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).