Hydrologist — Walpole, Western Australia

The one south coast estuary that never closes

Almost every estuary on this coast is bar-built and seasonally closed. Walpole and Nornalup are not. The mouth sits at the extreme western end of Bellanger Beach, sheltered from the south-westerly swell by a high granite headland, and in that protected position tidal exchange and river flow together keep the bar open year round. The estuary is always tidal. For anyone modelling the lower Frankland or Deep, that single difference inverts the usual south coast assumption: there is no seasonal impoundment behind a closed bar to represent, and the downstream boundary is a real tide rather than a stored water level.

What replaces it is a damping problem. The entrance channel narrows to about twenty metres where it is constrained by limestone and granite, and the inlets behind it are small and shallow — 13.2 square kilometres combined, with Nornalup’s central basin three to five metres deep and Walpole Inlet under a metre outside its dredged channels. The result is that water level variation inside the inlets runs at only about 40 per cent of the ocean range measured at Albany. A model that applies an open-coast tide directly at the inlet will overstate the tidal signal by more than a factor of two.

The tide reaches a long way inland. The Frankland is tidal for about twelve kilometres and the Deep is estuarine for about six, and in summer marine water reaches the head of both river estuaries. All three tidal reaches — Frankland, Deep and Walpole — sit inside the Walpole and Nornalup Inlets Marine Park, gazetted as a Class A reserve in May 2009. Works in those reaches are inside a Class A marine reserve, which is a materially different approvals path from works on an unreserved watercourse.

The two main rivers behave nothing alike, which matters when a combined inflow is being derived. The Frankland drains about 5,720 square kilometres of largely cleared agricultural country and is moderately saline — around 3,000 mg/L at the Mount Frankland gauge across 1993 to 2002, up from about 590 mg/L in the 1940s. The Deep drains around 1,580 square kilometres of mostly intact native forest; above its gauge the catchment is one per cent cleared and the water is fresh at about 220 mg/L. The system as a whole delivers roughly 558 gigalitres a year, and about 80 per cent of it arrives between June and October.

Rainfall here is the highest in the region and it is not uniform. Walpole averages about 1,324 millimetres a year against 921 millimetres at Albany, but the catchment spans a gradient from around 1,400 millimetres near the coast to under 500 millimetres in the north-east — roughly a factor of three between the mouth and the headwaters. Ground conditions add their own constraints: on the coastal plains the water table typically sits within a metre of the surface, the major drainage channels carry high-peat soils susceptible to compaction, the coastal calcareous sands are highly erosion-sensitive, and the flat poorly drained plains are conducive to Phytophthora dieback — which turns earthworks, drainage alignment and vehicle movement into a single combined design problem rather than three separate ones.