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Fine sediment plays a role in natural systems since it is involved in the morphologic change as well as the creation of abiotic conditions, e.g. by affecting the bed composition and water quality. The smart management of fine sediments can provide added value for a BwN design if the relevant natural processes are taken into account.
Sometimes fine sediment deposits are a welcome or even essential part of an ecosystem, for example on tidal flats and for salt marsh development. The deposition of fine sediments can create suitable conditions for certain species and may hence augment the biodiversity. Sometimes fine sediment deposits are unwelcome, for example on tourist beaches, or in harbours and access channels.
General Building Block Description
Smart handling of fine sediments is defined as a way of handling fine sediments that takes advantage of the natural processes in order to minimise costs and maximise ecological benefit. For example, local hydrodynamic conditions may be adapted in such way that siltation is either reduced or increased, whatever is desired. Or dredged material is placed locally in such a way that it enhances ecological habitats, decreases turbidity and/or reduces the need to ship this material over a large distance. In the following pages this is explained in more detail. This building block focuses on bed sediment composition and siltation. For impacts on the water column is referred to the building block assessment of dredging-induced turbidity. Both building blocks interact strongly, as suspended sediment is the source for siltation and bed sediment is a major source for turbidity.
Fine sediment characteristics
Fine sediment is defined as having a grain size diameter smaller than 63 um (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.
These characteristics have the following practical implications:
- Transport dynamics. Fine sediment is generally 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 particle, 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 light climate. This is important to acknowledge as turbidity and light climate affect primary production and visibility of 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 maintenance dredging. Instead, fine sediment contributes to mud accumulation on tidal flats, thereby increases 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.
Examples and benefits
Examples of siltation reduction measures are:
- Siltation reduction in harbours and access channels has been studies extensively. For further information and design rules reference is made to PIANC - 2008.
- Siltation in shallow areas such as constructed beaches can be avoided by a careful design of the wave climate. Examples: Amager Beach Park (Denmark), Sand Engine (Netherlands).
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 turbidity.
- The accreted area may provide a 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 are
- Construction of salt marshes, see Building Block –Site suitability for salt marshes
- Construction of deep pits, see Case - Ecosystem-based design of sand extraction sites – Pilot MV2 Sand Extraction site
- 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.
- Building Block –Site suitability for salt marshes
- Building Block - Assessment of dredging-induced turbidity
- Case - Galgeplaat nourishment
- Case - Shellfish reefs
- Case - Floating Marsh
- Case - Restoration of pioneer salt marsh (Spartina anglica) for coastal protection
- Case - Ecosystem-based design of sand extraction sites – Pilot MV2 Sand Extraction site
- Case - Sand Engine Delfland
- Morphological predictor for mixed beds
Guidelines and boundary conditions
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 sea bed or how much fine sediment moves around. Key parameters needed for a sediment availability and siltation assessment are:
- instantaneous current variability and residual current,
- wave climate,
- suspended sediment concentration (both during calm and rough weather, spring and neap tide),
- typical settling velocity (distribution)
- mud fraction in the sea bed.
- The main sources of fine sediment (e.g. fluvial input, cliff or sea bed erosion).
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
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, τcrit) 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, τ crit) 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)
- if bathymetric change alters the supply, or induced changes in siltation have substantial impact on mud balance, also take into account the change in suspended sediment concentration
- 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 bed module developed in the framework BwN). 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.
Understanding and controlling 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
siltation rate (cm/yr)
1 - 5
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 characterized by little fine sediment availability (i.e. clear water) than in a system characterized 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 resuspended within the water column, indicating high deposition and low resuspension, and hence a high trapping efficiency.
- A high fine sediment supply and/or low bed shear stress dynamics typically results in a muddy bed (high mud content)
- A low fine sediment supply and/or high bed shear stress dynamics typically results in a sandy bed (low mud content)
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 characterized 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
- 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 bed module developed in the framework BwN.
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 because of the risk of burying benthic flora and fauna. Note that the mentioned maximum accretion rate is only valid for Dutch intertidal areas.
Optimise design sand extraction pit
The question to answer first is what is the main objective of a sand extraction pit (apart from sand mining). Is it to provide a valuable seafloor habitat? Or is its main function to provide an effective sediment trap? The optimal design is probably quite different for these two answers.
Even with a fixed pit volume, there is still quite some room to optimise its depth profile, area and orientation.
In pits with a shallow depth, waves and currents may interact with the seabed. The final bed composition is likely to be close to surrounding bed composition. If a shallow pit attracts currents (e.g. orientation parallel to the dominant current direction), it may even become sandier (i.e. fines are resuspended from the seabed).
In pits with a very large depth, no resuspension at all may occur, if the critical shear stress for erosion is never exceeded. The ultimate bed composition is determined by the sediment quality of the settling material. In the middle of a pit of some size, this will be mostly mud. At the pit edges, there will be more sand. Very deep pits may experience stratification and low oxygen levels near the bottom.
By creating gradients in bed level, also gradients in hydrodynamic conditions and bed composition are invoked.
Essential input for the design are the hydrodynamics of the pit and the sediment transport rates at the pit's edge and bottom. Depth is an important design criterion, both with respect with trapping efficiency and habitat suitability.
Designing of Sand engine
The same principles apply as for extraction pits, but very different conditions occur. Sediment supply and transport are higher in the breaker zone. Also the typical bed shear stress is much higher. On long term, erosion and deposition of fines should be more or less matched. Only at locations that remain always sheltered from storm waves permanent mud deposits may be expected. With a careful design the wave and current dynamics can be established at a level optimal for habitat development.
Enhance siltation in marsh
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%.
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 of the pilot on rehabilitation of a mangrove-mud coast in Timbul Sloko (Java, Indonesia) 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.
Minimising siltation in harbours and access channels
This example is well-established, see for example PIANC report 102 (Minimising harbour siltation, 2008). Siltation depends on:
- harbour location
- harbour entrance geometry, dock length
- density differences
Most often, siltation reduction methods are applied to existing harbours. However, such methods can be applied most effectively during harbour design in which the harbour location along a coast or in an estuary is not yet determined.
Siltation reduction methods are often counteracted by ongoing deepening of harbours and access channels. As a result of deepening, both sediment supply and exchanges with docks typically increase.
How to Use BwN
A BwN design may have three different ambition levels with regard to fine sediment:
- level 0: neglect it;
- level 1: take it into consideration in order not to be hindered by it;
- level 2: adapt your design in order to profit from it.
The zero level is not recommended, unless it is evident that fine sediment does not play any role. The first level is the safe default level. However, when judged appropriate at level 1, a true BwN design should be considered, stepping up to level 2.
For example, if a modification of a coastal environment is planned, it is important to focus on the impact of the specific modification in connection with the natural system. The study of the natural system (serving as a background or reference situation) has the following important benefits:
- It puts the anticipated changes within the natural context, allowing for a sounder impact assessment;
- It allows taking advantage of natural processes (i.e. the enhancement of natural sedimentation by the extension of sheltered area may mitigate a turbidity increase by dredging works);
- It allows better modelling and model calibration. Understanding of the system based on in-situ data is essential for accurate modelling. A good model is essential for predicting the fate of modifications and the prediction of the response of the natural system to any human intervention. Proper predictions allow for better judgment of the need of mitigation and its optimum design.
Sufficient knowledge on the natural system, which is essential for an optimal design, necessarily requires field observations that are ideally combined with modelling to benefit most of both approaches.
Important pros and cons of field observations and modelling are:
- often substantial gaps in space and time
+ complete data at any place, any time
- not the 'truth'
- Ecoshape 2012: Assessment of dredging-induced turbidity
- Ecoshape 2012: Morphological predictor for mixed beds
- Ecoshape 2012: Sand Engine Delfland (Netherlands)
- Ecoshape 2012: site suitable for salt marshes
- Ecoshape 2012: Ecosystem-oriented Landscaping of Sand Extraction Sites
- Mangor, K., Broker, I., Haslov, D., Waterfront developments in harmony with Nature - Amager Beach Park (Denmark) , Terra et Aqua, June 2008
- PIANC report no. 102 – 2008, 'minimizing harbour siltation'