“…However we will now consider families of distribution functions f , which are parameterised by finitely many N Riemann invariantsλ i (x, t). We are interested in the case [9], [10] where the function λ(p, x, t) is such that only N of the moments are independent. Then there are N characteristic speeds, assumed real and distinct, and N corresponding Riemann invariants (p i ,λ i ), so Benney's equations reduce to a diagonal system of hydrodynamic type with finitely many dependent variablesλ i ,…”