Slit-Check Dams for Controlling Debris Flow and Mudflow

Abstract The paper presents a rational criterion to design the slit check dams to control mudflows and debrisflows. The method here presented is based on the scheme proposed by Armanini & Larcher (2001) for the caseof ordinary sediment transport and it is based on the balances of the solid and liquid phases of debris flow andmudflow and on momentum conservation. The theory is compared with the results of some laboratory testson scale reduced models of debris flows and of mudflows. The experimental results agree quite well with thepredictions of the theory.Keywords: slit check dam, debris flow, mudflow, defence works. Introduction Slit check dams are open check dams presenting one or more narrow, vertical openings, going from thedam’s base up to the weir Their purpose is mostly to dose the sediment transport rate.Although slit check dams have been used for many years in torrent restoration as a primary device toobtain a more effective sediment transport control, many aspects of the functioning of these devices are stillnot sufficiently clear. From a hydraulic point of view, open check dams are very often designed only on thebasis of the designer’s experience, who imitates similar structures built in analogous situations.In the active defence strategy against debris flow and mudflows, open check dams are widely employedwith the double purpose to reduce the kinetic energy of the flow (debris flow breakers) and to temporary storethe debris/mud volume. The first purpose is the target of the check dams located in the upper part of thebasins where the available volumes are small: in this case the dynamic impact is the most important designparameter. In the second case the reference parameter is volume retention. The check dams of this type arebuilt in the lower part of the basins, often at the exit of the stilling basins.A rational criterion for designing the opening of slit check dams to control the bed load and thesuspended load has been proposed by Armanini & Larcher (2001). The approach is based on the conservationof the mass of water and sediments and on the energy balance between an upstream section and the sectionof the opening of the check dam. In the present paper the authors extend the scheme, presenting a theoreticalapproach to the problem of designing the opening of a slit check dam when the solid transport rate exceedsthe typical limits of ordinary sediment transport. In particular two different types of flow are considered,characterized by markedly different features: debris flow and mudflow. In the case of granular debris flow,the interstitial fluid is represented by water and the effects of cohesion can be considered negligible. On thecontrary, the mudflow is represented by a homogeneous mixture composed by water and by the finer fractionof sediments with cohesion and yield stress that sharply affect the flow properties.