The Hjulström Curve is a graph devised by Filip Hjulström in 1935 that shows the velocity, and energy, of water needed to erode, transport or deposit varying sizes of sediment. For example, a small piece of sand takes a water velocity of only 20cm/s to erode, but a boulder takes a water velocity of around 600cm/s. This information can then be used by river engineers to design dams, bridges or flood defences. Boat builders can calculate how strong their boats needs to be in order to be safe in a powerful river.
The Hjulström Curve has logarithmic scale, which is because changes in the erosion, transportation and deposition of a particle changes quickly at a small scale, but at a large scale it changes very gradually. Therefore the best method to show the change is a logarithmic scale, which shows the smallest scale which greatest detail, and gradually loses detail as one travels away from the origin.
The Hjulström Curve is useful because it gives a model by which river engineers can follow when designing dams or bridges. This information is useful because the engineers can see what size sediment may be making its way towards their creation once they know the speed of the river. Therefore they can defend their creation with whatever protection is necessary. For example, a simple bridge over a river that only moves at 1cm/s is unlikely to need steel-enforced concrete pillars, but a bridge with water moving at 500cm/s probably would need such support. The fact that the graph is based on a model could be useful as this is would could happen in perfect conditions, that it, at the best conditions for erosion, transportation or deposition. Therefore those using the graph can be sure that if they prepare for what the graph predicts then they will not be affected by the actual events.
However, the fact that the Hjulström Curve is a model also means that it does not represent what would actually happen in a real case study. There are numerous factors that come into play...