SR. NO.ENROLLMENT NO.NAME PATEL HARDIK SHAH SIDHARTH SOLANKI SHIRAZ

Flood Management And Hydrologic Analysis

Flood Management A flood is relative high stage of the river that overtops the natural or artificials banks, due to spreading of the flood water over the flood plain causes damage to crops and property. Flood control is required to reduce the flood damage. However it is not economically feasible to have an absolute control over the flood

INDIAN RIVERS AND FLOODS The Indian rivers can be classified into four major groups:- 1.Himalyan rivers 2.Rivers on the central highlands 3.Rivers on the peninsula 4.Rivers on the coasts

HIMALYAN RIVERS Three important himalyan rivers are Indus, Ganga and Bhramaputra Indus flows for about 2900km. It has many tributaries like Jhelum, Sutlej, Ravi, Beas and Chenab. Ganga flows for about 2510km.It has a lot of tributaries like Yamuna, Sone, Gomti, Ghagra, Gandak, Kosi and Mahananda. River Bhramaputra rise in Mansarovar in Himalayas and flow for about 2900km before it meet Bay of Bengal in Bangladesh.

GANGA RIVER

BHRAMAPUTRA RIVER

INDUS RIVER

RIVERS OF CENTRAL HIGHLANDS This highlands have given rise to number of rivers like Narmada, Tapi, Sabarmati, Mahi, Mahanadi etc. All these rivers are non-perennial and are flooded after south;west monson rainfall only. The central highland consists of:- 1.Mewar highland 2.Bundelkhand plateau 3.Malwa plateau 4.Chotanagar plateau etc.

RIVERS ON THE PENINSULA Most of the peninsular rivers rise on the Western Ghats and flows towards the east to meet the Bay of Bengal. River Godavari, Krishna, Kaveri, Koyana etc. all are non perennial rivers. The regions as a whole doesn't have a serious flood problems except that on some rivers of Orissa state.

RIVERS ON THE COAST The rivers which originates on the Western Ghats and flows towards west are called as coastal rivers. These are Kali, Vashisti in Maharastra,Mandwi and Zuari in Goa,Sharawati, Tadri, Netrawati in Karanataka and Beypore Punuam, Peigar, Pamba in Kerala. They are not known for floods as their basins are not fully developed.

CAUSES OF FLOODS The causes of flooding in all the major river systems are : 1. Intense rainfall 2. Topography of the catchment 3. Sedimentation of rivers and reservoirs 4. Obstruction in the river flow 5. Failure of dam 6. Failure of river embankment 7. Inadequate cross drainage works 8. Contraction of waterway

FLOOD ALLEVIATION A flood is a relatively high stage of river which is usually accompanied by overtopping of the natural banks of the rivers in a particular reach. In India about 40 Mha of land is flood prone, which is about 12% of the total geographical area of 328 Mha. There are two types of measures to mitigate the flood disaster: 1. Structural mitigation measures 2. Non-structural mitigation measures

FLOOD ALLEVIATION

Structural mitigation measures are : 1. Storage reservoirs 2. Levees 3. Flood walls 4. Channel improvement works 5. Diversion of flood water to flood ways 6. Construction of high earth platforms 7. Sluices

LEVEES (EMBANKMENTS) Levees : A levees is an earth embankment. It is a sort of an earthen dam constructed along the river. In general, levees should satisfy the criteria of earth dam. The construction of levees is more common than flood walls as it is more economical and convenient.

EARTH EMBANKMENTS HAVE THE FOLLOWING ADVANTAGES : 1.They are easy to construct parallel to river bank by available earth 2.Initial cost of construction is less 3.They prevent overflowing the banks 4.Maintenance is easy 5.It increases the storage capacity of the river.

Drawbacks of earth embankments:- 1.Instead of flood disaster mitigation, sometimes it suddenly creates catastrophic flood exactly like dam-break flood when embankment fails during high stage. 2. During flood time, round the clock maintenance is very much essential against failure. 3. Due to increase of river capacity, velocity decreases, hence sediments flowing along with flood water are deposited in the bed thereby depth of river bed goes on. decreasing with time

FLOOD WALLS Flood walls are used in developed area where it is difficult to obtain enough land for construction of levees. Because of flat slopes, levees require a very large width. A flood wall is a sort of gravity dam constructed along the river. Flood walls are designed to withstand the hydrostatic pressure like gravity dams. Generally the section is designed for no tension.

CHANNEL IMPROVEMENT WORKS Channel improvement as a flood control measure can be used to reduce the river stage at a specific point in the reach. The flood stage of any section can be substantially reduced by improving the hydraulic capacity of the river.

Channel improvement can be done by: 1.Increasing the channel section 2.Realingment of the river 3.Increasing waterway at crossings 4.Increasing velocity in the river 5.Straightning the meandering type river by cutoff to reduce travel time of flood.

FLOOD DAMAGE ANALYSIS : Damages in the following forms occur due to the flood disaster: 1.Damage to corps 2.Damage to houses 3.Damage to human lives 4.Damage to live-stock 5.Damage to public utilities, roads, rails, etc. 6.Cost of relief measures

DIRECT DAMAGE It implies damage due to physical contact with water. for example, Damages in the following forms occur due to flood disaster: Erosion of agricultural land Loss of human life and animals Loss of property Breach of roads, canals, ponds, etc. Loss of forest and wild life INDIRECT DAMAGE In this case flood water does not come in direct contact of the property, etc. but causes damage by interruption and other causes.

Design Flood The design flood is the flood adopted for the design of a flood control project. It may be either the maximum probable flood or the standard project flood or flood corresponding to some desired frequency occurrence depending upon the degree of protection that should be provided by the flood control project

Maximum probable flood The maximum probable flood is the flood that may be expected from the most severe combination of critical meteorologic and hydrologic condition that are reasonably possible in the region It is estimated from the maximum probable storm applying the unit hydrograph principle. Standard project flood The standard project flood is the flood that may be expected from the most severe combination of meteorologic and hydrologic conditions that are reasonably possible in the region. It is computed from the standard project storm rainfall applying the unit hydrograph principle.

Methods of estimation of maximum flood The following methods are commonly used for the estimation of the maximum flood in the river 1.Past flood marks method 2.Empirical methods 3.Envelope curves method 4.Concentration time method 5.Rational method 6.Unit hydrograph method 7.Flood frequency methods

Past flood marks method The maximum flood that occurred in the past can be estimated from the flood marks left by the flood. These marks are generally in the form of floating debris sticking to the banks or to the walls of the structures at the river bank. The following procedure is used to estimate the flood discharge. 1.The flood marks left along the reach of the river are connected by levelling to determine the water surface levels. The slope of the water surface is calculated from the difference of high flood levels over a known distance

Envelop Curves Methods For plotting envelope curves, the maximum flood discharges for the catchments having similar topographical features and climatic conditions are determined. The maximum flood discharge is plotted against the area of the catchments. A curve is then enveloped the highest plotted scattered points.

Where A is the area of catchment.

Design Flood The design flood is obtained from the unit hydrograph and the design storm using the following procedure. 1.Obtain the hyetograph of the effective rainfall from the design storm. Select the lowest initial loss and the lowest infiltration index to obtain the maximum rainfall excess 2.Develop the hydrograph of direct runoff from the unit hydrograph and hyetograph of the effective rainfall. 3 Add the base flow to the direct runoff flood hydrograph to obtain design flood.

Flood Frequency Methods The frequency of rainfall is determined assuming that the rainfall is a random variable and mathematical theory of probability is applicable. To determine the frequency of a particular extreme rainfall, such as 24 hour maximum rainfall, the rainfall data of the past storms analysed. The rainfall data of the place is collected and arranged in the chronicle order. The storm flow peaks of the past years are collected for the flood frequency analysis. There are two methods of collecting the flood peak data. They are 1.Annual flood series 2.Partial flood series.

Annual Flood Series In the annual flood series, only the highest flood of each year is taken for analysis; whereas in the in the partial duration series, all flood peaks above a selected peak are collected irrespective of the year in which they occur. Partial Duration Series In the partial duration series, there may be number of such floods in a year. On the other hand, in some years, there may not be any such food at all. The partial duration series has an advantage over the annual flood series because of all floods above a particular peak are considered.

Probability plotting method In the probability plotting methods, the recurrence intervals are first computed for various flood peaks. A plot is then made between the flood discharge as ordinate and the recurrence intervals as abscissa. Recurrence Interval The recurrence interval is the average time interval that elapses between the two events that equal or exceed a particular level. The recurrence interval does not imply the periodicity of events. The recurrence interval is also known as return period.

EXAMPLE - 2 : what return period would adopt in the design of a bridge on a river if you are allowed to accept only 5% risk of flooding in the 25 years of expected life of the bridge ? SOLUTION : T r = return period R = risk = probability of occurrence at least once = 5% = 0.05

EXAMPLE – 5 : what is the return period that a highway engineer must use in his design of critical underpass drain if he is willing to accept only : 10% risk that the flooding will occur in next 5 years? 20% risk that flooding will occur in next 2 years ? SOLUTION : 10% risk that the flooding will occur in next 5 years n = 5 years R = 10% = 0.10

EXAMPLE – 7 : What will be the risk involved for a hydraulic structure having a design life of 100 years if it is designed for: 50 – year return period flood 1000 – year return period flood ? SOLUTION : 50 – year return period flood n = 100 years T r = 50 years

The process of computing the reservoir storage, volume, outflow rates and water levels in the reservoir corresponding to a particular hydrograph of inflow is known as flood routing.

Flood routing is carried out to obtain a hydrograph as moderate or modified by a reservoir when a particular flood passes through it. There are two broad categories of flood routing: 1.Reservoir routing 2.Channel routing

Reservoir routing is the process of determining the flood hydrograph at a location downstream of a reservoir, from the knowledge of the upstream flow hydrograph.

I − O = ds/dt ……(A) where, I = inflow rate O = outflow rate S = storage Alternatively, in a small time interval Δ t, the difference between the total inflow volume and the total outflow volume is equal to the change of storage volume. Î. Δ t − Ô. Δ t = Δ s ……(b)

Since, Î =I ₁ +I ₂ /2 Ô =O ₁ +O ₂ /2 Δ S=S ₁ -S ₂ where suffixes 1and 2 denotes the beginning and the end of the time interval Δ t equation (a) can be written as (I ₁ /2+ I ₂ /2) Δ t-(O ₁ /2+ O ₂ /2) Δ t =S ₂ -S ₁ ………………(c)

The following hydrologic methods may be used for reservoir routing: 1.Trial and error method 2.I.S.D curve method 3.Modified pul’s method 4.Goodrich method

I − O = ds/dt ……(A) where, I = inflow rate O = outflow rate S = storage Alternatively, in a small time interval Δ t, the difference between the total inflow volume and the total outflow volume is equal to the change of storage volume. Î. Δ t − Ô. Δ t = Δ s ……(b)

Since, Î =I ₁ +I ₂ /2 Ô =O ₁ +O ₂ /2 Δ S=S ₁ -S ₂ where suffixes 1and 2 denotes the beginning and the end of the time interval Δ t equation (a) can be written as (I ₁ /2+ I ₂ /2) Δ t-(O ₁ /2+ O ₂ /2) Δ t =S ₂ -S ₁ ………………(c)

The following hydrologic methods may be used for reservoir routing: 1.Trial and error method 2.I.S.D curve method 3.Modified pul’s method 4.Goodrich method

Trial and error method is widely adopted with the assistance of computers to reduce time taken in long calculation involving in this method This method arranges the basic equation as follows (I ₁ /2+ I ₂ /2) Δ t =(O ₁ /2+ O ₂ /2) Δ t + S ₂ -S ₁

For carrying out computation by this method following three curves are required: 1.Inflow flood hydrograph 2.Elevation storage curve 3.Elevation outflow curve

The inflow flood hydrograph is the maximum flood hydrograph at the reservoir site.

The elevation storage curve gives the reservoir storage at different elevation.

The elevation outflow curve gives the outflow rates for different elevation.

The inflow storage discharge curve method, also known as puls method. in this method following two graphs are to b prepared from the elevation storage curve and the elevation outflow curve

(2S/ Δ t + O) versus outflow curve (2S/ Δ t - O) versus outflow curve Where, S = Surcharge storage O =Outflow rate Δ t = Time interval

For preparing of these curves values of S and O for different elevation are obtained The value of (2S/ Δ t + O) and (2S/ Δ t - O) are then computed for different elevation. The two graphs are then plotted on same paper.

The elevation outflow curve gives the outflow rates for different elevation.

The ordinates represent the outflow rate & the abscissae represent the both (2S/ 2S/ Δ t + O) and the flood routing eqn.(c) is a slight modification form as follows : (I ₁ +I ₂ ) =(2S ₁ / Δ t - O ₁ ) = (2S ₂ / Δ t+O ₂ )

In this method of flood routing, the flood routing relationship is used i.e (I ₁ + I ₂ ) + (2S ₁ / Δ t - O ₁ ) =(2S ₂ / Δ t + O ₂ ) A curve is plotted between the elevation as ordinates and (2S/ Δ t + O) as abscissa. The value of (2S/ Δ t + O) are calculated foe different elevations.

The popular Goodrich method is much simpler than the conventional ISD method. The method requires 2 curves as compare to 5 curves required in ISD method. The popular Goodrich method rearranges the basic routing equation i.e

(I ₁ /2+I ₂ /2) Δ t - (O ₁ /2+O ₂ /2) Δ t = S ₂ - S ₁ As follows: I ₁ +I ₂ - O ₁ - O ₂ = 2S₂/Δt – 2S₁/Δt Rearranging the known values of I₁, I₂,O₁ and S₁/Δt we get I₁ + I₂ + (2S₁/Δt - O) = (2S₂/Δt + O₂) The above equation forms the basic equation of Goodrich method

For a given time step, the left side of the above eqn. can be computed from the known storage and outflow at the beginning. The computed value will indicate the value of function (2S₂/Δt + O₂) also from the known storage – elevation &elevation discharge data we can plot a curve between elevation & function (2S/Δt + O₂)

Channel Routing: In channel routing, the change in the shape of an inflow hydrograph is studied at a downstream location, as it travels down a channel. By considering a channel reach and an input hydrograph at the upper end. The flow in a river during a flood belongs to the category of gradually varied unsteady flow. The water surface in a channel not only parallel to the channel bed, but also varies with time.

When such river flow is considered, the storage between two section, will consists of two parts, 1.Prism storage 2.Wedge storage Prism Storage It is volume that would exist, if a uniform flow occurs at the downstream depth, i.e., the volume formed by an imaginary plane parallel to the channel bed, drawn upstream from the outflow section to the inflow section.

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