Slit Dams Research Articles

Enhancing sediment flux control and natural hazard risk mitigation through a structured conceptual planning approach

Sediment fluxes from mountain rivers contribute to shape the geomorphologic features of lowland rivers and to establish the physical basis for an optimal set of ecosystem functions and related services to people. Through significant public funding, the hydro-morphological regimes of mountain rivers in the European Alps have been progressively altered over the last century, with the aim to provide a safe dwelling space, to boost transport, mobility and to support economic growth. We claim that the underlying planning weaknesses contribute to determine these inefficient resource allocations, since flood risk is still high and the ecosystem services are far from being optimal. Hence, with the overall aim to enhance sediment flux control and hazard risk mitigation in such heavily modified alpine streams, we propose a structured design workflow which guides the planner through system analysis and synthesis. As a first step the proposed workflow sets the relevant planning goals and assesses the protection structure functionality. Then a methodology is proposed to achieve the goals. This methodology consists in characterising the hydrologic basin of interest and the sediment availability and determining the sediment connectivity to channels. The focus is set on the detailed analysis of existing river cross sections where the sediment continuity is interrupted (e.g. slit and check dams). By retaining relevant sediment volumes these structures prevent the reactivation of hydro-morphological and associated ecological functionalities. Since their actual performance can be unsatisfying with respect to flood risk mitigation (e.g. mainly old structures), we introduce specific efficiency indicators as a support for the conceptual design stage to quantify effects related to sediment flux control and risk management. The proposed planning approach is then applied to the Gadria system (stream, slit dam, retention basin and culvert), located in South Tyrol, Italy. This case study shows that design excellence is needed to re-establish the sediment continuity, while keeping flood risk below acceptable levels. Moreover, the detailed hydraulic analyses highlight that the slit dam is oversized and it could be redesigned to improve sediment continuity and to reduce maintenance costs.

Essential strategy for urban seasonal-river restoration

Recently, many urban rivers are being restored due to positive environmental effects while debris flow may occur in seasonal-rivers of cities in arid regions. Therefore, as a part of river restoration studies, the impacts of debris flow on an urban seasonal-river (Vardavard River, Tehran) were analysed based on field investigations and hydrologic, hydraulic and sediment analyses. The results show that high-concentration of sediments in debris flow increases peak flow and hydraulic roughness, resulting in river flooding. The steep slope of the river basin increases the debris flow peak up to 1.2 times more than an ordinary flood peak with the similar return period. Hence, designing a slit dam before the river enters the city is suggested as an essential strategy to control the debris flow and improve safety of the downstream urban areas. The proposed approach, with enforced watershed management schemes, can be implemented to restore other urban seasonal-rivers and to prevent probable debris flow in arid regions.

Essential strategy for urban seasonal-river restoration

Recently, many urban rivers are being restored due to positive environmental effects while debris flow may occur in seasonal-rivers of cities in arid regions. Therefore, as a part of river restoration studies, the impacts of debris flow on an urban seasonal-river (Vardavard River, Tehran) were analysed based on field investigations and hydrologic, hydraulic and sediment analyses. The results show that high-concentration of sediments in debris flow increases peak flow and hydraulic roughness, resulting in river flooding. The steep slope of the river basin increases the debris flow peak up to 1.2 times more than an ordinary flood peak with the similar return period. Hence, designing a slit dam before the river enters the city is suggested as an essential strategy to control the debris flow and improve safety of the downstream urban areas. The proposed approach, with enforced watershed management schemes, can be implemented to restore other urban seasonal-rivers and to prevent probable debris flow in arid regions.

Coarse granular flow interaction with slit structures

Coarse grains accumulate in geophysical flow fronts and have high solid fractions. Such fronts may arch in slit structures such as baffles and slit dams, leading to the rapid trapping of particles and potentially high-energy overflow. Existing empirical slit-structure design recommendations are limited and inadequate since they only focus on the slit size to particle diameter ratio (s/δ) and neglect the pileup height and pre-impact flow energy. Flume modelling was thus adopted to study coarse flow fronts impacting a slit structure. The characteristic Froude conditions, flow particle diameter and the ratio s/δ were varied. Results have shown the pileup height, and hence the confining stress is dependent on Froude conditions, but is not strongly influenced by s/δ. The flow particle diameter influences collisional and frictional stresses and hence the mean outflow rate, which is correlated with pileup height. Grain-trapping efficiency depends on both s/δ and Froude conditions. In contrast to existing continuum-based theory for slit-structure interaction, frictional contacts should be considered for coarse-grained flow fronts. High-energy supercritical flows lead to low trapping efficiency since stable arches cannot form at high shear rates. This implies that multiple slit structures may be more appropriate for attenuating high-energy supercritical flows.

Dimensionamento de barragens com fendas (“slit dams”) para o controlo de fluxos de detritos em bacias de regime torrencial

O artigo aborda aspectos conceptuais e de dimensionamento de barragens com fendas (“slit dams”) para o controlo de fluxos de detritos grosseiros de material sólido em bacias hidrográficas de regime torrencial, como medida estrutural para a mitigação dos seus efeitos. A avaliação do risco associado a um determinado perigo pressupõe a identificação do problema e do sistema – alvo de atenção. Neste contexto e após a apresentação de casos históricos de torrentes como um fenómeno global, faz-se uma breve caracterização das bacias hidrográficas de regime torrencial e dos mecanismos de ruptura de vertentes e de transporte de material sólido que estão associados às torrentes. A identificação dos factores potenciadores e a caracterização dos parâmetros básicos dos fluxos de detritos, necessários ao dimensionamento das estruturas de controlo e retenção, são aqui apresentados segundo diferentes técnicas e metodologias. De entre várias medidas estruturais de combate a estes eventos, o trabalho foca-se nas barragens com fendas e no seu dimensionamento, e apresenta o caso da “aluvião” de 20 de Fevereiro de 2010 na ilha da Madeira como um evento excepcional onde estas soluções foram adoptadas.

Experimental and numerical study of slit-check dams

This paper presents and discusses the results of laboratory tests and numerical simulations carried out to assess the efficiency of slit dams (open-type retention dams) and applicability as a short-term struc tural mitigation measure against debris flows in steep torrential channels. Inspired by common slit dam solutions, two different layouts of piers were tested in the experimental study – aligned and V-shaped. Some of the major experimental results about the sediment control efficiency are presented, showing that aligned layout is more appropriate to mitigate stony-type debris flows than the tested V-shaped solution. Actually, it was proved that slit-check dams mitigation efficiency is influenced by various variables and parameters, most of them previously studied by several authors, but also by pier layout in plan view. Considering aligned layout, 1.0d 95 to 1.4d 95 slit widths shall be considered to design effective slit dam solutions. Further, a numerical study was performed, providing relevant data that greatly enriched the experimental efforts and also support future slit dam design phases. The numerical results show similar quantities of retained solid material comparing with the experimental tests, indicating that the model provides reliable results, thus being able to support engineers and decision makers regarding debris flows mitigation.

The impact of slit and detention dams on debris flow control using GSTARS 3.0

Debris flows, often the result of environmental degradation in mountainous areas, can be an extreme geological catastrophe. The concept of building detention dams to control debris flows has emerged after numerous deaths and huge economic losses have accumulated due to the destruction of infrastructure. Detention dams are well known for their efficient flow control and relatively low installation cost. However, their efficiency in decreasing peak flow is adversely affected by sedimentation, which not only decreases the effective lifetime of dams but also causes obstruction of outlets. In this research, the capability of GSTARS3.0 (Generalized Sediment Transport model for Alluvial River Simulation) was evaluated for a semi-three-dimensional simulation of sedimentation and flow routing in the reservoirs of two different kinds of detention dams: a classic detention dam and a slit dam. Sediment transport, scour, and deposition processes were simulated and calibrated along an experimental flume that represented the reservoirs of the detention dams to give a semi-three-dimensional variation of the bed geometry after debris flow events. Finally, models were applied to the Mojen River, Iran, as a real case. The model results convincingly show the capability of the GSTARS3.0 model to simulate sedimentation in reservoirs and the superiority of slit dams in controlling debris flow.

Using Hydraulic Engineering Model Experiment to Study the Sediment Trapping Efficiency of Adjustable Check Dam

The construction of fully closed check dam (CD) is a conventional flood prevention mechanism implemented on rivers. Fully closed CDs trap large amounts of sediments in rivers to stabilize the river slopes and control erosion. However, fully closed CDs cannot selectively trap sediment and may easily overflow, causing them to losing their ability to mediate and hold sediments. Previous studies proposed the concept of “breathable CDs”. The researcher introduced metal slit dam (SD) that could be assembled and disassembled quickly and conveniently. Once a CD reaches maximum capacity, operators must ensure that the water channels of the dam are free from blockage. Moreover, they must inspect the internal accumulation conditions of the dam periodically or immediately following heavy typhoon rains. When necessary, either the sediment buildup in the upriver blockage must be cleared, or the transverse structure of the dam must be removed to allow fine particles to be discharged along with a moderate amount of water. These actions can free up the sediment-storing capacity of the dam for the next heavy typhoon rains. In addition, operators should also inspect the damages inflicted on the dam, such as erosion, wear and tear, and deformation conditions. Damaged components should be disassembled and repaired if possible, or recycled and reused. The present study performed channel tests to simulate closed CDs, SDs, steel pipe dam (SPDs), and steel pipe plus slit dam (SPSDs) for 50-year and 100-year frequency floods. Results were then analyzed to determine the sediment trapping (ST) effects of various CDs, the effects of “adjustable CDs”, and the changes of moderated riverbeds.

A Basic Study on Protective Steel Structures against Woody Debris Hazards

This paper presents a basic study for protective structures (slit dams) against woody debris hazards. First, a model experiment was carried out to examine the trap efficiency of a slit dam against woody debris with or without roots. Herein, the effects of length of woody debris, flow discharge and opening width of a slit dam on the trap efficiency were investigated. Second, a new three dimensional distinct element method (3D-DEM) was developed to simulate the trap efficiency of woody debris with or without roots by introducing cylindrical elements. Finally, the new 3D-DEM was applied to simulate an actual woody debris disaster site in Hiroshima, Japan.

Experimental study for debris flow capture function of steel slit dam focused on solid and plane structures

Experimental study for debris flow capture function of steel slit dam focused on solid and plane structures

Recent increases in sediment disasters in response to climate change and land use, and the role of watershed management strategies in Korea

Recent climate change caused by global warming has affected the environment in Korea, resulting in increased precipitation and frequency and magnitudes of typhoons. From 1994–2003, a mean of 2.3 days/year experienced heavy rainfall (≥80 mm), in contrast to a mean of 1.6 days/year from 1954–1963. Of the ten typhoons that have resulted in the maximum daily rainfall over the last ten decades, seven occurred between 1990 and 2007. The maximum instantaneous wind velocity of typhoons has greatly increased from 20 m/sec (in the 1970s) to 40 m/sec (in the 2000s). Furthermore, increases in the occurrence and scale of forest fires and landslides, as well as increased infrastructure and land use, contribute to sediment disasters. Accordingly, environmental changes and human-induced factors have resulted in significant increases in the magnitude and frequency of natural disasters, especially in the Gangwon region on the east coast of Korea. Therefore, in 2004 the Korea Forest Service instituted an integrated and environmentally-friendly system for forest management, which has helped prevent sediment disasters. Multiple prevention strategies are also required in addition to these systemic changes to forest structure management, including control of hill-slope erosion and torrent erosion, debris flow mitigation, water storage and slit dams, grade-stabilization structures and forest improvement, and watershed management.

Efficiency of slit dam prevention against non-viscous debris flow

This paper describes an experimental work in order to assess the efficiency of slit dam on non-viscous debris flow. Some results have been acquired as follow: ① there are three kinds of blocking type: Total-blocking, opening and part-blocking. The blocking conditions of slit dam are closely link tob/d max (the ratio of slit width to maximum diameter of solid matter), asb/d max is less than 0.739, the slit dam is total-blocking; andb/d max is more than 1.478, the slit dam will be opening; whereasb/d max from 0.739 to 1.478, the slit dam is part-blocking. ② variation of the mean density passing through slit dam is the most obvious asb/d max ranges from 0.739 to 1.232. ③ According to experimental results, slit dams have been shown to be effective in reducing debris flow density while slit density ∑b/B (B is slit dam width) ranges from 0.2 to 0.5.

Study on forecasting hydrograph of mudflow passing through a slit dam

Study on forecasting hydrograph of mudflow passing through a slit dam

FUNCTIONS OF SABO DAMS WITH A HORIZONTAL SLIT

An open type of sabo dams such as slit dams has been developed for the more effective sediment control works and the active sediment supply to the downstream reach. So far, many sabo dams with veritcal slits have already constructed. In this paper, sabo dams with a horizontal slit on the bottom is taken up as a new type of slit dams and the effect of the horizontal-slit dam on controlling sediment run off is studied with an experiment and the numerical simulation. The distinctive point of this type is that the dam does not work at all until the water level just upstream of the dam is over the slit height. This means that the horizontal-slit dam can store the sediment only at a high sediment discharge to be necessarily controlled. Some case studies on the sediment run off have explained such an effective function of the dam on the sediment control.

Slit Dam Design for Debris Flow Mitigation

Debris flows cause the destruction of engineering facilities including buildings, roads, and culverts. A slit dam may be used to control small volumes of debris that can damage or destroy small watershed infrastructure, such as culverts. The height and spacing of the dam posts and the location of the dam are the primary design factors. The posts may be arranged in single or double rows with the spacing of the slits a function of the culvert diameter being protected. The height of the dam is a function of the slope on which it is placed. The advantages of slit dams include the simplicity of design and installation and the relatively low cost. They are also stronger than other baffle systems and debris fences.