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storm_runoff.indd Page 1“Losing the Farm” by Edwin H. Price Losing the Farm: How Changes in Land Surface Aff ect Storm Runoff by Edwin H. Price Department of Physical and Life Sciences Nevada State College, Henderson, NV Part I – Leala’s Old Farm Leala and Jake raised their son, Jimmy, on Leala’s family farm in north Georgia. Jake built their modest home in the 1940s from recycled pieces of an old schoolhouse that were on the site. Summers in north Georgia are usually “green” because of high annual precipitation, about 53 inches. No one plowed the fi elds for several years and, through ecological succession, the fi elds grew into pine forest. Leala’s 30-acre property became fi lled with mixed hardwood trees and young pine forest. All soil was covered with vegetation and forest litter. Th e long driveway was through native loblolly pines and large oak trees. Th e old farm’s soil is typical for this area and is locally called “Georgia red clay,” a fairly poor-quality silty clay. After a rain, the soil is “sticky,” and low spots can hold puddles for days. Th e former farm is actually a small watershed. Th e west side (pictured on the left side of the aerial image on next page) is a ridge crest, and the land slopes northeastward (to the right) to the lowest spot on the property, below the house. In this northeast corner, all surface drainage exits the property through a single pipe under the railroad and highway, into a natural creek east of the highway. Questions 1. What characteristics of the farm may factor into how much water is retained or will run off during a rain/storm event? 2. Rainfall runoff is water that doesn’t soak into the ground or that gets trapped on plants or in surface puddles. It runs downhill off the land into streams. In the Southeast, one inch of rainfall or more during a day is common. What diff erences in runoff would you expect between one inch of rain that falls as light rain over a 24-hour period at the farm compared with a one-inch rainfall that occurs during a one-hour storm? NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE Page 2“Losing the Farm” by Edwin H. Price Th e U.S. Soil Conservation Service (SCS), now named the U.S. Natural Resources Conservation Service (NRCS), developed a widely-used method of determining the volume of storm runoff from diff erent land surfaces. Th is calculation method is detailed in the Addendum to this case study and is used in Question 3 below. 3. How much water, in cubic feet, would a pond in the northeast corner of the farm property have to hold below its spill point to catch all the runoff from a one-hour storm? Use the U.S. Soil Conservation Service (SCS) method for calculating storm runoff volume to determine the pond size. Assumption: average antecedent soil moisture condition (not wet, not dry). NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE Page 3“Losing the Farm” by Edwin H. Price Part II – If You Build It, They Will Come Jimmy inherited the old farm in 2006. Like many sons and daughters who have grown up in rural areas and then graduated from college, Jimmy found he had to live in a metropolitan area in order to pursue his career. Jimmy and his wife now lived in Atlanta and had no need for his old home place. Th ey decided that it was in their best interests to sell the property. Who do you think would buy such a place? A residential developer! “Mom would be proud that her dad’s old farm would help with our kids’ college tuitions,” Jimmy told a long-time neighbor. Th e developer had county-approved plans to pack the acreage with 128 small “aff ordable” single-family homes on 1/8 acre lots with small common areas, a project called Legacy Cove. Questions 1. After Legacy Cove is completed with nice green lawns, what characteristics of the landscape, with respect to runoff , will be diff erent from that of the original farm? Do you predict that the runoff from a storm event will be greater from the residential development or the original farm? How will the residential development infl uence the curve number as compared with that of the farm? 2. After the subdivision is built with houses, streets, and lawns, how would the volume of runoff from a similar 1-inch rain event compare with that which you calculated for the farm? What might be some consequences of this change downstream from the development? 3. What is a practical way to mitigate this change in runoff expected when Legacy Cove is completed? NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE Page 4“Losing the Farm” by Edwin H. Price Part III – Legacy Cove Legacy Cove started in early 2007. How do you make a subdivision out of an old 30-acre farm? Completely clear all 30 acres and remove all vegetation. Th e larger trees were cut and sold for lumber while the smaller pines and shrubs were scraped into piles and burned. “When we’re done, it’ll look nice,” Alvin, the dozer operator, said to a concerned neighbor. Th ere was no place in Legacy Cove for the house that Jake had built out of the old schoolhouse. It had to go. Th e de- veloper gave the home to the local fi re department to burn as a fi re rescue training exercise. Th e site of cheerful holidays came down quickly with billowing gray smoke. None of the family was there to watch; only disappointed friends. By the end of 2007, all 30 acres lay naked on the side of a ridge. Red and brown dirt had been skillfully shaped into parcels where new homes and streets would be. As seen in a Google Earth image, the once green spot is now totally barren. After the land was cleared and reshaped at the end of 2007, the developer never came back. Th e farm with its soil, forest, and fi eld ecosystems had been destroyed for an expected profi t and then abandoned. No homes, no streets, no lawns would be built. Th e legacy of Legacy Cove had changed. Alvin fi nally removed his equipment when it became obvious that he was not going to be paid by the developer. NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE Page 5“Losing the Farm” by Edwin H. Price As required by construction practices, Alvin had originally built a storm water catch basin in the northeast corner of the development to catch storm runoff from future lawns, roofs, and street pavement. Th is was the location of the hypothetical pond in Question 3 in Part I. A concrete drainage system had been built into the catch basin to allow storm water to drain out of the basin gradually. It was important not to overwhelm the natural creek into which the farm had drained. Questions 1. What do you predict is going to happen to the 30 acres of cleared dirt that has now been abandoned? 2. Would you predict that runoff from a one-inch storm event would be greater from the bare dirt or from the completed residential development? 3. How much runoff from a one-inch storm event will come from the abandoned dirt site? 4. How does your calculation for runoff from bare soil compare with the amount you calculated for Legacy Cove, which would have had grass lawns? Does that diff erence surprise you? Why is there such a diff erence? NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE Page 6“Losing the Farm” by Edwin H. Price Part IV – The Legacy of Legacy Cove During 2008 and through much of 2009, the abandoned site suff ered severe erosion. Much of the site lost soil. Gullies up to four-feet deep developed in the steeper slopes where the street beds had been. Th e catch basin totally fi lled with sediment, and its discharge plumbing system became overwhelmed and non- functional, totally clogged with sediment. Without any discharge from the basin, as little as 1 inch of rainfall could cause severe runoff and fi ll the catch basin to capacity. And then came the night of September 20, 2009. An extraordinary rainfall event occurred in northwest Georgia. Over 9.1 inches of rain fell. Th e U.S Geological Survey called it a fl ood of “epic” proportions. An 88-year-old neighbor said “I don’t know if it was the hundred-year-fl ood, but I know that it was at least the eighty- eight-year fl ood. I’ve never seen anything like that here in my lifetime.” Th e 9.1 inches of rain that collected on Legacy Cove’s bare dirt washed away the dysfunctional catch basin’s dam and then undermined the adjacent railroad bed. Even the highway was fl ooded and had to be closed for hours. Luckily, no one was injured. Questions 1. How does the volume of runoff from 9.1 inches of rain compare with your calculation of runoff captured by a 1-in rain on the bare dirt? 2. Calculations for these soil types and soil land uses have been made assuming average, not wet nor dry, antecedent soil moisture. Th e extraordinary rainfall event actually fell on saturated soils because it had rained the preceding afternoon. How would the curve number of a saturated soil compare with the curve number of the same soil that is not saturated and of average moisture? How would the runoff calculation result be diff erent from the one we have calculated above? NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE Page 7“Losing the Farm” by Edwin H. Price Part V – Stabilization By the end of 2010, the developer’s bank had foreclosed and taken possession of the property. Liabilities associated with uncontrolled erosion and sedimentation downstream became the bank’s responsibility. Th e Georgia Division of Environmental Protection (EPD) had been pursuing legal action against the developer for not maintaining the catch basin and erosion control structures. In 2011, fi ve years after the land was cleared, the EPD required the new owner, the bank, to take immediate action to stabilize the entire 30-acre site. Th e bank hired engineering and environmental consultants to rapidly stabilize the site so they could satisfy the EPD’s requirements and put the property on the market