Learning Packet 4 Hydrology PDF

i 4 | HYDROLOGY PREFACE The main purpose of this module is to provide the student with a clear and detailed presentation of the theory and application of hydrology. The course deals with the hydrologic cycle and the different processes such as p...

i 4 | HYDROLOGY PREFACE The main purpose of this module is to provide the student with a clear and detailed presentation of the theory and application of hydrology. The course deals with the hydrologic cycle and the different processes such as precipitation, evaporation, infiltration, overland flow, groundwater flow and surface runoff generation. To achieve this objective despite of this pandemic due to COVID-19, this work has been shaped by the comments and suggestions of the peer reviewer in the teaching profession, as well as the other faculty members who will ensure quality of the modules that will be distributed to the LGU.  1 4 | HYDROLOGY UNIT 7: RAINFALL-RUNOFF RELATION 7.0 Intended Leaning Outcomes 1. Define and describe basic principles related to rainfall-run-off relation 2. Solve problems related to rainfall-run-off relation. 7.1 Introduction The chapter focuses in discussing topics related to rainfall run-off relation such as hydrograph, hydrograph Separation, unit hydrograph, and methods used to estimate run-off such as rational method and SCS Curve Method. 7.2 Topics 7.2.1 Definition of Hydrograph A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs). It can also refer to a graph showing the volume of water reaching a particular outfall, or location in a sewerage network. Graphs are commonly used in the design of sewerage, more specifically, the design of surface water sewerage systems and combined sewers. Components of Hydrograph Discharge: the rate of flow (volume per unit time) passing a specific location in a river, or other channel. The discharge is measured at a specific point in a river and is typically time variant. Rising limb: The rising limb of the hydrograph, also known as concentration curve, reflects a prolonged increase in discharge from a catchment area, typically in response to a rainfall event. Peak discharge: the highest point on the hydrograph when the rate of discharge is greatest.  2 4 | HYDROLOGY Recession (or falling) limb: The recession limb extends from the peak flow rate onward. The end of stormflow (a.k.a. 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. Lag time: the time interval from the maximum rainfall to the peak discharge. Time to peak: time interval from the start of rainfall to the peak discharge. Types of hydrographs include: Storm hydrographs Flood hydrographs Annual hydrographs a.k.a. regimes Direct Runoff Hydrograph Effective Runoff Hydrograph Raster Hydrograph Storage opportunities in the drainage network (e.g., lakes, reservoirs, wetlands, channel and bank storage capacity) 7.2.2 Hydrograph Separation For the derivation of unit hydrograph, the base flow has to be separated from the total runoff hydrograph (i.e., from the hydrograph of the gauged stream flow). Some of the well-known base flow separation procedures are given below, Fig. 5.6 (i) Simply by drawing a line AC tangential to both the limbs at their lower portion. This method is very simple but is approximate and can be used only for preliminary estimates. (ii) Extending the recession curve existing prior to the occurrence of the storm up to the point D directly under the peak of the hydrograph and then drawing a straight- line DE, where E is a point on the hydrograph N days after the peak, and N (in days) is given by 𝑵 = 𝟎. 𝟖𝟑𝑨𝟎.𝟐 where A = area of the drainage basin, km2 and the size of the areas of the drainage basin as a guide for the values of N are given below:  3 4 | HYDROLOGY Area of Drainage basin (km2) 250 1250 5000 12500 25,000 Time after peak, N (days) 2 3 4 5 6 (iii) Simply by drawing a straight-line AE, from the point of rise to the point E, on the hydrograph, N days after the peak. (iv) Construct a line AFG by projecting backwards the ground water recession curve after the storm, to a point F directly under the inflection point of the falling limb and sketch an arbitrary rising line from the point of rise of the hydrograph to connect with the projected base flow recession. This type of separation is preferred where the ground water storage is relatively large and reaches the stream fairly rapidly, as in lime-stone terrains.  4 4 | HYDROLOGY Many a time a straight- line AE meets the requirements for practical purposes. Location of the point E is where the slope of the recession curve changes abruptly, and as a rough guide E is N days after the peak. In all the above four 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. In Fig. 5.7, ADE is the assumed base flow. Actually, when the water level in the stream rises due to floods, the stream feeds the ground water and the permeable boundaries (called bank storage, Fig. 5.8) and this is termed as negative base flow. But actually, the variation in base flow may be much less than indicated in Fig. 5.7 depending on the permeability of the boundaries and the gradient of the phreatic surface. The resulting variation will be a slight dip from the extrapolated depletion curve, followed by a gradual rise to a higher level than the initial value as the flood recedes as shown in Fig. 5.9. 7.2.3 Unit Hydrograph and Estimation of Discharge Using Unit Hydrograph Definition: The amount of run off resulting to 1 unit (1cm, 1mm, 1ft., etc.) of rainfall excess. Is essentially used for determining runoff response to rainfall. Once you know your water shed response to one storm, you can predict what its response for another will look like.  5 4 | HYDROLOGY Basic Assumptions: 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 hydrograph due to an effective rainfall of unit duration is constant. The ordinates of DRH are directly proportional to the total amount of DR of each hydrograph. For a given basin, the runoff hydrograph due to a given period of rainfall reflects all the combined physical characteristics of basin (time-variant) Procedure for derivation of uh (unit hydrograph) from hydrograph TYPE I. “Derivation of unit hydrograph from a given DRH or flood hydrograph” Example: Time (hr.) 0 6 12 18 24 30 36 42 48 54 60 66 Observed hydrograph 100 100 300 700 1000 800 600 400 300 200 100 100 (cu.meter/sec) Solution: STEP 1: COMPUTE DRH DRH = ORDINATES OF FLOOD HYDROGRAPH – BASE FLOW STEP 2: COMPUTE 6HR-UH ORDINATES OF UH = ORDINATES OF DRH/EXCESS RAINFALL  6 4 | HYDROLOGY 1 2 3 4 TIME OBSERVED Direct run-off Ordinates of UH HYDROGRAPH hydrograph hour cu. Meter per sec (2) - (base flow) (3)/excess rainfall 0 100 0 0 6 100 0 0 12 300 200 50 18 700 600 150 24 1000 900 225 30 800 700 175 36 600 500 125 42 400 300 75 48 300 200 50 54 200 100 25 60 100 0 0 66 100 0 0 1200 1000 Blue line shows the flood hydrograph 800 Orange line Direct Runoff Hydrograph Gray line shows the Unit Hydrograph 600 400 200 0 0 10 20 30 40 50 60 70  7 4 | HYDROLOGY Type II. “Derivation of DRH or flood hydrograph from a given unit hydrograph.” Example: The Ordinates of A 3hr-UH is given below. Assume base flow is 15 cu. meter / sec. Derive the DRH for 2cm, 6 cm and 4 cm excess rainfall. TIME ORDINATES OF A TIME ORDINATES OF A 3-HR UH 3-HR UH hour cu. meter / second hour cu. meter / second 0 0 30 96 3 12 33 87 6 75 36 66 9 132 39 54 12 180 42 42 15 210 45 33 18 183 48 24 21 156 51 18 24 135 54 12 27 144 57 6 Solution: TIME ORDINATES OF DRH DUE TO DRH DUE TO DRH DUE TO 4 A 3 HR UH 2 CM ER 6 CM ER CM ER hour cu. meter / (2) * ER (3) * ER (4) * ER second 0 0 0 0 0 3 12 24 72 48 6 75 150 450 300 9 132 264 792 528 12 180 360 1080 720  8 4 | HYDROLOGY 15 210 420 1260 840 18 183 366 1098 732 21 156 312 936 624 24 135 270 810 540 27 144 288 864 576 30 96 192 576 384 33 87 174 522 348 36 66 132 396 264 39 54 108 324 216 42 42 84 252 168 45 33 66 198 132 48 24 48 144 96 51 18 36 108 72 54 12 24 72 48 57 6 12 36 24 60 6 12 36 24 63 0 0 0 0 1400 1200 ORDINATES OF A 3 HR UH cu. 1000 meter / second 800 DRH DUE TO 2 CM ER (2)*ER 600 DRH DUE TO 6 CM ER (2)*ER 400 DRH DUE TO 4 CM ER (2)*ER 200 0 0 10 20 30 40 50 60 70  9 4 | HYDROLOGY 7.2.4 Rational Formula The Rational Method is widely used to estimate the peak surface runoff rate for design of a variety of drainage structures, such as a length of storm sewer, a storm water inlet, or a storm water detention pond. The Rational Method is most suitable for small urban watersheds that don’t have storage such as ponds or swamps. It is best for areas less than 100 acres, but is sometimes used for up to 2 mi 2 areas. The Rational Method Equation The equation that is the centerpiece of the Rational Method is: 𝒒 = 𝑪𝒊𝑨 Where, q is the peak surface runoff rate in cfs, from a watershed of area A acres, and runoff coefficient, C, due to a storm of intensity, i in/hr. The units on peak runoff rate, q, are actually acre-in/hr., but the conversion from acre- in/hr. to cfs is very nearly one, so the more common unit, cfs, is typically used for q. In order to calculate a value for peak runoff rate for a given drainage area, values are needed for the three parameters, A, C, and i. Each of these parameters will be discussed separately in the next three sections. The Drainage Area, A The drainage area, A, is often determined from a map which includes the drainage area of interest. It may be necessary to first determine the boundaries of the drainage area using a contour map. Once the boundaries are known, the area can be determined using the map scale. Since the area must be in acres for use in the Rational Method equation, a useful conversion factor is 43,560 ft2/acre. A map with a drainage area outlined is shown at the left. The Runoff Coefficient, C The runoff coefficient is the fraction of rainfall striking the drainage area that becomes runoff from that drainage area. It is an empirically determined constant, dependent on the nature of the drainage area surface. An impervious surface like a concrete parking lot will have a runoff coefficient of nearly one. A very tight clay soil will also have a relatively high runoff coefficient, while a sandy soil would have more infiltration and a lower runoff coefficient. In addition to the nature of the surface and the soil, the slope of the drainage area has an effect on the runoff coefficient. A greater slope leads to a higher runoff coefficient. There are many tables of values for runoff coefficient for a variety of types of drainage areas in handbooks, textbooks and  10 4 | HYDROLOGY on the internet. The table at the right shows some typical ranges of values for runoff coefficient. The article, “Runoff Coefficients for Use in Rational Method Calculations.” more details on the runoff coefficient and a more comprehensive rational method runoff coefficient table of values. Click the link below: Rational Method Runoff Coefficient Tables for Storm Water Runoff Calculation - Bright Hub Engineering The Design Rainfall Intensity, i The design rainfall intensity is the intensity of a constant intensity design storm with the specified design return period and duration equal to the time of concentration of the drainage area. Once the design return period and duration are determined, the design rainfall intensity can be determined from an appropriate intensity-duration- frequency graph or equation for the location of the drainage area. As you can see, determining a value for i is the most complicated part of using the Rational Method. The procedure for doing so is covered in a separate article, “Calculating Design Rainfall Intensity for Use in the Rational Method.” Click the link below for further discussion: Calculate the Rainfall Intensity of the Design Storm for use in the Rational Method - Bright Hub Engineering Example Calculation Consider a section of a downtown business area of 35,400 square feet that drains to a particular storm water inlet. The runoff coefficient for this drainage area has been estimated to be 0.85. Based on a specified design return period and the time of concentration of the drainage area, the design storm intensity has been determined to be 5.1 in/hr. What is the peak runoff rate from this area to be used for design of the storm water inlet? Solution: 𝒒 = 𝑪𝒊𝑨 = (𝟎. 𝟖𝟓)(𝟓. 𝟏)(𝟑𝟓, 𝟒𝟎𝟎/𝟒𝟑, 𝟓𝟔𝟎) 𝒄𝒇𝒔 = 𝟑. 𝟓𝟐 𝒄𝒇𝒔. One use for the Rational Method is in calculating design storm water runoff rate for storm sewer design. For an article with a downloadable Excel spreadsheet template for making storm sewer design calculations, see Use of Excel Formulas (S.I or U.S. units) for Storm Sewer Design in a Water Drainage System - Bright Hub Engineering  11 4 | HYDROLOGY 7.2.5 SCS Curve Method (𝑃 − 0.2 𝑆)2 𝑄 = (𝑃 + 0.8 𝑆) 1000 𝑆 = − 10 (Q, P, S, in inches) CN 25400 𝑆 = − 254 (Q, P, S, in mm) CN Where: Q = runoff (in) P = rainfall (in) S = potential maximum retention after runoff begins (in) CN = Curve number – reflects soil and cover conditions (0-100) Figure. Graphical Solution of SCS Method  12 4 | HYDROLOGY Advantages of SCS Method It is a well-established reliable method. It is computationally efficient. The required input is generally available. Various soil types, land use and management practices can be conveniently handled. Runoff Curve Numbers CN is a runoff curve number and can be evaluated from Tables developed by USSCS. Notes (1) Hydraulic condition is based on combination factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c)  13 4 | HYDROLOGY amount of grass or close-seeded legumes, (d) percent of residue on the land surface (good>=20%), and (e) degree of surface roughness. (2) Good: Woods are protected form, grazing, and litter and brush adequately cover the soil. (3) Good: >75% ground cover and lightly or only occasionally grazed. (4) CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type. The curve number is a dimensionless number defined such that 0  CN  100. For impervious and water surfaces CN = 100; for natural surfaces CN < 100. The curve number is a function of the following runoff-producing catchment properties: 1. Hydrologic soil group All soils are classified into four hydrologic soil groups of distinct runoff- producing properties. These groups are labeled A, B, C and D. Following is the brief of their runoff and infiltration properties: A Lowest runoff potential (Greater than 0.30 in/hr) B Moderately low runoff potential (0.15 – 0.30 in/hr) C Moderately high runoff potential (0.05 – 0.15 in/hr) D Highest runoff potential (0 – 0.05 in/hr) 2. Land Use and Treatment The effect of the surface conditions of a watershed is evaluated by means of land use and treatment classes. Land use belongs to watershed cover, including every kind of vegetation, fallow (bare soil), as well as nonagricultural uses such as water surfaces (lakes, swamps), impervious surfaces (roads, roof, and the like), and urban areas. Land treatment applies mainly to agricultural land uses, and it includes mechanical practices such as contouring or terracing and management practices such as grazing control and crop rotation. A class of land use/treatment is a combination often found in a literature. 3. Ground surface (Hydrologic) condition Hydrologic condition is based on combination of factors that affect infiltration and runoff, including: (a) density and canopy of vegetative areas,  14 4 | HYDROLOGY (b) amount of year-round cover, (c) amount of grass or close-seed legumes in rotations, (d) percent of residue cover on the land surface (e) degree of roughness Poor: Factors impair infiltration and tend to increase runoff Good: Factors encourage average and better than average infiltration and tend to decrease runoff. 4. Antecedent Moisture Conditions (AMC) Antecedent Moisture Conditions are grouped into three categories: AMC I - Low moisture AMC II - Average moisture condition, normally used for annual flood estimates AMC III - High moisture, heavy rainfall over preceding few days 4.2 CN(II) 23 CN(II) CN(𝐼 ) = CN(III) = 10 − 0.058 CN(II) 10 + 0.13 CN(II) Example 1: Determine the weighted curve for a watershed with 40% residential (1/4-acre lots), 25 % open space, good condition, 20% commercial and business (85% impervious), and 15% industrial (72% impervious), with corresponding soil groups of C, D, C, and D. Solution: Solution The corresponding curve numbers are obtained from SCS Tables. Land Use Soil Group Curve Number 40 % C 83 25 % D 80 20 % C 94 15 % D 93  15 4 | HYDROLOGY The weighted curve number is 𝐶𝑁 = 0.40(83) + 0.25(80) + 0.20 (94) + 0.15 (93) 𝐶𝑁 = 33.2 + 20 + 18.8 + 13.95 = 85.95 = 𝟖𝟔 Example 2: The watershed in example 1 experienced a rainfall of 6 in, what is the runoff volume? Solution CN = 86 P = 6 inches S = 1000/ CN - 10 = 1.63 (𝑷 − 0.2 𝑺)𝟐 𝑸 = = 𝟒. 𝟒𝟏 𝒊𝒏𝒄𝒉𝒆𝒔 (𝑷 + 0.8 𝑺)  16 4 | HYDROLOGY 7.3 Assessment Name: _______________________________________ Year & Section: ________ Instructor: ___________________________________ Date: ___ / ____ / _______ Test I. Explain the following items given below into at least 2-3 sentences. Criteria for evaluation: Originality of the answer - 80% Organization - 10% Cleanliness - 10% 100% 1. Identify the components of the hydrograph and explain each. (2 points each) 2. Identify at least three (3) different types of hydrograph and describe each. (2 points each) 3. Explain what is hydrograph separation. (3 points) 4. Explain unit hydrograph. (3 points) Test II. Answer the following problems given below. Avoid erasures. Criteria for evaluation: Correct Answer w/solution - 80% Organization - 10% Cleanliness - 10% 100% 1. Solve for the Unit Hydrograph from the given data. ( 10 points) Time (hr.) 0 8 15 19 26 31 38 44 49 55 60 66  17 4 | HYDROLOGY Observed 100 100 200 800 1100 700 600 500 400 300 100 100 hydrograph (cu.meter/sec) 2. Consider a section of a downtown business area of 35,000 square feet that drains to a particular storm water inlet. The runoff coefficient for this drainage area has been estimated to be 0.85. Based on a specified design return period and the time of concentration of the drainage area, the design storm intensity has been determined to be 4.1 in/hr. What is the peak runoff rate from this area to be used for design of the storm water inlet? (5 points) 3. Determine the weighted curve for a watershed with 35% residential (1/4-acre lots), 20 % open space, good condition, 25% commercial and business (85% impervious), and 20% industrial (72% impervious), with corresponding soil groups of A, C, B, and D. The watershed experienced a rainfall of 5 in, what is the runoff volume? (5 points)  18 4 | HYDROLOGY 7.4 References: K Subramanya (2008). Engineering Hydrology, Third Edition [E-book]. Tata McGraw-Hill Publishing Company Limited. https://kupdf.net/download/k-subrahmanya- engineering hydrology_5af43798e2b6f5d42e25cdd9_pdf H.M. Raghunath (2006). Hydrology Principles, Analysis and Design, Revised Second Edition [E-book]. New Age International (P) Ltd., Publishers.https://kupdf.net/download/hydrology-principles-analysis- design_59f21ecce2b6f5c576a9756f_pdf Hydrograph. Wikipedia, The Free Encyclopedia. Hydrograph - Wikipedia The Rational Method for Estimation of Design Surface Runoff Rate for Storm Water Control. Bright Hub Engineering. The Rational Method for Estimation of Design Surface Runoff Rate for Storm Water Control - Bright Hub Engineering 1-6-infiltration-2. SCS Curve Number Method - Altaf Rehman altafrehman.weebly.com › uploads › 1-6-infiltration-2 (google.com.ph) SCS Curve Number Method. https://engineering.purdue.edu/mapserve/LTHIA7/documentation/scs.htm 7.5 Acknowledgement The images, tables, figures and information contained in this module were taken from the references cited above.