What is the Impervious Area Curve Number?
The curve number (CN) of the impervious surface of a residential watershed in east-central Florida was determined by calibration against observed data. Evidently the curve number of the impervious surface was closer to 94 or 95 than 98 which is the value typically used. It is important to know if CN is 95 or 98 because a 1 -inch (25 mm) rainfall will produce approximately 0.79 in (20 mm) of runoff if CN = 98 and 0.56 in (14 mm) of runoff if CN = 95 yielding a rather large difference 28.8%. These percentage differences, which are even larger for smaller storm events, can become very significant if the curve number method is being used to design systems to accommodate the "first flush" from impervious surfaces. The first flush is the runoff generated during the rising limb of the storm events. It is also important to know the CN value if the SCS method is being used to estimate annual runoff or annual pollutant loads, since the bulk of the annual rainfall, and consequently pollutant loading is due to small (less than I inch or 25 mm) rainfall events. Observations supporting CN = 98 are scarce. This study indicates a need for further field investigations and more data, in all parts of the country, to determine better curve number values for impervious surfaces.
The objectives of this chapter are to:
1). discuss two calibration methods for estimating curve numbers, and
2). present results that appear to indicate that the curve number for impervious surfaces, under certain climates, may be lower than 98.
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