水利水电翻译

Design of Spillways

Spillways are ordinarily classified according to their most prominent

feature .Commonly referred to types can be listed as follows.

1. Free Overall (Straight Drop) Spillways

A free overfall or straight drop spillways is one in which the flow drops freely

from the crest. This type is suited to a thin arch or deck overflow dam or to a crest is

which has a nearly vertical downstream face. Occasionally the crest is extended in the

form of an overhanging lip to direct small discharges away from the face of the

overfall section.

Where no artificial protection is provided at the base of the overfall, scour will

occur in most streambeds and will form a deep plunge poll. The volume and depth of

the hole are related to the range of discharges, the height of the drop, and the depth of

tailwater.

A free overfall spillways is not adaptable for high drops on yielding foundations,

because of the large impact forces which must be absorbed by the apron at the point

of impingement of the jet. Vibrations incident to the impact might crack or displace

the structure, with danger from failure by piping or undermining. Ordinarily, the use

of this structure for hydraulic drops from head pool to tailwater in excess of 20 feet

should not be considered.

2. Ogee (Overflow) Spillways

The ogee spillways has a control weir which is ogee or S-shaped in profile. The

upper curve of the ogee ordinarily is made to conform closely to the profile of lower

nappe of a ventilated sheet falling from a sharp-crested weir. Flow over the crest is

made to adhere to the face of the profile by prevening access of air to the under side

of the sheet. For discharges at designed head, the flow glides over the crest with no

interference from the boundary surface and attains near maximum discharge

efficiency. The profile below the upper curve of the ogee is continued tangent along a

slope to support the sheet on the face of the overflow. A reverse curve at the bottom of

the slope turns the flow onto the apron of a stilling basin or into the spillway

discharge channel.

An ogee crest and apron may comprise all entire spillway, such as the overflow

portion of a concrete gravity dam, or the ogee crest may only be the control structure

for some other type of spillway. Because of its high discharge efficiency, the

nappe-shaped profile is used for most spillway control crests.

3. Side Channel Spillways

The side channel spillway is one in which the control weir is placed along the

side of and approximately parallel to the upper portion of the spillway discharge

channel. Flow over the crest falls into a narrow trough opposite the weir, turns an

approximate right angle, and then continues into the main discharge channel.

Discharge characteristics of a side channel spillway are similar to those of an

ordinary overflow and are dependent on the selected profile of the weir crest. The

discharge constriction may be a point of critical flow in the channel, an orifice control,

or a conduit or tunnel flowing full. Although the side channel is neither hydraulically

efficient nor inexpensive, it has advantages which make it adaptable to certain

spillway layouts.

4. Chute (Open Channel or Trough) Spillways

A spillways, whose discharge is conveyed from the reservoir to the downstream

river level through an open channel, placed either along a dam abutment or through a

saddle, might be called a chute, open channel, or trough type spillway. These

designations can apply regardless of the control device used to regulate the flow. Thus,

a spillway having a chute-type discharge channel,though controlled by an overflow

crest, a gated orifice, a side channel crest, or some other control device, might still be

called a chute spillway. However, the name is most often applied when the spillway

control is placed normal or nearly normal to the axis of an open channel, and where

the streamlines of flow both above and below the control crest follow in the direction

of the axis.

The chute spillway has been used with earth fill dams more often than any other

type. Factors influencing the selection of chute spillways are the simplicity of their

design and construction, their adaptability to almost any foundation condition, and the

overall economy often obtained by the use of large amounts of spillways excavation

in the dam embankment.

Chute spillways ordinarily consist of an entrance channel, a control structure, a

discharge channel, a terminal structure, and an outlet channel. The simplest form of

chute spillway has a straight centerline and is of uniform width. Often, either the axis

of the entrance channel or that of the discharge channel must be curved to fit the

alignment to the topography. In such cases,the curvature is confined to the entrance

channel if possible, because of the low approach velocities. Where the discharge

channel must be curved, its floor is sometimes superelevated to guide the

high-velocity flow around the bend, thus avoiding a piling up of flow toward the

outside of the chute.

5. Conduit and Tunnel Spillways

Where a closed channel is used to convey the discharge around or under a dam,

the spillway is often called a tunnel or conduit spillway, as appropriate. The closed

channel may take the from of a vertical or inclined shaft, a horizontal tunnel through

earth or roke, or a conduit constructed in open cut and backfilled with earth materials.

Most forms of control structures,including overflow crest,vertical of inclined orifice

entrances, drop inlet entrances, and side channel crests, can be used with conduit and

tunnel spillways.

With the drop inlet or orifice control, the tunnel or conduit size is selected so that

it flows full for only a short section at the control and thence partly full for its

remaining length. To guarantee free flow in the tunnel, the ratio of the flow area to the

total tunnel area is often limited to about 75 percent. Air vents may be provided at

critical points along the tunnel or conduit to insure an adequate air supply which will

avoid unsteady flow through the spillway.

Tunnel spillways may present advantages for dam sites in narrow canyons with

steep abutments or at sites where there is danger to open channels from snow or rock

slides. Conduit spillways may be appropriate at dam sites in wide valleys, where the

abutments rise gradually and are at a considerable distance from the stream channel.