Weir trial in a wetland to reduce nutrient movement into estuary.

Carlos Ocampo, Daniel Kidd, Megan Ryan

Friday 3 july 2015

14:00 - 14:15h at South America (level 0)

Themes: (T) Hydro-environment, (ST) Rehabilitation of water systems

Parallel session: 16J. Environment - Wetlands

Reducing nutrient (nitrogen and phosphorus) export to delta and estuarine aquatic environments has become a management priority over the last three decades due to eutrophication of waterways. Best management practices in catchments with agricultural areas traditionally focused on reducing nutrient export by controlling fertilizer application and export from areas prone to surface runoff. Rarely has there been an attempt to reinstate the natural capacity of the catchment to store water and recycle nutrients within shallow seasonal swamps and wetlands. This work presents the results from an ongoing research program aiming to assess how a constructed variable crest weir withholds surface water runoff from a 100-ha catchment to retrofit water into a small, shallow wetland area and slow the flow of nutrients into the Peel-Harvey estuary in Western Australia. A monitoring program collecting hydrometric data, water quality parameters, and nutrient concentration data was conducted between September 2013 and December 2014 to assess the weir effectiveness in reinstating wetland hydrology and reducing nutrient export. The weir was operated by the landowner, allowing them to control temporary inundation of the area during rainfall events in the wet season and the effects on water levels were continuously documented using field cameras. Results indicate that the weir helped to reinstate three basic hydrological functions of a natural wetland: reduction of peak flows and water discharge during the wet season, extended water availability for the dry summer season, and retention and infiltration of water from the first runoff event at the break of the wet season. Results also indicated that the weir made a positive impact by: 1) stopping the export of inorganic nutrients in water that results from runoff from the first heavy rains of the wet (winter) season; 2) reducing by up to 79% and 90% the annual load for total nitrogen and total phosphorus, respectively; 3) creating suitable conditions for changes in nutrient speciation (i.e. the chemical form of nutrients) on a seasonal scale; and 4) enhancing nutrient concentration attenuation from inflow water. The findings highlight the need to continue the work using water and nutrient mass balances and numerical modelling to confirm the benefits of weir installation and to examine whether the results are widely applicable by replicating the approach at other sites in similar catchments.