Abstract
Abstract. Information on sediment concentration in rivers is important for design of reservoirs and for environmental applications. Because of the scarcity of continuous sediment data, methods have been developed to predict sediment loads based on few discontinuous measurements. Traditionally, loads are being predicted using rating curves that relate sediment load to discharge. The relationship assumes inherently a unique relationship between concentration and discharge and therefore although performing satisfactorily in predicting loads, it may be less suitable for predicting concentration. This is especially true in the Blue Nile Basin of Ethiopia where concentrations decrease for a given discharge with the progression of the rainy monsoon phase. The objective of this paper is to improve the sediment concentration predictions throughout the monsoon period for the Ethiopian highlands with a modified rating type equation. To capture the observed sediment concentration pattern, we assume that the sediment concentration was at the transport limit early in the rainy season and then decreases linearly with effective rainfall towards source-limited concentration. The modified concentration rating curve was calibrated for the four main rivers in the Lake Tana basin where sediment concentrations affect fish production and tourism. Then the scalability of the rating type equation was checked in three 100 ha watersheds for which historic data were available. The results show that for predicting sediment concentrations, the (modified) concentration rating curve was more accurate than the (standard) load rating curve as expected. In addition loads were predicted more accurately for three of the four rivers. We expect that after more extensive testing over a wider geographical area, the proposed concentration rating curve will offer improved predictions of sediment concentrations in monsoonal climates.
Highlights
For a few rivers in the world and over a limited period, sediment concentrations have been measured at a daily or shorter frequency
After testing the sediment concentration rating curves for the Lake Tana watersheds, we investigated the applicability of the concentration rating curve for small watersheds
In the Ethiopian highlands sediment concentrations in the rivers decrease with progression of the rainy phase of the monsoon
Summary
For a few rivers in the world and over a limited period, sediment concentrations have been measured at a daily or shorter frequency. In order to determine sediment loads in the absence of these measurements, models and rating curves have been used. Knowing the total sediment loads from rivers is essential for evaluating the siltation of reservoirs (Ali et al, 2014), assessment of soil erosion and nutrient loss (Walling, 1977). As a result knowledge of sediment concentration is important in most environmental applications because among other things it hampers fish reproduction and reduces the esthetic value of surface waters (Vijverberg et al, 2012). In the Blue Nile Basin, where the construction of the Grand Ethiopian Renaissance Dam is and planning of other hydroelectric dams are under way, determining sediment loads is becoming more urgent.
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