3.3. Grain Size Change during Hurricane Harvey
Figure 5 shows the changes in grain type distribution in sediment samples collected before and after the five severe events in the studied segments. Among the studied storms, Hurricane Harvey showed completely different behavior compared to the other storms and will be discussed separately. Within the riverine segment of the system, the sand percentage increased while silt and clay percentages decreased during all events except Hurricane Harvey (Figure 5). For the same events, all metal concentrations (arsenic and cadmium were excluded) decreased in sediments collected from the riverine segment (Figure 6) suggesting a positive correlation between clay and silt percentage and trace metal concentrations. Within the Upper Bay segment, silt percentages decreased during TS Allison and MD+TD floods and increased during Hurricane Ike.
The opposite pattern was observed for sand. Changes in clay distribution were similar to silt except for a slight increase during TS Allison (<10%). Metal concentrations in these sediments decreased during TS Allison and MD+TD floods (except lead during TS Allison) when silt and clay percentages decreased and metal concentrations increased during Hurricane Ike when silt and clay percentages increased. The Lower Bay/Outlet segment had a different sediment redistribution behavior compared to the other two segments during rain-based events; increasing in clay and silt and decreasing in sand percentages. Metal concentrations in the Lower Bay/Outlet decreased which is not consistent with the observed correlation between clay and silt percentages and metal concentrations in sediment. However, changes in clay plus silt distribution were only +12% and changes in metal concentrations in sediments ranged from -15% to 15% in the segment during TS Allison. Metal concentrations increased during the MD+TD floods as clay and silt percentages were increased. During a surge-based event, the Lower Bay/Outlet showed similar redistribution to the riverine and opposite to the Upper Bay segments. Higher silt and clay percentages after Hurricane Ike led to higher metal concentrations in Lower Bay/Outlet sediments.
As noted before, Hurricane Harvey had different behavior in grain type redistribution. Sediment transport during Hurricane Harvey was mainly affected by the tremendous amount of silt carried by the floodwater from an unknown source. Silt percentages in sediments became up to two times higher than samples collected before the event in all three of the segments. The effect was more significant in the Upper Bay (Figure 5). Within the riverine segment, the average concentration of all metals except copper increased compared to the average concentration of samples collected within a year before Hurricane Harvey.
Figure S5 (Top) in the SI shows the changes in grain size distribution for six stations from upstream to downstream that had grain size data collected between Harvey and the Tax Day flood event (April 2016) (grain size distribution for all 11 samples collected post-Harvey is shown in Figure S5 (Bottom) in the SI.). Silt and sand percentages in sediment were up to four times higher and nine times lower than samples collected less than a year before Harvey, respectively, while clay percentages did not change significantly. The t-tests showed similar results when the percentages of silt, sand, and clay were compared before and after Hurricane Harvey with P-values of 0.001, 0.026, and 0.437, respectively. In addition, the results of RM-ANOVA showed that there was not a significant difference among samples collected before and after Harvey when considering all grain types and that there was no significant difference among grain types. However, when considering both factors (product of sampling event and grain type in IBM ®SPSS ®), the difference was significant (P-value= 0.048) indicating some of the grain types were significantly different before and after Harvey measurements. The observed changes in grain size distribution indicate the presence of newly introduced sediment to the system through the system boundary. These findings are consistent with other studies that reported on the effect of severe hydrological events on grain type distribution after Hurricane Sandy in New York and New Jersey (Reilly et al. , 2016; Romanok et al. , 2016; Artigaset al. , 2017) and after Hurricane Irene and Tropical Storm Lee on the east coast (Horowitz et al. , 2014).