Lecture 8: Federal Flood Insurance Studies by S55x5R


      Floodway Analysis

         Professor Scott Kenner
South Dakota School of Mines & Technology
        Federal Flood Insurance Studies
 National Flood Insurance Program
       Flood Insurance Studies
       Dealing with Existing Flood Insurance Studies

 Floodway and Encroachments
       HEC-RAS Floodway Encroachment Methods
       Special considerations at structures
       General modeling approach
     National Flood Insurance Program

Requires Local Flood Plain Management
Offers Federal Benefits to Participating
  Federally-Related Mortgages in Flood

   Hazard Areas
  Federally-Subsidized Flood Insurance

  Federally-Backed Flood Disaster Relief

    Federal Emergency Management
Administers National Flood Insurance
Ten Regional Offices
Technical Evaluation Contractors
         Flood Insurance Studies
 Water Surface Profiles for Major Streams
    Base Flood Elevations: 100-Year
    Others: 10-Year, 50-Year, 500-Year

 Flood Insurance Rate Maps (FIRMs)
    Base Flood Plain: 100-Year

    Critical Action Flood Plain: 500-Year

    Floodway: Limit of Encroachment

 Flood Hazard Factors and Flood Hazard Zones
 Dealing With Existing Flood Insurance
Appeals: Challenges to Proposed BFEs:
 During 90-Day Period
Map Revisions: Changes to Effective Maps
 Due to Changed Conditions
Map Amendments: Changes to Effective
 Map Due to Inadequate Mapping
        Justification for Appeals

Certified Data Showing “Mathematical or
 Measurement Errors”
Certified Demonstration of “Error in
Documented Demonstration of “Scientifically
 Incorrect” Results
       Categories of Map Revisions
Flood Elevations Not Affected
  Changes in Topographic Data (Minor
Flood Elevations Are Affected
  Channel Improvements

  Fill Projects

  Flow Rate Modifications (Diversion,
Floodway Revisions
           Map Amendments

Issued for Developed Property Only
May Be Submitted By Property
Corrects Inadequate or Incorrect Mapping
       Review of Proposed Projects

 Conditional Letter of Map Revision
    Based on Preliminary Hydrologic and
     Hydraulic Analysis
    Subject to Review Fees

 Final Letter of Map Revision
    Based on Final Hydrologic and Hydraulic
    Must Include As-Built Construction Drawings

     and Other Data
    No Review Fee
        Floodway Basic Procedure
Calculate a natural profile as first profile in
 multiple profile run
second (and subsequent) profile(s) are
 calculated using various encroachment
Evaluate Differences in Profiles
   Surcharge must be 1.0 foot or less

   Surcharge must not be negative (i.e.

    cannot repot a decrease in WSEL)
                Encroachment Method 1
 User specifies the exact locations of the encroachment
  stations at each x-section
 encroachment stations can be specified differently for each
                    Encroachment Method 2

 Method 2 is based on a fixed top width
 top width can be specified at each x-section
 left and right encroachment stations are made equal distance
  from the center of channel, center of channel is center between
  left and right channel bank stations
 maximum encroachment is left and right channel banks
 offsets - user can specify a left and right offset which establishes
  a buffer zone outside the main channel, limiting the
  encroachments to the channel bank stations plus the offset
             Encroachment Method 3
 Encroachment stations are calculated based on a
  specified percent reduction in conveyance
  (% K reduction)
 the % reduction is split evenly on both sides of channel
 again, encroachments are limited to the channel bank
  stations or the specified offsets
 if half the reduction in conveyance is greater than the
  overbank area conveyance, then additional reduction
  will be attempted in the other overbank area
 if conveyance reduction is greater than both overbank
  areas, the encroachments are set to the channel banks or
  specified offsets
         Encroachment Method 3

 % reduction can be changed for any x-section
 default of equal conveyance can be changed to a
  proportional reduction based on natural overbank
           Encroachment Method 4
 Method 4 calculates encroachments using equal
  conveyance reduction to meet a specified increase
  in natural (unencroached) water surface elevation
 a target increase of 1.0 indicates a 1 foot rise will
  be used to determine the encroachments based on
  equal conveyance
 as for method 3, a proportional reduction in
  conveyance can be specified
            Encroachment Method 5
 Similar to Method 4 except an optimization scheme is
  used to obtain the target difference in WSE
 maximum of 20 iterations
 equal conveyance attempted
 input target WSE and EGL increase
 the procedure attempts to match the target WSE
  increase without exceeding EGL increase, if the
  program cannot then it will match EGL increase
            Encroachment Method 5
 no target EGL entered - continues until WSE target is
 only target EGL entered - program will meet target
 if neither WSE or EGL can be met after 20 trials,
  program takes best answer from all trials
 can be changed at any x-section
     Bridge, Culvert & Multiple Opening
 Default is to use the downstream computed
  encroachments through the structure and at x-section
  just upstream (x-section 3)
 Exceptions
    using Method 1 - user can enter specified

     encroachment stations for x-sections 2, BD, BU, & 3
    using Methods 2-5 - program allows for separate
     encroachment calculations at a bridge when using
     energy based computation method
    encroachments can be turned off at any bridge,
     culvert or multiple opening
       General Modeling Guidelines
 Methods 4 & 5 are normally used to get a first cut
   increase in WSE will often exceed the target at
    some locations
   encroachment widths can be highly variable
    between x-sections
   from initial results new estimates can be tried or

    specified using Method 1
 Objectives
   met or be less than target increase in WSE

   generally uniform floodway width
     Mapping Floodway Boundaries at Confluences
                                       Main Stream 100-
Tributary 100-YR                       YR Flood Limits
Flood Limits
                   Floodway Limits

                                      Floodway Limits
                                      Main Stream
1. Edit Steady Flow Data, adding a second profile.
Note that flows will be the same for each profile. The
difference will be the second is encroached.
Profile 1 - 100yr base
Profile 2 - 100yr encroached
Under Run, Steady
Flow Analysis, options,
encroachments, you
enter the encroachment
information. Here
method 4 is used with a
target water surface
increase of 1.0.

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