 Wathi Ghadge, Assistant Professor, Department of Civil Engineering, Valchin Institute of Technology, Solapur. Topic for today's session, estimation of discharge of a sewer line by computing rainfall intensity, impermeability factor, or coefficient of runoff. At the end of this session, students will be able to estimate discharge of sewer line by computing rainfall intensity, impermeability factor, or coefficient of runoff. The amount of water flowing over the ground surface, pavement, house, is commonly known as runoff, or strong water. Strong water flow, it is also called as wet weather flow, is estimated by the following formula. There are two formulas. First formula is q is equal to 1 upon 360 into AIRI. Second formula is q is equal to 28 into AIRI. Here, in general, we can say q is equal to k into AIRI. k is the constant which permit the convenient unit of other parameter like discharge, area, and rainfall intensity. If the k is 1 upon 360 in that formula, keep discharge in meter cube per second, area in hectare, and RI in mm per hour. If the formula is 28 into AIRI, in that formula, you keep discharge in liter per second, area in hectare, and RI, intensity of rainfall, is in centimeter per hour. In the previous formulas, there are operative factors, cachement area, impermeability factor, and it is also called as coefficient of runoff, and intensity of rainfall. Cachement area, it can be found directly from the map of the town. And we'll see now impermeability factor. The percentage of rainwater that is available in the form of runoff is known as impermeability factor. Table one and table two are the guideline for the impermeability factor. Cachement area, the entire cachement area, if you see, it having the many types of surfaces. And so the area is divided into many parts. So how to find the impermeability factor in that case? We'll see here. Impermeability factor or the coefficient of runoff can be calculated by using the formula. A1, I1 plus A2, I2, and so on, divided by total area. That is summation of AI divided by summation of total area. And it is the formula is same for the coefficient of runoff. Table one, it is the guideline for the impermeability factor for the different type of surfaces. In this table, there are total element types of surfaces. And for that particular surface, you can assume the impermeability factor in the given range. Table two, it is the guideline for the impermeability factor for the different locality or different types of area. Now, a calculation of rainfall intensity. The value of rainfall intensity can be determined from the rainfall record of that city. Where rainfall record not available, the following empirical formulas are used. General formula is 25.4 into A divided by T plus B, where A and B are the constant shown in the table three. It is depend on the duration of the rainfall. And T is the duration of the rainfall. And there are many formulas for the storm occurring once in a year, once in a five year, and once in a 10 year. All these three are the general formulae. And there are the formulae derived by the Kuchlings. It is called as the Kuchlings formula. The storm occurring once in a 10 year formula is rainfall intensity is equal to 2667 divided by T plus 20. And if the storm occurring once in a 15 year formula is rainfall intensity is equal to 3048 divided by T plus 20. Table three, shown the constant used in a general formula that constant A and B are depend on the duration of the storm. If the duration of the storm is 5 to 20 minutes, A is 30 and B is 10. And the duration of the storm is 20 to 100 minutes, A is 40 and B is 20. Now you pause the video here and write the answer of question. What is the relation between rainfall intensity and the duration of the storm? We have seen the formulae, you write the relation. Here is the answer. In all the formulae, R i is inversely proportional to T. That is, shorter the duration of rainfall, greater will be the intensity during that period. Take one example. The following data is available regarding various type of area and corresponding I of a town. Types of area is given here, roof, pavement, lawn, unpaved area, and wooded area. And that area, roof is 15% of total area. roof, pavement is 20% of total area, and so on. Accordingly, I is given. Determine the average coefficient of runoff. If the total area of district is 20 hectare, this area is subjected to three storm, P1, P2, and P3, with equal frequencies of occurrence. The duration of the three storm is 20 minutes, 30 minutes, and 35 minutes respectively. Determine the maximum runoff of a storm. So you read numerical carefully, list out the given data. Here, first area, for example, first area is roof, and it is of 15% of total area. So A1 is 0.15 A. And I for the roof is given 0.9. So likewise, for all the area, you put A1, A and I, and total area is 20. So calculate I average, it is 0.39. And frequency of the storm is not given, so we have to use general formula that is 25.4 into A divided by T plus B for the calculation of intensity of rainfall. Consider storm one, that is P1. It is of duration 20 minutes. So according to the table number three, if the duration is equal to 20 minutes, A30 and B10. Put in a formula and find out the intensity of rainfall. After calculation, we'll get 25.4 mm per hour. So find out the discharge for the storm one. So according to your convenient unit, you decide the formula, which formula you want to use. Here discharge is in meter cube per second. And rainfall intensity is in mm per hour. So use a formula 1 upon 360 into AIRI. Put all the calculated parameters in the formula. Find out the discharge. So for the storm one, discharge one is 0.55 meter cube per second. Similarly calculate for the storm two. Storm two duration is given 30 minutes. So according to table, it is more than 20. So A is 40 according to table three. A is 40 and B is 20. And duration of the storm is given 30. So put in a formula for intensity, calculate it. After calculation, we'll get 20.32 mm per hour. Calculate the discharge for the storm two. So discharge two, that is Q2, it is 0.44 meter cube per second. And storm three is given. Storm three, it is of duration 35 minutes. So for 35 minutes, A and B will be same. That is 40 and 20. Put in the formula and calculate the RI. After calculation, we'll get RI is equal to 18.473 mm per hour. Calculate the discharge three for the storm three. It is 0.40 meter cube per second. So from the values of Q1, Q2 and Q3, we find the maximum discharge. Maximum discharge for the storm one, we found more discharge that is 0.55 meter cube per second. These are my references. Thank you.