Spatio‐Temporal Monitoring of Floodplain Environments Using Electromagnetic Methods: A Scaled Approach to Understanding Surface Water‐ Groundwater Interactions on the Chowilla Floodplain, South Australia
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Abstract:
Geophysical methods are used in Australia to provide detailed spatial information to help predict the impact of current and future irrigation developments, the design of salt interception schemes and protection of floodplain values. RESOLVE frequency domain helicopter electromagnetic data were acquired over the Chowilla Floodplains, in the Lower Murray region of southern Australia, to provide detailed baseline data on the spatial distribution of near‐surface salt stores and materials in the floodplain and their relationship with in‐river salinity. Degradation across the floodplain and wetlands has resulted primarily from a significant reduction in flood events, and overgrazing. Restoration of the floodplain will involve the reduction of salinity flow from groundwater into the river and increasing environmental flows across the floodplain. Conductivity models predicted from HEM data help identify local recharge and discharge areas, and links with river salinity. The baseline data provided by the airborne data are used with high resolution ground EM surveys including EM31 and time‐domain EM, over targeted areas. Ground methods can be repeated, to monitor affects of artificial flooding designed to restore vegetation health. Similarly the combination of airborne and ground data, allows piezometers to be effectively targeted with the resulting information interpreted within the context of the baseline conductivity structure defined form airborne data.Floodplain wetlands of low-gradient rivers are especially sensitive to alteration of hydrologic conditions. Although many studies have investigated changes in downstream hydrologic regimes caused by river dam construction, no study has specifically quantified the long-term effect of altered flood pulse on floodplain wetlands. Such knowledge is needed to develop effective dam operation strategies and plans for downstream wetland protection and restoration. We applied data time series from 1984 to 2018 to delineate the inundation extent of floodplain wetlands during the pre- (1984–2005) and post-dam (2006–2018) periods. Four segments of the 360-km river reach were selected to investigate how inundation frequency of floodplain wetlands responded to changes in flow regimes and flood pulse for both periods associated with dam operation. The results showed that the water area slightly increased as a result of recharging water from dam, while the inundation frequency of adjacent floodplain wetlands overall reduced because of lower magnitude of flood pulse. Inundation area of four segments show less variation during the post-dam period compared to the pre-dam period. These findings clearly indicate that decreased flood pulses caused by dam operations reduce both inundation frequency and areal extent of floodplain wetlands. Based on the findings, we propose an inundation threshold that can be used for dam operation to protect and restore river corridor and floodplain ecosystems.
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Abstract Rivers have been channelized, deepened and constrained by embankments for centuries to increase agricultural productivity and improve flood defences. This has decreased the hydrological connectivity between rivers and their floodplains. We quantified the hydrological regime of a wet grassland meadow prior to and after the removal of river embankments. River and groundwater chemistry were also monitored to examine hydrological controls on floodplain nutrient status. Prior to restoration, the highest river flows (∼2 m3 s−1) were retained by the embankments. Under these flow conditions the usual hydraulic gradient from the floodplain to the river was reversed so that subsurface flows were directed towards the floodplain. Groundwater was depleted in dissolved oxygen (mean: 0.6 mg O2 L−1) and nitrate (mean: 0.5 mg NO3 −-N L−1) relative to river water (mean: 10.8 mg O2 L−1 and 6.2 mg NO3 −-N L−1, respectively). Removal of the embankments has reduced the channel capacity by an average of 60%. This has facilitated over-bank flow which is likely to favour conditions for improved flood storage and removal of river nutrients by floodplain sediments. Editor Z.W. Kundzewicz; Associate editor K. Heal Citation Clilverd, H.M., Thompson, J.R., Heppell, C.M., Sayer, C.D., and Axmacher, J.C., 2013. River–floodplain hydrology of an embanked lowland Chalk river and initial response to embankment removal. Hydrological Sciences Journal, 58 (3), 627–650.
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