Floods and Ground Water

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.

• It is not possible to prevent the floods but it is possible to prevent or reduce the damage due to floods by controlling the floods. • Thus flood control or flood management is defined as the prevention or reduction of the flood damage.

TYPES OF FLOOD • Ordinary flood:- The flood that are sure to be equalled •

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in magnitude once or more times in the estimated life of the project Standard project flood (SPF):- Is the flood that is likely to be exceeded in magnitude only at rare occasions. And thus constitutes a standard for the design of structures that would provide enough flood protection. Although the SPF is of a such high magnitude. Probable maximum flood (PMF):- That might occur under the worst meteorological and hydrological conditions. This includes flood ranging in magnitude between SPF and PMF. As it is economically and practically unfeasible to provide flood control measures against all floods up to PMF value.

DESIGN FLOOD • While designing any important engineering structures 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 to think 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 Flood Discharge • The high flood discharge for smaller drain can be worked out by using empirical formulas; and for large 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

Determination of Maximum Flood Discharge • Physical Indications of past floods- flood marks and local inquiry: • 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 marks that the high flood in their memory in the past may have left on the river banks. • By noting the high water marks along the banks of the river the cross-section area and wetted perimeter of the flow section as well as the water surface slope may be computed and using the manning’s formulae, with suitable assumed value of the flood discharge may be determined.

Determination of Maximum Flood Discharge • Estimation of maximum flood discharge from rating curve: During the period of high flood, it is almost impossible to measure the discharge by making the use of markings of the high water marks on the banks of the river, the elevated water level, can be calculated. 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

Determination of Maximum Flood Discharge • (a) Simple Judgment The rating curve can be extended, by simple Judgment.

Determination of Maximum Flood Discharge

Simple Judgment method

Determination of Maximum Flood Discharge

• (b) Logarithmic method:

The following equation can be used to extend the rating curve Q= K d n Where, Q= Discharge (Cumecs) d= Stage in (m) K, n = Constants By taking logarithms of both sides, we get, Log Q= log k + n Log d If the available curve is plotted on a log-log paper, then it should be a straight line. This line can be extended to calculate the discharge at a higher stage.

Determination of Maximum Flood Discharge

Logarithmic method

Determination of Maximum Flood Discharge Empirical Formulas Several empirical formula have been developed for estimating the maximum or peak value of flood discharge. In these formulae 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=CAn Where, C is coefficient and n is index, Both C and n depend upon various factor, such as (i) Size ,shape and location of catchment , (ii) Topography of the catchment, (iii) intensity and duration of rainfall and distribution pattern of the storm over catchment area.

Determination of Maximum Flood Discharge

• Dicken’s formula:

Q = CA ¾ Where, Q= Maximum flood Discharge in cumec. A= Area of Catchment in sq. Km C= coefficient depending upon the region The maximum value of C= 35.

• Ryve’s Formula:

Q= CA 2/3 Where, Q= discharge in cumec A= Catchment Area in Sq. . Km C= coefficient depending upon the region

Determination of Maximum Flood Discharge

Rational Method:

In this method it is assumed that the maximum flood flow is produced by a certain rainfall intensity which lasts for a time equal to or greater than the period of concentration time. When a storm continues beyond concentration time every part of the catchment would be contributing to the runoff at outlet and therefore it represents condition of peak runoff. The runoff corresponding to this condition is given by: Q = 2. 78 C Ic A Where, Q = Discharge in Cumec, C= Coefficient which depends upon the characteristics of the catchment. Ic= The critical Intensity of rainfall (cm/hr) corresponding to the time of Concentration (tc) of the catchment for a given recurrence interval obtained from the intensity of duration frequency curves. A= Catchment Area in Km 2

Determination of Maximum Flood Discharge Rational Method:

Determination of Maximum Flood Discharge Inglis formula: • Q= 124 A = 124 A ½ √A + 10.4 Where 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 Cumec A=area of catchment insq .km C= Coefficient which varies from 48 to 60.

Determination of Maximum Flood Discharge • • • • •

Myer’s formula Q= 175 √A Where, Q= Discharge in Cumec A= Area of Catchment in Sq.. Km.

Determination of Maximum Flood Discharge Envelop Curves:

• Areas having similar topographical features and climatic conditions are grouped together. All Available data regarding discharges and flood formulae are compiled along their respective catchment areas. The maximum discharges are then plotted against the areas of the drainage basins and a curve is drawn to cover or envelop the highest plotted points, which is known as envelope curve. By using envelop curves the maximum flood discharge may be estimated if the area of the drainage basin is known.

Determination of Maximum Flood Discharge

Determination of Maximum Flood Discharge • 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 a particular 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 expressed in hours, days or months. Discharge is plotted on Y-axis and the corresponding 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 and specific areal distribution. • Unit Hydrograph is defined as the hydrograph of surface runoff of a catchment area resulting from unit depth of rainfall excess or net rainfall occurring uniformly over the basin at uniform rate for a specified duration.

Hydrograph

Determination of Maximum Flood Discharge • When a unit hydrograph is available for the catchment under consideration, it can be applied to the design storm to yield the design flood hydrograph from which peak flood value can be obtained. • Whenever possible it is advisable to use the unit hydrograph method to obtain the peak flood. It gives not only the flood peak but also the complete flood hydrograph which is essentially required in determining effective storage of reservoir on flood peak through flood routing.

Unit Hydrograph

Statistical or Probability method • In these method the prediction for the future floods are made on the basis of available records of the past floods. • These methods can be safely used to determine the max. 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 the recurrence interval • Probability of its non occurrence is q= 1-P • Probability of the event occurring r times in a n successive 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 (1P)n also the probability of an event occurring at least once in 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 two approaches:1. Constructing high earthen walls along the banks of the river to protect over spills into the adjoining areas. Such a measure can be adopted only for a particular length of the river. 2. Storing excess water during heavy rains at appropriate places in the river reach, so as to reduce the river flow downstream. This may be achieved by constructing a dam type obstruction across the river and storing water in the upstream portion to form a reservoir • Increasing the river capacity by improving the river cross section, so as to increase the flow velocity thereby reducing water depth development of cut-offs in meandering rivers to help reduce river length

• Construction of diversion channels to reduce the discharge in the main river • Constructing terraces to help increased ground infiltration • 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 be considered 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 new industries 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 flood control is thus comparable to life insurance, where the security received is the sufficient justification for payment 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 all aquifers 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 aquiclude and 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 excavated through 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 deposit below a pervious deposit is found to support a body of saturated material called as perched aquifer.

• Infiltration galleries:- An infiltration gallery is a horizontal conduit having permeable boundaries so that groundwater can infiltrate into the same. It is generally provided in highly permeable aquifers with high water table so that adequate head is available for gravity flow of ground water into the gallery. It is frequently located near a perennial recharge source and is sometimes placed along or under a river bed usual depth at which a gallery is placed range from 3to 6m

Water-Logging • In agricultural land, when the soil pores within the root zone of the crops gets saturated with the subsoil water, the air circulation within the soil pores gets totally stopped. This phenomenon is termed as water logging. • The water logging makes the soil alkaline in character 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 water logging: • Over Irrigation: In inundation irrigation since there is no controlling system of water supply it may cause over irrigation. The excess water percolates and remains stored within the root zone of the crops. Again, in perennial irrigation system if water is applied more than what is required than this excess of water is responsible for water logging.

Over Irrigation

Causes of Water-Logging • Seepage from Canals: • In Unlined canal systems, the water percolates through the bank of the canal and gets collected in the low lying areas along the course of the canal and thus the water table gets raised. This seepage is more in case of canal in banking

Seepage from Canals

Causes of Water-Logging • Inadequate Surface Drainage: • When the rainfall is heavy and there is no proper provision for surface drainage the water gets collected and submerges vast area. When the condition continuous for a long period, the water table is raised. • Obstruction in Natural Water Course: • If the bridges or culverts are constructed across with the opening with insufficient discharges capacity, the upstream area gets flooded and this causes water logging.

Causes of Water-Logging • Obstruction in Sub-Soil Drainage: • If some impermeable stratum exists at a lower depth below the ground surface, then the movement of the subsoil water gets obstructed and 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 percolate through it. So, in case of over irrigation or flood, the water retains in this type of land and cause water logging.

Nature of Soil

Causes of Water-Logging • Incorrect method of Cultivation: • If the agriculture land is not levelled properly and there is no arrangement for the surplus water to flow out, then it will create pools of stagnant water leading to water logging. • Seepage from Reservoir: • If the reservoir basin consists of permeable zones, cracks and 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

Causes of Water-Logging • Poor Irrigation Management: • If the main canal is kept open for a long period unnecessarily without computing the total water requirement of the crops, then this leads to over irrigation, which shall result in water logging. • Excessive Rainfall: • If the rainfall is excessive and the water gets no time to get drained off completely, then a pool of stagnant water is formed which might lead to water logging.

Excessive Rainfall

Causes of Water-Logging • 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 gets affected and this cause water logging.

Topography of the land

Causes of Water-Logging

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 sodium carbonate, sodium chloride and sodium sulphate come to the surface of soil. When the water evaporates from the surface, the salts are deposited there. This process is known as salinization of soil. Excessive concentration 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 salt efflorescence

Salination of Soil

Adverse Effects of Water-Logging • Lack of Aeration

• The crops require some nutrients for their growth which are supplied by some bacteria or microorganisms by breaking the complex nitrogenous compound into simple compound which are consumed by the plants for their growth. But the bacteria requires oxygen for their life and activity. When the aeration in the soil is stopped by water logging, these bacteria cannot survive without oxygen and the fertility of the land is lost which results in reduction of yield.

Aeration of soil

Adverse Effects of Water-Logging • 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 aquatic plants grow in plenty. These plants consume the soil foods in advance and thus the crops are destroyed.

Growth of weeds

Adverse Effects of Water-Logging • Diseases of Crops: • Due to low temperature and poor aeration, the crops get some diseases which may destroy the crops or reduce 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 be matured properly and the yield is reduced.

Adverse Effects of Water-Logging

Control of Water Logging • The following measures may be taken to control water 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 the course of the irrigation canals in place where the percolation of water is detected. The percolation water is intercepted by the drains and the water is carried to other natural water course.

Control of Water Logging • Prevention of percolation from the reservoirs: • During the construction of dams, the geological survey should be conducted on the reservoir basin to detect the zone of permeable formations through which water may percolate. These zones should be treated properly to prevent seepage, If afterwards it is found that there is still leakage of water through some zone, then sheet piling should be done to prevent the leakage.

Prevention of percolation from the reservoirs

Control of Water Logging • Control of Intensity of Irrigation: • The intensity of irrigation may cause water logging so, it should be controlled in a planned way so that there is 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 be high. 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 eliminating the possibility of water logging.

Remedial Measures: Against WaterLogging

Optimum Use of Water

Control of Water Logging • Fixing of Crop Pattern: Soil survey should be conducted to fix the crop pattern. The crops having high rate of evapotranspiration should be recommended 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-surface drains are provided which are connected to the surface drains. The surface drains discharge the water to the river or any water course.

Remedial Measures: Against WaterLogging

Leaching of Saline Soil Crop Rotation

Control of Water Logging • Improvement of Natural Drainage: • Sometimes, the natural drainage may be completely silted up or obstructed by weeds, aquatic plants, etc. The affected section of the drainage should be improved by excavating and clearing the obstruction. • 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 goes down to a safe level. The lifted ground water may be utilized for irrigation 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 sump well may be pumped to the irrigable lands or may be discharged to any river.

Remedial Measures: Against WaterLogging

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 soluble salt concentration and exchangeable sodium content. • These soils are formed from the normal soil through the accumulation of salts from applied irrigation water or by upward moving ground water. • The problem with these soils is that their crop yield is considerably low and gradually these soils become unsuitable for cultivation. • The formation of these soils may be prevented through proper 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 a land culturable after it gets converted to uncultivable area due to the bad effect of water logging. 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 with water to a depth of about 20-30 cm. the salt deposited on the surface are dissolved. Some portion of salt is then drained off through the subsoil drainage system and some portion of salt is then drained off through the subsoil drainage system and some portion is removed by surface drainage system. This operation is repeated several times at specific intervals • Generally, Leaching is followed by crop rotation as recommended 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 gypsum neutralizes the alkaline effect of the soil and yield of the crop is increased. • Surface Drainage • Proper surface drainage system should be provided in the agricultural land so that the water does not accumulates for a long time. The surface drains also help in draining the saline water in case of leaching operation.

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. It also 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 are very 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 towards the pond and thus the saturation in the root zone of the crop is reduced. In fact, these ponds control the water logging in rainy season and in dry season the water of the pond may be used for lift 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 this water 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

Ground water recharge and necessity • Artificial surface reservoirs are constructed by building dams in order to store the surplus surface waters in the same manner artificial underground reservoirs are now a days developed by artificial recharge for storing water underground. The development of such a reservoir may be advantageous 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 be considerably less than the cost of the surface reservoirs • The water lost in evaporation from an underground reservoir is much less than 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.

Methods of improving ground water storage • Spreading method:- this method consists in spreading the water over the surfaces of permeable open 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 permeability of the spread area and on the depth of water stored. Generally 1.5 m/day have been possible

Recharge well method • This method consist in injecting the water into bore holes, called recharge wells. • In this method the water is fed into recharge well by gravity or pumped under pressure to increase the recharge rate. • Recharge well method is certainly preferred when the spreading method can not yield appreciable recharge. • Because of low permeable areas the water to be used in the recharge well should be purer than that 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