DESIGN OF SURPLUS WEIR

Types of Weirs:

Weirs are classified according to:

1. Types of Weirs based on Shape of the Opening

•Rectangular weir

•Triangular weir

•Trapezoidal weir

2. Types of Weirs based on Shape of the Crest

•Sharp-crested weir

•Broad- crested weir

•Narrow-crested weir

•Ogee-shaped weir

3. Types of weirs based on Effect of the sides on the

emerging nappe

•Weir with end contraction (contracted weir)

•Weir without end contraction (suppressed weir)

Classification Based on Shape of Opening

Rectangular weir:•It is a standard shape of weir. The top edge of weir may be sharp crested or narrow crested.

•It is generally suitable for larger flowing channels.

Triangular weir:•The shape of the weir is actually reverse triangle like V. so, it is also

called V-notch weir.

•This type of weirs are well suitable for measuring discharge over

small flows with greater accuracy.

Trapezoidal weir:

•Trapezoidal weir is also called as Cippoletti weir. This is

trapezoidal in shape and is the modification of rectangular

weir with slightly higher capacity for same crest strength.

•The sides are inclined outwards with a slope 1:4

(horizontal : vertical)

Classification according to shape of the crest:

Sharp-crested weir

•The crest of the weir is very sharp such that the water will

springs clear of the crest.

•The weir plate is bevelled at the crest edges to obtain

necessary thickness. And weir plate should be made of

smooth metal which is free from rust and nicks.

•Flow over sharp-crested weir is similar as rectangular weir.

Broad-crested weir:

•These are constructed only in rectangular shape and are

suitable for the larger flows.

•Head loss will be small in case of broad crested weir.

Narrow-crested weir:

•It is similar to rectangular weir with narrow shaped crest

at the top.

•The discharge over narrow crested weir is similar to

discharge over rectangular weir.

Ogee-shaped weir:

•Generally ogee shaped weirs are provided for the

spillway of a storage dam.

•The crest of the ogee weir is slightly rises and falls into

parabolic form.

•Flow over ogee weir is also similar to flow over

rectangular weir.

DESIGN STEPS

• 1. Hydraulic Design

• 2. Structural Design

SURPLUS WEIR

• The surplus work of a reservoir or tank is usually a Weir.

• The length of such a weir must be such that the quantity of waterestimated as the maximum flood discharge likely to enter from thecatchment into the tank can be disposed off with a depth of waterover the weir equal to the difference between the Maximum WaterLevel (MWL) and Full Reservoir Level (FRL ) / Full Tank Level (FTL).

• Generally it is better to limit the difference between the MWL and FRLas small a height as possible. This difference is called the “Head Overthe Surplus Weir” with which the flood can be disposed off.

DESIGN STEPS

Hydraulic Design

• 1. Computation of maximum flood discharge.

• 2. Computation of length of weir based on type of weir.

• 3. Computation of downstream water level (TWL)

• 4. Computation of apron floor level

• 5. Computation of apron floor thickness (based on Khosla’s theory)

DESIGN STEPS

Structural Design

• 1. Design of body wall of weir.

• 2. Design of Abutment.

• 3. Design of Upstream Wing & Return

• 4. Design of Upstream Wing & Return

NEED OF FLOOD ESTIMATION

TO DESIGN THE WATER RESOURCE STRUCTURE SUCH AS DAM, SPILLWAYS, DIVERSION WORKS, BRIDGE ETC.

TO DETERMINE THE MAXIMUM DISCHARGE AND MAXIMUM POTENTIAL.

ESTIMATE COST AND SIZE OF STRUCTURE.

EMPIRICAL METHOD

• IT SHOULD BE EMPLOYED ONLY WHEN THERE IS INSUFFICIENT AVAILABLE HYDROLOGIC INFORMATION FOR PERFORMING THE DETAILED AND PRECISE ANALYSIS.

• THE VARIOUS EMPIRICAL FORMULAE COMMONLY USED IN INDIA ARE

(a) DICKEN’S FORMULA

(b) RYVE’S FORMULA

(c) MODIFIED REGIONAL FLOOD FORMULAE

(a)DICKEN’S FORMULA

• DICKEN (1885) MADE THE FIRST ATTEMPT IN INDIA TO DERIEVE A GENERAL FORMULA FOR DETERMINING THE MAXIMUM FLOOD ON THE BASIS OF STUDIES CONDUCTED FOR DETERMINING THE RELATION BETWEEN DISCHARGE RATE TO DRAINAGE AREA.

• Q=C A(3/4)

where,

Q=PEAK FLOW RATE

C,(3/4)=REGRESSION CONSTANT

A=AREA OF DRAINAGE(km2)

(b)RYVE’S FORMULA

• RYVE’S MODIFIED DICKEN’S FORMULA TO SUIT SOUTH INDIAN CONDITIONS FOR AREAS WITHIN 25 km FROM THE COAST, BETWEEN 25 TO 175 km FROM THE COAST, AND FOR LIMITED AREAS NEAR THE HILL

• Q=C A 2/3

• THE VALUE OF CONSTANT VARIES WIDELY BEING 6.8 IN FLAT TRACTS ALONG THE COAST AND ABOUT 42.4 IN THE WESTERN GHAT REGION.

(c) MODIFIED REGIONAL FLOOD FORMULAE

• KUMAR ET AL. (1999) DEVELOPED A METHODOLOGY WHICH MODIFY THE FORM OF THE EMPIRICAL FORMULA.

• QT=CT Ab

CT = REGIONAL PARAMETERS

QT =FLOOD OF T-YEAR RETURN PERIOD

A=CACHMENT AREA

b = TO BE ESTIMATED FOR REGION USING REGRESSION.

RATIONAL FORMULA

• IT IS MOST WIDELY USED FOR THE ANALYSIS OF RUNOFF RESPONSE FROM SMALL CATCHMENT

• THIS METHOD TAKE INTO ACCOUNT THE FOLLOWING HYDROLOGICAL CHRACTERISTICS OR PROCESSES

(a) RAINFALL INTENSITY

(b) RAINFALL DURATION

(c) RAINFALL FREQUENCY

(d) CATCHMENT AREA

(e) HYDROLOGIC ABSTRACTIONS

(f) RUNOFF CONCENTRATION

(g) RUNOFF DIFFUSION

THE REQUIREMENT OF THE RATIONAL METHOD IS THAT THE CACHMENT BE SMALL . FOLLOWING STEPS ARE INVOLVED IN THIS METHOD

i. DETERMINE THE TIME OF CONCENTRATION

ii. OBTAIN THE RAINFALL INTENSITY FOR THE DESIRED DURATION AND FREQUENCY FROM THE APPROPRIATE INTENSITY-DURATION-FREQUENCY CURVE.

iii. RUNOFF COEFFICIENT DEPENDS ON THE CONDITION.

Qp= C I A

FLOOD FREQUENCY ANALYSIS

• FLOOD FREQUENCY DENOTES THE LIKELIHOOD OF FLOOD BEING EQUALLED OR EXCEEDED.

• THE RETURN PERIOD (Tr)OF ALL FLOODS (Q1,Q2,…….QN) ARE CALCULATED.

• THE CURVE BETWEEN Q V/S Tr IS PLOTTED .

• THE CURVE CAN BE USED TO DETERMINE THE RETURN PERIOD FOR THE CORRESPONDING MAGNITUDE OF FLOOD.

UNIT HYDROGRAPH APPROACH

• UNIT HYDRO GRAPH WAS PROPOSED BY L.K. SHERMAN.

• UNIT HYDROGRAPH IS DEFINED AS THE HYDROGRAPH OF SURFACE RUNOFF OF A CACHMENT AREA RESULTING FROM UNIT DEPTH OF RAINFALL EXCESS.

• UNIT HYDROGRAPH IS A LINEAR MODEL OF THE CACHMENT .

• ASSUMPTIONS IN UNIT HYDROGRAPH METHOD

(i) RAINFALL IS OF UNIFORM INTENSITY WITHIN ITS SPECIFIED DURATION.

(ii)THE EFFECTIVE RAINFALL IS UNIFORMLY DISTRIBUTED THROUGHOUT THE AREA OF DRAINAGE BASIN.

S.No. Type Of Canal C.A. in upland Acres. CA in deltaic Tracts

1. MAIN CANAL Where Q=CM3/4

C=1400 for CA-1 Sq. Mile.C=1200 for CA between 1 to 30 Sq. Miles.

Q=CM2/3

Value of C=1000Velocities in vents restricted to 10 ft/sec

C=1060 for CA 30 to 500 Sq. Miles.For CA more than 500 Sq. MilesQ=7000 √M

Velocity to be allowed in vents upto 12’ to 13’ / sec.depending on stream bed strata.

2. BRANCH CANALSQ > 500 c/s

RYVE’S FORMULA Q= CM2/3

Where C=1000 With Velocity in Vents upto only10’/sec.

Same as forupland areas.

3. DISTRIBUTARIESQ < 500 c/s

RYVE’S FORMULA Q= CM2/3

Where C=750 Velocity upto only10’/sec.

Same as forupland areas.

FLOOD ESTIMATION• If the tank is an independent one, then the flood discharge can be easily estimated

using Ryve’s formula Q=C A 2/3

• In MKS system, the value of C varies from 6.8 to 15 depending upon the topographyof the catchment and the intensity of rainfall over the catchment.

• If the tank is not an independent one, but forms part of a chain of tanks, the flooddischarge likely to enter such a tank is calculated by using a modified formula givenby

• Q=C A 2/3 - ca 2/3

• Where Q is the estimated flood discharge in cubic metres / second that is likely toenter the tank.

• A is the combined catchment area of all the tanks above the surplus of the tankunder consideration in Square Kilometers.

• a is the intercepted catchment area is Square kilometres by the upper tanks.

• c is the modified coefficient which generally varies from 1/5 to 1/3 of C.

COMPUTING LENGTH OF WEIR

• Having calculated the flood discharge and also having fixed the FRLand MWL with reference to the storage capacity requirements, thelength of surplus weir is calculated by the following formula:

• (free discharge with out taking into account the head due to velocityof approach)

• Q = 2/3 Cd LH sqrt (2gH)

• = 2.95 cd L H 3/2

• Where L = Length of Surplus weir in metres.

• H = Head over the weir (MWL – FRL) in m

• cd = Coefficient of discharge

COMPUTING LENGTH OF WEIR

• cd = Coefficient of discharge depends on the type of Weir

For broad crest weir,

Q = 1.7LH 3/2

• For high coefficient Weir, Q=2.2 LH 3/2

COMPUTING LENGTH OF WEIR

• Problem:

• Compute the maximum flood discharge and length of broad crest weirwith the following data:

Combined catchment area of group of tanks: 25.89 sqkm

Intercepted catchment area of upper tanks : 20.71 sqkm

C value : 9 c value:1.5

• FTL of tank: +12.00 m

• Submersion of foreshore land is limited to +12.75 m.

Hydraulic JumpThe supercritical flow Froude number influences the characteristics of the hydraulic jump. Bradley and Peterka , after extensive experimental investigations, have classified the hydraulic jump into five categories as shown below. The hydraulic jump is the phenomenon that occurs where there is an abrupt transition from supercritical (inertia dominated) flow to sub critical (gravity dominated) flow.

The most important factor that affects the hydraulic jump is the initial Froude number F1. 1 1 V F gD = in which V1 is the longitudinal average velocity at the initial section, g is the acceleration due to gravity and D is the hydraulic mean depth (ratio of area of flow at free surface width).

Type of Jump Froude Number Remarks Critical flow F 1 =1 Wavy surface ,

celerity c =sqrt(gy)Undular jump 1 < F1<1.7 Undulations on the

surface Weak jump 1.7 < F1 <2.5 Small rollers, No

baffles. Oscillating Jump 2.5 < F1< 4.5 No periodicity. Rip

rap may get damaged. Canal drops, difficult to handle.

Steady jump 4.5 < F1< 9.0 Position, is sensitive to variation of Tail Water, Efficiency is 45 to 70 %.

Strong Jump F1 > 9.0 Efficiency is 85 %