I just saw this thread, I'll chime in with what some pond owners around here do for the "sand slipping" down to the bottom end of beach or further under water. Most build up a berm on the deep end of beach that will be under water, with clay or stone. On ours I shot pea gravel a few inches deep over the whole beach while still dry so it could settle in. At deep end I built up a 2' tall berm with 1"-2" gravel.
My pond builder said not to put sand on beach area until water was half way up it while it filled to keep from losing it. We shot the pea gravel and then sand over beach following year with a sling truck.
Every few years it seems most beaches still need a load of new sand added to keep nice.
For sand on the beach area above water that erodes bad I have seen people use rail road ties for a barrier.
