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Water and Waste Water Engineering

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Aditya Mistry

This document discusses methods for estimating stormwater runoff in urban areas. It begins by defining stormwater as water from rain, snow, or ice that flows over surfaces after falling to the ground. The rational method and empirical formulas are presented as common approaches to estimating peak stormwater discharge rates. The rational method estimates discharge as a function of rainfall intensity, runoff coefficient, and drainage area. Runoff coefficients vary based on surface type. An example problem applies the rational method to calculate sewage flow and stormwater flow for a drainage system.

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Subject:- Water and waste water engineering Subject code:- 2160601 Guided by:- Prof. Mamta Patel Prof. Manali Shah Name Enrollment No. Bambhroliya Rishabh 151103106001 Deshmukh Bhavik 151103106002 Mistry Aditya 151103106009 Pandya Dhrumil 151103106010 Patel Nirmal 151103106012
Topic: Estimation of storm water in urban area Outline What is storm water? Estimation of storm water discharge Rational method Empirical formula Example
What is storm water?  Water falls as rain, snow, or ice. Most seeps into ground.  If ground is saturated, frozen, or has paved surfaces, water flows & is called stormwater runoff.  Stormwater Flows over surfaces such as roads, highway.
Estimation of storm water discharge  The peak rate of runoff depends upon various factors such as; the type of precipitation, the intensity and duration of rainfall distribution, the soil moisture deficiency, the climatic conditions, the shape, size and type of catchment area, etc.  Due to these variable it is not precisely determine the runoff by exact equation.  In recent years, a rational method has been evolved to estimate the peak drainage discharge.  This method can be applied most precisely to areas smaller than 50 hectares.  However, for large areas, empirical formulae are used to determine the peak rate of runoff.  The most modern method for computing urban storm water is drainage is by ‘digital computer simulatios’.
Rational method:  If a rainfall is applied to an impervious surface at a constant rate, the resultant runoff from the surface would finally reach a rate equal to the rainfall.  In the beginning, only a certain amount of water will reach the outlet, but after some time, the water will start reaching the outlet from the entire area, and the runoff rate would equal to the rate of rainfall.  The period after which the entire area will start contributing to the runoff is called the time of concentration.  It is the time required by the water to reach the outlet from the most remote point of the drainage area.  The runoff resulting from a rain having a duration lesser than the time of concentration will not be maximum, as the entire area will start contributing to runoff in this case.
 Based upon this principles, the rational formula was developed by the efforts of kuichling of America, fruhling of Germany and Lloyd davis of England.  This formula states that, Qp= 1/36*K*Pc*A Where, Qp= peak rate of runoff in cumecs K= Coefficient of runoff Pc= critical rainfall intensity during the critical rain fall direction A= Catchment area contributing to runoff at the considerd point in hectare
Coefficient of runoff is in fact, the impervious factor of runoff, representing, the ratio of runoff to precipitation . The value of K increases as the imperviousness of the area increases. For perfectly impervious area K=1.  Runoff coefficient for different surfaces Sr. No. Type of Surface Value of K 1. Water tight roof surface 0.7 to 0.95 2. Asphalt pavement in good order 0.85 to 0.90 3. Stone, brick, wood-block pavement with cement joints 0.75 to 0.85 4. Same as above with uncemented joints 0.50 to 0.70 5. Water bound macadam roads 0.25 to 0.60 6. Gravel roads 0.15 to 0.30 7. Unpaved streets and vacant lands 0.10 to 0.30 8. Parks, lawns, gardens, etc. 0.05 to 0.25 9. Wooden lands 0.01 to 0.20
 Runoff coefficient for different types of localities Sr. No. Type of Surface Value of K 1. Business areas 0.85 2. Areas closely built up 0.75 3. Areas with 50% attached house and 50% detached houses 0.65 4. Suburban areas with widely detached houses 0.45 to 0.55 5. Extremely suburban areas with 20 to 40% parking and widely detached houses 0.35
Empirical Formulae  For the design of drains having catchment area larger than 400 hectares, it is generally advisable to use the empirical formula suggested for that region.  Various empirical formulae for calculating storm water run-off have been suggested by various investigators. Some if the important formulae are given below:  1. Burki zielgler formula  2. Dicken’s formula  3. Ryve’s formula  4. Inglis formula
Example:  In a a district area 40 hectares, the density of population is 300 persons/hectare. The quota of water supply is 225 lpcd.  20% of the are consists of roof with runoff ratio is 0.80. Maximum rainfall intensity is 5 cm/hr.  Calculate the quantity of  Sewage for which the sewers of a separate system should be designed.  Storm water for which the sewers of partially separate system should be designed.
Solution:  Case 1 Total area of the district = 40 hectares Population density= 40*300 = 12000 persons Average water supply= 225 litres/capita/day Average quantity of water required supplied to the district per day = 12000*225 = 2700000 litres per day = 2700 m3 per day Rate of water supplied= 2700/(2400*60*60)= 0.03125 cumecs
Assuming the sewage discharge as 0.8 times the water supplied. Average rate of sewage produced= 0.8*0.03125 = 0.025 cumecs. Now, assuming peak rate of sewage as 3 times the average, Peak rate of flow= 3*0.025 = 0.075 cumecs
 Case 2: In case of particularly separate system, the storm water from roofs and paved yards of houses will be allowed to enter the sewers. Area of roofs= 20*40/100= 8 hectares (K=0.9) Area of paved yards= 5/100*40= 2 hectares (K=0.80) The discharge from roofs and paved yards of houses is given by rational formula, Qp=1/36.K.Pc.A =[1/36*0.9*5*8+1/36*0.8*5*2] = 1+0.22 = 1.22 cumecs Hence, the storm water which must pass through the sewers of a partially separate system= 1.22 cumecs.
Thank You

More Related Content

Water and Waste Water Engineering

  • 1. Subject:- Water and waste water engineering Subject code:- 2160601 Guided by:- Prof. Mamta Patel Prof. Manali Shah Name Enrollment No. Bambhroliya Rishabh 151103106001 Deshmukh Bhavik 151103106002 Mistry Aditya 151103106009 Pandya Dhrumil 151103106010 Patel Nirmal 151103106012
  • 2. Topic: Estimation of storm water in urban area Outline What is storm water? Estimation of storm water discharge Rational method Empirical formula Example
  • 3. What is storm water?  Water falls as rain, snow, or ice. Most seeps into ground.  If ground is saturated, frozen, or has paved surfaces, water flows & is called stormwater runoff.  Stormwater Flows over surfaces such as roads, highway.
  • 4. Estimation of storm water discharge  The peak rate of runoff depends upon various factors such as; the type of precipitation, the intensity and duration of rainfall distribution, the soil moisture deficiency, the climatic conditions, the shape, size and type of catchment area, etc.  Due to these variable it is not precisely determine the runoff by exact equation.  In recent years, a rational method has been evolved to estimate the peak drainage discharge.  This method can be applied most precisely to areas smaller than 50 hectares.  However, for large areas, empirical formulae are used to determine the peak rate of runoff.  The most modern method for computing urban storm water is drainage is by ‘digital computer simulatios’.
  • 5. Rational method:  If a rainfall is applied to an impervious surface at a constant rate, the resultant runoff from the surface would finally reach a rate equal to the rainfall.  In the beginning, only a certain amount of water will reach the outlet, but after some time, the water will start reaching the outlet from the entire area, and the runoff rate would equal to the rate of rainfall.  The period after which the entire area will start contributing to the runoff is called the time of concentration.  It is the time required by the water to reach the outlet from the most remote point of the drainage area.  The runoff resulting from a rain having a duration lesser than the time of concentration will not be maximum, as the entire area will start contributing to runoff in this case.
  • 6.  Based upon this principles, the rational formula was developed by the efforts of kuichling of America, fruhling of Germany and Lloyd davis of England.  This formula states that, Qp= 1/36*K*Pc*A Where, Qp= peak rate of runoff in cumecs K= Coefficient of runoff Pc= critical rainfall intensity during the critical rain fall direction A= Catchment area contributing to runoff at the considerd point in hectare
  • 7. Coefficient of runoff is in fact, the impervious factor of runoff, representing, the ratio of runoff to precipitation . The value of K increases as the imperviousness of the area increases. For perfectly impervious area K=1.  Runoff coefficient for different surfaces Sr. No. Type of Surface Value of K 1. Water tight roof surface 0.7 to 0.95 2. Asphalt pavement in good order 0.85 to 0.90 3. Stone, brick, wood-block pavement with cement joints 0.75 to 0.85 4. Same as above with uncemented joints 0.50 to 0.70 5. Water bound macadam roads 0.25 to 0.60 6. Gravel roads 0.15 to 0.30 7. Unpaved streets and vacant lands 0.10 to 0.30 8. Parks, lawns, gardens, etc. 0.05 to 0.25 9. Wooden lands 0.01 to 0.20
  • 8.  Runoff coefficient for different types of localities Sr. No. Type of Surface Value of K 1. Business areas 0.85 2. Areas closely built up 0.75 3. Areas with 50% attached house and 50% detached houses 0.65 4. Suburban areas with widely detached houses 0.45 to 0.55 5. Extremely suburban areas with 20 to 40% parking and widely detached houses 0.35
  • 9. Empirical Formulae  For the design of drains having catchment area larger than 400 hectares, it is generally advisable to use the empirical formula suggested for that region.  Various empirical formulae for calculating storm water run-off have been suggested by various investigators. Some if the important formulae are given below:  1. Burki zielgler formula  2. Dicken’s formula  3. Ryve’s formula  4. Inglis formula
  • 10. Example:  In a a district area 40 hectares, the density of population is 300 persons/hectare. The quota of water supply is 225 lpcd.  20% of the are consists of roof with runoff ratio is 0.80. Maximum rainfall intensity is 5 cm/hr.  Calculate the quantity of  Sewage for which the sewers of a separate system should be designed.  Storm water for which the sewers of partially separate system should be designed.
  • 11. Solution:  Case 1 Total area of the district = 40 hectares Population density= 40*300 = 12000 persons Average water supply= 225 litres/capita/day Average quantity of water required supplied to the district per day = 12000*225 = 2700000 litres per day = 2700 m3 per day Rate of water supplied= 2700/(2400*60*60)= 0.03125 cumecs
  • 12. Assuming the sewage discharge as 0.8 times the water supplied. Average rate of sewage produced= 0.8*0.03125 = 0.025 cumecs. Now, assuming peak rate of sewage as 3 times the average, Peak rate of flow= 3*0.025 = 0.075 cumecs
  • 13.  Case 2: In case of particularly separate system, the storm water from roofs and paved yards of houses will be allowed to enter the sewers. Area of roofs= 20*40/100= 8 hectares (K=0.9) Area of paved yards= 5/100*40= 2 hectares (K=0.80) The discharge from roofs and paved yards of houses is given by rational formula, Qp=1/36.K.Pc.A =[1/36*0.9*5*8+1/36*0.8*5*2] = 1+0.22 = 1.22 cumecs Hence, the storm water which must pass through the sewers of a partially separate system= 1.22 cumecs.