You're right it wont work unless they seriously look at investing in some decent drainage. However 2 tonne of rhino walk over the top will probably break any pipe they lied under ground. Even soakaways or filter drains would not help enough.
Actually that reminds me, in the corner of the Black Rhino exhibit (by the meerkat statue) there is a large green lid which is fenced off.
I presume it is some type of drainage system as that area usually has deep floods whenever ive been after a period of bad rain.