1. 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

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

4. Iowa DOT’s Largest Highway Funding effort

5. Project overview
Program Progress

6. 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

7. 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

8. Background: Project components

9. Background: Field photographs and terrain

10. 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

11. 2D model setup: Existing and proposed conditions

12. 2D Model Setup: Proposed – Existing conditions

13. 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

14. 2D model results: 100-year water surface elevations

15. 2D model results: 100-year velocity magnitudes

16. 2D Model Results: increased velocities along levee toe?

17. 2D Model Results: increased potential for levee overtopping?

18. 2D Model Results: increased potential for levee overtopping?

19. 2D Model Results: does project require a map revision?

20. 2D Model Results: does project require a map revision?

21. 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

22. 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

23. Thank you. Questions? Dave Claman,
Iowa DOT, Ames, Iowa
Andy McCoy,
HDR, Des Moines, Iowa
Thank you.