INTRO & METHODS
Saline or Brackish water = 97.3%, Fresh = 2.7% , Total Water = 1.386 billion km³
Fresh water → Liquid form = 30%, Polar ice/glaciers = 70 %
Fresh water in india = 4% of the world's water resource
Sea water contain 80% of oxygen in freshwater stream
Crop yield → Quintel/ha or tonnes/ha
1 quintal = 100 kg, 1 ton = 1000kg
Productivity → Crop yield/mm of water applied
if water increases → Yield 1st increases then decreases
Tile drainage → Runs free gravity water that isn't directly available to the plants
Irrigable Land = 35% of Agriculture land ( world bank 2010)
Bhandara Irrigation scheme → Q=1.7LH1.5 m3/sec
Conjunctive use = Surface + Groundwater use
Consumptive use = Evaporation + Transpiration = Evapotranspiration
Perennial system → Constant + Continuous supply of water throughout crop period
Minimum depth of water required for safe and economical navigation = 2.70 meters
Surface drain → Design for storm run-off and surface flow resulting from excess irrigation
Type Of Irrigation Project.
Major > 10,000ha & > 5cr
Medium = 2k - 10k ha & 0.5 - 5cr
Small/minor < 2k ha & < 0.5cr
Types of irrigation
Mega is not a major irrigation method according to NRCS
Tile Drainage → Removes the free gravity water that is not directly available to the plants
Fertigation → process in drip irrigation to decrease evaporation
Sprinkler & drip are pressurised irrigation systems
i. Surface irrigation
Best for soil with low to moderate infiltration capacities.
in India 75% irrigation by Surface irrigation
Flow irrigation
By gravity
Ex → Flood/uncontrolled, perennial, canal irrigation, reservoir or tank irrigation, combined storage and diversion irrigation
Flood irrigation → Soil kept submerged
Direct irrigation → River into main canal directly
Lift irrigation
By mechanical or manual means. ex well irrigation
ii. Sub-Surface (Capillarity) irrigation
Suitable for highly permeable soil
Has high efficiency
Methods of irrigation
Free/wild/uncontrolled
Rolling land (irregular Topography), Steep land
No control on flow by means of levees
Low efficiency
Inundation/Diversion
Border flooding
Land divided into no. of strips
Time required to cover an area → t=2.303.(y/f).log(Q/(Q-fA))
Max area that can be irrigated → Amax=Q/f
f = Infiltration rate, y = depth, t = irrigation time require
Width = 10 - 20 m, Strip length = 100m- 300 m
Check flooding/Check basin flooding
Close growing crops-jowar paddy
Large Q ∴ for both permeable & less permeable soil
Water controlled by surrounding the area with low & flat levees
Cereal crops → Wheat, oat, barely
Basin flooding
Orchard trees, Special type of check flooding
Furrow or Corrugation
Crops are grown on ridges running on the side of the ditches
⅕ to ½ land surface is wetted by water
Aloo ki kheti yaad kar bas wahi hai
Depth of furrow = 20 -30 cm
Min furrow grade = 0.5%(1/2) to assure surface drainage
Raw crops → Sugarcane, sugar beet, groundnut, potatoes, tobacco
Furrow slope → Surface drainage ≥ 0.05% & Soil erosion ≥ 0.5%
Sprinkler irrigation/Overhead
Suitable → For land gradient is steep + Soil easily erodible (Uneven land surface,large undulation), Best for light soil, highly modulating land, Water is available with difficulty, land soil is excessively permeable or when soil is highly impermeable
Not suitable for soil (Clay) with low infiltration rate
Application rate → Limited by the infiltration capacity of the soil
Minimise erosion
80% water application efficiency, minimum water wastage
Tea, Coffee, Not for Rice & Jute (for rice & Jute Standing water is used)
Limitations → Heavy initial investment & strong wind disturbs sprinkling.
Drip/Trickle/Micro/Localised Irrigation
Directly to the root zone
Arid condition in hot & windy Areas, low rainfall and strong winds, irregular topography
η = 80 - 90%.
Less loss(evaporation, percolation) → Very high Duty
Tomato, Corn, Fruit, Orchard of mango
SOIL-MOISTURE & PLANT
Superfluous water → Water which drains down so deep that plant roots cannot draw it
Zone of areatⁿ → Root soil water zone, intermediate zone & Capillary water zone
Saturation Capacity → Max water holding capacity of soil , replacing all air pores
Field Capacity(FC) → Water holding capacity of plant roots, or MC after free drainage has removed most of gravity water, depends on → porosity and capillary tension
Capillary water → used by plants
Height of capillary fringe = 1 - 1.5 meters
Top of the capillary zone lies above the water table at every point
Readily available moisture → Most easily extracted by plants (appx 75-80% of available water)
Permanent Wilting Point(PWP) →Water content below which plant can't extract water for its growth & it is a soil characteristic, moisture tension = 7 - 32 Atm
PWP moisture content → Represents hygroscopic(bonding) water
Hygroscopic water → Can't be extracted by plants
Soil-Moisture Deficiency → Water depth required to Bring moisture up to field capacity.
Meteoric water → Water derived from precipitation (Snow & rain), Lake, River
i. Equivalent depth of water held at FC
dw =(d/w)dFC
d = Root zone Depth
Porosity=(d/w)FC
ii. Available moisture or Storage Capacity of soil
dw' =(d/w)d[FC-PWP]
iii. Readily available moisture to plant at OMC
=(d/w)d[FC-OMC]
iv. Readily available moisture at PWP
=(d/w)dPWP
Irrigation Water Quality
Sodium adsorption ratio
SAR =Na/(Ca+Mg)/2
10-18-26
Low sodium water = 0 - 10 ← Suitable for all crops
Medium = 10 - 18
High = 18 - 26
Very high sodium water > 26 ← Unsuitable for irrigation
Sulphate(ppm)
Good water for irrigation = 0 - 192
Satisfactory irrigation = 192 - 400
Not suitable > 400 → Soil becomes infertile
Electrical Conductivity (EC)
Total Salt Concentration → Expressed as EC
Low salinity water = 100 - 250 → Suitable for all crops
Medium = 350 - 750 Micromhos/cm
High = 750 - 2250
Very High Salinity > 2250 → Unsuitable for irrigation
Salts of Sodium, Calcium, Magnesium , Potassium
WATER REQUIREMENT OF CROPS
With the increase in the quantity of water supply → Crops yield increases up to a certain limit and then decreases
Mixed crop → To increase fertility of land
Command Area
Area over which canal irrigation water flows by gravity
Minor irri Project CCA < 2k ha
Medium irri Project = 2k - 10k
High irri Project ≥ 10k
Salinity → ↑es infertility
Gross Command Area
Total area enclosed by an irrigation canal that can be included in an irrigation project for supplying water to agricultural land by a network of canals.
GCA = CCA + Uncultivable area
Culturable Command Area (CCA)
Area in which crops are grown at a particular time or crop season
CCA = GCA - Uncultivable area
Net Command Area
NCA = CCA - Area occupied by canals, Networks, ditches
Consumptive use
Evaporation + Transpiration = Evapotranspiration
Measured → Vol of water per unit area, Depth of water on irrigated area
Supplied partially by precipitation and partly by irrigation
Crop Type
Rabi(Winter) →oct-march, Wheat, Barley, Gram, Pea, Mustard.
Zaid → march-june, Pumpkin, Cucumber, Watermelon, Bitter gourd.
Kharif (Monsoon) → june-sept, Paddy crop, Rice, jowar, bajra, Groundnut, jute, maze.
Water required Kharif = 2 - 3 x Rabi
Supplying water for R, K, Sugarcane → Capacity = max of i) R + Sugarcane or ii) K + Sugarcane
Cash Crop → Not used by farmers ex. jute tea sugarcane cotton tobacco, groundnut
Solid crop → Takes more than 4 months to mature
Crop Ratio = Area irrigated Rabi/kharif
Capacity of an irrigation tank depends on type of crop & duty
Crop period → Showing to harvesting, Crop period > Base period
Base period → 1st watering at time of showing to last watering before harvesting
Paleo irrigation → Prior to sowing of crop
Kor watering → 1st watering plants grown few cm, Crops require max watering during kor watering, about three weeks after sowing
Kor period → Rabi crops = 4 weeks = 28 days
Water requirement → wheat < paddy
Duty
Area(ha) irrigated with 1cumec (1m³/sec) of water, Place of measurement of water
D=A/Q
Unit → hect/cumec
D↑es → Efficiency↑es
Humidity ↑es → D ↑es
D ↓es → Supply ↑es
Duty is independent of Delta(∆)* → but by eqn D ∝ 1/∆
Duty is max at field & Minimum at barrage or weir
Full supply coefficient → Duty on Capacity or no. of hectares irrigable per cumec of the canal capacity at its head.
Outlet discharge factor = Outlet factor = Area/Q outlet = Duty at watercourse = Duty at field = Duty at outlet
Delta
Total depth of water Required/provided during base period (cm or m)
=8.64B/D
Where B = days, ∆ = m, D = ha/cumec
1ha = 10⁴ m²
Vol of water from Q of 1cumec per day = 8.64ha.m = 86400 m³
∆ → Well irrigation = 0.67 x Canal irrigation
Yearly intensity of irrigation = ioi1 + ioi2 + ioi3
irrigation frequency is function of Crop, Soil & Climate
kor depth → Rice = 19 cm, Wheat = 13.5cm, Sugarcane = 16.5cm
Root zone → Rice = 90cm, Wheat = 30-40 cm
Average ∆ → Rice = 120cm, sugarcane = 120cm, Tobacco = 75cm, Cotton = 50cm, Wheat = 40cm
Max base period = Sugarcane
Frequency of irrigation
Foi is so decided when the average moisture content in the root zone of soil is at optimum moisture content which is near the wilting point
Irrigation frequency is a function of → Soil, Crop, Climate
Interval of water to be irrigated Guava fruit → Summer = 7-10 days, Winter = 15 days
Irrigation Efficiencies
Canal → Field → Root → Used
Conveyance = Q field/Q canal
Application = Qroot/Qfield
Used = Qused/Qfield
Storage = Qroot/Field Capacity
irrigation eff. = Qused/Qcannal
d =(1-y/d)100 → Water distribution efficiency
y =xi-d/n
d =xi / d
Crop water use efficiency = Yield of crop / Amount of water depleted by crop in evapotranspiration
Irrigation Requirement of Crop
Water requirement for crops = Consumptive use + Application loss + Special needs for land preparation + Transplantation loss
★ GIR > FIR > NIR > CIR
i Consumptive irrigation requirement(CIR)
irrigation water require to meet evapotranspiration needs of the crop during its full growth
CIR =Cu-Re
Cu = Consumptive use, Re = Eff Rainfall
ii Net irrigation requirement
NIR =CIR+LR+PSR+NWR
LR → Leaching requirements
PSR → Pre showing requirements
NWR → Nursery water requirements
iii Field irrigation requirement
FIR=NIR/a
FIR = NIR + field application loss
iv Gross irrigation requirement
GIR=FIR/c
Design Discharge
Crops of diff season = max of all Q
Crop of same season = Sum of all Q
Design Q = Q in field / (Time factor x Capacity factor)
Design Q is controlled by kharif crop
CANAL DESIGN/IRRIGATION
Size of stone (d) ≥ 11 RS ← for stable sedimentation in alluvial channel or remain at rest in a channel
Critical tractive stress → The average shear stress acting on the bed of the channel at which the sediment particle just begins to move
Unlined Canal → τ = γw R S
Avg Shear stress on bed of a channel = γw R S = Critical shear stress
Avg Shear stress on bank of a channel = 0.75 γw R S = .75 x Critical shear stress
Avg τ → bed/bank = 4/3 > 1
Capacity factor = Avg supply Q/ Full supply Q.
Balancing depth → Depth of cutting for which Area of cutting = Area of filling
Darcy eqn → Analysis of groundwater movement in alluvial soils
Silt charge → The weight of silt carried by the river per unit volume of water
The canal system & drainage system are complementary
Designed capacity of an irrigation channel is usually controlled by Kor demand of kharif crops
Order of preference → Depth > Slope > width → while aligning a canal
Revetment → A facing of dry stone pitching laid on sloping face of earth to maintain the slope in position or to protect it from erosion
Culvert are provided on road embankment in areas having no defined channel to carry surface runoff
Canal automation system → 1st practised in gujarat
For design of major hydraulic structures on the canals → Khosla method of independent variables
Drainage density = Total channel length/Total drainage area
Types of canal
Rigid boundary canal → Designed on the basis of uniform flow
Most desirable alignment of an irrigation canal → Along the ridge line
Drains → Canals constructed for draining off water from waterlogged areas
Based on Canal Alignment
i. Ridge/Watershed Canal
Ridge/Watershed → The divide line b/w catchment area
Best alignment and Economical
Aligned along the ridge/natural watershed line → irrigate both sides of ridge
Used in Plain Areas → Land slopes are relatively flat and uniform
Not intercept any natural drainage → No cross drainage work are required
ii. Contour Canal
Parallel to contours
irrigate only one side
Maximum cross drainage work required
Used in Hilly area
iii. Side slope
Perpendicular to contours
irrigate only one side
Flow → Parallel to natural drainage of country
Neither along watershed nor in valley
No cross drainage work are required
Nature of source supply
i. inundation canal
Non-perennial canal
Obtain their supplies through open cuts in bank of river which are called heads,Flow occurs only when there's a rise of flow in river
Carry water in only rainy season
To divert flood or excess water
ii. Permanent
Perennial & Non-perennial
Perennial canal → Flow throughout the year, Canals taking off from ice-fed perennial rivers
Based on Function
i. Feeder
Constructed to feed other canals
Feeding two or more canal or when main canal divided into two canals
ex. Indira Gandhi Canal, Lower chenab canal
ii. Carrier
Based on Discharge
Canal system → Head work → Main Canal → Branch Canal → Distributary → minor canal
Canal in form → Water course or Field channel
Main canal → Not to do direct irrigation
Branch canal → Q > 30m³/sec
Major distributary → Q < 30 m³/sec
Minor distributary → Q < 2.5 m³/sec
Field channels or Watercourses → Small channel's excavated by cultivators in irrigation fields
Cross Section of irrigation canal
Sleep slope in cutting than feeling
Bank aim → To retain water
Service road → On canal for inspection purpose
Borrow pits → Central half width of the section of the canal
Pressure relief valves → Provided when canal is in full cutting
FSL of a canal at its head is kept 15 cm lower wrt parent channel
Freeboard
Margin b/w FSL and Bank level
FB ensures that water does not overlap bank
Unlined canal = FSL to top of bank
Lined canal = FSL to top of lining
IS 7112:1973 → Min FB = 0.50m (Q < 10cumec), = 0.75 (Q > 10cumec)
Berm
Side of canal serve as road
Provided in all situation
↑es factor of safety
Provided a scope for future widening of canal
Dowel
As a measure of safety in driving provided along the bank or on the service road
Helps in preventing slope erosion due to rain
Design of lined canal
Benefits → Helps increase the command area, Seepage reduction, prevent water logging
Earthen type lining → Compact earth lining, Soil cement lining
Hard surface lining → Cement concrete lining, Brick lining, Plastic lining, Boulder lining
Thickness of Concrete lining → Governed by requirement of imperviousness and structural strength, Canal capacity, Nature of canal, Slope of bank of canal
A lined alluvial canal is best designed on the basis of Manning's formula
Life of concrete lining canal = 60 years
Most Economical section of Lined canal
For Small Q (Q < 150m³/s) → Triangular section with circular bottom
For High/Large Q (Q > 150m³/s) → Trapezoidal section with rounded corners
Lining max velocity
Cement concrete = 2 - 2.5m/s
Burnt clay tile ≤ 1.8m/s
Boulder ≤ 1.5m/s
Manning's formula
Best → Design of lined canal or lined alluvial canal or impermeable soil
V = (1/n) R2/3 S
n = manning's coefficient → L-1/3T
Design of stable channel
By → kennedy theory and lacey's theory
Theoretical concept of silt transportation is the same in both the theories
Lacey improved upon kennedy formula
KENNEDY THEORY
Applicable to Irrigation channels only
Used the Kutter formula, Manning formula
RG Kennedy (EE pwd punjab) → 1895 → Upper bari doab canal system
Vo =0.55my0.64
Vo → Maximum permissible velocity or critical velocity
Silt transportation power → Qt ∝ Vo5/2
Section → Trapezoidal section
Recommended side slope → ½H : 1V
Kennedy found Upper bari doab canal → non silting non scouring
At critical velocity → non-silting and non-scouring
Garrett's diagram
Used by Kennedy to design Canals along longitudinal direction
Drawn for trapezoidal channel with slope 1/2H:1V
it gives graphical solution of kennedy’s eqn and kutter’s eqn
Critical velocity ratio (m)
cvr was introduce to take Silt grade effect in account
m = V/Vo = Actual mean velocity/Critical velocity
V > Vo (m > 1) ← Scouring
V < Vo (m < 1) ← Silting
V = Vo (m = 1) ← No scouring, No silting
V=CRS= (1/n)R2/3S1/2
Value of chezy’s constant
Mannings → C=R1/6/n Cn = R1/6
Kutters → C = [1/n + (23 + 0.00155/S)] / [1 + (23 + 0.00155/S)x n/√R]
Bazin’s →
Chezy’s → C=V/RS
Assumption of Kennedy theory
Silt is in suspension is due to eddy formed from the bottom/bed of the channel only
Limitations of Kennedy theory
Eddies are generated from Base/Bed only
Depth is known
Trial & error based
No formula for side slope
No explanation for CVR
LACEY'S THEORY
independent eqn = 03, Regime condtⁿ = 03
Applicable for both Alluvial rivers & Canal
Regime theory is applicable to channel in → True regime and Final regime
Silt supporting eddies → Generated from bottom as well as sides of channel
Wide & shallow w.r.t Kennedy theory
V = (Qf2/140)1/6=2fR/5
Regime (V) =10.8R2/3S1/3
Silt factor (f) =1.76d, d mm
R=5V2/2f=A/P V2=2fR/5
P=4.75Q
S=f5/3/3340Q1/6
A = Q/ V
Regime Scour depth → Any river width =1.35(q2/f)1/3
Regime Scour depth → Alluvial regime width =0.473(Q/f)1/3 → Normal regime scour depth
Max flood Q → V ∝ R2/3S1/3 → in a alluvial steam
General depth of scour calculated by lacey's formula in a river represents the depth below the maximum flood level in the river
Max scour depth
dmax = k x d
k → Nose of pier = 2
Ragoustiy coeff(N)
Defined by Lacey
N ∝ f1/4 =0.0225
Depends on Grade & density of boundary material
Lacey's Regime
Regime implies → Sediment comprises loose granular material
True Regime → Silt charge and Silt grade is constant, Discharge is constant, Flow is uniform, soil is incoherent alluvium
Final regime → Attains its section and longitudinal slope
Regime channel → No Scouring no silting
Permanent regime → Rigid boundary canals, Whose bed and banks are made with non-erodible material
Side slope in canal for different condition
For complete excavation → H : V = 1 : 1
For complete filling → H : V = 1.5 : 1
For partial filling → H : V = 1 : 1
CANAL REGULATION WORK
in barrages and canal head regulators → Weak and oscillating type hydraulic jump is formed
Capacity (Q) → Escape channel/ Parent channel > 50%
Compactness coefficient = P/2PA → P and A are Perimeter and area of drainage basin
Cistern → Designed on the downstream of a fall to dissipate the kinetic energy and prevent the damage to the bed and sides due to undesirable scour
Cross Regulator
in main canal increase the water head upstream when a main canal is running with low supplies
Head up water for adequate supply into the off-taking channel
Effectively controls the entire canal irrigation system
Absorbs fluctuations in various sections of the canal
Canal Head Regulator
Str at the head of canal @ 90 degree to the weir
Breast walls are provided with head regulator
Cross Drainage Work
At crossing of canal & natural drainage work
High flood drainage Discharge is small → Aquaduct or superpassage work
Fluming of canals → Contracting the waterway of the canal, reduces cost of cross drainage work
Syphon → Flow is under atmospheric pressure
1).Canal over Drainage
Aqueduct → HFL of drain much below bottom of canal
Syphon Aqueduct → FSL of D touches C bad, HFL of drain higher than canal bad flow under pressure through inverted syphon
Most severe condition of uplift on the floor occurs when canal runs dry and drain is at high flood level
2). Drainage over Canal
Super Passage → FSL of the canal is lower than drainage or stream
Canal Syphon → FSL of Canal touches D bad or FSL of canal higher than drain bed, flow is under pressure
3). Level Crossing
Canal water & drain water are allowed to intermingle with each other
Provided when canal & drainage approaches each other Practically at same level
Lift of material
i. Canal Drop/Fall
Control of bed grade
Provided → Ground slope > Design bed slope → Available ground slopes steep than design bed slope of channel
Vertical drop fall → Drop ht ≤ 1.5m
Baffle/inglish fall → Drop ht > 5m
Canal fall is located most economically → Where depth of cutting < Balancing depth
Flumed fall → length of body wall of fall < width of canal
Vertical drop fall (Sarda fall) → Designed to minimise the depth of cutting, Used as meter fall, Ht ≤ 1.5 m, Rectangular crest (Q < 14 cumecs), Trapezoidal crest (Q > 14 cumecs), Neither draw-down nor heading up of water, Width of trapezoidal crest = 0.55 H+d , width of rectangular crest = 0.55 d
Ogee fall → Minimise the disturbance & water impact
inglis/Baffle fall → A straight glacis type fall with baffle platform and baffle wall
Montague type fall → Uses parabolic glacis for energy dissipation
Glacis type fall → The energy is dissipated due to hydraulic jump
Rapid fall → Sudden fall of level of ground along the alignment of a canal joined by an inclined bed
ii. Canal Escape
It is Safety values for the canals → Bypass excess water entering a canal
Full supply level, Remove Surplus water
Weir type escape → Crest = FSL of the canal
Regulator / Sluice type escape → Scouring off excess bed silt deposited
Escape → Purpose of washing some of canal’s water
iii. Canal Cross regulator
Control flow Depth
To head up water of adequate supply into the off-taking channel
iv. Canal Outlets or Module or Sluice
Ctrl Discharge
Canal outlet is a structure built at the head of a watercourse that is used to release water from a canal
Types of canal outlets
Non modular outlets
Q depend on difference of head b/w distributary and water course
Fluctuation both side → Q varies with either change in water level of distributary or that of water course
Ex. Drowned or Submerged pipe outlets, Masonry sluices, open sluice
Flexible/Semi-modular outlet
Pipe outlet discharging freely in the Atmosphere
It is an adjustable proportional module
Q = Function of Distributary as long as min working head available
Q increases with rise distributary level
Q unaffected by WL in water course
ex. Kennedy’s gauge outlet, Venturi flume, pipe outlet, open flume outlet,
Rigid outlet/Modular outlets
Maintains Constant Discharge(Q) → No effect of fluctuations on any side
Q is independent of water level in distributors & watercourses
Eg. Gibb’s rigid module, Drowned pipe outlet, masonry orifice
Criteria for judging performance of module
Efficiency
n = Head recovered/head put
Flexibility
F=(dq/q)/(dQ/Q) = my/Hn
Proportional outlet F = 1 → Rigid or modular outlet
Sub Proportional outlet < 1
Hyper proportional outlet > 1
Flexible/Semimodular outlet ≠ 0
Sensitivity
S = (dq/q)/(dy/y)
Rigid module = 0
dq/q = Rate of change of discharge through outlet
dQ/Q = Rate of change of discharge through Distributary Channel
dy/y = rate of change of water level through Distributary Channel
Setting
S = m/n = H/y
Outlet → Q = KHm
Distributary → Q = Cyn
m → Outlet index, n → Channel index, y → Distribution depth
Wide trapezoidal channel → n = 5/3, m = 1/2(Orifice type outlet), m = 3/2 (Weir type outlet)
Triangular → m = 5/2
Blench Formula
X = 2Dc - D + (1/4)(HL - 3Dc/8)
DAM & RESERVOIR
Estimated avg life of a dam reservoir = 75 years
Mass curve analysis → Determine live storage requirement for a reservoir
Sequent peak algorithm → Estimation of minimum Reservoir capacity needed to met given demand
Usefulness of reservoir = Dead storage/Sediment deposition per year
Useful Storage → Water in reservoir b/w min pool level & Normal pool level
Useful/live storage → up to top level of sluice gate
Surcharge storage in Dam reservoir → Volume of water stored b/w FRL(full reservoir level) & MWL(Max water level)
Dead/inactive storage → Water stored in reservoir below the minimum pool level, provided to accommodate silt entry or sill-trapped in the reservoir
Dead storage is not provided in a dam of water resources project meant of irrigation and water supply
Valley storage → Vol of water held by natural river channel in its valley up to the top of its bank before construction of a reservoir
For flood control reservoir → Effective storage = Useful + Surcharge - Valley storage
Sequent peak algorithm → Estimation of minimum reservoir capacity needed to meet a given demand
Free board → Difference b/w top of dam and max water level(MWL), governed by size of the reservoir, runoff, wind velocity
Depth of scour is normally measured from mean water level
Talus → A protection work at the downstream end of the weir in form of blocks of concrete or masonry
Structure against overturning → Partial safety factor for dead load = 0.9
Inverted filter → to prevent dams from failing by water piping, permeability increases in upward direction
Design yield of a storage reservoir < Firm yield
Economical height of a dam → Cost per unit of storage is minimum
Capacity-inflow ratio for a reservoir → Decreases with the time
Safety valve of a dam → Spillway
Retarding basin → Dam reservoir not provided with gate controls on its spillway and other sluices
Ayacut → Area to be irrigated by a dam
Bank storage → increases the computed reservoir capacity in dam reservoir
Peak lag = Time for max inflow - Time for max outflow
Dam height increases → Cost per unit storage initially decreases and then increases
Water stop or Water bars → To prevent leakage through the transverse joints in a gravity dam
Overflow pipes → Provided at full reservoir level
Stilling basin → Provided at downstream floor to dissipate energy of flow
Mass curve method
Storage capacity of a reservoir
The demand line drawn from a ridge in the curve did not intersect the mass curve again → Represent the demand can’t be met by the inflow as the reservoir will not refill,
Trap Efficiency
TE → Function of Capacity/inflow ratio
TE = Total sediment deposited in a given period/Total sediment inflow in that period
Measure of reservoir sedimentation
Types of reservoir
Storage reservoir → Design yield < Firm or safe yield
Linear reservoir → Storage varies linearly with Outflow rate
Multipurpose Reservoir → Planned and constructed to serve various purposes
Conservation reservoir → Retains excess supplies during periods of peak flows and release them gradually during low flows, Protect the area lying on down stream, Supply water in a city
Balancing reservoir → To address the frequent fluctuations in the rate of consumption
Retarding reservoir → No gates at outlet
FOS = Firm or safe yield / Design yield
Classification of Dam
Ghatghar (Lower) dam is a type of roller compacted concrete dam
Base width → Rockfill dam << Earthen dam
Check dam → Flood control
i. Based on Structural behaviour
Gravity dam, Embankment dam, Arch dam, Buttress dam
Arch Dam → in a narrow V shaped gorge having sound and competent rock on side slope, idukki dam, Gorge Length < Height, Uplift on the base is not an important design factor
Buttress dam → Foundation pressure are less than gravity dam, it is a storage dam
Uplift pressure is of least importance in the designing of an arch dam
Concrete consumption → Constant angle arch dam < Constant radius arch dam
ii. Functional behaviour
Storage, Coffer, Diversion, Detention, Debris dam
Diversion dam → Used to raise the river water level to feed on off taking canal
Detention dam → To reduce flood peaks by impounding the flood flow excess of capacity of d/s channel
iii. Material of construction
Rigid, Non rigid dam
Rigid Dam → Arch, Timber, Steel
Non Rigid dam → Rock fill, Earthen dam
Beaver dam → A type of timber dam
iv. Hydraulic design
Overflow, Non overflow dam
Zoned (Non homogeneous) Embankment type dam
Made up of more than one material
Central impervious core → Most suitable Clay mixed with fine sand
Gravity Dam
Force exerted on it is resisted by its own weight
FOS → Sliding = 1.5 & Overturning = 2
Agg size ≤ 40mm (cement concrete dam)
Economic depth (Height) = Cost of dam per unit storage is minimum
Reservoir is full → Heel = Tension & Toe = Compression
Max ht. of masonry dam of triangular sectⁿ = b√Gs → b = base width, Gs = Specific gravity
Presence of tail water → Decreases Principal stress and shear stress
In extremely seismic zone acceleration due to EQ = 0.1 x g → for design of gravity dam
Narrow valley, fairly sound bedrock and stone masonry → favour a gravity dam
Bottom portion is stepped → in order to increase the shear strength of the base of the dam
Force acting on GD
Major resisting force → Self wt of dam
Main overturning force → Water pressure
i) Water pressure
At water surface = 0
Base = γw H
ii) Uplift pressure
At heel = γw H
At Drainage gallery = γw h + ⅓ (γw H - γw h) = ⅔ rd of toe + ⅓ rd of heel
At toe = γw h
Control of uplift pressure → Constructing cutoff under upstream face, Drainage channels b/w the dam and its Foundation, Pressure grouting in foundation
Uplift force increases FOS against overturning
iii) Earthquake force
India → 4 Zones → II, III, IV, V
Zone V is the most serious zone
Hydrodynamic pressure due to EQ =0.555KwH2 → at 4H/3π above base → opposite direction to acceleration imparted by EQ
Hydrodynamic pressure curve → Parabolic
Horizontal acceleration due to EQ → Results in Hydrodynamic pressure and inertia force(F = ma) into the body of the dam
Vertical acceleration due to EQ → Vertical inertia force opp direction of acceleration
iv) Silt pressure
Ps=0.5Ka sub H2→ at H/3 from base
v) Wave pressure
Waves are generated on the reservoir surface bcz of wind blowing over it
Pressure distribution → Triangle of Ht = 5hw/3
Total Pressure = =0.5whw2 → At 3hw/8 above the reservoir surface/max still water level
For wave action → ht. of free board = 1.5 hw and should not be < 0.9 m
hw = Wave ht → depends on wind velocity and fetch
vi) ice pressure
vii) The weight of the dam
Major Resisting force
Principal stress
V =0 1=vSec2-2tan2
Considering EQ → 1=vSec2-(P +Pe)tan2
Empty reservoir → 1=vSec2
H =0 = (v-2)tan
Considering EQ → = (v-(2e)tan
Criteria of stability & Modes of failure GD
i) Overturning about toe
if Σ Fx > Σ Fy
Mr = Mo
ii) Compression or Crushing failure
iii) Tension failure
For no tension σ min = 0 → 1 - 6e/B = 0
e ≤ B/6 → T = 0 (Middle third rule) → For no tension resultant force must pass through the middle third of Base
e ≤ B/6 → Max CS/Max TS = ∞
iv) Failure due to Sliding
Elementary Profile of GD
Elementary profile of GD → Right angle triangle
Base width is used with some extension on the upstream side
Resultant force will shift Towards heel → by providing a top width for roadways and free board, for full reservoir condition
Base width = 65 % of dam height → No tension criteria and neglecting uplift pressure
Limiting/Maximum height For elementary Profile
H=f/w(G+1) → Uplift Pressure not considered
H=f/w(G+1-C) → Uplift Pressure considered
f = Allowable stress
Height of dam > H → High gravity dam
a) Empty reservoir
Only Force due to Self wt is considered
Max compressive stress → At heel = 2W/B
At Toe → Zero stress
b) When reservoir is full
Max compressive stress/force → At the toe
Base width (B) = Max of [ H/G-C or H / (G-C)]
G = Specific gravity of dam material, C = Uplift coefficient, μ = Friction coefficient
i. For no Tension at base
BH/G-C
Critical condition → When uplit is not considered → C = 0
Critical → BH/G
ii. For no sliding
BH / (G-C)
Horizontal force < Foundation friction resistance → P < W
FOS = 1.4
iii. For no overturning
BH/2(G-C)
Max stress at Toe = Pv = γw H (Gs -C)
Min stress at heel = Zero
FOS = 1.5
FOS ≤ W(b - x)/(Ph/3)
Earthen Dam
As compare to gravity dam earthen dam requires less skilled labour, can be constructed on almost every type of foundation
Most adverse condition for stability of slope for u/s face → Sudden Drawdown
Most adverse condition for stability of slope for d/s face → Steady seepage when reservoir is full
Earthen dam are best type of dam structure to resist EQ shocks
Upstream face of an earthen dam → an equi-potential line
Q > 10m³/s → Freeboard = 0.75m
Seepage ↑es → Comp.... in filling
Hydraulic failure of earth dam → Overtopping, Crushing, Sliding, toe erosion, wave erosion
Max instances of failure of earth dams take place → Due to over toppling due to insufficient spillway capacity
Riprap → To prevent the Sloughing of an earthen dam, sloughing is not a hydraulic failure
Stability analysis for the downstream slope of an earthen dam is essentially carried out for Full reservoir condition
In zoned rolled earth-fill dam the central core zone serves → the purpose of checking seepage loss from reservoir
Filter or Rock toe → Provided at downstream bund, To collect and drain out the seepage water, to prevent slogging of the toe due to the seepage flow and increase the stability of dam
Flow through an earth dam is an example of unconfined flow
Cut-off → Reduces the uplift pressure in the base of the dam
Cut-off trenches for earth dam are effective → When the depth of pervious foundation strain below the dam is shallow
If the design flood is underestimated earthen dam may fail → Due to erosion of upstream face
Embankment seepage ctrl in Earth Dam
Horizontal drainage filter, Toe filter, Protective filter d/s of toe, Chimney drain
Focus of base parabola for dam having horizontal drainage filter → b from toe → b = width of Horizontal drainage filter
Foundation seepage ctrl in Earth Dam
Impervious cutoff, u/s impervious blanket, D/S Seepage berms, Drainage trench, Relief wall
Back water curve
it is the profile of the rising water on the upstream side of the obstruction/dam
Rate of change of depth = 0 → back water curve to occur
Hydropower Station
Load factor = Avg load/Peak load
Capacity/Plant factor = Avg load output/installed capacity of plant
Utilisation factor = Water actually utilised for power/Water available in river
Design speed → Max efficiency
Headrace tunnel → a channel of free-flow tunnel leading water to the fore bay or a pressure tunnel leading the water to the surge tank
Penstock → carry water from storage reservoir to the power house
Hydel/Hydro-electric Scheme Classification
Low head Scheme < 15m
Medium head = 15 - 70m
High head > 70
Runoff river plants → A low head scheme, Suitable only on a perennial river
Pumped storage plants → Generates power only during the peak hours
Dams Projects
Tehri dam → Highest(Tallest), Bhagirathi river (Uttarakhand)
Bhakra nangal dam → Concrete gravity dam, Largest dam in india, Satluj river (HP)
Hirakud dam → Longest dam, Reservoir for flood control, power and irrigation, Mahanadi()
Idukki → Non-overflow double curvature concrete arch, Periyar river(Kerala)
Rama-ganga → Uttarakhand
Sardar sarovar dam → concrete gravity dam, narmada river (Gujrat)
SPILLWAY
Spillway act as safety valves for the dam → Control excessive flood water
Provide str stability to Dam during reservoir flooding
Overflow Dam is also known as Spillway
Retarding basin → Provided with uncontrolled spillway & an uncontrolled orifice type sluiceway
Barrage & canal head regulators → Weak & oscillating type of jump formed
Reservoir with an uncontrolled spillway → The peak of plotted outflow hydrograph lies on the recession limb of the plotted inflow hydrograph
Solid roller bucket → Arranged at the end of a spillway to dissipate energy through hydraulic jump
When the outflow from a storage reservoir is uncontrolled as in a freely operating spillway the peak of outflow hydrograph → Occurs at the point of intersection of the inflow and outflow hydrographs
Chute spillways
The flow of water after spilling over the weir crest → At Right angle and parallel to weir crest
Flow → Supercritical
Side channel spillway
The flow of water after spilling over the weir crest → Parallel to weir crest
where long overflow crest required
where abutment are steep & control desired by narrow side Channel → High side and narrow gorge
Shaft spillway
In case of non availability of space due to topography → Shaft Spillway is most suitable
Ogee or Overflow spillway
Least suitable for Earthen dam
Mostly used with gravity Dam
minimise the disturbance & impact
Overflowing water is guided smoothly over the crest and profile of the spillway
Q=CLH3/2 → Q ∝ H3/2
Coeff of Discharge(C) depends on → Depth of approach, upstream slope, downstream apron interference, downstream submergence
Sharper crest → ↑es Cd & ↑es eff Head
Downstream curve → X1.85=-2yH0.85
Downstream profile with upstream face → Y/H= -0.5(X/Hd)1.85
Head of water < Design head → The coefficient of discharge < Design coeff of discharge
He = Hd → Cd = 2.2
Barrage type spillway
Used where a temporary and small storage above the crest is required
Gates
Fixed wheel gate → Most commonly used form of vertical lift gates on spillway crest
Radial gates → Least human/mechanical effort to operate
USBR drum gate → Can't be seen from a distance when lowered
Bear trap gate → for low navigator dam
Francis equation
Correction for end contraction for calculation of discharge capacity of spillway
Ln = Lt - 0.1n(hc + hv) → n = No. of deflecting corners
RIVER TRAINING WORK
River Training Work is generally required for meandering type of river
Width of launching apron = 1.5 depth of scour below original bed
Scour depth of protection works → D = yu/3 + 0.6
Rouse equation → Determining concentration of silt at any depth of river
Riprap → To protect river banks from erosion by river flow
Aim/Obj of RTW
To achieve ultimate Stability of river with the aid of river training
Safe passage flood discharge
Efficient disposal of sediment load, preventing the river from changing its course, to protect river banks.
Types of RTW
High water training → for Discharge (Q) → Flood control
Low water training → for Depth (d) → Navigation purpose
Mean water training → for Sediment → Preserve channel in good shape by efficient disposal of suspended and bed load
Method of River training
RTW → Guide bunds, riverbank protection, marginal guidebunds , Groyne or Spur, bandalling etc.
Rtw for depth (to increase depth) → by Groyne/Spurs and Bandelling
Bandelling → locally bamboo made str used for the river course stabilisation by river bank erosion protection
Bunds → Temporary Spurs
Leeves → Parallel to river flow, Earthen embankments constructed parallel to river at some suitable distances for flood protection
Guide banks or Bell’s bund
To Channelize the flow of the river, to confine the width of river, Prevent outflanking of structure
Sweep angle → Upstream = 120° - 145°, Downstream = 45° - 60° degree
Min length → Upstream = 1.1L, Downstream = 0.1L- 0.25L → L = length b/w abutments
Scour depth (D) → Nose = 2.25R, Transition from nose to straight = 1.5R, Straight reaches = 1.25R
Launching apron → Stone cover laid protect the face of the guide bank at river bed level
Groynes or Spurs or Dykes
Constructed Transverse to the river flow
Most widely used river training works
Spacing = (2 - 2.5) x Spur length
Function → Train river along a specified course by attracting, deflecting, repelling flow in river, protect river/canal bank by keeping the flow away from it, contract wide river channel, useful for improvement of depth
Spur are least effective in confining the flood water of alluvial rivers within reasonable waterway
Permeable spurs → River carrying heavy suspended load
Types of groynes
As per material of construction → Permeable and impermeable groynes
Permeable → Tree groynes, Pile groynes
As per height of groynes → non submerged groynes, submerged groynes
Attracting groynes → inclined towards downstream, θ = 45 - 60°C, not useful for bank protection
Repelling groynes → inclined towards upstream, θ = 60 - 80°C, Need strong protection directly subjected to current, protect upstream from scouring away
Deflecting groynes → Perpendicular to the bank, gives only local protection, relatively short length
Normal or Perpendicular groynes → vertically held
Sediment groynes → Permeable groynes, best for river carrying sediment, deposition of sediment carried by river without repelling/deflecting
Hockey groynes → with a Curved head
Types of river
Braided River → Two or more channel ex. Delta
Aggrading River → Silting rivers
Degrading River → Scouring, deficit of sediments
Ephemeral river → Flow for a short time after a rain strom and most of time their bed is dry
Meandering River → Erodibility of bed and banks of stream, Extra turbulence Generated by the excess of river sediment during floods, generally occurs in its through stage
A meander increase the river length but a cut-off reduces the river length
Tortuosity = Curved length/Direct axial length of river or Curved length/Meander belt > 1
Meander ratio = Meander belt/meander length
Meander pattern of river → Developed by Dominant discharge
Dominant Q = ½ - ⅓ of Qmax
Length of meander = 53.61Q
Width of meander = 153.42Q
Width of meander belt = Transverse distance b/w Apex point of one curve and the apex point of the next reverse curve
When a river starts meandering the sediment carrying capacity first decreases and ultimately increases
Deepest river portions will be available at the outer edges of meander loop
River bend in alluvial soil → Scouring on concave side, Silting on convex side
Dendritic → The pattern which involves irregular branching of tributary stream in many directions and at almost angle in the drainage basin
Juvenile Stage → River flow along an undulating topography and the gradient is very high
DIVERSION HEADWORK
Storage headwork → Dam, constructed to store water
Diversion headwork → To rise water level & divert to canal constructed across river and at head of offtaking canal
The most suitable location for canal headwork → The Trough stage (alluvial stage) of the river
A canal headwork in rocky stage of a river is not suitable → A costly head regulator is required, more falls are necessary to dissipate the energy, more cross drainage works are required
IS : 6004 - 1980 → Design of sediment ejector for irrigation and power channels
Suspended silt concentration in a channel carrying silt load → Higher at bottom layer than upper layers
Most commonly used vertical lift gates in modern day → Fixed wheel gates
For Smooth entry → Angle B/W head regulator and water/weir/barrage = 90 - 110 Degrees
Components of Diversion Headwork
Retrogression of downstream levels, generally considered in the design of weir or barrage → Higher at low water levels stage than at high flood stage
Effective control of silt energy into canal, the sill of the head regulator should be above the sill of the under sluice
If there are two canals taking off from each flank of a river → There will be two divide wall and two under-sluice
Devices used to reduce the silt entry into offtaking channel → King’s vanes, Gibb’s groyne wall, curved wings, desilting basins, vortex tube, cantilever skimming platform
Gibbs groyne wall → Denied entry of Bed load of the river into canal
Canal Head Regulator
Str at the head of canal @ 90 degree to the weir → Regulate supply of canal
Control entry of silt into canal, Completely exclude high flood entering into canal
Consist of no of spans separated by pier
Breast walls are provided with head regulator
The crest of a scouring sluice = 1.2 meter below the crest of a head regulator
Weir
To raise water level and divert to offtaking canal
Gravity weir → Wt of weir = Uplift pressure caused by head of water seeping below weir
Non gravity weir → Wt of concrete slab + wt of divide piers keeps the structure safe against uplift
Length of weir proper → Trought age > Boulder stage
Barrage
Function is same to that of weir → raise water level and divert to offtaking canal
The provision of a series of gates across the river for flow regulation
Crest level is kept at low level
In alluvial reach of river → Scour depth = 2 x lacey's scour depth
Downstream sheet piles → Provided to control failure due to piping by high value of exit gradient
Sluice gate
Removing excess sediment from entering the canal at its head regulator
Crest of under Sluice Portion of weir is kept at lower level then crest of Normal Portion and ponding is done by means of gates
Undersluice or Scouring sluice
Helps in removing silt near head regulator
Opening provided in weir wall with crest at low level
The crest level of the under sluice bays are kept at avg bed level of the river
Silt excluders
On River Bed & u/s of head regulator or front of head regulator
Exclude silt from entering into the canal
Silt ejector/extractor
On Canal bed & d/s of head regulator
Remove silt which already enters into canal
Divide wall or divide Groynes
Separate undersluices from the main weir → Provided Right angle to the axis of weir
Creating a still poll, To control silt energy into channel, To form still water pocket in front of canal head
can be designed as cantilever retaining wall subjected to silt pressure and water pressure from undersluice side
Fish ladder
Enable Fish to move freely and safely in the river
To check velocity flow in fish ladder Baffles or staggering device are provided
V = 3 - 3.5 m/sec
WATER LOGGING
in Loose saturated Sand
Capillary fringe reaches root zone of plant → Root zone become saturated
Drainage by gravity stops
Leads to → ↓es Temp, ↓es Crop yield, Soil become Alkaline, Growth of weeds, Accumulation of salt, defective air circulation in root zone
WL → Mild slope > Steep slope, Long rooted plants > Small rooted, Marshy area > Swamp
Lift irrigation → Preferable type of irrigation in water logged area, Reclaim a water logged area
Spacing of tile drain ∝ Coeff of permeability of the soil to be drained
Weeds in surface drains → Increases the value of coeff of rugosity
Drain → Canal Constructed for draining of water from WL areas
Water logged soils are infertile → Due to lack of aeration and reduced soil temperature
Vadose water → Water occurring in zone of aeration
Spacing of drain tile → L =4K(b2-a2)/q
Cause of WL
Excess rainfall
High water table
Seepage of canal
High Irrigation & frequent Irrigation
Flooding of field
Note → Excess tapping of groundwater is not responsible for Water Logging
Water logging control
↓es by providing Drains
Lining of canals is used to control water logging in Agricultural land.
reducing intensity of irrigation
Intercepting drains
Crop-rotation
Adopting consumptive use of surface & Subsurface water
Reclaimanation
Uncultivable land is made fit for cultivation
Alkaline soil → Gypsum + leaching
Acidic soil → Limestone as a soil amendment
Mulching → ↑es infiltration & ↓es evaporation by spreading of material on soil
Leachate → Generated from liquid present in Landfill
Leaching → Remove salinity (soluble salts) of the soil by downwards movement of water through the soil
Eutrophication → Plant Nutrition Accumulation, Excessive discharge of nutrients in water bodies
Bligh's Creep Theory
Design of Hydraulic structure on Permeable strata
t=FOS H/(G-1) = 4H/3(G-1) → Thickness of floor
Head loss = Hydraulic gradient x Creep length
Assumption → Equal weightage to horizontal and vertical creep → No distinction made b/w horizontal and vertical creep
Bligh's coff of creep C = 1/exit gradient
C = 18 ← Light sand & mud
Width of Launching Apron = 1.5 x D → D = scour depth below original bed
Creep length = B + 2(d1+d2+d3..) → B = horizontal floor length, d1,d2,d3 = pile lengths
Khosla's theory
Design of weir & barrages on permeable foundation
Exit gradient = H/d =H/Creep length
=(1+1+2 )/2
α = b/d
Exit gradient depends upon b/d, H/d ratio
Exit gradient ∝ Exit length → Horizontal length of apron is based on permissible exit gradient
ie ∝ Denseness of downstream cutoff
Exit gradient → in absence of downstream cutoff = infinity
Critical exit gradient depends on depth of upstream and downstream sheet
The Undermining or piping or sand boiling of the floor starts from the Tail end
Safety against piping failure Lrqr ≥ CH
Upstream pressure on upstream floor → By Bligh’s theory < Khosla’s theory
To increase inflow of water to Sub Surface water reservoir → Natural drainage of the area is improved
Pressure distribution under weir floors are based on conformal transformation of potential flow into the W plane
Critical hydraulic gradient → Stability of str against seepage pressure = 1.0, Alluvial soil = 1.0
Cut-off walls beneath a hydraulic structure reduces the exit gradient
For long range run off → Losses are function of temperature
Khosla’s safe exit
Silt(Shingle) = 0.25 - 0.20
Coarse sand = 0.20 - 0.17
Fine sand = 0.17 - 0.14
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