Bart Schultz
The Rhine is fed by both rainfall and meltwater, although the contribution from meltwater is diminishing due to the reduction of snow and glaciers in Switzerland. Accounting for 65%, the Rhine supplies by far the largest share of the country's surface water. The average discharge at Lobith - where the river enters the Netherlands - is 2,200 cubic metres per second. At this discharge rate, the water level there stands at nearly 11 metres above mean sea level.
Originally, the waters of the Rhine flowed through the Netherlands to the North Sea in broad channels filled with shallows and sandbanks, with the river occasionally bursting its banks. From 1100 onwards, the flow pattern became more irregular due to land reclamation, leading to more extreme water levels. The water flowed via various river branches; the largest of these was - and remains - the Waal. The others were the Oude Rijn, Vecht, IJssel, Lek, and Hollandse IJssel. Of these branches, only the Oude Rijn flowed directly into the North Sea. The Vecht and the IJssel discharged into the Almere, which later expanded to become the Zuiderzee. The IJsselmeer was formed following the closure of the Enclosing Dam in 1932. The Lek and the Hollandse IJssel flowed into the Merwede.
Before 1200, river flooding was not considered a disaster. The first recorded instance of a river flooding was the breach of the Noorder Lekdijk in 1233. Since then, the country has experienced numerous cases of flooding caused by the Rhine; the consequences of these events have been increased by land subsidence, population growth, and the rising value of buildings and infrastructure in flood-prone areas. Flooding from the Waal and the Lower Rhine/Lek could extend as far as the middle of the Alblasserwaard polder, southeast of Rotterdam. Downstream of that point, flooding was caused by storm surges from the sea.
Flood prevention measures have been taken since the 11th century. For instance, the dikes along the Rhine branches in the present-day Province of Gelderland were closed off after 1300. Since then, the approach and safety standards have been adjusted from time to time, and this will remain important in the future.
Between 1250 and 1600, not only did dike breaches occur more frequently, but the flooded area also became increasingly large. This was partly the result of land subsidence caused by the reclamation of peatlands.
Cases of Flooding occurring up to the 16th century are documented only in a fragmentary manner. It was not until the 16th century that an accurate account appeared of a catastrophic flood involving a chain of dike breaches that flooded the entire river region. Similar disasters occurred repeatedly in the centuries that followed.
Ice jams caused numerous cases of flooding, particularly between 1650 and 1850. Nowadays, the formation of ice jams is virtually impossible due to the warming of the water caused by the discharge of cooling water and shipping activity.
Over the course of the 17th century, an increasing volume of water flowed into the Waal - eventually reaching around ninety percent. The significantly reduced inflow into the Lower Rhine and the IJssel caused their discharge capacity to drop sharply. During periods of high discharge, this led to dike breaches and flooding. To remedy this, the Pannerden Canal was made in 1707, incorporating measures to achieve the desired distribution of water among the various Rhine branches. By the end of the 18th century, a distribution had been established in which two-thirds of the water flowed into the Waal, while the remainder - south of Arnhem - was split such that two-ninths flowed into the Lower Rhine and one-ninth into the IJssel.
Around 1800, the river region was struck by severe flooding roughly every ten years. For instance, large parts of the river region were flooded in 1809, 1820, 1855, and 1861. The flooding of 1809 was the most disastrous ever.
Following the floodng of 1809, the Central Committee for Water Management was established. After considerable discussion, it was decided to construct the Liemers spillway in the Oude Rijn dike, east of Zevenaar. By 1820, it was already evident that this overflow channel could not prevent dike breaches. Consequently, it was decided in 1852 to close the Liemers spillway.
Following the flooding disaster of 1820, a committee was established by Royal Decree to investigate the best river diversions. The organization of the dike defence system was significantly improved during this period; not only were the dikes reinforced, but the riverbed was also improved, and several bottlenecks in the Waal were eliminated.
In 1849, two inspectors from the Agency Rijkswaterstaat were commissioned to make recommendations. They recommended that each river branch be adjusted to a standard width with a single, continuous channel, enabling it to independently discharge upstream water and ice. For instance, the width of the Waal was to be 360 metres at Pannerden and 600 metres further downstream.
Following the devastating cases of flooding of 1855 and 1861, large-scale improvement works on the river branches and dike reinforcements were carried out to enable the rivers to accommodate peak discharges and ice flow, and to make them suitable for navigation. The prevailing idea was that these dike reinforcements and river improvements would largely eliminate the risk of flooding. This view proved overly optimistic, as demonstrated, for instance, by the flooding of 1926. In the wake of that flooding, further large-scale dike reinforcements were undertaken, raising the dikes to a level one metre above the peak water level in 1926.
Following the 1953 flooding disaster, it was decided that dikes in the river basins would be reinforced based on an annual flood probability of 1 in 1,250 years. Nevertheless, there were initially no large-scale dike reinforcements. This was due to the central government failing to make sufficient funds available and to the deep mistrust between the Water Authorities and activists campaigning for landscape preservation.
Water levels in various branches of the Rhine were very high in December 1993 and January 1995. In January 1995, the water rose about 25 cm higher than in 1993. Confidence in the safety of the dikes was lost, as certain sections had a level of safety with a failure probability of only 1 in 100 per year. Fortunately, the dikes did not fail. This situation led, among other things, to the Room for the River Project and the current Flood Protection Programmes. I previously presented this in greater detail in a contribution to Flevolands Geheugen regarding Safety against flooding in the Netherlands.
