Abstract

The significant characteristics controlling the variability in storm-generated suspended-sediment loads and concentrations
were analyzed for four basins of differing land use (forest, pasture, cropland, and urbanizing) in
humid-tropical Puerto Rico. Statistical analysis involved stepwise regression on factor scores. The explanatory variableswere
attributes of flow, hydrograph peaks, and rainfall, categorized into 5 flow periods: (1) the current storm
hydrograph, (2) the flowand rainfall since the previous stormevent, (3) the previous stormevent, (4) 2nd previous
storm event, and (5) the 3rd previous storm event. The response variables (storm generated sediment loads
and concentrations) were analyzed for three portions of the stormhydrograph: (1) the entire storm, (2) the rising
limb, and (3) the recessional limb. Hysteresis differences in sediment concentration between the rising and falling
limb were also analyzed using these explanatory variables.
Sediment availability in the study basins is related to land use and underlying geology. The supply of sediment
and its location in the watershed have a strong influence on how current and previous storm events,
and flow and rainfall between storm events, affect sediment loads and concentrations. In basins with limited
sediment availability (forest and pasture), previous storm events supply sediment to the channel. This
in-channel sediment becomes the source of sediment on the rising limb of the next event and clockwise hysteresis
occurs. In the cropland basin with high sediment availability, sediment delivered to the channel during
events becomes the source of sediment on the rising limb of the next event and clockwise hysteresis
occurs. In the urbanizing basin with high sediment availability, counterclockwise hysteresis is prevalent as
stormwater runoff dilutes suspended-sediment concentrations on the rising limb and upland sediment arrives
on the hydrograph recession. In the urbanized basin, previous storm events flush sediment from the
system. The statistical approaches presented here can be used to generate hypotheses on the location and delivery
of watershed sediment sources which can improve the design of appropriate field studies.

Citation

Gellis, A.C. (2012): Factors influencing storm-generated suspended-sediment concentrations and loads in four basins of contrasting land use, humid-tropical Puerto Rico. Catena. DOI: 10.1016/j.catena.2012.10.018