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<br />SECTION 1.0 - INTRODUCTION
<br />
<br />1.1 Authorization
<br />
<br />This study was authorized by the Urban Drainage and Flood Control District (District) and the City of
<br />Arvada in the agreement regarding Hydrologic and Hydraulic analysis for Ralston Creek Watershed, dated
<br />March 14, 2002 (Agreement No. 01-10.04). This report documents the Flood Hazard Area Delineation
<br />(FHAD) for a portion of Ralston Creek and a portion of its tributary, Leyden Creek. Boyle Engineering
<br />Corporation (Boyle) was retained to complete the scope of work summarized in Subsection 1.2. This
<br />FHAD study updates and replaces the previous floodplain and floodway studies performed for the Ralston
<br />and Leyden Creek watersheds.
<br />
<br />Upon acceptance of this report by the Urban Drainage and Flood Control District, this study can be used by
<br />the City of Arvada to regulate future floodplain construction, to guide future drainageway improvements,
<br />and to plan flood protection measures along the channels and floodplain of Ralston Creek and Leyden
<br />Creek.
<br />
<br />1.2 Purpose and Scope
<br />
<br />This report was prepared to assist government officials from various agencies in floodplain management for
<br />Ralston and Leyden Creeks. This report presents the results of an updated floodplain delineation based on
<br />recent topography and an updated hydrologic analysis for future basin conditions within the Ralston Creek
<br />watershed. This report only covers the areas along Ralston Creek from the Arvada/Blunn reservoir to the
<br />confluence with Clear Creek and the areas along Leyden Creek from Leyden Lake to the confluence with
<br />Ralston Creek.
<br />
<br />The hydrologic analysis was completed using the District's Colorado Urban Hydrograph Procedure
<br />(CUHP), Version 1.8.3 (Reference 1) to determine peak flows from over 200 subwatersheds in the Ralston
<br />Creek drainage basin. The results of the analysis were presented in the 2003 RalstonlLeyden Creeks
<br />Hydrologic Report (Reference 2). The resulting peak flows were used in the hydraulic modeling of the
<br />creeks to determine the updated floodplain boundaries as shown in this report.
<br />
<br />The following is a brief summary of the scope of work for this project:
<br />
<br />1. Calculation of water surface profiles for the 10-, 50-, 100-, and 500-year flood events;
<br />2. Delineation of the 100-year floodplain boundary on Flood Hazard Area Maps;
<br />3. Development of discharge profiles indicating the relative magnitudes of the 10-,50-, 100-, and
<br />500-year flood events;
<br />4. Determination of the 100-year event floodways based on encroachments resulting in a maximum
<br />increase in the water surface elevation or energy grade line of 0.5 feet and 1.0 foot;
<br />5. Preparation of a report showing the updated floodplain delineation and profiles, and tabulations
<br />indicating pertinent floodplain and floodway information.
<br />
<br />With the exception of the planned improvements to the confluence of Van Bibber Creek and Ralston Creek,
<br />which were incorporated in this study, the flood profiles and floodplain delineations presented in this report
<br />are based on existing floodplain and channel conditions. Flood elevations and the corresponding floodplain
<br />location are often changed by road and bridge construction, floodplain development, flood control
<br />improvements, or natural processes. Prior to utilization of this report for planning or design purposes, the
<br />user is advised to contact the Urban Drainage and Flood Control District or the City of Arvada to determine
<br />if the information in this report has been amended.
<br />
<br />1.3 Watershed Study History
<br />
<br />The Ralston, Leyden, and Van Bibber Creeks watersheds have been extensively studied. Previous studies
<br />were conducted by the U.S. Army Corps of Engineers (1970), Frasier and Gingery (1974), Wright-
<br />McLaughlin Engineers (1976), Gingery and Associates (1983), and Wright Water Engineers (1984).
<br />
<br />Of particular importance was the 1976 study by Wright-McLaughlin Engineers that resulted in Phase A of
<br />the Major Drainageway Plan for the watershed. That study used the MITCA T model to establish flow rates
<br />for all three basins. The model included Ralston Reservoir, but did not include the Arvada/Blunn Reservoir.
<br />Leyden Lake as it was built at that time was included in the model. The updated model by Boyle includes
<br />the improvements to Leyden Lake, which establishes it as a certified flood control facility.
<br />
<br />The 1984 study by Wright Water Engineers resulted in Phase B of the Major Drainageway Plan. That study
<br />was essentially an update of the 1976 study. The MITCAT model was still used, but a more detailed
<br />analysis was performed. The Arvada/Blunn Reservoir was still not included in the model. At the time,
<br />Leyden Lake was determined to have a significant risk of failure due to an inadequate spillway. For this
<br />reason, its function as a flood control element was in doubt. Therefore, two models were analyzed - with
<br />and without Leyden Lake. Flow rates and hydraulics used for the Flood Insurance Study (1989) utilized
<br />results without Leyden Lake from that study.
<br />
<br />In 2003, Boyle Engineering Corporation updated the hydrology for Ralston Creek by creating a new model
<br />in CUHPfUDSWM. The new model results were calibrated to the previous MITCA T results and
<br />documented in the 2003 hydrology study report. The storage capacity of Leyden Lake was recognized in
<br />the updated report, which reduced the flows directly downstream.
<br />
<br />1.4 Floodplain History
<br />
<br />The 100-year floodplain has been delineated in previous studies for various sections of Leyden and Ralston
<br />Creeks. Upper Ralston Creek, between the Arvada/Blunn Reservoir and the confluence with Leyden Creek,
<br />was last delineated in a September 1977 "Major Drainageway Planning" report by Wright-McLaughlin
<br />Engineers (Reference 3). Ralston Creek from the confluence with Leyden Creek to its confluence with
<br />Clear Creek, and Leyden Creek from Leyden Lake to its confluence with Ralston Creek, were delineated in
<br />the 1986 Major Drainageway Planning, Lower RalstonlVan Bibber/Leyden Creeks, Phase B Report by
<br />Wright Water Engineers (Reference 4). FEMA used the floodplains in their 1989 Flood Insurance Rate
<br />Maps. Since that time, various Letters of Map Revision (LOMRs) have been prepared to modify and update
<br />the 1989 FEMA floodplain. These LOMRs include the Cottonwood West Industrial Park along Ralston
<br />Creek, revised December 5, 2001, and a section of Ralston Creek just upstream of the confluence with Clear
<br />Creek, revised August 12, 1998. Boyle used the information provided in the LOMRs to compare data
<br />results for the new floodplain delineations.
<br />
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