Flood management strategies for the Rivers Rhine and Meuse in The Netherlands

Transcription

1 Destructive Water: Water-Caused Natural Disasters, their Abatement and Control (Proceedings of the Conference held at Anaheim, California, June 1996). IAHS Publ. no. 239, Flood management strategies for the Rivers Rhine and Meuse in The Netherlands JOS DIJKMAN, ROB KLOMP & MONIQUE VILLARS Delft Hydraulics, PO Box 177, 2600 MH Delft, The Netherlands Abstract Two recently conducted major studies onflooddamage mitigation in The Netherlands have resulted in concrete courses of action regarding flooding issues of the Rivers Rhine and Meuse and opened the way for a new approach for river and flood management in the future. This new approach deviates from historic attempts to control floods (i.e. emphasis on dike construction) and is oriented towards providing sufficient space for the river to develop under more or less natural conditions and allowing the riverine ecosystem and landscape to develop in a more natural way. These studies were conducted under the oversight of two independent Committees which advised the Government on issues that were much debated in Dutch society, namely (a) how to carry out the dike improvements along the River Rhine and (b) how to deal with theflooddamage recently experienced along the River Meuse. FLOODS AND FLOODING THREATS IN 1993 AND 1995 The events surrounding the extreme discharge of the Rivers Rhine and Meuse in The Netherlands in recent years will have escaped few of those interested in the field of flood mitigation. Over Christmas 1993, flooding of the River Meuse in the hilly southeastern part of the country inundated thousands of homes and businesses, directly affecting over 8000 people and causing about US$160 million in damage. Just 14 months later, in early 1995, similar events took place, again resulting in significant damages. Also in early 1995, flooding occurred in many European countries, including France, Germany, and Belgium. Along the River Rhine in The Netherlands, the events took a rather remarkable turn. Due to fears of dike instability and possible failure, close to people and cows and pigs were evacuated from the low lying areas adjacent to the river within a timespan of some 48 h. The dikes did hold due to vigilant surveillance and constant reinforcement, but four people lost their lives in unfortunate accidents. Damage reached a total of US$600 million, mostly related to the costs of the evacuation. The probability of occurrence of these Rhine and Meuse discharges was "only" about 1/100 per year. In order to understand how this all could happen in a country that has developed a reputation regarding flood mitigation, some historical perspective is needed.

2 372 Jos Dijkman et al. FLOOD PROTECTION HISTORY IN THE NETHERLANDS The severe coastal flooding that occurred in the southwest of The Netherlands in February 1953 claimed nearly 2000 lives and triggered the well known "Delta Project". Major investments in coastline protection were made, including the closure of various estuaries. The safety level along the coast was thus increased to 1/4 000 or 1/ per year, depending on the location. The majority of this work was conducted between 1955 and 1985, though the last project (construction of a movable gate across the entrance of Rotterdam Harbour) is currently being completed. Along (the southern reach of) the River Meuse, there have historically been no flood protection measures. Due to the natural profile of this river (shown in Fig. 1), flooding can cause damage, but safety is not usually an issue (inundation depths are limited to about 1 m). This has resulted in a relatively low priority for flood protection activities here. Along the Rhine branches in The Netherlands, the river level is several m above the adjacent "polder" land. If the river overtops or breaks through its dikes, the inundation depths can be 4-5 m, creating severe safety risks for the people living there (Fig. 1). In 1958, the Government decided to reinforce the river dikes to a safety standard of 1/3000 per year, but the urgency and expense of the Delta Project resulted in a secondary priority for the strengthening of the 570 km of dikes along the River Rhine. In addition, the plans developed in the 1960s for river dike strengthening had little public acceptance and often spurred widespread protests against the "bulldozer methods" applied to those dikes that run through beautiful natural landscapes or historical and cultural sites. In 1977, an independent Committee (installed by the Minister of Transport, Public Works and Water Management) advised the government to adjust the plans for dike strengthening to better reflect the values found in the region. This Committee recommended lowering the safety standard to 1/1250 per year, improving the technical design and revising the decision making procedures. Plan adjustments were made but to no avail. Years of legal procedures followed in which those opposing the plans were able to slow down the process considerably. As a result, little or no work on dike strengthening was performed during the period River Meuse - damage protection River Rhine - polder safety Fig. 1 Schematic of River Rhine and River Meuse cross-sections.

3 Flood management strategies for the Rivers Rhine and Meuse in The Netherlands 373 ANALYSIS TO SUPPORT RIVER RHINE FLOOD MITIGATION (BOERTIEN-I) By July 1992, the protests against river dike reinforcements prompted the Minister to install a new independent Committee (the Boertien-I Committee, named after the Chairman) to conduct a policy analysis study to verify the existing dike-improvement criteria. The project area primarily covered the dikes along the non-tidal stretches of the River Rhine and its tributaries, but also included the lower stretches of the River Meuse, as shown in Fig. 2. The study primarily had to answer questions regarding the dike safety standard to maintain and whether or not any new scientific and technological insights in dike construction could lead to new recommendations regarding dike improvement. The study was commissioned to the consortium of Delft Hydraulics and Rand European-American Center for Policy Analysis (AEC-Rand). Several other technical firms and organizations were associated with the consortium. The study was conducted along the lines of a structured policy-analysis method, as presented in Fig. 3. The analysis had to be performed within a tight 5 month time schedule and a budget of US$1.5 million.

4 374 Jos Dijkman et al. safety analysis boundary conditions structural design functions and values procedures over-all regional 1 selection 1 selection 1! mm IE 1 ^^^Ë^^^^^^^^^^^si^ m impact assessment IE score cards III 1 i' strategy design II mihiiiiii III ^HI^^^^^^^^B i' II m em 1! 1 Fig. 3 Boertien-I policy analysis method (project execution scheme). evaluation of procedures recommendations for improvement Tl Results Since most dikes were sufficiently high, but needed reinforcement, the consequences of a lower safety standard proved to be limited (see score card in Table 1). A reduction of the safety standard from 1/1250 to 1/500 per year would reduce damage to inventoried "landscape, nature, and cultural" (LNC) values only slightly. However, using a lower safety standard, personal risk and economic damage would increase. The Boertien-I Committee recommended that the existing safety standard of 1/1250 per year be retained for most regions but suggested applying lower safety standards for certain less sensitive areas. The existing guidelines for river dike design were also analysed in detail, taking new technology and state-of-the-art design practices into consideration. This resulted in recommendations for a new "optimal" design with a smaller dike cross-section, which was better for landscape preservation. Also, new technologies like sheet piles, diaphragm walls, and flap gates were assessed. Based on this analysis, several stretches of the dike system could be strengthened to the required safety level while preserving identified LNC values. Possibilities for the development of other values, including the improvement of dike vegetation by stimulating an environment-friendly management, were also created. Recommendations were given for a modification of the state-subsidy structure to stimulate creative design and nature-friendly management of the vegetation. Measures for improving the existing dike strengthening plans were combined into alternative strategies, which were assessed using a number of criteria, such as damage to the current functions and values, potential for flood-induced damage, and budget required for construction and maintenance. Policy options were fashioned by combining a value for the safety standard and strategies for dike improvement from which the decision makers could make their choice.

5 Flood management strategies for the Rivers Rhine and Meuse in The Netherlands 375 Table 1 Score-card of various effects (criteria) as a function of dike safety standard. Criterion Dike length to be raised/reinforced raised and reinforced reinforcement only Personal risk Preservation of LNC values: landscape nature culture Damage by flooding (NPV): dike ring no. 43 dike rings 36, 43 and 52 all dike rings Dike reinforcement costs: Unit km km km - % % % w M/ M/ M/ Safety standard (per year) 1/1250 1/500 gr=16 500m 3 s-' g^=15 200m 3 s-' Q m = 3650 m 3 s" 1 Qm = 3350 m 3 s R 2.5R Legend: Qr, Q = m design discharge of the Rivers Rhine and Meuse, respectively; R = personal risk for a safety standard of 1/1250 per year; NPV = Net Present Value; (1.7 M/ = 1.7 million guilders = US$ 1 million). 1/200 g^=13 650m 3 s-' Q m = 3050 m 3 s" ? In 1993, the Boertien-I Committee presented its conclusions in the form of a recommended strategy (plans) for the dike strengthening along the River Rhine (and lower stretches of the River Meuse). The plans incorporated a safety factor of 1/1250 per year and included various new design factors and technologies which would (at some extra cost) preserve many of the identified landscape, natural and cultural values. The Government decision on dike reinforcement closely followed the Committee's advice. The total dike reinforcement program required a budget of US$600 million. Thus in 1993, (publicly acceptable) plans were finally in place for the dike strengthening along the Rhine. ANALYSIS TO SUPPORT RIVER MEUSE FLOOD MITIGATION (BOERTIEN-II) After the 1993 flood along the River Meuse in the southern Netherlands, yet another committee (Boertien-II) was established to conduct an independent investigation. The objective of the (US$4 million) study was to prepare an analysis of potential administrative and technical measures to reduce flood damage along the Dutch stretch of the River Meuse that is not provided with levees (Fig. 4). A nine-month study period was made available.

6 376 Jos Dijkman et al. Delft Hydraulics was asked to lead the study and formed a consortium of many specialized institutes to conduct the technical analyses. The study was structured as a policy analysis (Fig. 5). After a data collection phase, in which specific attention was paid to the causes of the flood damage and the emotional effects of the inundation, a number of topics was addressed simultaneously, namely: water management (hydrology, hydraulics, and river morphology), landscape and ecology, institutional and "other" aspects (including agriculture, shipping and recreation). Fig. 4 Schematic map of the (upper) River Meuse in The Netherlands (Boertien-II study).

7 Flood management strategies for the Rivers Rhine and Meuse in The Netherlands 311 data collection / analysis causes of flooding damage T water management aspects T landscape ecological aspects T development of measures and impact assessment methodology T institutional administrative aspects T other functions ( agriculture navigation recreation ) selection of measures design of strategies impact assessment of strategies impact assessment cards Fig. 5 Structure of the Boertien-H study. Identification of potential measures A set of possible measures to reduce flooding and flood damage was identified (many proposals were also brought forward from society). The selection included both institutional and technical measures. The institutional measures considered spatial planning and technical specifications regarding building in the flood plain, as well as with flood forecasting, information dissemination to the population, evacuation procedures, and disaster mitigation. Measures were also accepted that could considerably reduce flood damage, including water proofing of the distribution network of utilities (power, gas, drinking water, telephone, cable television, etc.), constructing facilities to close sewer overflows to the river, and increasing the elevation of certain roads to better serve as evacuation routes. Two sets of large scale engineering measures were selected, namely: the deepening and widening of the river over a length of about 190 km, and the construction of low dikes (levees) to protect built-up areas. These measures are illustrated schematically in Fig. 6. Analysis of the River Meuse basin upstream The Netherlands In the public debate on floods, developments in the river basin in Belgium and France (urbanization, deforestation, changes in river geometry, road construction,

8 378 Jos Dijkman et al. winter bed summer bed winter bed Fig. 6 Illustration of possible engineering measures in the River Meuse. etc.) are often blamed for increases in flood stages. In the study, these hydrological aspects were studied in great detail. Comparing land use developments over the last 100 years led to the conclusion that the amount of forest in the basin had not decreased but increased by about 10%. Applying hydrologie and hydrodynamic models for the river geometry early this century and for the current geometry led to the conclusion that the total effect of developments in the basin on peak discharges as in 1993 is less than a 10% increase, equivalent to an increase in flood stage of (only) about 0.3 m. Practically speaking, extreme discharges can, therefore, only be attributed to (too) much rainfall, possibly aggravated by frost and snowmelt. The study concluded that there are no practical options to reduce flood damage along the River Meuse in The Netherlands by means of measures in Belgium and France, thus the flooding problem can only be solved by measures applied in The Netherlands. Definition and selection of strategies Based on selected potential measures and the upstream basin analysis, five strategies were developed, each corresponding to a certain overall policy, namely: focus on river deepening (strategy 1); focus on (extensive, less extensive and restricted) nature development (strategies 2a, 2b and 2c); and focus on embankments (levee construction) (strategy 3). The five alternatives were each investigated in the same level of detail, with respect to level of protection, costs, time of completion and opportunities for ecology and nature development (Table 2). The Committee selected strategy 2b as the best means for providing flood protection by making a compromise between costeffectiveness, ecological attractiveness, and time for completion. There was complete agreement among the Committee members about accepting limited extra project costs to preserve values of landscapes and cultural heritage and to provide options for nature development. The Committee selected a 1/250 per year flood protection level based on economic considerations (given that safety is not an issue). The length of the dikes required to achieve this protection level is about 62 km, of which only 6 km is higher than 2.5 m.

9 Flood management strategies for the Rivers Rhine and Meuse in The Netherlands 379 Figure 7 illustrates the significant reduction in flood stages after adoption of this strategy (compared to the flood stages in December 1993) along the river. Only at a fixed weir at km 69 along the river (Fig. 7) does lowering of flood stages remain unfeasible given the major construction changes that would be required. As a result of the selected strategy, which combines institutional and engineering measures, the expected value of flood damage is reduced by more than 90%. The institutional measures try to close the gap that has developed between spatial planning (allowing villages and towns to develop in the flood plain) and water management (warning against this but having no legal base to block such development). The engineering measures not only reduce flood damage but also restore or develop natural areas by focusing on widening and deepening the river in order to lower flood stages and developing low dikes only where additionally required. Table 2 Summary of Meuse strategies giving cores on main aspects and criteria. Aspect Unit Strategy: 1 2a 2b 2c 3 Protection - reduction flood damage % - reduction in number of evacuees % compared to reference situation at flood flow like Dec Changes in landscape, etc. - total length of dikes (levees) km - length of river works km - amounts of sand and gravel Mton - surface area of agricultural lands ha to be changed to nature development Damage and opportunities for nature and landscape - loss of existing values Qual. - potential for new values Implementation period vear - start of works within: vear - completion of works within: - damage reduction after 10 years % Cost - investments M/ - possible benefits (sand, gravel) W - annual maintenance costs W - financial risks Qual. - Net Present Value W Mton = million tons; M/ = million guilders (1.7 guilders = 1 US$); and Qual. = Qualitative scores: + + strong reason to select the strategy; + reason to select the strategy; 0 no reason to select the strategy; - reason not to select the strategy; - - strong reason not to select the strategy. 95% 95% 90% 65% 65% 99% 97% 96% 86% 86% % 60% 60% 65% 65% /

10 380 Jos Dijkman et al. ai > a) Fig. 7 Flood stage reduction after completion of the Boertien-II advice length profile Meuse (km) Because a total of 130 million tons of sand and gravel can be sold to the construction industry, the entire strategy can in economic terms be realized at no net cost, provided that the market is not spoiled by large quantities. Therefore, an implementation period of years was proposed, during which the flood protection level gradually would be increased as the work progresses. This advice was well received in society and politics. Ironically, the advice of the Committee was published in December 1994, just a few weeks before the flood of January 1995 hit the same area again. RESPONSE TO THE 1995 FLOOD AND FLOODING THREAT The near disaster along the River Rhine and the repeated flooding along the River Meuse in early 1995 made everybody in The Netherlands realize that something had to be done quickly to move forward with dike strengthening and other forms of flood protection. While the sand bags were still being carried by the army to the weakest stretches along the Rhine, parliament quickly approved emergency legislation. A new law, "The Delta Act for the Large Rivers" came into being in April 1995 requiring the enforcement of about 200 km of river dikes along the branches of the River Rhine in the central parts of The Netherlands before 1997 and completion of all dike strengthening by The levees along the River Meuse in the southern part of the

11 Flood management strategies for the Rivers Rhine and Meuse in The Netherlands 381 country had to be constructed before the end of Parliament decided to develop not just 62 km but 145 km of levees, and at short notice, in order to provide an initial 1/50 per year protection level. The widening and deepening of stretches of the River Meuse must be completed in the year 2005 (within 10 years). This act is exceptional in a number of respects. Firstly, it sets aside all other legal acts, rules, and regulations pertaining to the river stretches to be protected. It allows for immediate and forced transfer of private ownership of land and buildings to the water management authorities. The law also provides for only one round of appeals, to which independent judgement is applied. In addition, it requires that the design of the protective measures should explicitly take into account landscape, cultural and ecological values. The water managers in The Netherlands are now actively looking for ways to safeguard the flood-prone areas while at the same time create opportunities for the establishment and evolution of 4000 ha of natural river landscape before the year When all these works have been completed, the Dutch will rest more easily knowing that they should be safe from most potential floods. However, they will always remain aware of the fact that whatever safety level is provided, an even higher flood always remains possible.