Hydrograph_ for Design of structure pptx

hydrograph basic for design of hydroliv structure
measure of precipitation

1. HYDROGRAPHS LECTURE 11

2. HYDROGRAPHS A hydrograph is a continuous graph showing the properties of stream flow with respect to time Normally obtained by means of a continuous strip recorder that indicates stage versus time (stage hydrograph) and is then transformed to discharge hydrograph by use of rating curve. The term hydrograph generally means discharge hydrograph The hydrograph is a result of the physiological and hydrometerological effects of watershed

3. COMPONENT ELEMENTS OF HYDROGRAPH Direct surface runoff Interflow Ground water or base flow Channel precipitation

4. DEFINITIONS OF HYDROGRAPH Rising limb or Concentration curve Crest Segment Falling Limb Hydrograph in period of no DRO and where no reservoir regulation exists reflects discharge from ground water Point of inflection (where DRO ceases) Point of rise (where DRO starts) Time Discharge

5. RUNOFF FORMATION FROM A RAIN EVENT

6. FACTORS AFFECTING THE SHAPE OF HYDROPGRAPH Climatic Factors Rainfall intensity Rainfall duration Distribution of rainfall on the basin Direction of storm movement Topographic and geologic Factors Catchment size Catchment shape Distribution of water courses/ drainage Slope of the catchment Geology of the catchment Land use Land cover

7. FACTORS AFFECTING SURFACE RUNOFF The type of storm is important. Thunderstorms produce peak flows on small basins , whereas large cyclonic or frontal-type storms are generally a determinant in larger basins . Rainfall intensity: It affects the amount of runoff and the peak flow rate. For a given rainfall duration, an increase in intensity will increase the peak discharge and the runoff volume , provided the infiltration rate of the soil is exceeded. Rainfall duration: It affects the amount of runoff , the peak flow rate and the duration of surface runoff. For a rain of given intensity, the rainfall duration determines, in part, the peak flow. If a storm lasts long enough, eventually almost all the precipitation will become runoff (the time after which this occurs is called the time of concentration); consequently the peak flow will approach a rate equal to the product iA , where i is the rainfall intensity and A is the area of the basin. This situation is never reached in large basins, but may occur in small watersheds and is frequently used as the criterion for design of storm sewers, airport drainage or small culverts. 7

8. FACTORS AFFECTING SURFACE RUNOFF Areal distribution of rainfall can cause variations in hydrograph shape. If an area of high rainfall is near to the basin outlet , a rapid rise , sharp peak and rapid recession of the hydrograph usually result. If the same amount of rainfall occurs in the upper reaches of a basin, the hydrograph exhibits a lower and broader peak . The direction of storm movement with respect to orientation of the basin affects both the magnitude of the peak flow and the duration of surface runoff. Storm direction has the greatest effect on elongated basins. On these basins, storms that move upstream tend to produce lower peaks of a longer duration than storms that move downstream. 8

9. DIRECTION OF STORM MOVEMENT EFFECT Storms that move upstream tend to produce lower peaks of a longer duration than storms that move downstream.

10. D rainage area  has effect on that the time base of the hydrograph is lengthened.  The peak flow per unit area thus reduces with catchment size for a given rainfall depth .  This is partly due to the rainfall intensity being less for storms of extensive size, and partly due to the longer time required for the total catchment area to contribute to the peak runoff (time of concentration). The effect of  shape   can best be demonstrated by considering the hydrographs of discharges from three differently shaped catchments with the same surface area, subject to rainfall of the same intensity.  The lines of equal run-time ( known as isochrones ) to the outlet show that shape B has the smallest time of concentration (5 hours), and thus reaches the peak after 5 hours.  The most elongated catchment needs 10 hours to reach the peak.  The  pattern and arrangement of the natural stream channels   determine the efficiency of the drainage system. Other factors being constant, the time required for water to flow a given distance is directly proportional to length.  Since a well-defined system reduces the distance for which water must move overland, the corresponding reduction in time involved is reflected by an outflow hydrograph having a short time to peak . 10 FACTORS AFFECTING SURFACE RUNOFF B A C

11. The steeper the  slope  of the catchment, the more rapidly surface runoff will travel. The time to peak will be shorter and the peaks will be higher .  Infiltration capacities tend to be lower as slopes get steeper, thus accentuating runoff . Since  storage   must first be filled before it empties, it has a delaying and modifying effect on hydrograph shape.  Much of the variation in runoff is smoothed out by natural or artificial storage. The pedology and geology   of the catchment influence primarily the groundwater component and the "losses".  High infiltration rates reduce the surface runoff ; high permeabilities combined with high transmissivities substantially enhance the baseflow component.  The type of stream (influent, effluent or intermittent) can have a substantial impact on hydrograph Landuse ,   finally, can strongly influence the runoff coefficient.  Urbanized areas may have a runoff coefficient of almost 100 %, whereas natural vegetation may have low runoff. Ploughing, drainage, cropping intensity, afforestation etc. also have a considerable effect on runoff. 11 FACTORS AFFECTING SURFACE RUNOFF

12. SHAPE OF CATCHEMENT EFFECT Wide Narrow or long

13. SHAPE OF CATCHEMENT EFFECT The steeper the   slope  of the catchment, the more rapidly surface runoff will travel. The time to peak will be shorter and the peaks will be higher.   Infiltration capacities tend to be lower as slopes get steeper, thus accentuating runoff.

14. DRAINAGE OF WATERSHED The  pattern and arrangement of the natural stream channels   determine the efficiency of the drainage system. Other factors being constant, the time required for water to flow a given distance is directly proportional to length.  Since a well-defined system reduces the distance for which water must move overland, the corresponding reduction in time involved is reflected by an outflow hydrograph having a short time to peak.

15. LAND COVER

16. HYDROGRAPH AND TIME RELATIONS Time base: Time from which the Rising Limb (or concentration curve) begins (point of rise) until the direct runoff ceases.

17. HYDROGRAPH AND TIME RELATIONS Lag time or Basin Lag : difference in time between center of mass of effective rainfall and center of mass of runoff produced . Or Time interval from the maximum effective rainfall to peak rate of runoff.

18. HYDROGRAPH AND TIME RELATIONS Time of concentration: the concentration time is the time required, with uniform rain for 100% of a tract of land to contribute to the direct runoff at the outlet. Or the time for runoff to arrive at the outlet from the most remote point

19. HYDROGRAPH SEPARATION Hydrograph separation means to separate base flow and DRO in a hydrograph as a basis for further analysis techniques Several methods of base flow separation are used when exact amount of base flow is unknown. Such as Straight Line method Constant Discharge method/ Horizontal line method Base Length Method Fixed base length (time) with Constant Slope method/Inclined line method Fixed base length (time) with Variable Slope Method Concave method

20. Hydrograph Separation In Horizontal Line method ( Fixed Base Discharge ): A horizontal Line is drawn from point of rise of hydrograph In inclined Method : An inclined line is drawn by joining two points; i.e. where DRO starts and where DRO Ends . DRO is supposed to cease N days from the Peak discharge , where N = K (A) 0.2 , N is in days, and A is watershed area. If area is in mile 2 , K =1, if area in km 2 , K=0.8. In fixed base length method , a line AB is drawn in the extension of recession curve ending at peak of hydrograph (B). Another line BC is drawn from point B to C where DRO ceases at a distance N days from the peak. In Concave method , (Most realistic, but need judgment of experience hydrologist) An experienced hydrologist separates the base flow from the DRO, with reducing base flow till peak of hydrograph and increasing base flow till inflection point on receding limb of hydrograph. (REF: Lab Manual of Hydrology, exercise. 5)

21. UNIT HYDROGRAPHS A unit hydrograph is defined as the direct runoff (total discharge minus base flow) resulting from 1 inch (or 1 cm) of “ excess rainfall ” generated uniformly over the basin at a uniform rate during a specified period of time or duration. (USGS) The shape of the unit hydrograph is a function of the basin characteristics. The concept of unit hydrograph is based on linear systems theory and follows the principles of superposition and proportionality. It is incorrect to describe a unit hydrograph without specifying the duration, of the storm that produced it. An x hours unit hydrograph means a direct runoff hydrograph having 1.0 inch (or 1 cm) volume resulting from an x hours storm having a steady intensity 1/x in per hour Rainfall excess is that part of the rainfall that becomes direct overland runoff to the streams.

22. APPLICATION OF UNIT HYDROGRAPH A unit hydrograph is used for the prediction of flood peak and time to peak in the stream at a particular section due to any amount of effective precipitation Application of an x-hour unit hydrograph to rainfall excess amounts more than 1 unit is accomplished just by multiplying the excess amount by the unit hydrograph ordinates For example, a 3 hours rain event producing 2.0” effective precipitation would have runoff rates 2 times of a 3-hours unit hydrograph. Similarly a 3- hours storm having 0.5” net precipitation would produce runoff rates half of the 3 hours unit hydrograph. This assumption of proportional flows applies only to equal duration storms

23. ASSUMPTIONS OF UNIT HYDROGRAPH Following are the assumptions while deriving the unit hydrograph: Precipitation amount and intensity is uniform over the entire watershed Precipitation intensity remains uniform throughout the storm Base length of the hydrograph DRO for a particular catchment resulting from a storm of given duration is approximately constant Entire watershed is treated as a single unit

24. DERIVATION OF UHG

25. NUMERICAL PROBLEM Time (units) Total Runoff (cfs) Base-flow (cfs) Time (units) Total Runoff (cfs) Base-flow (cfs) 1 110 110 7 293 113 2 122 122 8 202 112 3 230 120 9 160 110 4 578 118 10 117 105 4.7 666 116 10.5 105 105 5 645 115 11 90 90 6 434 114 12 80 80 Determine UHG ordinates if effective precipitation is 1.4 in for a storm of 1.5 hour duration . C:\Users\Noor Khan\Documents\BSc\Hydrology\Lectures\Unit Hydograph_S Curves.xlsx

26. NUMERICAL PROBLEM Using the hydrograph developed in last problems derive a DRO hydrograph of a storm event as given below. 0.7” 1.7” 1.2” Each time unit is 1.5 hours

27. CONVERSION OF UHG DURATION Unit hydrograph developed by procedure outlined earlier is applicable only for a specified duration of storm The application to storms of larger or smaller duration might be required sometimes Instead of making hydrograph for the new duration there are two more methods Lagging method S-curve Method

28. CONVERSION OF UHG BY LAGGING METHOD The method of “ lagging ” is based on the assumption that linear response of the watershed is not influenced by previous storms one can superimpose hydrograph offset in time and flows are directly additive if a hydrograph of 1 hour is given, hydrograph for 2 hours duration can be obtained by plotting two 1 hour UHG with second UHG 1 hour lagged, adding ordinates and dividing by two if a hydrograph of 1 hour is given, hydrograph for 3 hours duration can be obtained by plotting Three 1 hour UHG with second UHG 1 hour lagged and third UHG 2 hours lagged, adding ordinates of all and dividing by Three Lagging procedure is restricted to the multiples of the original duration according to the expression D 1 = nD D 1 : possible durations of UHG by lagging method D : Original duration of UHG n : 1,2,3,….

29. CONVERSION OF UHG BY S-CURVE METHOD Construction of any duration of unit hydrograph Lagging system is the same as described in last method For construction of S-Curve A unit hydrograph is assumed to repeat indefinitely Continuous lagging of UHG is comparable to a continuously applied rainfall at a certain intensity S curve of a UHG of Tr duration, is constructed by adding together a series of unit hydrograph, each lagged Tr hours with respect to preceding one. Maximum discharge of S-hydrograph occurs at time D hours that is less than time base of the storm An S Curve is a Hydrograph that would result from an infinite series of unit runoff increments

30. S-CURVE METHOD (Summation Curve Method)

31. Suppose you have S curve made from UHG of D duration Now you have given it a shift of t duration. This is S curve made from UHG of D duration, but lagged by t duration (shifted S Curve). When we subtract both S curves, we get Hydrograph because of storm occurring in the lagged period (t duration. Adjust this hydrograph by multiplying with D/t to get UHG of duration t. Ref. Slides of Dr Ahmed Sana, SQU t t 1/t

32. S-CURVE METHOD Suppose we have a Unit Hydrograph (UHG) of D hours , and we want a UHG of t hours Make an S Hydrograph of D hours UHG, by adding ordinates of a series of UHG lagged at the D intervals from preceding UHG Simply lag the first S-hydrograph of D hours UHG by a second S-hydrograph a time interval equal to t hours Subtract the ordinates of second S-hydrograph by the first one And multiply these ordinate with a factor D/t Result is UHG of t duration storm

33. NUMERICAL PROBLEM Given the following 2-hr UHG, use S-curve procedures to construct a 3-hr UHG. Time (hours) Q (cfs) 1 100 2 250 3 200 4 100 5 50 6

34. 35 Average of Several UHG’s It is recommend that several unit hydrographs be derived and averaged. The unit hydrographs must be of the same duration in order to be properly averaged. It is often not sufficient to simply average the ordinates of the unit hydrographs in order to obtain the final unit hydrograph. A numerical average of several unit hydrographs which are different “shapes” may result in an “unrepresentative” unit hydrograph. It is often recommended to plot the unit hydrographs that are to be averaged. Then an average or representative unit hydrograph should be sketched or fitted to the plotted unit hydrographs. Finally, the average unit hydrograph must have a volume of 1 inch of runoff for the basin.