your topic sounds good, although more focused/based upon HSC course (world cities/urban dynamics) not prelim which tends to be more enviro stuff. Generally most ppl do erosion along various stretches of the coast, so its good your doing something out of the box. Topic is good, although i would probs focus more on the alts for second airport i.e. within syd (badgers creek, richmond) or upgrade newy or canberra etc - with pros n cons, feasibilty timeframe etc. Also fill it full geo based 'big' words - sounds good. Also probly better do one eco impacts or 2nd airport, as it will get TOO BIG - better to do smaller topic but well, as broad and poorly. ultimately its up to you and do what your more personally interested in.