SEASONAL PEAK STREAMFLOWS AND SEDIMENT TRANSPORT

The objective of this presentation is to demonstrate that an analysis of peak flows as part of a study to understand geomorphic processes in rivers should also include an analysis of peak flow types. In this presentation the geomorphic process is suspended sediment loads and the peak flow types are replaced by a seasonal analysis that captures most of the types of storms. A case study of the Rio Tanama in Puerto Rico shows there are two peak flow seasons: a season dominated by frontal storms from the northwest (March- 15 June) and a tropical cyclones season with storms from the east (16 June – November). An index to sediment transport capacity index based on the relation between suspended sediment concentration and streamflows, and the measured daily streamflows was developed. The index for the two seasons was then calculated for each year. Analysis of the relation between the seasonal suspended sediment loads (sl) and the seasonal sediment transport capacity index (x) showed the relation between the loads and index is not the same for the frontal storm season [sl = (388 x) – 14] as for the tropical cyclone season [sl = (134 x) -+ 55]. (The range of x is 0 – 16) Most of the sediment load is associated with the peak flows in each season. The tropical cyclone season, especially, may have multiple peaks which makes understanding the physical process even more complex. A frequency analysis of the peaks in each season shows the frontal season peaks are, on average, smaller than the tropical cyclone peaks. In any give year the frontal peak may be larger than the tropical cyclone peak. The median for the frontal season is 57 m³/s compared to 95 m³/s for the tropical cyclone season. The flows for an average return of 1 in 100 years is 125 m³/s for the frontal season and 560 m³/s for the tropical cyclone (hurricane) season. Major frontal storms and tropical cyclone storms have occurred in the same year (1985).