Text by Mark Voorendt
The city of Dordrecht was founded along the creek Turedricht, or Turedriht, which was connected with the river Dubbel and the river Merwede. After the floods of 1150, the creek turned into an important stream and Dordrecht was founded on both sides of its riverbanks. Dordrecht aimed to become a successful trade city, which succeeded within a century. Dordrecht changed into an industrial city during the industrial revolution and in the twentieth century, the city centre developed into an important shopping area. Nowadays it fulfils a social economic function on a regional level.
The initial river banks were raised and transformed into dikes, to form better protection against fluvial floods. From the 15th century onwards, several city expansions have been carried out towards the river. Until the present day, these expansion areas are unprotected against floods. Moreover, houses were built on both sides of the dike, called the Voorstraat, which has been used as an important shopping street (Figure 2.9). This made the required heightening of the dike around 1917 problematic. Therefore, stop logs in doorways and alleys were used for achieving a sufficient retaining height (Figure 2.10). These stop logs are placed in door openings at the street side of the houses of the Voorstraat, Prinsenstraat and Riedijk. In the Grote Kalkstraat, Houtsteiger and Boomstraat large bulkheads have been placed (Figure 2.11). In between the houses
The improved housing fronts could nevertheless not resist the storm surge of 1953. Sheet piles were then added and extra concrete walls reinforced the houses. During flood conditions, the doorways and alleys had to be closed by stop logs. With this stop log system the flood defence would be high enough, but the reliability of the system has not proven sufficiently. These stop logs, namely, were to be stored inside the houses, but some residents had used them for timber or for fire wood. Therefore, the water board presently organises a simulation of a flood event every year to stimulate the awareness of the residents. The stop logs, by the way, are stored in the unembanked area of the city.
During the last decades, the Dordrecht flood defence needed further improvement because the river discharge has increased and the sea level has risen. A major problem is formed by the monumental status of the city centre, making it impossible to heighten and widen the existing dike. Also a part of the town centre is situated outside the flood defence, along the river the OudeMaas. Several solutions have been studied to solve this problem, like the study of the ’work group 1981/1982’, Heidemij Adviesbureau 1985 and Bouwdienst Rijkswaterstaat 1985. An alternative for the solution of Heidemij is depicted in Figure 2.13: a concrete retaining wall has been integrated in the houses at the waterside of the houses along the Voorstraat. This alternative solution was rejected because of the high realisation costs and negative opinion of the residents. Graduate student Sjaak van ’t Verlaat made an inventory of structural solutions of previous studies. He divided the total alignment into sections and assigned the best solution to each section (Figure 2.14). For a representative section, Van ’t Verlaat made a design for a vertically sliding gate barrier, integrated in the quay wall (Van ’t Verlaat, 1998).
This idea has been elaborated by MSc-student Milan Hinborgh, who studied on a new flood defence through the part of the city of Dordrecht outside the dike. The newflood defence could be constructed on the quays in the town centre. Various alignments are possible,which protect a smaller or larger part of the city. In his report one variant has been elaborated in detail. This design is based on the assumption that the flood defence has to be strengthened on the short termfor the highest expected sea level rise and for a 1/10 000 safety level. The new flood defence would be able to retain a water level of NAP +4,35 m, where in the current situation a level of NAP +3,00mis prescribed. Hinborgh’s flood defence has been designed as a floating barrier: Under normal circumstances a gate is immersed in a recess of the quay wall. During extreme high water, the recess is filled and the gate floats up because of the buoyant force and is kept in place by the water pressure. In this way the quay wall height is extended in height automatically when necessary. When the water level drops again, the recess is emptied and the gate returns to its resting position in the recess (Hinborgh, 2010).
Finally a stormsurge barrier has been designed and constructed (completed in 1996). This surge barrier reduces the water levels during storm surge conditions (water entering from sea). A solution for the increasing flood discharge on the riverMerwede - OldMeuse has not been found yet (Stalenberg, 2010).
The Noordendijk, east of the city center of Dordrecht, has been improved too. Water board ’De Groote Waard’ initiated this project, called the ’DordtseWand’. In this project, the functions of flood defence, living and transport are combined. The reinforcement of the flood defence has been realised by excavating the existing dike slope and constructing an L-shaped wall (figure 2.17). This wall is founded on piles and a cut-off wall prevents piping. The retaining wall reaches up to NAP + 4,30 m and has a design life span of 100 years. At the place of the excavated slope, there is space for cycling and pedestrian paths and a green belt. The floor of the L-wall is at the same time the floor of the dwellings.
The ground floor of these dwellings can be used for shops, offices or storeroom. At several places, the ground floor extends further into the dike, creating sufficient space for a car park. The involved parties have signed a covenant to ensure safety against floods and enable maintenance of the flood defence. The purchase contract of the houses contains requirements to guarantee the preservation of the water retaining function. House owners, for instance, are not allowed to bore holes in the retaining walls (van der Veen, 2003).