Engineered Morphology: The role of hydrodynamic models in coastal ecological restoration
Xinpeng Yu (MLA I)
Venice lagoon, located at the end of the Adriatic Sea, has a tidal range between -0.5 meter to 0.8 meter from sea level. It has gone through centuries of morphological processes and is currently under threat by complex environmental changes. The lack of sedimentation and land subsidence, along with issues of sea level rise have deepened the tidal channels and will gradually change the salt marsh ecosystem into a bay if without human intervention. Various morphological restoration measures have been undertaken to oppose erosion. Meanwhile, hydraulic engineers have established several numerical modeling systems to study the tidal movements inside the lagoon. The aim of the project is to study and explore the role of hydrodynamic models in restoring and designing the coastal landscapes in Venice Lagoon. The research incorporates environmental science with the design of landscape systems through studies and implementations of morphological techniques. The ambition is to foster a new thinking of the dynamics of landscape in practice.
“[Venice] a ghost upon the sands of the sea, so weak -so quiet, -so bereft of all but her loveliness, that we might well doubt, as we watched her faint reflection in the mirage of the lagoon, which was the City, and which the Shadow. I would endeavor to trace the lines of this image before it be for ever lost, and to record, as far as I may, the warning which seems to me to be uttered by every one of the fast gaining waves, that beat, Eke passing bells, against the Stones of Venice.”
– John Ruskin, Stones of Venice, 1819-1900
Venice Lagoon, the largest lagoon in the Mediterranean, is now challenged by the ocean. From fifteen centuries ago, when the city of Venice was established, people has been altering the lagoon to suit their needs. Now the scale of human intervention threatens to destroy it. Giampaolo Di Silvio from the Department of Marine and Hydraulic Engineering, University of Padua, concludes that the Venice lagoon has experienced a progressive deepening of shallow areas due to the substantial sediment deficit of the basin compared to a century ago. It is in the process of transforming from a predominantly terrestrial landscape towards a predominantly marine landscape. The two major reasons of sediment deficit are the elimination of river sediment and the construction of the three jetties for navigation purposes. The area of salt marshes in the lagoon has fallen from about 115km2 in 1810 to 3.5km2 at present. After the great flood of 1966 when the water reached more than 2 meters above sea level for 24 hours, the Venice Water Authority and Consorzio Venezia Nuova announced a collaborative action plan to restore the lagoon environment. The two separate procedures are the plan for the morphological restoration (MAV-CVN, 1992c), and the plan to improve the quality of the sediment and water (MAV-CVN, 1993).
Within the past decade, many innovative morphological restoration techniques have been undertaken in Venice lagoon. One of them is the construction of salt marshes along the navigation channels to protect tidal flats from waves and to reduce maintenance dredging. The stabled structure used to construct the salt marshes helps to prevent siltation caused by the cross-flow transport of sediment eroded by wind waves and motor boat traffic. The other edge of the navigation channel is soft and not engineered, which could gradually develop into tidal flats and beaches. However, the design of the salt marsh restoration is mostly homogeneous along the navigation channels. Based on fluid dynamic principals, different bottom shear stress and wave velocity will cause a different suspension rate of the sediment, which will, in turn, lead to various erosion rates. The changing curvature and depth of the channel play an essential role in influencing the erosion rate. So could we use the SHYFEM (Shallow Water Hydrodynamic Finite Element Model ) to predict the erosion rate in the first place? The angle and width of the designed salt marshes could then respond to a mapping of the channel conditions. Before construction, several iterations of morphological simulations could also be calculated to evaluate different designs and to predict the possible growth direction of tidal flats.
With this hypothesis in mind, I firstly starts to build a three-dimensional bathymetry model of Venice Lagoon to prepare for the hydrodynamic simulation. The bathymetry of the lagoon, together with wind and tidal currents, are the major factors that determine the hydrodynamics of this ecosystem. As mentioned before, the average depth of the lagoon is only one meter, with some navigation channels as deep as 20 meters. It is characterized by the numerous tidal channels and navigation channels cutting through the shallow lagoon areas. In terms of stratification in section, the lagoon water could be viewed as a two-dimensional system. The tidal currents and the eddying flows caused by boat traffic are such strong forces that the lagoon water is well mixed together. Its biochemical and hydrologic characteristics are the same from the water surface to the bottom of the lagoon, making it unnecessary to simulate a three-dimensional situation. However, the edge conditions throughout the lagoon depend on the slope and sediment type of the underwater topography. From sandbanks to mudflats, to salt marshes, the tidal water sculpts the terrain, shaping the lagoon’s contour and creating a unique ecosystem. In a world, the lagoon ecosystem is a place of transition between terrestrial and aqueous environments. Its existence depends on a balanced natural processes.
From the manuscript of Prof. Pierre Lermusiaux from the MSEAS (Multidisciplinary simulation, estimation, and assimilation systems) at the MIT, it is clear that the biochemical aspects of the hydrodynamics of the lagoon is closed related to climatic change and the input from inland. The seasonal wind and runoff pollutants from the inland contribute to the variations of many environmental parameters of the lagoon, influencing the biodiversity of this ecosystem. However, from the point of view of morphological researches, the tidal currents are the biggest influential factor. During my visit of the SHYFEM development team at ISMAR-CNR( Institute of Marine Sciences – National Research Council) in Venice, Italy, I’ve consulted Christian Ferrarin, the project controller, about the implementation of the simulation model. Firstly, he confirms that the SHYFEM is a 2D simulation model. It aims at researching the hydrodynamics of shallow water systems, whose vertical stratification could be treated as homogenous. The establishment of the model requires a large effort at creating the control points of the bathymetry mesh. Since the situation of the lagoon varies a lot at different locations, it is almost impossible to automate the process. The variation of the mesh, which serves as a base for the simulation, is manually adjusted based on scientific considerations. It requires a high level of professional knowledge to understand and use the model properly. As to my original question of whether we could use the model to simulate a future scenario before construction, Ferrarin says that it is possible but very difficult to be precise. The only project that includes this approach is the MOSE (Experimental Electromechanical Module). MOSE is a series of mobile gates that locates at the three inlets. The major aim is to protect the whole area from storm surges and sea level rise. It will rise from the bottom of the sea at high-water events, separating the lagoon from the sea. The MOSE project has been under debate from the time it is proposed. By altering the mesh and the creating a bathymetry model to simulate the morphological influences of the MOSE, it gives some insights into the further application of the hydrodynamic models. But it is also pointed that the precision of the model is hard to determine. The simulation usually requires two inputs: the physical situation and the influential parameters, including climatic and tidal data. However, this two inputs are also mutually affected. The change of the terrain will inevitably change the tidal data in real situation. If the simulation system is still fed by the original data, the result will be questionable. It may only be used to tell the general trend of possible morphological changes.
After the preliminary researches of the Venice Lagoon, I took the trip to Venice in the spring of 2012. The moment when I was on the vaporetto, sliding into the very inside of the northern lagoon, my mind was emptied. All I could feel were the reflection of salt marshes in the tidal water, the smell of Mediterranean wind, and the sound of waves washing the sand banks. There’s no way to really experience and understand this place only from maps, aerials, and simulation data. For an instance, I seem to forget the science and simply let myself be amazed by this fascinating place, from water.
A.Scotti, Engineering Interventions in Venice and in the Venice Lagoon, ed. C.A. Fletcher and T. Spencer. London: Cambridge University Press, 2005, p253.
Bird, Eric. Coastal Geomorphology: An Introduction, Second Edition. England: John Wiley & Sons Ltd, 2007.
- Cecconi, Morphological Restoration Techniques, ed. C.A. Fletcher and T. Spencer. London: Cambridge University Press, 2005, p461.
- Umgiesser, G.L. AMOS, E. Coraci, A. Cucco, C. Ferrarin, D. Melaku Canu, I. Scroccaro, C. Solidoro and L. Zampato. An Open Source Model for the Venice Lagoon and Other Shallow Water Boides. ed. C.A. Fletcher and T. Spencer. Cambridge, UK: Cambridge University Press, 2005, p406 (cover image: Fig. 47.2).
- Carniello, A. Defina, S. Fagherazzi and L. D’Alpaos. A combined wind wave–tidal model for the Venice lagoon, Italy. From Journal of Geophysical Research, vol. 110, F04007, doi: 10.1029/2004JF000232, 2005.
Flooding and Environmental Challenges for Venice and its Lagoon: State of Knowledge. Editors: C.A. Fletcher and T. Spencer. Cambridge, UK: Cambridge University Press, 2005.
Shallow Water Hydrodynamic Finite Element Model: https://sites.google.com/site/shyfem/application-1/lagoons/venice
The Surface-water Modeling Solution: http://www.aquaveo.com/sms
Special Thanks to:
Christian Ferrain, Researcher, ISMAR (Institute of Marine Sciences, Italy). http://www.ismar.cnr.it/
David Mah, Lecturer in Landscape Architecture, Harvard Graduate School of Design
Panagiotis Michalatos, Lecturer in Architecture, Harvard Graduate School of Design
Pierre Lermusiaux, Doherty Associate Professor in Ocean Utilization, Massachusetts Institute of Technology
Silvia Benedito, Lecturer and Assistant Professor of Landscape Architecture, Harvard Graduate School of Design