A high concentration of particles in the water makes it difficult for light to pass through the water column, reducing the depth of the photic layer and affecting photosynthetic organisms. However, high turbidity, the optical property of water that represents the amount of light reflected by suspended matter, can also have negative effects on non-photosynthetic benthic organisms, once the suspended matter begins to sediment. Precisely the contribution of sediment that floods bring with them can produce high mortality rates in bivalves, with serious economic repercussions in shellfish production areas. This is the case of the Arousa estuary, where up to 700,000 kilograms of clams are extracted each year, a production that is very sensitive to floods, both due to the decrease in salinity, which causes the arrival of freshwater, and due to the contribution of sediment, given that the burial of cultivated organisms can cause high mortality rates in bivalves, with serious repercussions on the local economy.
With the aim of predicting turbidity in rivers, which would allow preventing possible economic losses for the shellfish sector, as well as damage caused to organisms, the doctor in Marine Sciences and coordinator of the Oceanography Unit of the Ecimat, José González, participated between 2008 and 2015 in research by the Oceanography Group of the University of Vigo, coordinated by Ramiro Varela, focused on the study of turbidity values in the final stretch of the Ulla River, specifically its mouth in the Ría de Arousa. “While monitoring salinity is an easy task to perform and predicting its evolution based on rainfall and river flow does not involve great difficulty, turbidity presents more problems,” explains González, in relation, on the one hand, to the need to keep the lens of the sensor responsible for monitoring clean at all times and, on the other, because the relationship between turbidity and flow or rainfall is not yet well known, which prevents a forecast of turbidity values based on the forecast of these variables.
To achieve a better understanding of the temporal variation of turbidity values, turbidity values in the final stretch of the Ulla River were monitored with an hourly frequency for seven years, “which allows the relationship to be established with the flow and precipitation values recorded”, explains José González, who is currently working on the processing of the collected data. With this objective, two measurement stations were used in the final stretch of the Ulla River and the precipitation data recorded at the Meteogalicia stations located in the river basin were used, as well as the flow data from the station located upstream.
A relationship has been established between increases in flow and increases in turbidity values.
“It is logical to think that water turbidity can depend on rainfall, due to the sediments carried by rain with runoff water, or on increases in flow, due to the sediments carried by rivers during floods”, explains the coordinator of the Oceanography Unit of Ecimat. However, over the seven years of study it was found that the monthly average of turbidity does not present significant differences throughout the annual cycle, nor was a direct relationship found between turbidity values with rainfall and flow. “Does this mean that there is no relationship between turbidity and rainfall or flow? The answer is no, what this implies is that the relationship is more complex”, explains González.
On the one hand, the researcher points out that the Ulla River has its flow regulated by dams, which means that it does not depend solely on rainfall, but that water can be released progressively or even before the rainfall occurs. “However, a relationship has been established between increases in flow and increases in turbidity values. That is, when there is an increase in flow, it has been observed that it is often associated with an increase in turbidity values, but after the increase in flow, even if it remains high, the turbidity values return to normal”, points out the researcher, who explains that this is because it is during the phase of increased flow when greater sediment is carried by the river, so that after this initial increase, the amount of suspended materials decreases, regardless of the flow value at that time. “This is the reason why a direct relationship cannot be established between the value of the flow and turbidity, since this depends solely on its variation and not on its absolute value,” explains the researcher.
Rainfall and flow forecasts to determine the risk level of increased turbidity
Research conducted in recent years has also confirmed that there is a relationship between the magnitude of the increase in flow and precipitation and the probability of an increase in turbidity. Thus, the greater the magnitude of the increase in flow or the greater the precipitation, the more likely it is that turbidity will increase in river waters. Likewise, the probability of an increase in turbidity is also greater when the increase in flow is associated with an episode of rainfall.
“As an example, we can say that the probability of an increase in turbidity in the river varies between 60% when there is an increase in flow, of any magnitude, and values greater than 90% when the flow increases by more than 50% and rainfall greater than 10 l/m2 is recorded,” details José González, who points out that these data with which they are currently working will allow, based on the rainfall and flow forecasts for the area, to predict the level of risk of an increase in turbidity that could cause damage to organisms and thus prevent possible economic losses for the shellfish sector.
Source: duvi
