Mineral. The 100-year design storm covers the entire basin with uniform constant rainfall intensity until the design dis-charge at the crossing is achieved. 20) (2. Q = Discharge or flow rate [L 3 /T]. The use of this method to compute 𝑄𝑄𝑄𝑄 requires three parameters: 𝑡𝑡𝑡𝑡, 𝑖𝑖𝑡𝑡𝑡𝑡. (1b) and can be used to estimate the volume flow and time used to drain a container or tank through an aperture. 2ft/sec 2 ) d = Depth of water over grate (in feet) Each subcatchment uses a Curve Number (CN) to characterize the runoff properties for a particular soil and ground cover. Multiply the result with 2. 849 for metric or 1. By considering flow rates, Manning's roughness coefficient, hydraulic radius, and slope, engineers can calculate the appropriate cross-sectional area for the trench drain. 18, if the water table declined 1 meter over a 1 square meter area, then 0. Soil Type The drainage coefficient is a key parameter used to design agricultural drainage systems. Distance between drains 2 days ago · First use the Hazen-Williams equation to find the velocity of the fluid: v = k × C × R 0. (a) Drainage path length, d = 5. /hr) for a storm with a duration equal to the time of concentration of Rational Equation Calculator Units: ft 3 =cubic foot, m 3 =cubic meter, mm=millimeter, s=second. You will also learn about the most C = runoff coefficient (percent of rainfall that becomes runoff) I = uniform rate of rainfall intensity (inches/hour) A = drainage area (in acres) Assumptions: 1. 8 to 2 mm) thick in the V. The Rational Method equation is: Q = C x I x A where: Q = Storm Water Runoff (in cubic feet per second) C = Coefficient of Runoff I = Rainfall Intensity (in inches per hour) Dec 2, 2015 · There are several method that can be used to calculate drainage, but the rational method is probably the easiest and most widely used method. 3 and 1. DRAIN D500S In order to determine the carrying capacity of the proposed Drain, the Manning’s Formula has been used. A well known steady-state drainage equation is the Hooghoudt drain spacing equation. Spacing drains closer together results in a higher drainage coefficient and faster drainage. The input information needed is: i) the drainage area, A in acres; ii) the runoff coefficient of the drainage area, C; iii) the design return period in years; and iv) the design storm duration, d, in minutes. 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. 2. Smooth Interior Pipe? Yes No. , 150 mm (6 inches) of HMA) using a layer coefficient (a) that represents the relative strength of the construction materials in that layer. 2ft/sec 2 ) d = Depth of water over grate (in feet) Using an Orifice coefficient of 0. 848. Land use - present and predicted future - as it affects degree of protection to be provided and percentage of imperviousness. If the drainage coefficient is 3 cm, the rate of flow through the drain (Q) will be: 5. 80, depending on the inlet type and configuration. Jun 1, 2022 · The runoff coefficient (C) related to a rainfall event was calculated by verifying the total volume of runoff, separated from the baseflow with the use of the digital filter, and the total precipitation in the catchment, according to the relation below: (2) C = V r V p Where, C – runoff coefficient; Vr –runoff volume; Vp – precipitation When using metric, the value of k = 1; when using English units, k = 1. Procedure for using the Rational Method. r include topography or structures tha. 1 Runoff Coefficient The runoff coefficient, C, in Equation 3-1 is a function of the ground cover and a host of other C = Orifice coefficient A = Open area of grate (ft 2 ) g = Acceleration due to gravity (32. 2: Select design storm rainfall intensity from an IDF curve given a design frequency and using the computed value for time of concentration as the duration. Example: An engineer wants to compute the weighted C value for the composite drainage area shown below. 𝑄𝑄, and C . Because of these inherent assumptions, the Rational formula should only be applied to drainage areas smaller than 80 ha (200 ac). Drainage coefficient (cm/day or in. drainage area, A, and the runoff coefficient, C, can typically be obtained without much trouble. The n value is determined Here, the constant k is known as the coefficient of permeability or simply permeability. Rational Method: In this method, the peak rate of runoff is given by the equation-. 80665 m/s 2) To determine your peak discharge requirements see: Rainfall Runoff Rate Formulas and Calculator. I = rainfall intensity, in/h (mm/h) A = drainage area, acres (m 2) Drainage area = land area (in 2) x depth + runoff volume onto land (in 3) Rainfall Intensity Formula. It is used in drainage design. coefficient to an area, such as a development site. 67 and correct open area from square feet to square inches and depth of water from feet to inches. 2. units) (ha for. Determine the footprint of your structure (F) in square feet: Measure along the length of the house or building and along the width of the house or building (in feet). Calculate the drainage coefficient. The coefficient of runoff or the maximum rate of runoff This calculator is based on eq. 23. Generally there are two methods by which the quantity of storm-water is calculated: 1. The Rational Method is limited to drainage basins 160 acre. The CN value is a primary input parameter for the SCS runoff equation, as used by HydroCAD. Q = Flow capacity (in cubic feet per second) C = Orifice coefficient; A = Open area of grate (ft 2 ) g = Acceleration due to gravity (32. Calculate the time required for (a) 90% consolidation settlement, (b) a settlement of 100 mm. Q = (C x I x A )/96. These runoff coefficients, known as runoff curve numbers (CNs), summarize the runoff potential of a particular area; the higher an area’s CN, the higher its runoff potential. In the Rational Method equation, “C” is the runoff coefficient, meaning it is your unknown. 95 is the runoff coefficient for impermeable surfaces; d p = the depth of the reservoir layer (feet); R = A c /A p The ratio of the contributing drainage area (A c, including the permeable paving surface), to the permeable pavement surface area (A p); t f = the time to fill the reservoir layer (day) – typically 2 hours or 0. Jul 10, 2024 · This open channel flow calculator will help you find water flow velocity and volumetric flow rate through an open channel, given the channel's roughness coefficient, slope, and cross-sectional area. Rational method involves following formula for computing the design runoff: Where, Q peak = peak runoff rate, m 3 /s. Uniformly graded soil has identical particles with Cu value approximately equal to 1. USBR (1997) suggests using the V-notch weir equations for the following conditions: Head (h) should be measured at a distance of at least 4h upstream of the weir. Mar 2, 2020 · The runoff coefficient is the variable of the Rational method that requires the most judgment and understanding on the part of the designer. The coefficient of discharge can be determined experimentally. C = Coefficient of runoff . 6 is for ideal flow without pressure loss in the orifice. Σqn = total theoretical load - all fixtures summarized (gpm, l/s) Jun 16, 2009 · The composite runoff coefficient reflects the surface characteristics of the contributing watershed. . There are many methods available to estimate the time of concentration including the Kirpich formula, Kerby formula Runoff coefficient, an empirical coefficient representing a relationship between rainfall and runoff . Re = ρuoL η Reynold's number (2. Q = A (m)2/3 (i)1/2 n Where A, is the cross-sectional area of the Drain n, surface roughness of the Drain, taken as 0. Method. Multiply the two figures together to obtain the square footage of your roof. Find the square root of that product. Area = length x width. The design watershed charac- Jul 10, 2024 · To calculate the flow rate of fluid passing through an orifice: Multiply the gravitational acceleration, g, with the mean centerline, H. 024 cfs. 55 % Carrying capacity of the Drain, Mar 2, 2020 · Time of concentration is a critical component in some analysis methods for calculating peak discharge from an area. In both the above methods, the quantity of storm water is a function of the area (in hectares): the intensity of rainfall and the coefficient of runoff. Orifice Discharge Coefficient. Q = peak discharge, ft 3 /s (m 3 /s) C = runoff coefficient = percentage of rain that appears as direct runoff. The rational formula is: Equation 4-20. 01 (a clean and smooth channel) to 0. The uniformity coefficient (Cu) is defined as the ratio of D60 to D10. (8) 3. less than 200 acres. g = acceleration of gravity, ft/s (32. 083 day; Streamgaging generally involves 3 steps: 1. The existing condition peak flow rate is 7. Step 2 Determine the runoff coefficient, C, for the type of soil/cover in the drainage area. Feb 5, 2024 · Calculating Runoff. = Definition. ADVERTISEMENTS: I = rainfall intensity (mm/h) for the duration equal to time of concentration of watershed and for a given recurrence interval. ( 1 ha = 10000 m 2 ) Water removal period = 2 days. Step 2. Tc = 2. Where, Q is the storm-water Run-off in Gallons Per Minute (GPM) C is the Run-off coefficient. Typical value are included for use with the CivilWeb Rainfall & Runoff Calculator This equ ation can be used to calculate a theoretical time to drain the container by setting h(t drain) = 0 and solving for t drain, and then comparing 9 to experimentally measured val-ues. The range of runoff coefficient values varies from 0 – 1. c = coefficient of discharge . Mar 21, 2024 · In this tutorial, you will be estimating the Tc and R. v 2 = c (2 g h) 1/2 (6b) where . 6 can be expressed with a coefficient of discharge - friction coefficient - as. A well known steady-state drainage equation is the Hooghoudt drain spacing The composite runoff coefficient, C, reflects the runoff potential of the drainage area. 20 to 0. 0144 m per day = 1. c) for a selected design storm . 4 in/hr. 2, increases exponentially as its velocity increases. One CFS is equal to one cubic foot of water passing a particular point over the course of a second. Previous work 8 relies on a standard ultrasonic motion detector12 to analyze the draining system. Consult a Water Table Management Professional for design criteria information. A is the Drainage Area in square feet. Dec 29, 2022 · 0. The Structural Number is converted to actual layer thicknesses (e. 01) R = hydraulic radius (cross sectional area of the fluid in the culvert divided by the wetted perimeter) He = entrance head loss = (. 0028 CiA (S. Parameters in Hooghoudt's drainage equation. Step 1: Determine the allowable discharge peak flow for 100-year storm. A uniformity coefficient value of 2 n = Manning coefficient of roughness - ranging from 0. 1/2 to 3/4 small, highly developed drainage areas. Its original publication is in Dutch. I is the Rainfall intensity in inches per hour. Checks the capacity of drain tile on existing drainage systems; Sizes the piping needed on the acreage to The Excel formulas also calculate i, and then use the Rational Method equation to calculate the design peak storm water runoff rate, Q. 06 (a channel with stones and debris, 1/3 of vegetation) R h = hydraulic radius (ft, m) S = slope - or gradient - of pipe (ft/ft, m/m) The Manning coefficients n in the English (Imperial) are equal to the coefficients in the SI system. Hf = barrel friction head loss. 3 Plastic Tubing Drainage Chart and should be used for estimating purposes only. Enter the Diameter of the pipe (inches): Enter the Grade (%): %. In geotechnical engineering k is commonly expressed in cm/s (although m/s is the preferred metric unit), and other possible units include m/s, m/day, and mm/hour. 3/8 to 1/2. P = Wetted perimeter [L]. 02. by subtracting basic infiltration rate from estimated consecutive day rainfall. Road drainage design has as its basic objective the reduction and/or elimination of energy generated by flowing water. Calculate your drainage install on-site. Calculate the Coefficient of Consolidation (cv) for 50 percent consolidation for each increasing load increment only using Where: d is the length of longest drainage path computed by taking one - half the average sample thickness (double drainage) for each load increment and time factor (Tv) for 50 percent consolidation is 0. 𝐴𝐴 = drainage area in km. Drainage Coefficient (inches/day) 1/4". The Rational Method 2. A = watershed area, hectare. 35 to 0. Full drainage design suite (50% Discount) including 7 spreadsheets; The Manning Coefficient for Sheet Flow is used with the Kinematic Wave Equation and other methods of calculating the velocity of sheet flow along the ground, typically during design storm events. Learn how the main pipe should be sized for best drainage performance. Why is It Important? Development Experience Clearinghouse (DEC) - Home • Coefficient of runoff is the same for all storms of all recurrence probabilities. 17405 ft/s 2, 9. 25/100) × 120000 × 2 = 3000 m3. Calculation: Given: Drainage coefficient = 1. /day) is calculated using the Manning equation and is dependent on the size, slope, and hydraulic roughness of the drains (and where pumped outlets are used, the pumping capacity). 40. Feb 6, 2021 · One way that we can quantify whether a fluid is laminar or turbulent is by calculating the Reynold's number for the flow, defined as. The runoff coefficient is determined by estimating the area of different land uses within each drainage area. A particular pipe size on a given grade will only carry a certain amount of water. Time to Drain Tank Calculator Units: cm=centimeter, ft=foot, hr=hour, m=meter, min=minute, sec=second. 44 cm/day, Ans. Choose the Coefficient of Dec 26, 2016 · The drainage coefficient was estimated for 5, 10, 20 and 25 years R. Drainage coefficient values typically range from 0. For the study area, the maximum value of Mar 30, 2020 · FEP Hydraulics Chapter 3 Homework, Problem 1 Determining the peak flow rate and volume of storm water runoff generated in a watershed for a given. Table 1 lists runoff coefficients for various combinations of ground cover and slope. 23 is a conversion factor when you want the flow rate in GPM and your driange area in square feet. It doesn't matter how thick the weir is except where water flows over the weir through the "V. Provide the hydraulic head value. The discharge formula, Q = A V, can be used to rewrite Gauckler–Manning's equation by substitution for V. In this calculator, you will learn how Manning's equation works and how to use it to determine water flow. The composite runoff coefficient is the weighted average of all of the land uses within the drainage area. The range of runoff coefficients varies from 0. The Engineer st roadway stormwater drainage systems, the Rational Method can be used to determine peak flow (Q). Also can solve for discharge coefficient, orifice diameter (or area), tank diameter (or area), initial liquid depth, final liquid depth. Then, to determine the amount of runoff in gallons Expected total load for sanitary drainage systems should instead be estimated by using empirical equations like. Step 3: Determine the appropriate runoff coefficient. A smaller, less pervious, area is tributary to the larger primary drainage area. condition, and flow rate of the runoff all play a major role in the sizing, configuration, and operation of. 50 m = 2500 mm For U 90, T 90 = 0. 2) Ke = inlet loss coefficient (see Section 2E-1, Table 2E-1. Field Crops. Example #1: Calculate the peak storm water runoff rate from a watershed of 15 acres, with a runoff coefficient of 0. qet = expected total drainage load (gpm, l/s) k = system coefficient describing the nature of the system. If drainage areas involve pump stations. Area = 12 ha = 120000 m 2. where: V = the average velocity in the culvert barrel, ft/s. Arcement, Jr. 2 ∗ ( L ∗LC Slope10−85− −−−−−−−√)0. The Rational Method says that Q=CiA, where: C = Runoff coefficient, a dimensionless value between 0 and 1 that represents the percentage of rainfall that becomes runoff. The steeper the grade of the installed pipe, the more water it will carry. While engineering judgment will always be required in the selection of runoff coefficients, a typical coefficient represents the integrated effects of many drainage basin parameters. b. The drainage coefficient depends on factors like rainfall, drainage area size and General Drainage Coefficients (inches/24 hours)* Without surface inlets . The units on peak runoff rate, q, are actually acre-in/hr, but the conversion from acre-in/hr To simplify the Orifice equation, use a coefficient of 0. The size of rainfall event, type of flow. 4859 for English units) [dimensionless] n = Manning roughness coefficient [s/ (ft 1/3 )] The Rational Method Equation. Since i is dimensionless, k has the unit of velocity. n is a function of the channel material, such as grass, concrete, earth, etc. 1/2 to 3/4 . A = Drainage area in acres Since our watershed is a relatively steep pasture with heavy soil, the C value is 0. A value of Cu greater than 4 to 6 classifies the soil as well graded. 1. Organic. 0, with higher values corresponding to greater runoff rate potential. , and Verne R. Flow rate Q Q can be described as flow volume per unit time Q=V/t Q = V /t, therefore: k=\frac {V} {t×A×i} k = t × A × iV. Values for the entrance loss coefficient, K e, are available in various hydraulic texts including HDS-5, and values range from 0. For convenience Step 1: Compute time of concentration for drainage catchment (we’ll discuss this procedure in 3 weeks). The more impervious surfaces that are proposed on a site, the higher C will be. The required parameters for the drainage coefficient calculator are as follows: The desired drainage coefficient. The calculator divides the container in "slices" and makes an iterative average calculation for each slice. nationally accepted methods. Measuring stream stage —obtaining a continuous record of stage—the height of the water surface at a location along a stream or river. Multiply the resultant with the area of orifice, A, and the coefficient of discharge, Cd. How do I calculate stormwater runoff? First, calculate the square foot of all hard, impervious surfaces such as driveways, sidewalks, and other roofed or concrete areas that do not allow water to soak in by measuring in feet and the multiplying length times width. The weighted C for a drainage area is computed by dividing the sum of all subarea CAs by the total area, where CA is the subarea C value multiplied by the area of the subarea. If the drainage coefficient is 3 cm, what would be the discharge through the drain? Solution: Drain discharge in 24 hours = 2. The value of Q is dependent on the intensity of the rainfall, which changes based on the 7. Typical Manning's Note: the Strickler coefficient is the reciprocal of Manning coefficient: Ks =1/ n, having dimension of L 1/3 /T and units of m 1/3 /s; it varies from 20 m 1/3 /s (rough stone and rough surface) to 80 m 1/3 /s (smooth concrete and cast iron). units) where: = the area of the watershed (drainage area) that drains to the point for which the peak runoff rate is needed (acres for U. Soil Type. g. “Q” is the value for the peak rate of runoff. Time of Concentration ( tc) There are a number of empirical equations available for the estimation of the time of concentration. storm event is an essential step in evaluating drainage design. These calculations are based on the Manning’s Roughness ASAE EP 260. The volume of water removed = Drainage coefficient × Area × Time = (1. Sep 2, 2018 · The coefficient of consolidation, whether it is vertical or radial drainage ( cv or cr) plays an important role in the design of vertical drains and it controls accurate prediction of rate of consolidation of soil. 96. If land use and soil cover are homogeneous over the drainage area, a C value can be determined from the following tables. 18 cubic meters would drain from the aquifer. ADVERTISEMENTS: Determining the proper trench drain size using Manning's equation provides a systematic approach to ensure efficient and effective stormwater management. where Tc = time of concentration (hrs); L =longest flow path (mi); Lc = Centroidal flow path (mi); Slope10−85 = average slope of the Nature of the crops grown. 54. Find the appropriate values for each variable. If there are multiple soil cover conditions, a weighted average must be performed. Step. High CN values (such as 98 for pavement) cause most of the rainfall to appear as runoff, with minimal losses. In this equation, k is either 0. 6 to 2. 💡 We can see from the above relationship that the hydraulic gradient i i is inversely proportional to A drainage equation is an equation describing the relation between depth and spacing of parallel subsurface drains, depth of the watertable, depth and hydraulic conductivity of the soils. Values of C for different slopes and land use conditions, determined from field observations are given in Table 3. 1. Rational Equation: Q=ciA. Where a drainage area is composed of The Prinsco Drainage Calculator estimates the capacity of tile drainage systems. Empirical Formulae Method. qet = k ( Σqn )1/2(1) where. The English form of the Manning equation uses the following variable units: V = average flow velocity [ft/s] k = unit conversion factor (1. = 0. The Manning Mar 2, 2020 · Equation 2E-2. Drainage Coefficient. Eq. Then Coefficients for Natural Channels and Flood Plains By George J. The peak runoff rate from a given drainage area is given by: Q y = C x I x A where: Q y = peak runoff rate (cubic feet per second) C = the runoff coefficient of the area (assumed to dimensionless) I = the average rainfall intensity (in. 955 mm²/min. 0 for SI, 1. 49 for USCS, Q p = design peak discharge, ft 3 /s (m 3 /s) S o = local slope, % f = friction factor, g = gravitational constant ( 32. Register to enable "Calculate" button. When this scenario occurs, two sets of calculations should also be made. Truck Crops . The volume of water removed from or added to aquifer storage over an area, A, for a change in head, ∆h, is determined as shown in Equation 48. The Rational Method equation is: Q = C x I x A where: Q = Storm Water Runoff (in cubic feet per second) C = Coefficient of Runoff I = Rainfall Intensity (in inches per hour) 2. General Drainage Coefficients (inches/24 hours)* Without surface inlets . If you want to calculate the discharge coefficient using hydraulic head: Enter the diameter of the orifice if it is circular. Drainage Density is performed using Digital Elevation Model (DEM). Inputs. 20) R e = ρ u o L η Reynold's number. In accordance with the Stormwater Management rules requirement for stormwater runoff The Rational Method is widely used to calculate the peak storm water runoff rate for a variety of storm water management applications. 67 and correcting open area from square feet to square inches and depth of water from feet to inches, this equation simplifies to: THE ORIFICE EQUATION. S. 4859. The use of average values has been adopted to simplify the determination of this coefficient. Roads will affect the natural surface and subsurface drainage pattern of a watershed or individual hillslope. The accuracy of the calculation can be increased by increasing the number of slices. The rational formula estimates the peak rate of runoff at a specific location in a watershed as a function of the drainage area, runoff coefficient, and mean rainfall intensity for a duration equal to the time of concentration. 08 inches (0. Use other. 318 if using imperial units, C is the roughness coefficient of the pipe material, R is the hydraulic radius (cross-sectional area divided by perimeter), and S is the slope of the pipe. Each of the factors that influence an area’s CN are discussed briefly below. The Reynold's number is found by considering the two terms in the momentum balance for the flow. 0/2 = 2. 1/2 to 3/4 In reality, the coefficient may vary with respect to prior wetting and seasonal conditions. The Rational Method equation actually used to calculate peak storm water runoff rate is: Q = CiA (U. 35, from a storm of intensity 2. This value is in CFS. (see below) Step 1 Determine the drainage area. The Rational equation is the simplest method to determine peak discharge from drainage basin runoff. The runoff coefficient (C) is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. 196. • First, calculate the Peak Q from the total drainage area with its weighted C value and the intensity associated with the longest time of concentration, Tc. This paper presents an overview of different methods used in the past for finding the coefficient of consolidation, using vertical May 25, 2024 · We will now show you how to calculate permeability using Darcy's law calculator: k=\frac {Q} {A×i} k = A × iQ. It represents the depth of water that must be removed from the drainage area in 24 hours to prevent stressing crops. Pipe Diameter (inches): % Grade: *. C = runoff coefficient. It is also called hydraulic conductivity. Sep 5, 2022 · The calculator now has everything it needs to compute the coefficient of discharge and flow resistance. " The weir should be between 0. Oct 1, 2021 · The Modified Rational Method is to be used to size a stormwater management basin for the 100-year storm. 5. Drainage area information should include: a. The rational method equation is: Q = C x I x A/96. 03 and 0. In the real worls - with pressure loss - eq. I. A drainage equation is an equation describing the relation between depth and spacing of parallel subsurface drains, depth of the water table, depth and hydraulic conductivity of the soils. The new NDS app is the perfect tool for determining runoff and system requirements on the go. It is not as sophisticated as the , but is the most common method used for sizing sewer systems. Otherwise, enter the cross-sectional area of the opening. 3. Area Drained (Acres) Use this calculator to compute the area that can be drained by a pipe of a given size and grade. Watershed runoff coefficients for calculation of storm water runoff rate can be obtained The coefficient of consolidation for the loading range is 0. For a specific yield of 0. retain or detain water, the Rational Method cannot be used. If there’s an abrupt transition from lighter to heavier soil, it’s better to keep the drains above the heavy layer, when possible. We will be using the equation which was derived for a Dam Safety Study in Pennsylvania. It is a larger value for areas with low infiltration and high runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat land). Values for exit loss coefficients, K x, can vary between 0. In this example the C values are not adjusted for storm n = Manning’s roughness coefficient, K n = constant; 1. Runoff coefficient C is defined as the ratio of the peak runoff rate to the rainfall intensity. 5 cm/day but can be up to 10 cm/day. The peak discharge occurs when all segments of the drainage area are contributing to the runoff from the site. Values for n can be found in the table of Manning's n coefficients shown below. The Rational Method Equation. Schneider Abstract Although much research has been done on Man­ ning's roughness coefficient, n, for stream channels, very little has been done concerning the roughness values for densely vegetated flood plains. For drainage areas greater than 200 acres, other methods can be used, or the drainage area can be divided into zones less than 200 acres. The discharge measurement —obtaining periodic measurements of discharge (the quantity of water passing a location along a stream) 3. 2 Determination of learn about Drainage Intensity and Drainage Coefficient with a simple example. The degree of surface drainage. There is two-way drainage, upward and downward. 013 m, Hydraulic gradient of the Drain i, mean slope of the Drain, 3. 5 ´ 60 ´ 60 ´ 24 m 3 /day. The drainage area is 20 acres. units), or Q = 0. M 2, M 3 = drainage coefficients for the base and subbase, respectively. i = Average intensity of rainfall for the time of concentration (T. Character of soil and ground cover as they may affect the runoff coefficient. For a sudden expansion of flow, the exit loss coefficient is set to 1. 95, with higher values corresponding to greater runoff potential. Where: 1. 23, where, Q is the storm-water run-off in gallons per minute (GPM) C is the runoff coefficient, I is the rainfall intensity in inches per hour. Tank (or pond or reservoir) is open to the atmosphere. Determination of the design rainfall intensity, i, usually requires the most effort. Chow lists 24 rainfall-intensity formulas of the form: Eq. The drainage area, runoff coefficient, and design rainfall intensity are needed for the storm water runoff calculation. 63 × S 0. When Cu is less than 4, it is classified as poorly graded or uniformly graded soil. The destructive power of flowing water, as stated in Section 3. system must be determined accurately. May 31, 2022 · Calculate Drainage Density and Prepare Drainage Density map using ArcGIS. P is the contact length between the water and the channel bottom and sides. n = Manning coefficient. 25 cm/day. The degree of protection required for them from waterlogging. * %Grade is the feet of fall per 100 feet of length. 0. xe ir pl nq ed jr yu ad pn li