[Swarm-Modelling] Swarm and Braided Rivers

[Crile Doscher]

Having recently gotten a basic Swarm model running (with a lot of help from the 
Support 
list), I think it's time I migrated questions over to this list as I move on to 
refinements.  So, 
hello all!  My interest is braided rivers, rivers with either sand or gravel 
riverbeds, with 
typically wide floodplains and relatively steep slopes.  Since the bed is made 
up of 
movable material, these sands and gravels are moved around by the flowing 
water, and 
in the process form multiple channels (hence the name braided).  You can see 
these 
processes at work next time you're at the beach and come across water flowing 
over the 
sand.  An interesting aspect of these rivers is their scaling properties.  
Regardless of 
whether it's a trickle of water flowing over sand or the Bhramaputra River (a 
rather large 
river), the behaviour seems to be the same: the channels tend to follow a power 
law 
distribution in both time and space (not unlike turbulence or earthquakes).  
There is 
debate about why this should be (and it's certainly not the only natural 
phenomenon that 
seems to do this) and I see Swarm as an ideal environment for some hypothesis 
testing.  
There are some feedback loops affecting how the river channels evolve, namely, 
the 
water moves sediment from certain places and deposits it elsewhere, which in 
turn 
affects where the water flows, which affects the sediment, etc.  It's a classic 
example of 
independent agents following (simple) local rules with global behaviour 
emerging from 
the interactions (or so I claim).   
  I currently have a model in which the agents are finite volumes of water that 
flow over a 
cell surface containing elevation values.  The agent moves about by going in 
the direction 
of steepest slope to the neighbouring cells.  A next step is to incorporate the 
ability of the 
water agent to pick up sediment in certain conditions and deposit it somewhere 
else 
under different conditions.  Then it gets a bit tricky.  There will likely be 
multiple agents in 
certain cells which are fatal (given Discrete2d) but I'm at work on that.  I'm 
sure 
everyone's Swarm model has unique requirments, and here are some of mine.
  One theory I'd like to test is that braided rivers are hiearchically 
structured, i.e. what 
happens at one scale can constrain what happens at another scale.  Is it 
possible to 
have multiple cell surfaces, each with its own resolution (cell size) living in 
the same 
space such that linkages can be set up between the scales?  The nature and 
direction of 
the linkages are things to be investigated.  What it might mean in effect is 
that before a 
water agent can move to a particular cell, it might need to check with what's 
happening at 
another scale (or scales) before doing so.  
   Another challenge - anyone who's watched a river flow will recognise that 
different 
parts of the river flow at different velocities, which gets us into the area of 
asynchronus 
updates.  Much of my preliminary reading of the Swarm documentation led me to 
believe 
that this is possible and is one of the main attractions of using Swarm.  Does 
anyone 
have any direction on how to carry that out? 
   My apologies if that's been a rather long-winded buildup and thanks for any 
responses.  
Crile

Dr Crile Doscher
Natural Resources Engineering
Lincoln University
Canterbury
New Zealand