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HYDRO28913
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HYDRO28913
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Last modified
8/24/2016 8:48:00 PM
Creation date
11/20/2007 10:04:41 PM
Metadata
Fields
Template:
DRMS Permit Index
Permit No
M1974015
IBM Index Class Name
Hydrology
Doc Date
12/17/1999
Doc Name
FLOODPLAN IMPACT ANALYSIS FOR LYONS PIT ROCKIN WP SAND & GRAVEL MINING & RECLAMATION PLAN
Media Type
D
Archive
No
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• The entire 4,000 cfs of flood flow in the south floodplain is considered to be spli~: from <br />the main channel west of North 51 °` Street, with no diversion occurring east of North 51 °` <br />St. The proposed "Reclamation Contours "map (see Figure 2) however, shows that the <br />open valley proposed to be created as part of the Phase II (East Plant} mining would <br />lower the topography across the currently mapped shallow flooding area east of ilorth <br />51°` St. This would effectively create a channel that would be expected to divert <br />additional and potentially significant flow from the main to the south floodplain. <br />This diversion could cause an increase in the south floodplain water surface elevations, <br />and therefore should be avoided. Measures to prevent this diversion from occurring are <br />discussed in Section VI (Recommended Mitigation Measures) of this report. <br />Due to the nature of the St. Vrain floodplain as discussed in Section III, consideration <br />should be given in the design of the reclamation plan with respect to the potential <br />realignment of the river during and after a flood event. Of particular concern wo~ild be <br />the potential for the river to be "captured" into the reclaimed open valley areas. ]f this <br />were to happen, the resulting scour and deposition of materials could create a situation <br />that would require a considerable post-flood restoration effort to restore the river to its <br />pre-flood alignment. Recommended measures based on the lJDFCD Technical Review <br />Guidelines for Gravel Mining Activities are discussed in Section VI of this report. <br />V. Off-Site Floodplaie Impacts <br />With implementation of the recommended mitigation measures, the proposed mining and <br />reclamation plan will not create or exacerbate any off-site flooding conditions. 7'he <br />additional floodwater storage volume provided in the reclaimed mining pits world <br />benefit downstream properties by providing attenuation of the 100-yeaz flood flows. This <br />attenuation would reduce the 100-year floodwater surface elevations downstream of the <br />project boundaries. <br />VI. Recommended Mitigation Measures <br />As discussed in Section II, the analysis and design of recommended flood mitigation <br />measures are based on the UDFCD Technical Review Guidelines for Gravel Miring <br />Activities (guidelines). The guidelines are directly applicable to sand and gravel mining <br />along the South Platte River, and may be modified on a case-by-case basis for a~~plication <br />at other locations to recognize differences in hydrology and site conditions. <br />The St. Vrain Creek 100-year flood flow of almost 10,000 cfs at the proposed n.ining site <br />is considerably less than the estimated 100-year flood flows of the South Platte :[liver <br />downstream of Denver. In addition, 100-yeaz floods on the St. Vrain are of mu<;h shorter <br />duration than those of the South Platte. Due to its close proximity to the foothills, the <br />materials found in the bed, banks and overbank areas of St. Vrain at the proposed site are <br />. composed of larger size rocks and gravels compared to the smaller particle size sand and <br />gravels found on the lower South Platte. The larger size materials found at the'.~t. Vain <br />
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