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1、Design of SpillwaysSpillways are ordinarily classified according to their most prominent feature .Commonly referred to types can be listed as follows.1. Free Overall (Straight Drop) SpillwaysA free overfall or straight drop spillways is one in which the flow drops freely from the crest. This type is
2、 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
3、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, becaus
4、e 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 poo
5、l to tailwater in excess of 20 feet should not be considered.2. Ogee (Overflow) SpillwaysThe 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 sh
6、arp-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.
7、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
8、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 SpillwaysThe s
9、ide 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 di
10、scharge 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
11、 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) SpillwaysA spillways, whose discharge is conveyed from the reservoir to the downstream river level through an o
12、pen 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
13、 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 abo
14、ve 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 foundati
15、on 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
16、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 讦 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
