Methodology - Response of Flood Events to Land Use and Climate Change - page 65

Fig. 3.5 Typical cross section configurations (Source EM 1120-2-1003 2002)

locations where there are changes in discharge, slope, shape, roughness, and at
locations where the dykes begin and end (Fig. 3.5 ).
Cross sections should be placed at representative locations to describe the
changes in geometry. Additional cross sections should be added at locations where
changes occur in discharge, slope, velocity, and roughness. Cross sections must be
also added at levees, bridges, and other structures. For example, if extensive flood
inundation studies are analyzed, then the cross sections should be extended longer
into the floodplain. Cross section elevations which have been constructed using
any model based on a DEM are not so accurate compared to the cross sections
which are measured in the field or digitized directly from photogrammetric stereo
models (EM 1120-2-1003 2002). Bed slope plays an important role in cross section
spacing. Steeper slopes require more cross sections. When there are not enough
cross sections, they are spaced far apart and if the changes in hydraulic properties
are great, the solution can become unstable. In general, cross sections spaced too
far apart will cause additional numerical diffusion, due to the derivatives with
respect to distance being averaged over too long a distance. Also, if the distance
between cross sections is too far, such as when the courant number is much smaller
than 1.0, then the model also becomes unstable. As cross section spacing is
increased, the hydrograph will show some numerical attenuation/diffusion.
If the cross sections are too close together, then the derivatives with respect to
distance may be overestimated (computed as steeper slopes than they should be),
especially on the rising side of the flood wave. This can cause the leading edge of
the flood wave to over-steepen to the point at which the model becomes unstable.
To create a cross section, the cut lines will first be applied. These cut lines should
always be considered perpendicular to the direction of the flow and start from the
left to the right bank.