Coeff. RunOff

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Appendix

CCExample Scenarios

1. Park ing Lot Exam ple

2. Podium Type Building Example


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C.1 Parking Lot Example

The example parking lot site description:

Th j i i 2 i h 1% l f d f l

Typical Parking Lot

Summary of

Stormwater Controls

Site Design Measures

Using drainage as a

design element

Source Controls

Stenciling storm drain

inlets

Landscape designer will

be asked to follow

Integrated Pest

Management principles

Treatment Measures

Bioretention areas

Introduction

This example shows a proposed parking lot in Alameda County with bioretention areas.

Beginning December 1, 2011, LID feasibility/infeasibility criteria (Appendix J) shall be used

to determine whether bioretention areas may be used and methods to design bioretention

areas to maximize infiltration and evapotranspiration.


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ALA MEDA COUNTYWIDE CLEAN WA TER PROGRAM

The site will be treating the 0.2 inches per hour event which is equivalent to 85% of annual runoff

from the site.

Using Chapter 5, the runoff coefficients are determined to be:

Standard Paving .7

Bioretention Areas .1

Sidewalks shall be graded toward landscaped areas.

The parking lot will have standard asphalt paving.

The parking lot will have landscaping as an amenity.

All areas will be graded to drain to bioretention areas along the perimeter of the site. Parking lot

slopes are approximately 1%.

Bioretention areas are sized following the Technical Guidance sheets presented in Chapter 6.

The following tables show sizing and calculations of the site and the treatment measures used.

The following figure shows locations and layout of treatment measures.

Typical Linear Bioretention Area


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Procedure for BMP sizing:

A1. List land types and locate runoff coefficient C from Table in Chapter 5 for use in determiningQ = CIA.

Landscaping C = 0.1

Paving C = 0.7

.

A2. List areas to each treatment measure. (A in Q = CIA)

Area A Landscaping 7,868 square feet

Area A Paving 6,788 square feetTotal Area A 14,656 square feet

Area B Landscaping 11,497 square feet

Area B Paving 24,491 square feet

Total Area B 35,988 square feet

Area C Landscaping 3,222 square feet

Area C Paving 11,442 square feet

Total Area C 14,664 square feet

Area D Landscaping 3,877 square feet

Area D Paving 18,265 square feet

Total Area D 22,142 square feet

A3. Flow through bioretention area (ignoring storage for a flow based BMP). Multiply the impervious

surface from A2. by a sizing factor of 0.04.

The 4% ratio is for planning review and is from 0.2 inches of rainfall per hour inflow versus 5inches/hour infiltration rate.

Area A 6,788 square feet * 0.04 = 272 square feet of bioretention treatment area.

Area B 24,491 square feet * 0.04 = 972 square feet of bioretention treatment area.

Area C 11,442 square feet * 0.04 = 458 square feet of bioretention treatment area .

Area D 18,265 square feet * 0.04 = 731 square feet of bioretention treatment area.

Total Treatment Area is 2,431 square feet using the flow based approach with no

allowance for storage.

A4. If calculating a treatment surface area including an allowance for storage, determine the compositeroughness coefficient.

Area A Landscaping 7,868 square feet * .1 = 787 square feet

Area A Paving 6,788 square feet * .7 = 4,752 square feet

Total, Area A, C*A 5,538 square feet

Total Area A 14 656


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Area C Landscaping 3,222 square feet * .1 = 322 square feet

Area C Paving 11,442 square feet * .7 = 8,009 square feet

Total ,Area C, C*A 8,332 square feet

Total Area C 14,664

Composite Runoff Coefficient 0.57

Area D Landscaping 3,877 square feet * .1 = 388 square feet

Area D Paving 18,265 square feet * .7 = 12,786 square feet

Total, Area D , C*A 13,173 square fee

Total Area D 22,142

Composite Runoff Coefficient 0.59

A5 Determine the Unit Basin Storage Volumes for 80 Percent Capture using 48-hour

drawdown. using Table 5.2 of Chapter 5. For Area A, the composite runoff coefficient is

0.38, the rainfall * C amount is 0.21 inches ((.34-.17)/(0.5-.25)*(0.28-0.17)+0.17

=0.26inches). Adjust this volume for mean annual precipitation. For a site near Oakland

with a mean annual precipitation of 20 inches, Use 0.26 rainfall for a 18.35 inch mean

annual precipitation and adjust to 0.28 inches (0.26 * 20/18.35) at 20 inch mean annual

precipitation. (C * I in Q=CIA). To calculate I, divide the C * I by I = .26/.38 -

= 0.74 inches For Areas B, C and D the rainfall amount is also 0.74 inches. For this

example, a rainfall intensity of 0.2 inches per hour is used to calculate runoff fro the flow-

based method. Other methods may be used to calculate the rainfall intensity for the flow-

based method.

A6. Use a constant infiltration rate of 5 inches per hour through the soil as required by the

Permit for use with treatment soils.

A7. Calculate the duration of treatment event rainfall with all rainfall from the 48-hour storm

occurring at a steady rate of 0.2 inches per hour at the start of the storm.

For Area A 48-hour rainfall total of 0.74 inches per 48 hours, all rainfall will occur during

the first 3.7 hours of the 48-hour period. (Duration = 0.74 inches/48 hours / 0.2 inches per

hour = 3.7 hours) The duration is also 3.7 hours, for Areas B, C and D.

A8. Compute Required Depth of Storage for a given treatment area. (Maximum Allowable Depth =

12 inches)

Area A

For a Bioretention Treatment Area of 150 square feet, infiltration rate of 5 inches per hour and

duration of 3.7 hours), Treated Runoff = 150 square feet * 0.42 feet per hour * 3.7 hours =


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Area B


duration of 3.7 hours , Treated Runoff = 450 square feet * 0.42 feet per hour * 3.7 hours =

694 cubic feet treated during rainfall duration.

Total Runoff Volume = C * A * Rainfall Amount = 18,293 square feet * 0.74 inches /12

inches/foot = 1,129 cubic feet

Stored Volume = Total Volume Treated Volume = (1,129 cubic feet 694 cubic feet =

435 cubic feet must be stored over 450 square feet. With a depth of 1.0 feet or 11.6 inch

average depth below the crest of the overflow riser. This compares with 972 square

feet treatment area with no allowance for storage.

Area C


duration of 3.7 hours, Treated Runoff = 300 square feet * 0.42 feet per hour * 3.7 hours =



inches/foot = 511cubic feet

Stored Volume = Total Volume Treated Volume = (511 cubic feet 460 cubic feet = 51

cubic feet must be stored over 300 square feet. With a depth of 0.17 feet or 2.0 inch



Area D


duration of 3.7 hours, Treated Runoff = 400square feet * 0.42 feet per hour * 3.7 hours =



inches/foot = 806 cubic feet

Stored Volume = Total Volume Treated Volume = (806 cubic feet 611 cubic feet =

194 cubic feet must be stored over 400 square feet. With a depth of 0.49 feet or 5.80 inch




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C.2 Podium Type Building Example

The example podium style building site description:

The project site is approximately 25,000 square feet.

The site is Type D soil with expected compaction of 95%.

Lot line is assumed to be to the edge of city right-of-way (sidewalks)

Introduction

This example is to show a proposed podium type building in Alameda County, with flow-

through planters. Beginning December 1, 2011, LID feasibility/infeasibility criteria

(Appendix J) will need to be used to determine whether the use of flow-through planters will

be allowed.

Typical Podium Building

Summary of

Stormwater Controls

Site Design Measures

Multistory building

above covered parking

Source Controls Covered trash storage

areas

Landscape designer will

be asked to follow

Integrated Pest

Management principles

Treatment Measures

Flow-through planters


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The proposed podium building is a zero lot line design with flow through planters in the

center of the building around a concrete patio and down at ground level.

The podium building is a mixed use building with residential units on the top floors, retailsites on the second floor and parking on the bottom floor. The building mechanical facilities

and trash facilities are also on the bottom floor.

The roof area of the podium building consists of approximately 9,000 square foot patio,

1,000 square feet of flow through planters and 15,000 square feet of roof.

Off site sidewalks and driveways will be graded toward street.

Using Chapter 5, the runoff coefficients are determined to be:

Patio Surfaces .7

Roof Surfaces .9

Flow through Planter .1

The ground floor is a concrete slab with buildings and a covered parking structure. There is

no potential for infiltration. The soils within the planter will be at least 18 inches of

treatment soil that must pass an in-situ percolation test with a rate of 5 inch/hour. A 12-

inch layer of drain rock will be placed around the perforated underdrain to allow for

dewatering of the flow through the planter.

The flow through planter areas will connect directly to the storm drain system through a

system of perforated underdrains and overflow pipes.

If a flow-based method is used without storage, flow through planters shall be sized to be

4% of the area of the impervious patio. The flow through planter shall have splash blocks at

rain water leader discharge points to protect against erosion.

The following tables show the sizes and calculations for the Podium building treatment

measures.

Source Control

Parking and trash shall be under the building and covered.


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Procedure for BMP sizing:

B1. List land types and locate runoff coefficient from Table in Chapter 5. (C in Q = CIA)

Landscaping C = 0.1

Patio Surface C = 0.7

Roof Surface C = 0.9

B2. List areas to each treatment measure. (A in Q = CIA)

Patio Surfaces 9,500 square feet

Roof Surfaces 15,000 square feet

Landscape 500 square feet

B3. Flow through bioretention area (ignoring storage for a flow based BMP). Multiply the

impervious surface from A2. by a sizing factor of 0.04.

The total impervious area from Step B2. is 24,000 square feet. 4% of 24,000 square feet is

960 square feet of bioretention treatment area.

B4. If calculating a treatment surface area including an allowance for storage, determine the

composite roughness coefficient.

C * A

Patio Surfaces .7 * 9,500 square feet = 6,650 square feet

Roof Surfaces .9 * 15,000 square feet = 13,500 square feet

Flow through Planter .1 * 500 square feet = 50 square feet

Total C * A = 20,200 square feet

Total Area = 25,000 square feet

Composite Roughness Coefficient = 0.81

B5. Determine the Unit Basin Storage Volumes for 80 Percent Capture using 48-hour

drawdown. using Table 5.2 of Chapter 5. For a composite runoff coefficient of 0.81,

the rainfall amount is 0.53 inches ((.81-.75)/(1-.75)*(.67-.5)+.5 =.53 inches). Adjust

this volume for mean annual precipitation. For a site near Livermore with a mean

annual precipitation of 16 inches, Use 0.53 rainfall for a 18.35 inch mean annual

precipitation and adjust to 0.47 inches (0.53 * 16/18.35) at 16 inch mean annual

precipitation. (C * I in Q=CIA). To calculate I, divide the C * I by I =

.47/.81 -= 0.58 inches For this example, a rainfall intensity of 0.2 inches per hour is

used to calculate runoff fro the flow-based method. Other methods may be used to

calculate the rainfall intensity for the flow-based method.

B6. Use a constant infiltration rate of 5 inches per hour through the soil as required by the

Permit for use with treatment soils.

B7. Calculate the duration of treatment event rainfall with all rainfall from the 48-hour storm


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B8. For a Bioretention Treatment Area of 500 square feet, infiltration rate of 5 inches per

hour and duration of 2.92 hours, Treated Runoff = 500 square feet * 0.42 feet per

hour * 2.92 hours = 609.cubic feet treated during rainfall duration. (5 inches per hour= 0.42 feet per hour).

Remaining volume (983 cubic feet 609 cubic feet = 375 cubic feet must be stored over 500

square feet. With a depth of 0.75 feet or 9 inch average depth below the crest of

the overflow riser. This compares with 960 square feet treatment area with no

allowance for storage.

Different ratios of treatment storage area to storage depth can be used. The maximum

allowable depth is 12 inches.


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