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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
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• between North 75`s Street and North 51" Street, additional flow will spill to the south <br />away from the main channel; however, this flow is small and is not quantified. Tlus <br />additional flow spill would not necessarily reach the south floodplain and results in an <br />area of shallow indeterminate flooding. The area between the main channel and :;oath <br />floodplain is designated as an area of shallow flooding with depths of between 0 and 3 <br />feet account for this flow spill. The main channel and the south floodplain converge into <br />one floodplain just upstream of North 75's Street. <br />The south floodplain azea is relatively flat. The most likely course for the 4,000 cfs was <br />followed from North 51" Street to its confluence with the main channel neaz 75's Street. <br />Because a single flow path for the south floodplain was difl;cult io determine, the area <br />between the south floodplain and the main channel is considered to be an azea of'shallow <br />flooding (corresponding to the shallow flooding area described above). <br />Independent floodways for both the main channel and the south floodplains were <br />determined based on Boulder County's criteria of a maximum of a 1-foot rise in the <br />energy grade line. The area along the split flow control line between U.S. Highway 36 <br />and North 51" Street was not encroached so that the distribution of flows at these: <br />locations did not change. <br />• IV. Imaacts of Minine and Reclamation on the Fooodolain <br />The proposed mining and reclamation plan can be expected to have an overall positive <br />effect on the St. Vrain floodplain by providing additional flood water storage volume in <br />the reclaimed "Open Valley" pit areas. This additional storage volume would be expected <br />to reduce downstream flood flows. The mining and reclamation plan does not propose <br />any increase in topographic elevations that would encroach or have any adverse effects <br />on the existing floodplain mode]. <br />The proposed reclamation plan the West Plant azea will result in the expansion ].00-yeaz <br />floodplain limits (inundation area) into the area to be excavated to the northwest of the <br />Southwest Portland Concrete Plant. The proposed future land use (wildlife habitat) of this <br />area however, is compatible with the floodplain designation. <br />As discussed in Section III, the St. Vrain floodplain downstream of North 51" St. is split <br />into a main and south floodplain, with an area designated as an area of shallow flooding <br />(0' to 3') separating the two floodplains. Although some flow is projected to spill from <br />the main to the south floodplain across the area of shallow flooding, the flow is assumed <br />to be small and therefore is not considered in the current floodplain model to contribute <br />• to the flow in the south floodplain (the area of shallow flooding is effectively considered <br />to be an area of higher topography separating the two floodplains in the hydraul is model), <br />4 <br />
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