FLOOD04049

• County: Denver
• Community: All
• Stream Name: All
• Basin: South Platte
• Title: Riverine Erosion Hazard Areas Mapping Feasibility Study Executive Summary
• Date: 9/1/1999
• Prepared For: State of Colorado
• Prepared By: FEMA
• Floodplain - Doc Type: Educational/Technical/Reference Information

I
I
I
I
I
I

In assessing the technical feasibility of mapping REHAs, each case study was analyzed for
applicability, Iimitabons, potential for mapping riverine erosion, cos~ and regulatory
potenbal. These documents revealed that numerous techniques are currently in use
covering geomorphic methods, basic engineering principles, and mathematical modeling.
This diverse collection of techniques is necessary because of the uniqueness of each site
and to address the objectives of the specific projects,

Assessment of Technical Feasibility

The case studies indicate that there are scientifically sound procedures for delineabng
riverine erosion hazard areas. Various geomorphic, engineering, and modeling
procedures can be applied, depending on stte-specific conditions. Specialized knowledge
and experience are needed to draw conclusions that would lead to delineabon of a hazard
area.

I
I
I

A time frame of 60 years has been specified in Section 577 of NFIRA as the interval of
interest for delineation of riverine erosion hazard areas. Although tt is feasible to use the
specified 60-year bme frame, the case studies and the opinions of the PWG indicate that
existing techniques may be better sutted for shorter bme frames, e.g., 30 years with
periodic revisions to the particular REHA study and delineation. This limitation arises from
data inaccuracies, imperfect knowledge of sediment transport mechanics, and unknowns
in future watershed development, hydrologic condibons, and magnttude and sequence of
future fiooding events. However, most structures have a useful life well over 30 years and
predictions should somehow address a looger bme span.

I
I

Given a suitable time frame, future erosion could be esbmated either extrapolating from
historic data or through the use of mathematical models. In both cases, an estimate of the
reliability of the prediction needs to be provided.

Cost

I
I

An approximate analysis was performed to esbmate the total cost to the Federal
govemment of mapping riverine erosion hazard areas. The sources of cost data include
information provided by the PWG, costs reported in the case studies, FEMA reports and
other literature. and cost data from previous studies performed by the project team
members, The data are not sufficient to make reliable nabonwide cost esbmation;
however, they can be used to perform an educated guess for total costs.

I
I
I
I
I

Average study values are $2,000-$3,000 per mile for geomorphic methods, $6,000-$7,000
for engineering methods, and $10,000-$12,000 for mathematical modeling methods, If
this effort were to be implemented as part of the NFIP, the cost to the Federal govemment
would be between 200 and 300 million dollars. Section 577 of NFIRA specifies that, if
REHA determination is found to be technically feasible, a cost-benefit study is to be
conducted, The current study does not include these cost-benefit analyses,

Implementation

There are at least two potential opbons for implementabon of a nabonwide REHA
delineation program: a federally run program and a locally run program. The federally run
program would be integrated into the NFIP. The fundamental principle of this first option is
to expand the current fioodplain regulabons to encompass riverine erosion. This opbon
emphasizes authority from the Federal govemment. The existing framework can be
modified to accommodate the new responsibilibes of regulabng erosion-prone areas.

10

I