WITH climate change and continued development on flood plains exacerbating flooding risk the Environment Agency estimates that damage costs could rise from their current level of £1bn to a worst case scenario of £20bn by 2080. Action is clearly needed.

In the past, flood defence was regarded as the answer. It remains part of the solution with over 100,000 properties in the UK receiving improved protection since 2003 and millions invested in flood-defence and coastal realignment schemes each year. But, as flooding is becoming an increasing threat, there has been a significant move away from a strategy of flood defence towards one of flood risk management.

A risk management approach is more holistic and can be introduced at planning stage. The recently published Planning Policy Statement 25 (PPS25) facilitates this approach. At its core, PPS 25 emphasises the need to consider  flood risk in an integrated way as early as possible in the planning process. This should mean that developments are not located on flood plains or designed in a way that increases flood risks. But what can be done for those properties that already are in flood risk areas?

Flood warning systems are essential to alerting people to potential flooding. These can provide accurate flood forecasts and timely warnings to the public with an adequate lead time to minimise flood damages. Depending on catchment characteristics and catchment response to rainfall, various types of flood forecasting models, including correlations, simple trigger flood forecasting and more sophisticated real time catchment-wide integrated hydrological and hydrodynamic models can be adopted.

These models provide flow and level forecasts at selected key locations, which are usually along major rivers or on streams near urban areas, with a history of flooding.

In recent years, Atkins has been working with the Environment Agency to develop real time flood forecasting models for the National Flood Forecasting System (NFFS) so that an effective flood warning service can be provided. These include simple trigger flood forecasting and catchment-wide integrated hydrological and hydrodynamic modelling.

The simple trigger flood forecasting method is a valuable tool for providing early warning of the onset of flooding to a specific flood warning area. It can also provide independent backup to more sophisticated forecasting methods, such as real time integrated catchment models combining rainfall runoff, flow routing and hydrodynamic processes. The simple trigger method provides information that a pre-defined flood-warning threshold will be reached if the current rate of rise is maintained. The application of the method for the Sankey Brook catchment helps to demonstrate how it works.

The headwaters of Sankey Brook catchment are detained by a complex system of tributaries that rise north of Rainford in Cheshire. They collectively form Sankey Brook proper in the township of St Helens. The watercourse then flows in a southeasterly direction for approximately 17.5 km until it converges with the River Mersey near Penketh. Flood risk assessment/modelling results identified three flood-warning areas for Sankey Brook. The nearest gauging station is located at Causey Bridges and gauges the river flow covering a catchment area of about 150km2.

The development of the flood warning triggers relied upon the data recorded at the Causey Bridges gauge only, which has been in operation since 1953. A short record also exists for Higham Avenue and Liverpool Road gauging stations. A peak-to-peak correlation analysis was undertaken to relate levels at Higham Avenue and Liverpool Road to the Causey Bridges gauge. Flooding thresholds for the three flood warning areas have been established following an analysis of return periods of flooding for each discrete flood risk area.

A rate of rise analysis was undertaken to determine the catchment response and the feasibility of developing flood warning triggers based on the rate of rise of the stage hydrograph at Causey Bridges gauge. The rate of rise information is then used to develop the flood forecast with the lead time requirement for each flood warning area. The water level forecast can then be compared with the flooding thresholds to determine if a flooding threshold will be reached during an event.

This simple trigger flood-forecasting tool provides a quick and easily implemented method to help the Environment Agency extend its flood warning coverage. A novel spreadsheet tool has been developed for easy implementation and verification of the simple trigger method. This tool was tested for nine flood events achieving adequate lead time of at least two hours for 75% of occasions when the lowest trigger level was reached.

The recent flooding in January 2005 that affected Carlisle and other parts of the North West was the first major event since the implementation of the simple trigger models developed by Atkins. A review of the flood warning performance in the north-west region revealed that 13 out of 18 simple trigger models provided at least two hours lead time and generated only one false alarm. The model forecasted the worst affected area at Carlisle would be flooded with 3.5 hours lead time.

The simple trigger approach is effective in providing a short lead time. For a longer lead time, however, the catchment-wide integrated hydrological and hydrodynamic modelling approach provides more robust and sophisticated flood forecasting. This includes a hydrological component in which flood discharges are estimated from rainfall measurements and forecasts and a hydrodynamic component in which the water levels along the entire river network are calculated. It simulates hydrological rainfall runoff processes and river hydrodynamic processes, which represent the channel and flood plain interactions and is able to represent the floodplain processes more accurately and provide accurate and timely flood forecasts.

During 2004-5, Atkins undertook one of the first real time catchment-wide integrated hydrological and hydrodynamic model development projects for the NFFS for the Sussex Ouse catchment in the Environment Agency’s southern region. The Sussex Ouse is a complex catchment with a significant number of braided channels and strong channel floodplain interactions. It covers an area of over 560 km2 and was represented by four separate flood risk mapping models. These were converted into four flood forecasting models to deliver real time flood forecasts for 17 forecast points.

Altogether, 38 hydrological rainfall runoff models were developed using the probability distributed moisture approach. The flood forecasting models were calibrated in off-line mode using daily rainfall, flow and level data over a period of 14 months and 15-minute data for three major events lasting 12 to 18 days. The model runtime improvement was a key technical challenge and also a key success of the project.

Following severe flooding in January 2005, Atkins has undertaken a similar project to improve the Eden and Carlisle hydrodynamic flood forecasting model, based on an existing defence strategy model. The newly-converted Eden and Carlisle flood forecasting model is robust and stable and retains all of the originally modelled floodplains within the required model extent.The accuracy of the model was demonstrated by historical events. The forecast point for Carlisle, Sheepmount, provides a typical example, and demonstrates the excellent agreement in both timing and shape of the flow and level hydrographs that has been achieved between the modelled and recorded data.

In order to work well, flood-forecasting models need to be appropriately calibrated using recorded data. It is also important to monitor their real time performance and update them regularly so that they can be an effective part of flood warning systems. Real time flood forecasting can form an integral part to a risk-based approach for communities living on flood plains or in flood-risk areas.