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Redelineation: What does it mean for me?
Kristen MacDougall, CFM, BakerAECOM

FEMA is updating the flood and wave data for the San Francisco Bay and Open Pacific Coast shorelines.  The results of the coastal analyses are used to delineate new coastal SFHAs.  Older SFHAs from riverine flood sources must be adjusted to merge with the new coastal floodplain boundaries at all coastal-riverine junctions.  In most cases, the coastal-riverine tie-in procedure at the merge point is straightforward.  The higher coastal backwater flood surface elevations supersede riverine water surface elevations up to the limit of coastal inundation.  However in some areas, minor redelineation of effective boundaries must be performed to accurately establish a transition between the new coastal inundation boundary and the effective riverine flood boundary.  This article discusses some fundamental concepts of redelineation and how those concepts apply to the CCAMP study areas.

What is Redelineation?
Redelineation is a FEMA-approved set of mapping procedures for revising effective floodplain information. It entails applying effective hydraulic modeling data over newer, more detailed, (and therefore improved) topographic data.  The new topographic data can be a DEM or TIN, derived from LiDAR or another FEMA-approved terrain source.  Redelineation does not alter any effective regulatory information; rather, the effective regulatory information is applied over the new terrain to revise the shape of the effective floodplain boundary (which represents effective base flood elevations).  Redelineation ensures that the SFHA boundary aligns more accurately with the improved terrain data.  All regulatory data (e.g., cross sections, BFEs, floodway widths) represented on the FIRM must continue to match the regulatory data listed in the FIS Floodway Data Tables, Flood Profiles, and regulatory FIS text within FEMA specifications.  Because portions of a stream channel may be realigned according to the improved terrain, the Profile Baseline feature is displayed on the FIRM to represent the effective stream stationing.  Stream centerlines matching the redelineated floodplain boundaries are visible on the underlying orthophoto FIRM base map.  Due to the NFIP regulations governing revisions of effective floodways, floodways are not changed with redelineation. 
 
Why is Redelineation Necessary?
Due to the age difference between some effective studies and the new topographic base data for the CCAMP study areas, discrepancies may come to light as some older effective flood boundaries are compared to the new study terrain.  Typically the preferred approach is to perform a new riverine detailed study using the new terrain to complement the new coastal study in these areas, but a new riverine detailed study is not always practical due to project scope, schedule, or funding availability.  Because of its cost effectiveness, redelineation is useful where the effective flood data and mapping is still an accurate depiction of flood risk and a full detailed study approach may not be warranted or feasible.  These discrepancies between old and new data sources do not mean that the effective riverine hydrologic and hydraulic data and modeling are bad data or modeling; this is simply an indication that a different, older, terrain source was used for the older effective study and mapping, which used the best-available terrain at that time.  Based on the new study terrain, small but effectual revisions and improvements can be made to the effective riverine boundaries at the locations of coastal-riverine tie-ins, where necessary.
 
Why is Redelineation Important?
To support reliable and defendable FIRMs, FEMA enforces a Floodplain Boundary Standard, which provides mapping partners a set of guidelines and standards for ensuring that the final flood hazard delineation of riverine BFEs and inundation boundaries matches the best-available terrain.  As effective riverine data is compared to the new study terrain, issues may become apparent where riverine streams are affected by new coastal data.  These issues may include:
1. Poor horizontal alignment between the new coastal boundary and the effective riverine boundary; and/or,
2. Poor representation of the effective riverine water surface elevation when compared with the new terrain.
Where necessary, redelineation on the riverine stream at the coastal tie-in location can help to overcome these issues (see examples below).
 
Where Can Redelineation Be Applied?
According to FEMA’s Guidelines and Specifications for Flood Hazard Mapping Partners, Volume 1: Flood Studies and Mapping (April 2003), redelineation is an effective technique for areas already studied and modeled in detail, where effective riverine discharges and Zone AE BFEs appear accurate, but the mapped SFHA appears to be inaccurate when compared to best-available terrain.  Re-alignment of approximate Zone A shapes may also be performed using the underlying terrain, using elevation or depth data where available.  The results of the redelineation process are typically presented on a Draft Workmap to FEMA and to communities during a community’s Flood Risk Review meeting. For any riverine streams where redelineation is necessary at the coastal-tie in point, or, for any riverine streams where further mapping issues may be evident upstream of the coastal-riverine tie-in point, the general area is marked for future detailed-study funding consideration.  Because the new CCAMP studies supersede all the effective coastal BFEs and mapping, redelineation is not applicable to the new coastal floodplain areas since the new SFHA delineation uses the new terrain.
 
Terms:
BFE: Base Flood Elevation
CCAMP: California Coastal Analysis and Mapping Project
DEM: Digital Elevation Model
FIRM: Flood Insurance Rate Map
FIS: Flood Insurance Study
LiDAR: Light Detecting And Ranging
NFIP: National Flood Insurance Program
SFHA: Special Flood Hazard Area
TIN: Triangulated Irregular Network

In this photo boundary alignment of effective data no longer matches new terrain.
redelineation 1.jpg
In the photo below the boundary has been redelineated to reflect current conditions in new terrain.
 

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