Unembanked Areas: A risk assessment approach

Unembanked Areas: A risk assessment approach

Wolthuis, M.P.

Van de Giesen, N.C. (mentor)

Civil Engineering and Geosciences

Watermanagement

Water Resources

2011-06-23

Areas outside the primary flood defenses, here called unembanked areas have a special status in the Dutch water safety policy. Whereas, primary flood defenses have to fulfill to legal standards and a functional manager is appointed for construction, maintenance and management. For unembanked areas this situation is different; some provinces have water safety policy and according to the national water plan residents and users are responsible for taking consequence reducing measures of floods. For the development of new areas decisions have to be made about the desired level of safety and how this is achieved. This leads to the issue of optimal adaptation strategies. What is the best level of safety so that unnecessary high risk levels and overinvestment in safety related infrastructure can be circumvented? This study presents a framework for municipalities and property developers how to deal with flood risk in unembanked areas. 952 developments are planned in unembanked areas of which 183 comprise urban dwelling projects. This thesis especially focuses on these urban dwelling projects where flood events can be regarded as a local, regional and direct tangible risk. The following research question is answered: How can we deal with the uncertainties of flood risk in investment decisions in the development of unembanked areas? 1. What is the current policy of building in unembanked areas and what are the responsibilities of the government? 2. Which strategies can be formulated to create the desired level of safety and how should they be compared? 3. How can a multi-layer safety approach contribute to the safety of the project area? 4. How do area specific characteristics influence the cost effectiveness of the measure? 5. How to deal with the residual risk? A multi-layer safety approach assumes three layers in flood control: 1. Prevention: characterized by structural measures which influence the boundary conditions of the project area. Surface level heightening and the construction of an embankment are discussed 2. Spatial planning, characterized by structural measures which influence the exposure and vulnerability. Wet proofing, dry proofing and an elevated configuration are discussed. 3. Disaster control, characterized by non-structural measures which influence the exposure and vulnerability. Organizational aspects and financial compensation are discussed. It was founded that the urban dwelling density of a project area determines the profitability between individual consequence reducing measures (layer 2 of the MLS approach) and collective probability reducing measures (layer 1). The profitability of collective measures grow linear and transcend individual measures at 24 dwellings/ha. An elevated configuration is preferred above wet and dry proofing. Considering the construction of an embankment it was founded that the profitability grows according to a power function and transcends surface level heightening at 35 ha. All proposed urban dwelling plans in unembanked areas are analyzed on these criteria and it was founded that for 23% of the plans individual measures are preferred above collective measures. 62% of these plans are located in areas where the province has no flood probability standards and therefore consequence reducing measures have a good chance. The other 38% of the plans are located in provinces with flood profitability standards and the profitability of extra consequence reducing measures is dependent on this standard. For the remaining 77% of the areas a probability reducing approach is preferred; of which for 6% the construction of an embankment is preferred and for the other a surface level heightening strategy is preferred. All criteria of insurability (grouped in actuarial, market-determined and societal) are analyzed for a flood damage insurance for unembanked areas. Due to the physical aspects and policy of unembanked areas the formulated criteria of insurability score better for unembanked areas. The actual realization will depend on the market determined criteria. This approach has been tested for a redevelopment project in Rotterdam, Heijplaat, where it was founded that surface level heightening is the only profitable measure.

unembanked areas
risk assessment approach

http://resolver.tudelft.nl/uuid:b825ae6e-c169-40f6-9d57-48f18ee48ef9

2011-07-21

Student theses

master thesis

(c) 2011 Wolthuis, M.P.