NOTES Module 14 Sustainable Design In this module, you learn about the hydrologic cycle, about stormwater runoff issues and why sustainable design techniques are vital to a civil engineer involved in land development. You learn how to use AutoCAD Civil 3D to apply sustainable design techniques. Objectives After completing this module, you will be able to: Describe the hydrologic cycle components. Explain surface water runoff volume and flow rate. Discuss the impact of land cover change on surface water runoff. Describe the traditional design emphasis on detaining stormwater volume to limit downstream damage. Discuss sustainable design strategies and structural control techniques.AutoCAD Civil 3D 2009 Education Curriculum NOTES 14-2 Notes to Instructor Data for this module resides in the \AutoCAD Civil 3D 2009 Education Curriculum\Module 14 – Sustainable Design\ folder. Student Exercises The following exercises are provided in step-by-step format. Open the AutoCAD Civil 3D program prior to beginning the lesson by double-clicking the Civil 3D icon on your desktop. The drawing data for this module is independent of units. You will find the required drawings in both the\Imperial and \Metric folders. The exercises in this module are as follows: 1. Visualize Runoff Flow Patterns 2. Inspect Roadway Drainage 3. Create a Rain GardenModule 14 - Sustainable Design NOTES 14-3 Sustainable Design Sustainable design is the practice of designing physical objects to comply with the principles of environmental, economical, social, and ecological sustainability. In the field of Civil Engineering and Land Development, the term sustainable design refers to applying new methods to: Reduce the use of nonrenewable resources and minimize environmental impact. Reduce the quantity of stormwater runoff. Improve the quality of stormwater runoff. Reduce erosion of soil. Infiltrate more stormwater to recharge groundwater aquifers. Through the process of designing improvements to a parcel of land, a knowledgeable engineer can minimize the impact of changing the land cover by using sustainable design techniques. In fact, depending on the predevelopment land use, designing and constructing with sustainable techniques can frequently improve the surface water and groundwater conditions at the site. The Hydrologic Cycle The hydrologic cycle is the ongoing process in which water is evaporated from oceans, lakes, streams, and rivers and then redistributed to the surface of the earth in the form of precipitation. When precipitation, such as rain or snow, falls on the land surface, it encounters a number of different fates. A portion of the precipitation returns to the atmosphere as water vapor, or evaporates. Some of the water vapor is consumed by trees and other plant matter in a process called transpiration. Some precipitation infiltrates into the earth's soil and may be stored there as groundwater or may continue flowing through the soil until it reaches a stream or river. Finally, some precipitation becomes surface runoff and enters our streams, rivers, and lakes. Civil and environmental engineers study the behavior of surface runoff to learn more about floods, droughts, and water pollution. Humans can greatly impact surface runoff patterns by altering the way land is used. When studying surface runoff, the basic hydrologic unit is the watershed. A watershed is the total area of land that contributes surface runoff to a particular point of interest. Surface Runoff Calculations Surface runoff is usually measured in one of two ways. The first measure of surface runoff is runoff volume. To understand this method of measuringAutoCAD Civil 3D 2009 Education Curriculum NOTES 14-4 surface runoff, envision a circular swimming pool that is 4 feet deep and has a radius of 10 feet. The total volume of water held in the pool is computed as: The second measure of surface runoff is volumetric flow rate. The volumetric flow rate describes how much water flows past a certain point within a given period of time. Usually, civil engineers abbreviate volumetric flow rate as Q and express it in terms of cubic feet per second (cfs). To understand this method of measurement, consider the previously described swimming pool. If it takes 2 minutes for the pool to completely drain, the volumetric flow rate is as follows: Therefore, we can say that the volumetric flow rate of the surface runoff near the pool's valve is 10.5 cfs. Impact of Land Cover Civil engineers use many different techniques to model surface runoff and determine the volume and/or volumetric flow rate of the runoff. Many of these techniques determine surface runoff as a function of a runoff coefficient. Typically, the runoff coefficient is assumed to be the ratio of runoff volume to precipitation volume: The runoff coefficient is determined by examining the land cover characteristics of a watershed. Land cover that does not allow the infiltration of precipitation is called impervious, while pervious land does allow water to migrate through it. For example, an asphalt parking lot does not allow rain to infiltrate into the soil below the parking lot. Therefore, when a watershed is covered by asphalt, it is commonly called impervious. On the other hand, a grass lawn may allow a significant portion of precipitation to infiltrate into the earth's soil. Grass and other types of vegetative land cover are often called pervious. The conversion of land cover from pervious to impervious can create serious runoff problems. An impervious land cover like asphalt may have a very high runoff coefficient, such as 0.9, while pervious land cover may have a very low runoff coefficient, such as 0.3.Module 14 - Sustainable Design NOTES 14-5 To see how land cover affects surface runoff, consider a rectangular watershed that is 100 feet long and 60 feet wide. If 1 inch of rain falls on this watershed, determine the volume of runoff if the watershed is covered by asphalt (runoff coefficient of 0.9) or if the watershed is covered by grass (runoff coefficient 0f 0.3). The volume of runoff is computed by multiplying the total volume of precipitation by the runoff coefficient. Step 1. Determine the total volume of precipitation landing on each watershed: Step 2. Determine the volume of runoff for each land cover condition: 1. Asphalt has a runoff coefficient of 0.9; therefore, the volume of
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