Rivers, water and sediments
Important rivers usually have a number of tributary streams which have their sources in the mountains. It is not just water that goes on the journey but sediments, stones and other material. These materials are transported dissolved, in suspension or deposited as sediment at the bottom of the river.
The turbidity and the concentration of sediments
A research team from the Department of Geodynamics at the University of the Basque Country (EHU-UPV) have begun a study in order to find out the amount of sediments transported from the three main river basins of the province of Gipuzkoa in the Basque Country, i.e. the Urola, Urumea and Deba river basins. The aim is to analyse how factors, both natural and non-natural, affect the presence and the transport of these sediments in these waters.
To this end, the upper basins of the aforementioned rivers were selected: Añarbe in the case of the Urumea river, Barrendiola in the case of river Urola and Aixola in the case of the Deba. These three basins were chosen after an exhaustive analysis of their physical parameters (geology, geomorphology, vegetation, hydrology, soils, and so on). Moreover, the three basins have different characteristics and, thus, are representative of different environments in Gipuzkoa.
In order to analyse these sediments, the water volume/flow of the selected streams are measured as well as the precipitations and turbidity thereof. Ongoing measurements have been taking place for two years now since the project started.
Turbidity depends on the amount of sediments transported in the water. This parameter is measured by means of a probe immersed in the water.
Each time the waters rise due to rainfall, the concentration of the sediments is measured. In order to carry this operation out, a device is immersed in the water to take samples from the stream. When it rains, this sampler is triggered and the bottles are filled with river water. The research worker collects these samples and replaces the emptied bottles for the next rise in water level.
These samples are then taken to the laboratory where they are filtered and the amount of sediment in them calculated. In this way the concentration of the sediments is known.
Although turbidity and the concentration of sediments are related parameters, it is not easy to fix this relationship directly. The size, type, colour and so on of the sediments may vary the turbidity while the concentration remains the same. Once the relation between these two parameters is established, the concentration of sediments can be worked out, given that the turbidity is being measured in an ongoing manner. This relation is usually different for each river basin.
Human activity versus natural basins
The concentration of the sediments transported by rivers is not only related to rainfall and the volume of water in the river; the features of the basin also have a bearing. Given that the three basins chosen for this study have different characteristics (size, lithology, soils, vegetation, land use, climatic conditions, geomorphology, and so on), a comparison of the results demonstrates that the main factors that influence the presence of the sediments for each basin are different.
Moreover, the use that the soil is put to and the human activity in the river basins also have considerable bearing on the results. For example, if trees are felled on a riverbank, the soil becomes unprotected and, thus, rainwater will wash out much more sediment into the stream. Continuously measuring the turbidity of the stream means that we can know the impact of such activities on the quantity of sediments present in the basin. Moreover, if the measurements are taken over a long period of time, it is also possible to observe the duration of the effects of such felling.
Another activity that can vary the amount of sediments is fill-in work. Fill-in earth is not usually compact nor is it protected by vegetation. So, until the earth becomes compact and covered by grass, greater amounts of sediments will reach the basin.
The three river basins studied supply water to their respective reservoirs. This is why another objective of the researchers was to find out how much of the sediments reached these artificial lakes. Particles in the water are deposited as sediment at the bottom of these and, little by little, the sediment builds up and will fill the reservoir. Thus, the data also calculates how long these reservoirs can continue to function as such. For example, in the Ebro basin there are reservoirs that have become full of sediment. The situation in our three basins is not so serious; it has not reached the stage of filling the reservoirs.
In future, researchers wish to undertake studies higher up in the basins in order to take measurements of the tributaries there. Moreover, they are planning to analyse the chemical characteristics of the water from these springs and rivers.
Garazi Andonegi | alfa