Constructing nature islands

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Sediment source and deposition

The entire strategy of building islands with fine sediments focuses on constructing natural islands with excess sediment. Usually, dredged sludge is thought to be useless and it is disposed at designated locations in the open sea, along rivers or estuaries or in designated deposits on land. However, mud can be re-used as a building block to form islands.

Fine sediment typically settles in protected areas such as ports, lakes or reservoirs as these are sheltered from currents and waves. Sedimentation rates can vary between a few centimetres to more than 100 centimetres per year. Areas like ports or specific spots in lakes therefore often have an excess of sediment, which can be used as a source for island building. The origin of the sediment (e.g. fluvial or coastal) can reveal more characteristics of the material, such as the potential presence of contaminants, the grain size and cohesiveness.

Hydrodynamic processes

Depending on whether the (is)lands are built in a lake, river or in a tidal environment, the processes in either of those environments are relevant. Below some of the most important processes are described for lakes and tidal environments. These are the dominant processes that drive sediment transport and morphology. The case of rivers is not regarded here in detail, but for rivers, flood intensity and return time determine peak flow velocities, inundation frequency and water level.

(A) Processes in lakes

For freshwater lakes, special attention should go to the water level of the lake and its variation over the year. The water levels will indicate at what elevations areas are inundated, what the inundation depth is and the duration of inundation. Moreover, attention should be given to the local wave conditions, which are the result of local wind direction and speed, fetch length and water depth. Strong flows or strong wave conditions can result in erosion of the islands and may require additional measures. On the other hand, some dynamics in flow and water levels may evoke dynamic processes that are beneficial to ecological development.

(B) Tidal processes

The tidal flow and sometimes also the wind transport the fine sediments towards or away from the islands. The strength of the flow determines the amount of sediment transport and the type of sediment that can be transported. Fine sediment is mostly transported in suspension. In general, fine sediment is much more likely to be limited by supply (i.e. available sediment) rather than the capacity to transport the sediment. In contrast, transport of coarser sediment is more often limited by the transport capacity. Therefore, the water often has a larger capacity to transport fine sediments.

Water levels and tidal currents are often seen as perfectly sinusoidally over an ebb and flood cycle, e.g. with equal durations for ebb and flood and with equal peak velocities. However, various factors cause deviations in the tidal signal, such as deformation of the tide on the shelf and in the tidal basin due to interaction with the bed (friction), but also the distribution of bed levels within the tidal basin. Systems with a large percentage of intertidal areas typically become ebb dominant (i.e. stronger and/or longer ebb flow), while systems with a low percentage are typically flood dominant (i.e. stronger and/or longer flood flow). A deformation of the perfectly sinusoidal tidal signal is called tidal asymmetry. Tidal asymmetry is an important process, because tidal asymmetry can cause asymmetry in the sediment transport over a tidal cycle. This can result in net sediment transport in either the landward or seaward direction (see for further information Friedrichs & Aubrey, 1988 or Dalrymple & Choi, 2013).  

As for the freshwater processes, wave climate also plays a role in sediment transport and morphology of coastal areas. It should therefore be assessed what the wave climate at the chosen location is and what the effect of the wave climate are on sediment transport and morphology.

Consolidation and rheological processes

Rheological processes are related to non-Newtonian (high concentrated) flows. While these may be relevant in capturing fluid mud currents in e.g. sediment traps or gullies, in the case of this concept sediment is then dredged to build the island.

Consolidation plays a role in both sediment traps as well as on the island, after the sediment has been dredged and deposited. Consolidation refers to the exudation of water from the porespace between the sediment particles by overburden pressure from the weight of overlying sediment. At dryfall, consolidation is sequenced by ripening. Ripening means further expellation of water mediated by evaporation. Ripening can of physical, chemical and biological nature dependent on environmental conditions. Both consolidation and ripening strengthen the soil and therefore change the rheological properties of the soil. The in-depth page on ‘Clay Ripening and Consolidation’ will elaborate more on this topic.

The water level on the island and water level fluctuations determine which vegetation will grow and which benthic species will occur on the islands. In turn, the vegetation (and biota in the sediments) will influence the consolidation behaviour.

Ecological processes and benthic life

Creating new islands provides habitat for terrestrial species. However, one should remember that these islands are placed at the expense of aquatic or marine habitats. Here, we will focus on the ecological benefits these islands could provide, neglecting the loss for the existing habitat. The latter is something that should be assessed in all cases where this concept is applied.

Under the right boundary conditions, new islands can facilitate a large biodiversity. With the absence of human interference and top predators (except for birds of prey), the ecosystem can flourish. Over the course of years, the local vegetation will transform from pioneer species to more long-living plants and threes. These will be able to hold the sediment, reducing erosion. On the shores of the islands, water plants may emerge. These are essential in the life-cycle of many fish (foraging, mating, egg deposition and shelter). For this all to be successful, the boundary conditions need to be optimised. This can be achieved by performing a system analysis.