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You will use a model to calculate storm water runoff. The model is in an Excel spreadsheet. When you open the runoff model, you will see a blue box with smaller yellow and light blue boxes in it. The yellow box is for you to enter data. The model returns the runoff in the light blue box.
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There are four categories of data: Land Use, Soil Group, Hydrologic Condition, and Rainfall. By clicking on the value under each category you open a pull down menu. Practice using the model with values for the Shodor office.
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First we will calculate the runoff before the Shodor office was built. Shodor is located in Durham North Carolina. Durham is in the piedmont. This area was originally covered by an oak forest.
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Open the Water Garden spreadsheet. Clicking on the value under Land Use will open a pull down menu. Select Oak Aspen.
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The soil at Shodor is a clay loam mix. The soil is classified C. Select C from the pull down menu under Soil Group.
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The Hydrologic Condition deals with the condition of the land. Good hydrologic condition has leaves, branches and small trees and bushes on the forest floor. Poor hydrologic condition is grazed by cattle leaving short grass. The pictures of the Shodor lot show un-mowed grass. Select fair from the Hydrologic Condition pull down menu.
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A 25 year storm is 6.6 inches of rain in central North Carolina. Select 6.6 from the Rainfall pull down menu.
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What is the runoff?
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Use the pull down menu for Predevelopment runoff to enter this number.
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Part of the land was left undeveloped. Approximately .3 acres was developed.
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This .3 acre is now impervious to water. Change the Land Use to parking lots.
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What is the runoff now?
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Use the Developed runoff to enter this runoff.
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The light blue box with New Runoff tells you the additional runoff caused by the development. The area affected by development is .3 acres. Use the development pull down menu to enter .3.
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The light blue Rain Garden box tells you the area needed to retain this extra water. The model calculates the size of the rain garden by first calculating the volume of extra runoff water. This volume of water needs to be held until it soaks into the soil. The water needs to soak into the soil in less than 3 days or the plants in the rain garden will start to die. If there is standing water for more than 7 days mosquitoes may mature. The soil type tells the model how fast the water can soak in. If the rain garden soil is type A, in 3 days the rain garden can drain 24 inches of water. Type B soil can drain 17 inches of water. Type C soil can drain 7 inches. Type D soil can only drain 2 inches of water.
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How much area is needed for Shodor's rain garden?
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Shodor is located on .75 acres. Is there enough land to construct this rain garden on Shodor's property?
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Most of the year, a rain garden is dry. Most days the rain garden is used for recreation. The rain garden could double as a soccer field, nature trail, rose garden, etc. During a heavy rain the excess runoff is collected in the rain garden. If the runoff isn't collected, it could cause a flood down stream. If the water runs off, it doesn't soak in and replenish the water table. Rain gardens help protect your neighbors downstream and replenish the ground water for wells and springs.
