1. Urban Drainage Design Issues and Codal Provisions
By:
Nishikant Singh,
Scientist – B,
Bureau of Indian Standards,
Hyderabad Branch Office

2. Introduction
Drainage- natural or artificial removal of surface or sub surface water to prevent water logging.
Natural drains- Rivers, streams etc.
Artificial drains- canals, nalas, etc
Problems due to water logging
-health hazards due to water borne diseases like cholera etc.
-adverse effect on agriculture – anaerobic condition

3. Drainage Surface Drainage: occurs in form of runoff above the surface.
Depends upon the location, topography, soil conditions, rainfall pattern, etc.
Sub Surface Drainage: occurs in form of elevated ground water level upto root zone depth.
Depends upon the soil characteristics, soil stratification, position of ground water table, etc.

4. Drainage issues faced
Mumbai Floods

5. Drainage issues faced… contd.
Mumbai Floods

6. Drainage issues faced… contd.
Chennai Floods

7. Drainage issues faced… contd.
Chennai Floods

8. Drainage issues faced… contd.
Hyderabad Floods

9. Drainage issues faced… contd.
Hyderabad Floods

10. Methodology
Carrying out detailed assessment of the existing Drainage System.
Identifying the issues, location of the inundation and possible remedies.
Finalization of the Design Criteria based on the rainfall data and other parameters.
Feasibility of most appropriate alternatives.
Evaluating preliminary capital cost and Operation & Maintenance cost.

11. Assessment of the Existing Drains
Review of existing maps
Details of natural watercourses, ponds, lakes, road side drains and their existing capacity
Detailed topographic survey showing details of major drains, their catchment areas, major outfall points, flow patterns etc.

12. Assessment of the Existing Drains… contd.
Type of drains
Originating point and final destination
Problems in the drain
Flooding
Silting
Bottom and side erosion
Solid Waste and sewage disposal
Encroachment
Existing methods for cleaning and maintenance.

13. Issues associated with drains…

14. Issues associated with drains…

15. Issues associated with drains…

16. Issues associated with drains…

17. List of Issues…
Absence of drainage system or lack of connectivity in conveyance
Physical constraints to flow in the drains viz. encroachments in the drainage channels
Solid waste dumping
Natural and man-made obstructions, including pipe culverts and low level causeways, road and rail over bridges, flyovers, canals etc.

18. Basic Design Parameters
Rainfall Analysis
Time of concentration
Frequency of storm/return period
Runoff coefficient for the project area
Estimation of Runoff

19. Rainfall Analysis Methods
CPHEEO Manual Method (Central Public Health and Environmental Engineering Organisation)
Gumbel’s Distribution Method

20. CPHEEO Manual Method Source: CPHEEO, 1993
The stepped line indicates the location of the storm occurring once in 2 years, i.e., 13 times in 26 years

21. Time intensity values of storms
The time-intensity values for this frequency are obtained by interpolation
Source: CPHEEO, 1993

22. Contd…
The relationship between intensity and time is then expressed by a suitable mathematical formula. As per CPHEEO, the following two types of equations are commonly used.
Where,
i : Intensity of rainfall in mm/hr.
t : Duration of storm in minutes
a, b, n : Constants
The available data on i and t are plotted and the values of the intensity (i) can then be determined for any given time of concentration, (tc).

23. The graph between duration and time intensity is plotted and the trendline of the form (i=a/t^n) is fitted and the values of a and n are found out from the regression analysis.
Duration (t)
Intensity (i)
679.09 5
496.00 10
315 15
166.88 20
110.00 25
95.00 30
51.00 40
12.22 50

24. Time of Concentration
It is the time required for the rain-water to flow over the ground surface from the extreme point of the drainage basin and reach the point under consideration
𝒕𝒄 = 𝒕 + 𝒕𝒇
where, t= inlet time
tf= time of flow in the drain
The duration of rainfall that is equal to the time of concentration is known as the critical rainfall duration.

25. Contd…
Indian Road Congress (IRC- SP 13) has suggested an empirical formula for the determination of time of concentration.
𝒕𝒄 = (𝟎. 𝟖𝟕 ×𝑳^𝟑/𝑯)^𝟎.𝟑𝟖𝟓
Where,
tc : Time of concentration in hours
L : Distance from critical point to the structure in km
H : Fall in level from the critical point to structure in m

26. Storm Frequency
Storm frequency is the number of times a storm of a certain frequency has occurred in the given/ adopted time frame.
The frequency of storm for which the sewers are to be designed depends on the importance of the area to be drained. Commercial and industrial areas have to be subjected to less frequent flooding.

27. Suggested frequency of flooding
S.No.
Type of Area
Storm Frequency 1.
Residential Areas
Peripheral Areas
Twice a year
Central and comparatively high priced area
Once a year 2.
Commercial and high priced areas
Once in two years

28. Runoff Coefficient
The characteristics of the drainage area such as imperviousness, topography including depressions and water pockets, shape of the drainage basin and duration of the precipitation determine the fraction of the total precipitation, which will reach the drain. This fraction is known as the coefficient of runoff.

29. Runoff coefficients for stated surfaces (CPHEEO)
S.No.
Type of Area
Percentage of Imperviousness 1.
Commercial and high priced areas
70-90 2.
Residential Areas
High Density
61-75
Low Density
35-60 3.
Parks and undeveloped areas
10-20

30. Indian Roads Congress has also recommended the following values for runoff coefficients in its special publication on Guidelines for Urban Drainage (IRC –SP 50) which is shown in the table below
S.No.
Type of Area
Coefficient of Runoff 1.
Most densely built up areas
0.7 to 0.9 2.
For adjoining area to built-up areas
0.5 to 0.7 3.
Residential areas
0.25 to 0.5 4.
Sub-urban areas with few buildings
0.10 to 0.25

31. When several different surface types or land use which comprise the drainage area, a composite or weighted average value of the imperviousness runoff coefficient can be computed,
Where,
A1, A2, …, An :Respective sub-drainage area
C1, C2, …, Cn :Respective coefficient of runoff
A : Total drainage area

32. The weighted average runoff coefficients for rectangular areas, of length four times the width as well as for sector shaped areas with varying percentages of impervious surface for different time of concentration is as shown below. Although these are applicable to particular shape areas, they also apply in a general way to the areas, which are usually encountered in practice.

33. Runoff Coefficient for times of concentration
Duration, t minutes 10 20 30 45 60 75 90
100
120
135
150
180
Weighted Avg. Coeff.
1. Sector concentrating in stated time
a. Impervious
0.525
0.588
0.642
0.7
0.74
0.771
0.795
0.813
0.828
0.84
0.85
0.865
b. 60% Impervious
0.365
0.427
0.477
0.531
0.569
0.598
0.662
0.641
0.656
0.67
0.682
0.701
c. 40% impervious
0.285
0.346
0.395
0.446
0.482
0.512
0.535
0.554
0.571
0.585
0.597
0.618
d. Pervious
0.125
0.185
0.23
0.277
0.312
0.33
0.362
0.382
0.399
0.414
0.429
0.454
2. Rectangle (length = 4 x width) concentrating in stated time
0.55
0.648
0.711
0.786
0.808
0.837
0.856
0.869
0.879
0.887
0.892
0.903
b. 50% Impervious
0.35
0.442
0.499
0.551
0.59
0.639
0.657
0.671
0.683
0.694
0.713
c. 30% impervious
0.269
0.36
0.464
0.502
0.53
0.552
0.572
0.601
0.614
0.636
0.149
0.236
0.287
0.334
0.371
0.398
0.422
0.445
0.463
0.479
0.495
0.522

34. Graph based on the table

35. Intensity Duration Frequency Curve
After interpolating and obtaining the relationship between intensity and duration of the storm, a table is formed for the various return periods of the storm. A graph is plotted between the various return periods with intensity on y-axis and duration on x-axis. This graph is the IDF curve.

36. Intensity values in mm/hr Storm frequency having a return period as
Duration in min.
Storm frequency having a return period as
1/2 Year
1 year
2 years
5 years
10 years
25 years
50 years 5 57 84
121
127
142
192
293 10 40 58 89
100
132
196 15 32 47 68 73 82
106
155 20 28 41 59 63 71 91 25 36 52 56 80
116 30 22 33 51
104 35 21 44 54 67 95 19 50 62 88 45 18 27 38 42 17 39 55 77 16 24 43 60 23 70
120 11 29 34
180 9 13 37
240 8 31
300 7 14

38. Estimation of Runoff
Runoff - Portion of the precipitation which drains over the ground surface.
Depends upon
intensity and duration of precipitation,
characteristics of the tributary area and
the time required for such flow to reach the drain
The design of drains begins with an estimate of the rate and volume of surface runoff. When rain falls on a given catchment, a portion of the precipitation is intercepted by the vegetation cover that mostly evaporates, a portion hits the soil and some of it percolates down below and the rest flows over the ground.
The higher the intensity of rain, the higher will be the peak runoff.

39. Contd…
The water flow for this purpose may be determined by using the rational method, hydrograph method, rainfall-runoff correlation studies, digital computer models, inlet method or empirical formulae. The empirical formulae that are available for estimating the storm water runoff can be used only when comparable conditions to those for which the equations were derived initially can be assured

40. Contd…
A rational approach, therefore, demands a study of the existing precipitation data of the area concerned to permit a suitable forecast. Drainage Systems are not designed for the peak flow of rare occurrence such as once in 10 years or more but, it is necessary to provide sufficient capacity to avoid too frequent flooding of the drainage area. There may be some flooding when the precipitation exceeds the design value, which has to be permitted. The frequency of such permissible flooding may vary from place to place, depending on the importance of the area. Though such flooding causes inconvenience, it may have to be accepted once in a while considering the economy effected in storm drainage costs.

41. Contd…
The maximum runoff, which has to be carried in a sewer section should be computed for a condition when the entire basin draining at that point becomes contributory to the flow and the time needed for this is known as the time of concentration (with reference to the concerned section. Thus, for estimating the flow to be carried in the storm sewer, the intensity of rainfall which lasts for the period of time of concentration is the one to be considered contributing to the flow of storm water in the sewer. Of the different methods, the rational method is more commonly used.

42. Rational Method
The rational formula for the relationship between peak runoff and the rainfall is given below.
Q = 10 x C x I x A
where,
Q : Runoff in m3/hr
C : Dimensionless runoff coefficient
I : Intensity of rainfall in mm/hr
A : Area of drainage district in hectares
Note: Q represents only the maximum discharge caused by a particular storm

43. Design Parameters
Velocity: Minimum velocity of 0.6m/s to be adopted to avoid siltation, while maximum velocity to be kept below 3.0m/s to avoid scouring of drain sections
Source: WPCF, ASCE, 1982

44. Design Formula: Since drainage systems are designed as open channel, Manning’s Formula has been widely adopted for gravity flow.

45. where,
Q : Discharge in l/s
S : Slope of hydraulic gradient
D : Internal diameter of pipe line in mm
R : Hydraulic radius in m
V : Velocity in m/s
n : Manning’s coefficient of roughness

46. Manning’s coefficient of roughness n, for different materials
Source: CPHEEO, 1999

47. Different Shapes of drains
a- Rectangular Section
b- Triangular Section
c- Trapezoidal Section
d- Circular Section
e- Parabolic Section
f- Egg Shaped Section

48. Materials for Drainage System
Factors influencing the selection of materials for sewers are flow characteristics, availability in the sizes required including fittings and ease of handling and installation, water tightness and simplicity of assembly, physical strength, resistance to scour, durability and cost including handling and installation.

49. Contd… Brick: used when size of section is large.
could be constructed to any required shape and size
prevent ground water infiltration, it is desirable to plaster the outside surface
Inside plaster can be with mortar using high alumina cement conforming to IS 6452 and the outer surface shall be plastered with mortar using sulphate resistant cement conforming to IS 12330

50. contd… Concrete: Relative ease of construction with required strength.
Shall conform to IS 456
Precast Concrete: speedy construction, easy to cast in shape, better quality control.
Cast-in-situ Reinforced Concrete: more economical, for non-standard sections or special shape is required.

51. contd… Stoneware or Vitrified Clay as per IS 651
Asbestos Cement as per IS 6908
Cast Iron pipes as per IS 1536 and IS 1537
UPVC Pipes as per IS 9271 and IS 15328
HDPE Pipes as per IS 14333

52. Rain Water Harvesting
Collection and storage of rainwater for future needs in surface or sub surface aquifers.
Rainwater from rooftops, paved and unpaved areas, etc. can be collected in recharge pits.
Helps in storing water, reduces surface runoff, ground water recharging, etc.