46 Monitoring at the outlet from the wetland at the earth embankment and culvert (Site TB4 in GHD 2022C), shows that there are periods of no discharge from the Gladstone Wetland: • Summer 2019 – not flowing for approximately 1 day on 2 occasions. • Summer 2020 – not flowing for approximately 5 months. • Summer 2021 – not flowing for approximately 2 weeks. Conceptually, GHD (2022C) envisages the wetland response as follows: • Significant rain events create rapid catchment run-off and saturation of soils and organic litter within the wetland. Excess water is rapidly passed through the wetlands, as high rates of surface water flow and high-water levels. Residual water, whether ponded or in wetland soils or litter, remains as stored water within the wetland. • Delayed flow from the catchment, such as from interflow, continues after rapid run-off has ceased. This continued inflow maintains effective saturation of the wetland. The catchment inflow reduces over a period of days to weeks since rainfall. • Water stored within the wetland drains from soils and organic litter at a slower rate than surface water flow. Water drains from the upper wetland areas towards lower parts of the wetland, where it contributes surface water flow in lower parts of the wetland and in the tributary, maintaining a higher degree of saturation in these areas. • The drainage of stored water is buffered by groundwater inflow to the wetland, any residual interflow from the catchment and any subsequent inflow created by rain events. The rate of drainage from wetland soils exceeds the rate of groundwater inflow. • During dry periods, sub-surface drainage continues to remove stored water, with greater reductions in stored water in the upper areas of the wetland first. Prolonged periods without rain see this ongoing drainage result in drying of the wetland, first at the upper wetland extents, and ultimately in lower areas, including at the tributary. • The occurrence of no-flow or dry conditions at the tributary coincides with that point in time when the rate of drainage from the wetlands has reduced to the point that it can be passed as groundwater flow beneath the tributary culvert i.e., there is no excess to generate surface water flow. • Re-saturation of the wetland is readily achieved by rainfall after a drought period. It is expected that when the tributary is dry, the wetland itself can be considered to have minimal standing water i.e., the wetland may also have dry parts. However, during such times the soil moisture within the wetland may still remain high.
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