8/11/2016 Mosquito Creek 2D hydraulic analysis: impacts to Tie-Back Levee and application of floodplain policy Andy McCoy, PHD, PE - HDR Dave Claman, PE - Iowa DOT Rusty Jones, PE - HDR

Does not meet current interstate design standards Program Need Does not meet current interstate design standards System does not accommodate traffic needs

Iowa DOT’s Largest Highway Funding effort

Project overview Program Progress

Mosquito Creek Section 408 Levee Sponsor Issues Increased Frequency of Levee Overtopping Flow Concentration between Proposed RR and Levee Levee Sponsor Expresses Concerns Regarding Changes to Flood Prone Areas (ZONE AH) Solution – TUFLOW Classic 1D/2D Hydraulic Model

Background: Hydraulic features and levee components Mosquito Creek Tie-Back Levee Reduces Risk to Council Bluffs from Missouri River flooding. Also reduces risk to area from Mosquito Creek flooding

Background: Project components

Background: Field photographs and terrain

2d model setup: grid resolution (TUFLOW CLASSIC) 15 ft grid cells plus break lines to force berms and roadways to act as hydraulic controls along their crests Tuflow samples terrain at the midpoints plus vertices plus the center – information every 7.5 ft

2D model setup: Existing and proposed conditions

2D Model Setup: Proposed – Existing conditions

2D Model setup: Upstream boundary hydrographs Upstream flow hydrograph came from effective hydrology. Peaks were adjusted to line up with the effective peak flow rates down mosquito creek. 28 hour hydrograph but peak happens at hour 8 10 –year through 100 year

2D model results: 100-year water surface elevations

2D model results: 100-year velocity magnitudes

2D Model Results: increased velocities along levee toe?

2D Model Results: increased potential for levee overtopping?

2D Model Results: increased potential for levee overtopping?

2D Model Results: does project require a map revision?

2D Model Results: does project require a map revision?

2D Model Results: does project require a map revision? Iowa DNR allowed development without Map Revision No fill in floodway Zone AH associated with shallow ponding. Fill in area allowed to cause 1-ft of impact above Base Flood Elevation – calculation shows that CBIS improvements cause less than 0.2-ft A few places received water that hadn’t before during the 100-year event due to removal of roadway/rail embankments already in Zone AH

conclusions Increased Frequency of Levee Overtopping - None Flow Concentration between Proposed RR and Levee causing potential erosion? Increase in velocities along west toe of levee investigated but were insignificant. East required riprap. Levee Sponsor Expresses Concerns Regarding Changes to Flood Prone Areas (ZONE AH) – Interesting discussion with Iowa DNR and Levee Sponsor – Map Revision not required, but information should be used during next revision Solution – TUFLOW Classic 1D/2D Hydraulic Model – Couldn’t have been done without 2D model

Thank you. Questions? Dave Claman, Iowa DOT, Ames, Iowa david.claman@dot.iowa.gov Andy McCoy, HDR, Des Moines, Iowa andrew.mccoy@hdrinc.com Thank you.