The land developer wishes to convert 25 acres of farmland in north York into a r

Question

The land developer wishes to convert 25 acres of farmland in north York into a residential neighbourhood. The ministry of the Environment requires as assessment of the impact of this development on runoff into a local river system. With the use of the Rational method, determine the peak flows for the pre-development and post-development conditions for the 25-year storm. The table for the runoff coefficients are provided. Surface = 35% roof, 55% asphalt, 10% parks in good conditions slope = 1% drained by circular 30-cm diameter pipes pipe flow velocity = 5 ft/s, total pipe length = 450 ft

Explanation / Answer

THIS WILL BE HELPFUL FOR YOU Rational Equation: Q=ciA The Rational equation requires the following units: Q = Peak discharge, cfs c = Rational method runoff coefficient i = Rainfall intensity, inch/hour A = Drainage area, acre Note that our calculation allows you to use a variety of units. The Rational method runoff coefficient (c) is a function of the soil type and drainage basin slope. A simplified table is shown below. See the references at the bottom of the page for more complete tables including impact of slope. The Rainfall intensity (i) is typically found from Intensity/Duration/Frequency curves for rainfall events in the geographical region of interest. The duration is usually equivalent to the time of concentration of the drainage area. The storm frequency is typically stated by local authorities depending on the impact of the development. A 10-yr, 25-yr, 50-yr, or even 100-yr storm frequency may be specified. Simplified Table of Rational Method Runoff Coefficients (see references below) Ground Cover Runoff Coefficient, c Lawns 0.05 - 0.35 Forest 0.05 - 0.25 Cultivated land 0.08-0.41 Meadow 0.1 - 0.5 Parks, cemeteries 0.1 - 0.25 Unimproved areas 0.1 - 0.3 Pasture 0.12 - 0.62 Residential areas 0.3 - 0.75 Business areas 0.5 - 0.95 Industrial areas 0.5 - 0.9 Asphalt streets 0.7 - 0.95 Brick streets 0.7 - 0.85 Roofs 0.75 - 0.95 Concrete streets 0.7 - 0.95

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