Climate change and changes in land use mean that the past can tell us very little about future extreme discharges of rivers. A clever combined approach of monitoring and management of delta areas improves safety, keeps shipping channels navigable, provides a supply of freshwater and strengthens biodiversity. This according to Prof. Ton Hoitink who was appointed to a personal professorship at Wageningen University & Research on 25 January.
For the Netherlands, but also for other delta areas in the world, the management of river discharges is of vital importance. In the ‘wild’, rivers are not fixed objects embedded in the landscape, filled with water. ”Rivers move”, says the new professor holding a personal chair at the Hydrology and Quantitative Water Management group in Wageningen. “The river bed changes continuously due to erosion and sedimentation. This means that sand banks are formed and shift, dunes grow and even the entire river may shift.”
The quantities of water and sediment carried along determine whether erosion or sedimentation occur. If there is more sediment in the water than the river can carry, then the heavier particles fall to the bottom or attach themselves to the banks; if there is less sediment in the river, erosion occurs. The river bed and banks are then eroded and the particles are carried downstream.
Safety is paramount, in particular in the Dutch rivers which together form one big delta. The rivers are fixed in the landscape by groynes and bank protection. In this area the river water must be managed, also in the event of high extremes. It is also in this area that the river is used for shipping, there are possibilities to extract sand for construction and where there is space for biodiversity. In delta areas closer to the coast, there is also the problem of the penetration of salt seawater via the river channels. The salt water can penetrate deep into the land through rivers, particularly down at the river bed, as salt water is heavier than freshwater. With a high tide and low river discharge in particular, as is the case in hot summers, the drinking water supply may be put at risk by salinisation. There are also potential problems with water use for irrigation and industrial applications.
Integrated river approach
“To maintain or even improve safety, navigability, drinking water supply, sand extraction and biodiversity in a dammed delta area, such as in the Netherlands, clever solutions and intensive monitoring are needed,” says Prof. Hoitink. A project that is both simple and effective involves a series of longitudinal dams constructed in the Waal – long, low dams between the bank and the middle of the river, over a length of 10 kilometres. These dams that run parallel to the river may offer a possible alternative to groynes placed across the river. With a longitudinal dam the water level rises with low water and the level falls with high water. In addition, with this kind of dam a natural environment for water flora and fauna is formed between the dam and the river bank. “It’s particularly difficult to predict how this process develops. It is highly dependent on the exchange of water and sediment from the main channel towards the side channel. We are currently carrying out measurements on this,” says Prof. Hoitink.
Surprise in Borneo
Sometimes rivers reveal unexpected secrets. An example of this is the Mahakam river on Borneo. We saw here that sediment had been deposited in the outside bend – completely against the ’rules‘. And erosion had taken place in the inside bend, which is also a violation of expectations. “A more detailed study of this sharp bend in the river showed that very deep hollows had been cut into the bed, up to a depth of fifty metres. Spectacular,” according to Prof. Hoitink.