ETL 1110-2-540
30 Jun 96
Table 3-1
Flood-threat Recognition Monitoring Systems
Preliminary
Monitoring System
Monitoring
Measurements
Small/Steep
Moderate
Large
Conditioning
Obtained
Streams
Streams
Rivers
1. Thunderstorms
Rainfall intensity
Network of Regional
Network of Regional/
Generally not
Cloudbursts
Recording Raingages
Watershed Recording
applicable,
Rainfall
Raingages
available system
acceptable
2. General-Steady
Rainfall intensity
Network of Regional/
Mix of Regional/
Available Raingage
Rainfall
& distribution
Watershed Recording
Watershed Recording,
System is generally
Raingages
Daily, and Field
applicable
Observed Raingages
3. Snow
Snow depth/
N/A
N/A
Field measurements,
moisture content
some automatic
Recording
Streams
1. Normal Flowing
Stages/rate-of-
Mix of Staff Gages,
Mix of Staff Gages,
Staff gages
rise
Upstream and Local
Upstream and Local
normally acceptable
Stage Alarms,
Stage Alarms,
Continuous Recording
Continuous Recording
Gages
Gages
2. Backwater/
Stages/rate-of-
Generally N/A
Staff gages, maybe
Staff gages
Ponding
rise
Continuous Recording
normally acceptable
location of mass care centers, management of vital
Table 3-2
services, and to estimate the number of structures and
Flood Hazard Rating
people impacted by an event. It is also used in defining
Physical
areas for potential implementation of temporary flood loss
Hazard
Low
Medium
High
reduction actions such as flood fighting, installation of
1% Flood Depth
<0.3 m
0.3-0.9 m
>0.9 m
(1 ft)
(1-3 ft)
(3 ft)
temporary barriers, and removal or raising contents of
Flood Rise Time
>24 hr
12-24 hr
<12 hr
structures, and potential hazardous material location. The
Flood Velocity
<0.3 ms
0.3 - 0.9 ms
>0.9 ms
threatened properties analysis helps refine response
(1 fts)
(1-3 fts)
(3 fts)
Flood Duration
<6 hr
6-24 hr
>24 hr
measures that might be dictated due to warning time
Site Access
good
fair
poor
available. The location of structures in the floodplain can
determine whether more or less warning time is available.
Structure located at lower elevation in the flood-plain
impact eroded soils around foundations, causing bridges and
generally have less time than structures located at higher
buildings to collapse. High velocities also sweep away mov-
elevations.
Taking a conservative approach when
able objects. These objects can become battering rams and
determining how much time is available is normally better.
cause further damage downstream.
d. Threatened properties analysis. Flood inundation
3-3. Assessment of Existing Flood Warning -
maps, aerial photographs, field surveys, etc., are used to
Preparedness Programs
identify locations where existing properties are threatened by
various levels of flooding. Existing damageable structures
Once the magnitude of the flood hazard is established and the
should be categorized by type and number for each flood
warning times determined, the existing flood warning -
event throughout the range of the flood-frequency relation-
preparedness program is defined. Once defined, the pro-
ship. Frequency-discharge-elevation-damage relationships are
gram's effectiveness can be evaluated. Program components
presented in tabular form. The information is used to help
currently in place are identified. Existing plans and Federal/
in the development of warning and evacuation plans,
local institutional arrangements must form the basis to
3-4
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