Page 176 - RoadCem Manual - PCT BV

6.4.2 Method
The capacity of drainage structures should ideally be calculated on the basis of local experience gathered over a long period of time and should be updated to cater for any recent changes in rainfall pattern and climate. Such information is often not readily available in many countries, prompting a need to develop standards for drainage design and calculations. In all cases it is advisable to combine calculations with observations on site, in addition to information from reliable local sources.
With the ever-increasing cost of maintenance, it is desirable to increase the size of drainage structures to a minimum of a 600 mm opening so that they can be easily maintained.
6.4.3 Return Period
The return period for a given flow of water is related to the estimated statistical risk of overtopping of drainage structures. It is part of the hydraulic calculations required for each type of structure and for each project on the basis of policy and anticipated consequences to the road or the public. The return period is therefore a critical parameter in the design of roads because it controls the level of risk in relation to cost of construction and the type of structure that is appropriate. As a broad guide, the following return periods can be considered for roads:
• Bridges: 20 - 50 years.
• Culverts: 5 - 10 years.
• Drifts or well-protected culverts: 0 - 5 years.
6.5 Drainage Structures 6.5.1 Introduction
This section gives some examples of the range of solutions available to designers and constructors of roads. The techniques shown are the results of innovative methods tried out and applied in many countries over a number of years. A basic requirement in the construction of structures for crossing water courses is to assess the need for protection of the structure against erosion. During the construction period as well as assessing the risk that structures that are not designed to withstand flooding will actually experience overtopping, so that additional protection measures can be taken. Construction of low, “sacrificial” points for overtopping should be considered where available resources do not allow for the provision of structures with adequate capacity.
6.5.2 Low-level structures
A low-level structure is designed to accept overtopping without damage, and is ideally suited for roads in locations where less than full all-weather passability is acceptable to the community. The two basic types have been used with success. Various alternative names are sometimes used to describe these structures.
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