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Human intervention, however well-intentioned, can dramatically alter the flow, water-quality and sediment regimes of dammed rivers, with severe and unpredictable impacts on aquatic habitat quality and quantity.
Exactly where overflowing rivers dump sediment on a floodplain may depend on numerous vagaries including flood duration, potential of flood-pond water formation and the time floodwater takes to travel from the channel to individual floodplain units. Sediment size, settling velocity, suspended sediment concentration, and critical shear stress (the minimum amount of shear stress exerted by stream currents that initiates particle motion) also have a major impact. This spatial variation in overbank floodplain sedimentation is a creature of the interaction between flow and sediment regimes as they respond to complex floodplain topographies. Human intervention superimposes further layers of uncertainty.
Consider the Goulburn River and its floodplain, where the channel has shifted dramatically across its valley under the influence of a combination of natural and anthropogenic factors. Not only has the reservoir imprinted changes on the river flow regime by slashing the total number of flooding days to less than a third; it has also impacted on the sediment regime, reducing the amount of sediment that can be transported downstream and deposited along the floodplain during overbank flood events.
While existing floodplain sedimentation models can potentially represent this variability, the relative importance of these causative factors on floodplain sedimentation rates is yet to be established. eWater PhD student Antonia Gamboa-Rocha set out to address this gap in our understanding of the spatial patterns of sediment deposition with a project exploring the post-European settlement impacts on the Goulburn River floodplain (Victoria).
The biggest tributary of the Murray River, the headwater flow of Goulburn River has been regulated for hydropower generation and for irrigation purposes since the early 1900s.
Lake Eildon, the upgraded irrigation reservoir with a capacity of 3,390,000 ML, was completed in 1955. Gamboa-Rocha selected a semi-confined floodplain between Lake Eildon and the Yea River confluence for study.
Her research aimed to:
(a) Expand our understanding on overbank sedimentation processes by investigating the relative influence of identified key processes on floodplain sedimentation and its spatial variability;
(b) Identify the geomorphic characteristics of the Goulburn River floodplain, including floodplain formation and changes during the post-European period, establishing overbank sedimentation patterns and estimating deposition rates; and
(c) Investigate the effects of flow regulation on the geomorphic processes involved in the evolution of alluvial, regulated rivers
Gamboa-Rocha used a combination of GIS analysis, sediment dating techniques and mathematical models developed for predicting sedimentation rates during development of this project.
An extensive historical analysis, including GIS, of old aerial photographs, maps, reports and surveys identified historic changes in the Goulburn River channel alignment.
“This work helped corroborate other research which shows that the Goulburn River is naturally an anabranching system,” she says. (An anabranch is a section of a river or stream that diverts from the main channel or stem of the watercourse and rejoins the main stem downstream.)
“These changes include nine channel cutoffs and two river avulsions (where an old river channel is abandoned and a new one created.) All (of the) identified meander cutoffs and avulsions occurred before operation of Lake Eildon (between the 1850s and 1936) but after the arrival of Europeans. One of the river anabranches and one cutoff were noted to have formed during the extreme floods that occurred in 1916 and 1917. Five of the cutoffs were formed between 1864 and 1936; two formed previously but close to 1864 and none of them after 1936 or 1955, which represents the major regulation period. This translates to the effect that the big dam has on the sediment and flow regimes, truncating the dynamics of the alluvial river.”
With early fieldwork failing to identify a distinctive pre-European versus post-European sediment horizon, Gamboa-Rocha estimated sedimentation rates using radionuclide analysis of isotopes Cs137 and unsupported Pb210.
An intense GIS analysis incorporating historical flood data and a Light Detection and Ranging, digital elevation model (LIDAR DEM) provided by the Goulburn Broken Catchment Management Authority facilitated selection of the floodplain units to be analysed (cored and dated). eWater CRC provided additional funding for sample collection during fieldwork and for the estimation of sample, grain size distributions. The Australian Nuclear Science and Technology Organisation (ANSTO) completed radionuclide analyses of 104 samples taken from 13 distinctive floodplain units sparsely distributed along the floodplain, located at various distances from the river channel and downstream from an irrigation reservoir.
“Deposition rates show a large spatial variation from 0.05 mm/yr to 10.07 mm/yr among the floodplain units,” Gamboa-Rocha says. “This data was obtained using reference Cs137 inventories from reference sites and is valid only for the post-Eildon regulation period (1955 to date). Estimation of pre-1955 and pre-European sedimentation rates was not possible to achieve with this technique due to a lack of detectable activities of unsupported Pb210 detected for those periods.”
She says the estimations from Cs137 analysis will be used for application of sediment deposition models, incorporating sediment settling velocity and accounts for sedimentation during recession pond formation.
Investigation of the key influential processes on floodplain sedimentation and its spatial variability is now underway, using two sediment deposition models (Nicholas and Walling, 1998 and Buttner's et. al. 2006) to calculate sedimentation rates. The sensitivity analysis involves investigation of the relative influence of the important factors on the spatial variability of overbank sedimentation.
“Each factor will be analyzed, set as a variable parameter at a time, estimating net sedimentation rates and subsequently identifying the range of sedimentation and its variability. Two scenarios will be scrutinized: the case in which sediment settles during flood receding phases along pond water and that in which sediment settles during flood water movement,” she says.
“For the latter, a post-peak discharge reconnection to the river channel exists.”
References: Nicholas and Walling, 1998; and Buttner et. al. 2006