Hydrologist — Esperance, Western Australia

A townsite in front of a wetland system with nowhere to drain

Esperance sits between the ocean and the Lake Warden wetland system, and that geometry is the whole hydrological problem. The lakes lie about five kilometres north-north-east of the townsite, entrapped between coastal dunes to the south and a granite escarpment to the north. There is no natural outlet. Eight major lakes and more than ninety satellite lakes make up a system Ramsar-listed in June 1990, fed by five creeks — Bukenerup, Melijinup, Coramup, Bandy and Neridup — draining a catchment of about 212,000 hectares that is 80 to 90 per cent cleared.

Clearing that catchment changed the water balance in a way that is still moving. Average lake depths have doubled since 1985. Lake Warden held unusually high levels of roughly 2.0 to 2.7 metres for a sustained period between 1999 and 2009, and about 65 hectares of wading habitat and 100 hectares of exposed shore habitat were lost between the early 1980s and 2005. Median depth to groundwater across the catchment is about 2.4 metres. Salinity has risen with the water: Lake Warden is around 30 per cent more saline than in the 1980s, and roughly 27 per cent of the catchment was projected at risk from salinity by 2020, rising toward 45 per cent by 2050. A gravitational pipeline installed at Lake Wheatfield in 2008 has discharged about 3.3 million cubic metres to Bandy Creek. This is a system where the receiving water level is not a fixed design assumption — it has been rising for four decades.

The rivers on this coast are saline in a way that is only partly natural. The Young River averages about 12,100 mg/L and the Lort about 26,900 mg/L — both highly saline, on catchments 55 and 64 per cent cleared. Their salt output-to-input ratios of 3.2 and 6.0 show net salt export, which is the signature of secondary salinisation from land use change rather than a purely natural baseline. West of town the Dalyup and West Dalyup catchment covers about 82,600 hectares, roughly 90 per cent cleared, delivering around 11,000 megalitres a year to Lake Gore — itself a Ramsar site.

The design event here is not the one the annual rainfall suggests. Esperance averages about 616 millimetres a year and is winter-dominant, peaking in July, much like Albany. But between 30 December 2006 and 5 January 2007 Tropical Low Isobel delivered 177 millimetres in a day at Esperance Aero and 224 millimetres over 48 hours — roughly two and a half times the entire mean summer rainfall, in January. The South Coast Highway was cut when parts of the bridges over the Young and Oldfield rivers washed away, and Bandy Creek broke its banks at the weir, flooding sand into the boat harbour and stranding vessels. The seasonal water balance is winter-driven; the extreme event exposure includes ex-tropical systems. Designing to the first and ignoring the second is how coastal infrastructure here gets caught.

The town’s drainage network was built from around 1966 as a system of pipes and pits discharging to the coast or to sumps, and has been extended with growth rather than redesigned. The Shire’s own Local Planning Strategy records that laser levelling and farm drainage installation have significantly changed surface water management on agricultural land, with consequences for roads, downstream properties and the wetlands, and states that further investigation into flooding in semi-rural areas is required. There is no DWER floodplain mapping for Esperance.