The purpose in using a PondVac is that you're preserving the water. A PondVac sucks the water from the pond, along with the suspended materials, and then passes the water/materials out of the outlet hose where the mesh bag traps the debris and the water (sans crapola) is then returned to the pond.
Simply vacuuming the water out of the pond and then dumping it somewhere doesn't require a vacuum. Hell, you can just use your pond pump to empty a pond.
My point is that if you want to remove the dirt/silt that was deposited in your pond from the run-off, you're going to have to empty the pond to remove the dirt, or partially empty the pond in order to scoop the dirt out by hand. Either way, it's a huge pain in the posterior opening of the digestive tract. The hard way is how I described, the easy way is to live with it.
What you need to think about is constructing some sort of a diversionary device, be it a dry well, a drainage ditch, whatever that will intercept any runoff that's going to get into the pond. Lawn fertilizer, for instance, will wreak havoc on a pond.
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