2. 6.1 Definition 6.1 Definition: runoff is defined as the portion of precipitation that makes its way towards rivers, or seas, or oceans, etc  In general sense it is the ppn excess after meeting ET and infiltration demands 6.2 Components of RO SRO Ground water RO according to the source from w/c the flow is derived Sub-surface Ro

3.  Effluent streamthe position of ground-water table is higher river  Effluent stream: when the position of ground-water table is higher than the river water level,  The stream receives water from groundwater reservoir(Fig. a).  Influent Stream: when the position of groundwater table is lower  Influent Stream: when the position of groundwater table is lower than the water level of a stream such that water from the stream contributes to the groundwater storage(Fig. b),

4. b) Effluent Stream Water table b) Influent Stream Fig.6.1: Effluent and Influent Streams Stream Hydrologic Year: groundsurface water storage Hydrologic Year: The period of one year starting with the time when the ground and surface water storage of a basin is the minimum is called hydrologic or water year.

5. Base flow to river

6. Time Loss Rainfall excess Direct Runoff Base Flow Time River Discharge Direct Runoff Fig. Catchment Rainfall and Runoff process

7.  The hydrograph has two main components, a broad band near the time axis representing base flow (contributed from groundwater), and the remaining area above the base flow, the surface runoff, ( produced by the storm).

8. HYDROGRAPH OF RUN-OFF  In order to represent discharge or flow, from a stream, river, watershed, etc. over time, a hydrograph is developed.  A hydrograph is graphical representation of the discharge versus time and it is a useful tool to engineers for the design of structures potentially in contact with the represented discharge.  A hydrograph may represent flow over along period of time, such as a month or year, or it may describe the flow of a single storm event. includes the integrated contributions from surface runoff, groundwater seepage drainage channel precipitation  It includes the integrated contributions from surface runoff, groundwater seepage, and drainage and channel precipitation

11.  Annual hydrograph:  Annual hydrograph: would depict discharge of a river over a full year, during dry and wet periods.

13.  Storm hydrograph  Storm hydrograph :would represent the discharge of a river just before a storm, during and after the storm, and finally as the discharge returns to its normal level before the storm.  Four major components are represented by a hydrograph are 1. Direct surface RO 2. Interflow 3. Ground water, or base flow,and 4. Precipitation  Base flow is the flow under the separation curve; direct surface run-off is above the separation curve;precipitation is the storm rainfall;interflow is part of the falling limb and recession curve.

15. peak flow rates volume of flow, or  For engineers: the parts of the hydrograph that are important are the peak flow rates indicated by the peak on the graph and the volume of flow, or the area under the hydrograph curve. vital information  The peak flow rates provide vital information about the limits for designing safe hydraulic structures.  By studying hydrographs for various storms of a particular site,an engineer is able to design above commonly exceeded peak flows and ensures the safety of his or her design. reservoir or storage structure spillways  The volume of flow is important for determining whether a reservoir or storage structure will be filled while also indicating the size of spillways necessary.  furthermore,water supply and flood control facilities are designed based on volumes of water to be available.

16.  The peak of the hydrograph is reached after the effective rainfall has reached its maximum.  Some important time relationships inherent in a hydrograph are:  The time to peak  The time of concentration and  Lag time  Time to peak: is  Time to peak: is the amount of time from the beginning of the hydrograph, or onset of rainfall, to the peak runoff or peak flow.

17. Time of concentration (Tc):  Concerns the drainage basin or watershed contributing to the flow in a channel,stream,or river.  At the onset of storm runoff,the area closest to the channel is only contributing,but as rain continues and more surface runoff is produced,the remaining parts of the watershed begin to contribute.  Therefore,tc indicates the time required by the runoff to reach the outlet from the most remote part of the drainage basin or in other words,the time required for 100% of the watershed to contribute to flow. the time lag.  The time difference between the maximum effective rainfall intensity and the maximum runoff is called the time lag.

18.  The inflection point  The inflection point is a change of slope i.e., there has been, inflow of the rain up to this point and after this, there is gradual withdrawal of catchment storage. By this time the ground water table has been built up by the infiltrating and percolating water, and now the ground water contributes more into the stream flow than at the beginning of storm,

19. Separation of Base Flow from the hydrograph of River to obtain Direct Runoff hydrograph Separation of Base Flow from the hydrograph of River to obtain Direct Runoff hydrograph  The separation of the components of flood hydrograph in to direct runoff and base flow is the important step in hydrograph analysis.  Sophisticated techniques like the master depletion curve may be useful for longer streams or rivers; however, they require a sufficient long observation period of discharge and rainfall.  There are some approximate methods of base flow separation that are in common use.

20. Straight Line Method  Draw a horizontal line from the point at which surface runoff begins to the intersection with the recession limb.  This method is very simple but is approximate and can be used only for preliminary estimates

21. Fixed Base Method  In this method the surface runoff is assumed to end a fixed time N after the hydrograph peaks.  The base flow before the surface runoff began is projected ahead to the time of the peak. A straight line is used to connect this projection at the peak to the point on the recession limb at time N after the peak.  Empirical equation for time interval N days from the peak to the point B is  N=0.83*A 0.2  Where A drainage area in Km2 and N in days

22. Variable Slope Method  The base flow curve before the surface runoff began is extrapolated forward to the time of the peak discharge, and the base flow curve after the surface runoff ceases is extrapolated backward to the time of the point of inflection on the recession limb. A straight line is used to connect the end points of the extrapolated curves.

23.  These simple techniques are normally leading to good results; however, it may require several steps or trials before reaching the condition of equal storm and runoff volumes.  In all the above separation procedures, the area below the line constructed represents the base flow, i.e., the ground water contribution to stream flow.  Any further refinement in the base flow separation procedure may not be needed, since the base flow forms a very insignificant part of high floods. In fact, very often, a constant value of base flow is assumed.

24. ANNUAL HYDROGRAPHS OF PERENNIAL,INTERMITTENT AND EPHEMERAL STREAMS  A study of the annual hydrographs of streams enables one to classify streams into three classes as (i) Perennial, (ii) Intermittent and (iii) Ephemeral. (i)A perennial stream is one which always caries some flow, there is considerable amount of groundwater flow throughout the year. Even during dry seasons the water table will be able to reach the bed of the stream..

26. Intermittent stream: (ii) Intermittent stream:  have mixed chxics,behaving as perennial at certain time of the year and ephemeral at the other times.  Carry BF contribution only during the rainy se

28. (iii)An ephemeral stream  is one, which does not have any base-flow contribution.  The annual hydrograph of such a river show series of short duration peaks marking flash flows in response to storms. The stream becomes dry soon after the end of the storm flow. Typically an ephemeral stream does not have any well-defined channel. Most rivers in arid zones are of the ephemeral kind.

30. The climatic factors include The physiographic factors are - Intensity of rainfall - Land use - Duration of rainfall - Type of soil - Areal distribution of rainfall - Area of the basin - Direction of storm movement - Shape of the basin - Antecedent precipitation - Slope - Other climatic factors that affect evapotranspiration Factors affecting runoff The runoff from a drainage basin is influenced by various factors which may be put under two groups, namely the climatic factors and physiographic factors

31.  Determination of accurate runoff rate or volume from the watershed is a difficult task, because runoff is dependant upon several factors related to watershed and atmosphere, prediction of whom is not so easy.  However, some common runoff estimation methods are given below: Rainfall-Runoff Correlation Empirical Methods Rational Method Infiltration Indices method Hydrograph Method  sc? Check other reference

32. I Rainfall-Runoff Correlation I Rainfall-Runoff Correlation  The relation between rainfall and the resulting runoff is quit complex and is influenced by a lot of factors relating the catchment and climate.  One of the most common methods is to correlate runoff, R with rainfall, P values. Plotting of R values against P and drawing a best fit line can be adopted for very rough estimates.  A better method is to fit a linear regression line between R and P and to accept the result if the correlation coefficient is near unity. The equation for straight-line regression between runoff R and rainfall P is R = a P + b …eq.1

33.  The values of the coefficients a and b are given by

34.  The value of r lies between 0 and 1 as R has only positive correlation with P. A value of 0.6 < r < 1.0 indicates good correlation. Further it should be noted that R  0.  For large catchment, it is found advantageous to have an exponential relationship as (5)  Where  and m are constants, instead of the linear relationship given by Eq. (1). In that case Eq. (5) is reduced to a linear form by logarithmic transformation as (6)  And the coefficients m and ln  determined by using the method indicated earlier.

35. II. Empirical Methods  With a keen sense of observation in the region of their activity many engineers of the past have developed empirical runoff estimation formulae.  However, these are applicable only to the region in which they were derived.  Empirical formulas can be classified in different ways depending upon the basis adopted.

36.  But they be considered under the following heads for the purpose of present discussion: 1) Formulae that take area of the basin only into consideration 2) Formulae that take one or more basin parameters apart from area and also rainfall characteristics into consideration rainfall-runoff 3) These formulae are essentially rainfall-runoff relations with additional third or fourth parameters to account for climatic or catchment characteristics.  Some of the important formulae are

37. A. Runoff Coefficient Method  This method involves the estimation of runoff by multiplying the runoff coefficient to the rainfall depth of the area. It is given by R = C  P Where, R = runoff,(cm) C = Runoff coefficient, and P = Rainfall depth, cm  Runoff coefficient depends on factors affecting runoff. The values of runoff coefficient for different land use conditions are given in the Tab. Below.

38. S.No.AreaC 1Urban area covered by (i)building (ii)garden apartment 0.30 0.50 2Commercial 6 industrial area0.90 3area0.50 to 0.20 4Parks, Farms, Pastures0.05 - 0.3 5Asphalt or concrete pavement0.85 Table 6.1: Values of Runoff Coefficient (C)

39. B)Formulae based on Area of the basin  There are several regression equation for predicting the runoff rate from the drainage basins.  The form of equation is given as under. Q = C  A n Where,Q = Peak flow for a given recurrence interval, (m 3 /s) n, C = are constants, known as regression constants A = Drainage area, (km 2 )

40. Dicken's Formula: Admasu'sFormaula(1989) Seleshi (2001): -

41. Khosla`s formula R = P – 4.811 T R m = P m – L m L m = 0.48 T m for T m  4.5 °C  For T m < 4.5, WhereR = annual runoff in mm P = annual rainfall in mm T = mean temperature in °C R m = monthly runoff in cm and R m  0 P m = monthly rainfall in cm L m = monthly loss in cm T m = mean monthly temperature of the catchment in °C. Khosla’s formula is indirectly based on the water- balance concept and the mean monthly catchment temperature is used to reflect the loss due to evapotranspiration. Tm ( o C) 4.5-7-12-18 Lm21181512.510

42. C)Rational Method  Among various types of empirical relations, rational formula is the most rational method of calculating peak discharge for small catchments.  In this method it is assumed that the maximum flood flow is produced by a certain rainfall which lasts for a time equal to or greater than the period of concentration time.  This concentration time is the time required for the surface runoff from the remotest part of the catchment area to reach the basin outlet.  When the storm continues beyond concentration time every part of the catchment would be contributing to the runoff at outlet and therefore it represents conditions of peak runoff.  The runoff rate corresponding to this condition is given by

44.  The maximum rainfall intensity depends on duration and frequency. The intensity of rainfall used in the equation above should therefore be corresponding to duration equal to concentration time and desired return period.  This requires an estimate of concentration time which is usually provided by an empirical equation given by Kirpich (1940).  Where t c is time of concentration in min, L is the maximum length of travel of water along the water course in m and S is the slope expressed as the ratio of difference in elevation between the remotest point and the catchment outlet to the length L.

46.  Runoff coefficient is the ratio of peak runoff rate to the rainfall intensity.  Its values are assigned on the basis of land use and soil type (Table 6.2).  When the watershed has different features regarding land use and soil types, then weighted value of runoff coefficient is determined. For instance, if a watershed area is divided into five sub-parts on the basis of soil type and land use practice adopted, having the area a 1, a 2, a 3, a 4 and a 5 and the value of runoff coefficient is C 1, C 2, C 3, C 4, and C 5, respectively for the five sub-watersheds.  Then the value of weighted runoff coefficient (C) is given by: In which, A is the total area of watershed

48. Land use and topography Soil Types loamClay and silt loamTight clay Cultivated land i)Flat ii)Rolling iii)Hilling 0.30 0.4 0.52 0.50 0.60 0.70 0.60 0.70 0.82 Pasture land i)Flat ii)Rollin iii)Hilling 0.10 0.16 0.22 0.30 0.36 0.42 0.40 0.55 0.60 land i)Flat ii)Hilling 0.10 0.30 0.50 0.40 0.60 Populated land i)Flat ii)Rolling 0.40 0.50 0.55 0.65 0.80 Table6.2 Values of C as a function of land use, topography and soil type for use in rational Method

49.  Infiltration Indices Method(as already seen in chapter 4)