Stepped channels and spillways have been used for over 3, years. Roller-compacted concrete , gabions and design techniques e. A bell-mouth spillway  is designed like an inverted bell where water can enter around the entire perimeter. These uncontrolled spillways are also called morning glory,  after the flower or glory hole  spillways. In areas where the surface of the reservoir may freeze, this type of spillway is normally fitted with ice-breaking arrangements to prevent the spillway from becoming ice-bound.
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Appendix 4. Labyrinth weirs 4. Existing labyrinths: One drawback of free overflow spillways is their low specific discharge. A standard Creager type profile will have a unit flow per metre length of spillway close to 2. The upstream head savings in these cases is close to 3 m.
The behaviour of these structures has been good but this solution has been used for only about 0. New solution: P. Weirs Since , studies and model tests have been made in: Algeria, China, France, India, Switzerland and Vietnam for labyrinth designs which may be placed upon normal gravity dams cross sections. The designs produced tried to optimise both hydraulic efficiency as well as structural and economic requirements.
This is favourable hydraulically especially for large discharges and also allows the base width of the structure to be reduced, thus favouring its utilization upon most spillways or gravity dams. A design which appears cost efficient for most existing or new spillways is represented below. This model model A has an upstream and a downstream overhang of same length. The proposed ratio N between the developed plan length of wall and overall spillway length is close to 5.
An increase of this ratio does not usually seem cost effective. Where this ratio of 5 is used, the proportions of other aspects of the PKWeir can be taken as follows, based on Pm in metres which is the maximum height of the labyrinth walls, see Fig.
The suggested upstream head over the weir crest, h, should be between 0. The saving in term of required head above spillway crest level as compared to a Creager weir is close to 0. To improve the hydraulic efficiency when the design conditions do not limit the height of the P. Weir, it is generally more economic to increase Pm and to keep same proportions as suggested, rather than to increase the value of the N ratio. Suggested dimensions are of course only indications and are subject to adaptation according to local requirements.
Weirs can be made of reinforced concrete, either precast or in situ. In the case of low height walls, thickness may be governed by practical steel fixing requirements. Such a solution is certainly advisable where Pm is less than 1m.
The cost per metre of spillway length is generally proportional to Pm. Alternative shapes and models It is possible for about the same cost and discharge, to slightly modify the weir shape as shown on Fig. For instance model B has a longer upstream overhang and no downstream overhang, see Figs 4 to 6.
This produced a slightly increased discharge for small values of h. Utilization Compared to Creager type weir profiles, P. Weirs may divide by a factor of about 3 the spillway lengths required at fill dams or by 2 the upstream head required at any weir. For existing free overflow spillways they may be used to enhance freeboard and increase safety or to increase storage at low cost. They may be used as emergency spillways associated with an existing or new gated spillway, using the freeboard for discharging part of any exceptional floods.
As there is also much air entrainment in flows from P. Weir and other labyrinth structures, energy dissipation is enhanced and, for moderate discharges, any downstream erosion may be much reduced.