Comparison of Storm-Generated Sediment Concentrations and Loads in an Urban Disturbed Basin and a Rural Undisturbed Basin, Puerto Rico
Abstract
The factors controlling storm-generated suspended-sediment loads and concentrations were examined for two basins of contrasting land use in Puerto Rico from 1989 to 1995. The Quebrada Blanca Basin (8.39 km2), a relatively undisturbed basin, drains pasture (54%), forest (21%), cropland (8%), and rural (15%). The Rio Piedras Basin (19.4 km2) is 73% urban and is considered disturbed because of extensive urbanization in the remaining 23% forested areas of the basin, involving many construction projects that expose bare soil. Twenty-three runoff events, defined by a peak over 0.42 m3/s, were examined in Quebrada Blanca and 26 events, defined by a peak over 1.4 m3/s, were examined in Rio Piedras. Three dependent factors were used to describe suspended-sediment transport: (1) suspended-sediment load, (2) discharge-weighted sediment concentration, and (3) time-weighted sediment concentration. Thirteen independent factors controlling sediment were delineated into three time categories: (1) characteristics of the previous event, (2) characteristics between events, and (3) characteristics of the current event. The dependent and independent factors were separated into quickflow and total runoff to determine if one was more significant in explaining sediment. Forward stepwise regression analysis for Quebrada Blanca showed that the most significant variables explaining sediment load and concentrations were directly correlated to the characteristics of the current storm event (for both the quickflow and total runoff aspects of the hydrographs). These included the total quickflow, sum of peak flows, and maximum rate of hydrograph rise for any peak. In Rio Piedras, the three dependent variables for both aspects of the hydrograph were inversely correlated to the rainfall since the previous runoff event. These are smaller rainfall totals that do not cause significant increases in streamflow but are flushing sediment from the system that would be available for the current sampled event. A principal components analysis was performed on the independent variables to simplify them into logical classifications. Forward stepwise regression of the principal components against the three dependent variables for Quebrada Blanca indicated that current event characteristics (PC2) was the most significant principal axis for both aspects of the hydrograph. Forward stepwise regression on the principal components against the three dependent variables for Rio Piedras showed that both previous event characteristics (PC1) and current event characteristics (PC2) are related to sediment transport. PC1 was inversely correlated to the dependent variables, suggesting that previous event characteristics are flushing sediment from the system. PC2 is directly correlated to the dependent variables. In the undisturbed basin (Quebrada Blanca) only characteristics of the current event were significant. In the more disturbed basin (Rio Piedras), where sediment sources are more widespread, previous event and between event characteristics were significant and are flushing sediment from the system. Many studies attribute the variance of storm-generated suspended-sediment transport to current storm event discharge, but results from this study indicate that aspects of the current hydrograph (such as peak flows), previous event characteristics, and between event characteristics may be significant. The degree to which each is significant may depend on the availability of sediment.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2002
- Bibcode:
- 2002AGUSM.H42F..06G
- Keywords:
-
- 4558 Sediment transport;
- 1815 Erosion and sedimentation;
- 1824 Geomorphology (1625)