measures for success in dam bottom outlet design for success in dam bottom outlet design . ... the...

Measures for Success in Dam Bottom Outlet Design

Pouya Amirsayafi

Abstract— The present paper investigates measures for successful design of dam bottom outlets. First, some of definitions and functions of bottom outlets are provided. Associated challenges and solutions will be discussed. Different parts of bottom outlets like intake structures, conduit, control structure and outlet structures are discussed and measures for successful design for each of them are presented with related design cases. Also risk management measures for ensuring successful performance of bottom outlets are discussed with proper examples.

Keywords— dams; bottom outlet; reservoirs

I. INTRODUCTION

Bottom outlets are constructed to ensure conveyance of water from dam reservoir into downstream river and to lower the level of the reservoir. They are called bottom outlet because of the fact that the intake of the structure is constructed at a low level close to dam foundation. Bottom outlet might work either pressurized or free flowing over some parts of the system. This depends on the position of outflow against tailwater level, and type and location of control structure (gates). In addition to emergency evacuation, bottom outlets can be used for providing compensation flow of downstream river. They can also be used to discharge sediment of the reservoir increasing the economic life of dams. One of the most important issues in successful design of bottom outlets is the arrangement and type of structures and gates [1].

II. FUNCTION OF DAM BOTTOM OUTLETS

Bottom outlets are one of the most important structures in dams which have important role in safety of dams and regulating reservoir water level. The function of dam bottom outlet can be one or combination of the following functions:

A. Reservoir evacuation

1) Reservoir evaculation for inspection and maintenance

Inspection and maintenance of structures upstream of dam such as the reservoir, spillway and intake structure for power plant and conveyance system is possible only when the water level is at the lower levels of the reservoir. Therefore, the main function of bottom outlet is reducing reservoir level to a suitable level and the ability for maintaining that level according to river flow. Further, reservoir evacuation after the first impounding is sometimes necessary for remedial works.

2) Emergency evacuation

Emergency evacuation of reservoir is undertaken with two major aims for safety of dam. One aim is reducing forces on the structure of dam in following cases:

Unconventional leakage in dam or foundation.

Substantial settlement in dam or foundation.

Slide in part of dam.

After earthquakes and crack formation.

Another aim is supporting flood discharge system (spillway) during:

Unexpected flood in upstream basin.

Failure of a dam in the upstream.

Occurrence of a floods larger than the design flood.

B. Control over first impounding of reservoir

Control over the extent of increase in water level in reservoir especially in the first impounding is required due to the reasons below:

Measurement and inspection of behavior reservoirs slopes and dam abutments.

Prevention of induced seismicity.

Prevention of quick consolidation of foundation and embankment.

DOI: 10.5176/2251-3701_3.3.150

GSTF Journal of Engineering Technology (JET) Vol.3 No.3, October 2015

©The Author(s) 2015. This article is published with open access by the GSTF

111

Received 02 Aug 2015 Accepted 22 Sep 2015

DOI 10.7603/s40707-014-0030-2

C. Provide downstream water demands during first impounding

This includes supplying environmental, agricultural, industrial and drinking water demands before reaching the predetermined water level in reservoir that is necessary for operation of dam.

D. Providing downstream needs under the condition that the reservoir must be kept at low levels

During emergency evacuation of reservoir, water level might be below the level of irrigation or hydropower plant intake or they might not be working due to maintenance activities. Under these conditions, bottom outlets provide downstream demands.

E. Flood evacuation during construction of dam

In some instances by quick construction of bottom outlets they can be used as part of river diversion works reducing associated costs. This is possible only when inflow and outflow of bottom outlets don’t make any problem for construction of intakes of conveyance system.

F. Flushing

If bottom outlets are constructed close to intake for conveyance system or hydropower plant, sediments that might block the intake structures can easily be removed. Bottom outlets can also evacuate sediments in reservoir increasing effective volume of reservoir.

G. Supprting for flood discharging system (spillway)

In general, use of bottom outlets as a part of flood discharging system is not proposed due to the following reasons:

In many dams, the flow velocity in bottom outlet conduit is so high and service gates work under high water pressure that cause risks to bottom outlets during floods.

Under floods, reliability of bottom outlet gates is less than spillway gates.

The probability of bottom outlets blockage due to reservoir sedimentation is high especially in the final years of operation.

Therefore, except for special cases, this system must not be used as flood discharging system. If such a function is considered, bottom outlets must be inspected and maintained with special requirements.

III. OPERATION

It is of great importance to have a proper operation of bottom outlets of dams and maintaining them under favorable conditions to assure function of outlets in necessary occasions, increasing lifetime of outlets and decreasing the costs for maintenance and operation. Continuous inspections to detect problems and adopt necessary implications to resolve them are among the most important operational activities. The stages for inspection of bottom outlets are at three levels of regular and continuous inspections, periodic and semi-professional inspections, and specialized inspections with necessary characteristics of inspectors at each inspection. Inspection of bottom outlet and its components must be regularly undertaken and the documents must be maintained by operator. Inspection and observance of the defects and any unusual behavior of the structures must be reported especially after special events and emergencies such as earthquakes and floods.

A. The problems during operation

The problems which have been reported in operation of bottom outlets include:

Cavitation due to high flow velocity. An example is 128 meter high Libby dam bottom outlet [2:705] in the U.S which the flow velocity reached 40 m/s and cavitation had occurred in 50 meter downstream of the service gate.

Erosion of outlet conduit which discharges flows with sediment. An example is erosion of Mud Mountain dam [2:708] outlet in the U.S. In this case, flow with large debris consisting of 60 cm rocks and velocity of 9 to 12 m/s caused severe damage to outlet tunnel.

Damage due to both erosion and cavitation. In most cases, damages are due to erosion caused by flow velocity that increases roughness on concrete surface beyond allowable roughness leading to cavitation. An example is outlets of Sefidrud dam in Iran that is shown in Fig. 1 [3].

Damages to downstream stilling basin due to clash of debris with concrete surface such as Navajo dam [2:702] in the U.S.

Erosion of the outlet channel downstream of energy dissipater.

Malfunction of hydro-mechanical equipment such as high leakage, vibration and corrosion of gates.



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Figure 1. Damages in Sefidroud dam outlet

B. Operation of bottom outlets

Some of the problems associated with operation of bottom outlets derive from inappropriate design and lack of suitable inspection and maintenance. Since bottom outlets are not constantly used, it is required to undertake bottom outlets operation at least once a year so as to get assured of suitable operational conditions of different components of bottom outlets. With regard to the problems and issues observed during operation, compliance with the following factors are necessary to get assured of suitable function of bottom outlets:

Free flow conduit downstream of control room must have a uniform and smooth surface; due to high velocity, any misalignment can cause cavitation.

Air vents must be inspected to make sure that there is no blockage for air conduit.

Downstream stilling basins must be cleansed from debris that might cause erosion.

Intake structure must not be blocked by debris.

Conduit upstream and downstream of service gates must be inspected.

Mechanical equipment must be inspected.

IV. MEASURES FOR SUCCESSFUL DESIGN

A. Intake structure

For intake structure design, general hydraulic, structural and geotechnical considerations must be included. Foundation strength especially for earthquake loads is important. Deterioration of concrete material leading to weakness of structure must be considered. Other measures can be as follows:

Providing debris control.

Providing diver friendly design for example designing overhead crane.

Considering 50% clogging of trashracks for hydraulic calculations.

Considering vibration and fatigue for design of trashrack or trashstrut bars.

If possible, designing bulkhead gate for inspection purposes and emergency situation.

As an example, intake structure of Nazloo dam bottom [4] outlet is shown in Fig. 2. The characteristics of the dam are presented in Table I. As it is shown in Fig. 2, the entrance after the trashrack is designed with bell mouth shape to minimize the head loss of entrance. Also separation of the flow from surface would be avoided. Also a bulkhead gate is designed to block the flow to the conduit so that inspection of conduit would be possible in emergency cases. The bulkhead gate is controlled from dam crest level.

Figure 2. Intake structure of Nazloo dam bottom outlet



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TABLE I. NAZLOO DAM AND ITS BOTTOM OUTLET CHARACTERISTICS

Nazloo dam characteristics

Location North West of Iran

Dam type Earthfil with clay core

Dam height 105 m

Reservoir volume 168 million cubic metres

Service gate dimensions 1.8 m × 2 m (width × height)

Emergency gate dimensions 1.8 m × 2.2 m (width × height)

Discharge capacity 109 m3/s

Velocity in tunnel 9.3 m/s

Velocity in steel pipe 29.5 m/s

Reservoir evacuation time 36 days

B. Conveyance system and control structure

Conveyance system usually consists of a conduit that is placed through dam body, at foundation level on abutments or dam body, and diversion tunnels on abutments. For concrete dams, conduit usually is placed in

dam body and for embankments dams are usually constructed as cast in place concrete structure or manufactured pipes with concrete cover. Size of the conveyance system is based on maximum release and inspectability and serviceability must be taken into account during design. For embankment dams it is important to have piping protection around conduits and for clay core embankment dams it is important to have free surface flow in the conduit downstream of clay core or grout curtain to avoid seepage into embankment. Other measures can be as follows:

Selecting appropriate conduit material. Providing measures to prevent seepage into

embankment Designing adequate transitions in the conduit to

reduce head loss and avoid separation of flow from surface

Gate chamber and conveyance system of Nazloo dam bottom outlet [4] are shown in Fig. 3. In this case, the gate chamber is designed in the middle of the conduit so that after the gates there is no pressurized flow in the conduit. As shown, it is important to have emergency guard facilities for the service gates.

Figure 3. Longitudinal profile of Nazloo dam bottom outlet



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Figure 4. Sections of outlet structure of Nazloo dam bottom outlet

C. Outlet structure

Outlet structure are constructed downstream end of bottom outlets to dissipate devastating energy of high velocity outflows. The degree of the need for energy dissipation and erosion protection depends on velocity of outflow, strength of foundation and frequency of bottom outlet operation.

In designing the outlet structure, flow condition in downstream flow has an important role. The flow condition in downstream river has uncertainties. The most important uncertainty is the Manning’s roughness coefficient. The

coefficient cannot be calculated perfectly and it might change over time. In order to cover all flow conditions in design of the outlet structure, upper and lower ranges for Manning’s coefficient discharge must be considered. For

calculation of outlet structure apron, the lower range can be considered and for calculation of level of outlet structure walls, upper range can be considered to avoid submergence of the structure and damages.

An example of the outlet structure of Nazloo dam bottom outlet [4] is shown in Fig. 4. In this project, the outlet structure of river diversion tunnel is actually used for outlet structure and energy dissipation of bottom outlet outflow. A transition from conduit outlet to the stilling basin is designed to avoid separation of the flow from surface.

D. Cavitation control

Formation of void in a flow is defined as cavitation. Local pressure lower than vapor pressure causes cavitation. Local pressure reduction along flow stream can be caused

by sudden change in elevation, local increase in velocity, turbulence, vortices and separation. Cavitation damage can be controlled by geometry modification and aeration. Steps in cavitation control are as follows:

Calculation of water surface profiles.

Computing pressure head.

Cavitation index Computation.

Improving geometry of the structure where cavitation is predicted.

If cavitation still predicted, the flow has to be aerated.

Modifying structure because of the increase of air-water depth.

V. RISK MANAGEMENT

Failure of bottom outlet could discharge reservoir water into downstream river. Possible scenarios should be considered for failure of bottom outlets. These could be failure of outlet gates to open or complete removal of gates causing uncontrolled flow release from the reservoir. In some cases failure of bottom outlet has led to sever damages downstream of dams like Massingir Dam [5] in Mozambique in May 2008 that caused huge structural damages to outlet structures (Fig. 5).



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Figure 5. Damages to outlet structure of Massingir Dam

Also bottom outlet failures can result in consequences like:

Inability to regulate required release of water.

Mechanical malfunction that might need evacuation of reservoir.

Uncontrolled release of water.

Most problems are related to high velocity condition such as erosion, vibration damages, cavitation and inadequate energy dissipation. Failure modes include foundation, structural and mechanical deterioration and failure; loss of power; hydraulic related failures; and operator error. Examples for failure modes of intake structure could be foundation failure, deterioration of concrete because of aging and exposure to water with unusual PH, cavitation, blockage of trahsracks or trashstruts, and blockage by sediment accumulation. Conveyance system and control structure failure modes could be erosion of embankment into the conveyance system, piping along conduit interface, differential settlement of conduit, leakage from conduit into embankment, failure to open or close control devices, corrosion and deterioration of conduit, and loss of power. For outlet structure most common mode of failure is uncontrolled erosion of exit channel.

IV. CONCLUSION

The role of dam bottom outlet is to provide efficient and flexible operation of the dam and reservoir. Measures for success in design of dam bottom outlet can be summarized as having smooth flow for maximum discharge, effective energy dissipation at outlet structure, no leakage, simple application, easy access, economic design and long life. Where the surrounding area of bottom outlet conduit is not saturated like downstream of dam impervious core or grout curtain, free surface flow must be designed to avoid leakage and associated risks. Flow condition in downstream river has important role in outlet structure of bottom outlet and uncertainty of the flow condition in river must be taken into account for deign of outlet structures. For risk management, failure modes of different part of the system must be taken into account to ensure reliable system throughout its economic life. All these measure are important and crucial in successful design of bottom outlets of dams.

ACKNOWLEDGMENT

The author wishes to thank Absaran Consulting Engineers for the opportunity to prepare and present this paper and for the input provided by the company reports.

REFERENCES

[1] W. Kipping,” Valves for bottom outlets of dams,” Proc. Workshop on Modern Techniques for Dams-Financing, Construction, Operation, Risk Assessment, vol. 1, GCOLD, Dresen, September 2001, pp. 241-253.

[2] P. H. Burgi, R. B. Jansen, R. P. Regan , E. T. Scherich, E. K. Scherade, “Outlet performance and remedial measures,” in Advanced dam engineering for design, construction, and rehabilitation, R. B. Jansen, Ed. New York: Van Nostrand Reinhold, 1988,ch. 22, pp 704-721.

[3] R. Kolachian, A. Abbaspour and F. Salmasi, “Aeration in Bottom Outlet Conduits of Dams for Prevention of Cavitatio” Journal of Civil Engineering and Urbanism, vol. 2, no. 5, pp 196-201. October 2012.

[4] Absaran Consulting Engineers, “ Hydraulic design report of Nazloo dam,” Absaran:Tehran, 2008.

[5] C. Seddon,”The cause of failure of the Massingir,” Civil Engineering, South African Institution of Civil Engineering, vol 13, no. 6, pp. 30-37, July 2010.



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AUTHOR’S PROFILE

Pouya Amirsayafi is a Senior Dam Engineer with 13 years of experience as project manager and design engineer. He holds master’s

degree in Civil Engineering and over his career successfully completed numerous projects; including planning and feasibility studies, project optimization, detailed design of new projects, upgrading of existing structures and support during construction. He currently studies Master of Engineering Science – Project Management; School of Civil and Environmental Engineering, University of New South Wales, Sydney, Australia.



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