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FLOOD10385
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Last modified
1/26/2010 10:13:20 AM
Creation date
10/25/2007 4:54:34 PM
Metadata
Fields
Template:
Floodplain Documents
County
Jefferson
Community
Golden, Lakewood
Stream Name
Upper Lena Gulch
Basin
South Platte
Title
Major Drainageway Planning - Upper Lena Gulch - Phase B Report
Date
3/1/1994
Prepared For
UDFCD
Prepared By
Boyle Engineering Corporation
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />HYDRAULIC ANALYSIS <br /> <br />IV. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Values of Manning's roughness coefficient, "n", were estimated for the channel and the left and right overbanks for <br />each of the study reaches. Estimates were based on field observations and were computed using the procedures <br />outlined in the Federal Highway Administration publication "Guide for Selecting Manning's Roughness Coefficient <br />for Natural Channels and Floodplains". This method estimates Manning's "n" by assigning a base value for a <br />particular soil type and applying adjustments for the degree of irregularity of the channel, the variations in channel <br />cross-sections, the effects of obstructions, the amount of vegetation, and the degree of meander of the channel. <br />Manning's "n" used in the hydraulic model for channels varied from 0.020 to 0.035. The values for the overbanks <br />varied from 0.020 to 0.050, <br /> <br />General <br /> <br />Hydraulic analyses have been performed to delineate the 1 DO-year floodplain along Upper Lena Gulch, Apex <br />Gulch, and the Pleasant View Tributary. The limits of the I DO-year floodplain represent the area in which <br />development is often regulated to prevent additional encroachments that increase water levels and cause increased <br />damages during a flood. Federal and state agencies such as the Federal Emergency Management Agency (FEMA) <br />the Colorado Water Conservation Board (CWCB), and the Urban Drainage and Flood Control District (OOFCD), <br />as well as local municipalities, use the floodplain delineations as a mechanism to identifY areas particularly prone to <br />damage from flooding and to oversee development activities in these floodplains. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />The hydraulic model assumes the channel has rigid boundaries. That means the cross-section shape does not vary <br />with time or flow rateto account for erosion or bank deterioration. This is generally appropriate for this stream <br />Any changes in channel geometry are transient and are not likely to significantly affect the floodplain limits, <br /> <br />I <br /> <br />I <br /> <br />The peak flow rates estimated from the hydrologic model are assumed coincident and are used in the hydraulic <br />model to compute the water surface profiles and floodplain limits in each reach, While this approach is <br />conservative, it represents the standard of practice and is appropriate here. The flow rates used to compute water <br />surface profiles reflected the effects of existing underground storm sewers, The capacity of existing storm sewers <br />were estimated based on available information. The capacity of the underground system was subtracted from the <br />gross peak flow rates to yield the surface peak flow rates. In areas with underground storm sewers, the surface <br />peak flow rates were used to compute water surface profiles. <br /> <br />Mapping <br /> <br />OOFCD provided 1"= I 00' mapping with a 2-foot contour interval for base mapping. The mapping coverage <br />extended from the upstream side of Maple Grove Reservoir upstream along Lena Gulch for 4.2 miles. Mapping <br />was also provided for Apex Gulch and Pleasant View Tributary from their confluences with Lena Gulch, upstream <br />for approximately 0.5 and 0.7 miles, respectively. <br /> <br />I <br /> <br />I <br /> <br />In some cases, flow left the main channel due to inadequate channel capacity or obstructions. These areas were <br />simulated using various methodologies. When overflows left the main channel and followed an independent flow <br />path unrelated to the main channel profile, the split flow option within HEC-2 was used. In those cases, an overflow <br />side channel section is defined and overflow rates are computed. The downstream discharge in the main channel is <br />reduced and the overflow discharge is diverted into the alternate channel. Significant overflows were evaluated and <br />a water surface profile established for the diverted flow using a separate HEC-2 analysis. In cases where overflows <br />were minor, no specific profile was computed and downstream main channel discharges were not decreased <br /> <br />IS <br /> <br />Methodology <br /> <br />The delineation of the I DO-year floodplain on Upper Lena Gulch and its tributaries was accomplished using the <br />U,S. Army Corps of Engineers HEC-2 Water Surface Profiles computer model. The HEC-2 Water Surface <br />Profiles model was developed for use in computing water surface profiles in natural or man-made channels. The <br />computational procedure used in the model is the Standard Step Method. A complete description of model <br />capabilities is beyond the scope of this report, however some basic assumptions used by the program are: I) flow <br />steady; 2) flow is gradually varied; 3) flow is one dimensional; and 4) channel slopes are small. <br /> <br />I <br /> <br />I <br /> <br />'I <br /> <br />At some locations, flow was divided but hydraulically connected. At those locations, flow splits were the result of <br />downstream conditions which affected the discharge along each flow path. The analysis for this condition required <br />the balancing of energy losses along both flow paths. A downstream section was identified where flows were <br />combined. A profile was computed along each flow path for a variety of discharges. The energy at an upstream <br />section where flow was once again combined was computed. When the sum of two discharges having the same <br />computed energy at the upstream section equaled the total discharge, the proper flow distribution had been <br />identified. This analysis served to identifY the discharge down each flow path and the water surface profile along <br />the path. <br /> <br />Flow in the channel was assumed to be sub-critical. This implies that flow is controlled by backwater effects and is <br />a function of downstream energy and losses in the intervening reach. This is a common assumption in the <br />delineation of floodplains and the determination offload profiles. Because of the sub-critical assumption, depths of <br />flow below critical depth are not mathematically possible. This assumption represents the long-term physical <br />condition in natural channels where super-critical flow is rare. Local anomalies such as scour-holes, debris snags, <br />and parked vehicles prevent flow from staying super-critical for significant distances <br /> <br />I <br /> <br />For <br /> <br />Flow through structures is often partially blocked during high flood flows due to debris accumulation. The <br />obstruction at structures was estimated using a reduction of the cross-sectional area of the structure opening. <br />structures less than 3 feet in diameter, the structure was assumed fully blocked and was not modeled. For <br />structures whose minimum dimension is between 3 and 10 feet, a 20 percent reduction in opening area was made. <br />Large structures were assumed to be capable of conveying all debris and were not modified in the model <br /> <br />I <br /> <br />I <br /> <br />the existing channel <br /> <br />both <br /> <br />100-Year Floodplain Description <br /> <br />The I DO-year floodplain is shown on the plan and profile drawings contained herein for <br />conditions and the conditions after Master Plan improvements have been implemented <br /> <br />into the model cross-section data, In developed <br />the obstruction resulting from entire blocks of <br /> <br />in the floodplain, such as buildings, were coded <br />within the expected I DO-year <br /> <br />Major obstructions <br />areas where residences were <br />homes was coded into the cross section <br /> <br />I <br /> <br />and the existing drainageway <br /> <br />OO-year floodplain is based on fully developed basin discharges <br /> <br />The existing <br /> <br />7 <br /> <br />1 <br /> <br />floodplain, <br /> <br />I <br /> <br />I <br />
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