That's strange.  When I look at a topo map for SA I see all sorts of places where it's not just a flat plain.  Did you folks bulldoze everything flat recently?  If you did, you still have thousands of abandoned mines that you could use.

Building backup would be a big job.  Building nuclear would be a big job.  The smart job that needs to be done now is to put aside opinions and work out an honest cost estimate.

I'm going to use US prices here because that's what I have at hand.  

New wind (unsubsidized) = <$0.03/kWh
New solar (unsubsidized) = $0.05/kWh
New nuclear (subsidized) = $0.13/kWh 
New pump-up hydro = $0.10/kWh

Assume 30% direct from solar and 30% direct from wind.  40% stored solar/wind.

30%@ 3 + 30%@5 +  40%@14 = $0.08/kWh

Eight is less than thirteen.  And I didn't include the storage which nuclear would need.