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Estimation of storm sewage

S
sidrarashiddar

This presentation includes the estimation of storm sewage generated as a result of storm/rainfall events. It includes the detailed usage of rational formula for quantity estimation with solved examples.

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Estimation of Storm Sewage Resource Person Engr. Sidra Rashid sidradar9@gmail.com
Textbook WATER SUPPLY AND SEWERAGE By E.W. Steel and T.J. McGhee 6th Edition 2
Factors Affecting Storm Sewage The surface run-off resulting after precipitation contributes to the storm water. The quantity of storm water reaching to the sewers or drains is very large as compared with sanitary sewage. The factors affecting the quantity of storm water flow are as below: i. Area of the catchment ii. Slope and shape of the catchment area iii. Porosity of the soil iv. Obstruction in the flow of water as trees, fields, gardens, etc. v. Initial state of catchment area with respect to wetness. vi. Intensity and duration of rainfall vii. Atmospheric temperature and humidity viii. Number and size of ditches present in the area 3
Estimation of Quantity of Storm Sewage Rainfall is the primary source of storm flow. Estimation of flow is the first step to design the Storm sewer. Rational Method is widely used Most widely used formula for urban areas Based on rainfall data For very large area empirical formula methods are used i.e. Burkli Zeiglar formula, Fullers formula etc. In both the above methods, the quantity of storm water is considered as function of intensity of rainfall and coefficient of runoff. 4
Cont. Rational Method The total volume which fall upon an area A per unit time under a rainfall of intensity i is Q = iA A portion is also lost by evaporation, percolation and ponding The actual amount which appears as run off may then be calculated from Q = CiA Where, C = Run off Coefficient 5
Cont. C for an area is not invariant but tends to increase as the rainfall continues. For impervious surfaces C = 1.75 t1/3 or C = t / (8 + t) These depends on duration of rainfall in minutes. For improved pervious surfaces C = 0.3 t / (20 + t) Where t is the duration of the storm in minutes. 6
Cont. Average values of C commonly used for various surfaces 7 Sr. # Type of Surface Value of C 1. Watertight roof 0.70 0.95 2. Asphalt cement streets 0.85 0.90 3. Portland Cement Street 0.80 0.95 4. Paved Driveways and Walks 0.75 0.85 5. Gravel Driveways and Walks 0.15 0.30 6. Lawn Sandy soil with slope 2% 0.05 0.10 2-7% 0.10 0.15 >7% 0.15 0.20 Heavy soil with slope 2% 0.13 0.17 2-7% 0.18 0.22 >7% 0.25 0.35
Cont. Some engineers use values of C 8 Sr. # Type of Surface Value of C 1. Densely built areas 0.70 0.9 2. Well-built areas 0.5 3. Residential areas 0.25 0.5 4. Suburban section 0.15 0.25
Sample Problem Determine the runoff coefficient for an area of 0.2 Km2. Out of this 3000 m2 is covered by buildings, 5000 m2 by paved driveways and walks and 2000 m2 by Portland cement streets. The remaining area is flat, heavy soil, covered by grass lawns. 9
Solution Surface Type C Avg C % of Area Runoff C of Area Roof 0.7 0.95 =(0.7+0.95/2) =0.825 =3000/(0.2*10002) =0.015 =0.825*0.015 = 0.012 Driveways & walks 0.75-0.85 0.80 0.025 0.02 Portland Cement Streets 0.8 0.95 0.875 0.01 0.00875 Flat, heavy soil grass lawns 0.13- 0.17 0.15 0.95 0.1425 Overall Runoff Coeff 0.1836 10 Select values of C for each type of area/surface form given values and calculate percentage of land area for each type Total area 0.2km2
Cont. Time of Concentration The time required for the maximum runoff rate to develop is known as the time of concentration or It is equal to time taken by a drop of water to run from the most remote point of the drainage area to the point for which the runoff is being estimated. The time of concentration has two parts Time of concentration = Inlet time + time of travel 11
Cont. 12
Cont. Consider two areas A and B as shown in Figure. I1 is entrance for area A and I2 is entrance for area B and I1 I2 The water flows from A enters the sewer at I1 and that from B at I2. Time of flow is a function of the velocity in the line I1 I2 and its length. The inlet time is time of concentration at I1. 13
Cont. The time of concentration at I2 is either the time of concentration for area B or the inlet time plus the time of flow from I1 to I2 , whichever is greater. The time of concentration for each sewer line is calculated in a similar fashion. The time of Concentration will largely depend upon the slope of the ground surface and slope of the sewer. 14
Cont. Nomogram is also used to calculate the time of concentration. In nomogram flow distance, type of surface and slope are used to calculate time of concentration. This procedure neglects effect of rainfall intensity but is adequate for most urban drainage project. 15
Rainfall intensity 16 Where, a , b and n are constants
Cont. 17
Problem 1 Determine the runoff coefficient for an area of 0.2 Km2. Out of this 3000 m2 is covered by buildings, 5000 m2 by paved driveways and walks and 2000 m2 by Portland cement streets. The remaining area is flat, heavy soil, covered by grass lawns. If the maximum intensity of rainfall is 40 mm/hour, calculate the quantity of storm water which will reach sewer lines. 18
Solution Surface Type C Avg C % of Area Runoff C of Area Roof 0.7 0.95 =(0.7+0.95/2) =0.825 =3000/(0.2*10002) =0.015 =0.825*0.015 = 0.012 Driveways & walks 0.75-0.85 0.80 0.025 0.02 Portland Cement Streets 0.8 0.95 0.875 0.01 0.00875 Flat, heavy soil grass lawns 0.13- 0.17 0.15 0.95 0.1425 Overall Runoff Coeff 0.1836 19 Select values of C for each type of area/surface form given values and calculate percentage of land area for each type Total area 0.2km2
Solution 20 i = 40 mm/hr. A = 0.2 km2 C = 0.1836 Plug the values in the following equation Q = C (i )(A) Q =0.1836 (40 )(0.2) = 0.40 m3/s
Problem 2 Drainage Area: 80 acres 30% - Rooftops (24 acres) 10% - Streets & Driveways (8 acres) 20% - Lawns @ 5% slope (16 acres) on sandy soil 40% - Woodland (32 acres) Time of Concentration (TC) = 15 min. Location: Tallahassee, Florida (Leon County), Zone 2 Calculate Peak runoff rate from 10-year frequency storm. 21
Cont. 22
Cont. 23
Solution Surface Type C Area Area * C Rooftops 0.9 24 21.6 Streets 0.9 8 7.2 Lawns 0.15 16 2.4 Woodland 0.1 32 3.2 Overall Runoff Coeff 24 Total area 80 Hectares C
Solution 25 i = 6.2 in/hr. A = 80 Acres C = 0.43 Plug the values in the following equation Q =C (i )(A) Q =0.43 (6.2 )(80) = 213.28 cfs.

More Related Content

Estimation of storm sewage

  • 1. Estimation of Storm Sewage Resource Person Engr. Sidra Rashid sidradar9@gmail.com
  • 2. Textbook WATER SUPPLY AND SEWERAGE By E.W. Steel and T.J. McGhee 6th Edition 2
  • 3. Factors Affecting Storm Sewage The surface run-off resulting after precipitation contributes to the storm water. The quantity of storm water reaching to the sewers or drains is very large as compared with sanitary sewage. The factors affecting the quantity of storm water flow are as below: i. Area of the catchment ii. Slope and shape of the catchment area iii. Porosity of the soil iv. Obstruction in the flow of water as trees, fields, gardens, etc. v. Initial state of catchment area with respect to wetness. vi. Intensity and duration of rainfall vii. Atmospheric temperature and humidity viii. Number and size of ditches present in the area 3
  • 4. Estimation of Quantity of Storm Sewage Rainfall is the primary source of storm flow. Estimation of flow is the first step to design the Storm sewer. Rational Method is widely used Most widely used formula for urban areas Based on rainfall data For very large area empirical formula methods are used i.e. Burkli Zeiglar formula, Fullers formula etc. In both the above methods, the quantity of storm water is considered as function of intensity of rainfall and coefficient of runoff. 4
  • 5. Cont. Rational Method The total volume which fall upon an area A per unit time under a rainfall of intensity i is Q = iA A portion is also lost by evaporation, percolation and ponding The actual amount which appears as run off may then be calculated from Q = CiA Where, C = Run off Coefficient 5
  • 6. Cont. C for an area is not invariant but tends to increase as the rainfall continues. For impervious surfaces C = 1.75 t1/3 or C = t / (8 + t) These depends on duration of rainfall in minutes. For improved pervious surfaces C = 0.3 t / (20 + t) Where t is the duration of the storm in minutes. 6
  • 7. Cont. Average values of C commonly used for various surfaces 7 Sr. # Type of Surface Value of C 1. Watertight roof 0.70 0.95 2. Asphalt cement streets 0.85 0.90 3. Portland Cement Street 0.80 0.95 4. Paved Driveways and Walks 0.75 0.85 5. Gravel Driveways and Walks 0.15 0.30 6. Lawn Sandy soil with slope 2% 0.05 0.10 2-7% 0.10 0.15 >7% 0.15 0.20 Heavy soil with slope 2% 0.13 0.17 2-7% 0.18 0.22 >7% 0.25 0.35
  • 8. Cont. Some engineers use values of C 8 Sr. # Type of Surface Value of C 1. Densely built areas 0.70 0.9 2. Well-built areas 0.5 3. Residential areas 0.25 0.5 4. Suburban section 0.15 0.25
  • 9. Sample Problem Determine the runoff coefficient for an area of 0.2 Km2. Out of this 3000 m2 is covered by buildings, 5000 m2 by paved driveways and walks and 2000 m2 by Portland cement streets. The remaining area is flat, heavy soil, covered by grass lawns. 9
  • 10. Solution Surface Type C Avg C % of Area Runoff C of Area Roof 0.7 0.95 =(0.7+0.95/2) =0.825 =3000/(0.2*10002) =0.015 =0.825*0.015 = 0.012 Driveways & walks 0.75-0.85 0.80 0.025 0.02 Portland Cement Streets 0.8 0.95 0.875 0.01 0.00875 Flat, heavy soil grass lawns 0.13- 0.17 0.15 0.95 0.1425 Overall Runoff Coeff 0.1836 10 Select values of C for each type of area/surface form given values and calculate percentage of land area for each type Total area 0.2km2
  • 11. Cont. Time of Concentration The time required for the maximum runoff rate to develop is known as the time of concentration or It is equal to time taken by a drop of water to run from the most remote point of the drainage area to the point for which the runoff is being estimated. The time of concentration has two parts Time of concentration = Inlet time + time of travel 11
  • 13. Cont. Consider two areas A and B as shown in Figure. I1 is entrance for area A and I2 is entrance for area B and I1 I2 The water flows from A enters the sewer at I1 and that from B at I2. Time of flow is a function of the velocity in the line I1 I2 and its length. The inlet time is time of concentration at I1. 13
  • 14. Cont. The time of concentration at I2 is either the time of concentration for area B or the inlet time plus the time of flow from I1 to I2 , whichever is greater. The time of concentration for each sewer line is calculated in a similar fashion. The time of Concentration will largely depend upon the slope of the ground surface and slope of the sewer. 14
  • 15. Cont. Nomogram is also used to calculate the time of concentration. In nomogram flow distance, type of surface and slope are used to calculate time of concentration. This procedure neglects effect of rainfall intensity but is adequate for most urban drainage project. 15
  • 16. Rainfall intensity 16 Where, a , b and n are constants
  • 18. Problem 1 Determine the runoff coefficient for an area of 0.2 Km2. Out of this 3000 m2 is covered by buildings, 5000 m2 by paved driveways and walks and 2000 m2 by Portland cement streets. The remaining area is flat, heavy soil, covered by grass lawns. If the maximum intensity of rainfall is 40 mm/hour, calculate the quantity of storm water which will reach sewer lines. 18
  • 19. Solution Surface Type C Avg C % of Area Runoff C of Area Roof 0.7 0.95 =(0.7+0.95/2) =0.825 =3000/(0.2*10002) =0.015 =0.825*0.015 = 0.012 Driveways & walks 0.75-0.85 0.80 0.025 0.02 Portland Cement Streets 0.8 0.95 0.875 0.01 0.00875 Flat, heavy soil grass lawns 0.13- 0.17 0.15 0.95 0.1425 Overall Runoff Coeff 0.1836 19 Select values of C for each type of area/surface form given values and calculate percentage of land area for each type Total area 0.2km2
  • 20. Solution 20 i = 40 mm/hr. A = 0.2 km2 C = 0.1836 Plug the values in the following equation Q = C (i )(A) Q =0.1836 (40 )(0.2) = 0.40 m3/s
  • 21. Problem 2 Drainage Area: 80 acres 30% - Rooftops (24 acres) 10% - Streets & Driveways (8 acres) 20% - Lawns @ 5% slope (16 acres) on sandy soil 40% - Woodland (32 acres) Time of Concentration (TC) = 15 min. Location: Tallahassee, Florida (Leon County), Zone 2 Calculate Peak runoff rate from 10-year frequency storm. 21
  • 24. Solution Surface Type C Area Area * C Rooftops 0.9 24 21.6 Streets 0.9 8 7.2 Lawns 0.15 16 2.4 Woodland 0.1 32 3.2 Overall Runoff Coeff 24 Total area 80 Hectares C
  • 25. Solution 25 i = 6.2 in/hr. A = 80 Acres C = 0.43 Plug the values in the following equation Q =C (i )(A) Q =0.43 (6.2 )(80) = 213.28 cfs.

Editor's Notes

  • #4: Factor affecting water consumption are responsible for the rate of sewage generation. Small/Large, Level of development Economic Status, Norms of society, Habits Extreme hot/ cold weather Pricing
  • #6: The portion lost is not constant, and determined for different conditions of temperature, soil moisture, and rainfall duration.
  • #9: Densely built area: walled city of Lahore Well built area: Mall road
  • #12: The period after which the entire catchment area will start contributing to the runoff is called as the time of concentration.
  • #13: The period after which the entire catchment area will start contributing to the runoff is called as the time of concentration.