Chum Saeng waterfall is the first signposted waterfall along the way, km's from the main highway interchange 80 km's before Ranong. It's on the south side of the road near km's 529/530. The waterfall is only 3,5 km from the main road.
The local roads department have foreseen larger things for Chum Saeng and have built a two lane highway from the main highway straight to the foot of the falls. Nothing has been left to coincidence and a once beautiful park-like surroundings would have existed. Not anymore unfortunately, as maintenance has failed to keep up with visitors resulting in a under utilization of the site as well as the slow accumulation of refuse making it increasingly desolate location.
It’s a nice view though, 50m or so of the fall. Further back the view is more impressive than being just underneath the fall.
RideAsia.net have gps coordinates (N10.30.030 E098.53.029) though they also mention there was no water in the falls during the dry season. Wikipedia adds:
'In the rainy season, a lot of water cascades down the boulders in rainbow splashes. This is how it is called “Namtok Sai Rung” or Rainbow Waterfall'.
Thai-tour.com adds:
'Sai Rung which means a rainbow in named according to the splashing figure of the waterfall resulting from the strong hit of the powerful water agains rocks'.