HYDROLOGY SHORT NOTES

 

INTRO, RAIN & GAUGE

• Isochrones → Time of travel of surface runoff to the catchment or outlet point

• Isobar → Pressure 

• Isohyets → Rainfall depth

• Isopluvial → Rainfall of a particular duration with a particular return period 

• Isopleths → Evapotranspiration

• Isotherms → Temperature

• Isonif → Snowfall

• Isobath → Depth in sea

• Isobront → Thunderstorm at same time

• Isohaline → Salinity

• Isohels → Sunshine

• Isoryme → Frost

• Iso Plastic → Piezometric surface

• Power = Qρgh

• Velocity of flow of underground water the most commonly used non-empirical formula is Darcy formula

Water Budget equation

• ∑inflow - ∑outflow = Change in storage

• P - R - G - E - T = ∆S

Hydrological Cycle 

• PIIDRETG → Precipitation → infiltration → interception → Depression storage → Runoff → Evaporation → Transpiration → Groundwater

• Coriolis force → Plays important part in completion of the hydrological cycle

Point to Remember

• Hydrology → Deals with surface and groundwater → occurrence, distribution and circulation 

• Humidity → Water vapour in air

• Avg annual rainfall india = 119cm

• Tropical grassland → Annual precipitation up to 150 cm

• Volume of average annual precipitation falling on the entire globe = 515000 km³

• Eff Rainfall → Precipitation falling during the growing period of a crop that is available to meet the evapotranspiration needs of the crop 

• Conjunctive use = Surface + Groundwater use

• Consumptive use = Evaporation + Transpiration

• Coeff of consumptive use = 0.9 (Wheat, barely,flax)

• Partial consumptive use of water → Greeneries

• Rain load = 5.2 (ds + dh) psf → ds & dh in inch

• Avg rainfall in world → 51,5000 km³

• Head ∝ Outflow → Storage ∝ Head

• Residence time = Storage vol/Q

• Residence Time →  Oceans > Global groundwater

• inlet time → T = (0.885L³/h)^0.385 → h = ht/diff level, L = length of overland flow

• Partial duration series → Mostly used for Rainfall analysis

• Variability of rain → Largest in regions of scanty rainfall

• IHP → international hydrological programme

• ASTRA → Application of science and technology to rural areas

• Absolute humidity in air decreases at higher altitudes

• Inverted syphon → To overcome obstruction in sewer lines due to road canal and railway lines

• Fugacity → A substances thermodynamic propensity to escape from one environmental component to another 

• Meteoric water → Water derived from precipitation which infiltrates into ground

• Dew point → Temp at which the air mass just becomes saturated if cooled at a constant pressure with moisture neither added nor removed

• USLE → Universal soil loss equation

• CWC → Flow in the major rivers of india is monitored by central water commission

• A tropical anticyclone in the Northern hemisphere → A zone of high pressure with anti clockwise wind 

• Absolute humidity → The amount of moisture present in the air expressed as mass per unit volume 

• Evaporation from ocean is the major source of atmospheric moisture for precipitation

• Bowen ratio = Heat flux /moisture flux near the surface

• In all groundwater exploration programmes one of the main objectives is to locate Phreatic water zones 

Precipitation

• Precipitation/rainfall → in terms of depth of water

• Rain in cold weather is due to high pressure

• Form factor = Area of catchment/(length of catchment)² = A/L² = B/L

• Daily factor = Average count of 7 days for 24 hours/ 24 hours count

• The moving average of annual precipitation record is carried out to determine Trend

Types of precipitation

• Orographic → Natural topographical Barrier (Hill), india

• Convective → Temp diff, Cumuliform clouds

• Cyclonic → Pressure diff → lifting of air mass

• Frontal → Warm + Cold air meets

Rain

• Rainy day > 2.5cm 

• light < 0.5mm/hr

• Moderate = 0.5-7.5

• Heavy > 7.5mm/hr

• Violent rain > 50 mm/hr

• Acid rain → pH < 5.6

Forms of precipitation

• Drizzle → Drop size < 0.5mm & Density <  1 mm/hr

• Rain → Drop size = 0.5mm-6mm

• Hail → Drop size = 5mm - 50mm

• Snow → Density = 0.1 gm/cc

• Sleet → Rain + Snow

• Glaze → Freezes on ground contact

Snow water Equivalent

• The amount of water contain within the snowpack 

• Water equivalent of snow = 10% → When specific information about the density of snowfall is not available 

Index of wetness

• iow = (Actual rainfall in a particular year/ Avg annual rainfall) x 100 

• 60% index of wetness → Rain deficiency of 40%

• Average annual rainfall In India → Mean of the annual rainfalls measured over period of 35 years 

Types of Raingauge /Ombrometer /Pluviometer /Hyetometer/Hyetometer/Udometer

• Collecting and measuring the amount of rain 

• Preferably be fixed → in an open space, Nearest object from a rain gauge should be a minimum distance equal to 30 m or twice its height, Must be set near the ground to reduce wind effect

• Standard RG in india → Natural syphon or Float type (Recording Rg)

• Commonly used Rg → Symons RG (Non recording Rg)

• Std Ht of std gauge = 30 cm

• Radar Rg → Used to measure the precipitation in the regions of difficult and inaccessible terrains

1. Non recording 

• Symon's rain gauge → d = 12.7cm

• IMD(india meteorological department) Non recording type → Symon's

2. Recording/Automatic rain gauge 

• Records cumulative depth of rainfall

• Gives mass curve (Accumulation vs time) of rainfall

• Ex. Tipping bucket, Weighing, Natural Syphon, Float type rain gauge

i. Tipping bucket 

• Dia = 300mm

• Remote area, Remote hilly inaccessible areas, Rainfall is associated with considerable amount of snow

ii. Weighing

iii. Natural syphon

• Natural syphon or float type → std RG in india

One raingauge station per

• Plain = 520km²

• Hilly & Heavy rainfall area = 150km²

• Region of an average elevation of 1km from sea level = 250 - 400km²

• Arid Zone = 

• As per WMO 10% of gauge stations should be self recording type.

Adequacy of Raingauge Station

• Mean rainfall → Pm=P /n = Rainfall /n 

• Std deviation → =(Pm-P)2/(n-1)

• Coeff of variation → Cv = (/Pm)100

• Optimum no of station → n=(Cv/)2 

• Error → =10%

• Additional Rain Gauge = Optimum no of station - installed rain gauge

Estimation of missing Data

i. Arithmetic mean method

• P=Pi /n → N is within 10% of missing data

ii. Normal ratio method 

• Px/Nx=(1/n) (Pi /Ni)→ N is beyond 10%

Average Depth or Mean Precipitation/Rainfall

• Importance to individual rain gauge station is given in thiessen method and isohyetal method 

i. Arithmetic mean method

• Pm=Pi /n

• Quick but least accurate

• Uniformly distributed on its area pattern

ii. Thiessen Polygon or Weighted average method

• Pm=PiAi /Ai

• Superior to the Average arithmetic method

• Theissen polygon is a representative area used for weighing observed precipitation at a rain gauge station 

iii. Isohyetal method

• Most accurate but very slow & laborious

• Best for Grouped amount Precipitation over an area 

• Used in the Hilly area & gives accurate results

• Variation of rainfall b/w two section → Assumed to be Linear

• To convert the point values of precipitation at various stations into mean precipitation over an area 

• linearly interpolated isohyetal m 

• Orographically weighted Isohyetal m → Best

vi. Station year method

• Used for extending the length of record for a frequency curve at a station

Rainfall intensity 

• I = Rainfall depth / Time

British formula for rainfall intensity

• I = a/(t+b) = R/(t+C)

• I = 760/(t+10) → if t = 5-20 min 

• I = 1020/(t+10) → if t = 20-100 min

• T → (minutes) Storm duration or time of concentration, I → mm/hr

Sherman intensity-duration-frequency eqn

• I=a/(d+b)n

Presentation of Rainfall 

• Hyetograph → Avg intensity (cm/hr) vs time, represented as Bar graph

• Hydrograph → Discharge/Runoff vs time

• Moving average → Gives trend of rainfall curve

• Mass curve → Accumulated precipitation vs time, Reservoir storage capacity, total amount of rainfall

• Flow mass curve → Cumulative Q, Volume and time in chronological order

• If Demand line drawn from ridge in a flow mass curve does not intersect the curve again it indicates → Demand can not be met by inflow

• Double mass curve → Check inconsistency of Raingauge records or rainfall is corrected.\

• Conventional flow duration curve → Flow vs % time flow exceeded

Depth Area Duration Curve (DAD) 

• Depth(cm) vs Area(km²)

• Maximise envelops through the appropriate data points

• Areal characteristics of a rain storm

• Depth ∝ 1/Area

• Indicates → For a given Area max avg depth of rainfall increases with storm duration

ABSTRACTION

• Evaporation (E), Interception(I), Transpiration(T), Depression storage(DS), Infiltration (IL)

• interception loss → Part of Precipitation that falls on plants and does not reach the ground surface and return to atmosphere by Evaporation, More towards end of a storm

EVAPORATION(E)

• E↑es → Patm↓, Temp increase, Surface area increase, Wind velocity increase, Density decrease.

• Lake Evaporation reduce by → films of Cetyl Alcohol(Hexadecanol), Stearyl Alcohol (octadecanol) and Acetyl alcohol→ Reduces surface area

• Under identical condition → E sea water < Pure water → due to salinity 

• Vapour pressure → Seawater < Freshwater 

• The highest rate of Evaporation is in winter  from deep water bodies.

• Epan > Eactual

• Max evaporation → Convex water surface

• Evaporation + Seepage loss = (B+d)2/3/200 → B = width & d = depth of a channel

Dalton law

• [E=k(ew-ea)] → mm/day

• Actual vapour pressure → [ea=% humidityew]

• Relative humidity = Actual vapour pressure/Saturation vapour pressure = ea/ew → At same temperature

Measurement of Evaporation

• Measured → Atmometer, Evaporimeter(Open pan)

a) Evaporimeter

• Class A evaporation pan, ISI Standard pan, Colorado Sanken pan, US geological survey floating pan

• Lake evapotranspiration = Cp x Pan evaporation

• Cp = 0.7 → Class A land pan (dia = 1210mm)

• Cp = 0.78 → Colorado sunken pan

• Cp = 0.8 → ISI/USGS floating pan (dia = 1225mm)

b) Empirical/meyer's equation

• E=km (ew-ea)(1+u₉/16)

• km → large deep of water = 0.36, Shallow, small water = 0.5

• u₉ → Monthly mean wind velocity at 9 m above the ground 

• uh = ch1/7wind velocity ∝ h1/7

c) Analytical methods

• Water budget eqⁿ, mass transfer, Energy balance

Evapotranspiration or Consumptive use

• Evapotranspiration = Transpiration + evaporation

• Crop field surrounded by dry fallow land > Surrounded by vegetation → due to Oasis effect

• Evapotranspiration is confined to daylight hours only 

• Transpiration → Water loss through the leaves of plants 

• Potential evapotranspiration → Evaporation where there is sufficient moisture available to a fully vegetated area

• Measured by → Penman’s equation, Lysimeter, Blaney-Criddle, Hargreaves class A pan

• Blaney-Criddle → Data set air-temperature , Annual evapotranspiration

• Penman’s eqn → Based on energy balance and mass transfer approach 

• Phytometer → Measurement of transpiration 

AET/PET 

• Range = 0-1

• When moisture is at FC → AET/PET = 1

• inadequate moisture → AET/PET < 1

• Clayey soil → AET/PET = 1

• At PWP → AET/PET ≈ 0

• Aridity index (AI) = ((PET-AET)/PET) x 100

• PET → Estimated by penman's equation & Blaney Criddle formula

INFILTRATION(I)

• Movement of water through the soil

• Rainfall simulator → measuring infiltration capacity

• fr < ff → Rainfall simulator type infiltrometer gives lower values then flooding type infiltrometer → Because impact of rainfall is considered

• Factor affecting infiltration of a formation → Thickness of saturated layer, temperature, vegetative cover, soil texture and structure, soil moisture content, condition at soil surface

• Infiltration capacity → The maximum rate at which a given soil, at a given time can absorb water, Changes with both time and location

• Potential infiltration→ Total infiltration along with initial basin recharge 

Horton's infiltration curve

• f=fc+[fo-fc]e-kt

• if i > fc → f = fc

• if i < fc → f = i 

• f = minimum of {i or fc}

•  i = intensity of rainfall

• Infiltration rate(f) ≤ Infiltration capacity(fc)

index

• ϕ-index = (P-R)/t 

• W-index = (P-R-S)/t

• W-index ≤ ϕ-index

• Step 1 → Find W -index

• Step2 → Assume ϕ = W-index & find ϕ-index

• ϕ-index = 0.1cm/hr → for max flood design

• ϕ-index → That separates runoff & rainfall intensity from particular strom, or rate of rainfall above which the rainfall volume = Runoff volume

• Note → Convert rainfall in mm/hr

STREAM FLOW

• Stream flow (Discharge) = Surface flow(Runoff) + Base flow

• Staff gauges → Meas water surface elevation

• Base flow → Flow in stream without contribution of direct runoff from precipitation, delayed flow that reaches the stream

• Water that percolate through the soil emerges as the dry weather flow in streams 

• Virgin flow → The flow unaffected by works of man, Artificial divergence 

• Infiltration gallery → To obtain water from perforated pipe laid in a trench in a river bed

• Traction → Flow exerted by the flowing water on the sediment particles to cause their motion

• Percolation tank → Constructed for artificial recharge of ground water

• Drainage density = Length of stream/Drainage area = L/A

• Flow through time → Avg time required for a batch of water to pass through the setting basin

• The rating curve applicable to a section of a stream → Depends on water surface elevation

• India has 12 major river Basin 

• Station rating curve → Discharge vs stage for a given point on stream usually at the gauging station 

• Radioactive isotope method → The velocity is measured by radioactivity on downstream at distance of 10 kms

• The Silt Load in the stream doesn't depend upon alignment of dam

Types of Stream

• Ephemeral Stream → Doesn't have any base flow

• Effluent stream → Receives some flow from the groundwater discharge 

• Artesian spring → Provide continuous flow of water

• Intermittent stream → 

• Perennial Stream → 

Methods of Base Flow separation

• Straight line method

• Fixed base method / Two line method

• Variable slope method / Curve extension method

• Base flow separation is performed on a unit hydrograph to get the direct runoff hydrograph 

Measurement of Q in stream flow

• Current meter  → Major velocity 

• Pygmy water current meter → in shallow streams, flume and small channel where velocity ≤ 1 m/sec

i. Direct methods

• Area velocity method, Dilution Technique, Electromagnetic method, Ultrasonic method, Moving boat method 

• Dilution Technique → Common salt (Sodium chloride) is used

Moving boat method 

• Suitable for Q measurement of fast moving surface of the stream for large alluvial rivers (Ganga)

• Measurement require → Velocity, direction of current meter, Depth and time interval b/w depth readings

ii. indirect methods 

• Hydraulic Structure

• Slope Area method → Used to estimate flood discharge based on high water marks left over in the past

Discharge - Frequency curve

• Q vs % of time the flow was equalled or exceeded

Rating curve 

• Q vs Stage (Surface elevation) for a given point

• To determine Q → Stage at section required

• For a given stage → Q ∝ √S → S = Slope

Flow duration curve 

• Plot of Stream Q vs % of time the flow equalled or exceeded

Spring

• Stratum spring → Formed when the downward passage of ground water in a permeable deposit is hindered by an underlying impervious layer

RUNOFF & DROUGHT

• Runoff unit = m³/s 

• Drainage coeff = Ratio of  total water discharge in 24 hrs(m³) to total land area(m²)

• Storage coefficient(Storativity) → Dimensionless

• Surface run-off → Water that reaches the stream channels

• Water lost → Trapped by building & Vegitatⁿ

• At eff Rainfall → Rainfall Vol = Run-off Vol

• Basin lag time is time Difference b/w centroid of rainfall excess and centroid of surface runoff

• Direct runoff → Consist of Surface runoff, Prompt interflow and precipitation over stream

• Hydrological drought → Surface water and groundwater deficit

• Permanent long term solution to drought problem may be found in the basic principle of transfer of water from surplus river basin to areas of deficit

• Drought Year → Total area affected > 20% of the total area of the country according to IMD 

• Drought affected area → Mean rainfall < 75% of normal value

• Drought prone area  → 0.2 ≤ P ≤ 0.4 → P = Probability

• Avg annual runoff potential → Godavari > Narmada > Tapi

• Best method of estimating runoff, Predicting flood of a given frequency the most reliable method → Unit hydrograph 

• Existence of building doesn't affect run-off

Runoff coefficient (K)

• K = Runoff/Precipitation

• K =(impermeability factor)/n

• impermeability(k)=kiAi /Ai   

Time of concentration 

• Time required by the drainage area to contribute to the runoff or Maximum time taken by the rainwater to reach the outlet of the basin 

• Tc = Time diff b/w entire basin starts contributing and rain starts

Kirpich equation

• To determine time of concentration in runoff Hydrograph

• t=0.0194L0.77S-0.385

SCS - CN Equation

• S=(1000/CN)-10

• Runoff = (P-0.2S)2/(P+0.8S)

• CN → curve number, S → Potential max retention, P → total Rainfall(inch)

• 1 inch = 2.5 cm → 1 mm = 0.03937 inch → 1 meter = 39.37 inch

Khosla method or Formula

• Monthly Runoff → Rm = Pm - Lm

• Lm = 0.48 x Mean Temperature(Tm) → Tm > 4.5 C

• Rm, Pm, Lm = Monthly runoff, Monthly Rainfall, Monthly losses in cm

• Annual runoff formula → F = R - K(1.81T +32)

Tubewells

• Shrouding of tubewells is generally done with pea gravels 

• Slotted type wells → Shrouding is provided

• Strainer tube well → Most important and widely used tubewell in india, Unsuitable for fine Sandy Strata 

• Deep well turbine pump → Pump to be installed if the depth of water table at a place is 50 m below the ground level

• The mode of sinking small diameter deep tubewells in Alluvial soil → Rotary drill with water jet

• Fastest method of constructing tubewells → Percussion drilling

• Infiltration well → The vertical walls provided along the banks of river to draw groundwater in dry season 

• Dug well → Used by small town for public water supply

• Avg yield from a tubewell → 40 - 50 litre/sec

• For list effect on water → one tube well should be dug in every 1.5 km² 

• Storage coefficient or Storativity of well  → Discharge per unit drawdown of well 

• Sy + Sr = porosity

• Drawdown = Double → if Q = double

• Coarse grain soil have more Sy but Sr ∝ 1/particle size 

• Yield → Volume of groundwater extracted by gravity drainage from a saturated water bearing material

• Yield of drainage basin is the runoff over long period

• Shallow well → Does Not rest on a mota formation 

• Deep Wells have more depth and more discharge as compared to Shallow Wells 

• Intakes → installed for drawing water from the source 

• The depression of water table in a well due to pumping will be maximum → Closed to the well

• Water obtained from tube well → Subsurface water

Artesian well 

• it is confined

• Has the highest Specific yield of water

• Water level b/w water table & ground level

• Performance of well is measured by its Specific capacity

• Coarse gravel aquifer highest Specific yield

1) Specific Yield

• Sy = Vol pumped/(Area x Δh) = Vol of water drain by gravity / unit drain vol of aquifer = vol of water that can yield/total vol of soil

• Max for coarse sand

• Sy = Q % drawdown

• Sy < Porosity

• Sy depends on → Compaction of stratum, distribution of pores, shape and size of particles

2) Specific capacity

• Sc = Well yield (Q) / Unit drawdown = Discharge per unit drawdown of well

• Sc of a well in an area of a stable weather conditions a function of time after starting of pumping

• Well yield = Sc x Depression head

• Specific capacity of a well decreases with time from the start of pumping 

• The specific capacity of well in an area of stable weather condition is →  A function of time after starting of pumping

• Confined well → Sc ∝ 1/Q

3) Specific retention 

• Sr = Vol of water retain / unit vol of aquifer against gravity = water retain by formation / vol of formation

• Quantity of water retained by sub-soil against pull of gravity 

• To transmit water through itself while considering unit width and full depth is under unit hydraulic gradient 

4) Specific storage 

• Amount of water that a portion of an aquifer releases from storage

5) Safe yield 

• Max water that can be .... during a critical dry day

a). Aquifer

• Yield as well as store 

• Water bearing strata is called an aquifer

• Eg → Coarse Sand and Gravel

b) Aquiclude 

• Highly porous but impermeable → Contain but not transfer

• eg. Clay

c) Aquitard

• Insignificant yield

• Partially impermeable and No yield or Poor permeability but seepage is possible

• Sandy clay

d) Aquifuge

• Neither porous nor permeable 

• eg. Rock

Type of Aquifers

i) Confined/Pressure aquifer

• Water is under pressure b/w two impervious strata → Confined at bottom and top 

• Patm/Pressure ↑es → Water level↓es

• Q ∝ R → R = drawdown

• Shape of water surface in confined aquifer for steady flow → linear

• Piezometric/potentiometric surface → Connects static water levels of a series of wells dug in a confined aquifer

ii) Unconfined/ Water Table/ Phreatic aquifer

• Dupuit's theory used

• Top water level → at water table (unconfined at top)

• Bottom →  impervious strata (confined at bottom)

• Water is under Atmospheric pressure

• Q ∝ (H² - h²) → H = Saturated depth, h = Drawdown

• Storage coefficient = Specific yield

iii) Leaky/semi confined aquifer

• b/w two semi-impervious layer

iv) Artesian Aquifer

• Water is under pressure b/w two impervious strata

• Pressure on water → Above atmospheric pressure

• Artesian system is confined source of water

• Piezometric surface of confined Aquifer above ground level

v) Perched Aquifer

• Found in an Unconfined Aquifer

HYDROGRAPH

• Hydrograph → Runoff discharge vs time

• To determine → Avg yield from stream

• Number of peaks may be More than two

• Vol.of Rainfall = Area of Hydrograph = Catchment area x 1cm

• Depth of rainfall (rainfall excess) = Vol of rainfall/Area of catchment = Graph Area/Catchment Area.

• Eff Rainfall = Direct runoff vol/Area of catchment

• Peak of direct runoff = Peak of flood hydrograph - Base flow

• Peak of unit hydrograph = Peak of direct runoff/rainfall excess 

• Inflation → Where direct runoff ends

• from study of annual hydrograph only intermittent classification of river is possible 

• Attenuation → owing to the storage effect the peak of the outflow hydrograph will be smaller than that of the inflow hydrograph 

• A flood wave a known inflow hydrograph is rated through a large Reservoir → the outflow hydrograph will have attenuated peak with increased time base 

• Peak Q ∝ Storm intensity

Factor Affecting Hydrograph

• Rising limb → Depends on climatic factor (intensity, duration & distribution of rainfall)

• Recession/Falling limb → On Basin/Catchment characteristics only

• The inflection point on the recession side of hydrograph → indicates the end of direct runoff, and the condition of maximum storage in catchment 

UNIT HYDROGRAPH

• Eff/Excess rainfall vs Direct Runoff

• Mr L.K. sherman

• a Hydrograph of direct runoff resulting from unit (1 cm ) of effective rainfall or one unit of rainfall excess

• Unit in UH Refers → Unit depth of direct runoff or unit precipitation

• Assumption → Time invariance & linear response, Rainfall is uniform all over the catchment 

• Use → Transformation of excess rainfall into direct runoff 

• For flood estimation → Hilly areas, large medium and small basin

• Limitations → Area b/w 2 km² - 5000km², No large storage, precipitation in the form of Rain only

• UH is best reliable method → For predicting floods of a given frequency

• Best unit duration = ¼th of Basin lag 

• Principle of linearity → Ordinate of the direct the direct runoff hydrographs of a common base period are directly proportional to the volumes of runoff represented by the respective hydrograph

• The unit hydrograph can be used to evaluate the hydrograph of Storms of any duration 

• In the derivation of unit hydrograph the flood hydrograph used should have the duration of rainfall as 20% - 30% of basin lag

• Peak ordinate or Q ∝ 1/time

• If two 2 hour unit hydrograph are staggered by 2 hour and added graphically → the resultant hydrograph will be 4 hour unit hydrograph with 2 cm runoff

• Hydrographs of direct runoff due to eff rainfall of equal duration have the same time base, Eff rainfall is uniformly distributed throughout the whole area of drainage basin, Hydrograph of direct runoff from a basin due to a given period of eff rainfall reflects the combination of all the physical characteristics of the basin

S-CURVE HYDROGRAPH

• Useful to obtain UH of shorter duration from longer duration & vice versa → To generate X hr hydrograph  from Y hr hydrograph

• S-curve is summation of unit hydrograph 

• Q = (Catchment area/Duration) x 1cm = (A/D) x 1cm

• Equilibrium Q = 2.778(A/D)

• Number of UH required to produce SH = T/D = Equilibrium Q/ UH duration

SYNTHETIC UNIT HYDROGRAPH

• By Synder

• 
  

INSTANTANEOUS UNIT HYDROGRAPH

• Unit hydrograph of infinity small duration(zero duration) or Hydrograph of unit Rainfall excess and infinity small duration

• Ordinate → IUH is the slope of S-Curve with eff rainfall intensity of 1 cm/hr

FLOOD & ROUTING

• CWC(central water commission) is the nodal agency for flood forecasting

• Peak drainage discharge → Maximum rate of storm run-off.

• Bunds are temporary Spurs

• Probable maximum precipitation (PMP) → Greatest of extreme rainfall of a given duration that is physically possible over a station or Upper limit of rainfall that is justified climatologically

• Intensity of storm ∝ Return period ∝ 1/Storm period

• IS 11223-1985 → Design of flood for dams

• Gauge-discharge curve → Estimate the flood discharge passing through a weir site

• During passage of flood wave → Qr > Qf → Discharge at the stage when the water surface was Rising > Discharge at the same stage when water surface was falling 

Types

• Standard Project flood (SPF) = 40-60% of probable max flood (PMF), Likely to be exceeded in magnitude only at rear occasion in the estimated life of the project 

• Design flood → Adopted for design of Hydraulic structure (Spillways, flood banks, bridge openings), max flood that any structure can safely pass

• Design flood in india for barrage and minor dam → Max of i). Standard project flood or ii). a 100-year flood 

• Probable max flood →  Extremely large but physically possible flood in the region, from severe-most combination of critical meteorological & hydrological condtⁿ

Rational Formula

• Qp = CiA/360 = kiA/360 = AIR/360

• A → Catchment area (hectare), I → Intensity of storm (mm/hr), R or C or K → Run-off coefficient = Runoff/Rainfall

Empirical formula for flood peak or Annual yield

• Qp → m³/s, A → Km² → in below formulas a, b, c, d

Dickens formula

• Central & Northern india

• Qp = CA3/4

• C = 11.4 (North india), C = 14-19.5(Central india), C = 6-30 in general

Ryve's formula 

• Tamil Nadu, Parts of andhra pradesh & karnataka.

• Qp = CA2/3

Faming

• Qp = CA5/6

Inglis & De Souza Formula

• Fan shaped catchment, Western ghat of Maharashtra(Former bombay presidency)

• Used only in Maharashtra

• Qp =123A =124A /A+10.4

Jarvis Formula

• Qp =CA

• Eastern india

Gumbel’s Method (Fisher triplet distribution)

• Estimation of design flood for a particular return period

• e-e-y=1-1/T Y= -ln.ln (T/(T-1))

• Y → Reduced variate, T → Return period

• Required data → Mean value, Std deviation, Length of record

• Based → on Extrapolation for large return period

Pettis formula

• Based on rainfall and drainage area

• Q = C(P.B)5/4

Lloyd-formula for design of stormwater drain

• Q = r P / 6tc

Fuller’s Formula

• Q = CA0.8(1+0.80logT)(1+2.67A-0.3 )

Risk and Reliability or Flood frequency analysis 

• X year flood → Means flood can we expected on an average once every 50 years 

• Return period(T)=(N+1)/m=1/P

• P=1/T=m/(N+1) → Probability of occurrence or exceedance of an event

• N → Total entries, m → position no or order no of rank 

• q = P - 1 → Probability of non occurrence 

• Risk = 1 - qⁿ = 1- (1-p)ⁿ → Probability of exceedance at least once or larger magnitude in next n years

• Reliability (Assurance) = qⁿ = (1-p)ⁿ → Probability of non occurrence in design life 

• Probability of exceedance of m times in n year = nCm pm qn-m  

• Probability of exceedance or exactly 01 time in n year = nC₁ pqn-1

• Weibull formula → P = m/(N+1) → m = Order number, N = No of years of record

Flood Routing  or Reservoir routing

• Integral part of → Flood forecasting, reservoir design, spillway design

• When the inflow is into a Reservoir with an uncontrolled outlet → The point of intersection of inflow and outflow hydrographs coincides with the peak of outflow hydrograph 

a). Lumped Routing (Hydrological method)

• Eqn used → Only Continuity eqn

i. Reservoir / Storage Routing 

• Storage is function of outflow discharge

• Graphical method → Goodrich method, Modified Puls method

ii. Channel Routing

• Storage is function of both inflow and outflow discharge

Muskingum method

• Most widely used Hydrological channel routing method

• Storage → Prism routing & reserved routing

• Involves concept of wedge and prism storage

• The prism storage in a river reach during the passes of a flood wave is function of outflow only 

• Three parameter modal

• S=K(XIm+(1-X)Qm

• Co + C₁ + C₂ = 1

b). Distⁿ Routing (Hydraulic fr)

• Eqn used → Both eqn of motion and Continuity eqn

• Stilling well → Flood Gauge recorder

Methods of mitigating floods

• By temporary evacuation of low lying areas and flood warnings

•  by construction of leaves, flood banks and dykes

•  by channel improvement 

Levees 

• Most frequently used flood control measure

• Earthen embankments constructed parallel to the river at some suitable distance

• ↑es Q at D/S, ↑es flow V, ↑es water surface elevation, ↓es flood storage