The dynamics of debris flows and hyperconcentrated currents, and mathematical models for the mapping of debris flow risk and for the evaluation of the effectiveness of defence structures
Aims and goals
- To define and characterize the rheology of the different types of motion of hyperconcentrated currents, debris flows, and mudflows.
- To identify the global relationships that exist between geometrical quantities and mean physical quantities; the use of an experimental system conceived by the proponents (the EU Debris Flow and Tharmit Projects).
- The fine adjustment of mathematical models integrated along the flow depth, which are more realistic than current models (Takahshi, 1980; Julien e O‘Brian, 1988; Armanini, 1999; Fraccarollo e Armanini, 1999), and are therfore capable of simulating the phase separation problem in a more correct manner.
The mountain areas of Italy cover almost two thirds of the national territory. Liquid and solid flows usually form in the mountain parts of catchment basins, where the transport processes are so tightly interdependent as to make a joint liquid-solid phase approach necessary.
Until not many years ago, mountain areas were considered marginal, particularly because of the scarce relevance that these areas had in the economy of the country. Indeed, they were areas where the only economic activities were silvopastoral and where, for a long time, emigration represented the most attractive prospect for the younger generations, they were areas, therefore, subject to ever-increasing depopulation. Since the 1960s, demographic growth and the increase in wellbeing have progressively assailed these regions with enterprises, that are growing in both number and financial commitment, in the tourism and craftsmanship sectors. In only a few decades these areas have assumed a very respectable economic role. This phenomenon has caused not only a halt to the abandonment, but also a progressive growth in the investments in the construction industry, both in the strictly tourist field and in the activities linked to it, in particular in the craftsmanship and minor manufacturing sectors. This social transformation has, however, consistently altered the hydrogeological equilibria of these areas, which are in themselves intrinsically fragile: not only have natural disasters increased in frequency, precisely because of the increased anthropic pressure and the progressive abandonment of the defused soil protection methods, but their cost in terms of human lives and destruction of instrumental goods has grown out of all proportion.
As opposed to what happens in the valley parts of a watercourse, the hydrogeological risk in mountain and piedmont areas depends in large part on the solid contributions: the solid flows are of such entity that it is not possible to tackle the two components separately. From a systemic point of view, one can imagine that intense liquid runoff moves debris from along slopes and streambeds due to geomechanical instability. In this phase, considerable solid discharges are generated, which can be up to one order of magnitude greater than the corresponding liquid discharge, and which often manifest themselves as debris flows or mudslides. These solid flows tend to be deposited on the alluvial cones, causing channel fill which puts human settlements at risk.
The finer part of the solid discharge flows to the piedmont channels causing the riverbeds to be raised dramatically, this often causes overbank flooding because of the reduced liquid discharge capacity. Sediment transport in the piedmont watercourses is also a determining factor in localized erosion, and in the fluvial morphology and environmental aspects linked to it.
Within this phenomenological-environmental context, the Centre intends to investigate the technical-scientific aspects that the various phases of the process entail. It also aims to prepare the instruments for forecasting, monitoring, and risk reduction, with particular mention to the following topics:
Analysis of the rheological and mechanical properties of debris flows by means of laboratory and field investigations and the adjustment of mathematical models integrated along the depth for the mapping of debris flow risk and the evaluation of defence structures. Regarding this topic, the European Union, within the V Framework Programme, has financed a project (THARMIT) which is coordinated by one of the Participants of the Centre and has two operational units which refer to professors that participate in CUDAM. The first systematic approaches go back to Bagnold (1954), who highlighted the role of intergranular interactions in the rheology of granular fluids. Other researchers have resumed Bagnold’s scheme, proposing rheological models that treat debris flows as an equivalent single phase fluid, as a two phase fluid, or as a multi-phase or particle fluid (McTigue, 1982; Jenkins and Savage, 1983; Savage and Sayed, 1982; Johnson, 1987). The proposed schemes, however, lack reliable physical confirmation of the various parameters that are present. The Proponents have recently conceived two new and original experimental systems (EU Projects, DEBRIS FLOW and THARMIT). The first, conceived for granular flows, allows one to reproduce stationary and uniform open channel conditions for high-concentration flows over an erodable bed in equilibrium (Armanini et alii,1999), which has produced innovative results, with respect to previous research work with annular channels (Bagnold 1954) and enclosed channels (Bakhtiary and Asano 1998), thanks to the various boundary conditions. Furthermore, the systems allow one to measure the concentration and velocity of the two phases, and the granular temperature, with a precision which had never been achieved previously.
With the aid of this new equipment, it is intended to define and characterize the rheology of the different typologies of motion of hyperconcentrated currents and of debris flows, and to identify the global relations that exist between geometric quantities and mean physical quantities.
The second installation, conceived for the study of hyperconcentrated suspensions, permits both the reproduction of uniform motion conditions on a mobile bed and the measure of the velocity profile. The precise identification of the behaviour of mudflows is investigated by means of this installation. Moreover, knowledge of the rheological relationships will allow for the fine adjustment of mathematical models integrated along the depth of the flows, which are more realistic than current models (Takahshi, 1980; Julien e O‘Brian, 1988; Armanini, 1999; Fraccarollo e Armanini, 1999), and which are capable of simulating the phase separation problem in a more correct manner. Even in this aspect, there is the intention to invest in the new Centre, especially in that which pertains to two phase modelling, but also in the development of some physical simulations in the laboratory which will serve as a test for the mathematical models.
CUDAM uses the new Hydraulics and Soil Protection Laboratory as its logistic structure, currently under construction at the Faculty of Engineering, as well as a series of equipped experimental basins in the Province of Trento. The new Laboratory covers an area of about 1000 square metres and, seeing its vocation, it could be dedicated almost exclusively to topics of interest to CUDAM, so as to become even formally a facility.
The project, though intent on the scientific problems of the various topics mentioned, also has applicative repercussions, especially in support of the monitoring, forecasting, and planning activities of those public bodies in charge of territorial management and safeguarding.