RICERCA ITALIANA     

Research program

Sand pits and beach nourishments: morphodynamic modelling and field applications

University Co-ordinator

Università degli Studi di BOLOGNA - INGEGNERIA DELLE STRUTTURE,TRASPORTI ,ACQUE,RILEVAMENTO DEL TERRITORIO - BOLOGNA(BO)

Research Unit Leader

Alberto LAMBERTI

Description

OBJECTIVES

The Bologna research unit (RU) focuses its activity on the surf zone, i.e. where most of the wave induced sediment transport takes place. Thanks to European (www.delos.unibo.it, thecoastviewproject.org) and Italian (www.idraulica.ing.unibo.it/PRIN) projects, the group gained experience on the near shore morphodynamic 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). The structures, analysed in the past as fixed, induce hydrodynamic effects, but since they were assumed invariable and the surrounding bottom has a secondary effect on the hydrodynamics, the interaction circle was not completely represented (as closed), precluding the possibility to represent relevant phenomena as -for instance- the structure instability due to erosion at its toe (peculiar problem of Igea Marina) or reaching the bed regime configuration at the end of the erosive process (a general problem).

The RU thus within this project wish to provide an applicative contribution to the analysis of the interaction circle among waves, currents and bottom, highlighting how bottom variation affects transport conditions and further bed evolution.

The research aims at defining and evaluating tools for predicting the evolution of a bed perturbation in/close to the surf zone. Bed perturbations are assumed to be caused by dredging (an entrance channel to a harbour or a pipeline trench) or nourishment of the submerged beach.
Due to environmental conditions of the Northern Adriatic sea, the research is addressed to study the transport of medium and fine sand.

A further purpose of the research is to provide suggestions for design:
· to define, in connection with sediment transport conditions, the best excavation width/length/depth ratios that, for a given dredged volume, would guarantee as long as possible the maintenance of the section necessary for harbour functionality;
· to evaluate the best dumping depth and the deposit shape that would let the shoreline benefit from the nourishment without accumulating on the emerged beach fine and anoxic sand, often associated to dredged material, and at the same time minimising costs.

METHODS

1. Surveys in prototype and in laboratory

The processes will be studied by means of a well documented and geometrically simple reference case in the Netherlands, of laboratory experiments carried out by Catania RU and of existing dredging and nourishment observations in Emilia Romagna region (most of which have been already performed and some are planned).
The literature case is a pipeline trench excavation carried out in March 1964 at Scheveningen (NL); the trench is straight, perpendicular to the shoreline and evolves similarly to a dredged harbour entrance channel. The case is well documented and was analysed using different models (Walstra et al., 2004; Blondeaux et al., 2004); it represents a benchmark case for sediment transport modelling.

The experimental tests are described by the Catania RU.

The prototype case selected for our analyses is located in Cervia municipality. Repeated dredging works were carried out under the urge of restoring the harbour functionality; they are generally well documented. A major intervention of approximately 0.1÷0.3 millions cum, whose details are not completely defined, is planned and associated to the nourishment of Milano Marittima beach, a resort placed some km North of the harbour suffering from erosion.

Since the entrance channel is frequently closed by sand and requires thus maintenance works, several bathymetries per year are available for the recent past and it is presumable that such information will be also available in the future. Even if of limited extension and performed without control of the RU, they provide a useful basis for RU analysis. Conversely, maintenance nourishment areas were not and presumably will not be surveyed. Additional bathymetries will then be carried out with the multibeam system, that provides a detailed description of the bed even in presence of sharp variations.

Two Acoustic Doppler Current Profilers (ADCPs), manufactured by RDI, are available for hydrodynamic monitoring. The instrument is provided with a pressure sensor and 4 divergent beams along which Doppler frequency and power is measured. They are designed to provide mean pressure (tidal level), directional wave spectra and vertical profiles of 3D current components and Doppler power (an indicator for sediment concentration). After a calibration suspended sediment concentration can be derived. Two sediment traps, characterised by a set of openings oriented along different directions are available for directional bed load transport measurements.

Further instrumentation will be provided for laboratory experiments.

2. Numerical models

There are several commercial codes available for morphological simulations: MIKE 21, TELEMAC, DELFT3D; among these, the first is available to the RU.

A Coastal Area Morphological model, as MIKE 21 CAMS (Coastal Area Morphological Modelling Shell) developed by DHI Water & Environment, integrates waves, flow and sand transport models into a full morphological model for the time-evolution of bed level changes at a given coastal area.

In particular, within CAMS, 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; 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 near shore 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, wave-current interaction. The basic equations in the model 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 to the mild-slope equation of 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, see e.g. Abbott et al. (1973) for details.
The ST-Q3 module that calculates the rates of non-cohesive sediment sand transport for combined waves and current situations. ST-Q3 implements a deterministic algorithm based on the model of Engelund & Fredsøe (1976) and evaluates separately bed load transport and suspension.

The numerical morphological models solve for wave propagation first, then evaluate the induced currents and the sediment transport field; on the basis of the transport divergence, the erosion or deposition rate is computed, with consequent updating of the bed.

Thanks to the advancement of the computing capacity, it is now possible to use such models to simulate on a PC with a resolution proportionate to structure size the hydrodynamic processes induced by a single wave attack over the whole physiographic area interested by the works in Cervia (7 km long). But the morphological modelling covering a long time period can not be carried out in a reasonable time. Therefore it will be run for the two restricted intervention areas with the boundary conditions that the hydrodynamic modules return for the regime bathymetry over the physiografic area.
The disadvantage of the cited numerical models is their rigid structure, so that nothing can be modified if not originally accounted for.

The mentioned necessary modules will then be implemented, within an open environment for solving PDE's, based on the finite element method: FEMLAB (Kremer, 2005; van Schijndel, 2003; Cohen & al. 2005). The implemented equations will describe the evolution of the excavation or nourishment, as for instance in the example shown by Hervouet (2000). First the nonlinear shallow water equations (Pironneau, 1989), the sediment transport equations (Bijker, 1971; Engelund & Fredsoe, 1976) will be implemented within a wave resolving approach; these equations neglect dispersive effets whereas fully represent the process nonlinearity. A restricted area close to the interventions will be modelled in order to limit the computational effort; the grid spacing shall be a small fraction of wave length and similarly the time step a small fraction of wave period; such a detail is comparable to the one necessary for describing the interventions and in particular the structures. Possibly effects due to vertical accelerations (Boussinesq equations: Peregrine, 1967; Madsen & al., 1999) or the separate dynamics of waves and currents (wave averaged model) will be implemented later.

The Scheveningen case and Catania laboratory experiments will be used for validation of the new finite element model, then it will be possibly calibrated by comparison with monitoring data and with results of the commercial code.

ACTIVITIES

The expected activities are listed below.
1. To develop the finite element model: modules for waves, currents and set-up, sediment transport and bed evolution.
2. To analyse the literature case, Scheveningen (NL), with MIKE21 and with the developed model.
3. To collect the relevant available information for the littoral between Milano Marittima and Cervia harbour, specifically: aerial views, bathymetries, sediment data, wave conditions, recent works (dredging and nourishments).
4. To collect and classify the bathymetries performed at Cervia harbour entrance in relation to the past dredging works.
5. To perform field campaigns in two places between Milano Marittima and Cervia of duration one month each. The 2 ADCPs and 2 sediment traps will be used to measure tide, waves, currents and sediment transport in order to provide the proper information for the calibration of numerical models.
6. To cooperate with Catania RU to perform laboratory experiments.
7. To integrate the monitoring activity of the local authorities with two bathymetric surveys and 1 field campaign for the analysis of the grain size distribution.
8. To calibrate the commercial code and the implemented model, on the basis of the data collected during the monitoring activity.
9. To simulate the annual evolution of the dredging performed at Cervia harbour, on the basis of a simplified wave climate, i.e. represented by a limited number of wave conditions. The result will be compared to the available bathymetries.
10. To derive, based on model results, suggestions for:
· defining the design criteria for the best dumping depth for the submerged nourishment;
·identifying the relation between the excavation geometry and the maintenance dredging frequency, with application to Cervia.
11. To participate in the project meetings.

TIME SCHEDULE

The activities 1, 2, 3 and 4 will be completed within the project first year.
The activities 5, 6, 7 and 11 will be carried out through the first and second year of the project.
The activities 8, 9 and 10 will be performed during the project second year.

RESULTS

The expected scientific results are:
- a 1 month monitoring of the two selected areas;
- two detailed bathymetries and one sedimentological survey;
- modules to be used within the general purpose finite element solver for the hydromorphodynamics simulation: hydrodynamic equations solver for waves and currents, sediment transport evaluator, bed evolution module.
- validation and calibration of the two numerical models involved in the analysis;
- suggestions for design and execution of submerged dredging and nourishments.
The results will be documented by means of:
- 2 internal reports, one on field monitoring and one on numerical modelling;
- 2 conference publications;
- 2 papers on international journals.

BENEFITS

The research outcomes may be used to improve the efficiency of dredging operations and the functionality of Cervia harbour and may be a sound basis for the management of other similar harbours in the area.