Adaptation with Small Steps Or a Big Step? a Probabilistic Approach of flood Risk Reduction in the Dutch Delta
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Adaptation with small steps or a big step? A probabilistic approach of flood risk reduction in the Dutch Delta J. Dokter1;2, T. Botterhuis2, M. Kok1 & H. Van Waveren3 1: Delft University of Technology, Delft, The Netherlands 2: HKV Consultants, Lelystad, The Netherlands 3: Ministry of Public Works, Lelystad, The Netherlands ABSTRACT: In view of expected socio-economic development of the Dutch Delta it has recently be decided to update the safety standards, in terms of a maximum allowable loss of life probability and more stringent acceptable flooding probabilities. In order to assess safety, a probabilistic approach is followed, where the un- certainties in hydraulic loads and strength of flood defences is described by probability distributions. Also, dependencies between various stochastic variables are taken into account. The approach will be described in the paper and applied to the Rotterdam area (South West Delta) in The Netherlands. Here, two divergent strate- gies are identified. The first strategy is to follow an adaptive approach with small temporal steps, with many strengthened inland flood defences as the main core. The second strategy is to continue with shortening the coastline by building a dam and pumping stations in the harbor of Rotterdam. The safety consequences of both strategies will be discussed. 1 INTRODUCTION ing years in order to control the flood threats in an effi- cient manner, see e.g. (Deltaprogramma 2014). In the Large parts of The Netherlands are exposed to the current situation, several river mouths are closed per- threats of flooding due to the influence of high wa- manently by dams or storm surge barriers. This holds ter coming from the North Sea region, due to high for the Haringvliet, Lake Grevelingen and Eastern discharge of rivers like the Meuse and Rhine or due Scheldt, as shown in Figure 1. The Western Scheldt to a combination of North Sea water levels and river is entirely open, providing free navigation to the port discharges. In order to cope these threats, The Nether- of Antwerpen, and the New Waterway is also open, lands is protected by flood defences. However, in but can be closed with the Maeslant barrier in case of 1953 a large flood occurred, where more than 1800 storm surges from the North Sea. The Maeslant bar- people lost their lives. As a reaction to this flood dis- rier is a closable storm surge barrier and is only closed aster, the Delta Works were constructed in the past when extreme high water levels are expected in the decades. The main philosophy of the Delta Works was cities Rotterdam or Dordrecht. As the Maeslant bar- to shorten the coast line by building large dams and rier is only closed for extreme events, ships are able storm surge barriers along the south west delta. to navigate to the Port of Rotterdam and the hinterland From 2017, new safety standards become effective, freely most of the time. stating the maximum allowable flood risk for the en- tire Netherlands. The decision context of the paper is how to implement the new safety standards. In the 2.1 Current strategy: ‘DP’- An open system with neighbourhood of Rotterdam the acceptable flooding closable barrier probability is 10−4 per year. Every twelve years, flood The current strategy by the Dutch government, the defences are assessed to check whether they fulfill the strategy that is elaborated in the Delta Programme requirements. and further referred to as strategy ‘DP’, accounts for flood safety of the Delta by means of the functioning 2 TWO STRATEGIES TO SAFEGUARD of the Maeslant barrier in combination with an ex- URBANIZED DELTAS tensive dike reinforcement program and room for the river measures, see also Figure 1. With a fully func- The Dutch government works with a strategy that de- tioning Maeslant barrier, i.e. failure probability of 0, scribes the measures that will be undertaken the com- the barrier is capable of reducing hydraulic loads in the system significantly. For the location of Rotter- 2.3 Impact of strategies dam, the design water level of 4.0m + NAP, (Nieuw Amsterdams Peil or Amsterdam Ordnance Datum), is The strategies have, amongst others, impact on hy- occurring with a return period of about 50,000 years draulics and strength of flood defences, investments in (Zhong et al. 2012), while it would occur with a re- infrastructural measures, navigation, fresh water, de- turn period of 600 years without a Maeslant barrier. velopment of old harbour area, water quality and ecol- Due to climate change the return periods will reduce ogy. For an indication on impact for these aspects, see in the future. Besides, the Maeslant Barrier has a cur- Van Waveren et al. (2015). rent failure probability of 1/100 per closure (Kallen This paper focusses on the aspects related to et al. 2012), reducing the calculated return periods. flood risk, specifically to the hydraulic loads and the This failure probability is not negligible and have to strength of flood defences. Risk is defined as fol- be taken into account when computing hydraulic load low: “Risk is a function of the probabilities and con- levels and design water levels in the system. sequences of a set of undesired events.” (Jonkman With respect to the functioning of the Maeslant bar- 2007). For the current ‘DP’ strategy - with an open rier, an additional research will be undertaken in or- situation and a closable barrier - a vast risk assess- der to improve its failure probability from 1/100 to ment is undertaken in which the current flood risks 1/200 per closure, taking partial functioning into ac- are determined (Vergouwe 2014). Insights of the risk count. On the long term, the Maeslant barrier will be assessment are used to determine the safety standards replaced at its technical and economic life time after for dike segments. Dike segments have a length of 20- 2070. It is assumed that the new structure will have a 30km and are part of a dike ring, which is defined as failure probability of at least 1/1000 per closure (Vos an area enclosed by a system of flood protections or 2014). high ground to protect the area from flooding (Ver- keer en Waterstaat 2008). For the determination of the safety standards, the following aspects are taken into 2.2 Alternative strategy: ‘Sluices’ - A permanent account: closed barrier 1. Individual Risk (IR) is related with the loss of life probability as consequence of a flood. Every- Opposed to the preferred strategy of the Delta Pro- where in The Netherlands, the maximum proba- gramme, an alternative strategy is proposed by six re- bility that causalities may occur as consequence puted Dutch engineers, led by Frank Spaargaren, a re- of a flood disaster is 10−5 per person per year. tired engineer who has been in charge of the construc- 2. Economic Risk (ER) is the risk related with an- tion works of the Eastern Scheldt barrier. According nual expected value of economic losses. In a to the engineers, the ‘DP’ strategy is insufficient to cost-benefit analysis the economic optimum for cope the threats on climate change in combination flood defences is found. with the waterway function for the Port of Rotter- 3. Societal Risk (SR) considers the number of dam. The engineers have the hypothesis that a cheaper deaths as consequence of a flood disaster. It strategy will be realized when the Maeslant barrier is is based on the perspective of people on risks; removed and replaced by a closed dam, with locks, an extreme event with many fatalities has a sluices and pumping stations, further referred to as al- higher impact than many smaller events with ternative ‘Sluices’. The dam prevents storm surges of few fatalities. the North Sea to penetrate into the system, whereas the sluices and pumping stations are implemented to discharge water from the rivers Lek, Waal and Maas. With above aspects, safety standards (Pmax) for all The second aspect of the alternative strategy is that dike segments in The Netherlands are derived. For the locations that are assessed in this paper, the safety the Eastern Scheldt is used as a retention basin by in- −4 creasing the flow profile from the Maas to the Eastern standard of the governing segment is 10 . Scheldt. This is done by realizing an open connec- The failure probability of a flood defence is a func- tion in the Volkerakdam, the construction of discharge tion of hydraulic loads acting at the flood the de- sluices at the Philipsdam and reducing the leakage fence and strength of the defence and is described by area of the Eastern Scheldt barrier. This strategy re- (CUR190 1997): duces the design water levels upstream of the com- ZZ plexes significantly. With the reduction in design wa- Pf = P (Z ≤ 0) = P (R ≤ S) = fR;S(R; S)dRdS ter level, the engineers want to reduce necessary dike reinforcements, leading to a cheaper strategy overall. Z≤0 The complexes are located nearby the Beneluxtunnel on the Nieuwe Maas and nearby Het Scheur on the (1) Oude Maas, which is more inland than the current lo- where Z = R - S, Pf = failure probability, Z = limit cation of the Maeslant barrier in order to let sea-going state function, R = strength parameter, S = load pa- vessels moor the Botlek area without the intervention rameter and fR;S = joint probability density function of locks, see also Figure 1. of R and S. ‘DP’: Maeslant Barrier The Hague ‘Sluices’: Dam, locks, sluices and pumping stations in Nieuwe and Oude Maas (2000m3/s and 1000m3/s) Rotterdam ‘Sluices’: Retention in Eastern Scheldt Dordrecht Volkerak North Zoommeer Sea Figure 1: System overview for both strategies, with the Maeslant Barrier in the ‘DP’ strategy and dams, locks, sluices, pumping stations and retetion in the ‘Sluices’ strategy.