Streambeds are the physical interface between surface water flow in streams and groundwater flow in underlying aquifers. A recent article in Reviews of Geophysics describes a new modeling blueprint to capture streambed dynamics in a manner consistent with observed processes. The journal’s editors asked the authors about the role of the streambed, its importance, and why a new modeling blueprint is needed.
Why do streambeds matter?
The topography, permeability, and porosity of a streambed controls water, mass, and energy fluxes between surface water and groundwater. A streambed’s physical characteristics also control residence times of nutrients within the hyporheic zone, with major implications for stream ecology, biogeochemistry, and water quality. Streambeds have therefore received significant attention in the fields of geomorphology, hydrology, hydrogeology and ecology.
What are the feedbacks and links between sedimentological, hydrological and hydrogeological processes?
The composition and structure of a streambed are the result of complex interconnected processes of surface water flows, groundwater flows, surface water-groundwater fluxes (i.e. downwelling and upwelling water), erosion, deposition, filtration, and biogeochemical processes. Streambed properties, such as hydraulic conductivity and porosity, are intrinsically linked to the sedimentary composition of the streambed. Meanwhile, the geometrical structure and type of streambed sediments impact upon how water flows over, into, out of, and across the streambed.
At the same time, water flow above and within the streambed drives the erosion, deposition and filtration of the streambed sediments. Most obvious are surface water controls: a major flood can scour a streambed and slowly flowing surface water promotes deposition. Flows through the streambed can also be important. For example, upwelling water can prevent the deposition of fine sediments while downwelling water can enhance this deposition.
The streambed is clearly changing in space and time. Physical changes of the sedimentological structure of the streambed as a result of erosion and deposition processes can change the flow across the streambed by orders of magnitude.
What are the current models for simulating flow and sedimentological processes in streambeds, what are their limitations, and could they be improved?
Our review illustrates that two families of streambed models have been developed, based either on a water flow perspective or on a sediment transport perspective.
The first family of models includes models that aim to simulate water flow within catchments by considering both surface and groundwater flow in a physically based way; for example, those based on the famous blueprint laid out by Freeze and Harlan . Those models typically simplify the representation of the streambed, by assuming that it is remains static, therefore neglecting sediment transport processes.
On the other hand, the second family of models have focused on fluvial geomorphology and sedimentological processes that encompass hydrodynamic flow and sediment transport processes, such as sediment erosion and deposition of the streambed. Those models, however, typically simplify or neglect the interaction between surface water and groundwater, such as upwelling and downwelling flows through the streambed, which influences the flow, streambed shear stress, and hence the erosion and deposition of the streambed, especially for fine particles.
Our review proposes a new modeling blueprint for streambeds that combines the two families of models to enable the coupled simulation of flow and sedimentological processes in streambeds. It therefore combines the advanced representation of flow and sediment transport processes already included in the two separate families of models.
What are the challenges in adopting or developing this modeling blueprint?
The modeling blueprint presents a number of exciting challenges. A key challenge is the establishing more reliable, quantitative links between directly observable streambed properties such as the grainsize distribution or its morphologic structure to physical properties, and also moving beyond the purely physical and incorporation of missing biogenic processes that also alter the properties.
The modeling blueprint also poses some numerical challenges associated with, for example, the coupling schemes for the various processes and also the disparate spatiotemporal resolutions required to, capture them accurately. The degree of accuracy and scale is tightly related to the scale of the processes of interest, i.e. the question asked of the model.
Last but not least, significant work will be required in dealing with the parameterization of such a model, not only in the context of the inverse problem but also in the unique observations of the streambed supporting such a model, which was discussed in detail in an associated review article [Brunner et al., 2017].
What are the implications?
An improved understanding of how fine particles redistribute in the streambed during both upwelling and downwelling will afford new insights into the transport of contaminants and on physical streambed clogging mechanisms. The latter process is of great concern from the perspective of fluvial ecology, as increased sediment loads pose a threat to the health of stream ecosystems due to clogging and permeability reduction of benthic and hyporheic zones.
From the perspective of water resources management, an improved understanding of streambed processes will allow for more informed assessment on the impacts to stream-aquifer exchanges, in response to management. A better understanding will also improve design of stream restoration projects, improve understanding of the fate of micropollutants and nutrient dynamics, but also how anthropogenic impacts to stream-aquifer interaction influence stream geomorphology.
–Daniel Partington and Craig T. Simmons, Flinders University, Australia; René Therrien, Université Laval, Canada; and Philip Brunner, University of Neuchâtel, Switzerland; email: [email protected]