Help to plan of Barrages

1. Steps for planning,
layout, design,
construction and
operation of barrages
The successful working of a barrage depends on a proper selection of the
location, alignment, layout, design and operation of the structure. Hence, the
following aspects have to be carefully looked in Barrage working
•Site investigation and data collection ;
This consists of :-
1- Study of availablemaps and satelliteimageries,
2- Regional and site geology ,
3- Study of foundation strata ,
4- Study of available hydrological data ,
5- Assessment of water needed for diversion ,
6- Effect of the barrageon environment ,

2. 7- Limitationson water withdrawal ,
8- Availability of construction material ,
9- Communication to thesite of work ,
10- Detailed topographical survey ,
11- Hydro-meteorological data ,
12- Sediment concentration data ,
13-Pond survey ,
14- Study of navigation and fish ,
15- Study for power generation , and
16- Study for provision of a rail or a road bridgeacross the barrage.
•Location and alignment selection of the barrage axis ;
The location for a barrageshould be decided on considerationsof suitability for
the
main structure(barrage) and itsappurtenant works, likesilt removing devices
and
intakefor canals(also called canal head regulators). An ideal location would
be that which satisfies the requirementsof all these three components. Some of
the pointsthat have to be kept in mind in selecting an appropriatelocation for a
barrageare as follows:
1- The canal head regulators(or head-works, as they are called) must planned to

3. be suitableto divert water to a canal for irrigation , by achievea barrageof
reasonableheight. The combined cost of construction of head-worksand that of
the canal fromthe barrageup to thepoint where the water is first used for
irrigation should besmall.
2- The favorablelocation for fixing thesitefor a barrageand canal head- works
may have to be selected due to largequantitiesof rock excavation required.
3- The river reach at theproposed location should be straight, asfar as possible,
so that velocitiesmay be uniformand thesectional area of theriver fairly
constant.
•Planning, layout of the barrage and its
appurtenant structures ;
The location and alignment of thebarrageaxis and that of thecanal head works
may be decided but the other details, likethe width of the barrageand
regulator,
levels crests, lengthsof floors, river training works, pond level , etc. have to be
finalized based on the hydraulic conditionsand geologic characteristicsof the
river

4. bed and banks of the site. The major
planning aspects are as follows:
1- Design flood ;
The diversion structurehasto be designed in such a way that it
may be able to pass a high flood of sufficient magnitude(called thedesign
flood) safely. It is assumed that when the design flood passes the structureall
the gates of thestructurearefully open and it acts like a weir across the river
with only the obstruction of thepiers between the abutments.

5. 2- Afflux ;
The structurewould cause a rise in the water level on the upstream
compared to level in the downstreamat the timeof passage of a high flood
(equal to or morethan thedesign flood) with all thegates open. This risein
water level on theupstream is called afflux.
3- Free Board ;
When the permissibleafflux is decided, the necessary upstream,
sufficient FreeBoard has to be provided so that there is no overtopping of the
water over the barrageand banks . The Free Board to be provided depends on
the importanceof the structuregenerally, 1.5 to 2 m Free Board abovethe

6. afflux water level on theupstream is provided.
4- Pond Level ;
Pond level is the level of water, immediately upstreamof the
barrage, which is required to divert a water into the main irrigation canal with
its full supply. The pond level is measured by adding the energy losses
through thehead regulator of canal to the full supply level of thecanal at its
starting point just downstreamof the head regulator .
5- Waterway ;
The waterway, is the clear opening of a barrageto allowflood
flowto pass has a bearing on the afflux. Hence, a maximumlimit placed on
the afflux also limitstheminimumwaterway. Many a times, theLacey’s
stable perimeter for the highest flood dischargeis taken as the basis of

7. calculating thewaterway , where :-
P = 4.83 Q1/2
Q is the design flood dischargein m3/s for the 50 year frequency flood.
6- River training works ;
This works needs to , guide the river to flow axially
through thebarrage, Proper alignment of guidebunds is essential to ensure
satisfactory flowconditionsin thevicinity of thebarrage. In case of wide
alluvial banks, the length and curvatureat the head of theguide bunds should
be kept such that the worst meander loop is kept away from either the canal
embankment or the approach embankment.

8. 7- Crest levels of spillway ;
The bays of a barragearein the shape of weirs or
spillwaysand the crest levels of these have to be decided correctly. Someof
the bays towardsthe canal end of thebarrageare provided with lower crest
(Figure12) in order to:
• Maintain a clear and well defined river channel towardsthe canal head
regulator .

9. • Enablethe canal to drawsilt free water from surface only as much as
possible .
• Scour thesilt deposited in front of the head regulator .
8- Spillway bays ;
This is the main body of the barragefor controlling the
dischargesand to raisethe water level to the desired valueto feed the canals.
It is a reinforced concretestructuredesigned as a raft foundation supporting
the weight of thegates, piers and the bridgeaboveto prevent sinking into the
sandy river bed foundation. The Figuresbelowillustratea typical featureand

10. section of a spillway bay .

11. 9- Cut-off ;
Cut-offs are barriers provided below the floor of the
barrage both at
the upstream and the downstream ends. They may be in
the form of concrete
lungs or steel sheet-piles, The purpose of providing
cutoff is to increases the
flow path and reduces the uplift pressure, that resulting
due to the differential
pressurehead between upstream and downstream
which cause uplift of river
bed particles against the barrage floor , thus the cut-off
ensuring stability to the structure .

12. 10- Silt excluding devices ;
The silt excluding tunnelscarry heavy silt down
the river belowthe under sluices. It is a thin, vertical, curved parallel walled
structuresconstructed of plain or reinforced concrete, it is provided as a
part of the under sluice bays of thebarragefloor in the river pocket adjacent to
the head regulator to minimizesediment entry into thecanal through thehead
regulator. Assuch, theexcludershave to deal with alluvial materialssuch as
boulders, gravel, and sand or silt depending upon theparent bed material and
that which is being transported by theriver.

13. 11- Canal Head Regulator ;
The water that enters acanal is regulated
through a HeadRegulator. A typical cross sectionthrougharegulator is
shown in Figure below. As it is desirable toexclude silt as muchas possible
from the head regulator, the axis of the head regulator is laidat an angle from

14. 90oto 110otothe barrage axis