FLOOD04543

• County: Statewide
• Community: Harrison County, Iowa
• Basin: Statewide
• Title: California Bend Missouri River Fish and Wildlife Mitigation
• Date: 11/1/1994
• Prepared For: US Army Corps of Engineers
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Data ‎(general)‎

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33. Model Calibration - Model appears to have been calibrated by adjustioa starting energy
slope until the correct elevation was reached. How well did it malch upstream? This method
of calibration is wholly incorrect. You are calibrating to synthesized water surfaces, DOl real
elevations. lust the opposite is the correct way to calibrate. Model should stan furtbt.r
downstream and extend upsllCam pasl projcct. TIle overbank "n" of 0.040 is likely 100 low.
AI the least, the model should have been calibrated to the measure profile, with discharae of
33,100 cfs, and then checked to see how wen the CRP is reproduCeCl, DOt vice versa.

34. What is the Ildvantl8' of IeCtill8 the chute invert sJiahtly Iteeptt tbaD the aVerll~ MI5.UlUri
River slope? The energy slope through the chule wouJc1 be essentiaDy rile same if rile inlet and
outlet were both 5 feet below CRP, and it is eDC11Y, DOl cba1mel, slope that determined !be
probabilil)' of scouring.

3S. Floo<lplain ADAlysis. See previous comments OD HEe-' Mocrlins.

36. OutpUI for SIS0-40K.DAT - There Is a major change in conveyance from Section 649.5
to 649.6. This can cause problems in model. A better means of modeling !be cbute would be
10 forget the Missouri River x-sections and stan the model at the mouth of the chute. This
would involve addins a few sections at the outI!.'t that repment the vade structure aoolJ1ttty and
developing a ralinB curve al the outlet to use in determining !be appropriate starting water
surface in the chule.

37. While the simplification of assuming that the Missouri River energy grade is COnstanl for
a given split flow situatioD is 8C'.ceptabl!.' given the level (If unoertainty involved. it is not
accurate. The energy grade in the Missouri will decrease as flow is taken out for the chute.
II appears thai the energy grade U$ed was from Ibe Missouri River assuming DO flow \\'IS
divened. At the least, the MlSsoun Kiver model should be nm with the Ipproprlate flow taken
OUI between the inlet and outlet to ensure the energy grade is not significantly lower. A
decrease of just 0.1 ft would reduce the determined chute flow by at least 10-1S" allower
flows.

Appendix D. Design Data.

38. Criteria in EM 1lIO-2-1601useo.d M5 heen !rupe=ded 3 years 110. Verify the results usine
the updaled version.

39. The velocity used to design the riprap in the sill was based on the average left overtr.ml
velocity. Since the sill lies within the main overbank conveyance zone, the local velocity may
~ considt.rably hieber. De~rihe how this is accOWlled for in selected riprap size.

40. Use of EM 1110-1-1601 for sizing riprap over which critk:al or supcrcriticaI flow is
assumed to flow Is invalid. Use HDC 712-1 (I~lJa~h) to sUe in turbulent flow.

41. Frequency mixina shown is from Texas Department of Highways. Is this valid
hydrologiCally for Iowa?

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