October 20～23 2008, Hohai University Nanjing, China
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DESIGN CONDITIONS FOR MORNING-GLORY SPILLWAYS: APPLICATION TO POTRERILLOS DAM SPILLWAY Claudio A. Fattor, Jorge D. Bacchiega National Institute of Water, Casilla Correo 21 – B1802WAA-Ezeiza – Argentina, e-mail:
[email protected]
Abstract.
Design of morning-glory spillways has special features, because the hydraulic behavior
of this structure is strongly related to the boundary topography, crest diameter, curve or bend downstream, spillway profile and the tunnel dimensions towards the outlet section. In the case of Potrerillos dam, located on Mendoza River, Argentina, a morning-glory spillway was selected. This spillway must be able to pass a design discharge of 1800 m3/s at free surface, being characterized by a 31,76 m-crest diameter, 13.40 m-cross section diameter, a vertical 90° curve of 44.67 m-radio and a 400 m-length horseshoe cross section tunnel of 11.80 m-diameter. A special study was developed at National Institute of Water in order to analyze the hydraulic performance of the spillway. The physical model studies allowed determining a final location different from that considered in the project, optimizing the boundary conditions close to the spillways, modifying the number of piles on the crest and improving by these means the hydraulic performance of the whole structure, including flow characteristics inside tunnel. Final hydraulic design was compatible with structural design, allowing besides that a strong reduction of rock excavation. Key words
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Morning-glory spillways; Potrerillos dam; physical model.
INTRODUCTION Ra
Morning-Glory spillways must be able of
profile, the radius of the downstream edge of the profile (Rt), curve radius (Rc), angle of the vertical
R
Dt
dimensions, as it can be observed in Figure 1, include radius at crest elevation (R), spillway
Dg y= f(x)
Dc
of spillways can be characterized by its spillway profile, a vertical curve and a discharge túnel. Main
Dv
Hd
discharging floods related to a determined return period under safe conditions for the dam. This kind
De
i
Figure 1 – Morning-glory spillway
bend, diameter D and the longitudinal slope (i). The discharge capacity of the structure is
The criteria mostly extended for this kind of
governed by the hydraulic regime, which can present three different phases, as it can be seen in
spillway is that must be designed for working in phase 1, it means at free surface flow, with the
Figure 2.
discharge as a function of the discharge coefficient
Reservoir elevation
16th IAHR-APD & 3rd IAHR-ISHS
at the inflow that could anticipate the submergence and the drowning of the spillway. If this were to happened, the efficiency is lower and the water surface elevation would be increased.
Phase III - Q=f(J)
On the other hand, inflow conditions are potentially able to influence on the flow inside the
Phase II - Q=f(H1/2) Phase I - Q=f(H3/2) Change of tendency: fromspillway regime to orifice regime
Discharge
tunnel, where high flow velocities and cavitation risks would be present. This conducts to try to get approximation flow conditions as symmetric as possible, which may reduce presence of waves and
Figure 2 – Discharge-elevation relationship
instabilities in a supercritical flow regime. Flow rotation at the inlet section of the
Cd, the effective length L and H3/2, where H is the hydraulic head. Following this design criteria,
spillway and vortexes are reduced by means of piles located at the crest. However, the singular boundary
instabilities caused by submergence are avoided (Fattor C. and Bacchiega J., 2001).
conditions involving the spillway geometry and the topography in the immediate surroundings can be a
If maximum flow discharge were exceeded, for instance due to the effect of river sedimentation in
source of instabilities inside the spillways and along the discharge tunnel.
the routing of the flood, the hydraulic behavior could be given by a total or partial submergence of
As a consequence, the design of morningglory spillways must make compatible the
the morning-glory spillway. These conditions should be carefully analyzed by means of a physical
optimization of the discharge capacity and the flow pattern, avoiding negative effects over its
model due to strong instabilities that could be occasioned, being able to reach water surface
performance and the presence of asymmetric flows along the tunnel.
elevations with some risks for the dam safety. The elevation-discharge relationship can be expressed by (1):
Q Cd 2 R H
3
2
(1)
2. BACKGROUND ANALYSIS Technical bibliography concerning morning-
, where: Cd: discharge coefficient, R is crst-
glory spillways is not so abundant, but, despite of that, there are references that allow analyzing main
diameter and H is the hydraulic head. Cd depends on Hd/R. For Hd/R
