Talkeetna Airport, Phase II Hydrologic/ Hydraulic Assessment Incomplete Draft January 2003 Prepared For: Prepared By: URS Corporation 301 W. Northern Lights 3504 Industrial Ave., Suite 125 Blvd., Suite 601 Fairbanks, AK 99701 CH2M Hill Anchorage, AK 99503 (907) 374-0303 TABLE OF CONTENTS Section Title Page 1.0 Introduction..........................................................................................................................1 2.0 Background Data .................................................................................................................4 2.1 Airport Construction ................................................................................................4 2.2 Past Floodplain Delineations ...................................................................................6 2.3 Past Suggestions for Floodplain Mitigation.............................................................7 2.4 Historical Floods......................................................................................................8 3.0 Flood-Peak Frequency .......................................................................................................12 3.1 Talkeetna River at its Mouth..................................................................................12 3.2 Susitna River Above and Below the Talkeetna River ...........................................13 3.3 Summary................................................................................................................15 4.0 Flood Timing ....................................................................................................................16 4.1 Susitna River Discharge Based on Talkeetna River Discharge.............................16 4.2 Talkeetna River Discharge Based on Susitna River Discharge .............................17 4.3 Summary................................................................................................................18 5.0 100-Year Flood Water Surface ..........................................................................................20 5.1 Two-Dimensional Surface Water Model ...............................................................20 5.2 100-Year Design Flood..........................................................................................21 5.3 General Flood Conditions During the 100-Year Design Flood.............................22 5.4 Flooding at the Talkeetna Airport During the 100-Year Design Flood.................26 5.5 Flooding at Proposed Airport Improvements During the 100-Year Design Flood..............................................................................................28 5.5.1 New Commercial Apron............................................................................31 5.5.2 New Helicopter Landing Pad.....................................................................31 5.5.2.1 Helicopter Landing Pad on East Side of Southwest End of Runway..............................................................................31 5.5.2.2 Helicopter Landing Pad on East Side of Northeast End of Runway..............................................................................31 5.5.3 New Commercial Access Road .................................................................32 5.5.4 New Automated Surface Observation System...........................................32 Talkeetna Airport, Phase II i January 2003 Draft, Hydrologic/Hydraulic Assessment URS Corp. TABLE OF CONTENTS (Continued) Section Title Page 5.5.5 New Maintenance Access Road.................................................................32 5.5.6 General-Aviation Parking Apron...............................................................33 5.5.7 Development Between the Railroad Embankment and the Southwest End of the Runway...................................................................33 6.0 Flood Mitigation Alternatives............................................................................................34 6.1 Possible Mitigation Alternatives............................................................................34 6.1.1 No Action...................................................................................................34 6.1.2 Relocate the Commercial Apron to Dry Ground .......................................34 6.1.3 Protect the Airport with Dike and Increased Drainage Capacity at the Talkeetna River Bridge ....................................................................34 6.1.3.1 Construct Dike ...............................................................................36 6.1.3.1.1 Dike Option 1...............................................................37 6.1.3.1.2 Dike Option 2...............................................................40 6.1.3.1.2 Dike Option 3...............................................................41 6.1.3.1.4 Dike Option 4...............................................................44 6.1.3.2 Increase Drainage Capacity at the Talkeetna River Bridge ...........46 6.1.4 Construct Drainage Swale..........................................................................46 6.1.5 Divert Flow to Twister Creek ....................................................................48 6.2 Flood Mitigation Alternatives Selected for Further Analysis................................49 7.0 References..........................................................................................................................50 Talkeetna Airport, Phase II ii January 2003 Draft, Hydrologic/Hydraulic Assessment URS Corp. LIST OF TABLES Table Title Page 2.4.1 1986 Flood High Water Marks ..........................................................................................11 3.1.1 Flood-Peak Discharge at the Mouth of the Talkeetna River..............................................12 3.2.1 Flood-Peak Discharge on the Susitna River Above and Below the Talkeetna River........14 3.3.1 Most Probable 100-Year Flood-Peak Discharge................................................................15 4.3.1 Three Possible 100-Year Flood Scenarios .........................................................................19 5.1.1 Two-Dimensional Model Calibration Summary................................................................21 5.2.1 100-Year Water Surface Elevations Upstream of the Talkeetna River Bridge..................21 LIST OF FIGURES Figures Page 1.0.1 Project Location Map...........................................................................................................3 2.1.1 Talkeetna Airport Layout.....................................................................................................5 2.4.1 1986 Flood Observations ...................................................................................................10 5.3.1 Water Surface Elevation Contours – 100-Year Flood Model............................................23 5.3.2 Velocity Contours and Flow Vectors – 100-Year Flood Model........................................24 5.3.3 Water Depth Contours – 100-Year Flood Model ..............................................................25 5.5.1 Location of Proposed Airport Improvements ....................................................................30 6.1.1 Relocate the Commercial Apron to Dry Ground ...............................................................35 6.1.2 Dike Options 1 and 2 and Increased Capacity at the Talkeetna River Bridge...................39 6.1.3 Dike Option 3.....................................................................................................................43 6.1.4 Dike Option 4.....................................................................................................................45 6.1.5 Drainage Swale & Diversion Alternatives.........................................................................47 Talkeetna Airport, Phase II iii January 2003 Draft, Hydrologic/Hydraulic Assessment URS Corp. LIST OF APPENDICES Appendix A Background Data B Flood-Frequency Analysis C Expected Probability D Flood-Peak Timing E Two-Dimensional Surface-Water Model F Susitna River HEC-RAS Model Talkeetna Airport, Phase II iv January 2003 Draft, Hydrologic/Hydraulic Assessment URS Corp. 1.0 INTRODUCTION The Alaska Department of Transportation and Public Facilities (ADOT & PF) wishes to make improvements at the Talkeetna Airport (Figure 1.0.1). However, past studies indicate that some of the airport property may lie within the 100-year floodplain of the Talkeetna River (USACE 1972, and Legare 1996, 1997, 1999). This is a problem for three reasons. (1) The Federal Aviation Administration is reluctant to fund improvements that are at risk of being flooded by the 100-year flood (Cinelli 1999a). (2) Title 17.29.180 of the Matanuska-Susitna Borough Code of Ordinances, requires that no improvements be made within a floodway1 that will result in any increase in the water surface elevation of the base flood2 (Hudson 2003). Although a floodway has not been defined for the Talkeetna River, it is the practice` of the Matanuska-Susitna Borough to enforce Title 17.29.180 for all permitted new construction within the floodplain of the base flood (Hudson 2003). (3) Title 17.29.170 of the Matanuska-Susitna Borough Code of Ordinances requires that the lowest floor of a structure3, built within a flood-hazard area, be located at or above4 the base flood elevation, or be flood-proofed (Hudson 2002, Lee 2002). This report summarizes the results of the first phase of a hydrologic and hydraulic assessment of