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INTRODUCTION
● Stilling basinsare used to dissipate the energy of water exiting the spillway of a dam
● Their purpose is to prevent scouring that occurs when high velocity water enters the downstream reach of the dam
● This scouring can damage the foundation of the dam, leading to overtopping , and also causes severe erosion downstream
● The primary method of dissipatingenergy is togenerate a hydraulic jump to
transition flow from supercritical to subcritical
● Stilling basins are placed at the ends of dam spillways and at the ends of steep- sloped canal sections where elevationchange has generated high kinetic energy
● The stilling basin is a hydraulic structure located between the outletworks of a dam and the tailwater, to where, should return excess flows safely.
● The stilling basin is a structure in which a hydraulic jump is generated and has been
designed economically in terms of length, tailwater level and scour.
● The selection of a stilling basin depends on
Approach flow conditions,
Tail water characteristics,
Scour potential and
Personal preference
The approach energy head should be between 10 and 30 m, in order that the
performance of the basin is successful.
A number of standard basins are available that have been tested extensively by Peterka
, (1958). Problems with stilling basins can occur for high approach velocity.
Froude number less than 2.5 with asymmetric approach conditions, non linear inflow or outflowor low tail water level
BASIC PRINCIPLE OF STILLING BESIN
A stilling basins are transition structures constructed to dissipate excess energy confined by high velocity flow at the outlet of conduit or tunnel so that the flow beyond the basin does not endanger the stability of bed and banks of downstream channel.
In a stilling basin kinetic energy causes turbulences and it is ultimately lost as heat and sound energy. there are several types of stilling basins which are used in various hydraulic structures like dam, canal, culvert etc. The type of stilling basin most suitable at a particular location mainly depends upon
initial Froude Number and initial velocity of flow.
Stilling Basin Elements
The different types of stillingbasins typically have the following common
elements
1. Chute blocks -
Concrete blocks built into the inclinedsections of the spillway.
These features are commonly placed at the head of the stilling basin to create
Turbulence prior to the hydraulic jump
Generate turbulence in the stillingbasin in order to cause a hydraulic jum p
Not used for flows greater than 20m/ s Flows >20m/s typically cause cavitation behind the blocks, damaging the stilling basin
The force on baffle blocks is:
F=2ɣA(d+hv)
F = force in lbs
ɣ = unit weight of water (lb/ft3)
(d+hv) = specific energy of flow entering basin (ft)
2. Straight Drop Stilling Basins
In straight drop stilling basins , water flows over the spillway crest andfalls onto a flat apron, often with blocks or other energy
Dissipating features
Straight drop basins are typically used for low-flow spillwaysbecause hig h flows can cause the nappe to submerge, reducingenergy
Dissipation and increasing potential for scouring
For intermediate flows , longerbasins are required, with baffle –blocks and sills
, in order to dissipate the energy further
3. USBR Type II Stilling Basins
Developed by the U.S Bureau of Reclamation Utilize s chute blocks and adentated end sill to dissipate energy
Does not utilize baffle blocksChute blocks lift flow and dissipate energy through eddies
Design Recommendations
Froude number of 4 to 14
The height of the chute blocks should be equal to the depth of the incoming flow
The width of the chute blocks should ben equal to the depth of the incoming flow
The spacing between each chute block should be equal to the height of the
incoming flow
A spacing equal to half of the incoming flow height is recommended between the chute blocks and the outside walls
The height of the dent ted sill should be equal to 0.2 time s the depth of incoming flow depth
The maximum width and maximum spacing of the dentated sill is 0.15 time s the incoming flow depth
Length of basin is dependent on Froude number and incoming flow depth
4. USBR Type III Stilling Basins
Developed by the U.S Bureau of Rec la mation
Utilize s chute b locks, baffle blocks, and an end sill to dissipate energy
Creates a steep hydraulic jump with minimam wave action downstream
The position of the baffle blocks are key in the success of type II stilling basins
Design Recommendations
Froude number of 4.5 to 17
Maximum velocity of 60 ft/ s
Maximum unit discharge of 200 ft^3/ s-ft
The height, spacing , and width of the chute blocks should be equal to the depth of incoming flow
The height and spacing of the bafflepiers are relative to the Froude number and depth of incoming flow
The distance from the back of the chute blocks to the front of the baffle piers should0.8 time s the the depth of incoming flow
The height of the end s ill is determine d by the Froude number and the depth of
incoming flow
Length of bas in is dependent on Froude number and incoming flow depth
The tailwater depth must be equal to or greater than full conjugate depth
1. Saint Anthony Falls (SAF)
Stilling Basin
Developed by model studies conducted by the Soil Conservation Service at
the St. Anthony Falls Hydraulic Laboratory of the University of Minnesota
Utilize s chute blocks, an end sill and either baffle or floor blocks
General used for smaller structures, such a s culverts or canals , to implement a more cost effective approach to energy dissipation
Effective for Froude numbers between 1.7 a nd 17
Failure Mechanisms
Stilling Basin Sweepout Tailwater is insufficient and does not allow hydraulic jump to form and stabilize in the stilling bas in which results in any or all of these failure s Eros ion in downstream channel Headcutting and progressive failure up spillway chute Eros ion of toe of embankment dam Up lift pressure causing the stilling bas into float Erosion of stilling bas in foundation Ultimate failure of the stilling basin If any of these mechanisms result, the stilling bas in needs modification a s it is not designed for the correct capacity
Ball Milling
Crushed material (sands, gravels , and cobbles) are drawn into stilling basin from downstreamand trapped in hydraulic jump
Trapped material erodes stilling basin concrete through grinding process
Over time , stilling basin foundation is undermined, headcutting progresses upstream and can eventually cause reservoir breach
Unlikely to cause complete failure because ball milling takes a very long time
and regular inspections will repair damagecaused
Case Study - El Guapo Dam
● Date : December 16, 1999
● Location: Rio Gua po Ba s in - 3 mile s South of El Gua po, Venezue la
● Events Progress ion
Water level of dam reaches 8 in. below dam crest
Overtopping of spillway crest occurs
Overtopping results in stilling basin sweepout
Headcutting progresses upstream of stilling basin as overtoppingover
spillway causes erosion of spillway backfill
Reinforced concrete chute, stilling basin, and approach channel all fail between 4:30 and 5:00 pm
The combination of these events results in breach of reservoir and ultimate failure of the dam
1) Stilling basin sweepout initiates headcutting upstream
2) Spillwayovertoppinginitiateserosionofspillwaybackfill
3) Headcuttingprogressestobreach reservoir
What went wrong
Initialhydrologicstudiesbasedon similarbasinbutnotRioGuapoBasin
During construction, spillway chute walls were overtopped which prompted
addition hydrologic studies and addition of a tunnel spillway
Basing designcalculationsoninaccuratedataresultedinastillingbasin
and spillway system that could not handle the volume of water necessary
CONCLUSION
It is conclude that several types of stilling basinsused in water resources projects.
design of none of these energy dissipators have been standardized and most
of them are larger in length. Hence, In the light of present knowledge of the energy dissipation, there is an
ample scope for evolving a shorter and simple design of an effective stilling basin based on systematic, rigorous and extensive experimental study. The new designs of shorter basins would save cost of construction of these basins to a great extent.