UNTOLD HISTORY: From stinking mess to engineering marvel. How the Peel estuary was saved

Motorists heading over the Port Bouvard Bridge south of Mandurah know it well. On one side of the elevated bridge a channel leads into the waterways of the Peel region. On the other side a channel leads out into the Indian Ocean. It’s a much loved region, popular for boating, fishing and just plain relaxing. But it wasn’t always that way. And the channel they see flowing under the bridge as they bowl along the road wasn’t always there. In fact it was designed and built as part of a visionary engineering project 30 years ago to save the Peel Inlet and Harvey Estuary from stagnation and collapse. It was a bold human solution to a human-created problem. In his booklet Saving the Peel Estuary — a public service; former State environment boss Barry Carbon described the problem. “In the 1980s the estuary waters of the Peel Inlet and the Harvey Estuary (together referred to as Peel) . . . were in danger of ecological collapse,” Mr Carbon wrote. “There were overgrowths of blue-green algae which rotted causing a stinking mess. “The once healthy waterway was no use for people and no use for nature.” Mr Carbon wrote that the leader of the Estuarine and Marine Advisory Committee, Ernest Hodgkin, and his colleagues, had shown that excessive mineral phosphorus in the water was feeding rampant algae growth. When the algae cells died, they rotted and sank. This process of too much plant nourishment and growth which chokes the water in algae is known as eutrophication. “Excess eutrophication can result in a smelly rotting mass,” Mr Carbon wrote. Much of the scientific input to tackling the problem came from Bruce Hamilton, who was at the time the major adviser to the environmental protection authority on the science needed to save Peel. Dr Hamilton told Agenda that excess nutrients in the catchment area, particularly phosphorus, leached through the sandy coastal soils, entering the estuary via rivers and the heavily drained coastal plain. “The 130 square kilometres of water was shallow, like a giant fish pond, and the nutrients caused large amounts of algae growth, firstly large (macro) algae and then the micro blue-green algae Nodularia,” Dr Hamilton said. “I remember going out in a boat on a calm day and the whole waterway was covered in what looked like pea soup. “The reason for the excessive use of superphosphate on the coastal plain sandy soils related to the discovery that the inland sandy soils around towns like Three Springs could be made productive by using superphosphate with trace elements in the 1950s and this led to large amounts of land being opened up for agriculture. “The rule of thumb for application of the super was a bag to the acre and this same rule of thumb was used for the west and south coast sandy soils. “But the difference was the rainfall, with less than 300 mm to 400 mm inland compared to 800 mm plus on the south west and south coasts. “This heavier coastal rainfall led directly to the leaching of much of the super from the sandy soils, which could not hang on to amounts being applied. “The initial growth of large algae caused by the excess nutrients resulted in large banks of weed that accumulated on the sandy shallows and rotted in the sun. “In turn, this caused foul odours and even toxic hydrogen sulphide gas. “The Peel Inlet Management Authority collected the rotting weed in the shallows with large front-end loaders and harvested the algae growing in the waterway, treating the symptoms not the cause. “The next phase were the massive blooms of the blue green algae Nodularia in late spring and early summer, causing the whole waterway to be closed for fishing, crabbing and contact recreation. “The Nodularia became toxic under certain conditions and there were cases of dogs dying overseas after licking their wet fur, so it was a pretty sensitive issue for recreation and tourism. “Large blooms of this type are seen as the final stages of a water body dying,” Dr Hamilton said. Action was needed. “A year-long analysis of a wide range of management options came down to two main choices,” Dr Hamilton said. “The first, catchment management, to work with farmers to reduce fertiliser losses was not seen as a short-term solution, because of the large amounts of phosphorus in the soil that would take time to run down, even if the superphosphate applications were stopped immediately. “The fix for the Nodularia became apparent when it was found that it did not survive in the salty water that came in from the ocean every summer,” Dr Hamilton said. Mr Carbon wrote that the EPA studied the options and recommended the construction of the Dawesville Channel. Its main benefit would be to increase the daily tidal inflow of salt water from the ocean into the estuary, and secondly there would be more direct outflow of phosphorus-laden estuary water to the ocean. “The target to stop system collapse and all of the things which went with it was to stop the blue-green algal blooms,” Mr Carbon wrote. “If the Nodularia blooms could be avoided in the short term, then gradual phosphorus flushing to the ocean could assist system recovery in the long term.” Trying to tackle the problem just by “catchment management” to reduce the phosphorus fertiliser from farms ending up in the Peel was not deemed sufficient. Nor was it deemed possible to achieve the required water exchange with the ocean via the existing Mandurah channel. WA State engineer responsible for the design and construction of the resulting project, Mike Paul, told The West Australian that one issue was that the Mandurah channel was too far from the southern tip of the Harvey Estuary to deliver the required results. A new cut to the ocean was needed. It was decided to make the cut through the narrow neck of coastal land at the junction of the Harvey Estuary and the Peel Inlet at Dawesville. It had not already been subject to major development, and agreement was reached with the landowners to build the channel in a way that would enable the development of adjoining areas. The channel had to be big enough to maximise tidal intake of seawater and tidal export of phosphorous, Dr Paul said. The cut also had to be designed so that sand drift along the coast — a potential net northerly movement of about 100,00 cubic metres a year — did not block the channel. In its final configuration the channel was about 2600m long, 200m wide at the estuary end and 130m wide at the ocean, Mr Carbon wrote. About 4.5 million cubic metres of sandy soil was shifted during construction, breakwaters and walls were built and private developers built and sold residential sites including with waterfront locations around land which was excavated. The Port Bouvard Bridge across the channel is 360m long, with eight spans on 19m-high piers. Construction cost the State $48 million including the bridge. Work started in 1991 and finished in April 1994. “The channel saved Peel,” Mr Carbon wrote. “Immediately.” “There was no waiting for the phosphorus to be washed out into the ocean with increased tides. “There was no waiting for winter rains to flush out the system. “There was just an immediate collapse of blue-green algae . . . caused by the salt water coming in from the ocean. “The estuary water was clean. “The fish and crabs came back. The birds came back. “The algae odour went away, so people could come back too.”