The Wahnbachtalsperre is a dam near Siegburg. The 52.5 meter high and 350 meter long dam wall was built in the middle of the last century.
The reservoir of the dam is the drink water supply for nearly 800,000 people in Rhein-Sieg-Kreis and Bonn. This fact is reason that, swimming ist not allowed there. You should observe the rule, if don't the punishment is very hard. But the nature arround and the view on the reservoir is still worth a visit. If you are interested in hiking you should take 'some' time an walk arround the Wahnbachtalsperre (ca. 30 kilometres, you need a whole day). Perhabs you will find some new spots for locationscout on your tour.
You can get there easily by car, bike or feet. Some Parkingspace is available nerarby.
I would recommend you to get there by feet or bike. The tracks and hiking paths to the dam leads you through an interesting environment, with lil creeks or e.g. a massive brige ruin wich destroyed in World War II.