However no information could be found regarding the reason for making such a shape, which is too perfect to be a co-incidence. May be some reader give some info about it.
I am thankful to Walter_V_R from Belgium for furnishing following information;
"About the shape of Schulensmeer: it is really a coincidence.The lake was created artificially in the 70's when they needed sand to construct the nearby highway. I suppose that the shape is only related to the parcels of land they could acquire for this purpose. (Belgium has lots of small parcels of land with different owners, so they have to negotiate a lot).
It now serves for watersport and fishing, but also as an overflow for the Demer river, in case of heavy rainfall."