Investigate sediment discharge discontinuity when erosion reaches bottom level
When the breach geometry curve reaches the fixed, unerodable bottom level, the sediment discharge drops by 10-25% (see screenshot below).
This is reflected both in the discharge curve (which uses the raw model data) and the visualisation of the breach cross section.
The following screenshot shows this error in version 6b2e1ed0, but it has been confirmed in b29f1b1c and is not a product of the recent refactors.