I was either listening to Irish radio or TV am Ireland and the resident vet (Pete the Vet from Bray) mentioned using balloons to give the cats a fright when they start digging your garden for you.
His answer was actually relating to a woman trying to stop her cat 'going' in a particular area of her garden. Pete mentioned putting a balloon under the soil - not sure how that would work.
Noise and water are, I think, your best idea.
I love cats but they can be a pain - dogs are social animals and always want to be our best friend. Cats don't care - as long as they are fed and pampered.
Frankie is a Scottish moggie - his sister is mainly black and very pretty and is incredibly stupid.
Now that you have found tiny pic what about a picture of your cowardly dog - can't even chase off a tiny cat.