Phase A - Alternatives Analysis Report - Urban Drainage and Flood ...

PHASE A – ALTERNATIVES ANALYSIS REPORTMajor Drainageway Planning Update for Hidden Lake-Bates Lake1 INTRODUCTION1.1 AuthorizationOn May 1, 2008 the Urban Drainage and Flood Control District (UDFCD or District), in joint sponsorshipwith the City of Arvada, the City of Westminster and Adams County contracted with Matrix Design Group,Inc. for the provision of engineering services for a major drainageway planning update for the Hidden Lakeand Bates Lake watersheds. Specific hydrology, hydraulics and alternatives analysis tasks were performedin accordance with Agreement Number 08-01.16. This major drainageway planning update replaces the1975 Major Drainageway Planning Study for Hidden Lake – Bates Lake.1.2 Purpose and ScopeThis report was prepared to provide a comprehensive master drainageway plan of improvements for theHidden Lake and Bates Lake watersheds. This Phase A investigation includes the compilation of existingdata, necessary field work, and the development and evaluation of reasonable alternatives so that the mostfeasible drainage and flood control plans can be determined for each of the reaches of the Hidden Lake andBates Lake watersheds. This report was closely coordinated with the project sponsors to incorporate the everchanging goals and objectives for each community. This basin is mostly built out with only a few in-fillprojects proposed. A major anticipated change within this drainage basin is the RTD construction of the“Gold Line” light rail tracks and station along the existing railroad tracks.The scope of this project generally consists of summarizing hydrology from the “Baseline HydrologyReport,” hydraulic evaluations, problem identification, alternatives evaluations and selection of a plan ofimprovements that can be recommended for conceptual design.Deliverables resulting from this master drainageway planning study include the following:• Baseline Hydrology Report• Phase A – Alternatives Analysis Report• Phase B -Planning Report1.3 Summary of Data ObtainedThe major drainageway planning is completed in two distinct phases. The alternative evaluation phaseincludes a summary of the hydrology that was developed in the Baseline Hydrology Report which wasutilized for the development of the floodplain data and associated problem identification. Alternativeimprovements are then identified and developed to help determine the most appropriate system to mitigateexisting flood-prone areas as well as planning for areas that have future development potential. A final planis then recommended. Project Sponsors will review the proposed alternatives and the recommended planand ultimately select a preferred plan of improvements. Once a plan is selected, the District will giveauthorization to develop a conceptual preliminary design of those improvements, which will be presented inthe Phase B - Planning Report. Once the Phase B report is adopted, it will supersede the previous masterplanning studies for the Hidden Lake – Bates Lake project area.Following the hydrology summary and the identification of floodplain problem areas, the alternativesdevelopment and evaluation phase was initiated. The entire study area was separated into six separatereaches. Improvement alternatives were considered on a reach by reach basis. Where inadequate capacitiesand flooding exist, alternatives were considered that included regional detention, improved conveyance, andwater quality enhancement. The possible alternatives were narrowed only to those that were consideredreasonably viable for the Hidden Lake – Bates Lake project area. Viable alternatives were evaluated byconsidering the impact on the reach hydraulics, adjacent existing and future development and by comparingthe costs with the benefits of each. The alternatives considered for the Hidden Lake and Bates Lakewatersheds are presented and discussed in this Major Drainageway Planning Phase A, Alternatives AnalysisReport.1.4 Mapping SourcesDetailed 2-foot contour topography was provided by UDFCD, Westminster, and Adams County for aportion of the watershed. The area that was not covered by the detailed 2-foot contour topography was filledin with the United States Geological Survey (USGS) 10-meter Digital Elevation Model (DEM). The aerialphotography, provided by UDFCD, was mapped by Accurate EngiSurv, LLC in 2007. The horizontalcoordinates are in Colorado State Plane Coordinate System, North American Datum of 1983 (SPCSNAD83), Central Zone. The elevations on these contours are on the North American Vertical Datum of1988 (NAVD88). All work has been completed on NAVD88.The 2-foot contour topography that was provided by Westminster was on the SPCS NAD83, North ZoneHorizontal Datum. The Westminster elevations for these contours were on the National Geodetic VerticalDatum of 1929 (NGVD29). The contours provided by Westminster had to be converted to the SPCSNAD83 Central Zone Horizontal Datum and the NAVD88 Vertical Datum. The 2-foot contour topographythat was provided by Adams County was on the NAVD88 Vertical Datum and the SPCS NAD83, CentralZone Horizontal Datum.Electronic land use and storm drain network data was provided by Arvada, Westminster and Adams County.Additional geographic data was obtained from CDOT’s website.1.5 Data CollectionMany sources for existing data were used to compile the hydrology, to complete hydraulic evaluations andfor completing the evaluation of improvement alternatives. Data was obtained from previous reportscompleted for and obtained from the District. The local sponsors from the cities and counties have providedexisting utility information in a digital format.The following District reports were utilized in providing background information for this study:• Hidden Lake Major Drainageway Planning Study Phase A Report, April 1975• Major Drainageway Planning, Hidden Lake-Bates Lake Phase B Report, November 1975• Hidden Lake Backup Data Vol. I, February 1976• Hidden Lake Backup Data Vol. II, February 1976• Revised Hydrology Study Hidden Lake Dam & Reservoir, December 1980• Hidden Lake Drainageway 100-Year Floodplain Revisions, May 19861-1