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<br />, <br /> <br />) <br /> <br />STEP FOUR: ESTABLISH NEW ELEVATION-FREQUENCY RELATIONSHIPS <br /> <br />, <br /> <br />J <br /> <br />Land use changes may cause major alterations in drainage <br /> <br />characteristics, particularly surface runoff. Hydrologic changes should <br /> <br /> <br />be projected up to the first 50 years of the project life, and will <br /> <br /> <br />primarily be based on land use changes. It is important that hydrologic <br /> <br /> <br />change be noted by time interval to reflect changes in the degree of <br /> <br /> <br />protection over time. <br /> <br /> <br />Hydrologic changes are critical when determining the level of <br /> <br /> <br />protection afforded by any particular measure. Consequently, conditions <br /> <br /> <br />should not be presented as averages, but rather shown as incremental <br /> <br />changes that are staggered over the period of analysis. <br /> <br />A. Calculate Land Use Chan~es Effect on Runoff <br /> <br />The rooftops, streets, and parking areas that_come with urbanization <br /> <br />can greatly reduce the amount of water that infiltrates into the ground. <br /> <br />The additional runoff can be reduced by retention and diversion schemes. <br /> <br />Otherwise, the increased runoff from urbanization will increase discharge <br /> <br />at points downstream of the development. <br /> <br />B. Identifv Other Physical Chan~es <br />Changes in the conveyance system that carries stormwater runoff can <br /> <br />also affect elevation-frequency relationships. Cleared and otherwise <br /> <br />smooth channels convey water more quickly and sustain fewer runoff losses <br /> <br />than channels with vegetation and other obstructions. Storm sewers also <br /> <br />cause more rapid conveyance, unless temporary storage is provided. <br /> <br />) <br /> <br />VI-ll <br />