DYNAMIC IMPACT HYPOTHESES FOR DESIGNINIG CHECK DAMS INVOLVED IN DEBRIS FLOW PHENOMENA

The scenario of front velocities and depths of a debris flow approaching a transverse structure is affected by some uncertainties that mainly depend on the rheological characteristics of the mixture. These uncertainties enlarge when an estimation of the forces acting on a check dam is requested. The paper refers on the available methods for the computation of forces and pressures to assume in the design of check dams against debris flow. After comparing the different approaches, some considerations are expressed to decide the design hypotheses in view of the check dam type and the level of safety advisable for the structure. INTRODUCTION The control of debris flow transit along the hydrographic net of a catchment can be achieved by design and construction of check structures with different functions. Not far from the potential triggering zone, consolidation dams are useful, particularly in case of deep talweg incision. This solution is conceived rather to limit the bed deepening and debris flow originating due to the bed fluidization (Jaeggi and Pellandini, 1997) than to reduce the stream gradient. In the middle and downstream reaches – sometimes upstream of the fan, sometimes even on the fan – check dams are constructed in view of a debris flow intercepting (Okubo et al., 1997). These dams provide a partial or total retention of the sediment yielded by the debris flow. Intercepting check dams have generally a central opening, sometimes associated or not to a filter, and they can be designed as isolated or grouped works. In the first case the location of a unique check dam gives the detention capacity requested by the scenario of the event. In the second case the debris flux control is progressive. The upstream check dams have a prevailing debris breaker function; the downstream ones magnify the mechanical retention function. Downstream of intercepting structures convey check dams may take place. The last structures have mainly the function of controlling the lateral instability of the residue debris flow passed trough the intercepting dams. Convey check dams also reduce the bed deepening induced by the debris flow and avoid the increasing in sediment volume of the fluent debris flow. It is advisable use this structure when the reach downstream of the intercepting structures is long and when the stream gradient after their construction still remains sufficient to avoid the deposition of the debris flow. For the above-mentioned categories of check dams the design engineer has to formulate appropriate hypotheses on the forces exerted by a debris flow. The path is not simple because many laboratory researches are available on these topic but they are not always immediately transferable to the specific problem under examination. The ultimate choice is in general oriented to a drastic simplification of the phenomenon and to a precautionary estimation of the involved physical parameter (e.g. viscosity, density, depth, velocity of the surge). The force of a debris flow on a transversal structure includes the hydrodynamic pressure of the flux and the random force of big boulders transported by the front. Hungr (2000) proved that a homogeneous mixture of a purely plastic fluid (Newtonian or Bingham type) flowing in a wide channel produces a wedge front tending asymptotically to the depth: