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FLOOD10390
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Last modified
1/26/2010 10:13:22 AM
Creation date
10/25/2007 5:33:15 PM
Metadata
Fields
Template:
Floodplain Documents
County
Boulder
Community
Boulder
Stream Name
Fourmile Canyon
Basin
South Platte
Title
Fourmile Canyon Creek - Major Drainageway Planning - Phase A Report - Alternatives Analysis
Date
6/1/2000
Prepared For
UDFCD, Boulder
Prepared By
Love & Associates
Floodplain - Doc Type
Flood Mitigation/Flood Warning/Watershed Restoration
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<br />I <br /> <br />I <br /> <br />After the base mapping was completed, new residential development occurred in both the North Briar <br />and Fourmile Creek subdivisions. Field surveys were completed in both these areas to determine <br />finish floor elevations of the new homes in relationship to the lOa-year flood water surface <br />elevations <br /> <br />HYDRAULIC ANALYSIS <br /> <br />4. <br /> <br />I <br /> <br />Finish floor and walkout basement floor elevations were surveyed for specific homes in the Fourmile <br />Creek subdivision, along South Hampton Circle and Savannah Place, which properties back up to <br />Fourmile Canyon Creek. A comparison of the walkout floor elevations and the lOa-year water <br />surface elevations indicated these homes are not in the lOa-year floodplain. <br /> <br />GENERAL <br /> <br />Hydraulic analysis previously defined for Fourmile Canyon Creek, as reported in the draft Fourmile <br />Canyon Creek Floodplain Study and Letter of Map Revision, dated May 18, 1999, prepared by Love <br />& Associates, Inc. (hereafter referred to as the Draft Restudy), was utilized as the hydraulic basis of <br />this study. The limits of the draft Restudy analysis extend from the Railroad bridge just downstream <br />of State Highway 119 to the westerly City Limits, a stream reach of approximately four (4) miles. <br /> <br />4.1 <br /> <br />I <br /> <br />I <br /> <br />Finish floor and garden level window elevations were surveyed for the homes in the North Briar <br />subdivision, which abut the lOa-year floodplain upstream of Broadway. A comparison of the garden <br />level window sill elevations and the lOa-year water surface elevations indicated three homes at the <br />corner of 7th Street and Yellow Pine Avenue are within the lOa-year floodplain. These three homes <br />were then included in the floodplain damage analysis. <br /> <br />Cross section field surveys were completed along the Fourmile Canyon Creek channel downstream of <br />30th Street to determine the extent of sediment deposition that has occurred since the base mapping <br />was completed in 1993. A comparison of the surveyed channel cross sections and base mapping <br />indicated the elevation of the channel bottom had risen between 0 and 1.5 feet due to the deposition <br />of sediment since the base mapping was completed. Field observations indicated the westerly <br />concrete drop structures no longer functioned as drops due to the sediment deposition. This is clear <br />evidence that the channel is aggrading and loosing flow capacity, probably due to lack of sediment <br />removal from the channel bottom and excessive build up vegetation. The Fourmile Canyon Creek <br />HEC-2 model was updated based on the field survey data. The revised Hec-2 model indicates the <br />sediment deposition resulted in only a slight rise in the lOa-year water surface elevation and no <br />additional homes were found to be in the lOa-year floodplain due to this deposition. <br /> <br />The Restudy was initiated to address increased development activity in North Boulder following <br />annexation and the discovery of mapping errors in the adopted regulatory floodplain study. Errors <br />discovered in the currently adopted mapping artificially contain flood waters along the south bank of <br />the creek west of Broadway. Because the south bank of the creek is lower in elevation than the north <br />bank, flood waters would be expected to physically spill to the south of the creek in major storm <br />events. This condition was confirmed in a review of 1971 historic floodplain mapping and <br />emphasized the need to update and restudy the Fourmile Canyon Creek floodplain mapping. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />The Draft Restudy originally completed 10-, 50-, 100- and SOO-year floodplain analyses of Fourmile <br />Canyon Creek using the regulatory hydrology as its basis. The Draft Restudy also completed a <br />floodplain analysis of the spill flows as they flow southeasterly from Fourmile Canyon Creek to <br />Wonderland Creek. The Draft Restudy utilized the U.S. Army Corps of Engineers' HEC-2 water <br />surface profile computer model for the floodplain analysis. This floodplain study assumed a "fixed" <br />bed, clear water and the floodplain was not studied as an alluvial fan floodplain. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />The base mapping does not reflect the grading recently completed for the City of Boulder <br />sports/soccer complex located west of 47th Street and south of Fourmile Canyon Creek. The grading <br />for this site included fill within the floodplain at the northeast corner of this site. The current Hec-2 <br />model in this area does not reflect this grading, however the model does include encroachments <br />between the creek and the sports complex to model the ineffective flow areas south of the creek. <br />Because the sports complex is located south of the encroachment, modifying the Hec-2 model to <br />reflect the recent grading would have no impact on the water surface elevations along Fourmile <br />Canyon Creek. The southern limit of the floodplain at the northwest comer of this site will generally <br />follow the limits of the fill placed for the sports complex. <br /> <br />Analysis. " <br /> <br />The mapping utilized for the Restudy, as well as this study, is the City of Boulder's 1993 digital base <br />mapping. This base mapping was based upon aerial photography completed in April of 1993 and was <br />completed to National Map Accuracy Standards for a 1"=100' scale and 2-foot contour intervals. <br />These standards require a horizontal accuracy of 2.S-feet horizontal and a vertical accuracy of 6- <br />inches. <br /> <br />in Appendix B, "Hydraulic <br /> <br />and documentation are presented <br /> <br />Supporting figures <br /> <br />BASE MAPPING <br /> <br />4.2 <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />The Hec-2 model does include the pedestrian underpass under 47th Street located approximately 350 <br />feet south of the Creek. A City of Boulder Floodplain Development Permit was approved for this <br />work and indicted no rise in the lOa-year floodplain water surface elevations. <br /> <br />The accuracy of the base mapping was spot checked by both the City and Love & Associates by <br />comparing known ground elevations such as manhole covers and survey monuments, with the <br />contours shown on the base mapping. The base mapping was determined to be in general <br />conformance with National Map Accuracy Standards. <br /> <br />I <br /> <br />I <br /> <br />surveys <br /> <br />actual field <br /> <br />Where greater accuracy was required, such as at bridges and drop structures; <br />were completed and incorporated into the hydraulic modeling. <br /> <br />I <br /> <br />4-1 <br /> <br />I <br /> <br />I <br />
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