FLOODPLAIN TECHNICAL MEMORANDUM FOR RTD SOUTHWEST CORRIDOR EXTENSION PROJECT. January Prepared By Southwest Corridor Extension Project Team

Transcription

1 FLOODPLAIN TECHNICAL MEMORANDUM FOR RTD SOUTHWEST CORRIDOR EXTENSION PROJECT January 2010 Prepared By Soutwest Corridor Extension Project Team In some cases, information in tis Environmental Tecnical Report may ave been refined or updated as preparation of te Draft EE advanced. In suc cases, te information and conclusions presented in te Draft EE supersede all previous background material included in tis Tecnical Report. 1

2 TABLE OF CONTENTS 1.0 INTRODUCTION Regulatory Environment AFFECTED ENVIRONMENT IMPACT EVALUATION AND MITIGATION METHODOLOGY Direct Impacts Indirect Impacts Construction Impacts Cumulative Impacts MITIGATION REFERENCES LIST OF FIGURES Table 1 Existing Bridges Witin te Study Area... 3 Table 2 Jurisdictional Requirements for On-site Detention... 6 LIST OF FIGURES Figure 1 Project Location Map... 2 Figure 2 Floodplain Map... i

3 1.0 INTRODUCTION Tis Tecnical Memorandum focuses on information regarding potential effects to floodplains tat would occur as a result of te Soutwest Corridor Ligt Rail Extension project. Te Soutwest Corridor Extension project will extend ligt rail service in te Soutwest Corridor along te C (Union Station to Mineral) and D (30t and Downing to Mineral) lines by 2.5 miles (Figure 1). Te project passes troug te City of Littleton and Arapaoe County, between te Mineral Avenue end-of-line station and County Line Road, were it crosses into Douglas County and Higlands Ranc. Between Mineral Avenue and County Line Road, te ligt rail alignment will run adjacent to Santa Fe Drive, crossing Dad Clark Gulc along te existing UPRR alignment. A new structure is added at Dad Clark Gulc to accommodate a double-tracked ligt rail crossing. Furter sout te alignment crosses County Line Road, bot freigt rail lines and C-470 on a fly-over bridge. Te tracks will follow CDOT s rigt-of-way sout of C-470, crosses Hig Line Canal over an extended box culvert, and continues east to Lucent Boulevard. 1.1 REGULATORY ENVIRONMENT Floodplains for tis analysis are te lands on eiter side of a water way tat are inundated wen te capacity of te stream cannel is exceeded during a 100-year precipitation event. Canges in te floodplain, suc as adding fill material, constructing buildings or bridges, or in any way limiting te natural conveyance of floodwaters, can cause a rise in te 100-year water surface and can subsequently impact properties not previously affected by a 100-year storm event. Areas of specific interest are te floodplains defined by te Federal Emergency Management Agency (FEMA) as special flood azard areas tat are inundated by a 100-year flood. FEMA floodplains are labeled by flood zones. Eac flood zone represents a type of flooding and level of detail used to create te floodplain boundary. Te following regulatory requirements apply to te floodplains located witin te study area. Executive Order (EO) 11988, Floodplain Management (1997), was autorized to direct federal agencies to provide leadersip and take action to reduce te risk of flood loss, to minimize te impacts of floods on uman safety, ealt and welfare, and to restore and preserve te natural and beneficial values served by floodplains. Tis EO was autorized to support te National Environmental Policy Act (NEPA), te National Flood Insurance Act of 1968 (amended), and te Flood Disaster Protection Act of Code of Federal Regulations (CFR), Title 44 Emergency Management and Assistance, Capter 1 FEMA contains te 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 aving special flood azards. It requires te design to be consistent wit FEMA protocols, and construction to be completed in a manner to avoid FEMA identified flood azard areas. 1

4 Soutbridge e Sout P la tt k Jac MINERAL ass H ill Rd Ave Ja G e Lin ck as s g Hi Mine ra l u Soutpark W Fe Dr Can al Ci ty Ditc lc Soutbridge Park n Lo g nt a Sout Platte Park LITTLETON Jackass Hill Park pa RTD rk n-r id Ri v e er C D Aspen Grove Sopping Center Sa r Ci lc Writers Vista Park Wolurst Sou tp a rk Cir u rk G d C la Low er D a 85 McLellan Reservoir County Line Rd Eri ck so n B lv d ARAPAHO E CO DOUGLAS CO Cit y of Little to n c Lu en t vd Bl INTERMEDIATE STATION Line Can al Pl az a Dr C-470/ LUCENT Rd Hig SF Wind Crest BN U PR R (Not part of FasTracks Plan) Vista Higlands Ranc Golf Club ,000 bo ns rou g Dr Wa y ee Gr Creeksi de Mill HIGH LANDS RAN CH 2,000 Feet Source: Soutwest Corridor Extension Project Team, 2009 Stations Existing Soutwest Ligt Rail Line County Boundary Soutwest Ligt Rail Extension City Boundary Freigt Railroad Proposed Future Figure 1 Soutwest Corridor Extension

5 2.0 AFFECTED ENVIRONMENT Te study area for te Soutwest Corridor Extension consists of te track alignment wit a 300- foot buffer on eiter side of te ligt rail rigt-of-way (ROW) or ligt rail station property lines. Te Soutwest Corridor Extension is a 2.5-mile project from te existing Mineral Station to Lucent Boulevard (Figure 1). Te Corridor will ave at least one ligt rail station, te Colorado State Higway C-470 (C-470) and Lucent Boulevard Station. An intermediate station, located on te sout side of C-470 east of Santa Fe Drive, is also being assessed as part of te Environmental Evaluation, but is not funded as part of FasTracks. Te Soutwest Corridor Extension alignment (Alignment) begins just sout of te existing Mineral Ligt Rail Station at te intersection of West Mineral Avenue and Santa Fe Drive (United States Route 85) and will continue sout virtually parallel to Santa Fe Drive and te existing BNSF Railway Company (BNSF) alignment. Approximately 0.5 mile nort of C-470, te alignment transitions to te east of te BNSF alignment. Once on te sout side of C-470, te Alignment continues east, running parallel to C-470, to Lucent Boulevard. Te alignment ends approximately 0.3 mile from te start of te C-470 off-ramp to Lucent Boulevard (Figure 1). Tere are several waterways tat are located witin te study area, including Jackass Gulc, Dad Clark Gulc, City Ditc, te Higline Canal, and an unnamed tributary to McClellan Reservoir. Te Sout Platte River is located approximately 0.4 mile to te west of te Alignment. Te 100-year floodplain only traverses troug te study area in two locations, at Jackass Gulc and Dad Clark Gulc (Figure 2). Te 100-year floodplain witin te study area creates an envelope tat varies between 80 and 800 feet. Tere are 5 bridges tat cross te floodplain witin te study area (Table 1). TABLE 1: EXISTING BRIDGES WITHIN THE STUDY AREA Name Location of Bridge in Study Area Bridge Use BNSF/Union Pacific Railroad Company (UPRR) Rail Bridge Regional Transportation District (RTD) Ligt Rail Bridge BNSF Rail Bridge UPRR Rail Bridge Santa Fe Drive Nort & Sout Bound Bridge Source: Soutwest Corridor Team, 2008 Continuation of Railroad Tracks over West Mineral Avenue Continuation of RTD Ligt Rail Tracks over West Mineral Avenue Continuation of BNSF Railroad Tracks over Dad Clark Gulc Continuation of UPRR Railroad Tracks over Dad Clark Gulc Continuation of Santa Fe Drive over Dad Clark Gulc 2 Track Railroad Bridge 1 Track Ligt Rail Bridge 1 Track Railroad Bridge 1 Track Railroad Bridge 4 Lane veicle travel way wit median and soulders Piers in 100- year Floodplain

6 C D Aspen Grove Sopping Center Soutbridge Sout P la tt e Ri v SD 57' UN Q 5 LG er H LITTLETON -DFNDVV +LOO 3DUN 0,1(5$/ k Jac ass H ill Rd Well Locations Domestic u Soutpark W Fe Dr Monitoring Can al 6RXWK 3ODWWH 3DUN G lc Ci ty ck as s Unknown e Lin Di tc Ja Ave g Hi Mine ra l 6RXWKEULGJH 3DUN Permit Number n Lo Sa r Ci nt a g rk G d C la Low er D a u lc :ULWHUV 9LVWD 3DUN Wolurst Sou tp a rk Cir 85 McLellan Reservoir County Line Rd Eri ck so n B lv d $5$3$+2 ( &2 '28*/$6 &2 &LW \ RI /LWWOH WR Q c Lu en $ t 1RW SDUW RI )DV7UDFNV 3ODQ Line Can al Pl az a Dr & /8&(17 Rd Hig 6) Wind Crest % vd Bl,17(50(',$7( 67$7,21 Mill Vista Creeksi de +LJKODQGV 5DQFK *ROI &OXE sb en g Dr Wa y e Gr ou or HIGH LANDS RAN CH )HHW Source: Urban Drainage & Flood Control District, 2009; Soutwest Corridor Extension Project Team, 2009 Stations Existing Proposed Soutwest Ligt Rail Line 100-Year Floodplain Soutwest Ligt Rail Extension Study Area Freigt Railroad County Boundary City Boundary Future )LJXUH 6WUHDPV )ORRGSODLQV DQG :HOO /RFDWLRQV

7 Hig water mark and flood istory data is not readily available for tese locations due, to te fact tat bot te Jackass Gulc and Dad Clark Gulc are minor tributaries to te Sout Platte River. Te largest flood along te Sout Platte River occurred on June 16, 1965, wen a stream gauge in downtown Denver recorded te Sout Platte River wit a flowrate of 40,300 cubic feet per second (cfs) and a peak flood stage of 15.0 feet (Perry, 2005). A flood stage is defined as te eigt of water outside te existing stream banks. Tis flood was te one of te most devastating floods in te Denver metropolitan area and spurred legislature to build more dams along te Sout Platte River to elp control te flooding. Te 1965 flood was caused by tree days of intense rains, wit some areas receiving upwards of 14 inces of rain (Sprengelmeyer, 1999). Following tat event te Catfield dam was constructed upstream of te project area to mitigate future flood events. Te most recent notable ig water mark occurred on te Sout Platte River on June 4, A flowrate on te Sout Platte was measured to be 9,710 cfs wit a peak flood stage of 7.2 feet at te United States Geological Survey (USGS) Stream Gauge Station on te Sout Platte River at Union Avenue (Perry, 2005). A variety of land uses are located witin te study area, including agricultural, residential, industrial and commercial. Tere are a few existing buildings located witin te floodplain in te study area; owever, West Mineral Avenue would become inundated by te 100-year flood. On te west side of Santa Fe Boulevard to te Sout Platte River, te land is mostly vacant, and is eiter agricultural or low lying floodplain open space. Te Alignment will cross te 100-year floodplain in two separate areas were it runs parallel to Santa Fe Drive. Te first location is at te intersection of West Mineral Avenue and Santa Fe Drive. Te existing Mineral Ligt Rail Platform was built outside of te floodplain to matc te existing grade of te BNSF and Union Pacific Rail Road (UPRR) tracks tat run nort/sout. Te existing railroad and ligt rail tracks cross over Mineral Avenue via a bridge. Te build-up of tis portion allows te existing railroad to be above te 100-year floodplain. Te floodplain in tis portion of te proposed alignment is caused by a localized low area along West Mineral Avenue tat transitions to Jackass Gulc and extends approximately 1.3 miles to te east. Jackass Gulc continues eastward to te intersection between West Mineral Avenue and West Long Avenue, and eventually across te Higline Canal. At te location of te intersection between West Mineral Avenue and West Long Avenue, Jackass Gulc receives stormwater discarge from an existing 54-inc reinforced concrete pipe (RCP) equipped wit an energy dissipater, connected to te West Mineral Avenue storm sewer. As te ditc proceeds downstream to te Sout Platte River, te ultimate outfall of Jackass Gulc, te ditc crosses troug multiple parcels witin te Freewood Subdivision in Littleton, and is ten piped underneat te BNSF-UPRR tracks and Santa Fe Drive via a 60-inc RCP along West Mineral Avenue. At te proposed Alignment crossing, te 100-year floodplain is approximately 5,382.5 feet above mean sea level (msl), wic is approximately 16 feet below te existing single track ligt rail bridge. At a minimum, wen a second ligt rail track is added to te crossing over West Mineral Avenue, it will mimic te existing ligt rail bridge. Te flowrate in Jackass Gulc, during te 100-year event in te area of te existing ligt rail bridge over West Mineral Avenue, is 1,240 cfs. Te approximate drainage area of Jackass Gulc is 500 acres (Centennial, 1990). Te drainage area associated wit te proposed Alignment ligt rail bridge structure would be sligtly less tan 500 acres, due to te Alignment bridge structure lying 0.4 mile upstream from were Jackass Gulc outfalls into te Sout Platte River. 5

8 Te proposed alignment would also cross te 100-year floodplain wen it passes over Lower Dad Clark Gulc. Tis portion of Dad Clark Gulc is located at te outfall point of McClellan Reservoir. Once water is discarged from te reservoir back into Lower Dad Gulc Clark, it passes underneat te existing UPRR, BNSF and Santa Fe Drive bridges. Dad Clark Gulc ultimately outfalls to te Sout Platte River, wic is located approximately 1.3 miles downstream from McClellan Reservoir. Dad Clark Gulc proceeds underneat te City Ditc, wic travels nort-sout on te western side of Santa Fe Drive. To avoid an intersection of waters, te City Ditc is conveyed via an aqueduct structure across te existing stream banks of Lower Dad Clark Gulc. It sould be noted tat tis portion of Lower Dad Clark Gulc is different from te portion of Upper Dad Clark Gulc tat feeds te McClellan Reservoir, wic crosses C-470. Te existing UPRR bridge would be removed and replaced wit a ligt rail bridge. Te UPRR bridge and railroad tracks would be moved approximately 75 feet to te west of te existing BNSF railroad bridge. At a minimum, te proposed ligt rail bridge and te proposed new UPRR bridge would mimic te existing UPRR bridge. Te 100-year floodplain elevation is 5,378.6 feet above msl at te proposed alignment crossing. Te water level of te 100-year flood level is approximately 12 feet below te existing UPRR bridge. Te flowrate in tis portion of Dad Clark Gulc during te 100-year event, in te area of te proposed alignment bridge, would be approximately 1,780 cfs. At te location of te proposed re-alignment of te UPRR bridge, te 100-year floodplain elevation is approximately 5,380.5 feet above msl. Te flowrate at tis point is approximately 1,780 cfs. Te existing BNSF bridge tat will lie approximately 70 feet to te east of te proposed UPRR bridge is approximately 19 feet above te 100-year flood waters (Centennial, 1990). Te above information does not take into account McClellan Reservoir flood storage capacity, and can terefore be looked at as a worst-case scenario. Flood storage can be defined as te water olding or containing capabilities of a reservoir. By not including te flood storage in te calculations, te flowrate and te 100-year floodplain elevation are increased. Te total drainage area to te McClellan Reservoir is approximately 7.5 square miles via te Upper Dad Clark Gulc Tributary (Raub, 1981). Tis would most closely resemble te total drainage area to te proposed Alignment bridge structure; owever, te total drainage area of Lower Dad Clark Gulc, witout taking into account te total drainage area to te McClellan Reservoir (primarily te Upper Dad Clark Gulc tributary), is approximately 290 acres (approximately 0.5 square mile) in size were it outfalls to te Sout Platte River (Centennial, 1990). Te C-470/Lucent Boulevard and Intermediate stations would not be located witin 300 feet of te 100-year floodplain. It sould be noted tat te C-470/Lucent Boulevard Station would be developed along an unnamed tributary to McClellan Reservoir. Te existing open area allows cannel flow from tis portion to te RCP culvert underneat C-470. As te proposed Alignment proceeds sout, and crosses to te sout side of C-470, te Alignment would not be located witin 300 feet of te 100-year floodplain. Te proposed Alignment passes over te Higline Canal as well as an unnamed tributary to McClellan Reservoir; owever, neiter of tese features is surrounded by te 100-year floodplain. Te unnamed tributary to McClellan is connected to te storm-sewer underneat C-470 were te proposed Alignment crosses it. Encroacment of any of te floodplains along te Alignment study area would be subject to te requirements of local jurisdictions. During construction activities, Best Management Practices 6

9 (BMPs) for erosion and runoff control would be implemented. Detention systems would allow for te storage and control of stormwater runoff. Requirements for onsite detention facilities are provided in Table 2, by jurisdiction. TABLE 2: JURISDICTIONAL REQUIREMENTS FOR ON-SITE DETENTION Jurisdiction Minor Flood Levels Major Freeboard Requirements Detention Sizing Metods Underground Detention Above Ground Detention Urban Drainage and Flood Control District (UDFCD) Arapaoe County Douglas County 5- and 10-year plus water quality capture volume (WQCV) WQCV and Excess Urban Runoff Volume (EURV) WQCV and Excess Urban Runoff Volume (EURV) 100-year plus no less tan one-alf WQCV 100-year plus te WQCV 100-year plus te WQCV 1 foot above te water surface elevation wen te emergency spillway is conveying te maximum design or emergency flow Sources: UDFCD, 2007; Arapaoe County, 2007; and Douglas County, 2008b. Simplified metod based on empirical equations in storage capter of District Manual Only in ultraurban setting were no onsurface metods are practicable Proibited Preferred 7

10 3.0 IMPACT EVALUATION AND MITIGATION 3.1 METHODOLOGY Several streams are located witin te Soutwest Corridor Extension study area. A baseline map of te floodplains associated wit tese water features was developed from FEMA Flood Insurance Rate Maps (FIRMs). Drainage Master Plans and Flood Hazard Area Delineations (FHADs) were obtained from te UDFCD, wen available, and incorporated into te floodplain maps for te Soutwest Corridor Extension study area, and te 100-year floodplain event was mapped. Floodplain impacts ave been categorized as direct, indirect, temporary construction, and cumulative impacts. Direct impacts are defined as tose effects caused by te action and occurring at te same time and place (40 CFR ). Tese impacts would alter or eliminate te original floodplain and can include acquisition of a floodplain for development. Te study area for direct impacts is defined by te limits of te construction footprint and te limits of te permanent ROW required for te project witin te 100-year floodplain.. Te No Action Alternative would not result in any direct, indirect, construction-related, or cumulative impacts to floodplains and drainage/ydrology. Te direct, indirect, construction-related, and cumulative impacts of te Build Alternative are described below. 3.2 IMPACTS Direct Impacts Only two FEMA- and Urban Drainage and Flood Control District (UDFCD)-designated 100-year floodplains are located witin te study area: Jackass Gulc and Lower Dad Clark Gulc. Te Build Alternative would cross bot of tem. Te Lucent/C-470 Station and te Intermediate Station and associated park-n-rides are not located witin tese floodplains; owever tese facilities would introduce new impervious surfaces. Portions of te track/guideway and traction power substations would also introduce small amounts of impervious surface. Historic drainage patterns will be maintained across te tracks. Impacts to 100-year floodplains include: Jackass Gulc: Tree new piers would be placed in te 100-year floodplain. No effects to te 100-year water surface elevation due to teir minimal cross section and because te Gulc already flows troug a 60-inc RCP in tis area. Lower Dad Clark Gulc: Six new piers (tree for eac bridge) would be placed in te 100-year floodplain. Given teir minimal cross-section, tey would ave a negligible impediment on te 100-year floodplain. Te Build Alternative would also affect existing and proposed detention ponds along Santa Fe and C-470. Impacts to tese ponds and te need for additional ponds will be determined in 8

11 more detail as te engineering design is completed. Possible modifications could include resaping, enlarging, and relocation Indirect Impacts Indirect effects to floodplains would result from additional impervious surfaces introduced by transitoriented development around stations. Impacts due to suc development would be avoided troug aderence to local development requirements Construction Impacts Te development of te Build Alternative witin te study area would cause some temporary construction impacts witin te 100-year floodplain; owever te use of temporary construction BMPs would minimize water quality impacts Cumulative Impacts Only minor cumulative impacts are anticipated for tis resource. 3.3 MITIGATION Mitigation measures include adering to all UDFCD and local jurisdiction requirements. Some requirements include completing detention and water quality treatment in accordance wit tese requirements, and to continuously implement BMPs. RTD will coordinate floodplain management wit UDFCD and local jurisdictions, and any new development will also be subject to te requirements of te local jurisdictions. RTD will coordinate wit Denver Water for construction activity restrictions in proximity to te Hig Line Canal wen water flows between April and November. RTD as adopted a sustainability policy tat includes objectives for environmental sustainability, one of wic is to enance water quality and lower water use. Accomplisment of tis objective is recommended troug developing and adopting best practices for sustainable design, construction, operations, and maintenance. Specifically, RTD encourages evaluating te incorporation of LEED practices and urban drainage and flood control district BMPs in transit centers, park-n-rides, selters, and corridor projects. Improving stormwater quantity and quality sould be accomplised using locally accepted/encouraged BMPs for protection of water quality (RTD, 2007). 9

12 4.0 REFERENCES Arapaoe, Arapaoe County Stormwater Management Manual. Arapaoe County Public Works and Development, Centennial, Colorado, adopted January 30, Centennial, Flood Hazard Area Delineation, Lower Dad Clark Gulc and DFA Prepared for Urban Drainage and Flood Control District, Centennial Engineering Inc., Centennial, Colorado, November Douglas, Douglas County Storm Drainage Design and Tecnical Criteria Manual. Douglas County Department of Public Works Engineering Division, Castle Rock, Colorado, adopted January 1986, amended July Perry, 2005, Summary of Significant Floods in te United States and Puerto Rico, 1994 troug 1998 water year. United States Geological Survey Scientific Investigation Report , Prepared for te United States Geological Survey, C.A. Perry, Reston, Virginia, Raub, 1981, Flood Hazard Area Delineation, Dad Clark Gulc. Prepared for Urban Drainage and Flood Control District, Jack G. Raub Company, Englewood, Colorado, May Sprengelmeyer, 1999, 1965 Flood left Deep Scars along Sout Platte article in Rocky Mountain News, M.E. Sprengelmeyer, December 14, UDFCD, Urban Storm Drainage Criteria Manual Volume 2. Urban Drainage and Flood Control District, Denver, Colorado, January [June Revision]. 10