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23 March 2013 Written by 

Addis Ababa Light Rail Transit System (LRT) flood risk issues and recommendations

Dr Manaye Ewunetu

This article is a sample schematic layout of LRT and urban motorway with its drainage arrangements.

One of the most important issues of the trackform design for the Addis Ababa LRT project is the drainage system. To ensure that LRT operations remain reliable and sustainable throughout the life of the project, close coordination in the detailed design phase of the project between alignment design, civil engineering, hydrology/drainage and geotechnical specialists within the Client (Ethiopian Railways Corporation (ERC)/Addis Ababa City Roads Authority (AACRA)) and its International Contractor/Consultant design groups is essential to ensure that site specific drainage design requirements are robustly modelled and developed to ensure that flood risk to the LRT network  during the Addis Ababa’s rainy season is avoided or minimised.

The LRT drainage design assessment should also take into consideration areas adjacent to the railway, where elements such as street surfacing type, parking facilities, roads, landscapes, wells etc exist. All have an impact on the drainage design of the railway area. LRT drainage design shall be based at least in part, on a comprehensive geotechnical engineering analysis. It is also imperative that investigation of historical flood levels along the Addis Ababa LRT network is carried out to ensure that an accurate assessment of the threshold flood level is undertaken.

Also, it is essential that for any major project such as the Addis Ababa LRT, a standard flood risk assessment report, supported by hydrological and hydraulic modelling, is produced as part of the system-wide drainage design by the Contractor and checked/approved by the Client, in this case Ethiopian Railways Corporation (ERC) and Addis Ababa City Roads Authority (AACRA), prior to commencement of works.

From information in the public domain for the LRT, some sections of the tunnel will be constructed for the project, and in a situation where there is no recorded flood level at the tunnel entrances (or portals, as they are called in civil engineering technology,) as will be the case for the LRT, it is too difficult, and indeed inappropriate to estimate the threshold flood level and design a robust tunnel flood protection system for such a complex project without drainage and flood risk modelling. A number of current technology computer software application packages are readily available for use in this aspect of the design to minimise flood risk. 

As the Addis Ababa city’s storm drainage system, into which the LRT drainage system is designed to discharge, is currently in a state of despair, for a reliable and uninterrupted operation of the tunnel, high capacity pumps and corresponding large volume sumps, combined with a correctly designed robust storm drainage system are required. This will require development and implementation of a comprehensive operation and maintenance regimes for the design life of the project, and it will be costly.

This should not be confused between the local drainage and the watercourse systems that unpredicted intense rainfall in urban areas could cause flooding, which may put the tunnel at risk of flooding.

The land use of Addis is constantly changing due to regeneration activities and new infrastructure developments which will change the nature of the surface area that will have a profound impact on the water run-off characteristic and volumes, which consequently make the design flood prediction significantly more challenging in Addis.  Surface water run off also increases with growth, creep, deterioration of existing storm drainage systems and climate change.
The traditional design approach is to install large diameter sewers and storage tanks to accommodate storm water, which very often generates deluges of surface water which we are all familiar with during our rainy seasons, but these may not cope with a changing environment and therefore may not be appropriate for the AA LRT.  Modern computerised systems and software packages such as ‘Integrated Urban Drainage Modelling’ allow a more calculated and definitive approach to drainage design and flood mitigation. Any drainage design strategy that does not take the future land use and climate change effect into account is likely to have disastrous consequences at some point during the design life of the project. Therefore, both client and contractor must ensure that robust and sustainable drainage systems are developed and delivered, as part of the network wide flood risk management strategy for the AALRT project.

 Consequently, it is recommended that, and indeed considered highly essential, flood risk from extreme event assessments for the LRT project are supported by industry standard software and the finished design platform level of the railroad to be designed above the 100 year plus 20 percent allowance for climate change, plus 0.3 metres “freeboard” (or the safe distance above the worst case modelled flood level) in order to make the railway immune to flooding during extreme storm events. Ignoring the above design principles will expose the Ethiopian Government and its Agent ERC to operational delays and very expensive maintenance cost throughout the design life of the project.

The LRT network will incorporate sections of “at grade” ballasted track and embedded rail for street running and level crossing (where the LRT intersects at highway) sections of track, the drainage designs for which are again very complex and require both a robust design and maintenance regimes to be put in place to ensure undisrupted operation and sustainable reliability. Arrangements for elevated sections are less onerous, but outfall volumes will bridge support positions and robust design and construction techniques are again essential.     
It is recommended that the Government’s Client (ERC/AACRA) makes sure that the most up-to-date technology and computerised design software is utilised on all sections of the project.

Given the urgency of the LRT project and time constraints to complete the project, it would be prudent to undertake a thorough review of all the drainage design work undertaken by the International Contractor to manage the flood risk to the project as a whole. These designs would then need to be signed off by an independent consultant that has the relevant experience on similar projects. This exercise manages the risk of poor design and will not expose the ERC to unnecessary risks.

Ed's Note: Manaye is the Managing Director of ME Consulting Engineers (Africa and Middle East). The views expressed in this article do not necessarily reflect the views of The Reporter. He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it. .