ETL 1110-2-540
30 Jun 96
selected historic flood events. The model will also be calibrated
Chapter 3
using the hypothetical events to gaged frequency statistics. If
Defining Base Conditions
there are no stream gages in the watershed of interest, the
analysis may include modeling of nearby, similar-gaged
watersheds and extrapolation of parameters. The assumption is
3-1. Overview
that at least some recording stream gage data are available in or
near the study watershed. If no data are available, synthetic unit
The initial task in evaluating the feasibility of a flood warning -
hydrograph parameters can be developed and applied, based on
preparedness program is to define the base year conditions.
watershed time-of-concentration. These parameters can be used
Base conditions refers to the year the project will go on-line.
to develop flood hydrographs from hypothetical frequency
In some cases, base year conditions will be different from
existing or current year conditions. The base conditions form
storm data published by the NWS.
the baseline for evaluation of incremental enhancements to
flood damage mitigation strategies for flood warning -
(2) Warning time. An important element of the flood
preparedness programs. The expected magnitude and fre-
warning - preparedness plan is the warning time available to
quency of flooding, the populations (people and structures) at
residents subject to flooding. Warning time determines how
risk, and the institutions involved in emergency management all
much time a threatened population has to respond to a specific
require evaluation.
event and, therefore, the amount of flood damage reduction
activities that can take place. The purpose of a flood
recognition system is to provide a means of increasing the
3-2. Flood Hazard Analysis
warning time and its reliability. This is accomplished by
reducing flood recognition and reaction time and allowing more
a. General. Hydrologic engineering investigations are
time to carry out response actions. These times must be defined
conducted to define the flood hazard and to develop
information for economic analysis of existing and proposed
to evaluated potential enhancements that increase warning time.
improved conditions as required for evaluation of any flood
Figure 3-1 shows warning time for a representative storm event
damage reduction measure. The flood hazard is defined by the
and for a specific threatened property location (flood stage).
This type of relationship can be developed for different flood
duration, and velocity of the floodwater. The information
stages and locations as appropriate for identified threatened
required includes discharge-frequency and stage-discharge
properties. Mathematically, warning time can be described as:
relationships, flood inundation boundary maps, and flood
warning time. The level of detail must be commensurate with
(3-1)
TW ' TWP & TR
the overall scope of the investigation, allocated time, and
resources. Flood hazard data available from previous water
where
resources investigations and developed for evaluation of other
alternatives are used to the extent possible.
TW = actual warning time
b. Hydrologic analysis. The hydrologic analysis required
TWP = maximum potential warning time
for defining the base condition flood hazard consists of
traditional, standard procedures that are well documented in
TR = flood threat recognition/reaction time
Corps' publications. EM 1110-2-1419 describes general
hydrologic requirements for flood damage reduction studies.
(a) The maximum potential warning time is defined
Elements of the hydrologic analysis that are important for flood
herein as the time from the beginning of the storm event to the
warning - preparedness programs and are described below.
time that the stream reaches flood stage.
(1) Flood hydrograph development. Flood hydrographs are
(b) The flood recognition and reaction time, TR, includes
developed at key locations to define the magnitude and timing
the time required to observe or measure developing conditions,
of flood flow. The relationship between rainfall and runoff
Tobs, the time needed to acquire the necessary data, Tdata, the
helps define warning times. A watershed rainfall-runoff model
time required to analyze the storm data and determine
such as the HEC-1 program (U.S. Army Corps of Engineers
(recognize) that a flood is likely to occur and to prepare a
(USACE) 1982b) is generally used to develop the flood
forecast, Tfp, and the time required to disseminate the flood
hydrographs. Historic and hypothetical frequency events are
warning, Tdis. The flood recognition and reaction time can be
selected for evaluation based on available stream flow records,
expressed as:
rainfall records, high water marks, etc. The analysis involves
developing runoff parameters from gaged data and the
(3-2)
TR ' Tobs % Tdata % Tfp % Tdis
calibration of rainfall and runoff parameters for each of the
3-1
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