Scott,

Using EXTRAN set up the channels and floodplain as a 2-dimensional grid
type network, no different than doing a storm sewer network. The
channels are modeled as irregular channels that extend from top of bank
to top of bank. The floodplain is broken up into grids, not necessarily
rectangular. Each floodplain grid is assigned a node in the center with
an invert elevation equal to the average ground elevation of that grid
and a ground elevation set a little higher than the expected highest
flood elevation to avoid surcharging the pipe. Each node has an
overland flow conduit to the four nodes (grids) bordering it. The
length of each connecting conduit is the actual distance between nodes.
The width is only half the actual width so the area of all the grids
comes out correct. The floodplain conduit between the channel flow
reach and the adjacent floodplain grid is modeled as length from the
center of the floodplain node to the top of channel bank x width to give
correct area for the wedge shaped area between the node and edge of
channel reach. Set the invert of the connecting conduit at the average
ground elevation of the floodplain node at each end. It had straight
canals instead of braided channels. If you choose, some of the
floodplain can be included with the channel, but if most of the flow
occurs in the floodplain, choose the channel upstream and downstream
invert elevations so they represent overland flow slope. Attached is a
sample file of a segment of the St. Johns River in Florida done hastily
for demonstration purposes that was never calibrated. The floodplain
"n" values are for sawgrass. I have a Visual SWMM version if you would
like it.

One other comment on meandering streams. Depending upon direction of
meander, Mannings "n" decreases up to the top of bank, then increases
above that. USGS stream gage measurement on the Ocklawaha River,
Florida show velocities increase with rising stages up to the top of
bank, then they decrease as the flow spills over the top of banks. I
can't recall the direction of the meander where they made this
measurement, but I expect it was made where the river flowed at more of
a right angle to the floodplain.

Robert Freeman
RFreeman@SJRWMD.COM