Noens, Leonard A.

English

Purdue E-Pubs

89th Annual Purdue Road SchoolDrainage, Drainage, Drainage— or —“A Primer on Base Flood Elevations and their Determination”Presented By:Leonard Noens, P.E.Christopher B. Burke Engineering, Ltd.National City Center, Suite 1368 South115 W. Washington StreetIndianapolis, Indiana 46204March 26, 20032GoalsOverview of BFE DeterminationKeep Things SimpleBe “Un-Boring”Presentation OutlineBasic DefinitionsWhy New MappingWhat Happens with New Mapping4Presentation OutlineFEMA Zone A AreasA “Minor Site Assessment”Detailed Hydraulic Analysis5Presentation OutlineExamplesSummaryQuestions…6Some Basic Definitions7Basic DefinitionsFlood - Temporary condition of partial or complete inundation of normally dry areas from overflow of inland or tidal waters, rapid accumulation or runoff of surface waters from any source8Basic DefinitionsSFHA - Special Flood Hazard AreaAreas subject to a 1% or greater annual chance of flooding in a given yearIncludes Zones A, AE, AO, and V500-year floodplain is NOT Special Flood Hazard Area9Special Flood Hazard Area10Basic DefinitionsBFE - Base Flood Elevation. The flooding elevation caused by the base flood which is the flood that has a 1% probability of being equaled or exceeded in any given year (also referred to as the 100-year flood or the 1% annual chance flood).11Basic DefinitionsFIRM - Flood Insurance Rate MapDFIRM – Digital Flood Insurance Rate MapFBFM - Flood Boundary Floodway MapFIS - Flood Insurance Study (includes FIRMs, FBFMs, and report with flood profiles)FDT - Floodway Data Table12Flood Insurance Rate Map (FIRM)13Flood Insurance Study (FIS)14Summary of Discharges Table15Floodway Data Table16Flood Insurance Study (FIS) Profile17Basic DefinitionsStructure -For floodplain management, walled and roofed building, including a gas or liquid storage tank, that is principally above ground (includes manufactured homes)For insurance purposes, walled and roofed building, other than a gas or liquid storage tank, that is principally above ground and affixed to permanent site (includes manufactured homes on permanent foundations)18Structure and Flood19Basic DefinitionsFloodway – The critical area of the SFHA that must be preserved to allow passage of floods.  Hydraulically, this is the area of most conveyance in the channel. The floodway is defined such that if fill is placed on both banks of the floodplain to the limits of the floodway, the new water surface elevation will be higher than the BFE without any fill placed by the amount of the Floodway Surcharge.20Basic DefinitionsFloodway surcharge – The difference in the elevation of the base flood and the floodway water surface elevation.  In Indiana, this value is 0.1 feet.Encroachment – Any fill, structures, or other obstructions that infringe upon the floodway or floodplain and cause an increase in the BFE.21100 year flood stageFloodway Surcharge100 year floodway stageFloodplainFillFloodplainFillFEMA Floodway Modeling22Basic DefinitionsFlood fringe – The area of the SFHA between the floodway and the 100-year flooding limits.  When fill is placed in the flood fringe, this results in an increase in the BFE.23Floodway LimitsFlood Fringe LimitsFloodway versus Flood Fringe24Why New Mapping?25Why New Mapping?Newly Studied AreasPhysical ChangesImproved Methodology/Data26What Happens with New Mapping?27What Happens with New Maps?Local regulatory agencies adoptFlood insurance requirements are based on the new mapsPermit requirements are based on the new maps28FEMA Zone A Areas29FEMA Zone A AreasWhat are they?Why are they important?What to do about them?30Floodway LimitsFlood Fringe LimitsFloodway versus Flood Fringe31A “Minor Site Assessment”32New IDNR Procedures• IDNR will calculate the BFE and floodway limits only under these conditions:• Single residence and/or out building• Single residential lot• No other work proposed in the floodway33New IDNR Procedures• Consultants now responsible for flood elevation recommendations – IDNR only reviews• Implementation of ‘Two Strikes’ policy34New IDNR Procedures• Consultants’ flood modeling submittals will  be held to higher standards• IDNR no longer participates in project specific flood model development – serve only as reviewers35Process for Minor Site Assessment• Use best-available mapping or survey data• Typically, one cross-section is all that’s required• Bridges/culverts require more data36Process for Minor Site Assessment• Calculate 100-year peak discharge• Estimate required values to calculate approximate flow depth/BFE• With Minor Site Assessment, IDNR considers all land below the calculated BFE as floodway37Detailed Hydraulic Analysis38Determining the Study ReachDownstreamStudy ReachThalwegStudy ReachUpstreamPropertyLimitAProject LocationSlopeLL=150 HD 0.8/SEstimated 100 YearFlood ElevationAreaTop WidthHD =Area/Top WidthA39Detailed Hydraulic Analysis1. Effective Model2. Duplicate Effective Model3. Corrected Effective Model (Base Model)4. Existing or Pre-Project Condition Model (Cumulative Effect Model)5. Proposed or Post-Project Condition Model40Detailed Hydraulic AnalysisWhy do modeling?• Flood Insurance Study• Revisions to Flood Insurance Study (LOMR)• Special Studies• Permits• FARA – Floodplain Analysis Regulatory Assessment41Detailed Hydraulic AnalysisA. Effective Model1) Published Flood Insurance Study2) IDNR approved detailed hydraulic study• IDNR Flood Insurance Study for areas not covered by FEMA mapping• Detailed models on file with IDNR - Division of Water used to evaluate permits• May be used for Regulatory Purposes (Permits)• Should not assume FEMA acceptance42Detailed Hydraulic AnalysisD. Existing or Pre-Project Condition Model1) Physical changes • Reflects any man-made physical changes (structure, fills, encroachments, etc.) that occurred or are expected to occur in the near future (i.e., recently permitted projects) within the floodplain since the date of the Effective Model.  It does not contain the proposed project; this model serves as the basis for evaluating cumulative effects.43SummaryDefinitionsNew MappingSimplified and Detailed Analyses44Examples!!!45Questions & Answers

Drainage, Drainage, Drainage

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1822&context=roadschool

Drainage, Drainage, Drainage

Abstract

Similar works

Full text

Available Versions

Purdue E-Pubs