Section 5 Complete
Section 5 Complete
Section 5 Complete
TABLE OF CONTENTS
FIGURES
Figure 5.1: A Typical Straight Drop Structure 5-4
TABLES
Table 5.1: Advantages and Disadvantages of Bridges 5-2
Table 5.2: Advantages and Disadvantages of Culverts 5-2
Waterway structures are comprised of bridges, culverts or flood ways, or a combination of these structures.
Floodway is an especially constructed low-level section of the road to convey floodwater where traffic
volumes are low.
This Chapter provides basic information on factors which should be considered for designing a waterway
structure. The factors discussed here relate to the environmental and design considerations.
Quality of the environment receives prime importance in current engineering practices all over the world.
Selection of a location for river crossing, its design and construction methods should aim at minimal adverse
impact on historical, archaeological, scenic, natural, recreational and social values and all resources of the
project area. A comprehensive environmental impact assessment should be an important element of the
selection criteria adopted for a river crossing. Following main steps can ensure proper coverage of
environmental issues:
Some factors require an expert assessment of their impact on the environment, warranting an
interdisciplinary investigation. The following description presents specific considerations for the site selection
for a river crossing.
Impact of bridge construction is generally short lived. In some cases the construction activities may produce
long-term damaging effects. Possible impacts of construction should be assessed and suitable control
measures adopted. Such measure include control of erosion and pollution at the construction site and
minimising possible impact on water supplies. The landscape should be restored after completion of
construction to eliminate source of long-term adverse impacts.
Choice between a bridge or a culvert as the waterway structure is quite clear in most cases and above
factors help in making this choice. Table 5.1 and Table 5.2 present advantages and disadvantages of the two
waterway structures in generally encountered conditions.
Short reaches of the stream can be slightly modified to suit the proposed waterway design. The controlling
criteria should be a minimal change in its hydraulic performance. This can be satisfactorily done by computer
modelling (Section 5.3.4) of the river reach with and without the bridge and compare the basic parameters of
flow velocity, flow depth and shear stress. Excessive increases in the flow velocity and shear stress along
the modelled reach represent potential for bank and bed instability as well as undesirable impact on the
stream ecosystem. Channel modifications must be avoided in such cases unless there is not practical
alternative.
HEC-RAS is an integrated package of hydraulic analysis programs, in which the user interacts with the
system through the use of a Graphical User Interface (GUI). The system is capable of performing Steady
Flow water surface profile calculations as well as Unsteady Flow computations. Sediment Transport, and
several hydraulic design computations are expected to form part of this model in the future.
5.4 References
Austroads-Australia (1994),“Waterway Design- A Guide to the Hydraulic Design of Bridges, Culverts and
Floodways”, AUSTROADS National Office, Sydney.
NAASRA (989), “Bridge Water Ways – Hydrology and Design”, National Association of Australian State
Road Authorities Working Group, Sydney.