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increased property values, and by actual and real savings in damages and <br />safeguards to the public health and welfare. <br />In accordance with the accepted practices, a two -year "design storm" has <br />been selected as the maximum storm that can be justified in this, a residen- <br />tial area. A two -year storm for the Greeley area, as recorded in Fort <br />Collins rainfall records, is a rate of one inch per hour for one hour dura- <br />tion. <br />RUN -OFF FACTOR <br />The selection of a "run -off factor" varies directly with the type of <br />surface material within the drainage basin. Paved streets, walks, drives, <br />and roofs will absorb only a minimum of water, whereas plowed fields, lawns, <br />parks, and wooded areas will absorb most of a storm's waters, At present <br />only about 20% of the drainage basins are developed as urban land so the <br />current run -off factor is very small. Under the 100% ultimate residential <br />development that is anticipated for this area, the run -off factor will be <br />considerably increased. <br />To arrive at a reasonable "run -off factor ", a section of a newly <br />developed subdivision has been analyzed. Aerial photographic maps showing <br />the actual areas of roofs, paved streets, drives, etc., were used. From <br />these maps an actual ratio of paved and impervious areas to grassed and <br />wooded areas was developed. From this ratio and standard factors for the <br />various materials, an actual run -off factor of 0.40 impervious was calculated. <br />For computing maximum run -off and sizing all pipes, this factor of 0.40 has <br />been used for all individual areas. <br />- 12 - <br />