The LiDAR points, used to generate the model, were numerous and dense in the upland

area of the digital elevation model. However, points were sparser in the forested portion of the

floodplain wetland and in the river corridor itself. A typical bend in the river and the lack of

corresponding LiDAR data within the floodplain are shown in Figure 4-5. This trend continued

for the length of the study corridor.

 In the downstream half of the study area, break lines were added along each bank. This

results in an approximation of the floodplain area by defining the upland portion with LiDAR

and the river's edge with a hard break line. This is useful in the TIN because it keeps the river

channel well defined, even in the absence of high-density LiDAR data near the river. However,

the more upstream portion of the study lacked these break lines and the river channel is more

poorly confined in this portion of the DEM. Survey data also provided localized improvement of

the topography TIN and resulted in better development of the DEM in areas where survey data

augmented the LiDAR data. However, these data resulted in only localized improvements in the

topography TIN.

 The hydraulic model estimates an inundated area by calculating the area of a trapezoid.

The trapezoid is bound by the water surface extent at two adj acent cross-sections. The distance

between cross-sections is defined in the model. In a GIS environment, it is possible to estimate

the inundated area between cross-sections by calculating the area where the water surface DEM

is higher then the topography DEM (Figure 4-6). The inundated area calculated by GIS and

HEC-RAS between two cross-sections is presented in Table 4-2.

 Discussion

GIS to HEC-RAS

 The use of LiDAR to augment survey data was initially viewed as a device for

supplementing partial floodplain survey work. The concept was that cross-sections could be