RICERCA ITALIANA     

Sand pits and beach nourishments: morphodynamic modelling and field applications

Università degli Studi di Genova

Abstract

The research project is aimed at improving the actual knowledge on the dynamics of large scale sand pits and large scale sand deposits or beach nourishment to formulate accurate theoretical/numerical models able to provide reliable predictions of the morphological time development of localised perturbations of the sea bottom.
To this aim we want
a) to formulate a three-dimensional model able to predict the time development of the sea bed configuration forced by tidal currents interacting with wind waves and to develop a solution procedure based on perturbation and numerical approaches.
b) to compare the results obtained by means of the three-dimensional model with those obtained using two-dimensional models based on the shallow water approximation (Boussinesq model and NSWE model).
c) 1) to collect field data, available in the literature, on the time development of coastal sand pits and beach nourishment to create a data bank to be used for the validation of the numerical codes. 2) to make accurate field measurements, because the data available in the literature do not give an exhaustive information on both the hydrodynamics of the coastal region and the morphodynamics of the sea bed.
d) to make laboratory experiments to collect further data in a controlled environment where it is possible to have a detailed knowledge of all the quantities which affect the phenomenon under investigation.

In particular, the research project is aimed at improving the actual knowledge of large scale coastal morphodynamics and to answer the following questions:
i) What is the optimal design of large scale sand mining operations ?
ii) What is the optimal design of a navigation channel ?
iii) What is optimal geometry of a beach nourishment ?
iv) What are the effects of large scale sand mining and beach nourishment on the physical environments ? <<<

Principal Investigator

Paolo BLONDEAUX Università degli Studi di GENOVA

Research Objectives

The main goal of the research project is to improve the actual knowledge of the dynamics of

1) large scale sand pits both on a flat sea bottom or on large scale bottom forms like the sand banks observed in the North Sea and the sand waves observed in the North Sea and in the Adriatic Sea;
2) large scale sand deposits or large scale beach nourishment

to formulate accurate models able to provide reliable predictions of the morphological time development of localised perturbations of the sea bottom.

Massive mining of sand from the middle and lower shore face in large-scale mining and borrow pits/areas will be required in a near future in many European countries. Mining of sand will be required to nourish beaches and coastal dunes in response to the increased coastal erosion due to the expected sea level rise (Eurosion; Eisenreich, 2005). Furthermore, the large scale reclamation of land and the construction of large scale islands (for industrial purposes, ports and airports) in coastal sea, which are presently being considered, will also require huge amounts of sands as building material. An analysis of the data describing the volumes of sand extracted from the Dutch continental shelf shows that up to the eighties the average volume of sand extracted every year was equal to about 2.5 million of cubic metres. Then, an almost exponential growth took place and in 2002, about 35 million of cubic metres were extracted. In Italy the first sand pits were identified in 1980 (Preti, 2002). More recently, for the protection of the lagoon of Venice, about 10 million of cubic metres have been extracted from
the ancient dunes off the coast of Venice (Cecconi & Ardone, 2000). Similarly 4 million of cubic metres have been used for the nourishment of beaches along the coast of Lazio and several million of cubic metres have been used in a single beach nourishment in Emilia-Romagna region. Moreover the Emilia-Romagna region foresees a need of about 2 million of cubic metres per year to protect its coast from coastal erosion (AA.VV., 1996) and similar volumes are required for other Italian regions. These figures are in common with many European countries. Hence, an estimate, made in the framework of the European research project SANDPIT, of the volumes required for the next 50 years leads to values of about 60 millions of cubic metres per year for every European country with costs equal to about 600 millions of Euro per year. To meet these demands, the existing areas for mining of sand need to be extended considerably and new potentially attractive areas should be explored and exploited. Massive mining of sand may take place by dredging artificial sand pits or channels or by dredging existing large scale sand banks/shoals in the offshore zone (middle and lower shore face).

Large scale mining pits have a significant impact on the near field and far field flows (up to the coast) and wave pattern. Indeed, the flow velocities inside the pit are reduced and the wave heights may also be reduced, depending on the depth of the pit. As a consequence, the sand transport capacity inside the pit decreases and sediment particles settle, resulting in deposition in the pit area. Thus, the pit acts as a sink for the sediment originating from the surrounding areas and erosion of the sea floor takes place in the immediate surrounding of the pit. Moreover recent studies performed in the framework of the European research project HUMOR have shown that sand extraction from shore face connected ridges (large scale morphological patterns present in the near-shore region) may lead to the erosion of the beach face and to a retreat of the coastline.

Considering the massive scale of future mining of sand and the large areas that will be affected by the mining activities, the mining areas need to be situated in the off-shore zone to minimise the effects of near-shore coastal erosion. On the other hand the mining of sand will be progressively more expensive at greater distances from the shore. Therefore, research is required to find the optimum solutions between the effects on the coast and the costs of mining. The technical evaluation of sand mining activities requires fundamental knowledge of morphological processes (bed load, suspended load, bed instability, flow fields on a wavy bed, …) and sand budgets in the offshore and near-shore coastal zones. Closure of sand budget estimates for the sand mining areas requires rather accurate information of sand transport rates at the boundaries. Often the accuracy of the computed sand transport rates is not sufficient, because of the application of relatively simple sand transport models. Furthermore, model verification and validation based on detailed field data is lacking in most cases. The main reason for this is that detailed field data sets of sand transport in offshore (deep water) and nearshore (shallow water) conditions are scarcely available.

Therefore, during the research project, we want

a) To formulate a three-dimensional model able to predict the time development of the sea bed configuration forced by tidal currents interacting with wind waves. We want to consider: 1) the oscillatory slowly varying flow induced by tidal currents, 2) the steady currents (residual or littoral currents) which often are present in the coastal area and play a significant role in sediment transport and the migration of the bottom forms (sand pits and beach nourishment), 3) the wind waves which induce mainly an oscillatory motions but also steady streaming because of nonlinear effects and hence contribute to the development and migration of sand pits and beach nourishment.

b) to compare the results obtained by means of the three-dimensional model with those obtained using two-dimensional models based on the shallow water approximation (Boussinesq model and NSWE model). To this aim, we want both to develop numerical codes to make a detailed comparison of all the quantities which control the phenomenon and to use commercial codes (MIKE21).

c) 1) to collect the field data, available in the literature, on the time development of coastal sand pits and beach nourishment to create a data bank to be used for the validation of the numerical codes. 2) to make accurate field measurements, because the data available in the literature are often only partial and do not give an exhaustive information on both the hydrodynamics of the coastal region and the morphodynamics of the sea bed.

d) To make laboratory experiments to collect further data in a controlled environment where it is possible to have a detailed knowledge of all the quantities which affect the phenomenon under investigation

The research project is aimed at improving the actual knowledge of large scale coastal morphodynamics and in particular to answer the following questions:

i) What is the optimal design of large scale sand mining operations ?
ii) What is the optimal design of a navigation channel ?
iii) What is optimal geometry of a beach nourishment ?
iv) What are the effects of large scale sand mining and beach nourishment on the physical environments ? <<<

Timescale

24 months

National and international background

The planned research project is an ideal continuation of the research activities carried out by Genova University in the framework of the European research projects HUMOR and SANDPIT, which ended in February 2004 and March 2005, respectively, by Catania University in the framework of the SANDPIT project and by Bologna University in the framework of the European projects DELOS and COASTVIEW, which ended in February 2004 and March 2005, respectively. These research projects were aimed at the study and the coastal processes in general while the SANDPIT project was aimed at the study of the dynamics of pits and trenches. Hence, the project will start by considering the results achieved in the framework of the above research projects and in particular in the framework of the SANDPIT project, both in the basic research (concerning sediment transport predictors) and the applied research on the time development of morphological patterns. A detailed description of the results obtained in the SANDPIT project is given in the proceedings of the final SANDPIT meeting (Van Rijn et al., 2005).

In particular, the starting point of the theoretical part of the present research project is the morphodynamic model developed by the research unit of Genova University in the framework of the research project SANDPIT (Blondeaux & Vittori, 2005 a,b). In the model, the sediment is supposed to move both as bed load and suspended load, since field surveys show that large amounts of sediment are put in suspension by the stirring action of sea waves and then transported by tidal currents. Also residual (steady) currents are considered because they play a fundamental role in the migration of bottom forms. The hydrodynamics considers a water body of small depth h* which extends indefinitely in the horizontal directions (herein `indefinitely' means distances much larger than the horizontal scale of the area we are interested in). By using the f-plane approximation the problem of flow determination is posed by continuity and momentum equations where Coriolis terms related to Earth rotation are taken into account because they affect tide propagation. The flow regime is assumed to be turbulent and viscous effects can be neglected. Moreover, Reynolds stresses are modelled by Boussinesq hypothesis and introducing the dynamic eddy viscosity m*T. The time development of the bottom is described by means of Exner equation derived by forcing the sediment mass balance: this equation simply relates erosion/deposition processes to spatial increases/decreases. To close the problem it is necessary to specify the eddy viscosity structure and to chose an empirical predictor for sediment transport. The actual version of the model considers in detail only turbulence induced by the tidal current and the sediment transport is that associated to the slowly changing velocity induced by tide propagation. During the present research project, the model will be improved by considering different approaches to describe Reynolds stresses, by introducing a better description of the mixing processes induced by sea waves and by using different methods to evaluate the sediment transport rates. Indeed, the results obtained so far suggest that the description of the mixing processes induced by turbulence should be improved to obtain prediction in good agreement with field surveys. Moreover, the estimates of the sediment transport induced by sea waves should be improved, too. Finally, also the description of the effects of sea waves on the hydrodynamics should be described with more accurate approaches. When these improvements will be achieved, it will be necessary to test the model by comparing its theoretical/numerical predictions with accurate laboratory or field data.

The research unit of Bologna University has the Coastal Area Morphological model MIKE 21 developed by DHI Water & Environment which integrates waves, flow and sand transport modules into a full morphological model for the time-evolution of bed level changes at a given coastal area. In particular the Near-shore Spectral Wave (NSW) or the Parabolic Mild Slope (PMS) modules can be adopted for simulating waves, usually offshore and near the coastal structures respectively. The hydrodynamic (HD) module is used for simulating currents and the ST-Q3 module simulates sediment fluxes and bottom variations. More in details, the NSW model is a wind-wave model, which describes the growth, decay and transformation of wind generated waves and swell in nearshore areas. The model is a stationary, directionally decoupled parametric model and takes into account the effects of refraction and shoaling, local wind generation, energy dissipation due to bottom friction and wave breaking, and wave-current interaction. The basic equations are derived from the conservation equation for the spectral wave action density and are solved using an Eulerian finite difference technique. The PMS module is based on the parabolic approximation of the mild-slope equation (Kirby, 1986), which assumes a predominant wave direction and neglects wave diffraction and back-scattering in the direction of wave propagation. It accounts for refraction and shoaling due to varying depth, diffraction along the perpendicular to the predominant wave direction, energy dissipation due to bottom friction and wave breaking, frequency and directional spreading effects. The HD module solves the full time-dependent non-linear equations of mass and momentum balance. The solution is obtained using an implicit ADI finite-difference second-order accurate scheme (a.o. Abbott et al., 1973). The ST-Q3 module implements a deterministic algorithm based on the model of Engelund & Fredsøe (1976) and evaluates separately bed load and suspended load sediment transports.

As far as the field data on the time development of coastal pits and trenches or beach nourishment is concerned, the expertise in obtaining field data gained by Catania and Bologna Universities during European research projects is a good starting point. The Bologna research unit has focused its activity on the surf zone, where the wave induced sediment transport is mainly confined. In particular, thanks to European and Italian research programs (www.delos.unibo.it, thecoastviewproject.org, www.idraulica.ing.unibo.it/PRIN), the group gained expertise on the mapping of the nearshore morphodynamics induced by beach defence structures. The effects of the works in Lido di Dante have been continuously monitored since 1996 and occasionally other sites in the region have been examined (Gabicce, Igea Marina, Cervia, Milano Marittima). During the present research project, field data will be collected at different sites, possibly offshore of Venice Lagoon. The region offshore of the Venice lagoon has been chosen because recently large amounts of sand have been extracted for beach nourishment and the reconstruction of coastal dunes along the islands which separate the lagoon from the open sea. Indeed, the lowering of the ground due to natural causes and to human activities (subsidence) and the sea level rise have caused a decrease of the ground level of the city of Venice with respect to the see of about 23 cm. Therefore the coast between Jesolo and Isola Verde (Jesolo, Cavallino, Lido, Pellestrina, Sottomarina e Isola Verde) has been eroded and has lost is capability to defend Venice Lagoon from sea storms. As already pointed out, these problems have been partially solved with beach nourishment and the reconstruction of the littoral dunes. Data on the time development of the pits and of the beach nourishment are available and they will be collected and a data set will be created. Finally, it may be worth pointing out that, in the framework of the interventions planned to withstand the important erosion processes which have taken place along the north-eastern Sicilian littoral in the last decades, numerous interventions of beach nourishment have been programmed. In particular, such interventions will be made by using sand and/or gravel materials coming from submarine pits located offshore of the site where the intervention is planned. In the frame of the present research project, it seems particularly interesting the possibility to use the available equipment for monitoring one or several sand-gravel submarine pits, in order to follow their evolution. Particular attention will be addressed to the analysis of the effects of the pit presence on the morphology of the nearby beach. Such a kind of pits for the extraction of a sand-gravel mixture are of particular scientific and engineering interest. Indeed the current submarine mining activity has been mainly focused, particularly in Italy, on the extraction of sand (see, as an example, the case of the Venice lagoon described in Cecconi and Ardone, 2000). Therefore only few data are available on the use of coarse material (gravel). A previous experience with gravel on the northern coast of Tuscany (Aminti et al., 2002) showed that a coarse material causes a modification of the beach profile, so that the beach can no longer be used for tourist and bathing purposes, due the remarkable berm height reached during storms. However it seems that the introduction of fine materials could generate a more gentle beach slope, more usable for recreational purposes (Aminti et al., 2002).

Finally, it is important to note that in the framework of the Sandpit research project, the research unit of Catania University set up a tank for the investigation of the flow structure due to the simultaneous presence of a wave field and an orthogonal steady current. The experimental tank can be run also in presence of an erodible bottom. Therefore, these laboratory facilities will be available during the present research project and will be used to investigate the hydrodynamics and morphodynamics due to a combined flow (waves plus an orthogonal steady current) in presence of several geometric configurations of an idealised submarine pit (dimension, shape, orientation). <<<