HEC-RAS 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 http://www.fema.gov/plan/prevent/floodplain/publications.shtm http://www.fema.gov/plan/prevent/fhm/index.shtm Floodway and Encroachments HEC-RAS Floodway Encroachment Methods Special considerations at structures General modeling approach National Flood Insurance Program Requires Local Flood Plain Management Regulations Offers Federal Benefits to Participating Communities Federally-Related Mortgages in Flood Hazard Areas Federally-Subsidized Flood Insurance Federally-Backed Flood Disaster Relief Funds Federal Emergency Management Agency Administers National Flood Insurance Program 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 Studies 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 Application” Documented Demonstration of “Scientifically Incorrect” Results Categories of Map Revisions Flood Elevations Not Affected Changes in Topographic Data (Minor Fill) Flood Elevations Are Affected Channel Improvements Fill Projects Flow Rate Modifications (Diversion, Detention) Floodway Revisions Map Amendments Issued for Developed Property Only May Be Submitted By Property Owners/Leases 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 Analysis 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 options 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 profile 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 distance 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 conveyance Encroachment Method 4 Method 4 calculates encroachments using equal conveyance reduction to meet a specified increase in natural (unencroached) water surface elevation (WSE) 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 met only target EGL entered - program will meet target EGL 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 Encroachments 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 Tributary 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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