title_body: How do you model language changes with wave theory (areal developments)? I'd like to hear from anyone who has built a dialect continuum, in which innovations spreading from different centres affect overlapping subsets of the range (see wave theory).  How do you model it?  How do you manage the results?
  
  I suppose I'd start with a list of towns and a table determining how likely a town is to adopt an innovation from each of its neighbors.  The table can change over time as towns lose or gain influence, or new roads are made.
  
  Then what? Maintain a separate grammar for each town?  Impractical.
  
  With a list of sound shifts, you can easily generate a given town's version of a given lexeme as needed; but managing changes in syntax or morphology is, I'd think, not so easy.
upvoted_answer: I have never tried to do it, but I think the approach you call "impractical" is the way to go. You need to maintain a grammar and a vocabulary for each town and for each period (say, a snapshot every 50 or 100 years) to keep consistency.
  
  The good news that can save a lot of time is, that you probably don't need complete reference grammars for each step in the evolution: Start with a good grammar of the oldest stage of your language continuum and than write the grammars of the evolving dialects/languages in terms of differences to the old grammar. 
  
  Only when accumulated differences become too large to be handled well, write a full grammar again.