2.2 Environmental data
The modeling system requires inputs from the terrain, Land Use Land
Cover (LULC), and soil type and depth. Among these variables, terrain
data arguably plays the utmost important role in hydraulic simulation
(Dullo et al., 2021; Schumann & Bates, 2018). There has been a thorough
investigation of terrain data affecting flood inundation modeling since
the early development of hydrologic/hydraulic models (Kenward,
Lettenmaier, Wood, & Fielding, 2000; Sanders, 2007). Lately, with the
increasing interest in deploying macro-scale flood inundation
simulations, global terrain data assessment has been again brought up
(Mohanty et al., 2020; Sampson et al., 2015). Generally, three types of
data are favored and available in the U.S.: 1) airborne light ranging
and detection (LiDAR) that resolves terrain with a high degree of
vertical accuracy (0.05–0.2 m) and comes with a high spatial resolution
but limited areal coverage, 2) spaceborne radar interferometry (IfSAR,
e.g., Shuttle Radar Terrain Mission) that provides global coverage but
poor vertical accuracy (~10 m) and spatial resolution
(~90 m), and 3) a mixed product such as the National
Elevation Dataset (NED) from the USGS that merges LiDAR surveys and the
USGS quadrangle maps, whose accuracy (~5-7 m) and
resolution (~5/10/30 m) sit in between the former two
products. A general consensus from these studies is that LiDAR data is
the most favorable DEM owing to improved vertical accuracy in flood
modeling (Mohanty et al., 2020; Sanders, 2007; Schumann & Bates, 2018)
but they have to be accompanied by surveyed channel profile. IfSAR,
however, degrades its quality because of poor vegetation penetration and
speckle noise while NED smooths some artifacts. The NED 10 m data
accurately represents the river channel morphology than high-resolution
LiDAR data that cannot penetrate water surface. Therefore, in this
study, we select the 10 m DEM data from the NED dataset in the study
area. To confirm the river channel bathymetry, 13 surveyed river
geometries from the Harris Country Flood Control are curated and
compared to NED 10 m, shown in Table 1. The average difference is found
to be small (~0.55 m).
The LULC and impervious area data are acquired from the National Land
Cover Database (NLCD) at 30 m resolution to derive a-priori parameter
sets. The soil type dataset is retrieved from the United States
Department of Agriculture.
[INSERT TABLE 1 HERE]
2.3 Study area
Greens Bayou Basin, located in the north of the Houston metropolitan
region, is one of the areas that are susceptible to regional flooding
because, firstly, landfalling tropical cyclones and hurricanes bring
torrential rainfall within a short period; secondly, the urban
development in the recent years have altered the local ecosystem (e.g.,
replacement of soil with built-up structures). The basin is relatively
flat (~1.5%), with an average elevation of around 23.65
meters, and the total drainage area is 457.9 km2.
Three main streams flow across this region. Reinhardt Bayou (drainage
area: 86.3 km2) flows from north to south, met with
Greens Bayou to form the longest river in this area. Halls Bayou
(drainage area: 225.1 km2), the second-longest river,
meets Greens Bayou at the basin outlet (Figure 1a). The five USGS stream
gauges, situated at each mainstream, monitor instantaneous streamflow at
a 15-min time interval. Nearly 90% of the area is well-developed,
especially in the western portion; forests and wetlands are present
downstream, close to the basin outlet (Figure 1b). The soil types are
dominated by a mixture of sand, clay, and loam (Figure 1c). The typical
runoff generation mechanism in this region is infiltration excess runoff
when extreme rain rates surpass soil infiltration capacity, indicated by
relatively low hydraulic conductivity values (Buchanan et al., 2018).
Meanwhile, the correlation between rainfall and streamflow is above 0.6,
pointing to a flashy hydrograph (Berghuijs, Woods, Hutton, & Sivapalan,
2016).
[INSERT FIGURE 1 HERE]
During the 500-year Hurricane Harvey event, this region is largely
inundated due to record-breaking 1600 mm rainfall over a one-week storm
lifespan (Chen et al., 2020; Li et al., 2020). According to the Harris
Country flood report, both Greens Bayou and Halls Bayou experienced a
500-year water level downstream and 50-year to 100-year in between
upstream. Greens Bayou broke previous water level records in 2002 and
observed flooding occurred along the entire channel.