Morphodynamic modelling of dryland non-perennial riverscapes, with implications for environmental water allocation

Abstract

Reach-scale river restoration or environmental water allocation (EWA) exercises typically address the magnitude and temporal dynamics (frequency, duration, timing, rate of change) of flows required to sustain desirable ecological conditions along a river. The role of geomorphology in this process is to broaden the gaze beyond flows to consider larger and longer-term interactions between valley lithological structure, and the feed and fate of flow-sediment mixtures. This paper proposes the integration of numerical morphodynamic modelling in evaluations of environmental water requirements for non-perennial riverscapes (channel–riparian–floodplain environments). The paper presents a methodological framework, and proof of concept case study from the Touws River, South Africa, for the application of morphodynamic modelling in EWA. The paper illustrates operational approaches to modelling the complexity of dryland mixed bedrock-alluvial (and mixed-load) riverscapes with highly variable non-perennial flow regimes, including an approach to generating initial bed conditions for numerical experiments by ‘morphodynamic spin-up’, and approaches to synthesising and presenting numerical experiment output in the form of a dynamic range of potential variability in metrics of physical habitat suitability and diversity, and disturbance/renewal regimes. Such efforts can assist in enhancing field observations and testing field-based hypotheses of flow-sediment regime–physical habitat associations, extending the timescales of analysis beyond field observation, and constraining uncertainty about the dynamic range of variability in responses to predicted future flow-sediment regime modifications. Further research is needed to develop growth models appropriate for key non-perennial river vegetation types, to support biomorphodynamic modelling of geomorphology–vegetation interactions, and to determine or predict appropriate inlet sediment concentrations for historical and future modification scenarios.

Spectral classification, mapping and physical habitat implications of a riparian invasion by Tamarix ramosissima Ledeb. in the Touws River, Klein Karoo, South Africa

“…Figure 6 shows the mapped results of the dNDVI 0.2 threshold classification for the study reach, and illustrates a generally linear, channel/pool margin-associated distribution of T. ramosissima. There is an increase in the cover of T. ramosissima through the middle to lower part of the reach, consistent with the distribution of deeper channels and pools (Grenfell et al 2021). The invasion covers an area of 80 300 m 2 , which constitutes only 8.21% of the total reach valley floor area mapped, but 65.50% of the total vegetated valley floor area (i.e.…”

“…Kleynhans et al 2008) in non-perennial rivers is a challenging task (Seaman et al 2016). High to extreme variability in flows (Puckridge et al 1998), physicochemistry (Day et al 2019), and geomorphology (Jaeger et al 2017) make it difficult to determine natural reference conditions for these driver variables (Seaman et al 2010;Grenfell et al 2021). Additionally, some response variables that are routinely and effectively applied in perennial rivers (e.g.…”

“…Mtengwana et al 2020), and to model the potential future spread of invasions (Mtengwana et al 2021). Such understanding is critical in evaluating the effects of riparian invasions on riverscape morphodynamics and physical habitat structure and function to enhance EcoStatus determination (Grenfell et al 2021). The objectives of this paper are to i) investigate the utility of a delta normalised difference vegetation index (dNDVI) in evaluating the status, pattern and process of invasion of a reach of the Touws River by Tamarix ramosissima Ledeb.…”

Using the water balance approach to understand pool dynamics along non-perennial rivers in the semi-arid areas of South Africa

“…The catchment is covered mainly in natural vegetation, predominantly shrubland and fynbos, with some parts of the floodplain being used for agricultural purposes. The mean annual rainfall is 244 mm/year (Grenfell et al, 2021). However, the study site received rainfall amounting to 112 mm/year in 2018, 91 mm/year in 2019, and 182 mm/year in 2020.…”

Predicting Wetland Occurrence in the Arid to Semi—Arid 1 Interior of the Western Cape, South Africa, for Improved 2 Mapping and Management

“…In both Karoo zones it was rare to find wetlands extending out of the channel across the valley floor, and where 262 present (Supplementary Plate 8), these were generally located within 10 km of the higher rainfall mountain areas, from which they likely receive surface flows following major rainfall events. In addition, some are supplied by groundwater, for which direct evidence is found for the Prins River 265 (Grenfell et al 2021). In the Great Karoo, the next most frequently occurring HGM type was the 266 depression, with all depressions being unvegetated, as was the case in the Little Karoo.…”