The Sustainable Sites InitiativeM-Station

Hydrology Credits 3.5 and 3.6

Prabhas Ranjan Gupta

Yvonne W. Xie

INTENT

Restore and maintain hydrological function

METHODOLOGY

Calculate reduction in site Curve Number (CN):

Pre- to Post-development condition

What is a curve number?

Composite Curve Number CalculationExisting conditions: M-Station

Land Use Category CN

Area covered by land use category

CN x AreaIn acres

As percentage of

total area

Impervious cover 98 3.88 45.87 380.24

Lawn in good condition 74 0.00 0.00 0.00

Woods 73 1.27 15.01 92.71

Water 98 0.42 4.96 41.16

Open space 79 2.89 34.16 228.31

Totals 8.46 100 742.42

Composite CN = 742.42 / 8.46 = 87.76

CREDIT REQUIREMENTS

• Attempt to achieve a target curve number

• Different for greenfields, greyfields and brownfields, and for different climatic regions

• SWMM modeling study

• Reduction in curve number decides points and improvement in water storage capacity (or reduction in runoff)

BMPs at M-Station to reduce runoff

• Rainwater Cistern

Stores water from roofs for re-irrigation

Can capture first 1.2 inches of rainfall

• Pervious Pavements

60% of sidewalks made of porous concrete

Depth: 4 inches Porosity: 0.30

Final/Post-Development Curve Number

• BMPs not accounted for by conventional land use classes

• BMPs have to be factored in: CN values taken from the Green Values Calculator

Final CN: Green Values Calculator

Land Use Category CN

Area covered by land use category

CN X AreaIn acres

As percentage of

total area

Impervious cover 98 2.02 23.88 197.96

Roof served by cistern 98 1.78 21.04 174.44

Pervious pavement 40 0.49 5.79 19.60

Lawn in good condition 74 1.70 20.09 125.80

Woods 73 1.27 15.01 92.71

Water 98 0.42 4.96 41.16

Open space 79 0.78 9.22 61.62

Totals 8.46 100 713.29

Composite CN = 713.29 / 8.46 = 84.31 (no points!)

Finding a CN for the roofs

Final CN: Another Estimate

Land Use Category CN

Area covered by land use category

CN X AreaIn acres

As percentage of

total area

Impervious cover 98 2.02 23.88 197.96

Roof served by cistern 75 1.78 21.04 133.5

Pervious pavement 40 0.49 5.79 19.60

Lawn in good condition 74 1.70 20.09 125.80

Woods 73 1.27 15.01 92.71

Water 98 0.42 4.96 41.16

Open space 79 0.78 9.22 61.62

Totals 8.46 100 672.35

Composite CN = 672.35 / 8.46 = 79.47 (7 points!)

What does the CN look like without the BMPs ?

Land Use Category CN

Area covered by land use category

CN X AreaIn acres

As percentage of

total area

Impervious cover 98 4.29 50.71 420.42

Lawn in good

condition74 1.70 20.10 125.80

Woods 73 1.27 15.01 92.71

Water 98 0.42 4.96 41.16

Open space 79 0.78 9.22 61.62

Totals 8.46 100 741.71

Composite CN = 741.71 / 8.459 = 87.67

Assessment and Suggestions

• Curve Number is good indicator of overall site

hydrology

• Documenting 3.5 should be Civil Engineer’s job

• Provide method to classify project site

• Determination of CN quite open to judgment

call: reviewing should also be easy!

• Considerable scope for confusion in wording

Intent

Protect the quality of receiving waters

Methodology

Calculate percentage of volume of stormwater captured and treated

Requirements Effluent should have less than 25mg/l TSS

Treat 80/90/95 percent of average annual volume of runoff

• Common stormwater pollutants– Landscape chemicals: pesticides, fertilizers,

herbicides, oil, grease– Metals: copper, zinc, lead– Nutrients: nitrogen, phosphorus– Pathogens: bacteria, viruses, protozoa– Regional pollutants: salts, alcohol, temperature– Solids: soil, tire particles, road abrasion material, etc

• Specific pollutant of concern– Refer to the Clean Water Act Section 303(d) impairment

lists

Stormwater Pollutant

BMPs

• Biofiltration System

Capture Capacity: 64,522 gallons

• Cistern and Re-irrigation

Capture Capacity: 56,439 gallons

All captured water in the cistern will be utilized for re-irrigation

How much of runoff is captured?

• Combined cistern and biofiltration capture volume: 120,961 gallons (16,170 cubic ft.)

• Area of impervious cover served by BMP’s: 165,528 sq. ft.

• Hence, depth of capture: 1.17 inches

• Based on a 21-year database of daily rainfall, this corresponds to 76.7% of runoff volume or 81% of runoff volume (if extreme storms are excluded)

Assessment and Suggestions

Assessment• Strictly on a volume basis, M Station will earn 0 point in

Credit 3.6.

Suggestions to SITES• Consider to evaluate the percentage of pollutant removal in

a stormwater treatment system as a second parameter as to ensure the effectiveness of the controls.– M Station captures: 1.17” / rainfall event– A typical rainfall event in Austin: 1/3”– 13 rainfall events exceeded a two year storm (3.44” in 24 hours)– The performance of treatment system various (soil, vegetation

selection)

Assessment and Suggestions

Suggestions to M Station

• Address vegetation selections (especially plants in biofiltration ponds) in regard of specific stormwater pollutants removal.

• Stormwater treated by biofiltration should be reused on site instead of going directly to municipal swage. (Also aims to decraese CN targeted in Credit 3.5)

• Apply further BMPs in capturing and reusing the bypassed stormwater on the site to reduce portable water consumption as well as ensure ongoing pollutant removal.