Lee Catchment Flood Risk Assessment and Management Study

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Hydrological assessment

Detailed hydrological analysis

The hydrological analysis of the Lee catchment used the scientific findings of the Flood Studies Report (FSR) together with recorded river flow and rainfall data to estimate rates and quantities of runoff into the catchment water bodies. The hydrological analysis provided river flow data for storm events of various magnitudes. This data has been input to our computer models of the rivers and used to estimate water levels along the river channel. The hydrological analysis has been carried out with the help of our project partners, J B Barry & Partners Ltd.

Hydrology concerns the occurrence and movement of water in the environment. For assessing flood risk in the Lee catchment, we are particularly interested in the effects of surface water hydrology, which looks at the relationship between rainfall on the land surface and runoff into the streams, rivers, lakes and reservoirs (water bodies). The catchment defines the area drained by a river or other body of water. The map below shows the Lee catchment broken down into a series of smaller adjoining sub catchments.

Hydrological schematisation map
Loaded on: 21/09/07 - Size: 350kb

These smaller sub catchments are separated from each other by higher ground such as a ridge, hill or a mountain. The sub catchments act like a funnel, collecting all the water within the area covered by the sub catchment and routing it into a water body. The rate and quantity of water reaching the water body depends on a number of factors:

If the ground is already saturated, then any new rainfall will have a faster rate of run-off.

Review and analysis of historic flood events

A review of available flood records relevant to the study has been undertaken. Information was made available from a number of sources including Cork City Council, Cork County Council and the OPW. The OPW National Flood Hazard Mapping website www.floodmaps.ie contains information on past flood events in the form of detailed reports, photographs, newspaper articles and minutes of meetings. Information on historic flood events was also gathered at our public information and consultation days held in December 2006 and through feedback to the project email address. This information has been used to calibrate the hydraulic computer models.

Climate and land use changes

There are a number of drivers that are likely to influence future flood risk in the Lee catchment, including changes in climate and land use, and urban growth. As these drivers are likely to change over time it is important to appreciate how they could affect future flood risk in the Lee catchment. Potential changes in these drivers will be assessed in the form of two future scenarios. The ‘Mid Range Future Scenario’ (MRFS) considers the more likely estimates of changes to the drivers by 2100.  To allow for future adaptability of flood defence measures, a ‘High End Future Scenario’ (HEFS) has been included, representing extreme changes in the respective drivers by 2100.  The future scenarios have been assessed through our hydrological analysis and hydraulic modelling to identify the future flood risk in the Lee catchment.

Climate change refers to the change in the earth’s global climate or in regional climates over time. An extensive quantity of research exists both internationally and in Ireland. Recent publications include the Intergovernmental Panel on Climate Change (IPCC) 4th Assessment Report (February 2007) and the Irish Committee on Climate Change Report by the Royal Irish Academy (RIA) (February 2007). Uncertainty remains with respect to future climate conditions, however current research indicates that climate change will cause an increase in rainfall depth and intensity and a rise in sea levels. An increase in rainfall depth and intensity will lead to bigger flows and higher water levels in the catchment rivers and lakes. A rise in sea level will result in higher water levels in Cork Harbour and the lower River Lee (downstream of the Waterworks weir in Cork City).

The amount of rain which makes its way into watercourses is directly related to the use of the land around the catchment. Land use changes can result in an increase in the flows and river levels in the catchment watercourses. Flood risk will also be increased by isostatic subsidence which is causing ground levels in the south of Ireland to gradually fall as they recover from the ice age. The map below shows the distribution of forestry and urban areas in the Lee catchment.

Landcover map
Loaded on: 21/09/07 - Size: 400kb

Urban development increases runoff rates and volumes, as impermeable surfaces causes the transfer of water to watercourses more quickly than rural and undeveloped lands. In the future the extents of urban areas are expected to increase as population rises. The impact of this urbanisation on flood generation in the catchment will depend on the spatial distribution of this urban growth.

Forestry cover is expected to rise over the next century. By 2035 it is expected that 17% of the Lee catchment will have forest cover (Forest Service, 2006). The impact of aforestation on flood generation varies over time. Once the forest has reached maturity, which takes approximately 20 years, it will help to reduce the peak flow and flood response in the catchment watercourses.

Hydrology Report

The Hydrology Report for the Lee CFRAMS is available to download below. (The report is 13 MB in size and will take some time to download).

Hydrology Report
Loaded on: 01/07/08 - Size: 13012kb

The report details the hydrological assessment that has been undertaken for this study with the objective of determining hydrological inputs for the Lee and its tributaries for specific design events and future scenarios.

Photos

Blarney River

Cork Harbour

River Lee

Waterworks weir.

Key Activities

The key activities to the project are

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