Lecture 3.2 Runoff PDF

Runoff  Runoff means the draining of flowing off of precipitation from a catchment area through a surface channel.  Runoff is the portion of rainfall which flows through the rives, streams, etc. Total Precipitation Precipitation...

Runoff  Runoff means the draining of flowing off of precipitation from a catchment area through a surface channel.  Runoff is the portion of rainfall which flows through the rives, streams, etc. Total Precipitation Precipitation Infiltration Abstractions Excess Surface Subsurface Deep Runoff Runoff Percolation Groundwater Prompt SSR Delayed SSR Ruoff Direct Runoff Base Flow Base Flow Total Runoff Types of Runoff  Surface runoff o Portion of rainfall (after all losses such as interception, infiltration, depression storage etc. are met) that enters streams immediately after occurring rainfall o After laps of few time, overland flow joins streams o Sometime termed prompt runoff (as very quickly enters streams)  Subsurface runoff o Amount of rainfall first enter into soil and then flows laterally towards stream without joining water table o Also take little time to reach stream  Base flow o Delayed flow  o Water that meets the groundwater table and join the stream or ocean o Very slow movement and take months or years to reach streams Factors affecting runoff  Climatic factors o Type of precipitation  Rain and snow fall o Rainfall intensity  High intensity rainfall causes more rainfall o Duration of rainfall  When duration increases, infiltration capacity decreases resulting more runoff o Rainfall distribution  Distribution of rainfall in a catchment may vary and runoff also vary  More rainfalls closer to the outlet, peak flow occurs quickly  Direction of prevailing wind o If the wind direction is towards the flow direction, peak flow will occur quickly  Other climatic factors o Temperature, wind velocity, relative humidity, annual rainfall etc. affect initial loss of precipitation and thereby affecting runoff  Physiographic factors o Physiographic characteristics of watershed and channel both o Size of watershed  Larger the watershed, longer time needed to deliver runoff to the outlet  Small watersheds dominated by overland flow and larger watersheds by runoff o Shape of watershed  Fan shaped, fan shaped (elongated) and broad shaped  Fan shaped –runoff from the nearest tributaries drained out before the floods of farthest tributaries. Peak runoff is less   Broad shaped –all tributaries contribute runoff almost at the same time so that peak flow is more o Orientation of watershed  Windward side of mountains get more rainfall than leeward side o Landuse  Forest –thick layer of organic matter and undercover –huge amounts absorbed to soil –less runoff and high resistance to flow  barren lands –high runoff o Soil moisture  Runoff generated depend on soil moisture –more moisture means less infiltration and more runoff  Dry soil –more water absorbed to soil and less runoff o Soil type  Light soil (sandy) –large pores and more infiltration  Heavy textured soils –less infiltration and more runoff o Topographic characteristics   Higher the slope, faster the runoff  Channel characters such as length, shape, slope, roughness, storage, density of channel influence runoff Natural Flow  Runoff representing the response of a catchment to precipitation reflects the integrated effects of a wide range of catchment, climate and rainfall characteristics.  True runoff is therefore streamflow in its natural condition ( without human interventio n) such stream flow unaffected by works of man, such as reservoir and diversion structures on a stream, is called natural flow or virgin flow. 𝑅𝑁 = (𝑅𝑜 − 𝑉𝑟 ) + 𝑉𝑑+ 𝐸 + 𝐸𝑥 + ∆𝑆 Where: 𝑅𝑁 = 𝑁𝑎𝑡𝑢𝑟𝑎𝑙 𝑓𝑙𝑜𝑤 𝑣𝑜𝑙𝑢𝑚𝑒 𝑖𝑛 𝑡𝑖𝑚𝑒 ∆𝑡 𝑅𝑜 = 𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 𝑓𝑙𝑜𝑤 𝑣𝑜𝑙𝑢𝑚𝑒 𝑖𝑛 𝑡𝑖𝑚𝑒 ∆𝑡 𝑎𝑡 𝑡ℎ𝑒 𝑡𝑒𝑟𝑚𝑖𝑛𝑎𝑙 𝑠𝑖𝑡𝑒 𝑉𝑟 = 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑟𝑒𝑡𝑢𝑟𝑛 𝑓𝑙𝑜𝑤 𝑓𝑟𝑜𝑚 𝑖𝑟𝑟𝑖𝑔𝑎𝑡𝑖𝑜𝑛, 𝑑𝑜𝑚𝑒𝑠𝑡𝑖𝑐 𝑤𝑎𝑡𝑒𝑟 𝑠𝑢𝑝𝑝𝑙𝑦 𝑎𝑛𝑑 𝑖𝑛𝑑𝑢𝑠𝑡𝑟𝑖𝑎𝑙 𝑢𝑠𝑒 𝑉𝑑 = 𝑣𝑜𝑙𝑢𝑚𝑒 𝑑𝑖𝑣𝑒𝑟𝑡𝑒𝑑 𝑜𝑢𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑡𝑟𝑒𝑎𝑚 𝑓𝑜𝑟 𝑖𝑟𝑟𝑖𝑔𝑎𝑡𝑖𝑜𝑛, 𝑑𝑜𝑛𝑒𝑠𝑡𝑖𝑐 𝑤𝑎𝑡𝑒𝑟 𝑠𝑢𝑝𝑝𝑙𝑦 𝑎𝑛𝑑 𝑖𝑛𝑑𝑢𝑠𝑡𝑟𝑖𝑎𝑙 𝑢𝑠𝑒 𝐸 = 𝑛𝑒𝑡 𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 𝑙𝑜𝑠𝑠𝑒𝑠 𝑓𝑟𝑜𝑚 𝑟𝑒𝑠𝑒𝑟𝑣𝑜𝑖𝑟𝑠 𝑜𝑛 𝑡ℎ𝑒 𝑠𝑡𝑟𝑒𝑎𝑚 𝐸𝑥 = 𝑛𝑒𝑡 𝑒𝑥𝑝𝑜𝑟𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑏𝑎𝑠𝑖𝑛 ∆𝑆 = 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑠𝑡𝑜𝑟𝑎𝑔𝑒 𝑣𝑜𝑙𝑢𝑚𝑒𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑠𝑡𝑜𝑟𝑎𝑔𝑒 𝑏𝑜𝑑𝑖𝑒𝑠 𝑜𝑛 𝑡ℎ𝑒 𝑠𝑡𝑟𝑒𝑎𝑚 Rational Method 𝑸 = 𝑪𝑰𝑨 Where: Q = the maximum rate of runoff  C = a runoff coefficient that is the ratio between the runoff volume from an area and the average rate of rainfall depth over a given duration for that year I = average intensity of rainfall A = area Runoff Coefficient Time of Concentration (Tc)  Time required to reach the surface runoff from remotest point of watershed to its outlet  At Tc all the parts of watershed contribute to the runoff at outlet  Have to compute the rainfall intensity for the duration equal to time of concentration  Several methods to calculate Tc 𝑇𝑐 = 0.02𝐿0.77 𝑆 −0.385 Where : Tc = time concentration L = Length of channel reach  S = Average channel slope  Computation of rainfall intensity for the duration of Tc 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 𝐷𝑒𝑝𝑡ℎ 𝑐𝑚 𝑜𝑟 𝑚𝑚 𝐼= = 𝑇𝑐 ℎ Runoff Volume Yield  The total quantity of surface water that can be expected in a given period from a stream at the outlet of its catchment is known as yield of the catchment in that period. 𝑌 = 𝑅𝑁 + 𝑉𝑟 = 𝑅𝑜 + 𝐴𝑏 + ∆𝑆 Where : RN = natural flow in time ∆𝑡 VR = volume of return flow from irrigation, domestic water supply and industrial use Ro = observed runoff volume at the terminal gauging station of the basin in time ∆𝑡 AV = abstraction from time ∆S = change in the storage volumes Rainfall runoff correlation 𝑹 = 𝒂𝑷 + 𝒃 𝑵(∑ 𝑷𝑹) − (∑ 𝑷)(∑ 𝑹) 𝒂= 𝟐 𝑵(∑ 𝑷𝟐 ) − (∑ 𝑷) (∑ 𝑷) − 𝒂(∑ 𝑷) 𝒃= 𝑵 Where: N = number of observation sets runoff (R) and rainfall (P) The coefficient of correlation r can be calculated as: 𝑵(∑ 𝑷𝑹) − (∑ 𝑷)(∑ 𝑹) 𝑟= √[𝑵(∑ 𝑷𝟐 ) − (∑ 𝑷)𝟐 ][𝑵(∑ 𝑹𝟐 ) − (∑ 𝑹)𝟐 ] Curve Number Method  Calculates runoff on the retention capacity of soil, which is predicted by wetness status (Antecedent Moisture Conditions [AMC]) and physical features of watershed  AMC - relative wetness or dryness of a watershed, preceding wetness conditions  This method assumes that initial losses are satisfied before runoff is generated (𝑷 − 𝟎. 𝟐𝑺)𝟐 𝑸= (𝑷 + 𝟎. 𝟖𝑺) 𝟐𝟓𝟒𝟎 𝑪𝑵 = (𝟐𝟓. 𝟒 + 𝑺) Where : Q = Direct Runoff P = Rainfall Depth S = Retention capacity of soil CN = Curve number  CN depends on land use pattern, soil conservation type, hydrologic condition, hydrologic soil group Procedure  Step 1 o Find value of CN using table o Calculate S using equation o Use equation and calculate Q (AMC II)  o Use correction factor if necessary to convert to other AMCs)  Three AMC Conditions Factors for converting AMC II to AMC I or AMR III  AMC I –Lowest runoff generating potential –dry soil  AMC II –Average moisture status  AMC III –Highest runoff generating potential –saturated soil  Soil A –low runoff generating potential, sand or gravel soils with high infiltration rates  Soil B –Moderate infiltration rate, moderately fine to moderately coarse particles  Soil C –Low infiltration rate, thin hard layer prevents downward water movement, moderately fine to fine particles  Soil D –High runoff potential due to very low infiltration rate, clay soils