Understanding fluvial aggradation styles through physical modelling: Are we able to distinguish changes in water discharge and/or sediment supply (upstream drivers) from changes in base-level (downstream forcing) on river aggradation?

<p>Fluvial stratigraphy is the product of changes in Earth’s history and inverting this record has often resulted in interpretations associated with changes in base-level caused by sea-level and/or basement subsidence (downstream drivers).  Similarly, environmental perturbations occurring in the upstream reaches of a fluvial system (i.e., the source region), such as climate driven changes of water discharge and/or perturbations of sediment supplied to rivers (upstream drivers), can also drive river bed evolution.  Therefore, both changes in upstream and downstream drivers can cause a river’s equilibrium profile to respond and adjust through aggradation or degradation and hence generate stratigraphy.  Furthermore, it is likely to have changes in both upstream and downstream drivers simultaneously because both drivers may be themselves driven by the same factors e.g., astronomical cycles can drive both sea-level variation at the downstream end of fluvial systems and water discharge variations in the upstream end.  Deciphering the effects of the two drivers is vital to be able to comprehensively interpret the narrative of Earth’s history preserved in fluvial successions.  We explore this issue with river flume experiments, where we are able to test the influence of both upstream and downstream drivers in isolation.  Furthermore, the small scale of physical modelling reduces the spatial and temporal timescales compared to natural systems and allows us to investigate how quickly the system responds.</p><p>We use a narrow (0.05 m), long (2.25 m) flume with an initial gradient of zero.  Side-profile photos are taken throughout the experiment run, which are then analysed and fitted to monitor river bed evolution.  Top view photos record channel dimensions.  We use low flow rates (~<600 ml/min) delivered by a peristaltic pump, to avoid turbulence and ensure bedload transport.  We have three aims with our experiments. Firstly, to investigate the role of changes in water discharge and sediment supply on equilibrium river profiles and the timescales associated.  Secondly, to carry out a series of perturbation experiments varying downstream drivers (i.e., sea-level), which theoretically produce the same amount of aggradation as the upstream parameters we have used in order to compare.  Thirdly to vary both upstream and downstream parameters simultaneously to investigate the effects. Results to date suggest that the growing wedge maintains a relatively constant slope and that the slope of the wedge is dependent on sediment concentration (sediment discharge/water discharge), when using the same grain-size distribution for each experiment.  Furthermore, results imply that the system is highly sensitive to perturbations when the setup of the system is with relatively low sediment concentrations. Therefore, a greater magnitude of response is seen than with setups of higher sediment concentrations. Currently we are undergoing perturbation experiments and downstream perturbations, the results of which will also be presented here. Ultimately we will use our findings to upscale our experiments into a fully 3-D flume tank that will grow as an unconfined fan in order to observe any similarities and differences.</p>

A Wave Input-Reduction Method Incorporating Initiation of Sediment Motion

The long-term prediction of morphological bed evolution has been of interest to engineers and scientists for many decades. Usually, process-based models are employed to simulate bed-level changes in the scale of years to decades. To compensate for the major computational effort required by these models, various acceleration techniques have been developed, namely input-reduction, model-reduction and behaviour-oriented modelling. The present paper presents a new input-reduction method to obtain representative wave conditions based on the Shields criterion of incipient motion and subsequent calculation of the sediment pick-up rate. Elimination of waves unable to initiate sediment movement leads to additional reduction of model run-times. The proposed method was implemented in the sandy coastline adjusted to the port of Rethymno, Greece, and validated against two datasets consisting of 7 and 20 and 365 days, respectively, using the model MIKE21 Coupled Model FM. The method was compared with a well-established method of wave schematization and evaluation of the model’s skill deemed the simulations based on the pick-up rate schematization method as “excellent”. Additionally, a model run-time reduction of about 50% was observed, rendering this input-reduction method a valuable tool for the medium to long-term modelling of bed evolution.

A coupled 1-D/2-D model for simulating river sediment transport and bed evolution

The paper details the method to couple a 1-D hydro-sedimentary model to a 2-D hydro-sedimentary model in order to represent the hydrodynamics and morphological processes during a flood event along a river. Tested on two field cases, the coupled model is stable even in the case of generalized overflow over the riverbanks or of levee breaching. For lateral coupling, the coupled model allows saving computational time compared to a full 2-D model and to provide valuable results concerning the flooding features as well as the evolution of the bed topography. However, despite a similar simplified representation of the sediment features in the 1-D and 2-D models, some discrepancies appear in the case of upstream/downstream coupling along a cross section perpendicular to the flow direction because the assumption of homogeneous velocity and concentration is not valid for estimating sediment transport. Further research is necessary to be able to define a suitable distribution of the sediments on the 1-D side of the boundary between the two models.