Why do streambeds matter?
On Integrating Sedimentology and Hydrogeology in Streambeds
A new modeling blueprint seeks to unify sedimentology, hydrology, and hydrogeology in the modeling of streambeds.
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 [1969]. 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.
By Daniel Partington, Craig T Simmons, René Therrien, and Philip Brunneron