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1. Highway Drainage System and Design CHAPTER 6 1 By: Taye A. (MSc), 2019 Email: [email protected]

2. Introduction 2 One of the most important factors in building a road is drainage. If every other aspect of highway design and construction is done perfectly but the drainage does not work well, the road will fail quickly . So to avoid such failure, proper drainage system should be provided The sources of water may be rain, snow, and/ or ground water . Water on a road always poses potential danger to moving vehicles, due to the longer distances required to stop under wet conditions. The risk of skidding due to friction reduced between tyre and wet carriageway . Highway design uses a combination of surface and subsurface drainage solutions. Subsurface drainage problems often lead directly to pavement failure. .

3. Introduction 3 Highways by their nature cut across natural drainage ways and streams… Adequate and economical drainage is absolutely essential for The protection of the environment Safeguarding the lives of the persons using the highway Problems resulting from poor drainage Standing water on pavements is a danger to traffic safety Seepage water into pavement and subgrade leads to development of soft spots & breakup of the surfacing Streams have the power to destroy roadways by force & holdup traffic by flooding

4. Introduction 4

5. Surface Drainage 5 The measure taken to control the flow of surface water is generally termed as surface drainage. Surface drainage encompasses all means by which surface water is removed from the pavement and right of way of the highway or street. The surface drainage system for rural highways should include. sufficient transverse and longitudinal slopes on both the pavement and shoulder longitudinal channels (side ditches), culverts, and bridges to provide for the discharge of the surface water to the natural waterways.

6. Surface Drainage (cont’d) 6 In urban areas, the surface drainage system also includes. enough longitudinal and transverse slopes, but the longitudinal drains are usually underground pipe drains designed to carry both surface runoff and ground water. Transverse Slopes The main objective for providing slopes in the transverse direction is to facilitate the removal of surface water from the pavement surface in the shortest possible time . This is achieved by crowning the surface at the center of the pavement, thereby providing cross slopes on either side of the centerline or providing a slope in one direction across the pavement width.

7. Surface Drainage (cont’d) 7 Transverse Slopes (cont’d) Table 6.1: Recommended Cross Slope by Surface Type

8. Surface Drainage (cont’d) 8 Longitudinal Slopes A minimum gradient in the longitudinal direction of the highway is required to obtain adequate slope in the longitudinal channels, particularly at cut sections. Slopes in longitudinal channels should generally not be less than 0.2 percent for highways in very flat terrain. Although zero percent grades may be used on uncurbed pavements with adequate cross slopes, a minimum of 0.5 percent is recommended for curbed pavements. This may be reduced to 0.3 percent on suitably crowned high-type pavements constructed on firm ground.

9. Surface Drainage (cont’d) 9 Longitudinal Channels Longitudinal channels (ditches) are constructed along the sides of the highway to collect the surface water that runs off from the pavement surface, subsurface drains, and other areas of the highway right of way . When the highway pavement is located at a lower level than the adjacent ground, such as in cuts, water is prevented from flowing onto the pavement by constructing a longitudinal drain (intercepting drain) at the top of the cut to intercept the water .

10. Side Drainage Design 10 The design of surface drainage system has two phases Hydrologic analysis and Hydraulic analysis Factors to be considered in drainage design include Size of the area to be drained Expected maximum rainfall Slope of the surrounding terrain / possible rate of runoff Characteristics of the soil (permeability, tendency to erode) Presence of springs or other underground water General elevation of the ground water level Minimum depth of cover required to protect pipes from traffic loads

11. Hydrologic Analysis: Rational Method 11 Several methods for estimating runoff are available. Two commonly used methods applicable to available data for Ethiopia are rational method and SCS Synthetic Unit Hydrograph. The Rational Method is most accurate method for estimating the design storm peak runoff for areas up to 50 hectares (0.5 km2). This method, while first introduced in 1889, is still widely used. Even though it has come under frequent criticism for its simplistic approach, no other drainage design method has achieved such widespread use. The rational formula estimates the peak rate of runoff at any location in a catchment area as a function of the catchment area, runoff coefficient, and means rainfall intensity for duration equal to the time of concentration.

12. Hydrologic Analysis: Rational Method 12 The rational formula is expressed as: Q = 0.00278CIA………………………(6-1) Where: Q = maximum rate of runoff, m3/s C = runoff coefficient representing a ratio of runoff to rainfall I = average rainfall intensity for a duration equal to the time of concentration, for a selected return period, mm/sec A = catchment (drainage) area tributary to the design location, in ha.

13. Hydrologic Analysis: Rational Method 13 Runoff Coefficient, C Less frequent, higher intensity storms will require modification of the coefficient because infiltration and other losses have a proportionally smaller effect on runoff. The adjustment of the Rational Method for use with major storms can be made by multiplying a frequency factor Cf . Q = 0.00278CC f IA………………………(6-2)

14. Hydrologic Analysis: Rational Method 14 Runoff coefficient, C

15. Hydrologic Analysis: Rational Method 15 The product of Cf times C shall not exceed 1.0 .

16. Runoff coefficient, C, Rural 16

17. Hydrologic Analysis: Rational Method 17 Rainfall Intensity(I) : is the average rainfall rate in mm/ hr for duration equal to the time of concentration for a selected return period. Once a particular return period has been selected for design and a time of concentration calculated for the catchment area, the rainfall intensity can be determined from Rainfall-Intensity-Duration curves .

18. Hydrologic Analysis: Rational Method 18 Time of concentration, Tc Time of concentration - the time required for water to flow from the most remote point of the basin to the location being analyzed. The time of concentration for a drainage area must be determined in order to select the average rainfall intensity for a selected frequency of occurrence. Initial time – time to drain across the land up to longitudinal drain Time needed to flow on longitudinal drain

19. Hydrologic Analysis: Rational Method 19

20. Basic Hydraulics 20

21. Basic Hydraulics 21

22. Basic Hydraulics 22

23. Basic Hydraulics 23

24. Basic Hydraulics 24

25. Basic Hydraulics 25

26. Basic Hydraulics 26

27. Basic Hydraulics 27

28. Example 28 The drainage area which contributes runoff to a proposed culvert installation is 130hectars in extent. The time concentration is 25minute for the return period of two years. The area has an average slope of about 4%. Assuming a runoff coefficient of 0.25, determine the quantity of flow and design the size of the culvert. [NB: If slope of culvert is not given, use S = 10% as a default value]