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.