Drainage Design for the road and Irrigation systerm

Drainage Design

1. Application of Low Volume Roads Manual APPLICATION OF LOW-VOLUME ROADS MANUAL Lesson 8: Drainage Eng. Lameck Methusela

2. PRESENTATION OUTLINE Application of Low Volume Roads Manual 2 Lesson 8: Drainage Introduction Importance of drainage and the control of water Elements in a drainage system Basic Information to be gathered from site and Office works Low Level Water Crossing drainage structures Culverts Vented Drifts and Causeways Design of Vented Drifts Box Culverts Small Bridges Structure Selection Rational method for estimation of Peak runoff =Q Manning’s Equation – For Sizing of Structures

3. 1. INTRODUCTION Application of Low Volume Roads Manual 3 Lesson 8: Drainage Hydrology and hydraulic analysis the estimation of flood ( rainfall) run-off from a catchment for a specified but rare storm and the design of drainage structures of appropriate capacity . A storm water drainage system collects and conveys run-off generated within a catchment area during and after rainfall events, for safe discharge into a receiving watercourse or the sea.

4. Application of Low Volume Roads Manual 4 Lesson 8: Drainage The magnitude of peak flows depends primarily on the intensity of rainfall and size, topography, soil type, configuration and land use of the catchment. Thus, Thorough information on the planning and investigation required for storm water drainage systems

5. 2. IMPORTANCE OF DRAINAGE Application of Low Volume Roads Manual 5 Lesson 8: Drainage Principal effects of Water to a Road. weakens most road building materials, especially unbound materials including the sub-grade causing the road to deteriorate quickly. erodes materials and transports them to somewhere they are not wanted. When not adequately controlled, water can cause landslides, wash away whole sections of road, generally cause immense damage .

6. 3. ELEMENTS OF A DRAINAGE SYSTEM Application of Low Volume Roads Manual 6 Lesson 8: Drainage 3.1 Transverse Drainage Camber slopes  Gravel roads 4 – 6%, Paved Roads 2.5 – 3% Transverse Drainage

7. Application of Low Volume Roads Manual 7 Lesson 8: Drainage 3.2 Longitudinal Drainage  Grades, Side Drains (Earth, Lined), Scour Checks, Catchwater Drains and Mitre Drains Longitudinal drainage

8. Application of Low Volume Roads Manual 8 Lesson 8: Drainage 3.3 Drainage Structures  Low level water crossings,  Culverts , Vented Drifts  Bridges

9. A low level water crossing is simply one that is designed to be over-topped.  Drift The ability to be over-topped with little risk of failure. 4 LOW LEVEL WATER CROSSING Application of Low Volume Roads Manual 9 Lesson 8: Drainage A drift consists of a flat slab and two inclined approach ramps. Cheapest form of watercourse crossing.

10. Key Features of Drifts Transfer water across a road without erosion of the road surface. Useful in areas that are normally dry with occasional heavy rain causing short periods of floodwater flow. Provide a cost effective method for crossing wide rivers which are dry for the majority of the year or have very slow or low permanent flows. Easier to maintain Act as traffic calming measures. Suited to areas where material is difficult to excavate, thus making culverts difficult to construct. Suited in flat areas where culverts cannot be buried because of lack of gradient. Application of Low Volume Roads Manual 10 Lesson 8: Drainage

11. 5 CULVERTS Culverts are usually constructed in narrow well defined water courses but they can also have many apertures in order to cross wide and shallow water courses. Culverts perform two basic functions. Relief Culverts and Stream Culverts Application of Low Volume Roads Manual 11 Lesson 8: Drainage

12. Can vary in number from about two per kilometre in dry and gently rolling terrain up to six or more for hilly or mountainous terrain with high rainfall. In addition to well-defined water crossing points, culverts should normally be located at low points or dips in the road alignment. Relief culverts may be required at intermediate points where a side drain carries water for more than about 200 m without a mitre drain or other outlet. Aprons with buried cut off walls are also required at the inlet and outlet to prevent water seepage The gradient of the culvert invert should be between 2 and 5% . Shallower gradients could results in silting whereas steeper gradients result in scour at the outlet because of high water velocity. Application of Low Volume Roads Manual 12 Lesson 8: Drainage Key Features of Culverts

13. 6 VENTED DRIFTS  A combination of a culvert and a drift.  They are suitable for carrying roads across water courses which have a perennial (permanent) water flow for most of the year and which have large flows for less than three days after heavy rains. Application of Low Volume Roads Manual 13 Lesson 8: Drainage

14. Designed to pass the normal dry weather flow of the river through pipes below the road. Occasional larger floods pass through the pipes and over the road, which may make the road impassable for short periods of time. The level of the road on the vented drift should be high enough to prevent overtopping except at times of peak flows. There should be sufficient pipes to accommodate standard flows. A vented drift should be built across the whole width of the water- course. A vented drift requires approach ramps which must be surfaced with a non-erodible material and extend above the maximum flood level. Application of Low Volume Roads Manual 14 Lesson 8: Drainage Key Features of Culverts

15. 7 SMALL BRIDGES  The manual covers small simply supported bridges (max. span 10m).  For bridges with spans more than 10m refer to the Bridge Management System for Tanzania, Handbook for Bridge Inventory, Ministry of Infrastructure Development (2007) and Overseas Road Note 9 (TRL, 1992) Application of Low Volume Roads Manual 15 Lesson 8: Drainage

16. Elements of a simply supported deck bridge include a superstructure (comprising deck, parapets, guide stones and other road furniture) and substructure (comprising abutments, wing walls, foundations, piers and cut off walls). Bridges are generally the most expensive type of road structure, requiring specialist engineering advice and technically approved designs. The main structure is always above flood level, so the road will always be passable. Bridges require carefully designed foundations to ensure that the supports do not settle or become eroded by the water flow. On softer ground this may require piled foundations which are not covered in this Manual. Application of Low Volume Roads Manual 16 Lesson 8: Drainage Key Features of Bridges

17. Should not significantly affect the flow of water (i.e. the openings must be large enough to prevent water backing up and flooding or over topping the bridge). Embankments behind the abutment must be carefully compacted to prevent soil settlement which would result in a drop off between the bridge deck and the road surface at the end of the bridge. Application of Low Volume Roads Manual 17 Lesson 8: Drainage Key Features of Bridges

18. Group Exercise Discuss Advantages and Disadvantages of Drifts Culverts Vented Drifts Bridges Application of Low Volume Roads Manual 18 Lesson 8: Drainage

19. 8 PEAK FLOW ESTIMATIONS  The storm that provides maximum flow for which a drainage structure is designed The risk of a severe storm occurring is defined by the statistical concept of its likely return period. (2, 5, 10, 15, 20, 25, 50 or 100 years) Application of Low Volume Roads Manual 19 Lesson 8: Drainage 8.1 Design Storm

20. 8 PEAK FLOW ESTIMATIONS Application of Low Volume Roads Manual 20 Lesson 8: Drainage 8.2 Return Period for different structures Type of Structure DC5 DC6 DC7 DC8 Side Ditches 10 5 5 2 Stone Drifts 10 5 5 2 Drift or Vented Drift 10 5 5 2 Culvert dia. <2m 15 10 10 5 Culvert dia. >2m 25 15 10 5 Gabion abutment bridge 25 20 15 - Short span bridge <10m 25 25 15 - Masonry arch bridge 50 25 25 - Medium span bridge (15 – 50m) 50 50 25 Long span bridge (>50m) 100 100 50

21. 8 PEAK FLOW ESTIMATIONS Application of Low Volume Roads Manual 21 Lesson 8: Drainage 8.3 Design for Climate Resilience To cater for effects of unpredictable climate change, (higher temperatures, higher rainfall, more intense storms, more frequent storms), advised to design structures based on estimates of storm characteristics with higher return periods (i.e. design for severe risk situations)

22. 8 PEAK FLOW ESTIMATIONS Application of Low Volume Roads Manual 22 Lesson 8: Drainage 8.4 Rational Method of Flow Estimation The flow of water in a channel, Q, is given by: Q = 0.278 x C x I x A (m 3 /s) C = Catchment run-off coefficient I = Intensity of the rainfall (mm/h) for the time of concentration A = Catchment area (km 2 )

23. 8 PEAK FLOW ESTIMATIONS Application of Low Volume Roads Manual 23 Lesson 8: Drainage 8.4 Rational Method of Flow Estimation Run-off coefficient C = C T + C S + C V C T (Slope-topography) C S (Soils) C V (Vegetation) Very Flat (<1%) 0.03 Sand and gravel 0.04 Forest 0.04 Undulating (1-5%) 0.08 Sandy Clays 0.08 Farmland 0.11 Hilly (5-10%) 0.16 Clay and Loam 0.16 Grassland 0.21 Mountainous (>10%) 0.26 Sheet rock 0.26 No vegetation 0.28

24. Application of Low Volume Roads Manual 24 Lesson 8: Drainage Worked Example Problem: Calculate the Peak Flow Q for the rainfall intensity of 8mm/h given the following catchment properties: Catchment area: 4.6km2 Land Slope: 2.7% Soil type: Sand and gravel Vegetation: No vegetation

25. Application of Low Volume Roads Manual 25 Lesson 8: Drainage Worked Example Solution: Step 1: Calculate the catchment run-off coefficient: From LVRM Table 11-4, For a slope of 2.7%, the Land slope coefficient C T = For Sand and Gravel, the Soil Type coefficient C S = For catchment with no vegetation, the C V = Catchment run-off coefficient, C = C T + C S + C V =

26. Application of Low Volume Roads Manual 26 Lesson 8: Drainage Worked Example Solution: Step 2: Calculate the Peak flow: From LVRM Equation 11-1, Peak Flow, Q, is given by Q = 0.278 x C x I x A = 0.278 x 0.40 x 8 x 4.6 = 4.09 m 3 /s Note: If the calculated peak flow is >30m 3 /s , the problems is beyond the scope of LVRM.

27. 9 MANNING’S EQUATION Application of Low Volume Roads Manual 27 Lesson 8: Drainage The cross section area of a drain or channel must be sufficient to accommodate the expected flow of water Q Q = A.V The Flow velocity is calculated from Manning Equation V=1/nR 2/3 S 1/2 V = average velocity (m/s) A = Cross sectional area of water (m2) R = Hydraulic Depth (m) S = Slope of the Drain n = roughness coefficient Q = Discharge volume flow rate

28. 9 MANNING’S EQUATION Application of Low Volume Roads Manual 28 Lesson 8: Drainage Definition of Hydraulic Depth

29. 9 MANNINGS EQUATION Application of Low Volume Roads Manual 29 Lesson 8: Drainage Roughness Coefficients (n), LVRM Table 11-5 Material in drain Coefficient Sand, loam, fine gravel, volcanic ash 0.022 Stiff clay 0.020 Coarse gravel 0.025 Conglomerate, hard shale, soft rock 0.040 Hard rock 0.040 Masonry 0.025 Concrete 0.017

30. Application of Low Volume Roads Manual 30 Lesson 8: Drainage Exercise Problem: Determine whether the following concrete lined drain can accommodate a peak flow of 4.09m 3 /s, assuming the slope of the drain is 3% and required free board is 0.3m 0.6 0.6 0.6 0.6m

31. Application of Low Volume Roads Manual 31 Lesson 8: Drainage Solution: Step 1: Cross-sectional area of water, A 0.6 0.6 0.6 0.6m Step 2 : Hydraulic Depth, R Step 3: From Table 11-5 determine roughness coefficient, n, for concrete

32. Application of Low Volume Roads Manual 32 Lesson 8: Drainage Step 4 : Determine Channel Slope, S From given problem, the channel slope is 3% S= Step 5 : Calculate the discharge volume, Q Step 6: Compare Peak Flow, Q max and Discharge Volume, Q and comment on capacity of the drain, propose new size if necessary Q = A.V = A(1/nR 2/3 S 1/2 )

33. Application of Low Volume Roads Manual 33 Lesson 8: Drainage Group Exercise Prepare an excel spread sheet to automate the Rational Method formula for calculation of Peak Flows and the Manning’s Equation for estimation of size of drains and channels

34. Application of Low Volume Roads Manual 34 Thank you! Lesson 8: Drainage