EstProc 13

1. EstProc 13/10/2004 Waves in estuaries Andrew Lane David�Prandle Judith�Wolf Waves in estuaries and their impact on sediments This presentation is an overview of the sediment modelling and wave modelling, applied to Liverpool Bay and the Mersey estuary. Other speakers will talk more specifically about waves in saltmarshes, bed shear stresses and modelling of sediments.Waves in estuaries and their impact on sediments This presentation is an overview of the sediment modelling and wave modelling, applied to Liverpool Bay and the Mersey estuary. Other speakers will talk more specifically about waves in saltmarshes, bed shear stresses and modelling of sediments.

2. Introduction Objectives POL�s contributions to EstProc 1-D model of tides and sediments 2-D cross-section model, incl. morphology 3-D whole-estuary model wave modelling extreme events Summary Our objective is to develop tools which include improved prediction of wave generation within estuaries, while accounting for wave and current interactions. Our challenge is to link together models of hydrodynamics, waves and sediment transport. This will enable us to study effects of waves on sediments in estuaries. Within EstProc, we have developed sediment models in one- and two-dimensions. More recently, we have concentrated on 3-D whole-estuary models, particularly of the Mersey. Waves are now being incorporated into the 3-D sediment transport model, and I will show results of how waves affect sediment concentrations in the Mersey.Our objective is to develop tools which include improved prediction of wave generation within estuaries, while accounting for wave and current interactions. Our challenge is to link together models of hydrodynamics, waves and sediment transport. This will enable us to study effects of waves on sediments in estuaries. Within EstProc, we have developed sediment models in one- and two-dimensions. More recently, we have concentrated on 3-D whole-estuary models, particularly of the Mersey. Waves are now being incorporated into the 3-D sediment transport model, and I will show results of how waves affect sediment concentrations in the Mersey.

3. Processes Each model builds upon the processes of the previous ones: 1-D Tides and sediments 2-D Tides, sediments, morphology 3-D whole-estuary model Tides, sediments, morphology and locally-generated wind-waves.Each model builds upon the processes of the previous ones: 1-D Tides and sediments 2-D Tides, sediments, morphology 3-D whole-estuary model Tides, sediments, morphology and locally-generated wind-waves.

4. �User-friendly� 1-D �z� model Suspended sediment concentrations (Lagrangian) Depth 20 m, current amplitude 0.5 m s-1, ws 10-4 m s-1 See also www.gotm.net, General Ocean Turbulence Model This simplified model was produced to enable project partners to explore, for example, the effect of tidal currents, diffusion and bed friction on suspended sediment concentrations. It is a random-walk particle tracking model that replicates solutions to the 1-D advection-diffusion equation, in which the change in sediment concentration is equal to the sum of advection, diffusion and sources (i.e., erosion minus settling). A more sophisticated model is available at the GOTM web site. ____________________ In detail: Erosion A simple �erosion potential� at each time step is assumed, which is proportional to the tidal current at the bed raised to some power. These are summed until a threshold value is reached, and then a �particle� is released at a height corresponding to a normal distribution with standard deviation equal to the diffusive path length. Diffusion The diffusive path length is twice the vertical diffusion coefficient multiplied by the model time step (see Fischer et al., 1979). Particles can be displaced up or down by this distance � if they reach the surface they are reflected back down; if they reach the bed they deflect upwards, but by a reduced distance. The diffusion coefficient is approximated by the product of bed friction, water depth and tidal current amplitude. Advection/settling Particles settle by a distance of the time step multiplied by the settling velocity multiplied, and is deposited when it reaches the bed.This simplified model was produced to enable project partners to explore, for example, the effect of tidal currents, diffusion and bed friction on suspended sediment concentrations. It is a random-walk particle tracking model that replicates solutions to the 1-D advection-diffusion equation, in which the change in sediment concentration is equal to the sum of advection, diffusion and sources (i.e., erosion minus settling). A more sophisticated model is available at the GOTM web site. ____________________ In detail: Erosion A simple �erosion potential� at each time step is assumed, which is proportional to the tidal current at the bed raised to some power. These are summed until a threshold value is reached, and then a �particle� is released at a height corresponding to a normal distribution with standard deviation equal to the diffusive path length. Diffusion The diffusive path length is twice the vertical diffusion coefficient multiplied by the model time step (see Fischer et al., 1979). Particles can be displaced up or down by this distance � if they reach the surface they are reflected back down; if they reach the bed they deflect upwards, but by a reduced distance. The diffusion coefficient is approximated by the product of bed friction, water depth and tidal current amplitude. Advection/settling Particles settle by a distance of the time step multiplied by the settling velocity multiplied, and is deposited when it reaches the bed.

5. 2-D �y-z� cross-sectional model Morphology evolution (Langrangian) Depth 10 m, tidal amplitude 3 m, ws 10-3 m s-1 Morphology evolution can be simulated with this 2-D cross-sectional �Lagrangian� model. This diagram shows the development and migration of channels, with positions after 2�, 5 and 10 years as a result of 3 m amplitude tides and sediment with settling velocity of 1�mm�s-1. The model can be used as a tool for testing different scenarios, for example, to examine the effects of changing the tidal amplitude, mean sea level, sediment type, waves and flood defences. It is interesting to note that sediment tends to accrete in the intertidal areas, with a channel forming adjacent to it. This feature is seen in both the Mersey and Dee estuaries. ____________________ Note: Another general model! Original slope is 1:1000 Slow adjustment Sensitive to a large range of parameters LLW to HHW breadth is 1 to 5, i.e., large intertidal areaMorphology evolution can be simulated with this 2-D cross-sectional �Lagrangian� model. This diagram shows the development and migration of channels, with positions after 2�, 5 and 10 years as a result of 3 m amplitude tides and sediment with settling velocity of 1�mm�s-1. The model can be used as a tool for testing different scenarios, for example, to examine the effects of changing the tidal amplitude, mean sea level, sediment type, waves and flood defences. It is interesting to note that sediment tends to accrete in the intertidal areas, with a channel forming adjacent to it. This feature is seen in both the Mersey and Dee estuaries. ____________________ Note: Another general model! Original slope is 1:1000 Slow adjustment Sensitive to a large range of parameters LLW to HHW breadth is 1 to 5, i.e., large intertidal area

6. Liverpool Bay model 3-D hydrodynamic model ~120-m resolution Estuary bathymetry from LiDAR and echo sounding Tidal elevations and currents good agreement with observations��� The 1-D and 2-D models can be applied to any situation. To look at a particular location [i.e., Liverpool Bay], this hydrodynamic and sediment model uses bathymetry data from echo soundings and LiDAR surveys for the estuaries, and has a resolution of approximately 120�m. It is driven by M2 tidal elevations at the open sea boundaries. Current profiles are calculated at 10 equally-spaced levels. Tidal elevations and currents show good agreement with observations. A module with processes similar to the 1-D vertical sediment model is incorporated, which allows the 3-D simulation of suspended sediments. ____________________ Note: Site-specific model � �bottom-up� [EstProc provides tools for researchers; not meant to be a full evaluation of model against data at this stage.] The 1-D and 2-D models can be applied to any situation. To look at a particular location [i.e., Liverpool Bay], this hydrodynamic and sediment model uses bathymetry data from echo soundings and LiDAR surveys for the estuaries, and has a resolution of approximately 120�m. It is driven by M2 tidal elevations at the open sea boundaries. Current profiles are calculated at 10 equally-spaced levels. Tidal elevations and currents show good agreement with observations. A module with processes similar to the 1-D vertical sediment model is incorporated, which allows the 3-D simulation of suspended sediments. ____________________ Note: Site-specific model � �bottom-up� [EstProc provides tools for researchers; not meant to be a full evaluation of model against data at this stage.]

7. Sediment in the Mersey Random-walk particle-tracking module sediment resuspension currents at bed, wind-waves vertical diffusion, advection vertical current profile settling ws and water depth The program currently tracks positions of up to 5 million particles, each representing 500�kg of sediment. Particles originate from the sea, and their supply is assumed to be unlimited. Sediment concentrations are closely linked to tidal currents, which control resuspension and advection. In the Mersey Narrows, suspended sediments concentrations remain high throughout the tidal cycle, ranging from about 200�(at low and high water) to 600�mg�l-1 (during flood and ebb). In the Inner Estuary, concentrations range from about 50�to 400�mg�l-1.The program currently tracks positions of up to 5 million particles, each representing 500�kg of sediment. Particles originate from the sea, and their supply is assumed to be unlimited. Sediment concentrations are closely linked to tidal currents, which control resuspension and advection. In the Mersey Narrows, suspended sediments concentrations remain high throughout the tidal cycle, ranging from about 200�(at low and high water) to 600�mg�l-1 (during flood and ebb). In the Inner Estuary, concentrations range from about 50�to 400�mg�l-1.

8. Sediment in the Mersey Random-walk particle-tracking module sediment resuspension currents at bed, wind-waves vertical diffusion, advection vertical current profile settling ws and water depth We can use this model tool to look at the pattern of sediment settling at the sea bed. Some persistent features are apparent, such as deposition of sediment over sand banks [and other intertidal areas].We can use this model tool to look at the pattern of sediment settling at the sea bed. Some persistent features are apparent, such as deposition of sediment over sand banks [and other intertidal areas].

9. Sediment in the Mersey Sediment transport is greatest in the deep channels at estuary entrances, peak value is about 10 tonnes s-1 With this model, we can answer questions such as �how much sediment is transported over the tidal cycle?�. This is a time series of the numbers of particles crossing a transect line in the Mersey Narrows. An advantage of a particle-tracking model over a traditional advection-diffusion model is that it is possible for sediment to move up and downstream simultaneously. Peak sediment transport is about 10 tonnes per second. [Dee, 2.5 t s-1, Ribble, 0.6 t s-1.]With this model, we can answer questions such as �how much sediment is transported over the tidal cycle?�. This is a time series of the numbers of particles crossing a transect line in the Mersey Narrows. An advantage of a particle-tracking model over a traditional advection-diffusion model is that it is possible for sediment to move up and downstream simultaneously. Peak sediment transport is about 10 tonnes per second. [Dee, 2.5 t s-1, Ribble, 0.6 t s-1.]

10. Sediment in the Dee Sediment model of the Dee Similar model runs have been completed for the Dee and Ribble estuaries. This picture of the Dee estuary shows a large number of sediment particles in the deep entrance channels. Younger sediments appear offshore and near the mouth of the estuary; older sediments are found further upstream. High particle densities (hence greater sediment concentrations) occur in the channels.Similar model runs have been completed for the Dee and Ribble estuaries. This picture of the Dee estuary shows a large number of sediment particles in the deep entrance channels. Younger sediments appear offshore and near the mouth of the estuary; older sediments are found further upstream. High particle densities (hence greater sediment concentrations) occur in the channels.

11. Wave models SWAN model and TOMAWAC model compared Choice of a wave model? HR Wallingford have compared third-generation wave models, SWAN with TOMAWAC. They�ve done this for waves propagating into the Thames Estuary from the open sea over the tidal cycle. Data for validation comes from CEFAS�s WaveNet: a buoy is sited in the centre of the modelled area. SWAN � �Simulating waves nearshore� was developed by TU Delft (see www.wldelft.nl/soft/swan). Delft Hydraulics have been using this to investigate the effects of bed friction and energy dissipation over mudflats and also interactions between flow and waves. TOMAWAC developed by EDF, is part of the TELEMAC finite element (unstructured grid) model suite.Choice of a wave model? HR Wallingford have compared third-generation wave models, SWAN with TOMAWAC. They�ve done this for waves propagating into the Thames Estuary from the open sea over the tidal cycle. Data for validation comes from CEFAS�s WaveNet: a buoy is sited in the centre of the modelled area. SWAN � �Simulating waves nearshore� was developed by TU Delft (see www.wldelft.nl/soft/swan). Delft Hydraulics have been using this to investigate the effects of bed friction and energy dissipation over mudflats and also interactions between flow and waves. TOMAWAC developed by EDF, is part of the TELEMAC finite element (unstructured grid) model suite.

12. Wave models SWAN model and parametric wave model compared Choice of wave model? At POL, the SWAN model has been compared with a parametric wave model (based on equations in Shore Protection Manual, 1984). This was done to provide �rules� for �cellular� models [in Cemcos]. Wave parameters from SWAN are regarded as being �correct�, and simulations were completed for wind speeds of 5 m s-1 (representing �background� conditions), 15 m s-1 (once in a year storm) and 25�m s-1 (once in a century storm). [Wind is from NW.]Choice of wave model? At POL, the SWAN model has been compared with a parametric wave model (based on equations in Shore Protection Manual, 1984). This was done to provide �rules� for �cellular� models [in Cemcos]. Wave parameters from SWAN are regarded as being �correct�, and simulations were completed for wind speeds of 5 m s-1 (representing �background� conditions), 15 m s-1 (once in a year storm) and 25�m s-1 (once in a century storm). [Wind is from NW.]

13. Wave models Parametric vs SWAN (small differences in estuaries) Wave height, peak period of fetch-limited locally-generated wind-waves ? stress at sea bed from waves Differences between wave heights produced by SWAN and the parametric model are generally small, except in the entrance channels where there is some transmission of the offshore waves. Fortunately, much of the wave energy inside estuaries is driven by local wind. We have chosen to use the parametric equations in our sediment model to help reduce the computation times for waves in estuaries.Differences between wave heights produced by SWAN and the parametric model are generally small, except in the entrance channels where there is some transmission of the offshore waves. Fortunately, much of the wave energy inside estuaries is driven by local wind. We have chosen to use the parametric equations in our sediment model to help reduce the computation times for waves in estuaries.

14. Application of wave model Wave height and peak period calculated from wind speed, water depth and fetch Dissipation in nearshore region wave�heights limited by shallow water and wave steepness Wave orbital velocity at sea bed, Ubed from�linear wave theory Wave stress on sea bed from�Ubed, bed�friction coefficient (sediment type), near-bed wave orbital diameter To calculate wave stresses at the sea bed, we need the peak wave period as well as the wave height. However, the processes of wave propagation and dissipation tends to reduce wave heights� this is shown to be more important in shallow water and for low wave heights. Wind-waves are generated locally, and so are fetch-limited. The fetch varies because of wetting and drying during the tidal cycle. It also varies with wind direction. Additionally, the wind interacts with the tidal currents to produce surges, but we don�t consider this here.To calculate wave stresses at the sea bed, we need the peak wave period as well as the wave height. However, the processes of wave propagation and dissipation tends to reduce wave heights� this is shown to be more important in shallow water and for low wave heights. Wind-waves are generated locally, and so are fetch-limited. The fetch varies because of wetting and drying during the tidal cycle. It also varies with wind direction. Additionally, the wind interacts with the tidal currents to produce surges, but we don�t consider this here.

15. Wave/current stress algorithms Sediment resuspension depends on stress at the sea bed stress from tidal currents stress from waves Combine wave and current stress? algorithms by Soulsby (1997), Soulsby�and�Clarke (2004) Sediment erosion at the sea bed is proportional to the stress, and we have stress from tidal currents [proportional to bed friction coefficient and current speed squared]. We also have stress due to waves (equations in Swart, 1974). But how do we combine the wave and current stress? We use an algorithm by Soulsby (1997) for rough turbulent conditions. A later method also considers cases of laminar flow and smooth or rough turbulent flow. [The additional stress due to waves is proportional to the ratio of stress due to waves alone to the sum of the wave and current stresses raised to the power 3.2 (Soulsby 1997).] Sediment erosion at the sea bed is proportional to the stress, and we have stress from tidal currents [proportional to bed friction coefficient and current speed squared]. We also have stress due to waves (equations in Swart, 1974). But how do we combine the wave and current stress? We use an algorithm by Soulsby (1997) for rough turbulent conditions. A later method also considers cases of laminar flow and smooth or rough turbulent flow. [The additional stress due to waves is proportional to the ratio of stress due to waves alone to the sum of the wave and current stresses raised to the power 3.2 (Soulsby 1997).]

16. Recap � effects of waves Waves increase bed stress Sediment more readily resuspended Sensitive to wind speed fetch, state of tide, wind direction Increase in sediment concentrations especially during extreme events Waves provide additional bed stress, resulting in more potential for sediment erosion. As well as wind speed and direction, the amount of erosion is sensitive to the state of tide where wetting and drying modifies the fetch and water depth. Extreme events are particularly important for raising concentrations and then transporting the sediment.Waves provide additional bed stress, resulting in more potential for sediment erosion. As well as wind speed and direction, the amount of erosion is sensitive to the state of tide where wetting and drying modifies the fetch and water depth. Extreme events are particularly important for raising concentrations and then transporting the sediment.

17. Extreme events Sediment concentrations, differences for N and E wind � no surges yet! Winds with speed 25�m�s-1 represent a once in a hundred year storm. Over two tidal cycles, it produces increased sediment concentrations in both the Mersey Narrows and the more exposed Inner Estuary. The effect is more prominent during low water in the sheltered Narrows. In the Inner Estuary, this occurs throughout the tidal cycle. But these results are not significantly different from the case of tides alone. [Mersey is flood-dominant]Winds with speed 25�m�s-1 represent a once in a hundred year storm. Over two tidal cycles, it produces increased sediment concentrations in both the Mersey Narrows and the more exposed Inner Estuary. The effect is more prominent during low water in the sheltered Narrows. In the Inner Estuary, this occurs throughout the tidal cycle. But these results are not significantly different from the case of tides alone. [Mersey is flood-dominant]

18. Summary Technical advances application of wave models wave-current bed stress algorithms Whole-estuary assessments Mersey,�Dee, Ribble Tools for investigating morphology evolution, Global Climate Change, coastal�management Summary EstProc has advanced the application of wave models and provided algorithms for bed shear stresses. We have used these to examine waves and sediments in the Mersey, Dee and Ribble. These tools will be useful to coastal managers in helping them to predict changes in morphology due to climate change [e.g., increase storminess, sea level rise].Summary EstProc has advanced the application of wave models and provided algorithms for bed shear stresses. We have used these to examine waves and sediments in the Mersey, Dee and Ribble. These tools will be useful to coastal managers in helping them to predict changes in morphology due to climate change [e.g., increase storminess, sea level rise].

19. Ways forward Assess models and results POL Science Programme Coastal Observatory cobs.pol.ac.uk Operational modelling of estuaries, sediments, ecology Forecasts of 2050 scenarios Development of Estuary Morphological Models (Defra project FD2107) www.pol.ac.uk/estmorph Where do we go from here? We intend to examine these models and results, within the framework of POL�s Coastal Observatory, and in the EstMorph Project. [More further work: Spring-neap cycle important to get water and sediment into upper estuaries and onto banks and saltmarshes. Different sediment types (sand, fine sand, silt, mud) including cohesive sediments, consolidation. Storm surges � winds modify tidal flow and waves � increases bed stress. Effect of limited sediment supply.]Where do we go from here? We intend to examine these models and results, within the framework of POL�s Coastal Observatory, and in the EstMorph Project. [More further work: Spring-neap cycle important to get water and sediment into upper estuaries and onto banks and saltmarshes. Different sediment types (sand, fine sand, silt, mud) including cohesive sediments, consolidation. Storm surges � winds modify tidal flow and waves � increases bed stress. Effect of limited sediment supply.]

20. This is an animation of suspended sediment in the Mersey Estuary over a tidal cycle (without waves). Each particle represents 500�kg of sediment with settling velocity ws 0.5 mm s-1 (fine silt, d50 22��m).This is an animation of suspended sediment in the Mersey Estuary over a tidal cycle (without waves). Each particle represents 500�kg of sediment with settling velocity ws 0.5 mm s-1 (fine silt, d50 22��m).

21. This is an animation of the settled (inactive) sediment in the Mersey Estuary over a tidal cycle (without waves). Each particle represents 500�kg of sediment with settling velocity ws 0.5 mm s-1 (fine silt, d50 22��m).This is an animation of the settled (inactive) sediment in the Mersey Estuary over a tidal cycle (without waves). Each particle represents 500�kg of sediment with settling velocity ws 0.5 mm s-1 (fine silt, d50 22��m).

22. PLEASE WILL ALL ESTPROC MEMBERS READ ALL OF THIS AS THIS IS VERY IMPORTANT, EVEN IF YOU ARE NOT SPEAKING AT THE MEETING See following templates for our presentations:: Slide 1 is the �title slide for your talk� and Slide 2 �for bullet point statements� Slide 3 is a blank version for inserting your own information Slide 4 is a �separator slide� if you want to use it Feel free to make the presentations personalised but keep the EstProc �theme� Instructions for presentations - print or keep this page and then of course you can delete it from your final presentation. Do not try to cram too much onto slides, use them as key points rather than read from them, include pictures and clear illustrations. If you have a 15 minute speaking slot then aim for 15 slides - better to explain them clearly to the audience rather than rush through. If you have a longer speaking slot then you can aim for more. Day 1 speakers generally have longer slots than day 2 speakers. Day 2 speakers are: 15 mins for presentation + 3 mins for questions + 2 mins for handover. Both Day 1 and Day 2 speakers do not need to introduce EstProc, I will have done that already. This means you can miss out slides on project background and funders acknowledgement. So you can have a title slide then bang straight through your objectives into the key points, results etc - the audience needs to be kept engaged and entertained. I think the introductory slide should retain project identity, subsequent slides can to but if you want to use a full page illustration maybe just leave �Estuary Processes Research Project� showing at top. The presentations need to cover your own work and the work of the project team who have worked in that area. The next draft of the final report will be circulated end of Sept 10th and you can see what is in there, look back to notes of other meetings we have had to help recall topics of interest, key results. Speakers need information to present - so I am relying on research teams to provide relevant slides and explanation to the presenter, it is not just down to the presenter to extract information from other team members. I am relying on the presenters and contributors to presentations to create their own communications - it is everyone�s interest to ensure that there work is represented and that we are seen to have worked as a team. Presentations completed before the meeting can be circulated to others for review/comment. I would like to receive copies of all presentations prior to the meeting. It is the speakers responsibility to bring their presentation on CD ROM to the meeting. Thanks and good luck. Any problems then let me know. Richard Whitehouse, 9 September 2004PLEASE WILL ALL ESTPROC MEMBERS READ ALL OF THIS AS THIS IS VERY IMPORTANT, EVEN IF YOU ARE NOT SPEAKING AT THE MEETING See following templates for our presentations:: Slide 1 is the �title slide for your talk� and Slide 2 �for bullet point statements� Slide 3 is a blank version for inserting your own information Slide 4 is a �separator slide� if you want to use it Feel free to make the presentations personalised but keep the EstProc �theme� Instructions for presentations - print or keep this page and then of course you can delete it from your final presentation. Do not try to cram too much onto slides, use them as key points rather than read from them, include pictures and clear illustrations. If you have a 15 minute speaking slot then aim for 15 slides - better to explain them clearly to the audience rather than rush through. If you have a longer speaking slot then you can aim for more. Day 1 speakers generally have longer slots than day 2 speakers. Day 2 speakers are: 15 mins for presentation + 3 mins for questions + 2 mins for handover. Both Day 1 and Day 2 speakers do not need to introduce EstProc, I will have done that already. This means you can miss out slides on project background and funders acknowledgement. So you can have a title slide then bang straight through your objectives into the key points, results etc - the audience needs to be kept engaged and entertained. I think the introductory slide should retain project identity, subsequent slides can to but if you want to use a full page illustration maybe just leave �Estuary Processes Research Project� showing at top. The presentations need to cover your own work and the work of the project team who have worked in that area. The next draft of the final report will be circulated end of Sept 10th and you can see what is in there, look back to notes of other meetings we have had to help recall topics of interest, key results. Speakers need information to present - so I am relying on research teams to provide relevant slides and explanation to the presenter, it is not just down to the presenter to extract information from other team members. I am relying on the presenters and contributors to presentations to create their own communications - it is everyone�s interest to ensure that there work is represented and that we are seen to have worked as a team. Presentations completed before the meeting can be circulated to others for review/comment. I would like to receive copies of all presentations prior to the meeting. It is the speakers responsibility to bring their presentation on CD ROM to the meeting. Thanks and good luck. Any problems then let me know. Richard Whitehouse, 9 September 2004