Floods and Ground Water

7/28/2019 Floods and Ground Water

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Floods

•

A flood is any relatively high flow that overtops the naturalor artificial banks in any reach of a stream.

• The flood is the result of runoff from rainfall or melting

snow in quantities too great to be confined in the low waterchannels or streams.

•When the banks are overtopped water spreads over the

flood plain and thus cause damage to crops and property

within the flood plain of the stream.



• It is not possible to prevent the floods but it is

possible to prevent or reduce the damage due tofloods by controlling the floods.

•Thus flood control or flood management is definedas the prevention or reduction of the flood damage.



TYPES OF FLOOD •

Ordinary flood:-The flood that are sure to be equalled

in magnitude once or more times in the estimated life of the project

• Standard project flood (SPF):- Is the flood that islikely to be exceeded in magnitude only at rare occasions.

And thus constitutes a standard for the design of structuresthat would provide enough flood protection. Although theSPF is of a such high magnitude.

• Probable maximum flood (PMF):- That mightoccur under the worst meteorological and hydrological

conditions. This includes flood ranging in magnitudebetween SPF and PMF. As it is economically and practicallyunfeasible to provide flood control measures against allfloods up to PMF value.



DESIGN FLOOD

•

While designing any important engineeringstructures provision must be made for the flood that

is likely to occur during the life time of that particular

structure.

• Therefore while designing structures we have tothink of a flood value against which these structures

can be designed as safe.

• We can neither choose a very high value nor we can

choose a very low value.



Determination of Maximum FloodDischarge

• The high flood discharge for smaller drain can beworked out by using empirical formulas; and forlarge drains other methods such as Hydrograph

analysis, Rational formula, etc may be used.• In general the methods used in the estimation of the

flood flow can be group as:

• Physical Indications of past floods

• Empirical formulae and curves

• Overland flow hydrograph and unit hydrograph




• Physical Indications of past floods- flood marks and localinquiry:

• The maximum flood discharge may be approximately estimated by enquiring from the residents in the village

situated on the banks of the river about the flood marksthat the high flood in their memory in the past may haveleft on the river banks.

• By noting the high water marks along the banks of theriver the cross-section area and wetted perimeter of the

flow section as well as the water surface slope may becomputed and using the manning’s formulae, with suitableassumed value of the flood discharge may be determined.




• Estimation of maximum flood discharge from rating curve: Duringthe period of high flood, it is almost impossible to measure thedischarge by making the use of markings of the high water markson the banks of the river, the elevated water level, can becalculated. Making use of this values high water marks in meters

the value of maximum flood discharge can be calculated, by extrapolation from the stage or rating discharge curve• The above mentioned curve needs to be extended for the higher

value of stage. It is done by using following methods.• Simple Judgment• Logarithmic method• The above mentioned curve needs to be extended for the higher

value of stage, it is done by using following methods.• (a) Simple Judgment• (b) Logarithmic Method




• (a) Simple Judgment

The rating curve can be extended, by simple Judgment.




Simple Judgment method




• (b) Logarithmic method:The following equation can be used to extend the rating curveQ= K d n

Where,

Q= Discharge (Cumecs)d= Stage in (m)K, n = ConstantsBy taking logarithms of both sides, we get,

Log Q= log k + n Log dIf the available curve is plotted on a log-log paper, then itshould be a straight line.

This line can be extended to calculate the discharge at ahigher stage.




Logarithmic method




Empirical FormulasSeveral empirical formula have been developed for estimatingthe maximum or peak value of flood discharge. In theseformulae the maximum flood discharge Q of a river is

expressed as a function of the catchment area A. Most of these formulae may be written in a general form as:• Q = C A n

Where, C is coefficient and n is index, Both C and n dependupon various factor, such as

(i) Size ,shape and location of catchment ,(ii) Topography of the catchment,(iii) intensity and duration of rainfall and distribution patternof the storm over catchment area.




• Dicken’s formula:Q = CA ¾Where,Q= Maximum flood Discharge in cumec.A= Area of Catchment in sq. KmC= coefficient depending upon the regionThe maximum value of C= 35.

• Ryve’sFormula:Q= CA 2/3

Where, Q= discharge in cumecA= Catchment Area in Sq. . Km

C= coefficient depending upon the region




Rational Method:In this method it is assumed that the maximum flood flow is producedby a certain rainfall intensity which lasts for a time equal to or greaterthan the period of concentration time. When a storm continuesbeyond concentration time every part of the catchment would be

contributing to the runoff at outlet and therefore it representscondition of peak runoff. The runoff corresponding to this condition is given by:Q = 2. 78 C Ic AWhere, Q = Discharge in Cumec,

C= Coefficient which depends upon the characteristics of thecatchment.Ic= The critical Intensity of rainfall (cm/hr) corresponding to the timeof Concentration (tc) of the catchment for a given recurrence intervalobtained from the intensity of duration frequency curves.A= Catchment Area in Km 2




Rational Method:




Inglis formula:• Q= 124 A = 124 A ½

√A + 10.4Where Q= discharge in cumec

A= area of catchment in Sq.. Km.Inglish formula is derived by using the data of rivers of Mahashtra, where it is commonly used.

Ali Nawab Jang Bahadur formula:Q= CA ( 0.993- 1/14 log A)

Where, Q= Discharge in CumecA=area of catchment insq .kmC= Coefficient which varies from 48 to 60.




• Myer’s formula

• Q= 175 √A

• Where,

• Q= Discharge in Cumec

• A= Area of Catchment in Sq.. Km.




Envelop Curves:• Areas having similar topographical features and

climatic conditions are grouped together. All Available

data regarding discharges and flood formulae arecompiled along their respective catchment areas. Themaximum discharges are then plotted against theareas of the drainage basins and a curve is drawn tocover or envelop the highest plotted points, which is

known as envelope curve. By using envelop curves themaximum flood discharge may be estimated if the areaof the drainage basin is known.







• Overland flow Hydrograph and Unit Hydrograph:• A Hydrograph is a graphical plot of discharge of a natural stream

or river versus time. It shows variation of discharge with time, at aparticular point of a stream. It also shows the time distribution of total runoff at the point of measurement. Discharge is usually

expressed in cumec or hectare-metre per day and time is expressedin hours, days or months. Discharge is plotted on Y-axis and thecorresponding time is plotted on X-axis.

• Unit Hydrograph: A unit Hydrograph is a hydrograph

representing 1 cm of runoff from a rainfall of some duration andspecific areal distribution.• Unit Hydrograph is defined as the hydrograph of surface runoff of

a catchment area resulting from unit depth of rainfall excess or netrainfall occurring uniformly over the basin at uniform rate for aspecified duration.



Hydrograph




• When a unit hydrograph is available for thecatchment under consideration, it can be applied tothe design storm to yield the design floodhydrograph from which peak flood value can beobtained.

• Whenever possible it is advisable to use the unithydrograph method to obtain the peak flood. It givesnot only the flood peak but also the complete floodhydrograph which is essentially required indetermining effective storage of reservoir on floodpeak through flood routing.



Unit Hydrograph



Statistical or Probability method

•

In these method the prediction for the future floodsare made on the basis of available records of the pastfloods.

• These methods can be safely used to determine themax. Flood that is expected on the river with a given

frequency.• Probability of occurrence or exceedence is

represented by P and equal to 1/T where T is therecurrence interval

•Probability of its non occurrence is q= 1-P

• Probability of the event occurring r times in a nsuccessive years



Pr.n= ncr.Pr.qn-r

Hence the probability of an event not occurring at all in “n” successive years would be equal to qn which is equal to (1-

P)n also the probability of an event occurring at least oncein n successive years (R) would evidently be equal to 1-qn or[1-(1-P)n]

R= 1-qn= [1-(1-P)n]

Probability plotting on empirical relations• Empirical California formula P=M/N

• (T)= 1/P=N/M

• Weibult formula:- P=M/(N+1)

•Hazen formula:- P= (M-0.5)/N

• Cbegodayev formula:- P= (M-0.3/N+0.4)

• Blom formula:- P= (M-0.44/N+0.12)

• Frequency of return period T= (N+1/M)

• Chance percentage = 100/T



Flood control measures

• Flood control measures generally consist of twoapproaches:-

1. Constructing high earthen walls along the banks of theriver to protect over spills into the adjoining areas. Such ameasure can be adopted only for a particular length of theriver.

2. Storing excess water during heavy rains at appropriateplaces in the river reach, so as to reduce the river flowdownstream. This may be achieved by constructing a damtype obstruction across the river and storing water in theupstream portion to form a reservoir

• Increasing the river capacity by improving the river crosssection, so as to increase the flow velocity thereby reducingwater depth development of cut-offs in meandering riversto help reduce river length



• Construction of diversion channels to reduce the

discharge in the main river

•

Constructing terraces to help increased groundinfiltration

• Adopting soil and water conservation techniques to

help increase the vegetative cover in the catchment

area of the river to reduce the runoff

• By construction of dikes and flood walls:- dikes also

called levees, are earthen embankments which are

raised parallel to the river flow• Provision of drainage sluices through dikes



Economics of flood control

• Flood protection should be considered as a problem of

applied economics and the safety against floods must beconsidered as the integral problem of the entire society

• The losses caused by flood divided into two categories

1. Intangible losses:- are those which are most important

and can not be evaluated in monitory values• Loss of human life

• Anxiety and general social distress

• Snake bites and other physical ailments

• Reduced chances left to industrialists for developing newindustries in flood prone areas



2. Tangible losses:-

(a) direct tangible losses

• Loss of cattle and live stock

• Destruction of personal properties• Loss of earnings and services

• Loss of growing and harvest in agricultural fields

• Reduction in property values

(b) indirect tangible losses• Increased expenditure in medical care

• Losses due to stoppage and disruption of business

• All these losses may become benefits as when flood

control measures are assured. Expenditure on floodcontrol is thus comparable to life insurance, where thesecurity received is the sufficient justification forpayment of premium.



Channel routing









Ground water

• Aquifers:- A permeable stratum or a geological

formation of permeable material which is capable of

yielding appreciable quantities of ground water

under gravity is known as an aquifer. Aquifer vary in

depth, lateral extent and thickness but in general allaquifers fall into one of the two categories.



Unconfined or Non artesian aquifers

• The top most water bearing stratum having no confined

impermeable over burden (aquiclude) lying over it is known as

unconfined aquifer. The ordinary gravity wells of 2 to 5 m

diameter which are constructed to tap water from the top most

water bearing strata, are known as unconfined or non artesian

wells. The water level in these wells will be equal to the level of the water table. Such wells are known as wells or gravity wells.



Confined aquifers or Artesian aquifers

When an aquifers is confined on its upper and under

surface by impervious rock formation that is aquicludeand is also broadly inclined so as to expose the aquifer

some where to the catchment area at a higher level for

the creation of sufficient hydraulic head. It is called as

confined aquifer or artesian aquifer. A well excavatedthrough such an aquifer called as flowing well.



• Aquiclude:- Geological formation of relatively

impermeable material which permits storage of water

but it is not capable of transmitting water in sufficient

quantity ex. Clay

• Aquifuge:- Geological formation of relatively

impermeable material which neither contains nor

transmits water. Ex. Granite• Aquitard:- geological formation of poorly permeable

material which permits storage of water but obstructs

ground water movement and does not yield water

freely to wells. Ex. Sandy clay



• Perched aquifer:- if within the zone of saturation on impervious depositbelow a pervious deposit is found to support a body of saturatedmaterial called as perched aquifer.

• Infiltration galleries:- An infiltration gallery is a horizontal conduithaving permeable boundaries so that groundwater can infiltrate intothe same. It is generally provided in highly permeable aquifers with high

water table so that adequate head is available for gravity flow of groundwater into the gallery. It is frequently located near a perennial rechargesource and is sometimes placed along or under a river bed usual depthat which a gallery is placed range from 3to 6m



Water-Logging

• In agricultural land, when the soil poreswithin the root zone of the crops getssaturated with the subsoil water, the air

circulation within the soil pores gets totally stopped. This phenomenon is termed as waterlogging.

• The water logging makes the soil alkaline incharacter and the fertility of land is totally destroyed and the yield of the crop is reduced.



Water-Logging



Causes of Water-Logging

• The following are the main cause of waterlogging:

• Over Irrigation: In inundation irrigation since

there is no controlling system of water supply it may cause over irrigation. The excess waterpercolates and remains stored within the rootzone of the crops. Again, in perennialirrigation system if water is applied more thanwhat is required than this excess of water isresponsible for water logging.



Over Irrigation




• Seepage from Canals:

• In Unlined canal systems, the water percolatesthrough the bank of the canal and gets

collected in the low lying areas along thecourse of the canal and thus the water table

gets raised. This seepage is more in case of

canal in banking



Seepage from Canals




• Inadequate Surface Drainage:• When the rainfall is heavy and there is no proper

provision for surface drainage the water getscollected and submerges vast area. When thecondition continuous for a long period, the watertable is raised.

• Obstruction in Natural Water Course:

•

If the bridges or culverts are constructed across withthe opening with insufficient discharges capacity,the upstream area gets flooded and this causes waterlogging.




• Obstruction in Sub-Soil Drainage:• If some impermeable stratum exists at a lower

depth below the ground surface, then themovement of the subsoil water gets obstructedand this cause water logging in the area.

• Nature of Soil:• The soil having low permeability, like black

cotton soil, does not allow the water to percolatethrough it. So, in case of over irrigation or flood,the water retains in this type of land and causewater logging.



Nature of Soil




• Incorrect method of Cultivation:• If the agriculture land is not levelled properly and there

is no arrangement for the surplus water to flow out, thenit will create pools of stagnant water leading to water

logging.• Seepage from Reservoir:

• If the reservoir basin consists of permeable zones, cracksand fissures which were not detected during the

construction of dam, these may cause seepage of water.This sub-soil water will move forward toward the low lying area and cause water logging.



Seepage from Reservoir




• Poor Irrigation Management:

• If the main canal is kept open for a long periodunnecessarily without computing the total water

requirement of the crops, then this leads to overirrigation, which shall result in water logging.

• Excessive Rainfall:

• If the rainfall is excessive and the water gets no timeto get drained off completely, then a pool of stagnantwater is formed which might lead to water logging.



Excessive Rainfall




• Topography of the land:

• If the agricultural land is flat, i.e. with no country slope and consists of depression or undulations, then

this leads to water logging.• Occasional Flood:

• If an area gets affected by flood every year and there

is no proper drainage system, the water table getsaffected and this cause water logging.



Topography of the land






Adverse Effects of Water-Logging

• The following are the adverse effects of water logging:• (i) Salination of Soil:

• Due to water logging the dissolved salts like sodiumcarbonate, sodium chloride and sodium sulphate cometo the surface of soil. When the water evaporates fromthe surface, the salts are deposited there. This processis known as salinization of soil. Excessiveconcentration of salts make the land alkaline. It does

not allow the plants to thrive and thus the yield of crop is reduced. This process is also known as saltefflorescence



Salination of Soil




• Lack of Aeration• The crops require some nutrients for their growth

which are supplied by some bacteria or micro-organisms by breaking the complex nitrogenouscompound into simple compound which areconsumed by the plants for their growth. But thebacteria requires oxygen for their life and activity.When the aeration in the soil is stopped by water

logging, these bacteria cannot survive without oxygenand the fertility of the land is lost which results inreduction of yield.



Aeration of soil




• Fall of Soil Temperature:• Due to the water logging the soil temperature is

lowered. At low temperature of the soil the activity of the bacteria becomes very slow and consequently the

plants do not get the requisite amount of food in time.Thus the growth of the plants is hampered and the yield also is reduced.

• Growth of weeds and aquatic plants:• Due to water logging, the agricultural land is

converted to marshy lands and the weeds and aquaticplants grow in plenty. These plants consume the soilfoods in advance and thus the crops are destroyed.



Growth of weeds




• Diseases of Crops:• Due to low temperature and poor aeration, the crops

get some diseases which may destroy the crops orreduce the yields.

• Difficulty in Cultivation:• In water logged area it is very difficult to carry out the

operation of cultivation such as tilling, ploughing. etc.

• Restriction of Root Growth:

• When the water table rises near the root zone the soil gets saturated. The growth of the roots is confined only to the top layer of the soil. So, the crop cannot bematured properly and the yield is reduced.






Control of Water Logging

• The following measures may be taken to controlwater logging:

• Prevention of percolation from Canals:• The irrigation canals should be lined with

impervious lining to prevent the percolation of water through the bed and banks of the canals.Thus the water logging may be prevented.

• Intercepting drains may be provided along thecourse of the irrigation canals in place where thepercolation of water is detected. The percolationwater is intercepted by the drains and the wateris carried to other natural water course.




• Prevention of percolation from the reservoirs:

• During the construction of dams, the geologicalsurvey should be conducted on the reservoir

basin to detect the zone of permeable formationsthrough which water may percolate. These zonesshould be treated properly to prevent seepage, If afterwards it is found that there is still leakage of water through some zone, then sheet pilingshould be done to prevent the leakage.



Prevention of percolation from thereservoirs




• Control of Intensity of Irrigation:• The intensity of irrigation may cause water logging so,

it should be controlled in a planned way so that thereis no possibility of water logging in a particular area.

• Economic Use of Water:• If the water is used economically, then it may control

the waterlogging and the yield of the crop may behigh. So, Special training is required to be given to the

cultivators to realize the benefits of economical use of water. It helps them to get more crops by eliminatingthe possibility of water logging.



Remedial Measures: Against Water-Logging

Optimum Use of Water




• Fixing of Crop Pattern: Soil survey should beconducted to fix the crop pattern. The cropshaving high rate of evapotranspiration should berecommended for the area susceptible to water

logging.• Providing Drainage System:• Suitable drainage system should be provided in

the low lying area so that rain water does not

stand for long days. A network of sub-surfacedrains are provided which are connected to thesurface drains. The surface drains discharge thewater to the river or any water course.




Leaching of Saline Soil

Crop Rotation




•

Improvement of Natural Drainage:• Sometimes, the natural drainage may be completely silted up or

obstructed by weeds, aquatic plants, etc. The affected section of thedrainage should be improved by excavating and clearing theobstruction.

• Pumping of Ground water:• A number of open well or tube wells are constructed in the water

logged area and the ground water is pumped out until the goesdown to a safe level. The lifted ground water may be utilized forirrigation or may be discharged to the river or any water course.

• Construction of Sump Well:• Sump Well may be constructed within the water logged area and

they help to collect the surface water. The water from the sumpwell may be pumped to the irrigable lands or may be discharged toany river.




Tile Drain Canal Lining

Salt efflorescence



• In salt efflorescence soils may be classified as saline,saline-alkali and alkali soils on the basis of their solublesalt concentration and exchangeable sodium content.

• These soils are formed from the normal soil through theaccumulation of salts from applied irrigation water or byupward moving ground water.

• The problem with these soils is that their crop yield is

considerably low and gradually these soils becomeunsuitable for cultivation.

• The formation of these soils may be prevented throughproper planning and management of irrigation.

• The soluble salt present in soils are mainly chlorides,sulphates and sometimes nitrates of sodium, calcium,magnesium and potassium, calcium sulphate(gypsum)and calcium magnesium carbonates



Land Reclamation

• The reclamation of land is the process of making aland culturable after it gets converted touncultivable area due to the bad effect of waterlogging. The following are the general methods of land reclamation.



Land Reclamation



Leaching

• Leaching is a process for reclamation of the saline soil.In this process, the agricultural land is flooded withwater to a depth of about 20-30 cm. the salt depositedon the surface are dissolved. Some portion of salt is

then drained off through the subsoil drainage systemand some portion of salt is then drained off throughthe subsoil drainage system and some portion isremoved by surface drainage system. This operation isrepeated several times at specific intervals

• Generally, Leaching is followed by crop rotation asrecommended by agricultural department



Leaching



Land Reclamation

• Addition of Chemical Agent• For improving he alkaline soil a chemical like gypsum is

generally added with irrigation water. The gypsumneutralizes the alkaline effect of the soil and yield of the

crop is increased.• Surface Drainage

• Proper surface drainage system should be provided inthe agricultural land so that the water does not

accumulates for a long time. The surface drains also helpin draining the saline water in case of leachingoperation.



Addition of Chemical Agent



Land Reclamation

• Sub-Surface Drainage• The sub-surface drainage system on the

agricultural land should also be provided for

draining the excess water from the root zone. Italso helps in draining of saline water in case of leaching operation.

• Addition of Waste Products

• Waste products like ground nut shells, saw dust,etc. are added to the alkaline soil and these arevery effective in removing the salinity of soil.



Land Reclamation

• Excavation of ponds• Ponds are excavated at suitable places within the

water logged area. The excess run-off is collected

in the ponds. The pore water also flows towardsthe pond and thus the saturation in the root zoneof the crop is reduced. In fact, these pondscontrol the water logging in rainy season and indry season the water of the pond may be used forlift irrigation



Excavation of Ponds



Land Reclamation

• Pumping of Water from Tube Wells• Some tube wells are sunk within the water

logged area. The water is pumped

continuously from the tube wells, Initially thiswater is discharged to a river or pond.

• When the reclamation of the land is complete,

the water may be utilized for lift irrigation.



Pumping of Water from Tube Wells

G d h d i



Ground water recharge and necessity

• Artificial surface reservoirs are constructed by building dams in order tostore the surplus surface waters in the same manner artificialunderground reservoirs are now a days developed by artificial recharge forstoring water underground. The development of such a reservoir may beadvantageous as compared to the development of a dam reservoir

• 1 much pure water can be obtained from an underground reservoir source

•No space is required for building such a reservoir

• The cost of building such a reservoir by recharging the aquifers may beconsiderably less than the cost of the surface reservoirs

• The water lost in evaporation from an underground reservoir is much lessthan the water lost from a surface reservoirs.

• The raising of the water table by artificial recharge may help in building

pressure barriers to prevent sea water intrusion in the coastal areas.

h d f d



Methods of improving ground water storage

• Spreading method:- this method consists inspreading the water over the surfaces of permeableopen land and pits.

• In this method the water is temporarily stored in

shallow ditches.• The stored water slowly and steadily percolates

downward so as to join the nearby aquifers.

• The recharging rate depends upon the permeabilityof the spread area and on the depth of water stored.Generally 1.5 m/day have been possible

h ll h d



Recharge well method

• This method consist in injecting the water intobore holes, called recharge wells.

• In this method the water is fed into recharge wellby gravity or pumped under pressure to increase

the recharge rate.• Recharge well method is certainly preferred

when the spreading method can not yieldappreciable recharge.

• Because of low permeable areas the water to beused in the recharge well should be purer thanthat is required in spreading method.

Induced infiltration method



• By this method the water table gradient from a source of recharge.

• In this method Renney type wells are constructed near the river banks.

• The percolating water is collected in the well through radial collectors and

is then discharged as recharge into a lower level aquifer

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Floods and Ground Water

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