Destructive Water: Water-Caused Natural Disasters, their Abatement and Control (Proceedings of the Conference held at Anaheim, California, June 1996). IAHS Publ. no. 239, 1997. 383

How to manage flood waves in the Dutch

JOOP H. GERRETSEN Rjkswaterstaat, Department, PO Box 25, MA 6200 , The

Abstract The River Meuse in The Netherlands is 225 km long from the Belgian border to the tidal area, and is under management of the Directorate Limburg. The upstream part, the Gravelmeuse, is hilly whilst the down­ stream part, the Sandmeuse, has a lowland character. The largest part of the Dutch Meuse does not have river dikes and there is hardly any protection against floods, while there are many building areas, industry and horti­ culture. The forecasting of flood stages mainly takes place by using a one- dimensional water motion model. After the flood warning messages the local authorities make technical arrangements. After the floods of 1993 and 1995, public pressure was so great that, as a quick reaction, about 145 km of clay walls were built in the Meuse valley. Safety has to increase still more and, therefore, the Gravelmeuse and Sandmeuse projects are under construction.

HYDROGEOLOGY AND HYDROLOGY OF THE DUTCH MEUSE

The part of the Meuse with a length of 225 km from the Dutch state border (Maastricht) to Bois le Duc (s'Hertogenbosch) (Fig. 1) is under the management of the Directorate General for Public Works and Water Management, Directorate Limburg. This paper has been written from the viewpoint of the manager of this water system. The largest part of the Dutch Meuse does not have dikes, while there are many building areas, industry and horticulture. In normal circumstances the Meuse has a width of 100 m but during periods of rain the width increases to a few kilometres with depths on the flood plain ranging from 1 to 3 m. Some million years ago the Meuse was a tributary of the River and the confluence was in the south of The Netherlands. During one of the glacial ages, a cut-off occurred and with tectonic activity the Meuse changed course to a westerly direction and was connected with the Rhine again at about 40 km upstream from the North Sea Delta. At the begin- ation of the Rhine and Meuse at the old location and by digging a channel to the sea. Because of the absence of glaciers, the runoff in the Meuse depends on rainfall in the basin and is, therefore, very capricious. The minimum discharge is about 10 m3 s"1, the median discharge is 140 m3 s"1 and the maximum measured discharge is 3100 m3 s"1. Often the natural discharge is too little to cope with the demand, as for example for navigation. That is why the Meuse in The Netherlands has seven weirs in order to provide sufficient water depth for shipping. On average, the weirs the are hoisted five days per year because of peak floods, but during wet winters these weirs can be hoisted 15-20 days.

THE FIRST INDICATIVE FLOOD FORECASTING

When the discharge increases to 1250 m3 s"1 at Maastricht all the weirs are hoisted and a natural hydraulic gradient is created. In this situation the flood plains are at the 384 Joop H. Gerretsen

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Fig. 1 The Dutch Meuse under management of the Directorate Limburg. How to manage flood waves in the Dutch Meuse 385

point of being inundated. If this discharge appears likely to be exceeded, the flood forecasting goes to an alert stage and there will be intensive consultation with Riza, (Institute for Inland Water Manegement and Waste Water Treatment) a Dutch specialized service in forecasting discharges from abroad. If there are sufficient data of precipitation and discharges of the Meuse tributaries from abroad available, an estimate of the peak discharge is made 24 h in advance with the aid of the mathematical forecasting model Flofom, and the time of passing at Maastricht is given. At Maastricht the discharges are measured continuously by an acoustic discharge measuring device and water levels are also measured. In this way a reliable stage-discharge curve at Maastricht is obtained. Knowing the empirical and calculated stage relation curves of the principal locations along the Dutch Meuse to the stage at Maastricht (Fig. 2) (Bastings & Calculating Group, 1996), the water levels along the Dutch Meuse can be derived from the forecasted peak discharge at Maastricht. Also, the propagation velocities of all the flood waves in the Dutch Meuse since 1910 are analysed and peak discharge-travel time relations are obtained (Fig. 3) (van der Made, 1967). A timely first message about crest stages and passage times, is very welcome to the authorities and citizens in preparing for a flood. An on-line connection with the measuring network abroad is not always available. In such cases an emergency solution is created with the help of precipita­ tion data in the river basin obtained from the Royal Dutch Meteorological Institute.

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CALCULATION MODEL FOR THE DUTCH MEUSE

The observed rise in stage, the forecasted peak discharge, the help of Riza, and local experience of guessing the falling stage, roughly give the shape of the expected flood wave about 24 h before the discharge peak arrives at Maastricht. This shape of the wave, being the discharge-time relation, is needed as upstream input at Maastricht to operate the one-dimensional water motion model to forecast reliably the crest stages at several locations along the Dutch Meuse. The boundary condition at the down­ stream end of the Meuse, 200 km further on, is the local measured stage-discharge curve. To handle the model, a river schematic was developed with respect to geometry, bottom level, roughness, stream and storage width, and hydraulic radius (van der Veen, 1993). The calculations are based on the principles of conservation of mass and of impulse momentum. During a flood period, at least one calculation is made every half hour. The more measured and forecasted discharge data are available at Maastricht during the flood, the more accurate the shape of the flood wave becomes and the more reliable the forcasting for the downstream crest stages. A reliability of 0.1 m is required. A sharp-peaked wave at Maastricht has more subsidence downstream than a flat wave, so for the same peak discharge the water levels downstream can differ. By analysing the shape of the floodwaves at Maastricht over a period of 80 years it was found that the possible shapes in the 95 % range vary much and, therefore, the water levels downstream can differ from 0.3 m to 0.5 m in the worst case. Often an acceptable shape has to be estimated based upon physical circumstances as forecasted from rain intensities, rain amounts, and storage potentials. Attention has to be paid that in forecasting water levels, there is a good How to manage flood waves in the Dutch Meuse 387

10 20 30 40 50 60 70 80

ERROR IN FORECASTING IN CM Fig. 4 Error in forecasting, depending on the forecasting moment before the peak. and wise cooperation between river experts and the computer. After a flood, the calculated results have to be evaluated by comparison to a dense measuring network. If there are big differences, it may be necessary to upgrade the model (Fig. 4) (Gerretsen, 1994). It was later found that a wrong downstream bottom level caused poor forecasting results at the location Grave, 160 km downstream from Maastricht.

FLOOD WARNING IN THE DIRECTORATE LIMBURG

When flood peaks are imminent, Directorate Limburg carries out pre-planned actions stated in a script. There are two stages: first, the alert stage, when the discharge at Maastricht exceeds the 1250 m3 s"1 (recurrence interval once in a year) and the regional fire service and municipal officials are warned so they may prepare their crisis centres in three locations in the Province of Limburg; and second, the alarm stage, when the discharge exceeds 1500 m3 s"1 (recurrence interval once in two years) and phone and fax warnings and alerts are sent to (a) the adjoining Provinces and Directorates, (b) the Flemish Service of Hydrology, (c) the Water Boards, and (d) the regional police and the media. The fax messages, which in extreme situations appear three times a day, contain forecasted flood crest stages with passing times for important locations along the Dutch Meuse and comparisons with remarkable historic flood peaks. Warnings are given to shipping that it is forbidden to navigate during floods of more than 2000 m3 s"\ recurrence interval once in six years during one 388 loop H. Gerretsen week. This rule helps to limit the extra damage to greenhouses and the water saturated river dikes, caused by screw water. Local authorities in the crisis centres make technical arrangements to protect the citizens against the rising destructive water. There is a team of cooperators working day and night in the Directorate Limburg office in Maastricht to answer about a thousand telephone calls a day from authorities as well as citizens.

THE GRAVELMEUSE PROJECT

The Gravelmeuse section, as shown in Fig. 1 [G] reaches from Maastricht to about Roosteren. The deepest points in the main channel form the state border between and The Netherlands. This part of the Meuse is not embanked and is not navigable. For shipping, the Juliana is available. The river bottom has a steep slope of 0.5 m km"1 and consists of much gravel and little fine sand. The flood plain consists of grassland and agricultural land. As a quick reaction to the floods of 1993 (Bleichrodt, 1994) and 1995 (Goudriaan, 1995) about 45 km of clay walls were built near building areas to protect the citizens and buildings. The walls give protection against floods with a recurrence interval of 50 years. The aim is to provide protection against floods with a recurrence interval of 250 years before the year 2006. The main river should be widened at 12 places and the flood plain should be lowered. Nature friendly development is an important goal (Conducting Group Gravelmeuse, 1996; Delft Hydraulics, 1994). Besides lowering of the flood water levels, the project yields a supply of gravel for the Dutch gravel market and this market also helps finance the works. To be able to forecast the morphological effects of these alterations, the Delft Hydraulics Laboratory built a physical-scale model for a 4 km river section in which simulations of the present situation and the discharges that occurred in 1993 and 1995 are examined. The test results for the present situation appeared to be sufficient. The model can be considered to be representative of the whole Gravelmeuse. A mathematical water motion model was calibrated with the physical parameters of the scale model for all modifications. From the experiments, it is shown that care must be taken in widening the main river at different places at the same time, because at narrow places the water velocities increase and there will be much scour. This scour will be severe at places where the underlayer of the river bed consists of finer material. Moreover, in lowering the flood plain one should be careful of cut-offs of river bends. This can happen after one flood, as in 1995, with great risk to the building areas. Besides the river problems, there are still a lot of other aspects that need attention. The conclusion is that much research still has to be done before the year 2000, at which time the design of "the preference alternative" has to be started, in order to obtain in the year 2006 a lowering of the water level such that floods with a recurrence interval of 250 years do not cause damage any more. After that, further nature friendly construction can take place, taking into account that the safety of the 250 year flood has to be maintained. It is hoped that in the next century it will be proved that the exchange "green for gravel" has been a good exchange, especially with respect to the water levels. How to manage flood waves in the Dutch Meuse 389

THE SANDMEUSE PROJECT

The Sandmeuse section reaches from Linne to Lith, are shown in Fig. 1[S]. The first 90 km are vmembanked, while the last 40 km are embanked, because of the low land. This is the main north-south ship channel and the draught is guaranteed by the effect of weirs. The river bottom is rather flat (hydraulic gradient 0.10 m km"1) and consists of much sand and little gravel. In the flood plain there are grassland and horticulture areas. Clay walls (100 km) have been built along the Sandmeuse too, under the same conditions as those for the Gravelmeuse. Nevertheless, some hundreds of solitary buildings are left unprotected. The aim of the project is to enlarge the main river in such a manner that in the year 2006 the protection against a flood with a recurrence interval of 250 years is achieved (Delft Hydraulics, 1994). As the water depths normally are levelled by the weirs, this reach is different from the Gravelmeuse where groundwater levels are affected. Here the project will only deepen the river bottom. The dredged sand generates necessary income to finance the project. It is estimated, that the amount of sand available is 70 x 106 tons from the intended deepening of the channel by 3 m. Because of a second flood peak within a short time, the clay walls were built with all speed and higher and longer than was intended in 1994, so either (a) the deepening could now be less than 3 m, (b) the safety becomes higher, or (c) the walls have to be lowered later. The decision has to be made after finishing the environmental impact assessment (EIA) in which two additional alternatives have to be considered: (d) only the existing walls will be heightened; and (e) (the nature friendly construction alternative) secondary channels are constructed in the flood plain with as little deepening of the mainstream as possible. By constructing the secondary channels on different inundation levels and taking into account the nature of the topsoil, a great variety of vegetation will develop (Tonneyck, 1996). It is the intention that, after the choice has been made, the project will commence in mid-1998. The morphological consequences of widening and deepening the river are roughly calculated by the one-dimensional sand transport calculation model, Meusemor. A calculation is made for a mid to long term process of 75 years. It appears that downstream at the outlet of the Sandmeuse the bottom will lower 0.5 m over a length of 30 km and also upstream at the beginning of the Sandmeuse lowering of 1.0 m over a length of 10 km will occur. In the largest part of the Sandmeuse, that is about 100 km, there will be aggradation of an average 0.3 m. This has consequences for flood levels, so upkeep is necessary. A big problem is the crossing of the Sandmeuse by 130 cables and conduit pipes. Although there are alternatives for a number of these cables and pipes, it could be a problem to deepen the channel by 3 m for a number of these crossings. Thus, protection of the conduit with coarse gravel or cobbles and constructing a secondary channel as compensation may be necessary. These compensations are also necessary at thresholds of weirs. It appears that a limited number of thresholds does not influence the morphologic behaviour of the river. It may be that the citizens support all these activities in order to make living near the river safer, whilst they still remember the flood peaks. It probably depends on the weather to maintain this support. 390 Joop H. Gerretsen

REFERENCES

Bastings, A. & Calculating Group (1996) Waterlevel and Discharge Calculations for the Meuse. Directorate Limburg, Maastricht, The Netherlands. Bleichrodt, G. et al. (1994) The Meuse Hits Out. Report of the December Flood 1993. Directorate Limburg, Maastricht, The Netherlands. Conducting Group Gravelmeuse (1996) "Green for Gravel" on the Way to a Beautiful Exchange. Province of Limburg, Maastricht, The Netherlands. Delft Hydraulics (1994) Policy Analyses MeuseMaln Report, vol. 4, vol. 5. Delft Hydraulics, Delft, The Netherlands. Gerretsen. J. H. (1994) Floodforecasting Meuse. Otar 79/10 Publishing business Debozet, Hoogmade, The Netherlands. Goudriaan. J. (1995) The Meuse Hits Out Again. Report of the January Flood 1995. Directorate Limburg, Maastricht, The Netherlands. Made, J. van der (1967) Traveltlmes of Floods in the Meuse. The Hague, The Netherlands. Tonneyck, M. R. (1996) Report of the Sketch Design Sandmeuse. Directorate Limburg, Maastricht, The Netherlands. Veen, R. van de (1993) "Zwendl" schematizing and calibrating. Work Documents 037 and 107, Riza, Arnhem, The Netherlands.