Mary River Salt Water Intrusion Project
Since 1987, the Northern Territory Government has conducted a major saltwater control programme aimed at halting the expansion of saltwater channels into the freshwater floodplains. In 1988 a major barrage was constructed at Shady Camp which significantly reduced further salt water intrusion past this point. On the coastal edge, simple earthen barrages where used to block the breaches in the Chenier ridge. This early effort was instrumental in restoring 2000 hectares of freshwater grassland and Melaleuca swamps.
Over the years, various national and international engineering consultants have been commissioned to investigate the problems of salt water intrusion with a view to finding a cost effective solution. Part of the problem is that the mouth of the river system and the floodplain, being a drowned river valley infilled with 30m of unconsolidated marine mud’s, does not provide a suitable foundation for rigid long term structures such as concrete weirs.
The type and size of works has changed over the years through trial and error. Much of the work has focused on the stabilisation of the beds and banks of affected streams using natural materials obtained on site to reduce the tidal influx and allow the natural vegetation to restore itself. Examples of design trials and engineering structures trialled to date include:
- Concrete barrage & Gabions Shady Camp
- Rock lined spillways, Floodgate and pipe – Swim Creek
- Gabion, reno mattress basket and rock lined – Bobbies Barrage
- Geo textile and mesh design & Plastic lined spillway
- Submerged weir – Tommycut Creek
- Fish way - Carmor Creek
There are a number of operational constraints and limitations in implementing the salt water intrusion project that must be managed annually including:
- High rainfall
- High tidal variation
- Stability of substrate
- Remoteness – access and supplies often require helicopter support
- Multiple land use – politically sensitive differing land uses
