Determination of the water level – discharge relationship in a river: Simple or complex?

Abstract

The water level – discharge relationship in a river, or rating curve, is very useful because it allows to calculate the discharges from measured water levels in gauge station. For a nearly permanent regime a discharge corresponds to a water level (simple relationship). However, frequently different circumstances and interventions can affect and invalidate this relationship, like a tributary discharging in the river, the backwater created by a control or regulation structure (dam, weir, etc.), floods or the operation of a reservoir. These factors originate a complex relation between waters levels and discharges and then a discharge can be referred to two water levels and vice versa. In this study the single and complex rating curves were determined in La Balsa hydrometric station, located 27 km downstream of Salvajina dam (Cauca river, Colombia). The simple curve was calculated by applying the logarithmic method and the complex curve by using the method of storage per unit rate of change in water levels. The results show significant differences in the estimated discharges through the two relationships. By applying the two rating curves for a moderate flood (01/01/1999) differences up to 19% were estimated between both calculated discharges. In consequence, in order to get the appropriated rating curve it should to carry out a rigorous analysis of the flow conditions on the river reaches of the hydrometric station.