Comprehensive Risk Assessment for Natural Hazards - Planat

22
Discharge (m 3 /s)
4 000
3 000
2 000
Annual exceedance probability
0.99 0.98 0.95 0.90 0.80 0.70 0.50 0.30 0.20 0.10 0.05 0.02 0.01
Chapter 3 — Hydrological hazards
the suddenness of its onset due to a lack of coordinated and
complementary systematic records for various aspects of
the hydrological cycle. In addition, bank erosion, sediment
transport and floodplain sedimentation are important topics
in their own right. Quantitative prediction of sedimentrelated
phenomena for specific floods and river reaches is
very difficult because of the complexity of the phenomena
involved and the lack of appropriate data.
3.5 TECHNIQUES FOR FLOOD HAZARD
ASSESSMENT
1 000
1.01 1.1 2 5 10 20 50 100
Return period (years)
Figure 3.2 — Example of a flood-frequency diagram plotted
on log-probability paper. Symbols represent data points; the
line represents the cumulative probability distribution, which
has been fitted to the data
by lowering a device into the water that measures water depth
and velocity. These are measured repeatedly along a line
perpendicular to the direction of flow. For any reasonablysized
river a bridge, cableway or boat is necessary for discharge
measurement. Discharge (m 3 /s) through each cross-section is
calculated as the product of the velocity and the crosssectional
flow area.
Most gauging stations are located such that there is a
unique or approximately unique relation between flow rate,
velocity and stage. The flow-rate measurements may, therefore,
be plotted against stage measurements to produce a
rating curve. Once the rating curve is established, continuous
or periodic stage measurements, made either
automatically or manually, can be converted to estimates of
discharge. Because measurements of discharge during
floods are difficult to make when the water levels and flow
velocities are high, it is common to have records of only
stage measurement during major floods. Consequently, the
estimation of discharges for such floods relies on extrapolating
the rating curve, which may introduce considerable
error. However, recent advances in the development and
application of acoustic Doppler current profilers for discharge
measurement have facilitated discharge
measurement during floods on large rivers, e.g., on the
Mississippi River during the 1993 Flood (Oberg and
Mueller, 1994). Also, stage measuring equipment often fails
during severe floods. In cases where no discharge measurements
have been made, discharge can be estimated using the
slope-area technique, which is based upon hydraulic flow
principles and the slope of the high water line. The high
water line is usually discernable after the event in the form of
debris lines.
The area of inundation can be measured during or
immediately after a flood event using ground surveys (local
scale), aerial photographs (medium scale), or satellite techniques
(large scale). With remote-sensing techniques, it is
best to verify the photo interpretation with ground observations
at a few locations. In general, it may be difficult to
establish the cause of flooding (cyclone, snowmelt, etc.) or
3.5.1 Basic principles
Well-established techniques are available for the assessment
of flood hazards (WMO, 1976 and 1994). The most comprehensive
approach to hazard assessment would consider the
full range of floods from small (frequent) to large (infrequent).
Such comprehensive treatment is rarely, if ever,
practical. Instead, it is customary to select one size of flood
(or a few sizes) for which the hazard will be delineated. The
selected flood is called the target flood (commonly referred
to as a design flood in the design of flood-mitigation measures)
for convenience. Often the target flood is a flood with
a fixed probability of occurrence. Selection of this probability
depends on convention and flood consequences. High
probabilities are used if the consequence of the flood is light
(for example, a 20-year flood if secondary roads are at risk)
and low probabilities are used if the consequence of the
flood is heavy (for example, a 500-year flood if a sensitive
installation or large population is at risk). In some countries
the probability is fixed by law. It is not strictly necessary,
however, to use a fixed probability of occurrence. The target
flood can be one that overtops the channels or levees, a
historical flood (of possibly unknown return interval), or
the largest flood that could conceivably occur assuming a
maximum probable precipitation for the region and conservative
values for soil moisture and hydraulic parameters
(sometimes known as the probable maximum flood). In the
simplest case, hazard estimation consists of determining
where the hazard exists, without explicit reference to the
probability of occurrence; but ignorance of the probability
is a serious deficiency and every effort should be made to
attach a probability to a target flood.
In terms of specific techniques, several methods and
combinations of methods are available for assessing flood
hazards. What follows is, therefore, a list of possible
approaches. In any practical application, the exact combination
of methods to be used must be tailored to specific
circumstances, unless the choice is prescribed by law or
influenced by standard engineering practice. Determination
of the most suitable methods will depend on:
(a) the nature of the flood hazard;
(b) the availability of data, particularly streamflow measurements
and topographic data;
(c) the feasibility of collecting additional data; and
(d) resources available for the analysis.
Even with minimal data and resources, it is generally
possible to make some type of flood hazard assessment for