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ENFORCE26865
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ENFORCE26865
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Entry Properties
Last modified
8/24/2016 7:34:40 PM
Creation date
11/21/2007 11:26:59 AM
Metadata
Fields
Template:
DRMS Permit Index
Permit No
M1978052
IBM Index Class Name
Enforcement
Doc Date
7/12/2001
Doc Name
MAY 05TH SOUTH PLATTE RIVER FLOW EVENT
From
APPLEGATE GROUP INC
To
MASSEY SEMENOFF SCHWARZ & BAILEY PC
Media Type
D
Archive
No
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Jeffrey W. Schwarz <br />Massey, Semcnoff, Schwas, and Bailey <br />Mobile Premix Concrete, Inc. <br />October 10, 2001 <br />Page d <br />The River System is Dynamic <br />The South Platte River in the vicinity of the MPC permit area is a dynamic reach of river. A <br />review of historical aerial photography suggests that there have been significant changes in <br />channel plan form over the period of record (reference Figure 2.1). The reach of interest has been <br />subjected to numerous natural and man induced stresses on the river system. Natural stresses to <br />the system include both annual peak and extreme flood runoff events and changes in sediment <br />load due to upstream natural events. Man induced stresses to the system include the construction <br />of the existing Brantner Irrigation Diversion structure in 1947 (a concrete dam with a 165 foot <br />concrete roll); construction (1940' and 1950's) of artificial levees along the river reach to protect <br />adjacent farm land from seasonal flooding; various bank revetment activities over the period of <br />record; reconstruction of artificial levees and river banks following extreme flood events (1965 <br />and 1973); in-channel mining and gravel extraction activities in the I970's and 1980's; grazing <br />and clearing of vegetation as part of agricultural practices over the period of record; off channel <br />or flood plain mining and encroachment/ revetment of channel banks (since 1970); construction <br />of Urban Drainage Flood Control Structures (period of record) and construction of the Metro <br />Aeration and Grade Control Structure in 2000. <br />The geomorphic behavior of a natural alluvial river is a function of changes in flow (water or <br />flood discharge QW) and sediment transport (sediment discharge Q,)(reference Figure 2.2, Lane's <br />relationship). The natural channel profile and channel geometry develops in a manner which <br />balances water and sediment discharge. Specifically, natural rivers will respond vertically <br />(changes in channel profile) and horizontally (changes in channel plan form) to ensure an <br />equilibrium transport of its bed material load under average hydrologic conditions. Where man <br />induced efforts restrain one direction of change, the river will respond in an alternate direction. <br />For example, the Brantner Diversion structure establishes grade at a location immediately above <br />the South Platte River's confluence with the Bull Seep Slough. Since the 1947 construction of <br />the diversion (grade control structure) channel degradation (lowering of the channel bed }has <br />occurred .below this structure. In the early 1980's in-channel mining below the Brantner <br />Structure contributed to further degradation of the channel bed. Cross section data (XS 14) <br />provided by the Urban Drainage and Flood Control District (UDFCD) are quantitative evidence <br />of the ongoing (post 1990) degradation. The Brantner Ditch Company has spent, reportedly, <br />thousands of dollars in attempts to control degradation below the diversion structure. Associated <br />with this lowering in the elevation of the main channel bed is a local drop in baselevel at the <br />mouth of the Bull Seep Slough. As the base level drops at the mouth of the Slough, the Bull Seep <br />channel slope ovetsteepens, resulting in channel degradation within Bull Seep Slough. <br />The Brantner Diversion and grade control structure influences geomorphic conditions in an <br />upstream direction. Since the construction of the structure, the channel bed has aggraded to a <br />level equal to the top of the structure. Over the course of this aggradation, the upstream channel <br />slope has flattened. Based on the Flood Hazard Area Delineation completed by Gingery <br />Associates, Inc., 1977, the channel slope above the structure was approximately 0.002 fUft. <br />Based on the October, 2001 survey completed by Peter Epp and Associates, Inc. the channel <br />slope for [he upstream reach of interest is approximately O.OOI fl/ft: a reduction of 50%. With <br />this reduction in slope there has been a reduction in sediment transport capacity or the ability of <br />the river to carry its sediment load. The channel response to the reduced sediment transport <br />
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