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.
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