You have been assigned the task to complete the preliminary design calculations

Question

You have been assigned the task to complete the preliminary design calculations for the sanitary sewer and drainage system for a new subdivision located on Fishinger Road within the City of Columbus. The property in question is east of the Scioto River and is 2,800 feet wide (west to east) and 1,500 feet deep (north to south). The property is fairly flat (3% average slope) and the soils are predominately tight clay (Class D) soil. The parcel has never been developed and is densely wooded (C=0.40).

a) Calculate the composite runoff coefficient if 65 percent of the area will be used for residential units (C=0.50); 15 percent Green space and parks (C=0.15); 15 percent street and parking areas (C=0.95); and the remaining 5 percent will remain wooded.

b) Determine the return interval for the critical storm.

Explanation / Answer

For question a

Runoff coeffecient for wooded = 0.4

Runoff coeffecient for residential = 0.5

Runoff coeffecient for green area and parks = 0.15

Runoff coeffecient for parking and street = 0.95

Composite runoff coeffecient = 0.65* 0.5 + 0.15*0.15 + 0.15 * 0.95 + 0.05* 0.4 = 0.51

Thus due to development the runoff coeffecient increase= 0.51-0.4 =0.11

Thus percentage increase in runoff = 0.11*100/ 0.4 = 27.5 %

Thus for question B as per critical storm method the return interval of critical storm will be 5 years.

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