2.4. Comparison of Post-Harvey Sediment and Metal Concentrations
to Historical Data
The GBES was divided into three independent segments as shown in Figure
S2 in the SI to account for the various hydrodynamic regimes; a riverine
segment, a mid-bay segment, and a bottom-bay/outlet segment. All
segments receive waters from Houston watersheds while Trinity Bay, East
Bay, and West Bay (Figure S2 in the SI) were excluded from the analysis
because they are affected by the Trinity River, the Intracoastal
Waterway and Chocolate Bayou, respectively.
Sediment Grain Size and Transport Associated with Extreme
Events . Data for several extreme events in the Houston area were used
to elucidate observations related to changes in sediment and metals
regimes after extreme events; the events included: Tropical Storm
Allison (Allison, 2001), Hurricane Ike (Ike, 2008), Memorial Day Flood
(MD, 2015), and Tax Day Flood (TD, 2016), and Hurricane Harvey (Harvey,
2017). Due to a lack of samples immediately before and after some of the
events, samples collected one year before and after the events were
used. The MD and TD floods occurred within a year of each other, thus
for the purposes of the analysis in the paper, they were combined using
a time period one year before MD to one year after TD flood. Similarly,
the TD flood and Hurricane Harvey were combined, the data that was
available between these two events was used as “after event” for TD
flood and “before event” for Hurricane Harvey.
For each segment in the GBES shown in Figure S2 in the SI, average
values of all existing data within the segment and before and after each
event was used. The percent change in metal concentrations in sediment
and changes in each grain type were calculated using:
\begin{equation}
\text{Percent\ changes}\left(\%\right)=\frac{\left(\ value\ prior\ to\ the\ event-\ value\ after\ the\ event\right)\ }{\text{value\ prior\ to\ the\ event}}\nonumber \\
\end{equation}The Pearson correlation test was performed on the percent change in
metal concentrations and the percent change in each grain type. Arsenic
and Cadmium were excluded from the correlation analysis because 46% and
58% of historical sediment samples showed values below the detection
limit for arsenic and cadmium, respectively.
Additionally, and because of the unique volume of transported sediment
during Harvey, additional analyses were undertaken for sediment data
collected after Harvey that included visual inspection, repeated
measures ANOVA (RM-ANOVA), and paired t-test statistical analyses using
IBM ® SPSS ® (IBM, 2017) to place
the data in context relative to historical patterns.