A friend sent me a question a while ago that got me thinking.
She was working with a flow chart about when to use <ch> and when to use <tch> and was wondering why the words <much> <such> <which> and <rich> did not seem to follow the convention. With a single vowel letter and no other letter after that vowel letter, we would expect these bases to use <tch>.
I remembered a Gina Cooke (LEX) presentation on function and lexical words in which she suggested that a number of the words that she used to not have an explanation for could be understood due to the convention that function words typically use as few letters as possible. (If function and content words are a new concept to you there is a film at this link that I recommend!). In the function words she identified, there were no trigraphs. So perhaps spellings that would otherwise use the <tch> use <ch> in the case of function words.
I remember being delighted with the logic of that conclusion. Conventions for the spelling of function words already are a special class that this reasoning fits right into. They are the only complete English words to use one or two letters. They are the only complete English words to have a final <i> (as in the word <I>or <u> (as in the word <you>. When a function word has a homophone pair, the function word almost always uses fewer letters. (<for> / <four>; <to> / <too> / <two>) and more. So the idea that function words would use a digraph where the content word would use a trigraph seems an elegant solution for a long unresoved question I had.
I think that this is an excellent candidate to explain <much> <such> and <which>. However, I am sure that <rich> is not a function word. So as my friend Avivia would say -- I wonder what the story is behind that spelling?