Floods •A flood is any relatively high flow that overtops the natural or 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 water channels 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.
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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.
• 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
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
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
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.
• 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.
• 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
• 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.
• 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
• 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.
• 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.
• 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.
• 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
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
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.
• 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
• 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.
• 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
• 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
• 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.