FLOOD08354 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I Alternative 2A was selected for further development during Phase B. Alter- native 2A recommendations included: (1) Establish a system of channels through- out the basin that have a 10-year historic runoff capacity; (2) diversion of all surface drainage flows to the new drainageway system; (3) sizing of culverts at all major drainageway and arterial roadway crossings to 100-year capacities and at local and collector roadway crossings to 10-year design capacities; (4) the utilization of existing drainage facilities wherever possible; (5) enforcement of a bas i nwi de detent i on po 1 icy whereby future development is requi red to 1 i mit the 2-, 10- and 100-year runoff peaks to historic (undeveloped) levels. 7. In developing needed channel cross-sections and longitudinal slopes, utilize the following limits: a) maximum depth 5 feet; b) maximum 100-year velocity 7.0 fps; c) side slopes of 4:1 or flatter. 8. New development in the basins provide on-site erosion control during construction and provide on-site detention control for the 2-, 10- and 100-year fl ows when the site is developed. 9. Work with the planning departments of Lafayette and Boulder County to develop a hydrologic land use in the Phase B report acceptable to both. Recommended Alternative During the review process of the Phase A study, the input of officials from the City of Lafayette, Boulder County and the Urban Drainage and Flood Control District was used to select a recommended alternative. Based on this review Basinwide Major Drainageway Plan Improvements The selected alternate drainageway plan utilizes 4 to 1 side sloped, grass-lined channels having concrete trickle channel inverts for all of the major drainage basins delineated during the development of Alternative 2A, All major drainage improvements are designed for future basin development conditions and assume that mandatory on-site detention requirements will be enforced to limit fl ows of The selected plan by the project sponsors included the following direction: the 2-, 10-and 100-year return frequencies to historic levels, The planned major basin improvements are briefly summarized below for each Major Drainage Sub-Basin shown in Figure 2. 1. Recommend in the plan the enforcement of the lOO-year floodplain regulations in the state designated floodplains. 2. Discuss clearly the need for regulating a residual 100-year floodplain that may remain after a plan to handle less than IOO-year flood is implemented, Point out that 100-year potential flood zones need to be controlled through the development process until modified by actual construction of drainageways, Planning or anticipating changes does not justify controlling anything less than the potential floodplain areas defined using existing ground conditions. 3. Encourage purchase of fl ood insurance for pro pert i es ident i fi ed as susceptible to flooding. 4, All storm runoff des i gn be in accordance wi th the "Storm Dra i nage Design and Technical Criteria." Major Sub-Basin 1: Construction of a 10-year capacity, 4:1 side sloped, grass-lined channel from Coal Creek to North 119th Street. The improvements include lOO-year capacity crossings under the Weld County Road, with 10-year capacity cul verts under all the other road crossi ngs and under the Chicago, Burlington and Quincy Railroad tracks, Major Sub-Basin 2: Construction of 10~year capacity, 4:1 grass-lined channel 6. Develop a major drainageway plan utilizing Alternatives 2 and 2A by identifying additional freeboard needs for adjacent structures when using the 2A concept, Provide for adequate maintenance access to all improvements. Any maintenance trail should have a minimum stabilized flat surface width of 10 feet. from Coal Creek to North lllth Street. The development of a park containing a trickle channel along the low point of the drainageway, from North 119th Street and North lllth Street has also been included, Other improvements include culverts with a 10-year capacity at Weld County Road and at the Chicago, Burlington and Quincy Railroad and 10-year capacity culverts at North 119th and 5. 4 5