Unit - 3

Floods

3.1 Floods:

• An overflow of water from a river or a stream is defined as flood.
• During the maturity stage of any river the depth of the river is less and the width goes on increasing e.g. From the source river Ganga runs through a deep and narrow valley upto Haridwar in Uttarakhand, where it comes down to the plain region.
• The load carrying capacity of river Ganga is reduced so the load (mud and other eroded material) is deposited on the bed of the river to make it shallow.
• During the Southwest monsoons as the supply of water increases the excess water carried by river Ganga gets spread crossing the right and left banks on both the sides to cause floods.
• During summer season when the show melts at the source region i.e., as Dev Prayag in Himalayas, t water supply increases to cause floods.

Causes of Floods:

• On the basic of causes the floods can be of two

A: Natural floods

B: Man-made floods

A: Natural floods-

• During rainy season as the supply of water, at the source region of the main river and its tributaries increases the river gets flooded.
• During summer season, when the high temperature makes the snow melts to increase the supply of water at the source region, causes floods.
• In India the rivers having their sources in Himalayas have flood, during summer season e.g., Deyprayag Ganga, Mansarovar – Brahmaputra.

B: Man-made floods-

• For many purposed we construct dams to the rivers.
• During the rainy season these dams are over flooded.
• The level of artificial stored water in the catchment area rises above the normal level of water to cause floods.
• It is mainly the result of over siltation which is the effect of cutting of trees in the source region e.g., most of the rivers in Himalayas are dammed in their lower courses.
• The forests are getting cut to make the topsoil loose which is carried by the river and gets stored over the dam-stored water.
• It reduces the capacity to store water as it becomes shallow due to heavy deposition of soil. This causes floods, river Sometimes these man-made construction of dams, due to the high pressure breaks to cause sudden rise in the water level in the down courses to get flood.
• The rivers flowing through the urban areas many a times changes their courses due to the construction work, mining work causing floods.
• E.g., In 2008 river Kosi was over flooded to put half of Bihar under water.
• In India some rivers so defamed due to their flooding nature to cause losses of human lives and of the properties e.g., River Damodara is called as ‘Tears of West Bengal Brahmaputra as the Tears of Assam’, River Kosi as the “Tears of Bihar’.

Classification of Floods:

Based on Intensity of Floods:

• On the basis of the intensity of floods they can be classified as,

A: Ordinary floods.

B: Standard project floods.

C: Design floods.

A: Ordinary floods

• The normal rise of water level above the bank of the river to make it spread over the shallow region along the banks, is called as ordinary floods.
• E.g., The rivers in south and central India like Mahanadi, Narmada, Tapi, Godavari, Kaveri get flooded during the rainy season to the addition of rainwater in the course of the river.
• After the rainy season the river goes back to its normal size. Unless, we make use of the empty dry, shallow land for construction purpose (unauthorized), these floods do not cause serious losses.

B: Standard project floods

• The floods occurred with higher estimated intensity with which the design of the reservoir has been made is called as Standard project floods.

C: Design floods-

• The design floods we should not choose very high or very low value of the construction of dams.
• If the very high value for design is selected it increase the cost of construction and unnecessary very large investments are needed to be used.
• On the other hand, if very low value of design flood is selected, it may cause failure of structure of the dam itself.
• The dams with low value designs cause more damage in the commend areas of the river.
• So, to fix the design flood, data of minimum 35 years, of floods to that particular river must be taken into account.

D: Probable Maximum Flood (P.M.F)

• P.M.F is the largest flood which can occur at a given place: It is estimated on the basis of the probable maximum precipitation and in case of some regions in the mid-latitude, the melting of snow, in summer e.g., Himalayan Rivers, like Ganga, Kosi, Brahmaputra, get flooded twice a year i.e., in rainy season and in summer, due to melting of snow.
• The P.M.F. Defines the extent of flood prone areas. e.g., in the flood plain areas on the banks of the river the flood lines are marked to indicate the maximum extent of the flood.
• The areas below this marked line, toward, the banks, are not to be used for any construction, to avoid the economical losses. Through the floods.

E: Design Storms

• Design storm is a mathematical representation of a precipitation event which reflects the conditions in a given region, for the design of infrastructure.
• It provides the guidelines for computing flows and for sizing the infrastructure. e.g., the roads, channels, ponds, pipelines etc.
• This helps for better planning to reduce the losses due to high floods. In India, North India, mainly, Bihar, Uttar Pradesh, Assam etc. state suffer from floods every year. Per them design storm system is very useful.

Key Takeaways:

An overflow of water from a river or a stream is defined as flood.

3.2 Estimation of Peak flow:

• "Design flood", is the flood, which has been adopted for design of a flood control project.
• It can be either maximum probable flood or may a standard project flood or a flood with some designed frequency of occurrence.
• It depends upon the degree of protection, which should be provided by the flood control project. Let us check these different types of floods, which can be designed.

A: The maximum probable flood

• If a flood, which occur under the most severe combination of critical hydrological meteorological and conditions.
• It is estimate from the maximum probable storm, by applying the unit hydrograph principle.
• It is an estimate of the physical upper limit to a storm rainfall in the catchment. This data is obtained from the studies of all the storms which have occurred in that catchment and by maximizing them for the most critical atmospheric conditions.

B: The standard project flood

• It a flood, may be expected from the most severe combination meteorological and hydrological conditions, that are possible in the catchment.
• It is computed from the standard project storm-rainfall by applying the unit hydrograph principle.
• This standard project storm rainfall, can be taken as the largest rainfall of the catchment, during the period of the weather record.
• It is still, not maximized for the most critical, atmospheric condition as in case of maximum probable flood. Generally, the standard project flood is about 50% of the maximum probable flood of that region.
• In reality, it is not possible to provide absolute flood protection by any such methods. So, there is way a risk which must be accepted, while selecting the design flood.
• Various methods are used for estimation of design flood, which are classified as under.

C: Estimation of Peak flow

• In India, most of the watersheds i.e., the catchments upto 500 km² total Geographical area (T.G.A) are considered as the ungauged catchments. This makes the watershed planning difficult i.e., flood modelling and flood forecasting become difficult.
• In hydrology Peak flow or Peak discharge is the highest concentration of the runoff from the river basin. If the Estimation of Peak flow is carried accurately the economic losses in the catchment areas can be minimized.

Key Takeaways:

"Design flood", is the flood, which has been adopted for design of a flood control project. It can be either maximum probable flood or may a standard project flood or a flood with some designed frequency of occurrence.

3.3 Rational formula and other method:

Rational method:

• It is a method derivation of design flood from storm studies and application of unit hydrograph principle.
• In this method following steps are taken,

1. Analysis of rainfall v/s runoff date for derivation of loss rate under the critical conditions.
2. Derivation of unit hydrograph by analysis (if the data are not available by synthesis).
3. Derivation of design storm.
4. Derivation of design flood, from the design storm (by applying the rainfall, excess increment to the unit hydrograph).

• In the regions, having snowfall, this unit hydrograph principle is not useful.

Application of suitable factors of safety, to maximum observed flood or the maximum historic floods:

• The safely factor is applied to observed or estimated historical floods, to obtain the design flood, at the project site or on any.
• Infact, the safety factors depend upon, the designer's Judgement and so, it is very subjective method as far as the selection criteria of the safely factors are concerned and also the results have limitations because the length of available stream flow record may not provide an adequatic sample of flood magnitude which is likely to occur over a long period of time.

The empirical flood formulae:

• The empirical formulae are based on the topography (relief), climatic conditions, (Precipitation, temperature), and geological conditions (infiltration).
• In India the most famous formula used for the estimation of design flood of a catchment, are Dicken's formula, Rational formula, Sir Inglis formula (mainly useful for the ghat-fed catch ents) etc.
• In these formulae, the peak flow (flood) is given as a function of the catchment area and of a coefficient.
• The values of the coefficient vary from region to region, so these values of coefficient to be used in a formula, are based on judgement (which may be right or wrong).
• So generally, the use of these empirical food formulae, is made when no other accurate method is applicable due to lack of dates.

Envelope curve:

• In this, the maximum flood is obtained from the envelope curve of all the maximum observed floods, for a number of catchment areas, in a region where in theme is a homogeneity in the climatic conditions and the numerical values are plotted against the drainage area.
• This method is useful for the generalizing the limit of the floods.

Key Takeaways:

It is a method derivation of design flood from storm studies and application of unit hydrograph principle.

3.4 Flood frequency analysis:

• This method needs to have a fairly long period's data (at least of 25 years) for the statistical analysis, to calculate the frequency of the floods.
• It is based, totally on the statistical method, and so when it is applied to a derive design flood for long recurrence intervals, which are several times larger than the data.
• It has a lot of limitations, so, the method is required to be used with all the necessary precautions.

Key Takeaways:

This method needs to have a fairly long period's data (at least of 25 years) for the statistical analysis, to calculate the frequency of the floods.

3.5 Design floods:

• When we study the hydraulic analysis the aim of such study is assess the causes of the over flooding, the effects of over flooding the measures to control such natural or man-made hazards to control the environmental and economical losses.
• Various methods have been introduced to control floods. (They are also known as flood management programs).
• A Broad classification has been given by the organization working at a nation level, namely "Rashtriya Barh Ayog".

• To attempt to modify flood.
• To attempt to modify the susceptibility to flood damage.
• To modify the loss burden.
• To bear the losses.

• Following are the commonly accepted and adopted methods to control floods.

      A: By using reservoirs.

      B: By constructing levees, flood walls.

      C: By evacuation of the flooded area (either temporary or permanently).

      D: By channel Improvement.

      E: By using flood ways.

      F: By flood proofing specific properties (by constructing ring levees).

      G: By reducing flood runoff by land management.

• In reality, in most of the flood control project, the combination of the above-mentioned method is used for better and assured results.

3.6 Brief introduction of hydrologic design of culverts and bridges:

Some time it is felt necessary to make the stream to carry more discharge at the lower levels or lower stages. It can be done just by improving its hydraulic conditions. Such type of improvement can be drought by using the following methods.

Methods of Channel Improvement:

      A: By increasing the size of the cross Section,

      B: By realigning the channel along a shorter rout.

      C: By increasing the velocity of the flow in the channel.

      D: By removing the deficiency of water ways in the drainage crossing.

A: By Increasing the size of the cross-section

• Only on the channels of small streams, which carry a manageable high flood discharge, the increase in sizing of the cross section to cater for a high designed flood; without spilling over the bank is possible for the economic consideration.
• In case of large rivers this method is not practicable due to its huge cost and also it is not practicable to dispose of the excavated material over any developed regions.

B: By realigning the channel along a shorter route

• It practicable only in case of small streams because for the larger the stream more would be the cost.
• It also has to face the problem of disposing the excavated material.

C: By increasing the velocity of the flow in the channel

• It can be increased by deepening, or by straightening or by shortening the length of the channel.
• This can be done by cutoffs, by removing the barriers in the channel section and by lining the channel to improve its coefficient of rugosity (for any unlined alluvial channel, this value of Rugostly coefficient (N) depend upon the nature of the material in the bed and sides of the channel and also on the condition of the channel).
• This mean can be used by keeping in mind the cost and also the feasibility.

D: By removing the deficiency of water ways in the drainage crossing

• The inadequacy in the water-ways of bridges, culverts, canal cross-drainage works etc. result in increase in the flood situation.
• These manmade obstruction can be avoided by proper evaluation of the ultimate drainage requirements of the basin, at the time of any proposed works, for crossing the drainage line.
• All the above-mentioned methods help to reduce the present water level or stage, for the flood discharge and so after adopting any one of the measures of channel improvement.
• The banks of the channel are not over topped (or the frequency of over topping is reduced) during the high flood discharges and so they help to provide protection to the areas which used to be over flooded.
• We must note down that although, the channel improvement is a well-recognized method, for controlling floods, still it must be used with maximum precaution.
• This is required because whatever measures we accept for improving channel capacity are the measures to protect from the floods at a local level i.e., at a micro-level, but it may increase the magnitude of flood at the downstream points due acceleration of flow of runoff, from the upper improved reaches of the channel.
• So, unless, well-planned and well programmed, such channel improvement work, will merely shift the problem of floods from one region to another region in the down-stream direction.

3.7 Hydrologic flood routing:

Flood routing can be defined as. "It is procedure to calculate the depth of the flow and the hydrograph, at a section knowing flood hydrograph at cross section and the hydraulic characteristics of the channel, in between or the hydraulic characteristics of a reservoir.

Types of Flood Routing

The flood routing can be divided into two as,

1. Flood routing through channel.
2. Find routing through reservoir.

Methods of Flood Routing:

Three methods are used for the procedure of flood routing:

1. A: Hydraulic routing.
2. B: Hydrologic routing.
3. C: Routing machines.

A: Hydraulic routing

• It can be carried out by using the three basic equations like,

1. Continuity equation
2. Energy equation
3. Momentum equation

• If the flow in a channel not uniform and is very unsteady. It is difficult to apply this method.

B: Hydrologic routing

• It is based on a very simple equation, such as

Inflow-Outflow= Change of storage

• Over given period of time interval At. Symbolically, as state below.

I-Q =

Where,

I=In flow;

Q = Out flow

t =Change in storage

• The inflow-hydrograph ordinates at a time interval of will be I1, I2, I3 these will be known because the inflow-hydrograph is known.
• The outflow originates may he Q₁, Q₂, Q3 as there are not known, they are to be calculated.

Over a time interval

t the inflow volume will be =

Over the same time interval,

t the outflow volume will be = (Q₁ + Q₂)/2 x t

• The difference between these two inflow and outflow volumes will be the change in the storage = S2-S1
• (In this case neither energy equation non the momentum equation is used only the relationship between inflow-outflow is used.

So, -(Q₁ + Q₂)/2 x t

= S2 - S1

C: Routing machines

• The mechanical flood routers have mechanical gears which represent the reservoir system in reality. The mechanism is shown in the Fig.

Fig. 1: Mechanical flow router

• Recently, electrical voltage system or/and electronic voltage system Fig. Have been introduced for the flood routing procedure.

Fig. 2: Electric Analogue routing machine

Key Takeaways:

Flood routing can be defined as. "It is procedure to calculate the depth of the flow and the hydrograph, at a section knowing flood hydrograph at cross section and the hydraulic characteristics of the channel, in between or the hydraulic characteristics of a reservoir.

3.8 Muskingum method:

• The Muskingum Routing unit fashions the glide of water in herbal and man-made open channels the use of the Muskingum approach to path the glide.
• The Muskingum Routing calculates the release inside a river or channel attain given the influx hydrograph on the upstream quit.
• The unit is primarily based totally at the continuity equation and the Muskingum garage relationship (McCarthy G.T. (1938)).
• Cross phase info isn’t required and most effective the Muskingum parameters ok and x are special. Both parameters are fixed.
• A minimal of Muskingum Routing sections are required for every quit of the river or channel attain.
• Intermediate discharge factors may be special through the use of extra Muskingum Routing nodes.
• The equations used in the Muskingum Routing are the continuity equation:

Where:

I = inflow to the reach [m3/s]

O = outflow from the reach [m3/s]

S = storage in the reach [m3]

t = time [s]

Key Takeaways:

The Muskingum Routing unit fashions the glide of water in herbal and man-made open channels the use of the Muskingum approach to path the glide.

3.9 Q-GIS software application in hydrology (watershed delineation):

Q-GIS software application in hydrology (watershed delineation) are as follows:

1. Geo reference scanned maps
2. Digitize vectors
3. Import tabular data
4. Join characteristic tables
5. Interpolate factors to a raster
6. Apply map algebra
7. Delineate streams and catchments
8. Find and use Open Data
9. Calculate the share of land cowl in keeping with sub catchment
10. Design stunning catchment maps

References:

1. A textbook of Hydrology, Dr. P. Jaya Rami Reddy, USP Publisher

2. Irrigation, Water Resources and Water Power Engineering, P.N. Modi.

3.Irrigation and Water Power Engineering, Dr. Purnima and Dr. Pande

4. Irrigation Engineering, Bharat Singh, Nem Chand & Bros. India

5.Irrigation Engineering, H.M Raghunath, Wiley