Modeling Braided Rivers in Presence of Exotic Weeds and Dams – GWC Mag

Editors’ Highlights are summaries of recent papers by AGU’s journal editors.

Source: Journal of Geophysical Research: Earth Surface

Human activities such as urbanization, construction of dams, levees, major roads, or the introduction of exotic species, may cause changes in, among other things, river pattern and geometry. River pattern is the plan view of a river as seen from an airplane and, in general, it is classified as meandering or braided. Meandering rivers have a single, sinuous channel surrounded by a floodplain. Braided rivers, on the contrary, have multiple channels that divide and re-join around islands. Channel geometry is commonly described in terms of width, depth and slope of different sections. Plants can stabilize islands and influence bank erosion with consequent impact on channel pattern and geometry.

Understanding if and how humans are responsible for change in river properties, as well as the time intervals over which this change may occur, remains a challenge. This challenge is in part related to predicting and modeling river dynamics because not all processes are well understood and quantified by mathematical expressions, and in part to the fact that rivers and riparian ecosystems may naturally evolve. Identifying natural and human induced change in river characteristics remains matter of debate.

Numerical models are used to forecast river dynamics over different spatial and temporal intervals. Very complex models describe, for example, changes in channel geometry around structures or in relatively short reaches, as well as flow depth and velocity during a flood. These models, however, require too much information and details to reliably forecast how river planform and geometry change in response to events occurring in the 1900s or early 2000s. Finding a reasonable balance between model complexity, data availability, and computational power is critical for model applications and this balance varies from one case to the other.

Stecca et al. [2023] illustrate how a numerical model can be used to distinguish between causes of change in channel pattern and geometry notwithstanding uncertainties associated with limited data availability, use of empirical predictors, and limited knowledge of complex interactions between flow, sediment, and vegetation. The problem is understanding what caused the different response of two large braided, gravel bed rivers in New Zealand to the introduction of the same exotic vegetation. In the Waitaki River downstream of dams, vegetal encroachment induced a transition toward a single channel pattern. The unregulated Waimakariri River remained braided with minimal vegetal encroachment on the islands. Is this different response caused by flow regulation for hydropower generation on the Waitaki River?

Using an innovative approach, the authors thoroughly calibrated and validated a numerical model to reproduce vegetation dynamics at the study sites, and then confidently used the model to study the response to the introduction of exotic weeds that the Waitaki River might have had in the absence of flow regulation. Numerical results clearly show that the different flow regime between the two rivers alone (flashy floods on the Waimakariri River vs naturally dumped floods on the Waitaki River) is sufficient to justify the different response of two gravel bed rivers to the introduction of exotic weeds. Thus, flow regulation was not the cause of the different river response but helped the growth of exotic weed on the Waitaki River islands.

This is one of the few (if not the first) documented attempts to isolate and quantify impacts of flow regulation on channel pattern over several decades and tens of kilometers. The authors clearly demonstrate how integration of field observations and physics-based modeling can effectively be used to isolate and quantify causes of river change over spatial and temporal intervals that are commonly considered too large and too long for physics-based model applications.

Citation: Stecca, G., Hicks, D. M., Measures, R., & Henderson, R. (2023). Numerical modeling prediction of vegetation trajectories under different flow regimes in New Zealand braided rivers. Journal of Geophysical Research: Earth Surface, 128, e2023JF007397. https://doi.org/10.1029/2023JF007397

—Enrica Viparelli, Associate Editor, JGR: Earth Surface

Text © 2023. The authors. CC BY-NC-ND 3.0
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