| 23 Mar 2026
Evolution of seepage driven networks in the lab
Abstract. During rain, water infiltrates the ground, where it flows as groundwater toward nearby rivers. There, its emergence can entrain sediments, triggering seepage erosion and thereby influencing the development and expansion of river networks. To investigate this process, we construct an experimental aquifer, made of erodible plastic sediments. A reservoir beneath the aquifer supplies water at a controlled recharge rate. We find that seepage erosion, driven by the resulting groundwater flow, is sufficient to initiate the formation and growth of a drainage network. For a given recharge rate, network growth eventually ceases as the drainage system reaches a steady-state morphology, in which sediments are everywhere at the threshold of motion. This observation indicates that the recharge rate of the aquifer selects the size of the network. In our experiment, the depth of the aquifer is small compared to its lateral extent, so that the flow of groundwater obeys the Dupuit-Boussinesq equation. As in natural systems, the water table in our experiment intersects the drainage network at the elevation of the streams. This condition provides the necessary boundary conditions to solve for the Dupuit-Boussinesq equation and reconstruct the shape of the water table around the river network. The resulting numerical solution agrees well with piezometric measurements carried out in the experimental aquifer and reveals that groundwater flow converges toward channel tips, where velocities are maximal. These results suggest that seepage erosion occurs only when groundwater velocity exceeds a critical threshold.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Earth Surface Dynamics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
Romon et al. describe an experiment in which they allow channel networks to develop through groundwater seepage, then model the water table elevations and groundwater flow. They posit that networks will grow until the system reaches steady-state and the groundwater velocities are at or under the threshold for mobility of sediment. Since the groundwater velocities are a function of total groundwater discharge, this means that the size of a seepage-driven network is a function of aquifer recharge (and thus groundwater discharge) rates.
The paper is short and to the point. The experimental set-up is fairly straightforward, the results are clearly described, and the modeling seems appropriate. The research is of interest to those studying drainage network evolution, particularly in low-gradient areas where seepage erosion might dominate over overland flow-driven erosion. I have a few questions about some points of the experimental set-up, data analysis and modeling, and a desire for more discussion on the implications of the experimental work.
In terms of data collection, repeat photogrammetry was taken, and channel networks were analyzed through manual delineation of the tops of the channels only. Was there any effort to track the volume of the channel network? If multiple images were taken, they could be processed using Structure-from-motion to get topography. If not, was there anything else done to try and collect topographic data? Or monitor the sediment export?
The discharge measurements were made by measuring the mass of water coming out of the basin over a set period of time (line 86). What time period did you use to measure discharge? Did that water also contain sediment? If so, how was that accounted for in the mass/time measurements?
The experiment ran for 35 days. Did you run it continuously? Overnight? How often did you measure things like discharge?
On line 116-117, you note that you waited until the network had achieved a stable morphology before increasing the aquifer recharge rate. Please explain how you determined when the network was no longer eroding and had reached a stable morphology. Was it just through visual observation? Did you compare photos between different time periods?
For the modeling, the assumption was made that the channel slope was not important as it was only 2% and thus the water table at the outlet was used to set the elevation of the water table throughout the channel network, making the water table slope essentially zero in the channel network (line 152-155). Looking at Figure 5a, it looks like there is about a 3cm drop over the experimental domain (150cm x 150cm), which is a 2% slope on the water table. Thus, the channel slope that is neglected is the same as the water table slope. It is thus not negligible. The result is that anywhere there is a channel, the water table gradient in the model will be artificially high adjacent to it because the water table elevation drops to zero there (same as the outlet). This will generate higher velocities along the edges of the channel because of the artificially high hydraulic gradient. This seems rather important. In lines 175-177, you note that the difference between the water table heights in the channels and the modeled heights is quite high, indicating that the channel slope cannot be neglected. Why did you neglect it then? Can you please explain how you handle this discrepancy in a way that does not impact the results and ensuing interpretation of the water table gradients in the vicinity of the channel network?
The paper states that 6 experimental runs were conducted (line 104), yet only one of the runs is described here. While the approach of focusing on one experiment to highlight a particular process is reasonable, there are no data presented at all from the other 5 experiments. Could you combine some of the data from those experiments to bolster the data being presented here from a single experiment? I was left wanting some confirmation of the observations from this single experiment with more experiments, and since you ran more experiments, perhaps you can include some information on whether they support or contradict the observations seen in the one experiment presented here.
Lastly, there is no discussion section in the manuscript, tieing your observations back to the literature and to the natural world. The second paragraph in the conclusions section highlights this a little bit. I think the paper would be stronger if you could spend more time relating the results from here back to other experiments or the natural world. What does it mean in terms of landscape evolution if seepage erosion reaches a steady-state and essentially stops unless aquifer recharge increases? How might the results here inform channel network evolution models in heterogeneous aquifers? How might these results combine with surface flow in natural systems to set the pace of channel network evolution?
A few minor notes:
The term “ramified” is not one I was familiar with to describe dendritic or branching rivers. Consider using a different term, like branching. (This may be a regional issue – I am based in the USA.)
What kind of plastic were you using? Did it have any cohesion?
On figures 2, 5, and 6, please include scale for the images or explain what area is covered in the figure captions.
Line 212 should be “split” not “splitted”
Line 168 remove word “with” or “to” (we compare it to the piezometric…)
Line 191 should be “velocity” not “vel”