4 runoff and floods

Introduction, Factors Affecting Runoff, Hydrograph, Baseflow, Techniques to determine runoff

1. Unit IV RUNOFF 1 Prof. Pradeep T. Kumawat BE Civil, ME Geo-Tech. (Assistant Professor) Late G. N. Sapkal College of Engineering, Nashik , Maharashtra, India .

2. Objective Definition of runoff Runoff process Surface runoff Classification of runoff Factors affecting runoff Methods of Estimating runoff Summary of Rainfall-Runoff process Base Flow Seperation Unit Hydrograph

3. Introduction: Stream flow representing the runoff phase of the hydrological cycle is the most important basic data for hydrologic studies. Its occurrence and quantity are dependent on the characteristics of the rainfall event, i.e. intensity, duration and distribution . Runoff can be defined as the portion of the precipitation that makes it’s way towards rivers or oceans etc., as surface or subsurface flow. Surface runoff can be generated either by rainfall, snowfall, rainstorms or by the melting of snow, or glaciers. 3

4. 4 Runoff is that portion of the rainfall or irrigation water which leaves a field either as surface or as subsurface flow. When rainfall intensity reaching the soil surface is less than the infiltration capacity, all the water is absorbed in to the soil. As rain continues, soil becomes saturated and infiltration capacity is reduced, shallow depression begins to fill with water, then the overland flow starts as runoff.

5. 5 Surface detention / Detention storage: The amount of water on the land surface that can form a film of water or in transit towards stream channels is called detention storage or surface detention . Surface Flow: Surface flow is water that has remained on the surface and moves as overland or channel flow. The surface runoff process: As the rain continues, water reaching the ground surface infiltrates into the soil until it reaches a stage where the rate of rainfall (intensity) exceeds the infiltration capacity of the soil. Thereafter, surface puddles, ditches, and other depressions are filled with water (depression storage) and after that overland flow as runoff is generated.

6. 6 Flooding: Flooding occurs when a watercourse is unable to convey the quantity of runoff flowing downstream. Floods can be both beneficial to societies or cause damage . Importance of Runoff: water balance calculation Irrigation scheduling The magnitude of flood flows to enable safe disposal of the excess flow. The minimum flow and quantity of flow available at various seasons . The interaction of the flood wave and hydraulic structures, such as levees, reservoirs , barrages and bridges.

7. Types of  Runoff Types of Runoff : There are three major types of runoff depending on the source: Surface runoff Sub-surface runoff or Interflow Base flow

8. 8 Surface Runoff: That portion of rainfall which enters the stream immediately after the rainfall. It occurs when all loses is satisfied and rainfall is still continued and rate of rainfall [intensity] in greater than infiltration rate . Sub-Surface Runoff: That part of rainfall which first leaches into the soil and moves laterally without joining the water table , to the stream, rivers or ocean is known as sub-surface R unoff. It is usually referred is inter-flow.

9. 9 c . Base flow: It is delayed flow defined as that part of rainfall, which after falling on the ground the surface , infiltrated into the soil and meets to the water table and flow the streams, ocean etc. The movement of water in this is very slow. Therefore , it is also referred a delayed runoff . Total runoff = Surface runoff + GW Base flow.

10. 10 Factors Affecting the Runoff: Runoff rate and volume from an catchment area or drainage basin are mainly influenced by following factors: Precipitation characteristics Shape, size & location of the catchment Characteristics catchment surface Topography Geological characteristics Meteorological characteristics Storage characteristics.

11. 11 1. Precipitation characteristics: A precipitation which occurs in the form of rainfall starts immediately as surface runoff depending upon rainfall intensity while precipitation in the form of snow does not result in surface runoff . If the rainfall intensity is greater than infiltration rate of soil then runoff starts immediately after rainfall. While in case of low rainfall intensity runoff starts later. Thus high intensities of rainfall yield higher runoff. It has great effect on the runoff . Duration of Rainfall: It is directly related to the volume of runoff because infiltration rate of soil decreases with duration of rainfall. Therefore, medium intensity rainfall even results in considerable amount of runoff if duration is longer .

12. 12 Rainfall Distribution: Runoff from a watershed depends very much on the distribution of rainfall. It is also expressed as “distribution coefficient ”. Near the outlet of watershed, runoff will be more . Direction of Prevailing Wind: If the direction of prevailing wind is same as drainage system, it results in peak low. A storm moving in the direction of stream slope produce a higher peak in shorter period of time than a storm moving in opposite direction .

13. 13 2 . Shape, size & location of the catchment: Size of Watershed: A large watershed takes longer time for draining the runoff to outlet than smaller watershed and vice-versa . Shape of Watershed: Runoff is greatly affected by shape of watershed. Shape of watershed is generally expressed by the term “form factor” and “compactness coefficient ”. Form Factor = Ratio of average width to axial length of watershed. Compactness Coefficient: Ratio off perimeter of watershed to circumference of circle whose area is equal to area of watershed .

14. 14 Two types of shape: A) Fan shape [tends to produce higher runoff very early ] B) Fern shape [tend to produced less runoff ] Slope of Watershed: It has complex effect. It controls the time of overland flow and time of concentration of rainfall. E.g. sloppy watershed results in greater runoff due to greater runoff velocity and vice-versa . Orientation of Watershed: This affects the evaporation and transpiration losses from the area . The north or south orientation, affects the time of melting of collected snow.

15. A ] Fan shaped catchment : All the tributaries are approximately of the same size . Gives greater runoff because the peak flood from the tributaries is likely to reach the main stream approximately at the same time.

16. B ] Fern leaf catchment : The tributaries are generally of different lengths and meet the main stream at the regular intervals. Such a narrow catchments the peak flood intensity is reduced since discharges are likely to be distributed over a long period of time.

17. 17 3. Characteristics catchment surface: Land Use: Land use and land management practices have great effect on the runoff yield. E.g. an area with forest cover or thick layer of mulch of leaves and grasses contribute less runoff because water is absorbed more into soil . The runoff also depends upon surface condition of the catchment which may be cultivated or natural. Soil moisture: Magnitude of runoff yield depends upon the initial moisture present in soil at the time of rainfall. If the rain occurs after along dry spell then infiltration rate is more, hence it contributes less runoff .

18. 18 4. Topographic characteristics: It includes those topographic features which affects the runoff . E.g. inclination or slope of catchment also upon whether the catchment area is smooth or rugged terrain. Undulating land has greater runoff than flat land . Drainage Density: It is defined as the ratio of the total channel length [L ] in the watershed to total watershed area [A]. Greater drainage density gives more runoff. Drainage density = L/A

19. 5. Geological characteristics: Soil type: In filtration rate vary with type of soil. So runoff is great affected by soil type. It is one of the important factor. It includes the type of surface soil, subsoil, type of rock and their permeability characteristics . If the soil & subsoil is porous, seepage will be more, resulting in reduction of runoff or peak flood. If the surface is rocky or impermeable then absorption will be nil which resulting more runoff. If rocks have fissures, are porous in nature, have lava funnels water will be lost resulting less runoff .

20. 6. Meteorological characteristics Runoff may also be affected by temperature, wind speed and humidity. If temperature is low and ground is saturated then runoff will be greater. If temperature is high and greater wind velocity give rise to greater evaporation loss and resulting in less runoff. Other factors such as temperature wind velocity, relative humidity, annual rainfall etc. affect the water losses from watershed area.

21. 21 7 . Storage Characteristics: Depressions in Ponds , lakes and pools Capacity of the reservoir Stream or Channels Check dams in gullies Flood moderation Upstream reservoirs or tanks. Ground water storage in deposits/aquifers . The artificial storage such as dams, weirs etc. and natural storage such as lakes, ponds etc. tend to reduce the peak flow .

22. 22 Measurement of Runoff : River discharge, the volume flow rate through a river cross section, is perhaps the most important single hydrologic quantity. Measurements of river discharge are required for flood hazard management, water resource planning , climate and ecology studies and compliance with transboundary water agreements . The discharge (or stream flow ) of a river relates to the volume of water flowing through a single point within a channel at a given time . Understanding this information is essential for many important uses across a broad range of scales , including global water balances, engineering design, flood forecasting , reservoir operations, navigation, water supply , recreation and environmental management .

23. METHODS OF ESTIMATING RUNOFF Stream flow measurement techniques can be broadly classified into two categories as: 1. Direct determination 2. Indirect determination (Notes: Direct measurement of runoff is already explained in unit 1 stream gauging.)

24. 24 Direct determination of stream discharge: Area- Velocity Method Dilution techniques Moving Boat Method Electromagnetic method Ultrasonic method . Indirect determination of stream flow: Slope-area method Empirical Formula Infiltration Method Rational Method Unit Hydrograph Method Hydraulic structures, such as weirs, flumes and gated structures .

25. By Empirical Formulae, tables In the past, many empirical formulae have been developed, but these are applicable only to the region where they were derived.  Further more , attention must be given in their application if the characteristics of the region have been subjected to manmade disturbance (e.g., settlement , construction activity, land use in irrigation). These are essentially rainfall-runoff relationships with additional third or fourth parameters to account for climatic or catchment characteristics. Some of the important empirical runoff estimation formulae used in various parts of The India are given below : (Notes: Slope area method and hydraulic structure's method of Indirect measurement of runoff is already explained in unit 1 stream gauging.)

26. By Empirical Formulae, tables 1. Binnie’s Percentage Method: Sir alexander Binnie (1869) after carrying out experiments on the river in the M.P. has established following relation. Annual rainfall in mm 500 600 700 800 900 1000 1100 Runoff in percentage 15 21 25 29 34 38 40

27. By Empirical Formulae, tables 2. Runoff Coefficient: Runoff is a function of rainfall as a equation in the form R = kP Where, K is a constant and depends upon the type of surface. Sr. No. Types of surface Value of constant (k) 1 Urban residential 0.2 to 0.3 2 Commercial & Industrial 0.9 3 Parks, Farm etc. 0.05 to 0.30 4 Concrete or asphalt pavement 0.85 to 1.00

28. By Empirical Formulae, tables 3. Barlow’s Table: T. G. Barlow the first Chief Engineer of the Hydro-Electric Survey of India (1915 ) after carrying out experiment on catchment below 130 square km area in U.P. Class Description of catchment area % of runoff A Flat, cultivated & B. C. Soil 10 B Flat partly cultivated soil 15 C Average type 20 D Hills & planes with little cultivation 35 E Very hilly, steep with no cultivation 45

29. By Empirical Formulae, tables 4. Strange’s Tables: W. L. Strange (1892) after carrying out experiment in Maharashtra has established ratios between rainfall and runoff.

30. By Empirical Formulae, tables 5 . Inglis and DeSouza formula: As a result of careful stream gauging in 53 sites in Western India, Inglis and DeSouza (1929 ), they recommended the two regional formulae. For plain areas: R = (P/254) × (P-17.8) For ghat areas: R = 0.85 P – 30.5 Where, R and P represent average annual runoff and rainfall in mm

31. By Empirical Formulae, tables 6. Laceys formula: According to Lacey, Sr. No. Type of monsoon Recommended value of (F/S) of catchment A B C D E 1 Very short 2 0.83 0.5 0.23 0.14 2 Standard length 4 1.67 1 0.58 0.28 3 Very long 6 2.50 1.5 0.88 0.43

32. By Empirical Formulae, tables 7. A. N. Khosala’s Formulae: A. N. Khosala (1960) analyzed the rainfall, runoff and temperature data for various catchments in India and USA to arrive at an empirical relationship between runoff and rainfall. The time period is taken as a month. His relationship for monthly runoff is: R = P (T/2.08) Where, R & P are in cm and T is in °C

33. By Infiltration Method Surface runoff = R ainfall - infiltration

34. Infiltration Indices P - R - I a W index = -------------------- t r Where, P = Total Precipitation in cm R = Total storm runoff in cm Ia = Initial loss Tr = Rainfall duration in hrs

35. By Rational Methods Consider a rainfall of uniform intensity and very long duration occurring over a basin. In this method, runoff and rainfall are correlated by following equation: Q = C · i · A Where, Q = Flood flow in cubic meters per second A = Drainage area that contributes to run off ( km 2 ) i = Intensity of rainfall in cm per hour C = Coefficient of runoff depend upon catchment characteristics.

36. Unit Hydrograph Method Hydrographs: A hydrograph is a graph displaying some property of water flow, such as stage (i.e. water level), discharge, velocity, etc., versus time . For displaying runoff characteristics of a watershed, the hydrograph is one of discharge (cubic meter per second) versus time (hours). It represents watershed runoff at a certain point in the flow and includes only the rainfall upstream of the point in question . After determining the infiltration index and unit hydrograph from the rainfall runoff observation, the flood hydrograph for a given rainfall excess can be calculated.

37. HYDROGRAPH There are three basic parts to the hydrograph: the rising limb or concentration curve the crest segment, and the recession curve or falling limb or depletion curve Such hydrographs are commonly used in the design of sewerage, more specifically, the design of surface water sewerage systems and combined sewers .

38. 38

39. HYDROGRAPH Components of hydrograph: Rising limb: The rising limb of hydro graph, also known as concentration curve, reflects a prolonged increase in discharge from a catchment area, typically in response to a rainfall event Recession (or falling) limb: The recession limb extends from the peak flow rate onward. The end of storm flow (aka quick flow or direct runoff) and the return to groundwater-derived flow (base flow) is often taken as the point of inflection of the recession limb. The recession limb represents the withdrawal of water from the storage built up in the basin during the earlier phases of the hydrograph .

40. HYDROGRAPH Peak discharge: The highest point on the hydro graph when the rate of discharge is greatest. Lag time: The time interval from the center of mass of rainfall excess to the peak of the resulting hydrograph. Time to peak: The time interval from the start of the resulting hydro graph. Discharge: The rate of flow (volume per unit time) passing a specific location in a river or other channel.

41. Unit Hydrograph Method The concept of unit hydrograph first suggested by Sherman in 1932 is of immense use in the prediction and estimation on of the flood hydrographs of known rain storm from a catchment. A unit hydrograph of a catchment is defined as a hydrograph of direct run off (i.e. the total runoff minus base flow) resulting from one centimeter of effective rainfall of a specified interval occurring uniformly over the entire catchment area at a uniform rate .

42. Unit Hydrograph Method The basic assumption of the theory of Unit hydrographs (U.H.G.) states that if two identical storms occur over a catchment with exactly identical condition prior to the rain, the hydrographs of runoff resulting from these two storms would be expected to be the same. The important characteristic of unit hydrograph is its specified duration i.e. a 6 hours unit hydrograph implies a unit hydrograph resulting from a rainfall of 6 hours duration i.e. it is a hydrograph obtained by surface runoff from a storm of 6 hours duration that results in a rainfall excess of one centimeter depth.

43. Unit Hydrograph Method Utility of unit hydrograph: It e nables to estimate the maximum flood discharge of a given stream or channel. It assists in the preparation of a flood hydrograph for any anticipated rainfall in the catchment. The Unit Hydrograph (UH) technique is widely used for runoff estimation , especially for determining peak discharges.

44. Unit Hydrograph Method Basic Assumption Of Unit Hydrograph: The effective rainfall is uniformly distributed within its duration. The effective rainfall is uniformly distributed over the whole drainage basin. The base duration of direct runoff hydrograph due to an effective rainfall of unit duration is constant. The ordinates of DRH are directly proportional to the total amount of Direct Runoff of each hydrograph (i.e. linear). For a given basin, the runoff hydrograph due to a given period of rainfall reflects all the combined physical characteristics of basin (time-invariant ).

45. Unit Hydrograph Method Construction of a Unit Hydrograph: 1. From the past rainfall records select an isolated intense storm-rainfall of specific or unit duration. For this storm using isohyetal or Thiessen polygon method calculate average depth of precipitation over the drainage basin. 2. Using (SRRG) self recording rain gauge data of all the available stations plot mass curves of rainfall for this storm and obtain, average mass curve of rainfall. From the average mass curve of rainfall construct hyetograph. 3. To construct hyetograph incremental rainfall quantities during successive units of time are obtained from the mass curve. The average depths of rainfall per unit of time are then plotted on ordinate against time as abscissa. 4. Using stage hydrograph and stage-discharge relationship obtain a complete discharge hydrograph at the drainage outlet for the selected storm. 5. If the recession limb is not smooth and contains bumps, make recession limb smooth or normal by removing the bumps. 6. Separate the base flow from total storm hydrograph using suitable empirical method and Subtracting the base flow components plot and obtain ordinates of direct runoff hydrograph .

46. Unit Hydrograph Method 7 . Either by planimeter or by mathematical calculations find out the area of the catchment or drainage basin. 8. Either by planimeter or by mathematical calculations find out the volume of the direct runoff. Volume of direct runoff = Area within the hydrograph = ∑ Ordinates × t × (60 × 60) = ∑0 × t Where, ∑ Ordinates = Sum of the ordinates of direct runoff hydrograph at equal time interval t = Time interval between successive ordinates To get volume of runoff in m3, ordinates have to be converted in cumec and time interval in seconds .

47. Unit Hydrograph Method This volume can be converted into cm of runoff by dividing the same by the area of the drainage basin sq. m  x 100. 9 . The duration of effective rainfall of the storm of specific or unit duration is determined by drawing a horizontal line on the hyetograph in such a way that the area of the hyetograph above the horizontal line is equal to the volume of direct runoff. Obviously the area below the horizontal line gives the abstractions. This is an arbitrary method. For elaborate analysis infiltration indexes and curves will have to be used. 10. Measure the ordinates of direct runoff hydrograph. Divide these ordinates of direct runoff hydrograph by the obtained depth of runoff in cm to get ordinate of unit hydrograph . Mathematically, Ordinate of direct runoff hydrograph = Ordinate of direct runoff hydrograph/Depth of direct runoff in cm 11. Plot these ordinates against uniform and the same time interval as the one used in direct runoff hydrograph to get unit hydrograph.

48. Factors Affecting Shape of hydrograph The Shape of the hydrograph is influenced by various factors such as: A circular shaped drainage basin leads to rapid drainage whereas a long drainage basin will take time for the water to reach the river. Topography & relief (e.g. slope, inclination , hilly, Rock type) Initial losses such as interception, infiltration, soil moisture etc. Variation in the direction and Heavy Storms Duration and intensity rainfall Duration and intensity Snowfall Vegetation cover

49. METHODS OF BASE FLOW SEPARATION The surface-flow hydrograph is obtained from the total storm hydrograph by separating the quick-response flow from the slow response runoff. It is usual to consider the interflow as a part of the surface flow in view of its quick response. Thus only the base flow is to be deducted from the total storm hydrograph to obtain the surface flow hydrograph. There are three methods of base-flow separation that are in common use. By straight Line Method By Extension of base curve By Backward Extension of base flow

50. METHODS OF BASE FLOW SEPARATION 1. By straight Line Method: In this method the separation of the base flow is achieved by joining with a straight line the beginning of the surface runoff to a point on the recession limb representing the end of the direct runoff .

51. METHODS OF BASE FLOW SEPARATION 2. By Extension of Base C urve: In this method the base flow curve existing prior to the commencement of the surface runoff is extended till it intersects the ordinate drawn at the peak (point C in Fig. 2). This point is joined to point B by a straight line. Segment AC and CB demarcate the base flow and surface runoff. This is probably the most widely used base-flow separation procedure.

52. METHODS OF BASE FLOW SEPARATION 3. By Backward Extension of base flow: In this method the base flow recession curve after the depletion of the flood water is extended backwards till it intersects the ordinate at the point of inflection (line EF in Fig. 23.3). Points A and F are joined by an arbitrary smooth curve. This method of base-flow separation is realistic in situations where the groundwater contributions are significant and reach the stream quickly.

53. DIRECT RUNOFF HYDROGRAPH The flood hydrograph obtained after the separation of base flow is called as direct runoff hydrograph.

54. EFFECTIVE RAINFALL Effective rainfall (also known as Excess rainfall) (ER) is that part of the rainfall that becomes direct runoff at the outlet of the watershed. It is thus the total rainfall in a given duration from which abstractions such as infiltration and initial losses are subtracted . For purposes of correlating DRH with the rainfall which produced the flow, the hyetograph of the rainfall is also pruned by deducting the losses. The initial loss and infiltration losses are subtracted from it. The resulting hyetograph is known as effective rainfall hyetograph (ERH). It is also known as excess rainfall hyetograph.

55. Part of the rain water is lost through deep percolation and run off

56. 56 Determination of runoff coefficients: The runoff coefficient from an individual rainstorm is defined as runoff divided by the corresponding rainfall both expressed as depth over catchment area (mm ): Run-off coefficients: The percentage of rainfall that appears as storm water run-off from a surface is called the run-off coefficient. The run-off coefficient of roofed areas (Cr) is 1.0. The run-off coefficient of paved areas ( Ci ) is 0.9. Depending on the soil type and rainfall intensity the run-off coefficient from pervious areas ( Cp ) could be as low as no run-off at all (low rainfall intensity, sandy soil) or up to 80% (high rainfall, heavy clay soil ).

57. 57 You need to know the run-off coefficient to size the storm water drainage system on the site. Effects of surface runoff: Erosion and deposition: Surface runoff can cause erosion of the Earth's surface; eroded material may be deposited a considerable distance away. Environmental effects : The principal environmental issues associated with runoff are the impacts to surface water, groundwater and soil through transport of water pollutants to these systems . Agricultural issues: The transport of agricultural chemicals (nitrates, phosphates, pesticides, herbicides etc.) via surface runoff. The resulting contaminated runoff represents not only a waste of agricultural chemicals, but also an environmental threat to downstream ecosystems .