Importance of Drainage on Roads
Drainage in roads is very important because of the following reasons:
i. The speedy removal of surface water which implies safety and fewer nuisances.
ii. Effective sub-surface drainage which implies durable pavement and earthworks.
iii. Minimization of the impact of runoff on the recovery environment.
iv. Minimization of the impact of runoff on the receiving environment.
The major function of a drainage system is to allow water to flow to suitable points of discharge that may include:
i. Adjacent land via infiltration (rural areas).
ii. Water course e.g. river.
iii. Ponds (usually called balancing ponds).
iv. Surface water sewer or combined sewer (surface + foul) treatment facility.
The components of the drainage system include:
i. The collector e.g. gullies, linear drain, adjacent land, ditch, etc.
ii. The transporter e.g. pipe, linear drain, ditch, etc.
iii. Suitable receptor e.g. adjacent land, watercourse, pond, surface water sewer or combined sewer, SUDs systems.
Information for Design of Drainage
Catchment Area (A)
A catchment area is an entire surface area that will discharge its stormwater to one point known as the discharge point. Before the design of a drainage system, it is necessary to determine the catchment area that the drainage system will drain and the amount of stormwater this catchment area will produce. The pipe system includes the impervious area contributing to each pipe length and the gradient of each pipe length. This is necessary so that an adequate-sized drainage system can be designed to effectively remove the water.
Determining the catchment area requires obtaining a map of the catchment area with gradient lines or a study of the catchment area from which it is possible to calculate its gradients and boundaries.
Contour lines on the map can help to determine the catchment area because water normally flows from the high point to the low point. When the catchment area is determined, the surface should be calculated by transferring the contours of the catchment area on paper with gridlines and counting the grids. Catchment area tools in software such as Autodesk Civil 3D can be used to estimate catchment areas automatically. If manual calculation is used, the next step is to identify the catchment area’s average gradient, which would enable the determination of the runoff coefficient.
Runoff Coefficient (C)
After determining the average gradient, it is also necessary to decide on the surface of the terrain to enable the determination of the runoff coefficient. The runoff coefficient is that part of rainwater or rainfall that becomes stormwater (water that drains off a land area from rainfall). The coefficient may be regarded as the combination of two separate coefficients, C = CvCR
Where:
Cv = the volumetric runoff coefficient
CR = dimensionless routing coefficient
The value of Cv is an average of 0.75 and ranges from 0.6 on catchment with rapidly draining soils to about 0.9 on catchment with heavy soils.
CR has a constant value of 1.30.
A runoff coefficient of 0.65 means that 65% of the rainwater will turn into stormwater. The runoff coefficient generally depends on the type of terrain and its slope. However, in determining this value, the anticipation of future changes in the terrain must be made to ensure that the drainage system does not have problems in the future.
Table 1 shows approximate values of the runoff coefficient assuming low permeability.
Table 1: Approximate values of runoff coefficient (applicable where it is not possible to determine the exact value of runoff coefficient of a catchment area)
| Terrain Type | Runoff Coefficient | |
| Gradient < 0.05 (flat terrain) | Gradient > 0.05 (steep terrain) | |
| Forests and pastures | 0.4 | 0.6 |
| Cultivated land | 0.6 | 0.8 |
| Residential areas and light industry | 0.7 | 0.8 |
| Dense construction and heavy industry | 1.0 | 1.0 |
Rainfall Intensity of the Catchment Area
Intensity-duration-frequency (IDF) curves which show the intensity of rainfall (mm/hour) against the duration of rains (minutes) for given return periods are used to determine the rainfall intensity. Return periods for drainage system designs are usually in years and they represent an estimate of how long it will take between rainfall events of a given magnitude. A curve with a return period of 1 year shows the worst storm that will occur on average every year. A return period of 2 years is the worst storm that can be expected in a 2-year period.
The intensity of rainfall data can be obtained from the IDF curve with the knowledge of the time of concentration, which represents the time water needs to flow from the furthest point of the catchment to the discharge point.
Mathematically,
Time of concentration,
Where:
tc = time of concentration (minutes)
lmax. = the maximum length of flow in the catchment area (m)
save. = average gradient of the catchment area
When the time of concentration is known, the suitable curve with an appropriate return period is chosen from which the intensity of rainfall can be obtained based on the duration of rainfall that equals the time of concentration.
For pipe drainage system, the expressions below may also be considered
Time of concentration tc = te + tf
Where,
te = time of entry
tf = time of flow through the pipe system to the point under consideration
| Return period | Time of entry |
| 5 years | 3-6 |
| 2 years | 4-7 |
| 1 year | 4-8 |
| 1 month | 5-10 |
For each return period, the larger times of entry are applicable to large, flat sub-catchments (area greater than 400 m2, slope less than 1 in 50) and smaller values to small, steep sub-catchments (areas less than 200 m2 slope greater than 1 in 30). These values of area and slope refer to the sub-catchments contributing to each pipe length.
The time of flow, tf, through the pipe system may be determined from the pipe full velocity given in suitable design tables. This velocity gives a good approximation to the actual water velocity at all depths likely to occur under design conditions.
Where the IDF curve is not available, the rainfall intensity of 100 mm/hr may be assumed (the value is typical for tropical countries with catchment areas less than 150 ha).
Rainfall intensity can also be determined by hand calculation using the formula below:
Where:
i = rainfall intensity (mm/hr)
N = specified return period (years)
T = critical storm duration (minutes)
2minM5 = the rainfall depth in mm occurring at the site in a period of 2 minutes with an average return period of 5 years.
Design Stormwater
The stormwater that would give maximum runoff at the various location of the catchment can be determined using the modified rational method outlined below:
Where:
Qp = the peak discharge which is the maximum flow of stormwater that the system would be designed for.
C = the runoff coefficient (dimensionless constant that is calculated as described above or obtained from Table 1)
i = average rainfall intensity during the time of concentration
A = contributing catchment area
If Qp, I, and A are expressed in l/s, mm/hr, and ha respectively. The eqn (3) above becomes,
Steps in the Design
Step 1: Sizing of Drainage-peculiar to all drainages
After determining the necessary parameters, the next step is to size the drainage system channel. This can be determined using the formula below:
This test can be repeated a number of times to determine the adequate size of the drain. After determining the suitable size of the drain, some allowances have to be made to ensure that the drain is not completely filled and to take care of the incidence of deposited solids and lack of maintenance which would reduce the efficiency of the drainage system.
The processes above apply to all kinds of drainage systems: surface rectangular/trapezoidal drains and sub-surface pipe drains.
In an example given below, processes for the design of a typical pipe drainage system will be presented:
Steps in the Design of a Typical Pipe Drainage System
A pipe drainage system consists of pipes, gullies, manholes, and inspection chambers. These are considered in the design of pipe drainage systems.
Step 2: Determination of the Gully and Manhole Spacing
Gully spacing: this is based on the longitudinal gradient, cross-fall, and areas drained. Gullys should be provided at:
i. Low spots – on vertical alignment
ii. Intervals such that the flow does not come out too far from the kerb (typically 600 mm)
iii. Intervals such as the flow gets taken in by the gully and water does not flow over or by the gully.
Gully spacing can be determined using the expression below:
m = maintenance factor and has a value of 1 if there is no blockage in the gully
We = effective width of the catchment (m)
Q = peak discharge
n = Manning’s constant
l = lmax
Otherwise, use the area drained by each gully as provided in Table 2 to calculate gully spacing using the expression:
Where,
w = carriageway width
f = footway width
Sp = gully spacing
N.B: Maximum gully spacing should be 40 m. If the calculated value is less than 40 m, use the value. If the calculated value is more than 40 m, set the value to 40 m.
Manhole spacing: there should be a manhole at each change of horizontal alignment, at each change of vertical alignment, and at not more than about 100 m intervals for maintenance purposes. Manhole spacing should be chosen by considering the maximum length of rodding which is 100 m. Choose a manhole spacing between 50 m and 100 m.
Step 3: Establishment of Preliminary Layout
With the information on manhole spacing and gully spacing, establish a layout showing the pipes, manhole positions, and gully positions (see Figure 1).
Step 4: Iteration for the Suitable Pipe Diameter
A calculation sheet is designed to be used iteratively to obtain suitable pipe diameters for drainage (see table below).
The pipe contains the following information:
1. Pipe number: each pipe network is given a reference system of branch numbers and pipe numbers. These can be obtained from pipe manufacturers.
2. Pipe length (m): obtained from the pipe data in (1).
3. Pipe gradient: obtained from the pipe data in (1) and may be adjusted if necessary.
4. Assumed diameter (mm or m): the initial diameter should be assumed to be the minimum permitted diameter. Further downstream, an initial diameter equal to the diameter of the largest oncoming pipe at the upstream manhole is assumed.
5. Pipe full velocity (m/s): the pipe full velocity is obtained from the pipe data in (1).
7. Time of concentration (min): information on this has been previously described
8. Rainfall intensity (mm/hr): information on this has been previously described
9. Impervious area (ha): information on this has been previously described
10. Cumulative impervious area (ha): information on this has been previously described. Additional area is added at the upstream end of the pipe length to which it contributes.
11. Calculated discharge (l/s): information on this has been previously described
12. Dry weather flow (l/s): the data which can be determined from local information or standard calculations can be added to the calculated discharge if it is available.
13. Total discharge (l/s): the sum of calculated discharge and dry weather flow.
14. Required diameter (mm or m): the smallest diameter of pipe that would convey the peak discharge is determined from the design tables. If the diameter is less than or equal to the originally assumed diameter of the pipe, the diameter given by the design tables is accepted (if permitted by local restrictions on minimum diameter and non-decreasing diameter), otherwise, repeat steps 1 to 13 with larger assumed diameter.
Note: for pipes downstream of a junction, the cumulative time of flow should be determined along the branch with the longest time of concentration. In exceptional circumstances, this may lead to a lower calculated discharge than that from a major branch entering the upstream junction. In these circumstances, the design discharge should not be reduced below the largest value entering the junction. This check may have to be repeated for several subsequent pipe lengths downstream.
References
The Wallingford Procedure: Design and Analysis of Urban Storm Drainage. Department of the Environment, National Water Council, Standing Technical Committee Reports No 31.
