Introduction, HYDROGRAPHS

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2 HYDROGRAPHS

3 Sequence of lecture Introduction Types of Hydrograph Components of Hydrograph Effective Rainfall Basin Lag or Time Lag Parts of Hydrograph Hydrograph Analysis Factors Affecting Hydrograph Shape

4 Introduction, HYDROGRAPHS The graphical relation between any hydrological quantity (stage, velocity, discharge, etc.) & the time is known as a hydrograph. The Hydrograph is a relationship of discharge and time at a particular location on a stream. A runoff hydrograph is a continuous record of stream flow over time. A complete runoff hydrograph contains information on runoff volume as the area under the hydrograph and peak runoff rates as the maximum flow or peak of the runoff hydrograph. The hydrograph is an integral expression of physiographic and climatic characteristics that govern the relation between rainfall and runoff of a particular drainage basin (Chow, 1964). 4

5 Figure #1. Distribution of Precipitation input (Viessman and Lewis, 1997) 5

6 TYPES OF HYDROGRAPH Discharge Hydrograph It is the graphical representation of discharge against time. Generally a hydrograph means discharge hydrograph. Stage Hydrograph It is the graphical representation of stage against time. Stage hydrograph is useful only for the design of flood-protection works like embankment. Velocity Hydrograph It is the graphical representation of velocity against time.

7 Parts of Hydrograph Duration Lag Time Time of Concentration Rising Limb Recession Limb (falling limb) Peak Flow Time to Peak (rise time) Recession Curve Separation Base flow

8 Hydrograph Components Rising limb: The rising limb of hydrograph, also known as concentration curve, reflects a prolonged increase in discharge from a catchment area, typically in response to a rainfall event. The shape of the rising limb depends upon the storm characteristics which are the duration of rainfall, intensity of rainfall, areal distribution of the rainfall, etc. Recession limb: The recession limb extends from the point of inflection at the end of the crest segment to the commencement of the natural groundwater flow (base flow). It represents the withdrawal of water from the storage built up in the basin during the earlier phases of the hydrograph. The falling limb depends upon the control on storage release, which itself depends upon the geological structure.

9 Hydrograph Components Peak discharge: The highest point on the hydrograph when the rate of discharge is greatest. The crest segment is controlled by the storm characteristics and the distribution of streams in the area, which further depends upon geological structure of the area. Lag Time Time interval from the center of mass of the rainfall-excess to the peak of the resultant hydrograph Time to Peak Time interval from the start of the rainfall-excess to the peak of the resultant hydrograph Rainfall Duration Time interval from the start to the end of rainfall-excess Time of Concentration Time required for runoff to travel from the hydraulically most distant point on the watershed to the point of interest

10 EFFECTIVE RAINFALL All of the rain usually does not go into the stream but a certain part of it reaches the stream & causes rise in the stream flow while the remaining part of rain is accounted for in various forms of precipitation losses. The portion of rainfall which contributes to stream flow is called Effective Rainfall.

11 Discharge, Q BASIN LAG OR TIME LAG A useful index to define the location of the peak of the hydrograph is the basin lag, which is the time difference between the center of mass of the rainfall and the center of the mass of the hydrograph. Rise of the stream flow depends upon the effective rainfall and type of the soil, and time lag depends upon type of area & also upon characteristics of rainfall Lag Hydrograph Time, t DRO Hydrograph Continued..

12 COMPONENTS OF HYDROGRAPH A hydrograph normally consists of the four components: Channel Precipitation Direct Runoff (DRO) Inter Flow or Sub-surface Flow Ground or Base Flow

13 HYDROGRAPH ANALYSIS Separation of Hydrograph Components Hydrograph Analysis means dividing total runoff indicated by the hydrograph into its (abovementioned) components, of which direct runoff (DRO) and Base Flow are more important. The separation of hydrograph components is done by one of the following four methods. Straight Line Method Fixed Base Length Method Variable Slope Method Recession Curve Equation Continued..

14 Discharge, Q Straight Line Method A hydrograph as shown in Figure. From point A draw a straight horizontal line which meets the falling limb of hydrograph at point B. Above line AB is the direct runoff and below this line is the base flow. The volumes of direct runoff and base flow (i.e. area under the curve) can be calculated by any suitable method A Hydrograph Time, t DRO Hydrograph Horozontal Line from the point of rise in rising limb of hydrograph The Straight Line Method for separation of hydrograph has the advantage of producing an extremely long time base for the direct runoff hydrograph. The time base varies from storm to storm depending on the flow at the point of rise. It is a computationally easy method. B

15 Discharge, Q Fixed Base Length Method Let us have the hydrograph shown in Figure 4. From point A (existing prior to the storm) extend the curve to point B which is vertically below the peak. Now draw a line from point B to point C on the hydrograph equal to a distance on X-axis given by equation below: N = 0.83*A d 0.2 Where, N = Length of time in days, and A d = Drainage area in square miles This line will meet the falling limb at, say point C. Join the point B to C by a line BC. Area below ABC is the base flow and that above it is the direct runoff Extended Recession Curve A Hydrograph B A Time, t DRO Hydrograph C

16 Variable Slope Method This method is based on the fact that if water rises in the stream the water level in the stream becomes greater than the adjoining ground water table level. As a result the flow occurs from the stream to the groundwater and is stored there as bank storage. This method is not a well defined one. It depends upon experience, so it may vary from one expert to another. Continued..

17 Discharge, Q Contd.. Take a point A on the previous recession curve, and draw the line AE by extending the previous recession curve. Point E is under the peak. 600 Now take a point B on the recession curve and extend it 500 backward. Take a point C on recession curve where the 400 recession curve is changing 300 slope. This point is called the point of 200 inflexion. Draw a vertical line from the point C. 100 It cuts the line drawn backward 0 from point B, at point D. Join point E and point D. The area under the line AEDB shows the base flow component Extended Recession Curve A Hydrograph E C D Time, t DRO Hydrograph Point of Inflexion B

18 Contd.. In general, Straight Line Method is the simplest and easiest but also the least accurate. Fixed Base Method is widely used and gives practically good results. Whereas Variable Slope Method is not commonly used and is difficult but can give better results depending upon the expertise of the person using it.

19 FACTORS AFFECTING HYDROGRAPH SHAPE Hydrograph shape depends on Metrologic and catchment characteristics. Among the Metrologic characteristics the most important are the rainfall intensity, duration. Among the catchment characteristics, the topography and geological conditions are more important. These are discussed below: Storm Characteristics Catchment Characteristics

20 Intensity of Rainfall Storm Characteristics The intensity of rainfall has a predominant effect on the shape of the hydrograph. If other conditions remain the same, a higher intensity storm will produce a rapid rise in the hydrograph and a higher peak than that in case of a low intensity rainfall. Such floods give little warning time and so are dangerous. Continued..

21 Duration of Rainfall Contd.. The duration of rainfall is important if it is greater than the time of concentration (defined as the time of travel from the farthest point in the catchment area to the gauging station). In such a situation, if the rainfall of certain intensity occurs uniformly, the whole of the catchment area is contributing runoff at the gauging station and the hydrograph attains its high peak. The runoff will remain constant at the peak rate if the rainfall occurs for duration longer than the time of concentration. If rainfall of the same intensity occurs for duration less than the time of concentration, the hydrograph will rise to a smaller peak. Continued..

22 Weather Contd.. The shape of the hydrograph greatly depends on the seasonal distribution of rainfall. During summer losses due to evaporation may produce a small peak hydrograph. On the other hand, in winter, losses are small with the result that even a small intensity storm may produce a relatively rapid rise and high peak of the hydrograph. Antecedent soil moisture conditions (generally being higher in winter) are important as for high soil moisture; the storm will produce more runoff.

23 Size of Catchment Catchment Characteristics The catchment area affects the stream flow in a variety of ways. Even if the intensity and depth of precipitation is assumed to be constant, The hydrograph of a smaller catchment rises to its peak and then recedes more rapidly than that for a larger catchment, because for the latter, it takes longer for the runoff to reach the gauging station. The hydrograph of a larger catchment area, therefore, has broader base than that of a smaller one. Moreover, the larger the area, the greater will be the heterogeneity in soil and vegetation cover, with a consequent effect on the stream flow of each part of the catchment. The vegetation cover increases the time of concentration. Continued..

24 Contd.. The hydrograph of a larger catchment area, therefore, has broader base than that of a smaller one. The vegetation cover increases the time of concentration. Continued..

25 Contd.. Shape of the Catchment Area The shape of the catchment area affects stream flow by altering the time of concentration and the pattern of drainage tributaries. If the length of the catchment along the main stream is less than the width across the same stream, separate runoff peaks generated by a heavy rainfall are likely to reach the gauging station at the same time from the tributaries, with the result that the flood peak in the main stream increases. Thus a hydrograph with a high peak and narrow base is obtained. Continued..

26 Contd.. However, if the length of the catchment along the main stream is larger than width, the tributaries will be of shorter length and contribution from each tributary area is likely to reach the gauging stations at intervals. After an intense storm over the whole of the catchment area there will be some lag between the times at which the peak from each tributary reaches the gauging station. The hydrograph will therefore, have a low peak and broader base, because the remote tributaries will continue contributing for some time even after the rain fall has stopped. Continued..

27 Contd.. Elevation of the Catchment The elevation of the catchment also plays an important role in some hydrologic characteristics of hydrograph, particularly due to variation in temperature and precipitation. The temperature reduces with the increase in elevation with the result that above a certain elevation, the temperature becomes so low that all precipitation falls as snow. In the Indo-Pakistan Sub-continent, contribution from areas, at altitude 15,000 ft. above the mean sea level, is mainly due to snowmelt. The floods from snowmelt are of usually low peak and broader base. Continued..

28 Contd.. Catchment Slope The slope of the catchment affects stream flow by reducing the rate of infiltration due to increased speed of water-movement towards drainage channels. The steeper the slope the faster will be the flow and rise in the hydrograph. The hydrograph in such a situation will have higher peak and smaller base.