Morphodynamics, Vegetation, Habitat

Group Leader: Murray Hicks

Morphodynamics

  1. Braided systems tend to have multiple pathways of adjustment, including bed roughness, local slope adjustments, braiding intensity, lateral shifts, etc. From a modelling perspective, how do we develop rational criteria to estimate which pathway a river is likely to take, and simulate essential feedbacks among these?
  2. Better understanding of the effects of (in-reach and external) bed-material supply on braiding morphodynamics.
  3. Better understanding the medium-long term (50-100 year) morphodynamics of braided channels – to better understand current processes as well as for prediction of future channel behaviour.
  4. Understanding the morphodynamic behaviour of the active braid belt –better understanding of the active belt dynamics is needed to manage the boundaries between humans and braided rivers. Example issues: avulsion precursors and prediction; braid-belt erosion and active-belt migration.
  5. Braided network morphological evolution at base flows.  Anecdotal observations that side-braids become disconnected from main braid network during extended periods of base flow, thus reducing braiding index and changing habitat. This links with recent progress in the numerical modelling of diffluences, and may also relate to surface-water / ground-water interactions.
  6. Braided river mouth morphodynamic behaviour. Many New Zealand braided rivers enter the sea on wave-dominated coasts, and the mouth morphology typically has an elongated lagoon (“Hapua”) fronted by a gravel barrier with a narrow and very dynamic outlet channel. The outlet channel is vulnerable to closure by wave-deposited gravel, particularly at low river flows. Extended mouth closure events can impact on fish migration, therefore “mouth maintenance” flows are important issues when setting water extraction limits from these rivers (and may over-ride minimum flows for in-stream habitat). The problem is to predict these, since both river and coastal processes are involved.

Vegetation

  1. We require better measures of the type of morphological change caused by vegetation and also better understanding of the main processes and thresholds involved with this interaction. Science questions include:
    (i) What is the morphodynamic signature of vegetation, and how does it depend on the types and distribution of vegetation present?
    (ii) What physical properties of vegetation matter for morphodynamics?
    (iii) What process do most of the work removing braidplain vegetation (e.g. bank erosion and lateral migration, surface scour, current drag, abrasion) and how does this relate to species and life-stage?
    (iv) What hydraulic thresholds (e.g. shear stress) apply for given species and life stages, and how these relate to total discharge?
    (v) How important is suspended sediment in the feedback between vegetation growth and morphological change?
    (vi) How is braiding influenced by the effects of vegetation on bed-material availability?
    (i)-(v) are all ingredients for morphodynamic models that seek to simulate braidplain morphologic evolution due to the individual or combined effects of flow regime change, dams, and vegetation invasion.
  2. What causes variation in vegetation cover in braided rivers? Vegetation cover varies greatly in braided rivers (e.g., from bar braided to island-braided configuration). The question is: how much vegetation, if any, do we expect for a given river and reach?
  3. Recruitment processes. This is controlled both by local conditions (access to groundwater, moisture) and external factors (climate, hydrological regime). It helps explain why some rivers are more prone than others to vegetation encroachment.
  4. The influence of riparian vegetation on meso-scale braided river topography and consequent effects on habitat. Examples: (i) the aggregate role of riparian vegetation on the form, elevation distribution and habitat structure of braided rivers. This would allow papers that go beyond a single habitat type and a local (albeit large) patch of vegetation).

Habitat

  1. Improved understanding of the space-time disturbance frequency of braided river bed habitat, and identification of the optimum disturbance frequency for biota endemic to braided rivers. This pursues the idea that optimal braided river habitat and refugia occur in the less active areas of braidplain.  It also relates to the characteristic morphologies that occur in variously active areas of braidplain (e.g. less active areas can evolve dendritic drainage networks). The application is towards defining, within a regulated regime, the minimum frequency of small floods to maintain substrate quality and so benthic habitat quality. This links also with changes in riverbed vegetation patterns under regulated flow regimes.
  2. Relationships between benthic habitat quality, surficial bed flushing of periphyton and fine sediment, general bed mobilisation, and water discharge. Ecologists in New Zealand tend to use multiples of the median discharge as an index flow for these processes (e.g. 3XQ50) – because it is easily done, but the connection with hydraulic parameters and thresholds (e.g. shear stress and threshold stress to scour periphyton and entrain bed material) is not well substantiated – so the reliability of such flow-based indices is questionable. This needs to be sorted one way or the other.
  3. What are the relative roles in braided river ecosystems of (i) spatial (ii) ‘hydrologic’ (induced by variation in discharge), and (iii) ‘autogenic’ (associated with internal morphologic change) variability?
  4. Do braided rivers and their associated ecosystems co-evolve?
  5. The effect of surface-water / ground-water exchanges on aquatic habitat patchiness, community diversity, and ecological potential. This is a key issue stemming from human pressure on water use.

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