floodplain implementation

[nmizukami]

Now, mizuRoute uses a rectangular channel with unlimited channel depth.

By implementing bankfull depth of the channel, allow the water to spill over the channel to the floodplain. This allow additional feedback of floodwater to Land model, as well as impact discharge simulation.

Rough tasks

1 conceptualize floodplain.
simple one- sloped floodplain (1:S slope means height at 100 meter from channel edge is 100*S meter higher than channel top)
complex one- look at table (height vs area based on topography). See Yamazaki et al., 2011 WRR (floodplain inundation paper)

2 need to implement new channel parameter
bankfull depth and also floodplain slope. MERIT-hydro may have this information, or somehow parameterize just like channel width.

3 modify diffusive wave, kinematic wave (Euler one) and muskingum cunge routing routine
Use bankfull storage, and if computed water volume exceeds this, flood happens and compute water height based on water spilled over the floodplain. Based on the water height adjusted by flood, compute celerity and diffusivity (for diffusive wave) and change flow rate. As far as I see, IRF is not working with floodplain. KWT may not also.

4 hydraulic variables
final hydraulic variables are discharge, water volume, flood water volume, water height.

Reference:

Yamazaki, D., Kanae, S., Kim, H., and Oki, T. (2011), A physically based description of floodplain inundation dynamics in a global river routing model, Water Resour. Res., 47, W04501, doi:10.1029/2010WR009726.