DESIGN FLOOD

The flood adopted for design of hydraulic structures is called Design flood. Any hydraulic structure is generally made safe against  the maximum flood possible in the catchment. Smaller structures such as culverts, storm drainage systems can be designed for relatively small floods (i.e., more frequent floods) as the consequences of a higher than design flood may only cause temporary inconvenience and some repair works without any loss of life and property. However, failure of structures such as spillway would cause huge loss of life and property and, therefore, such structures should be designed for relatively more severe floods having relatively larger return period. The table below  provides guidelines for selecting design floods. The terms PMF and SPF in the Table below have the following meaning...

PMF or probable maximum flood is the extreme large flood that is physically possible in a region as a result of severe most (including rare ones) combination of meteorological and hydrological factors.

SPF is the standard project flood that would result from a severe combination of meteorological and hydrological factors. Usually, SPF is about 40 to 60% of PMF.

General Guidelines for designing floods is as following…

Sl No	Structure	Recommended design flood
1.	Spillways  [storage more than 60 Mm3]	(a) PMF determined by unit hydrograph and probable maximum precipitation (PMP) (b) If (a) is not applicable or possible, flood frequency method with T = 1000 years to 5000 years
2.	Permanent barrage and minor dams [capacity less than 60 Mm3]	(a) SPF determined by unit hydrograph and standard project storm (SPS) which is usually the largest recorded storm in the region (b) Flood with a return period of 100 years.(a) or (b) whichever gives higher value.
3.	Pickup weirs	Flood with a return period of 100 or 50 years,depending on the importance of the project
4.	Aqueducts (a) Waterway (b) Foundations and free board	Flood with T = 50 years Flood with T = 100 years
5.	Project with very scanty or inadequate data	Empirical formulae

RUNOFF

When soil in is infiltrated to full capacity and excess water from rain, melt-water or any other precipitation flows over the land is called runoff or surface runoff.

After satisfying the requirements of evapotranspiration, interception, infiltration into the ground, and detention storage, precipitation (or rainfall) drains off or flows off from a catchment basin as an overland flow (or surface runoff which includes precipitation falling on the stream system too) into a stream channel. Part of the infiltrating water moves laterally through the upper layers of the soil and returns to the ground surface as interflow or subsurface runoff at some place away from the point of infiltration into the soil. Part of the infiltrating water percolates deep into the ground and joins the ground water storage. When water table intersects the stream channels of the catchment basin, some ground water may reach the surface or join the stream as ground water runoff, also called base flow or dry-weather flow. Thus, the runoff from a catchment includes surface runoff, subsurface runoff and base flow. The surface runoff starts soon after the precipitation and is the first to join the stream flow. Subsurface runoff is slower and joins the stream later. Depending upon the time taken by the subsurface runoff between the infiltration and joining the stream channel, it may be termed as prompt subsurface runoff or delayed subsurface runoff. The groundwater runoff is the slowest in joining the stream channel but, is responsible in maintaining low flows in the stream during dry season. Based on the time interval between the precipitation and runoff, the runoff is categorized as direct runoff (that enters the stream immediately after precipitation i.e., surface runoff and subsurface runoff) and base flow (i.e., ground water runoff). Runoff, thus is the response of a catchment to the precipitation reflecting the combined effects of the nature of precipitation, other climatic characteristics of the region, and the physiographic characteristics of the catchment basin.

The chief characteristics of the precipitation that affect the stream flow are
1.Type,
2.Intensity,
3.Duration
4.Areal distribution of precipitation over the catchment,

Precipitation in the form of rainfall is quicker to appear as stream flow than when it is in the form of snow. For the surface runoff to start, the intensity of rainfall (or precipitation) must exceed the infiltration capacity of the soil which decreases with the increase in the duration of rainfall. It is, therefore, obvious that a longer duration rainfall may produce higher runoff even if the intensity of rainfall is less but, of course, exceeding the infiltration capacity of the soil. Heavy rainfalls in the downstream region of the catchment will cause rapid rise in the stream levels and early peaking of the discharge. A rare occurrence of uniformly distributed rainfall may result in increased infiltration and, therefore, increased subsurface runoff and base flow resulting in slow rise in levels and delayed peaking of the discharge. Likewise, antecedent higher soil moisture conditions at the time of precipitation would hasten the rise in the stream levels.

HYBRID CROPPING

By the term Hybrid we mean an offspring of parents belonging to different characteristic groups of the same genetic group. Plant and animal breeders have developed special techniques for producing hybrids artificially in laboratories, zoos, and farms.

Hybrids generally tend to be sterile. Even if they can produce, the first generation offspring may resemble their parents but next generation may not. The second generation usually shows different combination of the characteristics of the original crossbred parents.

Growing of a crop with hybrid seeds is called hybrid cropping. The hybrid seeds may unite the desirable traits of both parents. For example, a gardener may crossbreed an ornamental large flower with a sweet-smelling variety to produce a hybrid variety of large aromatic flowers.
The hybrid seeds have what is called hybrid vigour i.e., they generally tend to be large, faster-growing and healthier than their parent. This fact has been exploited commercially in the cultivation of corn (maize), potatoes, cotton, and several varieties of flowers. However, the hybrid seeds are very costly and, therefore, cannot be adopted on a mass scale in the country. Hybrid seeds, however, appear more promising for glasshouse cultivation of plants.

MEASUREMENT OF EVAPOTRANSPIRATION

        The measurement of Potential evapotranspiration is done by a instrument called Lysimeter. It is a large container having crops inside it.The evapotranspiration is calculated by   measuring these crops water loss and gains. Natural conditions are simulated in these containers as closely as possible. The operator measures water added, water retained by the soil, and water lost through evapotranspiration and deep percolation. Weighings can be made with scales or by floating the lysimeters in water. Growth of roots in lysimeters confined to the dimensions of lysimeters, the disturbed soil in the lysimeters and other departures from natural conditions limit the accuracy of lysimeter measurements of potential evapotranspiration.

Potential evapotranspiration from a cropped surface can be estimated by the two processes described below. 
1.Correlating potential evapotranspiration with water loss from evaporation devices
2.Estimating the potential evapotranspiration based on various climatic parameters.
Correlation of potential evapotranspiration is baesed on the assumption  that the climatic conditions affecting crop water loss (D_et) and evaporation from a free  surface of water (E_p) are the same. Potential evapotranspiration (D_et)can be correlated to the pan evaporation E_p as below,
                                D_et = KE_p
where, K is called the crop factor for that period. K depends on the crop and its stage of growth. The main limitations of this method are the differences  in physical features of evaporation surfaces compared with those of a crop surface.

In the absence of pan evaporation data, the consumptive use is generally computed as follows:
(i) Compute the seasonal (or monthly) distribution of potential evapotranspiration, which is defined as the evapotranspiration rate of a well-watered reference crop which completely shades the soil surface. It is thus an indication of the climatic evaporation demand of a vigorously growing crop. Usually, grass and alfalfa (a plant with leaves like that of clover and purple flowers used as food for horses and cattle) are taken as reference crops.
(ii) Adjust the potential evapotranspiration for the type of crop and the stage of crop growth. Factors such as soil moisture depletion are ignored so that the estimated values of the consumptive use are conservative values to be used for design purposes.
Thus, evapotranspiration of a crop can be estimated by multiplying potential evapotranspiration by a factor known as crop coefficient.
Potential evapotranspiration can be computed by one of the several methods available for the purpose. These methods range in sophistication from simple temperature correlation (such as the Blaney-Criddle formula) to equations (such as Penman’s equation) which account for radiation energy as well. Blaney-Criddle formula for the consumptive use has been used extensively and is expressed as                                   u = kf
in which, u = consumptive use of crop in mm,
k = empirical crop consumptive use coefficient, and
f = consumptive use factor.
The quantities u, k and f are determined for the same period (annual, irrigation season,growing season or monthly). The consumptive use factor f is expressed as                                       f =p/₁₀₀(1.8t + 32)
in which, t = mean temperature in °C for the chosen period, and
p = percentage of daylight hours of the year occurring during the period.

EVAPOTRANSPIRATION OR CONSUMPTIVE USE

       Evapotranspiration is the combined loss of water from earth surface and crop by vaporisation. Crops uses water for transpiration and evaporation activates. During the growing period of a crop,there is a continuous movement of water from soil into the roots, up the stems and leaves, and out of the leaves to the atmosphere. Only a very small portion (less than 2 per cent) of water absorbed by the roots is retained in the plant and the rest of the absorbed water, after performing its tasks, gets evaporated to the atmosphere mainly through the leaves and stem. This process is called transpiration. In addition, some water gets evaporated to the atmosphere directly from the adjacent soil and water surfaces and from the surfaces of the plant leaves (i.e., the intercepted precipitation on the plant foliage). The water needs of a crop thus consists of transpiration and evaporation and is called evapotranspiration or consumptive use.

Consumptive use is the sum of water need of crop and volume of transpirated in  a specific time. Consumptive use is also described as amount of water needed to meet the water loss through evapotranspiration. It generally applies to a crop but can be extended to a field, farm, project or even a valley. Consumptive use is generally measured as volume per unit area or simply as the depth of water on the irrigated area. Knowledge of consumptive use helps determine irrigation requirement at the farm which should, obviously, be the difference between the consumptive use and the effective precipitation.
Evapotranspiration is dependent on following conditions
           1.Temperature
           2.Daylight Hour
           3.Humidity 
           4.Wind Movement
           5.Type of Crop
           6.Stage of Growth of Crop
           7.Soil moisture depletion
           8.Physical and chemical properties of soil

For example, in a sunny and hot climate, crops need more water per day than in a cloudy and cool climate. Similarly, crops like rice or sugarcane need more water than crops like beans and wheat. Also, fully grown crops need more water than crops which have been just planted.

UNIT HYDROGRAPH

First proposed by Sherman in 1932 unit hydrograph (UH) is the hypothetical unit response of a watershed (in terms of runoff volume and timing) to a unit input of rainfall.The Unit Hydrograph of a drainage basin is defined as a hydrograph of direct runoff resulting from one unit of effective rainfall which is uniformly distributed over the basin at a uniform rate during the specified period of time known as unit time or unit duration. The unit quantity of effective rainfall is generally taken as 1mm or 1cm and the outflow hydrograph is expressed by the discharge ordinates. The unit duration may be 1 hour, 2 hour, 3 hours or so depending upon the size of the catchment and storm characteristics. However, the unit duration cannot be more than the time of concentration, which is the time that is taken by the water from the furthest point of the catchment to reach the outlet.

Assumptions
The following assumptions are made while using the unit hydrograph principle:
1. Effective rainfall should be uniformly distributed over the basin, that is, if there are ‘N’ rain gauges spread uniformly over the basin, then all the gauges should record almost same amount of rainfall during the specified time.
2. Effective rainfall is constant over the catchment during the unit time.
3. The direct runoff hydrograph for a given effective rainfall for a catchment is always the same irrespective of when it occurs. Hence, any previous rainfall event is not considered. This antecedent precipitation is otherwise important because of its effect on soil-infiltration rate, depressional and detention storage, and hence, on the resultant hydrograph.
4. The ordinates of the unit hydrograph are directly proportional to the effective rainfall hyetograph ordinate. Hence, if a 6-h unit hydrograph due to 1 cm rainfall is given, then a 6-h hydrograph due to 2 cm rainfall would just mean doubling the unit hydrograph ordinates. Hence, the base of the resulting hydrograph (from the start or rise up to the time when discharge becomes zero) also remains the same.

limitations Under the natural conditions of rainfall over drainage basins, the assumptions of the unit hydrograph cannot be satisfied perfectly. However, when the hydrologic data used in the unit hydrograph analysis are carefully selected so that they meet the assumptions closely, the results obtained by the unit hydrograph theory have been found acceptable for all practical purposes.
In theory, the principle of unit hydrograph is applicable to a basin of any size. However, in practice, to meet the basic assumption in the derivation of the unit hydrograph as closely as possible, it is essential to use storms which are uniformly distributed over the basin and producing rainfall excess at uniform rate. Such storms rarely occur over large areas. The size of the catchment is, therefore, limited although detention, valley storage, and infiltration all tend to minimize the effect of rainfall variability. The limit is generally considered to be about 5000 sq. km. beyond which the reliability of the unit hydrograph method diminishes. When the basin area exceeds this limit, it has to be divided into sub-basins and the unit hydrograph is developed for each sub-basin. The flood discharge at the basin outlet is then estimated by combining the sub-basin floods, using flood routing procedures.

Application
Calculations of direct runoff hydrograph in catchment due to a given rainfall event (with recorded rainfall values), is easy if a unit hydrograph is readily available. Remember that a unit hydrograph is constructed for a unit rainfall falling for a certain T-hours, where T may be any conveniently chosen time duration. The effective rainfall hyetograph, for which the runoff is to be calculated using the unit hydrograph, is obtained by deducting initial and infiltration losses from the recorded rainfall. This effective rainfall hyetograph is divided into blocks of T-hour duration. The runoff generated by the effective rainfall for each T-hour duration is then obtained and summed up to produce the runoff due to the total duration.

ESTIMATION OF INFILTRATION

     Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour.

     The rate at which water infiltrates into a ground is called the infiltration capacity. When a soil is dry, the infiltration rate is usually high compared to when the soil is moist. For an initially dry soil subjected to rain, the infiltration capacity curve shows an exponentially decaying trend as shown in Figure 10. The observed trend is due to the fact that when the soil is initially dry, the rate of infiltration is high but soon decreases, as most of the soil gets moist. The rate of infiltration reaches a uniform rate after some time.

Interestingly, if the supply of continuous water from the surface is cutoff, then the infiltration capacity starts rising from the point of discontinuity as shown in below.

For consistency in hydrological calculations, a constant value of infiltration rate for the entire storm duration is adopted. The average infiltration rate is called the Infiltration Index and the two types of indices commonly used are explained in the next section.

Infiltration indices
The two commonly used infiltration indices are the following:
φ – index
W – index
The φ - index
This is defined as the rate of infiltration above which the rainfall volume equals runoff volume, as shown in Figure

The method to determine the - index would usually involve some trial. Since the infiltration capacity decreases with a prolonged storm, the use of an average loss rate in the form of - index is best suited for design storms occurring on wet soils in which case the loss rate reaches a final constant rate prior to or early in the storm. Although the - index is sometimes criticized as being too simple a measure for infiltration, the concept is quite meaningful in the study of storm runoff from large watersheds. The evaluation of the infiltration process is less precise for large watersheds. The data is never sufficient to derive an infiltration curve. Under the circumstances, the - index is the only feasible alternative to predict the infiltration from the storm.
The W – index
This is the average infiltration rate during the time when the rainfall intensity exceeds the infiltration rate.
Thus, W may be mathematically calculated by dividing the total infiltration (expressed as a depth of water) divided by the time during which the rainfall intensity exceeds the infiltration rate. Total infiltration may be fund out as under:
Total infiltration = Total precipitation – Surface runoff – Effective storm retention The W – index can be derived from the observed rainfall and runoff data. It differs from the - index in that it excludes surface storage and retention. The index does not have any real physical significance when computed for a multiple complex watershed. Like the phi-index the - index, too is usually used for large watersheds.

HYDROGRAPH

     Hydrograph is the plot of the stream flow at a particular location as a function of time. Although the flow comprises of the contributions from overland flow, interflow and groundwater flow, it is useful to separate only the groundwater flow (the base flow) for hydrograph analysis.

     The hyetograph is the graphical plot of the rainfall plotted against time. Traditionally, the hyetograph is plotted upside down as shown in Figure below, which also shows a typical hydrograph and its components. Splitting up of a complete stream flow hydrograph into its components requires the knowledge of the geology of the area and of the factors like surface slope, etc. Nevertheless, some of the simpler methods to separate base flow are described subsequently.

  

  Hydrology of rivers is required by engineers for estimation of water, design of dams, diversions, and flood control through reservoirs or dykes, etc. Information is gathered through a network of stream gauges. Hydrograph analysis deals with the study of runoff records at a stream gauge. Hydrograph analysis is often combined with rainfall analysis to investigate how a watershed responds to rainfall. In many cases, hydrometric information is not available. This is especially true for small watersheds. In such situations, rainfall information must be combined with rainfall-runoff models

MEASUREMENT OF RAINFALL

      One can measure the rain falling at a place by placing a measuring cylinder graduated in a length scale, commonly in mm. In this way, we are not measuring the volume of water that is stored in the cylinder, but the ‘depth’ of rainfall. The cylinder can be of any diameter, and we would expect the same ‘depth’ even for large diameter cylinders provided the rain that is falling is uniformly distributed in space.
     Now think of a cylinder with a diameter as large as a town, or a district or a catchment of a river. Naturally, the rain falling on the entire area at any time would not be the same and what one would get would be an ‘average depth’. Hence, to record the spatial variation of rain falling over an area, it is better to record the rain at a point using a standard sized measuring cylinder.
     In practice, rain is mostly measured with the standard non-recording rain gauge the details of which are given in Bureau of Indian Standards code IS 4989: 2002. The rainfall variation at a point with time is measured with a recording rain-gauge, the details of which may be found in IS 8389: 2003. Modern technology has helped to develop Radars, which measures rainfall over an entire region. However, this method is rather costly compared to the conventional recording and non-recording rain gauges which can be monitored easily with cheap labour.

     Rainfall measurement is commonly used to estimate the amount of water falling over the land surface, part of which infiltrates into the soil and part of which flows down to a stream or river. For a scientific study of the hydrologic cycle, a correlation is sought, between the amount of water falling within a catchment, the portion of which that adds to the ground water and the part that appears as streamflow. Some of the water that has fallen would evaporate or be extracted from the ground by plants.

Average rainfall depth
     The time of rainfall record can vary and may typically range from 1 minute to 1 day for non – recording gauges, Recording gauges, on the other hand, continuously record the rainfall and may do so from 1 day 1 week, depending on the make of instrument. For any time duration, the average depth of rainfall falling over a catchment can be found by the following three methods.
     • The Arithmetic Mean Method
     • The Thiessen Polygon Method
     • The Isohyetal Method
Arithmetic Mean Method
     The simplest of all is the Arithmetic Mean Method, which taken an average of all the rainfall depths as shown in Figure 1.

Average rainfall as the arithmetic mean of all the records of the four rain gauges, as shown below:

The Theissen polygon method
     This method, first proposed by Thiessen in 1911, considers the representative area for each rain gauge. These could also be thought of as the areas of influence of each rain gauge, as shown in Figure 2.

     These areas are found out using a method consisting of the following three steps:
1. Joining the rain gauge station locations by straight lines to form triangles
2. Bisecting the edges of the triangles to form the so-called “Thiessen polygons”
3. Calculate the area enclosed around each rain gauge station bounded by the polygon edges (and the catchment boundary, wherever appropriate) to find the area of influence corresponding to the rain gauge.
     For the given example, the “weighted” average rainfall over the catchment is determined as,

The Isohyetal method
     This is considered as one of the most accurate methods, but it is dependent on the skill and experience of the analyst. The method requires the plotting of isohyets as shown in the figure and calculating the areas enclosed either between the isohyets or between an isohyet and the catchment boundary. The areas may be measured with a planimeter if the catchment map is drawn to a scale.

For the problem shown in Figure 3, the following may be assumed to be the areas enclosed between two consecutive isohyets and are calculated as under:
Area I = 40 km²
Area II = 80 km²
Area III = 70 km²
Area IV = 50 km²
Total catchment area = 240 km²
The areas II and III fall between two isohyets each. Hence, these areas may be thought of as corresponding to the following rainfall depths:
Area II : Corresponds to (10 + 15)/2 = 12.5 mm rainfall depth
Area III : Corresponds to (5 + 10)/2 = 7.5 mm rainfall depth
For Area I, we would expect rainfall to be more than 15mm but since there is no record, a rainfall depth of 15mm is accepted. Similarly, for Area IV, a rainfall depth of 5mm has to be taken.
Hence, the average precipitation by the isohyetal method is calculated to be

Please note the following terms used in this section:
Isohyets: Lines drawn on a map passing through places having equal amount of rainfall recorded during the same period at these places (these lines are drawn after giving consideration to the topography of the region).

Planimeter: This is a drafting instrument used to measure the area of a graphically represented planar region.

Mean rainfall
     This is the average or representative rainfall at a place. The mean annual rainfall is determined by averaging the total rainfall of several consecutive years at a place. Since the annual rainfall varies at the station over the years, a record number of years are required to get a correct estimate.
Similarly, the mean monthly rainfall at a place is determined by averaging the monthly total rainfall for several consecutive years.

PRECIPITATION

     Under certain favourable condition when a warm air mass and cold air mass meet,the wormer air mass is lifted over the colder one with the formation of a front.The ascending warmer air cools adiabatically with the consequent formation of clouds and precipitate.
     Precipitation is any form of solid or liquid water that falls from the atmosphere to the earth’s surface. Rain, drizzle, hail and snow are examples of precipitation. In India, rain is the most common form of precipitation.
     Evapotranspiration is the process which returns water to the atmosphere and thus completes the hydrologic cycle. Evapotranspiration consists of two parts, Evaporation and Transpiration. Evaporation is the loss of water molecules from soil masses and water bodies. Transpiration is the loss of water from plants in the form of vapour. We proceed on to discuss precipitation, and its most important component in India context, the rainfall.

Causes of precipitation
    For the formation of clouds and subsequent precipitation, it is for necessary that the moist air masses to cool in order to condense. This is generally accomplished by adiabatic cooling of moist air through a process of being lifted to higher altitudes. The precipitation types can be categorized as.

• Frontal precipitation This is the precipitation that is caused by the expansion of air on ascent along or near a frontal surface.
• Convective precipitation Precipitation caused by the upward movement of air which is warmer than its surroundings. This precipitation is generally showery nature with rapid changes of intensities.
• Orographic precipitation      Precipitation caused by the air masses which strike the mountain barriers and rise up, causing condensation and precipitation. The greatest amount of precipitation will fall on the windward side of the barrier and little amount of precipitation will fall on leave ward side.
     For the Indian climate, the south-west monsoon is the principal rainy season when over 75% of the annual rainfall is received over a major portion of the country. Excepting the south-eastern part of the Indian peninsula and Jammu and Kashmir, for the rest of the country the south-west monsoon is the principal source of rain.
     From the point of view of water resources engineering, it is essential to quantify rainfall over space and time and extract necessary analytical information.