Floodgate

Abstract: The Three-Georges Dam holds many records in the history of engineering. While the dam has produced benefits in terms of flood control, hydropower generation and increased navigation capacity of the Yangtze River, serious questions have been raised concerning its impact on both upstream and downstream ecosystems. It has been suggested that the dam operation intensifies the extremes of wet and dry conditions in the downstream Poyang Lake, and affects adversely important local wetlands. A floodgate has been proposed to maintain the lake water level by controlling the flow between the Poyang Lake and Yangtze River. Using extensive hydrological data and generalized linear statistical models, we demonstrated that the dam operation induces major changes in the downstream river discharge near the dam, including an average "water loss". The analysis also revealed considerable effects on the Poyang Lake water level, particularly a reduced level over the dry period from late summer to autumn. However, the dam impact needs to be further assessed based on long-term monitoring of the lake ecosystem, covering a wide range of parameters related to hydrological and hydraulic characteristics of the lake, water quality, geomorphological characteristics, aquatic biota and their habitat, wetland vegetation and associated fauna.

Abstract: We analyse the potential impacts of sea-level rise on the management of saline coastal wetlands in the Hunter River estuary, NSW, Australia. We model two management options: leaving all floodgates open, facilitating retreat of mangrove and saltmarsh into low-lying coastal lands; and leaving floodgates closed. For both management options we modelled the potential extent of saline coastal wetland to 2100 under a low sea-level rise scenario (based on 5 % minima of SRES B1 emissions scenario) and a high sea-level rise scenario (based on 95 % maxima of SRES A1FI emissions scenario). In both instances we quantified the carbon burial benefits associated with those actions. Using a dynamic elevation model, which factored in the accretion and vertical elevation responses of mangrove and saltmarsh to rising sea levels, we projected the distribution of saline coastal wetlands, and estimated the volume of sediment and carbon burial across the estuary under each scenario. We found that the management of floodgates is the primary determinant of potential saline coastal wetland extent to 2100, with only 33 % of the potential wetland area remaining under the high sea-level rise scenario, with floodgates closed, and with a 127 % expansion of potential wetland extent with floodgates open and levees breached. Carbon burial was an additional benefit of accommodating landward retreat of wetlands, with an additional 280,000 tonnes of carbon buried under the high sea-level rise scenario with floodgates open (775,075 tonnes with floodgates open and 490,280 tonnes with floodgates closed). Nearly all of the Hunter Wetlands National Park, a Ramsar wetland, will be lost under the high sea-level rise scenario, while there is potential for expansion of the wetland area by 35 % under the low sea-level rise scenario, regardless of floodgate management. We recommend that National Parks, Reserves, Ramsar sites and other static conservation mechanisms employed to protect significant coastal wetlands must begin to employ dynamic buffers to accommodate sea-level rise change impacts, which will likely require land purchase or other agreements with private landholders. The costs of facilitating adaptation may be offset by carbon sequestration gains.

Abstract: Summary 1. Estuarine wetlands are important nurseries for fish and decapod crustaceans. Flood mitigation structures (such as levees, culverts and floodgates) that fragment wetland habitat can reduce fish and crustacean passage and subsequently impact biodiversity. 2. Remediating structures to enhance connectivity, tidal flushing and fish and crustacean passage are assumed to be important ways to rehabilitate estuarine wetlands, but they are rarely evaluated with a robust sampling protocol. Furthermore, studies are inconsistently applied across different barrier types, and success is variable. Consequently, those rehabilitating wetlands are left with an incomplete understanding of what trajectories of change (if any) may be expected from barrier remediation. 3. In collaboration with landholders and managers, ‘floodgate remediation’ (structural and operational changes to increase tidal flushing and connectivity) was undertaken in three tidal creeks in two coastal river systems in northern New South Wales, Australia. Changes in fish and crustacean passage were measured for two different techniques (flap gates built into larger gates and the intermittent opening of gates with manual winching) using a BACI design over 2 years. Temporal changes in assemblages and species richness in managed creeks were compared to those in reference creeks (i.e. without floodgates) and control creeks (with closed floodgates). 4. Both types of floodgate remediation enhanced the passage of fish and crustaceans and had significant impacts on assemblages in managed creeks when compared to control and reference creeks. This shift was sustained for the duration of our study in two of the three creeks and was driven primarily by an increase in the number of estuarine–marine-dependent species. 5. Synthesis and applications. Our study demonstrates that floodgate remediation can facilitate fish and crustacean passage and rehabilitate aquatic assemblages in defaunated, tidally restricted wetlands. Given that the vast majority of floodgates throughout south-eastern Australia can be altered to promote connectivity, such remediation may play an important part in guarding against future declines in estuarine connectivity arising from climate change.