Study On Measures Of Reducing The Difficulty Of River Closure

The investigation about reducing the closure difficulty of end-dump method, which is generally applied in great rivers with large drop and high flow velocity, is always an important technique problem concerned by engineers. In this paper, referring to some closure engineering instances in the world, and applying the theory analysis, scientific experiments and practice validation, the factors which affect the difficulty of closure are analyzed in detail, and then the estimation indexes of closure difficulty are discussed and the measures and applicability of decreasing difficulty of closure is summarized and presented.Due to the complex boundary conditions, the obvious three-dimensional flow characteristic and the specialty of different closure engineering, it is difficult to adopt a single index for reflecting the difficulty of whole closure engineering. Therefore, it is reasonable to considering the closure difficulty, closure scale and the safety degree of closure synthetically, and using systemic judgment method according to the specialty of closure engineering. The fundamental hydraulic factors of estimating the difficulty of end-dump method are closure discharge Q, final water drop Z, maximum velocity of flow V_max at river closure gap and depth H. The physical and mechanical properties of closure materials and the coarseness degree of river bed are also the pivotal factors affecting the stability of rubble.There are many measures to decreasing the closure difficulty and they can be classified into two broad categories. The first category is measures of improving the hydraulic characteristic of river closure gap and contain: (1) the measures of decreasing minimum discharge at river closure gap, such as excavating open channel or tunnel for constructing good distributary conditions, constructing closure gate and burying penstock in closing dike or using frame structure as closure materials for increasing the permeability and seepage quantity of embankments, and attempering the upstream cascade reservoir for decreasing the streamflow. (2) The measures of reducing flow velocity at river closure gap such as adopting broad embankment to increase the on line resistance of river closure gap. (3) The measures of decreasing theclosure drop, such as adopting bi-embankment or tri-embankment to separate the closure drop, which have been applied in open channel closure of the three gorges project and Itaipu project respectively. (4) Decreasing the discharge per unit width by using horizontal closure with broad river closure gap. For example, the width of river closure gap of Stalin grad project reached 300m. (5) Changing the throwing position for improving the flow pattern at rive closure gap. For example, in some cases that the vertical closure method is difficult to be implemented directly, boulders are always thrown at the upstream corner firstly or both upstream and downstream corners synchronously to steer clear of the torrent and form areas of slack water near the embankment end, so as to adopt common stones to complete the closure. The second category is measures of reinforcing the stability of foundation and closure materials, such as increasing the roughness of river bed protection, bedding course, setting interception barriers and steel tube bars, using anchorage cable, increasing the weight of rock blocks (block string, gabion, caisson or increasing unit weight), etc. In addition, increasing the throwing strength can restrain the running off and reduce the quantity of large closure materials. And bedding course can enhance the closure safety degree through not only increasing the roughness of river bed, but also reducing the depth of water and the possibility of dike end slump.The selection of measures about decreasing closure difficulty should consider different engineering practices and adjust measures to local conditions to make it scientific, effective, practicable and economical.