Modeling groundwater response to beach dewatering at Ravenna shoreline
F. Piccini, M. Gonella
Med Ingegneria S.r.l., Ferrara, Italy
P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
G. Gabbianelli
Environmental Science Faculty, University of Bologna, Ravenna, Italy
ABSTRACT
The effects of water-table position on accretion/erosion of a beach have been well documented in the
literature. A lower groundwater-table facilitates deposition of the sand transported by waves since it
reduces flow velocities during backwash and prolongs laminar flow. On the contrary, a high water-table
results in a condition favoring beach erosion. Beach dewatering (BD), i.e. the artificial lowering of the
water-table within beaches by a systems of drains, has been studied and applied over the last few decades
as a possible methodology of shore stabilization. However, the installations carried out worldwide do not
provide a convincing demonstration of the system capability. The initial position of the drain respect to
the beach profile and the flow rates through the drain system are the key design parameters. A BD system
has been installed on the Ravenna coastline in 2004. A 100 m long drain has been established on the
beach just north of the Fiumi Uniti river mouth, a portion of the Adriatic Sea coastline characterized over
the last decades by significant erosion due to land subsidence, of both natural and anthropogenic origin,
and a decrease of sediment supply from the river. A finite element variably-saturated groundwater flow
model is implemented to the actual vertical litho-stratigraphy of the beach and BD configuration. The
drain effect on the beach groundwater table is simulated using the results by a Boussinesq wave
propagation model as external boundary condition. The results show that the presence of a shallow
relatively low-permeable layer, originated from the sedimentation in a marsh environment occurred till 25
years ago, significantly decreases the water volume drained by the system, thus being responsible for a
relatively-low efficiency of the BD strategy. The study allows to conclude that an advanced modeling
approach is very useful to support the BD system design that, by contrast, has been usually based mainly
on empirical criteria up to date.
