Floodplain Technical Memorandum

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Derick Lloyd
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1 Southeast Extension Project Lincoln Station to RidgeGate Parkway Prepared for: Federal Transit Administration Prepared by: Denver Regional Transportation District May 2014

2 Table of Contents Page No. Chapter 1.0 Introduction Background Regulatory Environment Chapter 2.0 Alternatives Evaluated Chapter 3.0 Affected Environment Chapter 4.0 Impact Evaluation Methodology No Action Alternative Preferred Alternative Direct Impacts Indirect Impacts Construction Impacts Cumulative Impacts Chapter 5.0 Mitigation Chapter 6.0 References List of Figures Figure 1-1 FasTracks Plan Figure 1-2 Project Study Area Figure 2-1 No Action Alternative Figure 2-2 Preferred Alternative Figure 3-1 Floodplains List of Tables Table 5-1 Jurisdictional Requirements For On-Site Detention TOC-i

3 Chapter 1.0 Introduction This Technical Memorandum was prepared in support of the Southeast Extension Environmental Assessment initiated by RTD in This Technical Memorandum focuses on information regarding potential effects to floodplains that would occur as a result of the Southeast Extension project. 1.1 Background In November 2004, Regional Transportation District (RTD) voters approved the FasTracks initiative to expand and improve public transit service within the Denver Metropolitan Region (Metro Region). The comprehensive FasTracks Plan, which formed the basis of the FasTracks ballot initiative, includes the construction and operation of new fixed-guideway transit lines, improved bus service, and park-n-rides throughout the Metro Region. The Southeast Extension was included in the RTD FasTracks program and is in the currently adopted Regional Transportation Plan (RTP). The fixed-guideway transit elements (rail and bus rapid transit) of the FasTracks Plan are shown in Figure 1-1. The proposed action is to extend transit service south into the City of Lone Tree to serve the increased population and employment generated by planned development in the City of Lone Tree. The Southeast Extension study area is located in northern Douglas County, and includes the City of Lone Tree and portions of Highlands Ranch and the Town of Parker. It begins at the existing end-of-line Lincoln Avenue LRT station and extends south along I-25 to the I- 25/RidgeGate Parkway interchange. It includes areas of planned development south of Lincoln Avenue on the east and west side of I-25 (see Figure 1-2). The Southeast Extension floodplains study area differs from the larger project study area. The floodplain study area consists of the track alignment with a 300-foot buffer on either side of the light rail right-of-way or light rail station property lines, and is shown on Figure 3-1 in Chapter Regulatory Environment Floodplains for this analysis are the lands on either side of a water way that are inundated when the capacity of the stream channel is exceeded during a 100-year precipitation event. Changes in the floodplain, such as adding fill material, constructing buildings or bridges, or in any way limiting the natural conveyance of floodwaters, can cause a rise in the 100-year water surface, and can subsequently impact adjacent properties not previously affected by a 100-year storm event. Areas of specific interest are the floodplains defined by the Federal Emergency Management Agency (FEMA) as special flood hazard areas that are inundated by a 100-year flood. FEMA floodplains are labeled by flood zones. Each flood zone represents a type of flooding and level of detail used to create the floodplain boundary. 1-1

4 Figure 1-1 FasTracks Plan 1-2

5 Figure 1-2 Project Study Area 1-3

6 The following regulatory requirements apply to the floodplains located within the study area. Executive Order (EO) 11988, Floodplain Management (1997), was authorized to direct federal agencies to provide leadership and take action to reduce the risk of flood loss, to minimize the impacts of floods on human safety, health and welfare, and to restore and preserve the natural and beneficial values served by floodplains. This EO was authorized to support the National Environmental Policy Act (NEPA), the National Flood Insurance Act of 1968 (amended), and the Flood Disaster Protection Act of Code of Federal Regulations (CFR), Title 44 Emergency Management and Assistance, Chapter 1 FEMA contains the basic policies and procedures of FEMA to regulate floodplain management and to analyze, identify, and map floodplains for flood insurance purposes. Flood Management and Protection (FHPM and 23 CFR 650) applicable to any federally assisted construction project in an area identified as having special flood hazards. It requires the design to be consistent with FEMA protocols, and construction to be completed in a manner to avoid FEMA identified flood hazard areas. 1-4

Chapter 2.0 Alternatives Evaluated This technical memorandum evaluates the effects of two alternatives a No Action Alternative and the Preferred Alternative. These alternatives are described below.

7 Chapter 2.0 Alternatives Evaluated This technical memorandum evaluates the effects of two alternatives a No Action Alternative and the Preferred Alternative. These alternatives are described below. The No Action Alternative assumes no new improvements would be constructed other than currently committed projects identified in the 2035 RTP. This alternative includes the existing bus routes in the area and a new bus route (Route 411) connecting Parker and the Lincoln Station along RidgeGate Parkway. The CDOT project that includes I-25 widening from RidgeGate Parkway to C-470 is also included as part of this alternative. Figure 2-1 shows the No Action Alternative. Figure 2-1 No Action Alternative 2-1

8 The Preferred Alternative includes a 2.3-mile, double-track light rail extension that runs south from the existing Lincoln Station along the west side of I-25, crosses to the east side of I-25 just north of the Sky Ridge Medical Center, and continues south to the RidgeGate Parkway interchange. This alternative provides three new stations. The Sky Ridge Avenue Station across from the Sky Ridge Medical Center and the Lone Tree City Center Station situated in the core of the RidgeGate planned development are both planned as kiss-n-ride stations without parking. A new end-of-line station at RidgeGate Parkway would provide a park-n-ride. Access to the RidgeGate Station would be provided from Havana Street via two access roads. All three stations would accommodate feeder bus service. The light rail tracks would be grade separated via an overpass where they cross Lincoln Avenue, I-25, and RidgeGate Parkway. One at-grade crossing is proposed on a minor roadway near the Sky Ridge Station. Two parking design options are being considered at the RidgeGate Station, as described below: Option 1: This option would provide a 1,300-space surface parking lot on opening day (2019). In 2035, the surface parking lot would be replaced with two parking structures that would accommodate a total of 2,100 parking spaces. The southern parking structure would consist of four levels and the northern parking structure would consist of three levels. Option 2: This option would provide one 4-level, 1,300-space parking structure on opening day (2019). In 2035, an additional 3-level parking structure would be built north of the first structure that would provide 800 parking spaces, for a total of 2,100 spaces. The two parking structures in 2035 would be the same design and configuration under both parking options. The Preferred Alternative is shown on Figure

9 Figure 2-2 Preferred Alternative 2-3

10 Chapter 3.0 Affected Environment The floodplains study area for the Southeast Extension consists of the track alignment with a 300-foot buffer on either side of the light rail right-of-way or light rail station property lines (Figure 3-1). There is one waterway located within the floodplains study area, Cottonwood Creek. The closest major waterway and ultimate outfall of Cottonwood Creek is Cherry Creek, which is approximately 4.5 miles to the east of the alignment. The 100-year floodplain surrounds Cottonwood Creek on the east side of I-25. Cottonwood Creek The 100-year floodplain within the study area creates an envelope that varies between 30 and 120 feet. There are no existing bridges that cross the floodplain within the study area. High water mark and flood history data is not readily available because Cottonwood Creek is considered a minor tributary to Cherry Creek. The largest flood along Cherry Creek occurred on June 16, 1965, when a stream gauge along Cherry Creek in Melvin (Arapahoe Road) recorded a flow rate of 39,900 cubic feet per second (cfs). A peak flood stage at this point is unknown; it can be assumed that not only Cherry Creek, but Cottonwood Creek was flowing well outside of the existing stream banks during this event (Gingery, 1975). A flood stage is defined as the height of water outside the existing stream banks. This flood was one of the most devastating floods in the Denver metropolitan area and spurred legislature to build more dams. The 1965 flood was caused by three days of intense rains, especially over the Cherry Creek and Plum Creek drainage basins, with some areas receiving up to 14 inches of rain (Sprengelmeyer, 1999). The most recent notable high water mark occurred on Cherry Creek on July 30, A flow rate on Cherry Creek was measured to be 900 cfs and water was at an elevation of 5,814.7 feet above mean sea level (msl) at the United States Geological Survey (USGS) Stream Gauge Station on Cherry Creek, near Parker Colorado (USGS, 2008). The alignment would cross the Cottonwood Creek 100-year floodplain in one location, where the alignment is on the east side of I-25. The ultimate outfall of Cottonwood Creek is Cherry Creek, upstream of the Cherry Creek Reservoir. The total drainage area of Cottonwood Creek where it outfalls into Cherry Creek is approximately 8.7 square miles. At the point the alignment crosses over Cottonwood Creek, the drainage area is approximately 1.0 square mile, and the 100-year discharge was calculated to be 1,300 cfs with a 100-year floodplain elevation of 5,974.5 feet above msl and a 100-year floodplain width of approximately 35 feet (Gingery, 1975). 3-1

11 Figure 3-1 Floodplains 3-2

12 Chapter 4.0 Impact Evaluation 4.1 Methodology Several streams are located within the Southeast Extension study area. A baseline map of the floodplains associated with these water features was developed from FEMA Flood Insurance Rate Maps (FIRMs). Drainage Master Plans and Flood Hazard Area Delineations (FHADs) were obtained from the UDFCD, when available, and incorporated into the floodplain maps for the Southeast Extension study area, and the 100-year floodplain event was mapped. Floodplain impacts have been categorized as direct, indirect, temporary construction, and cumulative impacts. Direct impacts are defined as those effects caused by the action and occurring at the same time and place (40 CFR ). These impacts would alter or eliminate the original floodplain and can include acquisition of a floodplain for development. The study area for direct impacts is defined by the limits of the construction footprint and the limits of the permanent right-of-way required for the project within the 100-year floodplain. 4.2 No Action Alternative The No Action Alternative would not result in any direct, indirect, construction-related, or cumulative impacts to floodplains and drainage/hydrology. 4.3 Preferred Alternative The direct, indirect, construction-related, and cumulative impacts of the Preferred Alternative are described below Direct Impacts Only one FEMA- and UDFCD-designated 100-year floodplain is located within the study area: Cottonwood Creek, which is designated on FEMA s FIRM as Zone A, which indicates a 1 percent annual chance of flooding. Because detailed analyses are not performed for such areas, no depths or base flood elevations are determined. The Preferred Alternative would span the 100-year floodplain associated with Cottonwood Creek and not intersect the 100-year floodplain associated with Cottonwood Creek. The footprint of the Preferred Alternative is located entirely outside of the 100-year floodplain; no elements of the Preferred Alternative would be located within the floodplain. None of the proposed light rail stations along the alignment, including Sky Ridge, Lone Tree Town Center or RidgeGate Parkway stations, would be located within the 100-year floodplain. All station footprints and park-n-ride parking lot footprints are located at least 300 feet from the nearest floodplain Indirect Impacts Indirect effects to floodplains would result from additional impervious surfaces introduced by transit-oriented development around stations. Impacts due to such development would be avoided through adherence to local development requirements. 4-1

13 4.3.3 Construction Impacts The Preferred Alternative would have temporary construction impacts to the floodplains of Cottonwood Creek. These temporary impacts include the loss of vegetation and soil disturbance because heavy equipment would likely be operated within the floodplain during construction activities associated with the structure over Cottonwood Creek. These impacts would not alter the floodplain hydraulics or increase flooding risks Cumulative Impacts There are no cumulative impacts anticipated for this resource. 4-2

14 Chapter 5.0 Mitigation Although not anticipated, encroachment of any of the floodplains along the alignment study area will be subject to the requirement of local jurisdictions. A variety of Best Management Practices (BMPs) for erosion and runoff control will be implemented, as determined during final design. Detention systems will allow for storage and control of stormwater runoff. Requirements for onsite detention facilities are provided in Table 5-1 by jurisdiction. Table 5-1 Jurisdictional Requirements For On-Site Detention Jurisdiction Urban Drainage and Flood Control District (UDFCD) Douglas County Flood Levels Minor 5- and 10-year plus water quality capture volume (WQCV) WQCV and Excess Urban Runoff Volume (EURV) Major 100-year plus no less than one-half WQCV 100-year plus the WQCV Freeboard Requirements 1 foot above the water surface elevation when the emergency spillway is conveying the maximum design or emergency flow Detention Sizing Methods Simplified method based on empirical equations in storage chapter of District Manual Underground Detention Only in ultraurban settings where no onsurface methods are practicable Prohibited Aboveground Detention Preferred Sources: UDFCD, 2011 and Douglas County, 2008b. Coordination with the RidgeGate developer will continue as engineering design is completed. The drainage design for the RidgeGate Planned Development was oversized in anticipation of the Southeast Extension project. 5-1

15 Chapter 6.0 References Douglas, Douglas County Storm Drainage Design and Technical Criteria Manual. Douglas County Department of Public Works Engineering Division, Castle Rock, Colorado, adopted January 1986, amended July FEMA. FEMA_100-Year_Flood_Zones_in_the_USA. Accessed September 27, Gingery, Flood Hazard Area Delineation, Piney Creek, Cottonwood Creek, Lone Tree Creek, and Murphy Creek. Prepared for Urban Drainage and Flood Control District, Gingery Associates, Inc., Englewood, Colorado, October HNTB, Flood Hazard Area Delineation, Happy Canyon Creek, Badger Gulch, Newlin Gulch, Baldwin Gulch, and Tallman Gulch. Prepared for Urban Drainage and Flood Control District, Howard, Needles, Tammen, & Bergendoff, Denver, Colorado, November RidgeGate, RidgeGate, 2nd Amendment, Planned Development District Prepared for RidgeGate Investment, Inc, Coventry Development Corporation, Lone Tree, Colorado, May 7, Sprengelmeyer, 1999, 1965 Flood left Deep Scars along South Platte article in Rocky Mountain News, M.E. Sprengelmeyer, December 14, United States Geological Survey (USGS), USGS National Water Information System: Web Interface, Station # Cherry Creek near Parker, Colorado, Accessed December 4, UDFCD, Urban Storm Drainage Criteria Manual Volume 2. Urban Drainage and Flood Control District, Denver, Colorado, December

Right-of-Way Acquisitions and Relocations Technical Memorandum

Right-of-Way Acquisitions and Relocations Technical Memorandum Southeast Extension Project Lincoln Station to RidgeGate Parkway Prepared for: Federal Transit Administration Prepared by: Denver Regional