| 03 Jul 2025
Wave-induced sediment resuspension in the Finnish Archipelago, Baltic Sea: Combining small-scale in situ measurements and large-scale numerical model simulations
Abstract. Sediment resuspension, driven by wind-wave-induced shear stresses, plays a crucial role in coastal water quality, biogeochemical cycles, and the dispersal of pollutants and organisms. If the shear stress from waves exceeds an erosion threshold, or critical shear stress, sediments are resuspended from the seabed. This critical shear stress is an essential parameter in sediment transport models, as it determines sediment erodibility. In this study, we implemented a high-resolution (20 m) spectral wave model to simulate wave-induced near-bottom velocities across the complex archipelago of southwestern Finland. Near-bottom shear stresses from the model and their respective critical values were estimated using seabed data, with results compared to critical shear stress values obtained through laboratory testing of in situ sediment samples. Model data suggested that the critical shear stress could be exceeded over 70 % of the time in certain areas. However, laboratory-determined critical shear stresses were 3–8 times higher than those derived from the model based on median grain size, with modelled shear stresses rarely exceeding the measured critical values. These discrepancies likely stem from unaccounted-for biological and biogeochemical properties of the sediments, which cannot be captured by a simple grain size-based model. We estimate that the accuracy of the wave model data used in this study are of secondary importance compared to the uncertainty of determining the critical shear stress.
Review of: “Wave-induced sediment resuspension in the Finnish Archipelago, Baltic Sea: Combining small-scale in situ measurements and large-scale numerical model simulations”
Dear Editor,
The authors present an interesting case study on wave resuspension of sediment in the Finnish Archipelago. The authors have used a spectral wave model to calculate bottom orbital velocities and bed shear stresses, and use this to estimate the threshold of motion for the sediments in the region. Interestingly, the authors have also measured the threshold of motion from sediment samples using a laboratory which has been published (Joensuu et al. 2020). As I have little experience of spectral wave modelling, I will keep my reviews focused on the sediments. However I would ask if wind forcing at a temporal resolution of 3 hours is enough to adequately resolve the storms?
The authors find a large discrepancy between the predicted and measured critical shear stresses, and attribute this to biological and/or biogeochemical properties of the sediments. Concerningly, there is no reporting of the biogeochemical properties of the sediments, so it is difficult assess how accurate their claim is.
The authors seem to think this is the only way to explain the differences in measured and predicted threshold of motion for the sediments – but this is unfounded. Bulk density of sediment has been shown to have the largest effect on the threshold of motion (Thompson et al. 2019). Mixed grain sizes (for example, a bimodal distribution) can increase the threshold of motion (Staudt et al. 2017; McCarron et al. 2019). In short, the manuscript is, at present, not representing the state of the science. The authors need to address their simplification of the problem.
Upon reading (Joensuu et al. 2020), I see that all the relevant information has been collected, but not used in the present work. I encourage the authors to utilise this information, particularly to follow the work of Thompson et al. (2019), who were able to adjust the threshold of motion for their sediment based upon an array of sedimentary and biological variables. As such, I recommend major revisions as a reanalysis of the data is necessary for the manuscript to be up to date with the state of the science.
Comments to the authors.
Dear authors, I read your manuscript with interest. This is tricky subject, and I think you need to include some of the sedimentary information from (Joensuu et al. 2020) into your estimate of the threshold of motion of the sediments and use the work of (Thompson et al. 2019) to recalculate your thresholds of motion - it seems like an approach similar to their “model 1” would work well. You seem to have limited your explanation of the difference between modelled and measured threshold of motion to only biological processes, yet there is little reason given in the manuscript to justify this. There are numerous reasons why such a difference could occur, much of this is covered in Thompson et al. 2019, and other papers.
Some specific comments:
Equation 8 – the Soulsby Whitehouse equation is incorrectly written, it should be:
(see attached image)
I also highlight to the authors that this is the equation for initiation of bedload, not suspended load. There is a Soulsby-Whitehouse like equation for this, fitted by Van Rijn (unpublished, the source is his website):
(see attached image)
I have calculated the suspension values for their grain sizes (Their table 1, my numbers in red). It would appear they have used the correct version of the Soulsby Whitehouse equation, and it was just written wrong in the manuscript.
Sediment class
Grain size (mm)
theta crit (N m−2)
theta sus(N m−2)
Mud to muddy sand
0.09
0.14
0.16
Sand
0.34
0.21
0.37
Coarse sediments
2.00
1.20
3.06
Mixed sediments
0.15
0.16
? mixed ? (0.21)
Boulders
200.00
178.10
323
The difference between theta crit and theta sus could account for most of the discrepancy the authors have found, especially for the larger grain sizes. It is unclear how they have arrived at their bed shear stress estimate for “mixed” sediments, it appears to be just using the median particle size, which is unjustified. This is unwise as larger clasts (such as gravels and boulders) can “armor” the bed and reduce mobility of all sediments (Wiberg et al. 1994; Vericat et al. 2006). Likewise fine sediments (< sand size) can add cohesion to the bed and reduce hyporheic flow, impacting sediment mobility (Blois et al. 2014; Fox et al. 2014; Parsons et al. 2016; Perret et al. 2018; 2023).
In particular, i recommend to the authors that they read the paper by Thompson et al., (2019), as this paper works through many of the issues with defining a threshold of motion in complicated sediments, including those with biological controls (for instance, Thompson et al., include the concentration chlorophyll-A in their “Model 1”). Moreover, that work found that the bulk density and porosity of the sediment was the overriding control on benthic resuspension, I suggest you try and include this in your work based on the data available in (Joensuu et al. 2020).
References.
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Fox, Aryeh, Fulvio Boano, and Shai Arnon. 2014. ‘Impact of Losing and Gaining Streamflow Conditions on Hyporheic Exchange Fluxes Induced by Dune‐shaped Bed Forms’. Water Resources Research 50 (3): 1895–907. https://doi.org/10.1002/2013WR014668.
Joensuu, Mari, Conrad A. Pilditch, and Alf Norkko. 2020. ‘Temporal Variation in Resuspension Potential and Associated Nutrient Dynamics in Shallow Coastal Environments’. Estuaries and Coasts 43 (6): 1361–76. https://doi.org/10.1007/s12237-020-00726-z.
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