Eco-friendly closure of tidal river systems

Abstract

To protect the hinterland, flood defences are often located on the border between water and land. This same border however is also an attractive place to live, which may hamper improvement of flood protections. Closing off a tidal (river) system, especially in case of little discharge, may in this case be a logical consideration. This would however result in the disappearance of tidal action, while ecological value in urban areas is already under pressure. Environmental organisations are representing the increasing voice and power of society and its cry for nature conservation. The Hollandsche IJssel is one of the side branches of the Dutch delta. It is a perfect case of the conflicting interests mentioned. The aim in this research was to design a hybrid barrier for this case that provides (1) the security of a close-off to the hinterland and (2) the openness of a surge barrier (tide and flow are possible) to nature and thus conserves this unique system. The core of this research is to come up with a preliminary design of a hybrid barrier that is appropriate from an ecological point of view. To do so, boundary conditions were distilled that ensure conservation of ecological value of the freshwater tidal river system. Focus was laid on the unique aspects of the ecosystem at hand. Ecological value is considered to be conserved if the hybrid barrier conserves the tidal range as much as possible (90 %) and if fish migration is still possible. The line of thought on how to take into account ecological value presented in this research could be used by others seeking to fully incorporate ecological value in their design. Several concepts and locations for the eco-friendly barrier are discussed. Furthermore, in order to increase the available intertidal area, a proposition is made to narrow the river. The proposed strategy consists of three aspects, namely (1) an eco-friendly barrier with pumping capacity, (2) river narrowing and (3) no dike reinforcements after implementation of the new barrier. Until the proposed strategy is implemented, dike reinforcements may still be necessary however. The proposed strategy was compared to the current or reference strategy. The reference strategy consists of three aspects, namely (1) improvement of the current barrier, (2) replacement of the barrier with a similar barrier in 2050, and (3) continuation of dike reinforcements. In both strategies the current barrier is replaced. It was found that for the proposed strategy, a single gate solution is not an option, hence multiple small gates are required. A single gate solution would lead to too high water levels in the Hollandsche IJssel. The final design consists of 31 square culverts with sides of 2.7 m. To maintain maximum tidal range pumps are required between 2105 and 2140 (depending on the severity of sea level rise). Therefore, a pump structure with 6 square inlets with sides of 3.7 m is included in the design. Approximately 90% of the original tidal amplitude is maintained and the available intertidal area almost triples. Furthermore, fish are able to migrate the majority of the time. The slight reduction of the tidal range is compensated by creating extra intertidal area. Therefore, the design is considered to be acceptable from an ecological perspective. The cost of implementation of the proposed strategy in 2030, 2050 and 2070 was compared to the reference strategy. Three points of comparison were taken into account, namely: (1) societal cost of shipping delay (2) cost of construction and (3) dike reinforcement cost. It was found that the proposed barrier including the creation of new intertidal areas is significantly cheaper than the reference strategy in two cases considered (implementation in either 2030 or 2050) or just as expensive as the reference strategy (implementation in 2070). In total, societal cost ranging from 400 M€ up to 700 M€ can be saved when implementing the new barrier complex in 2030. Although the societal cost due to shipping delay are higher, construction cost of the barrier itself are lower. More importantly, expensive dike reinforcements can be averted in the proposed strategy. Earlier implementation results in lower investment cost, mainly because less dike reinforcements need to be carried out.