Swannanoa River Flood Risk Management Study

Transcription

1 Swannanoa River Flood Risk Management Study Measures Evaluated to Reduce Future Flood Damages City of Asheville U.S. Army Corps of Engineers

2 Flooding History Part of the 132 square mile Swannanoa River watershed lies within the Asheville city limits. The river has a history of flooding and significant related damages. In 2004, back-to-back hurricane remnants hit the area especially hard, with the greatest damages occurring in Biltmore Village, a historical area just outside the Biltmore Estate, in the City of Asheville. The Corps estimated that flood damages in the watershed in 2004, to commercial businesses alone, were $25 million

3 Study Purpose Flood Risk Management Study was initiated after the 2004 hurricanes in partnership with the State of North Carolina, the City of Asheville, local contractors, and the US Army Corps of Engineers. Intent is to develop and evaluate alternatives to reduce flood-related damages and the risk of loss of life in the Swannanoa River watershed. With public and stakeholder input, prepare a Detailed Project Report and Environmental Assessment that recommends a plan to reduce flood risk. 3

4 Flood Risk Management Measures Identified 4

5 Structural Measures Evaluated Biltmore Avenue Bridge Channel Modification Removing historic fill upstream and downstream of the bridge. Allows greater flow capacity, reducing overtopping flood events. Looking Downstream Sweeten Creek Detention Reconfiguring existing culverts through railroad embankment or constructing a new structure upstream of railroad. Would detain floodwater upstream of existing embankment thereby reducing flows downstream. Lake Craig Dam Rehabilitation or Replacement Modify outlet structure to efficiently reduce flooding downstream while passing base flow (peak flow control structure). 5

6 Structural Measures Evaluated - Cont I-40 Construction Disposal Area Removal Removes constriction on Swannanoa River. Also provides additional storage capacity for Lake Craig Dam. Peak Flow Control Structure at Warren Wilson River Bend Field Passes normal flow but detains floodwater upstream of dry dam, reducing flows downstream. The recommended site is the only location in the basin with capacity (largest amount of available storage area) and is located above damage centers where rainfall can be captured to temporarily slow flood water. 6

7 Structural Measures Evaluated - Cont Bee Tree Dam Operations Modifications Modifying dam and/or dam operations to provide temporary flood control storage before a large storm event. Install pump station to provide ability to draw down reservoir in anticipation of imminent heavy rains. Pump Station Black Mountain Levee/Floodwall Constructing levees and/or floodwalls along the confluence of Flat Creek and the Swannanoa River to reduce flood damages and life-safety concerns in Black Mountain. Levee and/or floodwalls

8 Non-Structural Solutions Evaluated Elevation Raising structures in place using fill, walls, piers, posts, or columns Relocation Moving structures to locations away from flood hazards Buyout/Acquisition Purchasing and eliminating flood prone structures Local Berms or Floodwalls Dry Flood Proofing Sealing building walls with waterproofing materials to prevent the entry of floodwaters into damageable structures Wet Flood Proofing Allows floodwater to enter structure but vulnerable items such as furnaces are located and moved to higher locations Flood Warning & Preparedness Alert inhabitants in flood prone areas of impending high water 8

9 Combine Measures into Alternatives Measures were eliminated because costs > benefits Sweeten Creek Detention Structure I-40 Disposal Area/Beneficial landfill removal as standalone measure Will be combined with Lake Craig Dam as mitigation instead of purchasing upstream real estate Bee Tree Dam Operations Modification Concerns over water quality for water treatment and storm warning time Black Mountain Levee/Floodwall High induced damages Five measures were combined to create 30 alternatives Biltmore Avenue Bridge Channel Modification Non-Structural Lake Craig Dam Rehab Peak Flow Control Structure I-40 Construction Landfill Removal 9

10 Results of Cost Benefit Analysis Alt # Alt 6 Swannanoa & Tributaries Economic Analysis Dollars = FY 2014, 3.50% Annual Description of Measure or Alternative Benefits Total Cost BCR Net Benefits Biltmore Ave. Bridge Channel Modification, Lake Craig Dam Rehabilitation, Peak Flow Control Structure 2,857,890 21,528, ,940,072 Alt 5 Lake Craig Dam Rehabilitation, Peak Flow Control Structure 2,366,190 20,493, ,492,497 Alt 3 Biltmore Ave. Bridge Channel Modification, Peak Flow Control Structure 1,677,920 11,657, ,180,939 Alt 14 Biltmore Ave. Bridge Channel Modification, Buyout 10 Justifiable Structures, selected by BCR 1,331,867 7,663, ,063 M1 Biltmore Ave. Bridge Channel Modification 583, , ,537 There are five cost effective alternatives ranging from a local solution to a basin-wide alternative with three project sites. Alternative 6 consists of a channel modification near Biltmore Avenue Bridge, rehabilitating Lake Craig Dam so that it functions as a peak flow control structure, and construction of a new peak flow control structure at upstream of Warren Wilson College Road. Alternative 6 is estimated to cost $22M, has a 3.11 benefit-cost-ratio, and reduces almost half of the damages in the basin. 10

11 Alt 6: Benefits of Project Significant flood reductions throughout the Swannanoa River Up to 3.5 reduction in flooding in historic Biltmore Village Area Protects communities downstream of Warren Wilson to the mouth of Swannanoa Reduces average annual damages in the basin by 34% Lessens environmental degradation brought by extreme flooding including impacts to cultural resources. 11

12 Peak Flow Control Structure River Bend Location 12

13 Peak Flow Control Structure Artist Rendering To Scale 13

14 Peak Flow Control Structure Artist Rendering To Scale The opening is 24 feet wide by 12 feet tall, large enough for the river and associated species to pass through unobstructed. 14

15 Peak Flow Control Structure Inundation 2 Year Event

16 Peak Flow Control Structure Inundation 5 Year Event

17 Peak Flow Control Structure Inundation 10 Year Event

18 Peak Flow Control Structure Inundation 25 Year Event

19 Peak Flow Control Structure Inundation 100 Year Event

20 Peak Flow Control Structure Depth Comparison 2 Year Event Flooding Current 0 Hrs Add l Depth With Project 0 Hrs 3.6

21 Peak Flow Control Structure Depth Comparison 5 Year Event Flooding Current 3 Hrs Add l Depth With Project 3 Hrs 0.7 Flooding Current 0 Hrs Add l Depth With Project 5.25 Hrs 7.5

22 Peak Flow Control Structure Depth Comparison 25 Year Event Flooding Current 3.5 Hrs Flooding Current Add l Depth With Project Hrs Hrs Add l Depth With Project 10 Hrs 4.1 Flooding Current Hrs Add l Depth With Project Hrs 1.1 Warren Wilson College Road Flooding Current With Project 0.75 Hrs 1.5 Hrs

23 Peak Flow Control Structure Depth Comparison 100 Year Event Flooding Current Flooding Current Hrs Add l Depth With Project Hrs Hrs Add l Depth With Project Hrs 19.5 Flooding Current 25 Hrs Add l Depth With Project 25 Hrs 6.2 Warren Wilson College Road Flooding Current With Project 4.75 Hrs 5 Hrs

24 Emergency Access Proposed peak flow control structure will not change flood event that overtops Warren Wilson Road. Warren Wilson Road will be flooded starting at the 25-year flood event. Duration and depth of flooding is slightly increased. Flood Event Warren Wilson Road (Elev. = 2123) Current Conditions With Proposed Project Increased Max Flood Depth (ft) Duration Flooded (hr) Max Flood Depth (ft) Duration Flooded (hr) Flood Depth (ft) Duration (hr) year event flow decreased with proposed project from 9665 CFS to 9180 CFS Fire protection unchanged. Hospital access increase no more than 5 minutes. Flooding impacts to routes along Davidson Road, Riverwood Road, and Whitson Avenue are unchanged. 24

25 Cultural Resource Compliance and Considerations National Historic Preservation Act Identify and assess effects to historic properties archaeological sites, buildings, structures, objects, and landscapes All steps involve consultation with the State Historic Preservation Office, Tribes, Warren Wilson College, and other consulting parties such as historical societies. Outcomes may include recommendations for site management and preservation, recovering information, or public interpretation. Tribal Consultation Occurs at a Government-to-Government level USACE has Trust responsibility to consult with Tribes Consultation with Tribes is pre-decisional Eastern Band of Cherokee Indians, Catawba Nation, Cherokee Nation, United Keetoowah Band of Cherokee Indians 25

26 Environmental Compliance and Aesthetics Archeological investigations conducted during study and design and implementation phase Cultural Resource Management Plan developed includes presence and absence of sites within area of inundation and recommended management practices for continued preservation Educational signage could be added to the project at the request of the sponsor and land owner US Fish and Wildlife Service and other environmental organizations coordinated through Environmental Assessment Example Educational mural at Murfree Springs, TN 26

27 Real Estate Requirements 27

28 Summary In order to reduce flood risk in the Swannanoa River Watershed, a three part project is recommended: Biltmore Avenue Bridge channel modification, rehabilitation of Lake Craig to serve as a peak flow control structure, and construction of a peak flow control structure at Warren Wilson river bend. Results in significant flood reductions throughout the Swannanoa River Up to 3.5 reduction in flooding in historic Biltmore Village Area Reduces average annual damages in the basin by 34% Lessens environmental degradation brought by extreme flooding including impacts to cultural resources. 28

29 Summary Close coordination will occur throughout study, design, and implementation to ensure a mutually beneficial design for peak flow control structure site. Access road location Cultural resource survey results Trail functionality Operation and maintenance by City of Asheville 29

30 Summary Significant flood reductions are not possible in the community without Warren Wilson s involvement. Without the support of the College flood depths can only be reduced up to a maximum of a foot and benefits in the region will be cut in half. Proposed project would increase community partnerships to decrease flood related risk and damages in the Swannanoa River Watershed. 30

31 Feasibility Study Timeline Feasibility Study Design and Construction Select Recommended Plan Environmental Investigations And Design Reviews Report Approval Detailed Design Property Acquisition Construction Significant work remains before construction of a flooding solution can begin. Before additional money can be spent on investigations and engineering designs to complete the feasibility study, the viability of implementing a peak flow control structure downstream of Warren Wilson Road needs to be determined so a plan can be selected for recommendation. 31

32 Partnership Reducing flooding to protect the environment, reduce flood damages, and help the community can only be accomplished through partnerships with local, state, and federal offices. A letter of support from the Warren Wilson Board of Trustees would provide the city and agencies with the information it needs to move forward with the feasibility study with the goal of strengthening the greater Asheville and Buncombe County community. 32