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Estuaries shaped by biomorphodynamics, inherited landscape conditions and human interference

Periodic Reporting for period 3 - ESTUARIES (Estuaries shaped by biomorphodynamics, inherited landscape conditions and human interference)

Reporting period: 2018-12-01 to 2020-05-31

ESTUARIES are shallow coastal water bodies with river inflow shaped by biomorphological processes, with patterns of channels and shoals, sand/mud flats, tidal marshes, vegetated banks and peat. Development was influenced by early Holocene landscape that drowned under sea-level rise, and by human interference.

Estuaries harbour highly productive natural habitats and are of pivotal economic importance for food production, access to harbours and urban safety. Accelerating sea-level rise, changing river discharge and interference threaten these functions, but we lack fundamental understanding and models to predict combined effects of biomorphological interactions, inherited landscape and changing drivers.

We do not understand to what extent present estuary planform shape and shoal patterns resulted from biomorphological processes interacting with inherited conditions and interference. Ecology suggests dominant effects of flow-resisting and sediment de/stabilising eco-engineering species. Yet abiotic physics-based models reproduce channel-shoal patterns surprisingly well, but must assume a fixed planform estuary shape. Holocene reconstructions emphasise inherited landscape- and agricultural effects on this planform shape, yet fossil shells and peat also imply eco-engineering effects.

My aims are to develop models for large-scale planform shape and size of sandy estuaries and predict past and future, large-scale effects of biomorphological interactions and inherited conditions. Objectives are:
1. to characterise biomorphological patterns and dynamics on shoals, bars and channels
2. to identify critical eco-engineering species and formulate rules for their habitats and effects
3. to create predictive numerical models for large-scale effects of biomorphological interactions
4. to identify large-scale effects of eco-engineers in experimental analogue models of estuaries
5. to formulate testable geological hypotheses about underlying conceptual models in reconstructions
6. to systematically unravel effects of biomorphological interactions from imposed conditions
• We are developing a topological analysis method for multi-temporal channel networks based on graph theory incorporating directional propagation of reversing tidal flow, sediment and disturbances. So far, the network structure appears sensitive to direction in the topological sense but further work on graph theory is necessary.
• We are developing object-based image analysis (OBIA) ruleset to discriminate shape characteristics of patch-, tidal flat- and marsh patterns based on spectral properties in aerial photographs of systems in nature and in the Metronome images. Using these, we aim to characterise changes of object properties through time, distinguishing at least between channels, sand bars, mud flats and vegetated flat.
• The first analyses characterising and comparing patterns of numerical and experimental results are underway.
• We identified the main ecosystem engineers and their habitats for temperate climate, sand-dominated estuaries.
• We found rules that predict biomass and biostabilisation or bioturbation effects and are mapping these on existing estuaries for verification of our method of application to entire estuaries, which is novel.
• We are building code to apply the species and their effects to any given estuary for application to field data and our numerically modelled estuaries.
• We are testing hypotheses of large-scale effects of biomorphological interactions for selected ecosystem engineers and compare these against control runs without vegetation. Furthermore we are testing interactions with mud and conducted runs for tidal rivers and for tide-dominated estuaries at a timescale of centuries.
• We designed proper lighting for growing and photography in the flume for analogue models in the Metronome and we have developed algorithms for data filtering, reduction and analyses.
• We discovered several ‘weed’ species that pioneer different habitats in our laboratory analogue models when distributed by flow and by seeding. These work considerably better than the alfalfa used until now by the international community and we already observed interesting eco-engineering effects including enhanced mud sedimentation.
• Through field site visits and review we are in the process of identifying which lab species represent which habitat in nature. We have one species that mimicks Spartina well and another species that mimicks Salicornia, which pioneers before Spartina, and a third species that may well represent riparian trees. Furthermore we may have a species that will form the experimental equivalent of peat.
• We are experimentally testing hypotheses of small- and large-scale effects of biomorphological interactions from runs with limited numbers of species compared to control runs, all run by protocols representing recruitment at a regular interval of a few years and excluding extreme events.
• We studied to what degree alternative reconstructions are possible for specific Holocene estuaries based on different conceptual models and unravelled unexplored connections with potentially applicable models from the morphodynamics community.
• We formulated hypotheses how specific aspects of estuary planforms are attributable to externally imposed constraints (and timing where relevant):
o confining effects of valley geometry on estuary dimensions and planform geometry, partly determined by human interference in peat that initiated ingressive estuaries
o Coastal sand supply, partly determined by substrate, has a large effect on the response of estuaries to early Holocene sealevel rise
o We identified significant effects of cohesive or resistant substrate underlying growing estuaries but have not yet found that estuarine margin stability is imposed by substrate except in bedrock cases.
o river water and sediment supply, accounting for avulsion, strongly determines estuarine development as evidenced by contrasting cases in the Holocene coastal plain.
Progress beyond the state of the art is found most clearly at this stage of the project in the experimental work in the new Metronome facility. Here we managed to create entire estuaries on a scale of about 1:1000 with not only water and sand but also mud and real live vegetation.
We expect the pattern analysis work to lead to novel tools and characterisations that are potentially useful for all channel networks in the geosciences.
The numerical modelling is expected to, for the first time, build entire landscapes from rivers and tidal currents including mud and the eco-engineering species. Models with multiple species are underway.