Threatened Shore

Waves that hit the shore stir not only the hearts of poets, but also the sand on the beach. Thus put in motion, the sand can be removed so the beach erodes. To counteract beach erosion the law in, e.g., Florida allows sand to be dredged from the sea floor and put on the beach, as long as it contains no more than 10% fines (<0.062 mm). If it contains 10% to 20% fines it can still be placed in the nearshore.

The intention is that the fine fractions be washed away from the beach sand by the wave action. The problem is, though, that this clouds the nearshore water, i.e., it increases the turbidity. It seems beyond doubt that this is what killed Florida's nearshore coral reefs. Those farther offshore are now being killed by the tens of tons of total nitrogen that comes out every day with sewage, none of which passes tertiary treatment.

Beach restauration and nourishment has for years been considered more environmental-friendly than the alternatives, but when sand is becoming sparse and the coral reefs are dying, what shall we do? A first step is to require that beach replenishment, and other coastal construction projects, do not harm the coral reefs. It can be accomplished by demanding continuous siltation monitoring of the bottom conditions in and near the reefs, and that the work is stopped when the levels are unacceptable. A long-term solution may be to eliminate the need for beach nourishment by installing wave energy converters that attenuate the waves, slows sand transport, and perhaps also produces renewable energy. Use the pull-down menus to find more information.

Regulated Rivers

Hydropower is a renewable and clean energy source, but the reservoirs have the negative impact of stopping much of the sediment transport in the river. Typically all the bedload and a large part of the suspended load is trapped. The proportion of the sediment discharge that is trapped is called the trapping efficiency.

Reservoir sedimentation gradually causes loss of storage capacity. Especially the loss of active storage affects the economy of the hydropower project. It is therefore essential to manage this negative impact. Dr Ulf Erlingsson, CEO of Lindorm Inc, has research experience in this field, in measuring sedimentation and erosion, creating a sediment budget, and creating an error budget of the measurement uncertainty.

Several types of studies deal with this problem. In the planning stage it is important to measure sediment transport and estimate the trap efficiency. It is also important to predict where the sedimentation will take place, and what environmental impact it will have upriver, in the reservoir, and downriver. Factors such as delta sedimentation and turbidity currents must be considered. Post constructions it is equally important to follow up on the sediments, as regards sediment yield in the drainage area, sediment discharge, sedimentation and trap efficiency, and erosion downstream of the hydropower reservoir.