Mechanicsville has obtained a grant to build a condominium complex for the retir

Question

Mechanicsville has obtained a grant to build a condominium complex for the retired community. The total land set aside is 74,010 m2. Of this area 15,831 m2 will be used for Swiss chalet condominiums with slate roofs. Public streets and drives will occupy 18,866m2. The remainder will be lawns. The area is considered flat and has clayey soil. Using the most conservative values for C, find the peak discharge from the development for a 2-year storm. Assume the overland flow distance is 350m and use the IDF curves provided to calculate your answer. ( 2. 200- 180 160 40 10-Year Storm -120- -5-Year Storm 2 100 2-Year Storm 60 40 20 1-Year Storm ¼ Year Storm ½ Year Storm FIGURE P- 10 20 30 40 50 60 70 80 90 100 IDF Curves fo Mechanicsvill Time of Duration (min)

Explanation / Answer

For lawns, runoff coefficient (c) = 0.35

For a 2-year storm, considering rainfall duration of 60 min, intensity of rainfall (i) = 30 mm/hr = 1.181 in/hr

Drainage area (A) = 74010 - 15831 - 18866 = 39313 m2 = 9.714 acre

Using Rational equation : Q = ciA

where Q is peak discharge (cfs)

c is rational method runoff coefficient

i is rainfall intensity (in/hr)

A is drainage area (acre)

So, peak discharge = ciA = 0.35 x 1.181 x 9.714 = 4.015 cfs

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