The strategic placement of dredged sediment in a shallow tidal environment can optimise the capacity for wind and waves to transport and disperse the material naturally to a desired location. Strategic placement of mud can be used to enhance ecological habitats, decrease turbidity and/or reduce the need to ship this material over a large distance thereby maximizing ecological benefits and minimizing costs. Throughout the world, there are locations that have too much fine sediment (causing a decline in water quality and high dredging costs) or a fine sediment shortage (for example sediment is needed to raise the land to compensate for sea level rise and is needed as building material for dike constructions and land reclamations). With strategically placing mud, we transform sediment from waste into a resource and contribute to the circular economy.
Fine sediment (mud) characteristics
Fine sediment (mud) is defined as having a grain size diameter smaller than 63 µm (as defined in The Netherlands; this threshold varies between countries). Fine sediment consists of silt (between 63 µm and 2 µm) and clay (smaller than 2 µm). Fine sediment is treated as a separate category from sand and coarser material because of its different properties and behaviour, which require a distinctly different approach for successful BwN design and implementation. The main characteristics of fine sediments are:
- Small grain size;
- Low settling velocity;
- Presence of clay particles, which have a large surface to volume ratio and are chemically active;
- Cohesive behaviour of the seabed and long-term consolidation, with implications for erosion behaviour;
- Flocculation and break-up of flocs in the water column, which influences settling and deposition behaviour.
Practical implications of fine sediment characteristics
These characteristics have the following practical implications:
- Transport dynamics. Fine sediment is mostly transported in suspension. In general, fine sediment is much more likely to be supply-limited rather than transport-limited such as is the case for coarser sediment, i.e. fine sediment transport often occurs much below saturation concentration. This implies that a different modelling approach is generally needed opposed to the ones used for modelling sand or coarse material.
- Density effects. At sufficiently high suspended sediment concentrations, its influence on water density may exceed that of salinity and temperature gradients. Turbulence may be damped and settling enhanced. In such 'saturated' conditions, near-bed fluid mud layers may be formed which strongly contribute to sediment transport.
- Contaminants. Because of the large surface area and chemical properties of clay particles, contaminants are often adsorbed to clay particles and may be transported. Therefore, pollutants are generally found in cohesive fine sediment deposits.
- Environmental impact. Because of the low settling velocity, and consequent tendency of remaining in suspension for a long time, fine sediment has direct implications on turbidity and underwater light climate. This is important to acknowledge as turbidity and light climate affect primary production and visibility for predators and prey.
- Deposition in low-energetic environments. Fine sediment tends to settle in sheltered areas such as harbour basins or tidal flats. In harbours, fine sediment contributes to siltation and hence increases the need for maintenance dredging. Instead, fine sediment contributes to mud accumulation on tidal flats, thereby increasing biodiversity. Fine sediment is therefore important for the ecological functioning of tidal flats and is ecologically (e.g. as foraging area of migratory birds), as well as economically (e.g. mussel beds and oyster reefs) beneficial.
Benefits
There are at least two reasons to promote fine sediment accretion:
- The more is deposited, the less is available in the water column contributing to lower turbidity and improved underwater light climate.
- The accreted area may provide valuable habitat for species such as plants or wading birds, thus increasing the ecological system value.
A sediment budget analysis should demonstrate the potential of these measures. If the objective is a local habitat improvement, small-scale measures may be effective. If the objective is an improvement on a system scale, the captured fluxes should be significant at this large scale. Only large-scale measures may thus be effective. Methods or designs to enhance sediment accretion include:
- Introduction of local sheltering structures such as small islands, dams or vegetation;
- Agitation dredging.
The last method is meant to increase sediment supply, the other methods are meant to increase sediment trapping.
For a prediction of the siltation of fine sediment and changes in bed composition, the following steps are recommended. These steps include both data analysis and the set-up of a conceptual framework with first-order quantification of siltation and bed composition change. The method described below provides a rough framework for the estimation of changes in bed level and bed composition caused by the erosion and deposition of fines.
Step 1: Understand the system
The present situation informs you about how much fine sediment is available at the seabed and in the water column. Key parameters needed for a sediment availability and siltation assessment are:
- depth;
- instantaneous current variability and residual current;
- wave climate;
- salinity;
- temperature;
- suspended sediment concentration (both during calm and rough weather, spring and neap tide);
- typical settling velocity (distribution);
- mud fraction in the sea bed;
- dredging activities and volumes;
- the main sources of fine sediment (e.g. fluvial input, cliff or seabed erosion, dredging activities).
From these parameters an estimate can be made of:
- sediment supply:
- horizontal transport flux C x U (product of concentration and current velocity);
- vertical settling flux w s x C (product of settling velocity and concentration);
- shear stress climate:
- typical current speed;
- typical wave height and period.
Sedimentation/erosion analysis
The combination of sediment supply and shear stress climate determines the bed composition of a stable or accreting bed. The composition of a gradually eroding bed is determined by the composition of old (sometimes even geologic) deposits with little to no relationship with the present conditions regarding sediment supply and shear stress climate. The state of the bed (i.e. accreting, stable or eroding) can be determined with bathymetric surveys.
For a stable or accreting bed, erosion properties (M, Tcrit) are the missing link between bed composition on the one hand and sediment supply and bed shear stress climate on the other hand. For a stable situation, cumulative deposition equals cumulative erosion:
∫ (w s C) dt = ∫ (M (τ/τ crit - 1)) dt
By setting this balance, the erosion properties (M, Tcrit) can be estimated. They can also be measured directly with erosion tests, but this requires sampling and lab tests (with the benefit of more certainty, as the consistency of this analysis can then be tested).
Step 2: Understand the desired changes to the system
With the understanding gained in step 1, the effect of new changes within the system can be estimated from the adapted shear stress climate or adapted sediment supply. The following approach is used to evaluate the impact of any changes to the system:
- Determine the changes in bed shear stress originating from:
- bathymetric change;
- change in typical current speed;
- change in typical wave height and period;
- Re-compute siltation and bed composition assuming supply does not change (e.g. with 1DV model);
- Take into account the change in suspended sediment concentrations if bathymetric change alters the supply or induced changes in siltation have substantial impact on mud balance;
- If the new system is very different from the present one, the present one may not be representative anymore and 3D predictive modelling is required (see Step 3).
Step 3: Consider more detailed analysis
If more detailed answers are required, consider modelling, e.g. with a 3D hydrodynamic and suspended sediment transport model including a bed module (for example the tool 'morphological predictor for mixed beds'). In such 3D predictive model sediment supply, erosion and deposition are dealt with interactively. This has the advantage that limitations for the Step 2 approach are solved. However, it requires more time and input data.
Deposition rates
Acceptable or desired deposition largely depends on the specific intervention, system or application.
For harbours and access channels, deposition should be kept at a minimum. However, the larger the desired siltation reduction, the higher the costs. An economic analysis considering construction and maintenance of the new design compared to maintenance of the present design will reveal which is feasible thereby including the cost of fine sediment removal.
For tidal flats and marshes the optimal deposition is less obvious. Both too little and too much deposition is undesirable, as it threatens the survival of the design on the long term and may have unwanted ecological and economical effects on biota. Successful (natural) examples of ecological rich flats or marshes in the vicinity of the design provide the most reliable benchmark for the desirable level of deposition. However, local peculiarities in sediment supply and properties, sea level rise, tide and wave forcing and benthic communities will increase uncertainty in deposition prediction and should be taken into account when feasible.
Table 1: Typical siltation rates for several environments
environment | siltation rate (cm/yr) |
|---|
harbour | > 100 |
access channel | >20 |
tidal flat | 1 - 5 |
marsh | 0.5 - 2 |
Sediment supply and trapping efficiency
The deposition rate of fine sediment is essentially determined by the interplay between sediment supply and trapping efficiency. Sediment supply refers to the availability of the sediment in the system. It is apparent that fine sediment deposition is generally lower in a system characterised by little fine sediment availability (i.e. clear water) than in a system characterised by high fine sediment availability.
The trapping efficiency is mostly determined by the energy of the currents and waves (i.e. the bed shear stress climate). For example, an average low bed shear stress will result in less hydrodynamic energy. This implies that less fine sediment can be kept in suspension, but also that less bed sediment can be eroded and re-suspended within the water column, indicating high deposition and low re-suspension, and hence a high trapping efficiency. Therefore, a high fine sediment supply and/or low bed shear stress dynamics typically results in a muddy bed (high mud content) whereas a low fine sediment supply and/or high bed shear stress dynamics typically results in a sandy bed (low mud content).
Deposition regimes
The bed shear stress (i.e. the hydrodynamic force that acts on the bed) is determined by the interplay between currents, waves and bed roughness. The bed texture and bedforms, but also vegetation and benthic species, may influence bed shear stress and strength, thereby influencing the friction between water and bed.
With reference of what is described above, the deposition regime is classified into 4 categories:
- low supply, low trapping efficiency: deposition very low
- low supply, high trapping efficiency: deposition determined by supply
- high supply, low trapping efficiency: deposition determined by trapping efficiency
- high supply, high trapping efficiency: deposition very high
Change in deposition regime
If a change in deposition rate is desired, the best way depends on the deposition regime:
- Category 1 and 4: focus on both trapping efficiency and sediment supply.
- Category 2: focus on supply if you want to increase deposition and focus on trapping efficiency if you want to further decrease deposition.
- Category 3: focus on supply if you want to decrease deposition and focus on trapping efficiency if you want to increase deposition.
Be aware that sediment supply and trapping efficiency may not be independent. For example, if the exchange of water between a channel and a tidal flat is reduced, the sediment supply will decrease but the trapping efficiency will increase. If the aim is to increase deposition, measures that reduce bed shear but maintain exchange are to be preferred (e.g. reduction of wave energy).
Additionally, supply often depends on the characteristics of the natural environment, and may be difficult to influence significantly. E.g. some rivers and coastlines are characterised by a large amount of suspended fine sediment, therefore large siltation rates in low energy areas (e.g. within a new harbour) occur that are difficult to reduce.
Understanding and controlling bed composition
The speed at which the bed composition changes due to fine sediment deposition depends on the rate of concurrent sand deposition and the intensity of bed mixing (e.g. due to biological activity). If no sand is deposited and bed mixing is negligible, a mud layer that gradually grows in thickness will be formed on the bed. If also sand is deposited or intense mixing occurs with a sandy sublayer, bed composition may only slightly change. These processes can be evaluated with field data and physical consideration, and can be quantified with the tool 'morphological predictor for mixed beds'.
Apart from recreational use, a higher mud content in the seabed is often favourable from an ecological point of view. However, the accretion rate should typically not exceed 0.5 cm/month (valid for Dutch intertidal areas) because of the risk of burying benthic flora and fauna.
To safeguard navigation, about 1.3 million m3 of mainly fine sediments are dredged in the harbour basins of the Port of Harlingen. The dredged sediment is placed in a designated area in the Wadden Sea, in the vicinity of the harbour. Within the Building with Nature case “Ports of the Wadden Sea” the idea was raised to place the dredged sediments further north of Harlingen as a semi-continuous source of sediment: The mud motor. The sediment is expected to be transported by natural processes further into the area. The extra input of sediment is expected to lead to the formation and extension of salt marshes. This would yield three favourable effects:
1. Less re-circulation towards the harbour, hence less maintenance dredging;
2. Promotion of the growth and stability of salt marshes, improving the Wadden Sea ecosystem; and
3. Stabilising the foreshore of the dykes, and therefore less maintenance of the dyke.
An underlying hypothesis of this Mud Motor concept is that mud (e.g. dredged from a nearby channel or port) that is supplied to a tidal current can be picked up by that current so that it achieves its maximum transporting capacity. Higher mud concentrations in the currents that feed a salt marsh will likely speed up marsh development processes, while maintaining the desired gradients that are associated with natural salt marsh development.
Rehabilitation of a mangrove-mud coast in Timbul Sloko (Java, Indonesia)
Mangrove belt establishment has been widely promoted as an alternative means to enhance coastal resilience. However, mangroves can only be successfully restored if the abiotic conditions are optimal (bathymetry, fresh water and sediment input). Most pilots, where this principle has been trested, do not reinstate these conditions and they fail to stabilise eroding coastlines. Long-term success rates of mangrove replantation are as low as 5-10%.
This pilot tests whether siltation in marshes can be triggered by building grids of permeable structures as a means of land reclamation and eventually, at higher elevations, mangrove growth. The ultimate aim is restoration of the natural mangrove – mud coastal system in Java. This approach is inspired by the Dutch salt marsh works. Fine sediments (mud) play a crucial role in the hypothesis tested in this pilot project.
Tidal flats are valuable habitats for different plants and animals and are important for coastal protection. However, the total area of tidal flats is decreasing worldwide due to various problems like sea level rise, coastal squeeze, subsidence by gas extraction and erosion initiated by man-made constructions. The construction of a storm surge barrier and compartmentalisation dams in the Eastern Scheldt in the 1980's is one example of a man-made structure that resulted in a change in hydrodynamic conditions of the Eastern Scheldt estuary and hence the sediment equilibrium. As a result, channels are filling in and tidal flats inside the estuary are eroding. Nourishing tidal flats with sediment might be a promising solution to mitigate these effects.
To test this approach, a small area of the Galgeplaat, a tidal flat in the Eastern Scheldt, was nourished in 2008 with 130.000 m3 sand dredged from adjacent channels over a total area of 150.000 m2. The processes of sediment distribution on the flats and benthic recolonization interact with each other. Therefore the design challenge is to find an optimum to reduce the initial impact of the nourishment on the benthic fauna, while optimizing the distribution of the sand over the tidal flat by wind and waves and the subsequent recovery of benthic life.
