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FLOOD06965
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Last modified
1/25/2010 7:10:28 PM
Creation date
10/5/2006 2:39:59 AM
Metadata
Fields
Template:
Floodplain Documents
County
Adams
Community
Northglenn, Thornton
Stream Name
Grange Hall Creek
Basin
South Platte
Title
Major Drainageway Planning
Date
12/1/1977
Prepared For
Adams County
Prepared By
UDFCD
Contract/PO #
&&
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />I <br />I <br />I <br />I <br />. <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />2. In the channel, where the side slopes are 2: I or steeper (grass or earth banks), <br /> <br /> <br />a floodway narrower than the flood plain was not defined due to the potential <br /> <br /> <br />erosion hazards of filling a channel on such a steep slope. <br /> <br /> <br />3. A floodway equal to the flood plain width is defined for the reach between 1-25 <br /> <br /> <br />and the Railroad Spur on Grange Hall Creek. This is because future building <br /> <br /> <br />development is unlikely in the park areas owned by Northglenn. <br /> <br /> <br />A tabulation of the floodway widths, as well as the supporting data, is presented in <br /> <br /> <br />Table 111-5. The section numbers and the sheet numbers refer to the flood plain mapping <br /> <br /> <br />presented on the master plan drawings of this report. <br /> <br /> <br />F. Erosion <br /> <br /> <br />Ninety-five percent of the soils in the Grange Hall Creek basin range from loam to <br /> <br /> <br />clay, with the remaining five percent consisting of alluvial soils and some gravel deposits. <br /> <br /> <br />For the surface soil layer (0-9 inches) over 70% of the soils pass the No. 200 sieve. This <br /> <br /> <br />very fine-grained soil is susceptible to erosion, which can be witnessed by inspection of <br /> <br /> <br />the steep, highly eroded banks of Grange Hall Creek. This erosion potential presents a <br /> <br /> <br />problem from the standpoint of bank stabilization for channels or sedimentation from <br /> <br /> <br />construction and from the standpoint of water quality. <br /> <br /> <br />G. Groundwater Recharqe <br /> <br /> <br />Due to the loamy, clay type soils in the Grange Hall Creek basin, the soils have a <br /> <br /> <br />low permeability (0.06 to 2.0 in/hr range), but have a good water holding capacity (0.14 to <br /> <br /> <br />0.20 in/hr). Therefore, the groundwater recharge is minimal in the steep channel area. <br /> <br /> <br />The existing stock water ponds have very minimal effect on the groundwater recharge due <br /> <br /> <br />to the sealing effect of the fine-grained soils on the pond bottom area. The bedrock <br /> <br /> <br />geology of the basin as discussed in Chapter II does not favor significant amounts of deep <br /> <br /> <br />aquifer recharge. <br /> <br />CHAPTER IV <br />MAJOR DRAINAGEWAY IMPROVEMENTS <br /> <br />A. General <br /> <br /> <br />The description of the major drainageway improvements which was selected by the <br /> <br /> <br />participating agencies to reduce the flood damages was presented in the Summary Table in <br /> <br /> <br />the Synopsis. The design criteria used for the improvements and the detailed description <br /> <br /> <br />of the master plan drawings is presented in this chapter. <br /> <br /> <br />8. Design Criteria <br /> <br /> <br />The criteria used for the preliminary design of the improvementsNas obtained from <br /> <br /> <br />the Urban Drainage Criteria Manual and the requirements of Adams County and the cities <br /> <br /> <br />of Northglenn and Thornton. Specifically, the criteria for design is as follows: <br /> <br /> <br />I. Open Channels <br /> <br /> <br />Where natural channels of adequate capacity to pass the 100-year storm exist no <br /> <br /> <br />improvements were recommended. Where modifications of the channel section were <br /> <br /> <br />required the channel section which minimizes right-of-way acquisition and erosion <br /> <br /> <br />potential was recommended. All modified channel sections are to be lined with natural, <br /> <br /> <br />drought resistant grasses where the average channel velocities are less than approximately <br /> <br /> <br />7 fps and a concrete low flow or trickle channel is included. The exact velocity is <br /> <br /> <br />dependent upon soil conditions, type of grass, and flow frequency. Channel side slopes are <br /> <br /> <br />a maximum of 4: I for the grass-lined portion. Since open channels are highly susceptible <br /> <br /> <br />to erosion before a good stand of grass is achieved, all the seeded channel slopes should be <br /> <br /> <br />stabi lized by an erosion protection blanket. <br /> <br /> <br />Trickle flow is considered to be the runoff from activities such as snowmelt, lawn <br /> <br /> <br />watering, and short duration, low intensity summer storms. Trickle flows of 10 to 15 cfs, <br /> <br />12 <br />
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