The Benefits and Appropriate Use of Base Flood Approximate Shapefiles to Calculate Zone A Base Flood Elevations. Jeremy Kirkendall, CFM June 12, 2013

Transcription

1 The Benefits and Appropriate Use of Base Flood Approximate Shapefiles to Calculate Zone A Base Flood Elevations Jeremy Kirkendall, CFM June 12, 2013

2 Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs) Identify flood hazards Zones identify the type of flooding or study Show Base Flood Elevation (BFE) 100 year or 1% chance flood Zone AE Detailed study Shows BFEs Zone A Approximate study In remote or sparsely populated areas No BFEs on FIRMs Public cannot easily determine BFE

3 *Not all counties have DFIRMs

4 Kansas Solution to Zone A Flooding: Base Flood Approximate (BFA) Cross Sections Paper FIRM Not on FEMA s paper FIRMs Found in Digital Flood Insurance Rate Maps files (DFIRMs) DFIRM Layers

5 Comparison of Study Types Approximate Studies Cheaper and faster Remote data acquisition Ideal for low population density If LiDAR data exists, BFAs may be easy to implement Public must use online map viewer to see BFA values Detailed Studies More accurate Requires local survey and cross sections Ideal for high population density BFEs published on paper FIRMs

6 BFA Cross Section Accuracy vs. Detailed Studies BFA and detailed studies have slightly overlapped, allowing BFE comparison Values usually close BFA studies are in sparsely populated areas, lower accuracy impacts fewer people

7 FEMA s Flood Map Viewer hazards.fema.gov Located under Map Viewer tab, no direct web address BFA Cross Section values are unlabeled Unintuitive interface Lengthy process to determine cross section value

8 Determining the BFA Cross Section Value Using FEMA s Flood Map Viewer Select Identify All Visible Layers tool Click cross section to identify Click the Identify tab Expand Cross Section Lines section Have experience working with FEMA s DFIRMs in order to understand the results

9 Kansas Department of Agriculture s Digital Floodplain Map Viewer gis.kda.ks.gov/ksfloodplain Direct web address to viewer BFA Cross Section values labeled automatically More responsive and intuitive interface

10 How to build a BFA Template in ESRI s ArcMap Required Shapefiles from FEMA DFIRM: s_fld_haz_ar.shp Flood Hazard Area s_fld_har_ln.shp Flood Hazard Boundary Lines s_wtr_ln.shp Stream Centerline (used in developing BFA Cross Sections, do not use aerials in lieu of this shapefiles) s_xs.shp Detailed and Approximate Study Cross Sections Additional Data Needed: Recent aerial photo to locate structures Parcel boundaries in order to determine BFEs for properties Symbology: s_fld_haz_ar.shp by fld_zone value field s_fld_har_ln.shp by ln_type value field Label s_xs.shp by wsel_reg field to display BFA values

11 Calculating a Zone A BFE When the structure or property is located between BFA Cross Sections, use interpolation Interpolation formula: BFE = E2 + EL EL = D1((E1-E2)/D2) E1: Upstream BFE E2: Downstream BFE D1: Distance between downstream BFE and upstream edge of structure or property D2: Distance between upstream and downstream BFE

12 Calculating a Zone A BFE Step 1 Identify structure or property and appropriate upstream and downstream cross sections All units are in feet E1 = E2 =

13 Calculating a Zone A BFE Step 2 Create cross section at upstream edge of structure or property using the Sketch Tool in the Editor Toolbar

14 Calculating a Zone A BFE Step 3 Select stream line(s) between upstream and downstream cross sections, merging into 1 line if necessary

15 Calculating a Zone A BFE Steps 4 and 5 Turn snapping on Use the Split Tool to split the stream line at the intersections with the upstream, downstream, structure s cross section Calculate the stream line length in feet in the attribute table D1 = 739 D2 = = 4648

16 Calculating a Zone A BFE Step 6 Interpolation formula: BFE = E2 + EL EL = D1((E1-E2)/D2) E1: Upstream BFE E2: Downstream BFE D1: Distance between downstream BFE and upstream edge of structure or property D2: Distance between upstream and downstream BFE E1 = E2 = D1 = 739 D2 = = 4648 EL = 739*(( )/4648) = 1.86 BFE = = feet

17 Limits of BFA Cross Section Interpolation Structure or property is up or downstream of the last BFA Cross Section in the study, no second cross section is available for interpolation Extreme distances between BFA Cross Sections lowers accuracy If a stream line does not exist between BFA Cross Sections, user must estimate distances using aerial, stream centerline may not be obvious If a dam or culvert is between BFA Cross Sections it can greatly impact the BFE Drastic slope changes between BFA Cross Sections lowers accuracy

18 Would Base Flood Approximate Cross Sections Benefit Connecticut? Connecticut has a number Zone A Flood Hazards BFA Cross Sections may already exist for some counties, but are not included in the DFIRM Maryland developed BFA Cross Sections with their DFIRMs, but they are stored separately from the DFIRM BFAs would be beneficial in areas where development is less likely, especially if acceptable LiDAR data already exists A simple online DFIRM viewer would maximize their utilization

19 Questions? Jeremy Kirkendall Sources: FEMA Mapping Information Platform FEMA Map Service Center Kansas Department of Agriculture Crawford County, Kansas GIS Department Geo.Data.gov