Question

TPE3601 Assignment 3 Question 1 1.1 Name and discuss the three important aspects of road surface drainage. (8) 1.2Calculate the peak runoff using the rational method with the following values as design input (3) C 0.52; A 2500 m; l = 60 mm per hour 1.3 How can the adverse effects of urban development on downstream runoff be minimized? (2) Name the principle and discuss. 1.4 Explain how the catchment area is determined when calculating surface runoff. (2) 1.5Calculate the time of concentration, with the following design inputs: Hydraulic length 500x0 m The area mostly consists of thick bush Level at the highest point of the catchment 1211 m above sca level. Lovel at the lowest point of the catchment 1026 m above sca level Use the Kirby formula (6) 1.6If the catchment in the above question mostly consistod of moderate grass, would the time of concentration decrease or increase? Explain your answer (2) (6 1.7 Discuss the application of woven and non-woven geofabrics for use in road drainage. 12

Answer 1

Ans 1.1) Three important aspects of road surface drainage are :

a) Cross fall - A cross fall is a type of slope given to road surface in order to remove water immediately away from road surface.The recommended cross fall will depend on type of road material .On straight road cross fall will range between 3 - 5% .On asphalt, recommended cross fall is 3% and for gravel road it is 5%.On a straight road cross fall is applied as crown section. On curves, cross fall is applies in a form of super elevation.

b) Side ditches - Side ditches collects road water and lead it onward to outlet ditches .The longitudinal gradient of side ditch should be at least 5%

c) Outlet ditches - These are the drainage structures that lead the water from the side ditch away from the road area .The water from the outlet ditch is normally discharged to existing waterway system such as river,channel and lakes. It is one of the important part of drainage system because if this is clogged then it can create significant problem in road .It is recommended to provide longitudinal gradient of 4% to the outlet ditch

Ans 1.2) We know, according to rational method :

Q = C i A / 360

where, C = discharge coefficient

i = rainfall intensity (mm/hr)

A= catchment area (hectare)

1 hectare = 10000 m²

=> A = 2500/ 10000 = 0.25 hectare

=> Q = 0.52 x 60 x 0.25

=> Q = 7.8 m³/sec

Ans 1.3) Adverse effects of urban development on downstream runoff can be minimized by reducing velocity and flow of storm water and reducing pollutant discharge with the techniques like Best Management Practice (BMPs) and pollution prevention practices which includes low impact development and green infrastructure.

Ans 1.4) The catchment area is determined with the help of contour map. The watershed lines which indicate the drainage basin of a river pass es through the ridge saddles of a terrain around the river.Therefore , it is always perpendicular to the contour lines . The catchment area contained between the watershed line and river outlet is then measured with the help of planimeter.

Ans 1.5) We know, according to Kerby method,

t_c = 0.8268 (L r / S^0.5) ^0.467  

where, L = water course length ( in ft) = 5000 m or 16404 ft

r = Kerby retardance roughness coefficient = 0.80 for thick bush

S = Slope = (1211 - 1026) / 5000

= 0.037

=> t_c = 0.8268 ( 16404 x 0.80 / 0.037^0.5 ) ^0.467

= 149.57 minutes

or 2.5 hours

Ans 1.6) We know, according to Kerby method,

t_c = 0.8268 (L r / S^0.5) ^0.467  

where, L = water course length ( in ft) = 5000 m or 16404 ft

r = Kerby retardance roughness coefficient = 0.40 for moderate grass

S = Slope = (1211 - 1026) / 5000

= 0.037

=> t_c = 0.8268 ( 16404 x 0.40 / 0.037^0.5 ) ^0.467

= 108.2 minutes

or 1.80 hours

Hence, time of concentration for moderate decreased due to easy flow passage comparative to dense bush

Ans 1.7) Woven geofabrics have high load load carrying capacity and less porosity due to which they cannot be used for drainage purpose but they can help to reduce corrosion . Non-woven geofabrics are used for providing drainage and also used in filter and separation applications.The non woven geofabrics will break down faster as compared to woven geofabrics .