Burnham

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Burnham Floodplain Study
October 23, 2009 Presented byMarty Spongberg,
PhD, PE, PGAMEC Geomatrix, Inc.
Prepared by
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Burnham Parcel
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Water Code Section 8610.5

• (c) The board shall, in any evidentiary hearing,
  consider all of the following, as applicable, for
  the purpose of taking any action pursuant to
  Section 8560
• .
• .
• (2) The best available science that relates to
  the scientific
• issues presented by the executive officer, legal
  counsel, the
• department, or other parties that raise credible
  scientific issues.

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Precedence for Modification to Floodway Maps
Kaweah River Map 1SA
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Precedence for Modification to Floodway Maps
Kaweah River Map 3SA
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Basis for Establishing River Stage

• CVFPB Staked locations reported to be high
  water
• marks
• FEMA Hydraulic modeling calibrated by reported
  high
• water marks

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Basis for Establishing Flow Rate

• CVFPB
• 80,000 cfs
• Basis reported high water, topography
• Return Interval - ?
• FEMA
• 73,000 cfs
• Basis USGS gaging station
• Return Interval 140-year

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Comparison of Designated Floodway and FEMA FIS
Maps
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HEC-RAS Study (Flow 73,000 cfs)
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HEC-RAS Study (Flow 80,000 cfs)
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CVFPB Issues

• The current request for modification of the
  floodway boundary will move the boundary within
  the actual river channel, thereby reducing any
  inherent factor of safety provided by a
  conservative estimate of the boundary location.
• (b) There is significant staff work associated
  with an approval of a designated floodway
  modification. Affected map sheets must be revised
  and renumbered, the revised maps sent to Tulare
  County recorders office as an official document,
  and the Boards records and website updated.

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CVFPB Issues (continued)
(c) Calculations by the consultant and Board
staff, show that the Island Building Pad would be
0.2 feet to 1.00 feet below the 100 year storm
(1 chance). In storms that exceed this
frequency, the Building Pad would be inundated.
This does not consider Wind Wave Run-up (1.0 feet
to 2.0 feet) nor margins of error within the
Hydrology calculations.
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CVFPB Issues (continued)
(d) There has been no discussion of the
likelihood of large boulders which are carried
through this river course which do damage and
present a clear danger to life and property.
The U.S. Army Corps of Engineers, Sacramento
District, California, prepared a Flood Plain
Information packet for the Kaweah River at Three
Rivers, California on October 1967. They state
that Kaweah River velocities can reach up to 20
feet per second and velocities would be
slightly higher on all streams and would be
extremely dangerous to life and property. On
page 3 of the above mentioned report, the U.S.
Army Corps of Engineers states Hazardous
Conditions would occur during large floods as a
result of rapidly rising streams, high velocities
and deep flows.
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• Cross Section 8

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CVFPB Issues (continued)
(e) If a residential structure were to be
allowed, it would create an in-stream blockage
which would decrease flow area and increase
velocities causing negative impacts both upstream
and downstream.
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Summary

• The current Designated Floodway Map is
  inaccurate, and should be modified to reflect
  detailed survey information and engineering
  studies.
• Safety Factor - Our request is to modify the map
  to reflect river stage for flows that are 10
  greater than the 140-year flow.