|Friends of Alewife Reservation (FAR) Join Email List||
Belmont Uplands Project - Project Impact Calculations
CJK M E M O R A N D U M
On November 25, 2003, I made a presentation to the Belmont Planning Board regarding the functions and values of the Belmont Uplands project site and the potential impacts of the office/R&D building proposed. That presentation included, among other data, a preliminary estimate of the annual increase in stormwater runoff volume attributable to the project (8,550 m3/yr) and preliminary estimates of the amount of three (3) heavy metal pollutants to be added to the project's stormwater drainage system annually (3.3 lbs. zinc, 3.38 lbs. lead, 0.88 lbs. copper). The purpose of this memorandum is to provide the rationale, calculations, and assumptions under which these preliminary estimates were developed for that presentation.
Increase in Stormwater Runoff Volume
It is widely recognized that increases in impervious surfaces within a watershed result in increases in both the peak rate and total volume of rainfall runoff (Leopold, 1968). Commonly applied engineering methodologies, such as The Rational Method, NRCS TR-55 and TR-20 methods, and others, are available to compute runoff quantities on an event-by-event basis but are not regularly used to model changes in annual runoff.
In order to increase recognition that the project represented not only a simply change in land cover, but specifically the loss of forested land cover, I first calculated the potential evapotranspiration of forest land cover, at this latitude, using the Thornthwaite Method (Dunne & Leopold, 1978).
Calculations according to the above equation result in a potential evapotranspiration value of
The area of existing forest to be converted to impervious surface under the proposed office R&D building scenario is 4.4 acres (Epsilon, 2003, pg. 3-1). Area unit conversions, at 2.471 acres = 1 hectare = 10,000 square meters, provide that the 4.4-acre forest area onsite to be converted to pavement is 17, 806 sq.m. or 178,060,000 sq.cm.
Multiplying the annual potential evapotranspiration (48 cm/yr) by the area of the forest affected (178,060,000 sq.cm) yields an estimated annual evapotranspiration volume of 8,546,880,000 cubic centimeters, or 8,546.88 cubic meters per year.
For the purpose of this simple estimate, I have assumed that the impervious cover to be created in the 4.4-acre area currently forested will provide no evapotranspiration (parking lot landscaping has been ignored) and that the entire volume estimated as forest evapotranspiration to be lost will be conveyed through various stormwater drainage systems, to the receiving water wetlands. This estimate of increased volume discharge due to loss of forested land cover, calculated above as 8,546.88 cubic meters per year, was reported to the Belmont Planning Board as 8,550 cubic meters per year or, by unit conversion at 264.5 gallon per cubic meter, 2.26 million gallons per year.
To simplify mass transfer calculations, I have assumed that all of the 8,546.88 cubic meters of water from forest evapotranspiration is reallocated as surface runoff from project impervious surfaces and is conveyed to the project stormwater drainage system.
Stormwater runoff from urbanized impervious surfaces is widely recognized to contain a variety of contaminants, including heavy metals (US EPA, 1983). Schueler (1987) reports heavy metal concentrations in stormwater runoff, in milligrams per liter, for a variety of urban and urbanizing areas. The concentrations reported as study averages for the Nationwide Urban Runoff Program for zinc, lead, and copper are as follows:
Zn = 0.176 mg/l
Simple volume conversion, at 1 cubic meter = 1,000 liters, provides that the 8,546.88 cubic meters of water from forest evapotranspiration, now reallocated as runoff from impervious surfaces, will contain 8,546,880 liters of urban runoff. Multiplying by the appropriate concentrations of heavy metals reported from urban stormwater results in the following estimates of heavy metals delivered to the project stormwater drainage systems.
Zinc - 8,546,880 liters/yr X 0.176 mg/l ÷ 106 mg/kg = 1.5 kg/yr or 3.3 lbs./yr
These estimated values for increased pollutant loading to the project's stormwater drainage system were reported to the Belmont Planning Board as calculated above.
Dunne, T., and L.B.Leopold; 1978. Ch. 5, "Water Use By Vegetation", in Water in Environmental Planning. W.H. Freeman and Co., New York.