By Murray Hicks
Braided rivers, defined by networks of channels that are forever changing and shifting, are iconic features of the New Zealand landscape. Their existence depends on abundant supplies of gravelly sediment and frequent disturbance by floods and freshes.
They also support unique communities of in-stream and terrestrial organisms (fish and birds) that have adapted to this dynamic physical environment. For example, the channel changes are sufficiently intense that riverbed weeds are naturally controlled, leaving bare gravel bars and islands which make relatively predator-safe habitats for river birds, including rare/endangered species such as the black stilt.
There is concern, though, that water-use schemes that “de-energise” braided rivers by reducing the size and/or duration of flood flows– e.g. main-stem or tributary dams, or diversions into off-channel water storage reservoirs (collectively termed “flood harvesting”) – might alter the balance between vegetation encroachment and the work done by floods, and so substantially change the river form, behaviour, ecosystems, and other environmental services. For example:
- with reduced flood size and frequency, riparian vegetation may establish semi-permanently on islands, degrading bird habitat and stabilising channels
- channels may be affected by algal blooms (e.g. didymo) for extended periods
- the braided rivers’ mouths, called hapua, may be closed by shingle deposited by waves, inhibiting fish migration and flooding river mouth settlements
- the supply rate of river gravel to beaches may be reduced, promoting coastal erosion.
The first of these effects is captured by the contrasting images of the pristine upper Rangitata River (left) and the regulated lower Waitaki River below Waitaki Dam (right). The challenge is to identify how the magnitudes of these responses relate to the extent of flow change, so that limits and rules can be set on water use that will allow these environments and services to be sustained at acceptable levels.
NIWA has been using numerical ‘morphodynamic’ modelling to explore the geomorphic responses of braided rivers to flow change and to discover the limits of tolerable change. The models are built on robust 2D and 3D hydraulic models, but also incorporate sediment transport and processes such as bank erosion to evolve both the riverbed morphology and the sizing of the gravel mixtures that form the surface of the riverbed and the layers underneath. In addition, the model domain can be covered in vegetation, which increases flow drag thereby causing changed flow patterns and velocities.
Two videos show an example of a modelled flood event in the Waikmakariri River at Crossbank, near Christchurch Airport. The flood occurred in April 2000, and its flow peaked at over 900 m3/s. The first is a time-lapse video taken by a camera mounted on power pylons and directed upstream (note that the pylon shadow acts as a sun dial!). It features shifting channels, removal of lupins from elevated gravel bars, migrating in-channel lobes of gravel, and an “avulsion” event in which an initially minor braid on the north (true left) bank progressively enlarged and “stole” more water from a main braid that had swung towards the south bank.
The second video is of the same event and location simulated with the Delft3D model (the model reach spans 4 km; the camera view in the first video looks upstream to the left from mid-reach). It maps evolving water depth (blue to red shading indicates deepening water), also shows migrating braids and gravel lobes, and reproduces the avulsion event. Detailed topographic surveys of the modelled reach made before and after this flood with aerial remote-sensing enable NIWA to validate the morphological change predicted by the model, which, in turn, provides confidence in our ability to predict the effects of flow regime change on other braided rivers.
Dr Murray Hicks is a fluvial geomorphologist at NIWA.