Fluvial Discontinuity from Downstream Hydraulic Geometry Analysis

Código

I-EBHE0205

Autores

Fernando Grison, Aline de Almeida Mota

Tema

WG 2.6: Water systems analysis for integrated planning & management

Resumo

The theory of downstream hydraulic geometry was analyzed in several reaches of the Bugres River, in Rio Negrinho ? SC. Eleven study points were defined along the river, where stream gauges were installed. To analyze downstream hydraulic geometry, it is necessary to estimate discharge of the same frequency and from them calculate the variables width, average depth and velocity. However, as there are no historical series of flow data for the Bugres River, several water levels were observed at all study points almost simultaneously. This was possible due to the ease of access to the study sites, which allowed a quick tour through all sections, measuring the water level on the stream gauges. This procedure was carried out over eight days, not consecutively, and at the time of the measurements it was not raining. Width, average depth, velocity and discharge were calculated (using the stage-discharge relation) for each water level. Also, the bankfull level was identified from field observation on each cross section. The exponents and coefficients of the downstream hydraulic geometry were calculated. The results showed that the discharge is on average accommodated in 27% by the width and 23% by the average depth of the sections and in 50% by the velocity. Therefore, velocity was the most sensitive variable to changes in discharge along the river. Furthermore, for all the water levels analyzed, it was found that the discharge does not naturally increase along the river. In some reaches, it was observed that the discharge on bankfull level decreased, that is, there was a discontinuity in the increase of discharge. The most significant decrease was from 7.58 m³/s to 2.47 m³/s, a loss of 5.1 m³/s in approximately 960 m of the river channel, even with an increase of 3.45 km of drainage network between these points. A decrease in discharge was also observed in some reaches along the river for the other water levels. Considering that no significant water withdrawal was observed along the river, which could cause this variation, the hypothesis is that factors such as, for example, i) changes in the shape of the bed river, increased resistance to discharge in the reach, due to deposition of eroded soil from the banks; ii) variability of the Manning roughness coefficient with the change in discharge; iii) existence of a geological fault; and iv) intense exchanges with the hyporheic zone. In this sense, the work shows that the traditional idea that the discharge in a river system always increases with the increase in the drainage area, cannot always be accepted.