From outer delta to Biesbosch

The area of Haringvliet-Hollands Diep-Biesbosch comprises an entire river delta from the Outer Delta to Biesbosch. This is the area where the influences of sea and river meet, where freshwater and salt water mingle. An area of contrast: idyllic villages and towns in vast polders, Moerdijk’s high-tech petrochemical industry next to mud flats and salt marshes, peacefulness alternated with busy activity.

Expected sea level rise caused by climate change and changes in river management require increased retention capacity for the Haringvliet-Hollands Diep area. The banks of its so-called ‘bathtub’ need to be higher, or the bathtub itself should expand. Central question is how to use the preferred strategy of ‘continuation on the current trajectory’ to tackle problems of water security, and simultaneously make the area as attractive and adaptive as possible.

For our International Architecture Biennale Rotterdam (IABR) entry, BoschSlabbers studied the area’s water and security challenges, and developed two scenarios in more detail.

Expanding the bathtub

For the short term (2020-2030), dikes in the area are strong enough, and those that are not can be relatively easily reinforced. Technically, the time-honoured strategy of reinforcement can be kept up for a long time to come. This will keep the area safe and secure, but it does not necessarily make it more interesting.

From 2030 onwards, the region’s water retention area (the ‘bathtub’) will undeniably need expansion. There are three options:

• A bathtub with higher banks: increasing the height of the dikes along the Hollands Diep-Haringvliet to form a ‘superdike’, allowing for higher water levels.
• An extra bathtub: diversion of excess water towards the Volkerak-Zoommeer and Grevelingen. This option allows for a later (circa 30 years) and less radical reinforcement of dikes along the Haringvliet-Hollandsch Diep.
• Expanding the bathtub: enlarging the retention surface by dike replacement. In this option, a broad stretch of the current primary dike is lowered and a new dike is constructed further inland.

In our study, both the first and the third option were explored.

Superdike – The height of the dikes on the northern bank of the Haringvliet-Hollandsch Diep are increased and broadened to form a plateau. Together with the adjacent foreshore and hinterland, this creates a dike landscape with lots of potential. Three types of dike are created:

Dune dikes. To the west of Hellevoetsluis, sand suppletion strengthens the dam. The result is a broad and dynamic dam with woodlands and shifting sands, and room for a broad program.

Saltmarsh dikes. Around the mouth of the Spui, the dike is reinforced with clay and sand extracted from the foreshores. This allows the tide to reclaim the foreshores, and restores the regular flooding of salt marshes and mudflats at high tide. The salt marshes and mudflats in turn have a positive influence on the dam: they cushion the beating waves and thus increase the dam’s stability.

Biesbosch dikes. In the east, dikes are reinforced with dredging material from the river channel. On and in front of the dike, tidal woodlands develop, connected to the Biesbosch ecosystem. Apart from their ecological value, these tidal woods also have a cushioning effect against the beating of the water.

Dike replacement – On the northern banks of the Haringvliet-Hollandsch Diep, a new inland dike system is constructed, over a length of 25 kilometres. Existing dikes are lowered to a level that allows regular flooding. This expands the foreshores, serving as water retention area of 30 square kilometres.

The dike replacement places several towns and villages back at the waterfront. New, proud promenades are developed, extending outwards from the dikes like bastions. The areas between both dikes change into delta wildernesses: terrains growing gradually wetter and rougher. These ecological zones also create room for the development of part-time residency and more innovative forms of long-stay tourism. In the easternmost polders in the zone, tidal woodlands will grow, while closer to the mouth of the Spui, new mudflats and salt marshes develop along the foreshore. West of Hellevoetsluis, a new dune valley arises along the foreshores, to merge with the rest of the gouged Dutch coastline.

Perspective 2100: Green Delta Edge as front garden of Greater Rotterdam

Dikes still play a central role in 2100. After all, on and at the dike, everything comes together. The dike keeps water out, guarantees water security and connects land with water. Along its length, new ecosystems develop. On the dike, or at its foot, people live, enjoy, invest. In 2100, the dike is no longer a monofunctional line, but part of a multifunctional dike landscape, founded upon existing features of the landscape.

Urban heating forces urbanites outwards, to find coolness along the waters of the delta. In 2100, the Haringvliet-Hollandsch Diep has developed into the vibrant ‘front garden’ of Rotterdam. The area flourishes, due to its accessibility from the city centre: attractively designed ‘threads’ (by foot, bike, horse, car, public transport or boat) connect the inner city Coolsingel with the ‘Rotterdams Koffiehuis’, a bistro pub on the island of Tiengemeten.

Along its waters, the ‘Green Delta Edge’ has been developed, boasting new and innovative residential and leisure types; private parties invest in summer-cities and delta estates, and the Rotterdam Port Authorities regard healthy living conditions as an integral part of its business climate and competitive position.

In 2100, the area between outer delta and Biesbosch is an integrated ecological zone of considerable size; a ‘Blue Carpet’, where seal and beaver meet. Sandy islands in the outer delta improve the mixture of fresh and saltwater, and the Biesbosch’s freshwater tidal areas have expanded.

Towns and villages have reinforced their links with the water in 2100, through restoration of old harbour connections and development of new promenades. On the island of Voorne and in the Hoeksche Waard polder, inland creek systems have been restored, for water retention purposes and as expansion of boating networks.