Why should water be stored before the diversion tunnel is blocked?

In the dam construction of water conservancy and hydropower projects, most projects use diversion tunnels for diversion.

If the diversion tunnel does not need to be converted into a permanent hydraulic structure after the diversion task is completed, it must be blocked to ensure the effective function of the project. The blocking building of diversion tunnel is a permanent building, and its importance is equivalent to barrage. In order to block the building, it is necessary to set a gate at the entrance of the diversion tunnel and block the river water in the reservoir by lowering the gate.

Therefore, the closure of diversion tunnel consists of two parts. First, the river water entering the tunnel is intercepted by the intercepting gate (concrete gate or steel gate), and then the diversion tunnel that has completed its mission is completely blocked with concrete (usually called plug) at a suitable position to ensure that the water stored in the reservoir does not leak downstream.

Extended data:

The river is led downstream through the hydraulic tunnel on the shore. Tunnel diversion is applicable to:

1, the valley is narrow, and the topographic conditions are not conducive to the layout of diversion open channel;

2. The geological conditions of riverside mountain are good, and tunnels can be excavated;

3. Some projects require cutting off the whole riverbed at one time and construction all the year round. Compared with open channel diversion, the flow of tunnel diversion is smaller, but with the development of construction technology, the diameter of diversion tunnel tends to increase. As of 1985, the largest diversion tunnel in the world is the diversion tunnel of Breil Hydropower Station in the Soviet Union, with a width of 17m and a height of 22m.

The engineering features of tunnel diversion are as follows:

1, because the tunnel is built in the mountain body on the shore, the layout of the hub can be simplified; 2. When the diversion flow is large, multiple tunnels can be built;

3. In order to meet the flood discharge requirements of different construction stages and different reservoir water levels, multiple diversion tunnels can be set at different elevations.

4. The diversion tunnel can be combined with the permanent project or converted into a permanent project;

5. The water flow conditions in the tunnel are generally complicated, and the operation situation is also difficult to observe;

6. The upstream cofferdam is relatively high, and the water level difference between upstream and downstream is relatively large during the construction period.

Diversion tunnel layout should be adapted to local conditions, and attention should be paid to:

1. Make full use of the terrain conditions and shorten the tunnel line length as much as possible. When the dam site is located in the river bend, the tunnel line should be arranged on the convex bank;

2. The layout of the entrance and exit of the tunnel should make the water flow smooth, and keep a necessary distance from the toe of the upstream and downstream cofferdam to prevent scouring;

3. The selection of import and export elevation should consider the closure gap, navigation, crossing trees and blocking conditions;

4. Good geological conditions, stable rock mass, few faults and undeveloped cracks are required along the tunnel. In hard and complete rock stratum, temporary tunnel can be unlined or partially lined. In order to reduce tunnel smoothness, improve drainage capacity and reduce leakage, smooth blasting and bolting and shotcreting support can be used in construction.

5, important projects when the flow or velocity is larger, should pay attention to the export energy dissipation measures.

References:

Baidu encyclopedia-diversion tunnel