Now consider how urbanization significantly increases flooding danger by increasing the runoff into streams, while decreasing the amount of water infiltrated into groundwater aquifers. Performing the following calculations will help illustrate this point.

1. Over the course of one summer storm, approximately 1 cm of precipitation soaks into the ground. An average house has a roof area measuring 12 m x 18 m and the precipitation that falls on the roof is diverted down the gutters and into a storm drain. Calculate the amount of water that is diverted from the ground into drains in m3. Note: Remember that 1 cm = 0.01 m. You must calculate the area (length × width) and then multiply by the depth of rainfall in meters. This will give the rain volume in m3. Now convert this amount into gallons knowing that 1 m3 = 264 gallons.

1cm/100=.01m of rain x 18m x 12m= 2.16 cubic meters x 264= 570 gallons of water diverted.

2. What if an entire subdivision of homes had 12 m x 18 m roofs as above? Calculate the volume of water that goes down the drain instead of infiltrating into the ground in both m3 and gallons if it rains to a depth of 1 cm on 20 homes in a new subdivision.

2.16 cubic meters x 20 = 43.2 cubic meters x 264 = 11,405 gallons

3. Now calculate the volume of a 1 cm rainfall over downtown Denver which has an approximate area of 401 km2. The conversion of km2 to m2 is 1 km2 = 1,000,000 m2. Calculate the amount of water in m3 and gallons.

.01 x (401 x 1,000,000) = 4,010,000 cubic meters x 264 = 1,058,640,000 gallons

4. In this case, runoff is defined as the amount of water that does not soak into the ground. Runoff promotes erosion when not controlled and intricate storm drains are required when building in an urban or suburban area to prevent erosion and damage to property and buildings. In an urban area, runoff accounts for about 90% of the precipitation that falls to the ground versus 20% in a rural area or an area of undeveloped land. How many gallons of water are going into the storm drains of Denver instead of infiltrating into the ground?

4,010,000 x .90 = 3,609,000 cubic meters x 264= 952,776,000 gallons

What are some ways we could reduce this problem, assuming that simply abolishing the city and all its dwellings isn’t realistic?

Answer rating: 100% (QA)

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