River Dodder CFRAMS

The River Dodder CFRAMS is a catchment-wide flood study undertaken by RPS for the River Dodder and its five major tributaries (Owendoher, Whitechurch, Dundrum/Slang, Little Dargle and Tallaght Stream) in South Dublin.

RPS were commissioned by Dublin City Council (DCC) in 2009 to undertake this study to assist with Ireland’s implementation of the EU Floods Directive. The entire catchment area consisted of 116km2. Two other Local Authorities made up the project team, from Dun Laoghaire-Rathdown County Council and South Dublin County Council.

Challenge

RPS undertook all aspects of this study which incorporated a significant data collection exercise for use in both the hydrological and hydraulic analyses. This included topographical surveys (land and hydrographic), LiDAR, CCTV, meteorological, hydrometric, coastal, historical flooding and climate change information. The hydrological analysis was based on the standard statistical approach to estimating river flows using FSR and FEH methodologies but also incorporating the development of a wide range of rainfall run-off models using MIKE NAM.

Using this data, coupled hydrodynamic models were established in MIKE Flood for the River Dodder and each of the five major tributaries.  Hydrodynamic models were produced that are fully dynamically linked, exchanging data in each time step of the simulation. MIKE 11 models defined the river channel and all “in-line” structures and MIKE 21 defined the floodplain and the 2D element of the models.

A separate MIKE 21 model defined the tidal boundary by undertaking tide and storm surge analysis within Dublin Bay. A sensitivity analysis was undertaken to ascertain the influence of a number of parameters on the model results. Models were created for both present day and future catchment conditions. The models where then utilised to produce a wide range of flood risk and flood hazard maps for consultation using ArcGIS software.

Solution

The suite of flood maps were presented for consultation with key stakeholders and members of the public and all comments and observations, where appropriate, were incorporated within the final maps.

The next stage of the project involved the development of flood risk management options which incorporated a detailed feasibility study with a view to establishing engineering solutions to the project.

Over 30 different flood risk management options were considered for the catchment, each of which was designed, costed and assessed against a wide range of criteria including a detailed cost benefit analysis and assessment against both social and environmental criteria. The environmental assessment integrated the project with the statutory Strategic Environment Assessment process and ensured compliance with that particular EU directive. 

The most economically advantageous options were consulted on with both the public, in a series of public information days across the catchment, and also the statutory and key stakeholders through a more formalised approach. All comments on the options were received and incorporated within the final flood risk management plan which will be utilised in reporting to the EU as part of Ireland’s Flood Directive Implementation. The plan has recently been consulted on, comments incorporated and was released in final form in 2013. The Plan has been subsequently adopted by all three Local Authorities involved in the project.