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Ideally, hydrologic studies to determine volume and rates of runoff should be based on long-term stationary streamflow records for the area being investigated. Such records are seldom available for small drainage areas, and because of the time involved in converting a watershed from rural to urban conditions, available records normally are not adequate. It becomes neces- sary to estimate the magnitude and frequency of peak rates of runoff through modeling of measurable watershed characteristics. An understanding of these characteristics is required for judging how to alter parameters to reflect changing watershed conditions. Urbanization of a watershed changes its response to precipitation. The most common effects are reduced infiltration and decreased travel time, which result in significantly higher peak rates of runoff. The volume of runoffis determined primarily by'the amount ofprecipi- !ation and by infiltration characteristics related to soil type, antecedent rainfall, type of vegetal cover, imper- vious surfaces, and surface retention. Travel time is determined primarily by slope, flow length, depth of flow, and roughness of flow surfaces. Peak rates of discharge are based on the relationship of the above parameters as well as the total drainage area of the watershed, the location of the development in relation to the total drainage area, and the effect of any flood control works or other man-made storage. Peak rates of discharge are also influenced by the distribution of rain- fall within a given storm event. PLANNING CONSIDERATIONS A. Select a Suitable Site I. A void construction on areas subject to flooding. 2. Consider erosion hazards and potential land slides, especially on steep slopes. 3. Wet, shallow, and unstable soils are not desirable building sites. Recognize and overcome these soil limitations or develop these areas for parks, wild- life preserves, or other such uses. 4. Developers should be encouraged to contact the local Soil Conservation District regarding soil limitations for the intended land uses and for as- sistance in preparing conservation plans for de- veloping areas. B. Keep Site Modification to a Minimum I. A void heavy grading that would alter drainage patterns or create very steep slopes. 2. Use diversions to direct water away from erosive areas to points for safe disposal. 3. Expose as small an area of soil for as short a time as possible. 4. Stockpile topsoil removed by grading and redis- tribute over any disturbed areas. 5. Use culverts when crossing streams with con- struction equipment. 6. Protect bare areas with temporary vegetation or mulches during construction. 7. Save and protect as much of the desirable native vegetation as possible. Mark or erect barriers to prevent damage from construction equipment. C. Minimize Pollution of Air and Water I. Use debris basins to trap trash and sediment, preventing these materials from moving off-site. 2. Keep dust levels low by sprinkling or mulching as necessary. 3. Place construction facilities and store equipment away from drainageways. D. Protect Area After Construction I. Establish and maintain needed structural conser- vation practices. 2. Establish lawns or other suitable vegetation on all disturbed areas. 3. Maintain vegetated areas using proper horticul- tural practices. 7