HYDRAULIC STRUCTURES 

  ASSIGNMENT 1 

SPILLWAY 

GROUP MEMBERS:

ZAHIR B IBRAHIM 2010482426NOORFARHAH MD DOM 2009659748

PREPARED FOR: ASSOC. PROF. HAMIDON AHMAD

Introduction

• Spillway is a passage in a dam through which the design flood could be disposed off safely to the downstream.

•In general, spillways comprise five distinct components namely:

(i) An entrance channel,(ii) A control structure, (iii) A discharge carrier,(iv) An energy dissipator, and (v) An outlet channel.

• Spillways are provided for storage and detention dams to release surplus water or floodwater that cannot be contained in the allotted storage space, and for diversion dams to bypass flows exceeding those turned into the diversion system.

A small spillway

Chute spillway 

• A spillway, 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 spillway.

• a spillway having a chute-type discharge channel, though controlled by an overflow crest, a gated orifice, a side channel crest.

Chute spillway 

• The chute spillway has been used more often with earthfill dams than with 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.– the overall economy often obtained by the use of

large amounts of spillway excavation in the dam embankment.

Chute spillway 

• Chute spillways ordinarily consist of– an entrance channel.– a control structure– a discharge channel– a terminal structure– an outlet channel.

Advantages• economical for both a high q and a high overfall height,

but if only one feature is necessary other structures are more appropriate

• usually built as a monolithic reinforced concrete structure

Disadvantages• skilled labor required • need good soil conditions for foundation; still, the life

expectancy is less than for the other structures

Labyrinth Spillways

• The concept behind the labyrinth spillway is to provide added crest length for a given total spillway width, so that less head is required to pass a given discharge.

• A labyrinth design is particularly beneficial when the spillway width is fixed, upstream water surface elevations are restricted, and large discharges must be passed.

Labyrinth Spillways

• Labyrinths have also been used as control or diversion structures on canals.

• Storage capacity can also be increased because the labyrinth crest can be set at a higher elevation than a straight crest while still passing the required discharge.

Lake Brazos Dam

Labyrinth Spillways

• The primary parameters affecting flow patterns and, thus, spillway performance are– the length magnification– crest length per cycle width– the discharge and head over the spillway– the angle of the spillway side walls with respect to

the flow– the ratio of the spillway cycle width to the spillway

height.

Labyrinth Spillways

• Advantages Labyrinth– Labyrinth spillways can be an economical solution

for increasing spillway capacity as they provide increased unit discharge over conventional weirs for a given head.

– Cost savings may be realized during initial construction and in future operation and maintenance costs.

– suitable for use anywhere an overflow structure is required depending upon the site conditions.

Labyrinth Spillways

• Disadvantages– This design was however not much developed for

two reasons:-• The shape was not optimized hydraulically and is

expensive structurally for large discharges• They cannot be placed upon gravity dams cross

sections, i.e. upon most spillways.

Stepped or Cascade Spillways 

• Stepped channels and Spillways are used since more than 3000 years. Stepped spillway is generally a modification on the downstream face of a standard profile for an uncontrolled ogee spillway.

• Stepped spillway was quite common in the 19th century and present practice is confined to simple geometries (e.g. flat horizontal steps in prismatic chutes)

Stepped or Cascade Spillways 

• Generally, stepped channel geometry is used in channels with small - slope: – for river training.– in sewers and storm waterways and channels

downstream of bottom outlets,– launder of chemical processing plants, – waste waterways of treatment plants – step –pool streams.

The hydraulic features of the cascade spillway as compared to chute flow are:

• The flow depth is much larger than in a chute due to the highly turbulent cascade flow, and higher sidewalls are required.

• More air is entrained and the spray action may become an important issue.

• Abrasion can be a serious problem for flows with sediment or with floating debris.

• Napped flow: is the flow from each step hits the next step as a falling jet;

• Skimming flow: the flow remains coherent over the individual steps.

Stepped spillway

• Advantages– is the higher energy dissipation along the chute compared with

conventional, smooth chutes. The energy dissipation is different if the flow is aerated or not.

– they can be placed over the top of an existing embankment without causing significant changes to the dam or spillway dimensions,

– they provide considerable energy dissipation in the chute, potentially reducing the size of the stilling basin

– they permit shorter, more efficient, and feasible construction schedules than other design options.

• Disadvantages– The only disadvantage with stepped spillway is that at large

discharges, as the jet is not aerated for some distance downstream of the spillway, low pressure may occur and lead to cavitations damage.

Morning Glory Spillway

• Most of the shaft spillways in the world were constructed in United States, Portugal and Italy in earlier days

• As the inlet funnel resembles the morning glory flower, the spillway is termed as morning glory spillway

• In general, a morning glory shaft spillway consists of an inlet funnel, conical transition shaft, bend, outlet tunnel and a stilling basin.

Definition Sketch of Morning Glory Shaft Spillway

• Advantages of Shaft Spillway– the large carrying capacity and the feasibility of constructing thespillway after the main constructionhas been completed, for instance, in the event it

becomes necessary to expand the spillway facilities. - – Water is not passing over embankment. – This also has the potential advantage of retaining an

appropriate flood wave speed through the flood storage area.

Siphon Spillway

• A spillway with one or more siphons built at or near the crest of the dam.

• This type of spillway is sometimes used for providing automatic surface-level regulation within narrow limits or when considerable discharge capacity is necessary within a short period of time.

• Siphon spillways can be used when large capacities are not required, space is limited and fluctuations of reservoir level must be maintained within close limits.

Max Negative pressure Head

Energy grade Line

HYDRAULIC GRADE LINE

Siphon Spillway

• Disadvantages– Construction is expensive– Sudden appearance of flood water downstream– Large flood debris can block outlet– A the siphon is primed the flow would result excessive

vibrations in the dam body which may cause expansion problems in the joints.

– There is a possibility of cavitations for negative pressures, which is affected by the head between upstream and downstream water levels.

– Repair and maintenance of siphon spillways are difficult.

Bellmouth Siphon Spillway

Ogee (Overflow) Spillways

• The ogee spillway has a control weir that is ogee-shaped (S-shaped) in profile.

• The upper curve of the ogee spillway ordinarily conforms closely to the profile of the lower napped of a ventilated sheet falling from a sharp-crested weir.

• This type of spillway is the most common type adopted in the field. It divides naturally into three zones. Crest spillway face and the toe.

Flow over a thin weir

Crystal Dam in Colorado

Advantages of Overflow SpillwayThe advantages to designing uncontrolled ogee crests based on minimum pressure criteria are as follows:– A smaller structure can adequately pass the design flood when

compared to the standard 75 % "under design" crest.– The spillway crest can be set at a higher elevation and still provide

discharge capacity to maintain the original maximum pool elevation for a given inflow hydrograph.

– An engineer can chose which minimum pressure is acceptable and design the ogee crest accordingly.

– An engineer can determine the discharge capacity of an existing ogee crest for heads up to 5 times the design head.

– An engineer can estimate the cavitations potential of an existing ogee crest for heads up to 5 times the design head.

• Disadvantages of Overflow Spillway– The main disadvantage with the glory-hole

spillway is that beyond a certain surcharge the discharge only increases slowly with increased head. It does not provide any substantial margin for underestimation of the maximum flood.

Spillway System Problems

• Undermining– Undermining is the removal of foundation material

surrounding a conduit system. Any low areas or unexplained settlement of the earthfill in line with the conduit may indicate that undermining has occurred within the embankment.

– As erosion continues, undermining of a conduit can lead to displacement and collapse of the pipe sections and cause sloughing, sliding or other forms of instability in the embankment. As the embankment is weakened, a complete failure of the conduit system and, eventually the dam may occur

Spillway System Problems

• Pipe deformation– Pipe deformations are typically caused by external

loads that are applied on a pipe such as the weight of the embankment or heavy equipment.

– Collapse of the pipe can cause failure of the joints and allow erosion of the supporting fill. This may lead to undermining and settlement. Pipe deformation may reduce or eliminate spillway capacity.

Spillway System Problems

• Misalignment– Excessive misalignment may result in other

problems such as cracks, depressions, slides on the embankment, joint separation and seepage.

– Misalignment can also result from poor construction practices, collapse of deteriorated conduits, decay of organic material in the dam, seismic events or normal settlement due to consolidation of embankment or foundation materials.

Mitigation Measures for Spillway Problems 

• Overall Design and Safety Considerations– Alignment

• For good hydraulic performance, abrupt changes should be avoided. This applies to sudden changes in vertical elevation of the chute floor, abrupt widening or narrowing of the chute, and sharp turns in the chute.

– Settlement and Movement• The concrete sidewalls and floor of the chute must have enough

strength to withstand water loads, soil/fill loads, uplift forces, weathering, and abrasion.

• As necessary, plans and specifications for repair to the spillway should also be promptly developed and implemented by a registered professional engineer.

Mitigation Measures

• Undermining– If the spillway is located in the abutment, erosion and

lowering of the lake level may result. A registered professional engineer should be hired to develop plans and specifications to repair undermining of the chute.

• Cutoff Wall and Endwall– A cutoff wall should be placed at the entrance to the

concrete chute to prevent the flow approaching and entering the chute from flowing beneath and undermining the floor slabs.

Mitigation Measures

• Seepage– The rate and content of flow from weep holes and

relief drains must be monitored and documented regularly.

– Plans and specifications to properly control the seepage and repair the drain(s) and embankment should also be developed

END……