Taking the Mystery out of Base Flood Elevation (BFE ...

1 April 30, 2009. Abigail Margelofsky Taking the Mystery out of base Flood Elevation (BFE) Calculations Initial Reconnaissance Determine the status of the community in the NFIP program Identify all past actions by FEMA or contractor that may affect the request. ( : past studies on site, upstream or downstream). Required Data Required data includes, but is not limited to: 1. MT-2 for (with municipality floodplain managers approval/signature). 2. Certification by a Registered Professional Engineer or Licensed Land Surveyor. 3. Topographic work map (includes site area, floodplain, BFEs, cross-section locations, etc.). 4. The effective map panels with the proposed floodplain overlaid 5. Adjacent property owner notification. 6. Hydrologic analysis Topography Requirements Existing Topographical Maps The greater level of detail on the map, the more accurate the BFE determination The minimum topographic survey should be as detailed as the US Geological Survey quadrangle maps, if better data is not available Field Surveys Must be on the same datum as the FIRM map Cross sections must be < 1 ft of water surface Elevation change between two cross-sections, or less than 500 ft apart Cross section perpendicular to the flow path, at major changes in channel characteristics, changes in discharge, or where a structure is located.

2 Technical Review Revisions involving the addition of detailed Flood hazard information or changes to flooding sources originally studied by detailed methods, may required additional data/studies. Hydrologic Analyses Discharge-Drainage Area Relationship Regression Equations TR-55. Rational Formula HEC-HMS computer program Hydraulic Analyses Normal Depth Critical Depth Step-Backwater Analysis (HEC-RAS). Technical Review Hydraulic Structures Hand calculations are also accepted for each of the following flows: Weir Flow Flow over a road, railroad, embankment, dam or canal Culverts HEC-RAS requires 2 cross sections. One upstream and one downstream of the road/crossing Bridges HEC-RAS requires 4 cross sections. Two upstream and two downstream of the road/crossing. Accepted Hydrologic Analyses Discharge-Drainage Area Relationship Use existing FIS report to find discharge flows Plot flows on log-log graph to create a straight line Regression Equations Q = K*AX*BY*CZ. Where: Q = the discharge, K= the regression constant, A, B, and C = watershed variables, and X, Y, and Z are exponents TR-55.

3 Graphical Peak Discharge method Tabular Hydrograph method Rational Formula Q=ciA. HEC-HMS computer program Accepted Hydraulic Analyses Normal Depth The depth expected for a stream when the flow is uniform, steady, one-dimensional, and is not affected by downstream obstructions or flow changes Critical Depth If the velocity head from the normal depth computation is equal to or more than half the hydraulic depth, the flow is supercritical and the critical depth should be used to establish the BFE. Step-Backwater Analysis (HEC-RAS). Based on conservation of energy. The energy at the upstream cross section is equal to the energy at the downstream cross section plus the losses between the two cross sections. Flood Zone Labels *Zone A Areas of 100-year Flood inundation as determined by approximate methods. (no BFE associated with delineation). *Zone AE Areas of 100-year Flood inundation as determined by detailed hydraulic analyses. BFEs are shown at select intervals on the FIRM. and are listed at lettered cross sections in the Floodway Data Tables of the FIS.

4 Zone AO Areas of 100-year shallow flooding (usually sheet flow on sloping terrain), where average water depths during the base Flood event range between 1 and 3 ft. Average depths of inundation are shown on the map. Flood Zone Labels Zone AH Areas of 100-year shallow flooding (usually areas of ponding), where average water depths are between 1 and 3 ft and BFEs are shown. Zone A99 Areas subject to 100-year Flood inundation to be protected by a federal Flood protection system under construction (BFEs are not shown). Zone AR Areas of special Flood hazard that result from the decertification of a previously accredited Flood protection system now being restored to provide a 100-year or greater level of protection. Flood Zone Labels *Shaded Zone X Areas between limits of the 100-year floodplain and 500-year floodplain. This zone also includes areas protected by levees, 100-year floodplains where water depths are less than 1 ft, and areas with drainage areas less than on square mile.

5 Flood insurance is available is available in this zone, but is not required. (Shaded Zone X has replaced Zone B). *Unshaded Zone X Areas of minimal flooding;. land elevations exceed the 500-year Flood level. (Unshaded Zone X has replaced Zone C). Zone D Areas where Flood hazards are undetermined, but flooding may be possible Flood Zone Labels *Zone V Areas of 100-year coastal Flood with wave action where BFEs have not yet been determined. *Zone VE Areas of 100-year coastal Flood with wave action where BFEs have been determined. Importance of Vertical Datums There are two datums currently used by FEMA. NGVD29 The National Geodetic Vertical Datum of 1929. NAVD88 The North American Vertical Datum of 1988. The average difference in Chatham County is '. Therefore: Elevation 10 on NGVD29 would be the same as Elevation on NAVD88. Letter of Map Revision LOMR process for requests made in a Zone A area. Must create 1-2 models: Existing Conditions Proposed Conditions LOMR process for requests made in a Zone AE area.

6 Must create 3-4 models: Duplicate Effective Corrective Effective Existing Conditions Proposed Conditions LOMR in a Zone A. LOMR in a Zone AE (Floodway). Digital Elevation Model (DEM). Digital Elevation Models (DEM). What are DEMs used for? Watershed delineation Stream network Watershed boundaries Characteristics determination Slopes (watershed and stream). Lengths Old DEMs Quad Maps Finding the direction of flow FIRM on NGVD29. Floodplain: Zone A. dFIRM on NAVD88. Floodplain: Zone A. Preparing DEM/LiDAR data Elevation correction with survey data Hydrologically correct DEM. Sinks Fill sinks to allow surface water flow through. Streams Are the drainage patterns correct? Catchment boundaries Are ridge lines in the appropriate locations? Preparing DEM/Lidar data Generating Basins using GeoHEC-HMS. Using the parent DEM grid create child grids including flow direction, flow accumulation, stream grid, stream link grid, and catchment grid. Quad Map with Basin Layout Extracting Hydrologic Parameters Hydrologic features such as the Land Use grid are used to create the Curve Number grid.

7 The final output for CN. calculations is a composite CN per basin. The Watershed slope gird generated by GIS is used with the land cover to calculate Time of Concentration. Final output for TCs shows longest flow path per basin. GeoHEC-HMS HMS Schematic Using HEC-HMS symbols, the model schematic is created in GeoHEC-HMS and then exported to HEC-HMS with the data attached including basin area, CN, TofC, reach length, and reach slope. HEC-HMS. Once the GeoHEC-HMS schematic is imported into HEC-HMS (the basin model), the file is edited to include: Basin model additional channel or node characteristics or distances Meteorologic Models design storm method (type 3 in Chatham) and depth Control Specifications Start and End Times for the Hydrology run and time interval. Time-series Data and Paired data . used for additional calibration. HEC-HMS. GeoHEC-RAS. GeoHEC-RAS is A GIS extension Used to generate: Stream centerlines Cross-section locations Channel, left bank, and right bank flow paths Bank stations Bridges/culvert locations Ineffective flow areas Land use areas Etc.

8 GeoHEC-RAS. Cross Section Locations Cross sections are placed at Changes in channel geometry Every 500 lf throughout streams At drastic changes in water surface Elevation Upstream and downstream of bridge crossings At points of interest along a stream Cross-Section Exhibit with Basins HEC-RAS. Enter/Edit Geometric Data Add Junctions Cross Sectional data Manning's n Bank stations Downstream reach lengths Bridge / Culvert data Inline Structures Lateral Structures Storage Areas and connections Pump Stations HEC-RAS Cross section Editor HEC-RAS Bridge Editor Combining HEC-HMS & HEC-RAS. By setting DSS Connections, you can easily import both steady and unsteady flow data after each manipulation to the hydrologic and hydraulic change. Final Floodplain mapping in GeoHEC-RAS. Proposed Floodplain vs. dFIRM. Proposed Floodplain vs. FIRM. Challenges Direction of flow during design storm. Are your basins routed the correct way? Are off-site properties accounted for in the basins? Changes in minimum cross-section Elevation .

9 Is the water ponding between two cross- sections or is there relief somewhere else? Do you need to add weir's to move water between parallel streams? Width of Cross-section. Is the entire floodplain covered? Do parallel streams have concurring BFEs? Challenges Which model is best to use in the situation, steady state model or unsteady state model? Is the model stabilized and calculating viable answers? Are off-site properties affected by the changes in floodplain delineation? Property Owner Notification letters. LOMR process for requests made in a Zone AE area Request current models from FEMA. Re-build current FEMA models in the same program that you will be using for subsequent models. (Duplicate Effective Model). Locate Existing Surveyed cross section locations Re-survey at existing cross-section locations Or, extract wetted data from existing model Site Location Project Site FIRM on NGVD29. Floodplain: Zone AE. Floodway dFIRM on NAVD88. Floodplain: Zone AE. Floodway LOMR in a Zone AE.

10 Quad Maps . Finding the direction of Flow Cross-Section Exhibit Overview of process Use GeoHEC-HMS to update watersheds if needed Create HEC-HMS model Use GeoHEC-RAS to cut cross- sections in newest available DEM data Export data into HEC-RAS and merge surveyed sections into DEM data Correct HEC-HMS and HEC-RAS data and export to GeoHEC-RAS for mapping New proposed Floodplain LOMR in a Zone AE (Floodway). Floodway using HEC-RAS Methods HEC-RAS Methods include: Method 1: User enters right and left encroachments stations Method 2: User enters a fixed top width Method 3: User specifies the percent reduction in conveyance Method 4: User specifies a target water surface increase. Method 5: User specifies target water surface increase and maximum change in energy. Floodway on a cross-section Floodway by location Proposed Floodplain vs. FIRM. Final Approval Letter Final Approval Letter Final Approval Letter Final Approval Letter FEMA Guidelines for Property Owner Concerns: Property Owner Notification Letters are sent when the LOMR is sent to FEMA for acceptance.