Estuarine Processes and Coastal Flooding

Within an estuarine system, tidal and river flows interact to produce complex and nonlinear hydrodynamic and sedimentary processes. In this way, the present estuarine shape and structure is defined. The response of an estuary to extreme conditions, such as heavy rainfall or large storms, is becoming ever more important for us to understand, as coastal regions become heavily managed and protected. Further, estuarine environments are vulnerable to climate change due to complex catchment-river-ocean interactions. Looking forward, sea levels around the Welsh coastline are set to rise by up to 1 m over the next century, and the frequency of storms and storm waves, high rainfall, and also drought events is projected to increase. Of particular concern, therefore, is increased flood risk of low-lying regions surrounding many of Wales’ estuaries. Additionally, the following processes are likely to happen in some Welsh estuaries in the future: increased fluxes of nutrients from agriculture, mobilisation of previously deposited flood plain sediments including their contained pollutants, run-off from agricultural land and mobilisation of pathogens, submergence of wetlands, disappearance of salt marshes, changes in suspended sediment and macro-nutrient concentrations, changes in estuarine morphodynamics, and consequent impacts on estuarine and coastal ecosystems.

 

Historically, land reclamation has occurred around the coast of Wales. For example, the natural surface area and tidal prism of the Dyfi Estuary (Fig. 1) was substantially reduced during the 19^th century due to the construction of a rail network and associated embankments, acting as de-facto coastal defence structures. Coastal managers are now looking for the best course of action as the embankments are increasingly being overtopped by tidal and rainfall events. ‘Hold the line’, or coastal realignment strategies, will protect properties, agricultural land, and ecologically important areas, but at high economic costs and with unknown consequences further afield (for instance, upstream in the tidally-influenced river). A ‘do nothing’ approach will allow nature to take its own course, but also have unknown far-field effects.

 

All these issues are being investigated by CAMS, using high-performance computational models, in conjunction with detailed observational measurements. Local scale, high-resolution, morphodynamic models have been developed by CAMS for several of Wales’ estuarine environments. For application of the models, CAMS has worked closely with the Countryside Council for Wales and the Environment Agency Wales (now merged as National Resources Wales), and additional research initiatives, such as the Centre for Catchment and Coastal Research (CCCR), the Climate Change Consortium for Wales (C3W), SEACAMS, HPC-Wales/Fujitsu, and the NERC Macronutrients Programme (in collaboration with the Centre for Ecology and Hydrology (CEH)). Many of these initiatives are joint ventures with other Welsh Universities. Through research studies at CAMS, a greater understanding of complex coastal and estuarine sedimentary processes has been achieved in recent years, which has helped policy makers to establish efficient and sustainable management for the future (see ‘References’ below).

 

Around Wales, CAMS has developed estuarine models of the Dee, Conwy, Menai Strait, Mawddach, Dyfi, Milford Haven, Burry Inlet (e.g. Fig. 2), and Bristol Channel. Such models typically extend ‘up-river’ into the catchment and also offshore to resolve tidal propagation on the Irish shelf. Our estuarine models are either nested within shelf-scale models (using structured grids) or extended onto the shelf using unstructured grids (i.e. high grid resolution within the estuary and low resolution offshore). CAMS is also developing ‘Catchment-to-coast’ models which link atmospheric, catchment, sub-surface, and hydrological processes to the marine system, thereby enabling the entire hydrological and carbon cycles to be investigated. Our case studies for this approach are the Conwy Estuary and the Dyfi Estuary.

 

 

Figure 1. The Dyfi Estuary, mid-Wales. TELEMAC-2D model simulation, at high resolution (i.e. 10 m, requiring 150,000 model nodes), showing peak spring tide velocities flowing into the estuary. The model was also used to look at sediment transport and sand evolution within the sandy estuary region. The natural estuary shape would have covered most of the model domain, but has been reduced by the construction of rail embankments running along the present southern side of the estuary.

 

 

Figure 2. The Burry Inlet, south Wales. TELEMAC-2D model simulation, at high resolution (i.e. 50 m, requiring 130,000 model nodes), showing residual velocities around Whiteford Point on the southern side of the estuary mouth. Three scenarios have been modelled: (1) present-day mean, (2) present-day ‘1 in 100 years’ extreme storm surge and high rainfall, and (3) future conditions with 100 years sea-level rise. A breach scenario has been simulated through Whiteford Sands, running east-west across Whiteford Sands, south of Whiteford Point.

 

References

 

Dausse, A., Garbutt, A., Norman, L., Papadimitriou, S., Jones, L.M., Robins, P.E., and Thomas, D.N. 2012. Biogeochemical functioning of grazed estuarine tidal marsh along a salinity gradient. Estuarine and Coastal Shelf Science, 100, 83-92.

 

Baas, J.H, Davies, A.G. and Malarkey, J. 2013. Bedform development in mixed sand-mud: The contrasting role of cohesive forces in flow and bed. Geomorphology, 182, 19-32.

 

Brown, J.M. and Davies, A.G. 2007. Flood/ebb tidal dominance in an estuary: sediment transport and morphology.  In Proceedings of 5th IAHR International Symposium on ‘River, Coastal and Estuarine Morphodynamics’, University of Twente, Enschede, The Netherlands, 17-21 September 2007, 779-787.

 

Brown, J.M. and Davies, A.G. 2010. Flood/ebb tidal asymmetry in a shallow sandy estuary and the impact on net sand transport. Geomorphology, 114, 431-439.

 

Brown, J.M. and A.G. Davies, 2009. Methods for medium-term prediction of the net sediment transport by waves and currents in complex coastal regions.  Continental Shelf Research, 29, 11-12, 1502-1514.

 

Davies, A.G. and Brown, J.M. 2007. Field measurement and modelling of scour pit dynamics in a sandy estuary. Coastal Sediments’07: proceedings of the 6th International Symposium on Coastal Engineering and Science of Coastal Sediment Processes, May 2007, New Orleans, American Society of Civil Engineers, 1609-1622.

  

Davies, A.G. and Thorne, P.D. 2008. Advances in the Study of Moving Sediments and Evolving Seabeds. Surveys in Geophysics, 29, 1-36. DOI 10.1007/s10712-008-9039-x.

 

George, R.L.S.F., Robins, P.E., Davies, A.G., and Roberts, J.C. 2013.
Visual Analytics of the Hydrodynamic Flux for Coastal Flooding Prediction and Management. Workshop on Visualisation in Environmental Sciences (EnvirVis) (2013), pp. 1-5. O. Kolditz, K. Rink and G. Scheuermann (Eds).

 

Gomiz Pascual, J.J.  Simulating the impacts of climate change on estuarine dynamics using an integrated catchment-to-coast model, PhD project in the Dyfi Estuary and Catchment;  Supervisors: R. Hashemi, P. Robins, S. Neill and C. Jago. Funded by HPC-Wales (2013-16).

 

Hasanvand, K., Hashemi, M.R., and Abedini. M.J. 2013. Development of an Accurate Time Integration Technique for the Assessment of Q-Based Versus H-Based Formulations of the Diffusion Wave Equation for Flow Routing. Journal of Hydraulic Engineering 10.1061/(ASCE)HY.1943-7900.0000773.

 

Hashemi M.R. and Badiee, M. 2012. Effect of urban development on river discharge using deterministic and stochastic models. In: From Catchment to Coast Conference, Bangor, UK, 2012.

 

McCann, D.L., Davies A.G., and Bennell J.D. 2011. Bed roughness feedback in TELEMAC-2D and SISYPHE. Proceedings of XVIIIth Telemac & Mascaret User Club, 19-21 October 2011, Chatou, France, 99-104.

 

Neill, S.P., Litt, E.J., Couch, S.J., and Davies, A.G.. 2009. The impact of tidal stream turbines on large-scale sediment dynamics. Renewable Energy, 34, 2803-2812.

 

Robins, P.E. and Davies, A.G.. 2009.  Morphodynamic processes in shallow estuaries: influence of tidal flats and channels on sand transport. In Proceedings of the 6th IAHR Symposium on River, Coastal and Estuarine Morphodynamics (RCEM 2009), Santa Fe, Argentina, 21-25 September 2009, CRC Press/Balkema, Volume 1, 161-167.

 

Robins, P.E. 2009. Development of a morphodynamic model of the Dyfi Estuary to inform future management decisions. CCW Contract Science Report 898a, pp. 86.

 

Robins, P.E. 2009. Development of a morphodynamic model of the Burry Inlet to inform future management decisions. CCW Contract Science Report 898b, pp. 89.

Robins, P.E. and Davies, A.G., 2010.  Morphological controls in sandy estuaries: the influence of tidal flats and bathymetry on sediment transport. Ocean Dynamics, 3, 503-517.

 

Robins, P.E., Davies, A.G., and Jones R. 2011. Application of a coastal model to investigate the effects of sea level rise and area management. Journal of Coastal Conservation 15:1–14. DOI 10.1007/s11852-010-0113