Abstract
Protection works by stones or gravel against local scour of sand bed around river structures are effective only if the protected bed sand is not drawn out through the cover layer of stones or gravel which is stable against tractive force by the local flow. Hydraulic conditions for the effective protection works are studied experimentally. Bed sand can not geometrically move through the stones or gravel layer, when the ratio, D/d, of stones or gravel scale D to sand scale d is less than 2. The tractive force on the bed sand is exponentially reduced through the protection layer as a function of the ratio, LID, of the protection layer thickness L to D.
