Surface drainage system is most important in Highway engineering. A pavement without proper drainage facilities will not serve for long time. The water or rainfall on road should be collected by side drains which carries the drain water to nearest stream or any water course.
So, prior to the construction of road, the designer should leave required space for providing proper drainage facilities as well as the pavement should also be constructed with minimum camber.
|Fig 1: Surface drainage system on a highway|
Design of Surface Drainage System for Highway
The design of surface drainage system carried by two types of analysis:
- Hydrologic analysis
- Hydraulic analysis
Hydrologic Analysis of Drainage for Highway
Whenever there is a rainfall, some of the rain water infiltrated into the ground and stored as ground water and some of the portion may evaporate into the atmosphere. Other than these losses, the water left on the surface is called as run off.
The method of estimating the run off is called hydrologic analysis. To estimate the maximum quantity of water expected to reach the drainage system is the main objective of hydrologic analysis. For this, one need to know the factors affecting run off and they are:
- Rate of rain fall
- Moisture condition
- Soil type
- Ground cover presence
Q = C i AdWhere;
Q = run off (m3/sec)
C = run off coefficient
i = intensity of rain fall (mm/sec)
Ad = area of drainage (m2)
|Fig 2: Pavement Drainage Design|
|Type of Surface||Coefficient of run off|
|Pervious soil surface||0.05 – 0.30|
|Soil covered with turf||0.30 – 0.55|
|Impervious soil||0.40 – 0.65|
|Gravel & WBM roads||0.35 – 0.70|
|Bituminous & C.C roads||0.80 – 0.90|
Fig 3: Types of Surfaces & their Coefficients
C = (A1C1+A2C2+A3C3) / (A1+A2+A3 )Where C1, C2, C3 are run off coefficients for different surfaces and A1, A2, A3 are their respective areas.
In the Next stage, Intensity of rainfall “i” is to be calculated. To find this, first we need to know the time taken by water to reach drainage inlet from the drainage area. This can be found out from the below graph. This is called as inlet time.
|Fig 4: Rainfall Time-period|
|Fig 5: Rainfall Intensity|
|Fig 6: Hydrologic Analysis of Drainage for Highway|
Hydraulic Analysis of Highway Drains
Now comes the second stage hydraulic analysis, in which the dimensions of drainage channels or culverts are designed based on “Q” obtained in the above stage of analysis. Now we have discharge which is designed run off “Q”.
If we know the allowable velocity “V” in the channel, then the area of channel can be calculated from below formula:
Q = A.VBut the allowable velocity is not same for all types of channels. If the channel is lined, then the allowable velocity can be kept at normal. But if the channel is unlined it may cause severe damage to the channel in the form of silting or scouring.
So, the allowable velocity for different cases of unlined materials is as follows:
|Soil type||Allowable velocity (m/sec)|
|Sand or silt||0.30 – 0.50|
|Loam||0.60 – 0.90|
|Clay||0.90 – 1.50|
|Gravel||1.20 – 1.50|
|Soil with grass||1.50 – 1.80|
Fig 7: Soil Types & their Velocities
|Fig 8: Hydraulic Analysis of Highway Drains|
V = Allowable velocity (m/sec)
N = Manning’s roughness coefficient
R = Hydraulic radius (m)
S= Longitudinal slope of channel
In the above formula, we already know the “V” value. Hydraulic radius “R” is the ratio of area of the channel to its wetted perimeter. Now comes, the roughness coefficient which is again varies according to lining material as follows:
|Lining material||Manning’s roughness coefficient, n|
|Soil with grass layer||0.05 – 0.10|
Fig 9: Rough Coefficients
Finally, longitudinal slope “S” is known and all the dimensions of drainage channel are known. Thus, the design of surface drainage system is complete. This method is mostly used for designing side drains of roads.