Comprehensive Risk Assessment for Natural Hazards - Planat

Comprehensive risk assessment for natural hazards
almost any river or stream, although the quality of the
assessment will vary. As a rule, it is better to examine several
sources of data and estimation methods than to rely on a
single source or method. Local conditions and experience
must be taken into account.
3.5.2 Standard techniques for watersheds with
abundant data
In rivers with abundant data, the standard method for floodhazard
analysis begins with a thorough analysis of the quality
and consistency of the available data. Upon confirming the
quality of the data, the peak flow rate of the target flood, which
may be the 100-year flood (or possibly the 20-, 50- or 200-year
floods), is determined. The peak flow rate of the target flood
will be called the target flow rate, for convenience. The watersurface
elevation associated with the target flow rate is then
determined using hydraulic analysis techniques, which
account for the relation between discharge, velocity and stage,
and how these quantities vary in time and space. Finally, the
inundated area associated with the stage of the target flood is
plotted on a topographic map.
Flood-frequency analysis is used to estimate the relation
between flood magnitude (peak) and frequency
(WMO, 1989). The analysis can either be conducted using a
series of annual peaks or a partial duration series. The former
consists of one data value per year: the highest flow rate
during that year. The later consists of all peaks over a specified
threshold. In either case the observations are assumed
to be identically and independently distributed. The discussion
given here is restricted to analysis of the annual series
because this series is used more frequently than the partial
duration series.
As the flow record is almost always shorter than the
return interval of interest, empirical extrapolation is used to
predict the magnitude of the target flood. A frequency distribution
is most commonly used as the basis for
extrapolation, and various distributions may be fitted to
observed data. These data with the theoretical distribution
fitted to the data are plotted on probability paper (Figure
3.2). Observed peaks are assigned probability of exceedance
using approaches described in section 2.4.2. Commonly
used distributions in flood frequency analysis include the
generalized extreme value, Wakeby, 3 parameter lognormal,
Pearson Type III and log-Pearson Type III. Flood frequency
analysis has been discussed extensively in the literature
(Potter, 1987; NRC, 1988; Stedinger et al., 1993; Interagency
Advisory Committee on Water Data, 1982). Recognition
that floods may be caused by several mechanisms
(snowmelt or rain on snow; convective or frontal storms)
has led to the concept of mixed distributions. The mixed
distribution permits a better statistical representation of the
frequency of occurrence of processes, but is, at times, difficult
to apply to the problem of hazard assessment due to a
lack of supporting evidence allowing the categorization of
causes of each flood event.
Once the target peak flow has been determined, the
corresponding areas of inundation can be calculated
(Figure 3.3). First, the water-surface profile is determined.
The water-surface profile is the elevation of the water
surface along the river centreline. Estimation of the profile is
determined from the target discharge (or hydrograph) and
hydraulic-analysis techniques. In many cases the flood
duration is sufficient to warrant the assumption that the
peak discharge is constant with time; in other cases, the
assumption is made for convenience. Therefore, steadystate
analysis is applied to determine, for a given discharge,
how the water-surface elevation and average cross-sectional
velocity vary along the length of the river. It is assumed that
discharge is either constant or only gradually varying in a
downstream direction. The step-backwater method is a
steady-state method that is commonly used in hazard
assessment. Among other factors, the method takes into
account the effect of channel constrictions, channel slope,
channel roughness and the attenuating influence of
overbank flow. Step-backwater and other steady-state
methods require topographic data for determining the
downstream slope of the channel bed and the crosssectional
shape of the channel at a number of locations.
O’Conner and Webb (1988) and Feldman (1981) describe
the practical application of step-backwater routing. More
complex methods may be adopted, depending on local
conditions that might make estimation by these simpler
methods inaccurate.
Once the water-surface profile has been determined,
the associated areas of inundation are indicated on a topographic
map, perhaps along with the velocities that are
calculated during the hydraulic computations. In some
countries it is customary to divide the inundated area into a
central corridor, which conveys the majority of the discharge
(the floodway) and margins of relatively stagnant
water (the flood fringe). To obtain accurate estimates of
velocity, more complex hydraulic modelling procedures are
employed.
All or part of the above techniques are unsuitable under
certain conditions, such as: river sections downstream from
major reservoirs,alluvial fans,lakes,areas oflow reliefand
few channels subject to widespread shallow flooding, areas
subject to mudflows, water bodies subject to tidal forces and
floods related to ice jam. In the case of reservoirs, the target
flow must be estimated on the basis of expected reservoir
outflows, rather than natural flows. Even though reservoirs
frequently serve to reduce downstream flooding by storing
floodwaters, it is customary to conduct flood analyses under
the assumption that the target flood will occur while the
reservoir is full. On active alluvial fans, one must consider
the possibility that sedimentation and erosion will cause the
channel to suddenly change position. For small lakes, the
water surface is assumed to be horizontal; the change in lake
volume is computed as the difference between flood inflows
and lake outflow. In the case of rivers that flow into lakes,
reservoirs or oceans, the water-surface elevation of the
downstream water body is used as the boundary condition
for hydraulic calculations. In principle, the joint probability
of high river discharge and high downstream boundary
condition should be considered, but this is difficult in
practice. In the case of oceans, mean high tide is often used
as the boundary condition, but it is better to use a
representative tidal cycle as a time-variable downstream
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