A = WQf / [C * (2gh)1/2] = 0.14 cfs / [0.6 * (2*32.2 ft/s2 * 1 ft) 1/2] = 0.03 ft2 Diameter = 2.3 in. Use 6in pipe with 2.5 in restrictor.

Figure 4.10 Example Flow Splitter Design

Step 7: Design pretreatment.

Pretreat with a grass channel. Assume trapezoidal grass channel dimensions of 2 ft bottom width, 2:1 side slopes, and 1 ft depth, with a cross-sectional area = 6 ft2. Size for 25% of WQv (1,786 ft3). The length can be determined by the following equation (Volume I, Section 3.2.4.4c):

L = 0.25 * WQv / A = 0.25 * 1,786 ft3 / 6 ft2 = 74.4 ft

To Offline Bioretention Facility for Water Quality Treatment

To Discharge Pipe that Outlets to Coastal Waters

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Given the site layout, 175ft is available for a grass channel along the parking lot, which is greater than the required length, OK.

Step 8: Size underdrain area.

Because this bioretention area is designed as an exfilter, an underdrain may not be necessary.

However, because the underlying volcanic soils are HSG “D”, an underdrain system should still be installed to ensure proper drainage for storm volumes that exceed the limited infiltration capacity of the soil. As a rule of thumb, the length of underdrain can be based on 10% of the Af or 84 ft2 and a 3 ft wide zone of influence. See Figures 4.8 and 4.9. Using 4-in perforated plastic pipes surrounded by a 3ft-wide washed, rounded limestone aggregate bed, 10 ft on center (o.c.), yields the following length of pipe:

(84 ft2)/3 ft per foot of underdrain = 28 ft of perforated underdrain

Step 9: Overflow design.

Since the bioretention is designed as an off-line practice sized to treat only the WQv, it theoretically should not overflow. However, should filtering rates become reduced due to facility age or poor maintenance, an overflow weir is provided to prevent flooding until the filter media and plants can be replaced. The overflow weir elevation is set so that a maximum of 6 inches of ponding occurs above the bottom of the bioretention area. In this case, the overflow weir is set at 10.0 ft. The overflow riser is connected to the underdrain system discussed in Step 8, which would be connected with the site’s drainage system that ultimately discharges to coastal waters in a dock area.

Step 10: Choose plants for planting area.

Choose plants based on factors such as whether native or not, resistance to drought and inundation, cost, aesthetics, maintenance, etc. Select species locations (i.e., on center planting distances) so species will not “shade out” one another. Do not plant trees and shrubs with extensive root systems near pipe work. Planting guidelines for this practice are presented in Chapter 5.

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4.2.2 Bioretention Design Example with Filter Strip Pretreatment This example focuses on the design of a bioretention system associated with catchment area “B” of the North Beach Shopping Center commercial development (see Figure 4.7), which drains to the SE portion of the site and is comprised of 0.46 acres with 79% impervious cover. This step-by-step example shows how the recharge volume and water quality volume requirements for this portion of the site will be met by the bioretention system. In addition, this example will show how to design a filter strip for pretreatment. For this example, the channel protection and peak control requirements are waived since the ultimate discharge flows to coastal waters. In general, the primary function of bioretention facilities is to provide water quality treatment and not large storm attenuation. As such, flows in excess of the water quality volume are typically routed to a downstream facility, such as conventional detention basins or some other facility such as underground storage vaults.

Follow the sizing calculations from Section 4.2.1 for the bioretention design. Pretreatment and overflow designs for catchment area “B” are discussed below.

Figure 4.11 Plan View of Bioretention with Filter Strip

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Step 1: Design pretreatment.

Volume I, Table 3.4 (also provided below).

Guidelines for Filter Strip Pretreatment Sizing

Parameter Impervious Parking Lots Residential Lawns Maximum Inflow Approach Length (ft.) 35 75 75 150

Filter Strip Slope <2% ≥2% <2% ≥2% <2% ≥2% <2% ≥2% Filter Strip Minimum Length 10’ 15’ 20’ 25’ 10’ 12’ 15’ 18’

The maximum inflow approach length is 60 ft with a slope of 2%, so a filter strip length of 20’ is required. However, our site only has 12’ available for a filter strip, so use a washed, rounded limestone aggregate curtain in addition to meet pretreatment requirements.

Step 2: Overdrain and Emergency Spillway design.

Stormwater treatment practices can be either on or off-line. On-line facilities such as this one receiving direct runoff from the parking lot are generally sized to receive, but not necessarily treat, larger storms. Thus, an overflow weir is provided to pass the 25-year event. The overflow weir elevation is set so that a maximum of 6 inches of ponding occurs above the bottom of the bioretention area. Outlet protection in the form of riprap or a plunge pool/stilling basin should be provided to ensure non-erosive velocities downstream.

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4.2.3 Sand Filter Design Example

This example focuses on the design of a perimeter sand filter associated with catchment area “C” of the North Beach Shopping Center commercial development (see Figure 4.7), which drains to the middle portion of the site and is comprised of 0.19 acres with 89% impervious cover. This step-by-step example shows how the water quality volume requirements for this portion of the site will be met by the sand filter. It is assumed that this catchment area is located in an area of higher-pollutant load generation. For this example, the recharge volume, channel protection and peak control requirements for this catchment area are not presented. The recharge requirement can be waived given that this site is a hotspot. Channel protection and peak control requirements can also be waived since the site discharges to coastal waters.

In general, the primary function of sand filters is to provide water quality treatment and not large storm attenuation. As such, flows in excess of the water quality volume are typically routed to bypass the facility. The post-development 2-yr peak discharge is provided to appropriately size the necessary by-pass flow splitter. If quantity control is required, bypassed flows are typically routed to conventional detention basins (or some other facility such as underground storage vaults).

Figure 4.12 Plan View of Sand Filter

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Figure 4.13 Profile and Plan Views of Sand Filter

40’

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Step 1: Compute water quality volumes using the Unified Stormwater Sizing Criteria.

Use the 90% capture rule with 1.2in of rainfall for hotspot areas.

WQv = [(P) (I) (A)] / 12 = [(1.2 in) (0.89) (0.19 ac) (1ft/12in) = 0.017 ac-ft = 741 ft3

Note: For this design example, the 2-year peak discharge will be used to size the bypass flow splitter. Any hydrologic models using SCS procedures, such as TR-20, HEC-HMS, or HEC-1, can be used to perform preliminary hydrologic calculations

Table 4.10 Summary of Post-development Hydrologic Data for Catchment C

Condition P(2-yr) Area CN Q2 in ac cfs

Post-developed Catchment C 7.2 0.19 96 0.33 perimeter sand filter.

Site Specific Data:

Existing ground elevation at practice location is 11.0 ft, mean sea level. Soil boring observations reveal that the seasonally high water table is at 2.0 ft.

There are no additional requirements for this site.

Step 4: Compute WQv, available head, & peak discharge (Qp-wq).

• Determine available head (See Figures 4.12 and 4.13)

Low point at edge of facility is 11.0 ft. Subtract 1 ft to pass Q2 discharge (10.0 ft). Set outfall underdrain pipe at 3.5 ft and add 4 in to this value for drain slope (3.83 ft). Add to this value 8 in for the washed, rounded limestone aggregate blanket over the underdrains, and 18 in for the sand bed (6.0 ft). The total available head is 10.0 ft – 6.0 ft or 4.0 ft. Therefore, the average depth, hf, is (hf) = 4.0 ft / 2, and hf = 2.0 ft.

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Step 5: Size flow diversion structure:

Size the 2-year overflow as follows:

The 2-year wsel is set at 11.0 ft. Use a concrete weir to pass the 2-year flow (0.33 cfs) into an overflow chamber using the Weir equation. Assume 1 ft of head to pass this event.

Q = CLH3/2

0.33 = 3.1 (L) (1ft)1.5

L = 0.11 ft; use L = 0.5 ft which sets the flow diversion chamber dimension.

Weir wall elevation = 10.0 ft.

Step 6: Size filtration bed chamber (see Figure 4.13).

Use sizing equation provided in Volume I, Section 3.2.4.4d:

Af = (WQv) (df) / [(k) (hf + df) (tf)] df = filter bed depth (ft), use 1.5ft k = coefficient of permeability of filter media (ft/day), use 3.5ft/day hf = average height of water above filter bed (ft), use 2.0ft tf = design filter bed drain time (days) (40 hours = 1.67 days is recommended)

Af = (741 ft3) (1.5ft) / [(3.5ft/day) (2ft + 1.5ft) (1.67 days)] (With df = 1.5ft, k = 3.5ft/day, hf = 2.0ft, tf =

1.67 days)

Af = 54.3 ft2 = use 55 ft2 with a width of 1.75 ft and length of 32 ft.

Step 7: Size sedimentation chamber.

From Camp-Hazen equation shown in Volume I, Section 3.2.4.4c: As = 0.066 (WQv)

As = 0.066 (741 ft3) or 48.9 ft2 given a width of 1.75 ft, the minimum length will be 48.9 ft2/1.75 ft or 30 ft (so, use 32 ft to match filter length for easy construction)

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Step 8: Compute Vmin.

Vmin is the minimum volume that must be held by the entire system at one time (see Volume I, Section 3.2.4.4d).

Vmin = ¾(WQv) or 0.75 (741 ft3) = 556 ft3

Step 9: Compute volume within practice (iterate if necessary).

Volume within filter bed (Vf): Vf = Af (df) (n); n = 0.4 for sand

Vf = (1.75 ft * 32 ft) (1.5 ft) (0.4) = 34 ft3

Temporary storage above filter bed (Vf-temp): Vf-temp = 2hfAf

Vf-temp = 2 (2.0 ft) (1.75 ft * 32 ft) = 224 ft3

Compute remaining volume for sedimentation chamber (Vs):

Vs = Vmin - [ Vf + Vf-temp] or 556 - [34 + 224] = 298 ft3

Compute height in sedimentation chamber (hs): hs = Vs/As

(298 ft3) / (1.75 ft x 32 ft) = 5.32 ft which is larger than the head available (4.0 ft); increase the design length to 40 ft for filter and sedimentation chamber;

Volume within filter bed (Vf): Vf = Af (df) (n); n = 0.4 for sand

Vf = (1.75 ft * 40 ft) (1.5 ft) (0.4) = 42 ft3

Temporary storage above filter bed (Vf-temp): Vf-temp = 2hfAf

Vf-temp = 2 (2.0 ft) (1.75 ft * 40 ft) = 280 ft3

Compute remaining volume for sedimentation chamber (Vs):

Vs = Vmin - [ Vf + Vf-temp] or 556 - [42 + 280] = 234 ft3

Compute height in sedimentation chamber (hs): hs = Vs/As

(234 ft3) / (1.75 ft x 40 ft) = 3.34 ft which is less than the head available (4.0 ft); OK.

Use 1.75 ft x 40 ft for filter bed and sedimentation chamber dimensions.

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4.3 Case Study #3: Commercial Site

This case study represents a commercial site located in Kloulklubed, on the island of Peleliu.

The Sunshine Market (see Figure 4.14) is a hypothetical commercial development consisting of a fish and produce market with a total site area of approximately 3.0 acres with 52% impervious cover. On-site soils as determined by the NRCS soil maps are Ngedebus – Majuro and are classified as HSG “A”. Infiltration chambers with a grass channel and oil/grit separator for pretreatment are the design examples presented for this case study.

Figure 4.14 Sunshine Market Site Plan discharge pipe to beach

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Table 4.11 Sunshine Market Base Data Location: Kloulklubed, Peleliu

Site Area: 3.0 acres Impervious Area: 1.56 acres

Soils Type: Ngedebus – Majuro, “A”

4.3.1 Infiltration Chamber Design Example with Grass Channel and Oil/Grit Separator

Pretreatment This example focuses on the design of subsurface infiltration chambers associated with the Sunshine Market in Kloulklubed, Peleliu (see Figure 4.14). This step-by-step example shows how the recharge and water quality volume requirements for this portion of the site will be met by the infiltration chambers. In general, the primary function of the chambers is to provide water quality treatment and groundwater recharge and not large storm attenuation. As such, flows in excess of the water quality volume are typically routed to bypass the facility. In this example, channel protection and overbank flood requirements are waived because flow from these larger storm events is discharged overland to the beach. The infiltration chambers are located in a landscaped area adjacent to the parking lot. The infiltration chambers receive runoff from the parking lot, walkways, and rooftops on the site.

Figure 4.15 Typical Detail of an Oil/Grit Separator

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Figure 4.16 Typical Detail of Infiltration Chambers

Step 1: Compute recharge and water quality volumes using the Unified Stormwater Sizing

Criteria.

Recharge, Rev

The site is located in a limestone region (Volume I, Figure 2.2), so use 1.2 in multiplied by the impervious area.

Rev = [(P) (I) (A)] / 12 = [(1.2 in) (0.52) (3.0 ac)] (1ft/12in) = 0.156 ac-ft = 6,795 ft3

Water Quality, WQv

This site discharges to a Class AA marine area, which falls under the 90% capture rule.

WQv = [(P) (I) (A)] / 12 = [(1.2 in) (0.52) (3.0 ac) (1ft/12in) = 0.156 ac-ft = 6,795 ft3

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Rev is equal to WQv, so 100 % of the recharge volume is contained within the WQv.

Step 2: Determine if the development site and conditions are appropriate for the use of infiltration chambers.

Site Specific Data:

Table 4.12 presents site-specific data, such as soil type, percolation rate, and slope, for consideration in the design of the infiltration chamber.

Table 4.12 Site Specific Data for Sunshine Market Criteria Value Soil Loamy Sand In-situ Infiltration Rate at Site 4.5 in/hr Design Infiltration Rate (fc) 2.41 in/hr* Ground Elevation at BMP 20 ft Seasonally High Water Table 9 ft Soil slopes <1% *Determine fc from Table 3.5 in Volume I.

Table 4.13, below, summarizes the requirements that need to be met to successfully implement infiltration practices. On this site, infiltration is feasible, with restrictions on the depth and width of the chamber.

Table 4.13 Infiltration Chambers Feasibility Criteria Status In-situ infiltration rate greater than or equal to 0.5 in/hour.

Infiltration rate is 4.5 in/hour. OK.

Soils have a clay content of less than 20% and a silt/clay content of less than 40%.

Loamy sand meets both criteria.

Infiltration cannot be located on slopes greater than 6% or in fill soils.

Slope is <1%; not fill soils. OK.

Hotspot runoff should not be infiltrated. Not a hotspot land use. OK.

The bottom of the infiltration facility must be separated by at least 3 ft vertically from the seasonally high water table.

Elevation of seasonally high water table: 9 ft Elevation of BMP location: 20 ft.

The difference is 11 ft.

Thus, the facility can be up to 8 ft deep (but use maximum of 4 ft). OK.

Infiltration facilities must be located 100 ft horizontally from any water supply well.

No water supply wells nearby. OK.

Maximum contributing area generally ≤5 acres. Area draining to facility is less than 5 acres. OK.

Setback 25 ft down-gradient from structures. Chamber edge is > 25 ft from all structures. OK.

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Step 4: Size the infiltration chamber facility.

In general, infiltration chambers can be sized by the equation below. For product-specific sizing, please refer to the manufacturer’s instructions.

Vw = L*[(w*d*n) – (#*Ac*n) + (#*Ac) + (w*fc*T/12)]

Where:

Vw = design volume (e.g., WQv) (ft3) L = Length of infiltration facility (ft) w = width of infiltration facility (ft) d = Depth of infiltration facility (ft) # = number of rows of chambers Ac = Cross-sectional area of chamber (ft2) n = Porosity (assume 0.4) fc = infiltration rate (in/hour) T = Fill Time (time for the practice to fill with water), in hours

Assume that:

n = 0.4 d = 4 ft (see above; feasibility criteria) fc = 2.41 in/hour (design rate, see above; site data) T = 2 hours (this is the recommended default value to be used unless site-specific data exists) Ac = 3.445 ft2 (supplied by manufacturer) # = 15 ft (available width) / 3.25 ft (supplied by manufacturer) = 4.6, only 4 rows can fit

Solve for Length given that we have 15 ft – 20 ft of width that we want to use at our site.

Therefore:

L = 6,795 ft3 / [(15 ft*4 ft*0.4) – (4*3.445 ft2*0.4) + (4*3.445 ft2) + (15 ft*2.41 in/hr*2 hr/12)]

L = 6,795 ft3 / [(24 ft2) - (5.5 ft2) + (13.8 ft2) + (6.0 ft2)] L = 178 ft

Add one foot to each end and each side to give room for a stone buffer (this distance could be more or less depending on manufacturer’s specifications). Thus, the facility dimensions will be 17 ft x 180 ft. Check to ensure that there is sufficient room for the infiltration chamber facility alongside proposed parking lot. The proposed parking lot is 250 ft long, Ok. (Refer back to Figure 4.14 for a site plan view).

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Step 5: Size pretreatment.

For pretreatment, use a grass channel and oil/grit separators as shown in Figure 4.14. These should be sized to pretreat 50% of WQv given the infiltration rate of the soil (see Volume I, Section 3.2.4.3c).

Pretreatment volume = 0.5 * WQv = (0.156 ac-ft) * (43,560 ft2/ac) * 0.5 = 3,398 ft3

Use 2, 5,000 gallon oil/grit separators = 2 * (5,000 gal / 7.5 gal/cf) = 1,333 ft3

Size the grass channel to treat the remaining 2,065 ft3. The grass channel is 235 ft long, which means that the cross-section area needs to be (2,065 ft3) / (235 ft) = 8.8 ft2. Use a swale with a 3 ft bottom width, 1.5 ft deep, and 2:1 side slopes.

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4.4 Case Study #4: Single Family Residential Site

This case study represents a single-family residential site located in Melekeok, Babeldaob. This single family home (see Figure 4.17) is a hypothetical site consisting of a ¼-acre lot. The drainage area used for this example consists of the entire lot and is comprised of 32% impervious cover. On-site soils as determined by the NRCS soil maps are Aimeliik-Palau and are classified as HSG “B”. Palau regulations do not require that stormwater management practices for single or dual-family residences be designed by an engineer, so a less rigorous approach is acceptable.

The stormwater practice design examples presented for this case study are mostly qualitative in nature and are suitable for design and construction by qualified contractors and even enthusiastic homeowners. This example consists of calculations for a cistern to a drywell and descriptions of a rain garden, permeable pavers, and a dry swale.

Figure 4.17 Single-family Residential Site Plan on Babeldaob

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Table 4.14 Babeldaob Single Family…