arch dam is constructed when dam abutment has rocky strata and have higher bearing capacity to resist trust from arching action of an arch dam.
see arch dam in figure
it is constructed when river run between two hills and the dam site has great depth, and less width at that location arch dam is very economical to another type of dam.
buttress dam is a dam in which has straight alignment and which have buttressed support on the downstream side.
buttress is provided to additional support of dam structure .
the main purpose of the buttress is to provide additional safety against the overturning of the dam. this type of dam is constructed at where dam height is high.