Chapter 4 - Infiltration PDF

HYDROLOY(CEE335) CHAPTER 4 INFILTRATION 1 INFILTRATION Overview of Infiltration Process Variables affecting Infiltration Infiltration Models Measuring Infiltration 2 Rainfall: 3 Possibilities Evaporatio...

HYDROLOY(CEE335) CHAPTER 4 INFILTRATION 1 INFILTRATION Overview of Infiltration Process Variables affecting Infiltration Infiltration Models Measuring Infiltration 2 Rainfall: 3 Possibilities Evaporatio n Rainfall Surface Runoff Infiltration 3 ZONES OF SATURATION u Important processes – Infiltration creating soil moisture – Subsurface flow through soil – Groundwater flow u Saturation = % of void space occupied by water u Zone of aeration (pores contain water & air) u Soil water zone – Water moves down (up) during infiltration (evaporation) u Vadose zone – Water held in place by capillary forces – Saturation is at or near field capacity except during infiltration u Capillary zone – Completely saturated at base – Near field capacity aat the top – Water is pulled up from the water table by capillary forces 4  THE PROCESS OF WATER ENTRY INTO THE Infiltration rate: SOIL volume of water per unit area that enters the soil, per unit time (Flux) Infiltration capacity the maximum rate at which infiltration can occur under specific moisture conditions. 5  The infiltration rate/capacity Starts out at a maximum, then drops off and approaches a constant with time That water which cannot infiltrate is then free to runoff In this way the land surface partitions flow in two directions 6 Infiltration Motion of water is driven by gravity and capillary action What is capillary action? Water is attracted to soil surfaces That attraction causes water to move into the pores between soil particles The smaller the pore, the greater the attraction, and the greater the suction force Net movement of water into soil When rainfall rate (i) > infiltration rate (f) then water infiltrates surface soil at a rate that generally decreases with time Infiltration rate usually reaches a constant value 7 Infiltration (From: Hydrology and Floodplain Analysis, P.B. Bedient and W.C. Huber, 2nd. Ed., Addison-Wesley Publ. Co., © 1992) 8 Infiltration Alphalpha loamy sand Fuquay pebbly loamy sand Dothan loamy sand 9  Infiltration Rate Ksat = Saturated Hydraulic Conductivity (mm/hr) Infil. Rate, f (mm/hr) Ksat Time Hydraulic Conductivity : The property of a material that describes how easily water (or any fluid) can move through it 10 Infiltration Rate Saturated Hydraulic Conductivity is a constant value Unsaturated Hydraulic Conductivity varies with moisture content, causing heaps of computational problems (which we will return to) Infiltration rate also related to depth (or rather, volume) of infiltrated water Infil. Rate, f (mm/hr) Cumulative Infiltration, F (mm) 11 Time Many factors may affect the amount and the rate at which water infiltrates: Soil Type and Condition Saturation Level Texture 12 Spatial Variability of Infiltration Influenced by: Soil Properties Land Management Temperature Slope Vegetation 13 Spatial Variability of Infiltration Soil type Massive Blocky Soil structure Granular Platy 14  MEASURING INFILTRATION CAPACITY Double Ring Infiltrometer Proble ms entrapped air preferential flow around ring edge flooding can cause redistribution of fines, Minimizes lateral spreading need source of water to maintain constant head ensures downward vertical flow 15 Effect of Saturation Level Infil. Rate, f (mm/hr) Dry Soil Moist Soil Time 16 Effect of Texture Infil. Rate, f (mm/hr) Sandy Soil Clay Soil Time 17 Many factors may affect the amount and the rate at which water infiltrates: Soil Type and Condition Surface Condition Sealed vs. Soil Surface Crust vs. Open Structure Rainfall Intensities 18 Effect of Rainfall Intensity Infiltration will occur at the rainfall intensity as long as the rainfall intensity is less than the infiltration capacity 19 Effect of Rainfall Intensity Infil. Rate, Infiltration Capacity f (mm/hr) Time 20 Effect of Rainfall Intensity If rainfall intensity is greater than the infiltration capacity, then water enters the soil at the infiltration capacity 21 Effect of Rainfall Intensity Infil. Rate, Infiltration Capacity f (mm/hr) Time 22  As the rainfall continues…. Infil. Rate, Infiltration Capacity f (mm/hr) Time 23 Infiltration Rate vs Time ff INFILT RATIO oo N RATE (f) DEPT H/ TIME f c TIME 24 Effect of Storm Intensity and Duration on Runoff Storm Intensity Stor f m o Inten Storm runoff sity and Infilt HIGH INTENSITY ratio SHORT DURATION n Rate f c TIME 25 Effect of Storm Intensity and Duration on Runoff Stor F Low intensity m o Short duration inten Low runoff sity Storm intensity and Storm runoff Infilt ratio n rate F c Time 26 Effect of Storm Intensity and Duration on Runoff Stor F m o inten Low intensity sity Longer duration and Infilt ratio n Storm intensity Storm runoff rate F c Time 27 Effect of Storm Intensity and Duration on Runoff Stor F m o inten sity and Infilt Low intensity ratio No runoff n rate Storm intensity F c Time 28 Effect of Storm Intensity and Duration on Runoff Stor F m o inten sity and Infilt Low intensity ratio Longer duration n rate Storm intensity Storm runoff F c Time 29 Effect of Storm Intensity and Duration on Runoff Stor f m o Inten sity and Storm runoff Infilt ratio n Rate f c TIME 30 Effect of Storm Intensity and Duration on Runoff Stor f m o Inten sity and Storm runoff Infilt ratio n Rate f c TIME 31 Modelling Infiltration Models vary in sophistication from average rates for a different soils to the use of differential equations governing unsaturated flow in porous media Most efforts are of two approaches: Empirical equations based of observations Solution of equations based on mechanics 32 Horton’s Infiltration Model (1933) In this model, infiltration rate is assumed to start at some rate, fo, and then falls off exponentially to a constant rate, fc To calculate the infiltration rate at a certain time, t, then, 33 HORTON'S MODEL FOR INFILTRATION CAPACITY u f : infiltration capacity [in/hr] u fo : initial infiltration capacity [in/hr] u fc : ultimate infiltration capacity [in/hr] u k : a constant representing the rate of decrease in infiltration capacity [hr-1] Cumulative infiltration depth [in or cm]: 34  Horton’s Infiltration Equation When i > f, at all times f(t) = fc + (fo - fc)e-kt u The equation indicates that if the rainfall supply exceeds the infiltration capacity, infiltration tends to decrease in an exponential manner 35 Typical Values for the Horton Model Example 1 A soil has the following characteristics fo = 1.5 in/hr fc = 0.20 in/hr k = 0.35 hr -1 What are the values of f at t = 12 min, 30 min, 1 hr, 2 hr, and 6 hr? What is the total volume of infiltration over the six hour period? 37 WHAT DO WE NEED TO KNOW? Assume i > f Need equation to calculate the total volume 38 Example 1 - Infiltration Curve 39 40 Example The precipitation and infiltration rate are given in Table below noting that period j = 1 corresponds to a time t between 0 and 1 hours; period j =2 corresponds to t between 1 and 2 hours, etc. The infiltration rate for each period is measured in the middle of the time interval. Calculate 1) the cumulative precipitation depth for the storm 2) the cumulative infiltration for the storm and 3) the effective precipitation depth or overland flow depth 41 1) p (8)= ( 52 mm/hr+ 38 ,mm/hr + 45 mm/hr + 53 mm/hr + 55 mm/hr+ 20 mm/hr + 30 mm/hr ) x 1 hr = 335 mm 2) I (8) = (29 mm/hr + 18 mm/hr+ 12 mm/ hr + 10 mm/hr + 9 mm/hr + 8 mm/hr + 7.5 mm/hr + 7.5 mm/hr) x 1 hr = 101 mm 3) R (8) = P(8) – I(8) = 335 mm – 101 mm = 234 mm 42 INFILTRATION INDICES u The infiltration curve expresses the rate of infiltration as a function of time. u The area between the rainfall graph and the infiltration curve represent the rainfall excess, while the area under the curve gives the loss of rainfall due to infiltration. 43  Due to the different states of wetness of the soil after the commencement of the rainfall. There are two types of infiltration indices: - index - W index 44  INDEX Φ index is the average value of infiltration 45  INDEX u index: is defined as the average value of infiltration rate above which the rainfall volume equals the runoff volume. u The index is derived from the hyetograph with the knowledge of the runoff volume. u If the rainfall intensity (i) < index, then the infiltration is equal to the rainfall intensity.. u If the i > index, the difference between rainfall and infiltration in an interval of time represents the runoff volume. The amount of rainfall in excess of the index is called rainfall excess. 46 W-index w-index=(P-R-S)/tf where P=total storm precipitation (cm) R=total surface runoff (cm) S=depression and interception losses (cm) tf=time period (in hours) The w-index is more accurate than the - index because it excludes the interception and depression 47 -index method Definition: a constant infiltration capacity units of [in/hr] or [cm/hr] Rain intensity Infiltration rate 48 Example Use the rainfall data below to determine the - index for a watershed that is 0.875 mi2, where the runoff volume is 228.7 ac-ft. Time Intensit [hr] y [in/hr] 0-2 1.4 2-5 2.3 5-7 1.1 7-10 0.7 10-12 0.3 49  Solution runoff A = 0.875 sq mi 640 acres/sq mi = 560 acres 50  Area above -index must equal 4.9 inches. Note: Only positive terms are considered. Trial-and-error method: 51  Example: A storm with 10 cm precipitation produced a direct runoff of 5.8 cm. Given the time distribution of the storm as below, estimate the index of the storm. 52 Solution Amount of abstractions= 10- 5.8 = 4.2 cm Assume t = time of rainfall excess = 8 hr index = 4.2/8 = 0.525 cm/hr For the first and last hour the rainfall is ineffective 1, 8 < 0.525 Assume t = 6 hour Abstractions = 10 - 0.4 - 0.5 – 5.8 = 3.3 cm index = 3.3/6 = 0.55 53  Total rainfall excess = 5.8 cm (checked) Another index called W index which is also called the average value of the infiltration rate: W = (P – R – Ia )/ te Where P = total storm precipitation (cm) R = total storm runoff Ia = initial losses te = duration of the rainfall excess I.e. the total time in which the rainfall intensity is > W (in hours) W = average rate of infiltration 54  Example For a storm of 3-hr duration, the rainfall rates are as Time period (min) follows 30 30 30 30 30 30 Rainfall rate (cm/hr) 1.4 3.4 4.8 3.2 2.0 1.2 If the surface runoff is 3.4 cm, determine the -index and w- index. Solution Assume that -index is more than 1.4 cm/hr. Therefore, surface runoff R=[(3.4- )+(4.8- )+(3.2- )+(2.0- )]30/60 3.40=6.7-2 =1.65 cm/hr 55 As the computed value of -index is greater than 1.4 cm/hr, the assumption was correct. (Check the total amount of excess rainfall = 3.4cm) For W-index total precipitation P=(1.4+3.4+4.8+3.2+2.0+1.2)30/60 =8 cm W-index=(P-R-S)/tf, assume S = 0 =(8.0-3.4)/3.0 =1.53 cm/hr 56 Thank you 57