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<br />I <br /> <br />I <br /> <br />I <br /> <br />VIII. ALTERNATIVE IMPROVEMENT PLANS <br /> <br /> <br />Alternate improvement plans and the conceptual design to implement the <br /> <br /> <br />plans are presented. First, the general considerations used to formulate and <br /> <br /> <br />design the alternatives are considered. Conceptual improvement plans for the <br /> <br /> <br />drainageway were formulated to alleviate flood damages in the Fossil Creek <br /> <br /> <br />drainage basin. In the formulation of alternative improvement plans the <br /> <br />following factors have been considered. <br /> <br /> <br />1 . Long term relief of flood problems. <br /> <br /> <br />2. Adequacy of natural detention and conveyance. <br /> <br /> <br />3. Increase in storm water flows due to development upstream. <br /> <br />4. Available right of way. <br /> <br /> <br />5. Ease of maintenance and access. <br /> <br /> <br />6. Water quality improvements. <br /> <br />7. Erosion control. <br /> <br /> <br />8. Sedimentation. <br /> <br /> <br />9. Embankment stability. <br /> <br /> <br />10. Existing water rights. <br /> <br /> <br />The design criteria utilized to formulate the alternative improvement <br /> <br /> <br />plans are those contained in the City of Fort Collins "Drainage Criteria <br /> <br /> <br />Manual," (ref. 3) and additional criteria for detention storage as recommended <br /> <br /> <br />by the technical committee. Specific considerations for the Fossil Creek <br /> <br />improvements are summarized below. <br /> <br />3. Detention Storage: In many cases, detention of storm water is a very <br /> <br /> <br />practical method of reducing peak flows. Planning of detention ponds should <br /> <br /> <br />consider use of detention ponds as parks or other recreational facilities. <br /> <br /> <br />Storm water detention facilities in general should be capable of storing the <br /> <br /> <br />100-year runoff for developed conditions in excess of a desired release to <br /> <br />reduce downstream flows to meet the proposed or existing channel capacity. <br /> <br /> <br />Natural storage behind road embankments should be designed to avoid over- <br /> <br /> <br />topping during the 100-year runoff for developed conditions. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />4. Irrigation Ditches: City of Fort Collins drainage policy states that <br /> <br /> <br />discharge into canals should be avoided. Therefore, all irrigation ditch <br /> <br /> <br />crossings should be capable of passing the lOa-year flood without combining <br /> <br /> <br />irrigation and storm water, if possible. Special considerations should be <br /> <br />given to irrigation ditches which have water rights on tributaries. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />5. Access to Improvements: A maintenance access road should be incorporated <br /> <br /> <br />into all master plan programs in order to allow the implementation of the <br /> <br />maintenance program. <br /> <br />I <br /> <br />I <br /> <br />1. Open Channels: Two types of open channels are commonly used - natural <br /> <br />and constructed. Constructed channels may be lined or unlined. Unlined open <br /> <br /> <br />channels with vegetative cover are preferred due to lower velocities and their <br /> <br /> <br />natural appearance. In addition, the shape of unlined channels can be chosen <br /> <br /> <br />to create additional sociological benefits by establishing greenbelts or <br /> <br /> <br />recreational opportunities. <br /> <br /> <br />Lined open channels are sometimes required due to limited rights of way <br /> <br /> <br />and topographical or hydraulic considerations. <br /> <br />6. Sedimentation and Erosion: Planning of drainage improvements should con- <br /> <br /> <br />sider sedimentation and erosion processes. The sediment supply changes during <br /> <br />urbanization. In general the sediment supply decreases following urbaniza- <br /> <br /> <br />tion, but may be increased during development. If the sediment supply exceeds <br /> <br /> <br />the capacity of the drainage system, deposition will result, which may reduce <br /> <br /> <br />the capacity of the system. When the sediment supply is less than the <br /> <br /> <br />transport capacity, channel erosion may occur, resulting in lateral migration, <br /> <br /> <br />headcutting, or bank failure. Hydraulic structures often change flow charac- <br /> <br />teristics, resulting in scour or deposition. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />2. Hydraulic Structures; Flow characteristics may be controlled with the <br /> <br /> <br />proper use of hydraulic structures. Hydraulic structures such as energy <br /> <br /> <br />dissipators, grade controls, bridges, culverts and flumes should be incor- <br /> <br />porated in the alternative development plans. <br /> <br />7. Nonstructural Measures: Nonstructural measures are most suitable for <br /> <br /> <br />undeveloped areas where encroachment on the flood plains has not yet occurred. <br /> <br />Nonstructural measures include flood plain zoning, subdivision regulations <br /> <br /> <br />and flood insurance. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />55 <br />