ABSTRACTS 'Extreme Discharges' - CHR-KHR

Impact assessment of flood mitigation measures
Matthijs Kok
HKV Consultants, the Netherlands
P.O. Box 2120, NL 8203 AC Lelystad, the Netherlands
m.kok@hkv.nl
Assessing impacts of flood mitigation measures (such as dike strengtening, lowering flood plains, building watertight
compartments, retention areas and spatial planning) can be done with traditional cost/benefit methods.
These methods are in decision making for two reasons not sufficient. First, flooding does also influence
“goods’ having not only a purely economical value, such as human lives, cultural goods and the value of keeping
control of the situation. Second, measures do not have only economic costs, but may provide other noneconomical
costs and benefits. Examples are the impacts on landscape of dike strenthening (see for example
Walker et al, 1994) or the increase of nature values by lowering the flood plains. However, in this presentation
we concentrate ourselves on the economic approach. In this approach the expected yearly risk is calculated,
where risk is defined as the product of the probability of flooding and the economic damage of a flood. Assessing
these probabilities is not easy, because there are many failure mechanisms and many dike sections (and
structures). For an adequate assessment these factors have to be taken into account. Assessing flood damage is
often done by assuming a ‘flood scenario’, calculating flood depths. These depths are combined with damage
functions, which give the relation between depth and damage. Next, the Present Value of the expected yearly
risk is calculated to assess the total benefits, taking into account economic growth and sometimes also climate
change is included. Incorporation of economic growth and climate change changes the problem into a dynamic
investment problem, as already described by Van Dantzig (1956). Repeated investments in flood mitigation
measures are necessary from an economical point of view.
In the presentation we will discuss three new methodological developments in the economical impact assessment
of flood mitigation measures. First, the relation between the exceedance frequency of water levels (which
is often used as the indicator of flood probability) and the flood probability will be discussed using some examples.
In the examples we will not consider all failure mechanisms, but only the mechnism ‘overflow and wave
overtopping’. In this example we consider the natural variability of water levels, but also knowledge uncertainties.
Second, the approach of Van Dantzig is recently extended by Eijgenraam (2003), and we will present the
consequences of his approach. In his approach, the ‘optimal’ protection level decreases in time (safety will increase),
due to economic growth. And by climate change the flood probability increases, and additional investments
are neceassary from an economical point of view. Thirdly, assessment of flood damages has a lot of uncertainties,
and we present results of an uncertainty analysis of flood damage using, carried out by HKV Consultants
and Delft University of Technology.
Literature
Dantzig, D. van., 1956. Economic decision problems for Flood prevention. Econometrica, 1956.
Eijgenraam, C.J.J. “KBA Ruimte voor de Rivier. Hoogte van dijken”, Centraal Plan Bureau, Memorandum,
6 september 2004.
Walker, W., M. Kok et al., 1994. A Policy Analysis of Dutch River Dike Improvements: trading off Safety, Cost,
and Environmental Impacts. Operations Research 42, 5, 823-836, 1994.
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