Ethiopian Roads Authority Drainage Design Manual

Summary

In order to a establish a design rainfall and flow estimation methodology compatible to the Ethiopian environment and facilitate standard design practices in all road design works in Ethiopia, the Ethiopian Road Authority (ERA) had commissioned Louis Berger Group to prepare a road drainage design manual in 2002 under a credit from the International Development Agency (IDA).

This technical note has been prepared for Ethiopian Roads Authority (ERA) as a result of a brief review of the above manual published in 2002.

The brief received from ERA required me to review the drainage design manual referred above and make comments and recommendations if any.

In preparing this technical note, I have read the ERA drainage design manual-and various road drainage design manuals produced by other countries including USA, UK, Australia, Gahanna, South Africa, Uganda and Kenya.

Clearly, in the time available, I have not been able to carry out a full technical review of the manual. However, I have used my local knowledge and past experience to review the manual with critical eye, with the aim of identifying information and technology gaps, any uncertainties, misconceptions or missed opportunities.

During the technical review it was revealed that the ERA drainage design manual contains most of the necessary relevant technical information which is required for design of road drainage structures in Ethiopia. The design methodologies had been adopted from the American Association of State Highway and Transportation Officials (ASSHTO) and the design charts were adopted from the Federal Highway Administration of the Bureau of Reclamation Hydraulic Laboratory.

Road Development in Ethiopia has vastly accelerated in recent ...

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...of tools available for the prediction of both rainfall runoff volume and peak flow rate from both rural and urban catchments.

However it should be noted that no method is considered to be “right”, while the rest are “wrong”. However to avoid confusion a preferred method is recommended as good practice. It is important to stress that using more than one method can provide additional information on the decisions to be made. This can

be particularly useful in situations where the consequences of “failure” of a drainage structure would be particularly serious (e.g. culverts and bridges).

Comments and recommendations made in this technical review note are as a result of careful screening of many hydraulic and hydrological modeling methodologies, hydrological and hydraulic modeling software programs and design procedures available to the drainage engineer today.