The Scottish Government

Listen

Natural Flood Storage and Extreme Flood Events

Research Summary 2005/1

S. Rose and R. Lamb, JBA Consulting
K. Conlan, Cascade Consulting
This study was funded by the Scottish Executive
Environment and Rural Affairs Department

ISBN 0 7559 3936 0

This document is also available in pdf format (140k)

In June 2004 the Scottish Executive commissioned JBA Consulting and Cascade Consulting to undertake a project on the natural attenuation of extreme floods on river floodplains. Flood attenuation provided by 'natural storage' has increasingly been considered as a useful complement to conventional flood defences in certain situations. The overall aim of this research was develop a robust method to assess the applicability of 'natural flood storage' as an environmentally sustainable method of flood risk management using four case study rivers in Scotland as examples.

Main Findings

• 'Natural flood storage' was defined as water held back on an area of the floodplain that would currently be inundated during flood events up to some assumed size. However, a better way to view the functioning of the floodplain is to consider natural floodplain attenuation, which is the overall impact of the floodplain to change the shape of flood hydrograph (reducing flood peak and increase flood duration) due to a combination of storage and resistance. Making additional use of areas that flood naturally has to involve some form of engineering or active flood management, such as maintaining flood banks or increasing roughness on the floodplain. These would need to be designed appropriately within the surrounding landscape features.
• A generalised method has been devised to assess natural flood attenuation that could be applied to any catchment, assuming there is a digital elevation model (DEM), with or without channel data. It does not try to identify locations for engineered storage solutions, but could help to show broadly where the natural floodplain could be used, both in terms of providing the required volumes and in terms of economic costs to agriculture.
• A combination of 1-D and 2-D modelling has been used to calculate the extents of the assumed 'natural' floodplain and the average depth of water that would have to be stored to achieve a desired reduction in flood risk. These results have been plotted in relation to the distance upstream of flood risk locations. The graphs were used to provide a concise, broad scale summary of areas for potential floodplain enhancement in the case study catchments.
• Any proposed scheme to enhance the natural storage must take due regard for all natural and man-made assets on the floodplain affected. The generic assessment produced by this project provides guidance on how all the appropriate datasets should be reviewed and stakeholder consultations undertaken to ensure that all the environmental, economic and social issues are given adequate consideration.
• The managed use of natural floodplains for attenuation of extreme floods with long return periods cannot provide concomitant benefits for biodiversity. By their nature, extreme floods occur infrequently and would not provide the regular inundation (usually at least yearly) required to promote changes to existing biodiversity, particularly where land is currently in productive agricultural use.

Case studies

The generic methodology for the assessment of natural flood attenuation and storage was developed on the following case study rivers:

• River Clyde, draining through Glasgow
• White Cart Water, a tributary of the Clyde, draining through southern Glasgow
• River Tay, draining through Perth
• The South Esk, draining through Brechin

These catchments represented a range of catchment characteristics, river modification and regulation regimes, and degree of floodplain development. The detail of relevant datasets and existing routing models on the case study rivers was also variable, a feature which was incorporated into the generic methodology.

Modelling

The modelling component of the generic assessment of 'natural flood storage' that was undertaken can be represented by the following flow chart:

Economic assessment

The impact of flood inundation on a particular agricultural or rural land cover is dependant on the timing, extent, frequency and duration of flood events. The economic assessment of the modelled inundation areas was based on two methods:

1. The agricultural damage calculation method in the Modelling and Decision Support Framework (MDSF), developed for England and Wales, was revised to utilise Scottish land cover data on productive agricultural systems (i.e. horticulture, arable and grassland).
2. A single compensation payment method, based on recent publications about 'natural' flood storage schemes and typical payments within the current Environmentally Sensitive Area (ESA) scheme in England and Wales.

Environmental assessment

The generic methodology also included an assessment of the impact of any enhanced floodplain inundation on all natural or man-made assets. This included an analysis of all the appropriate spatial datasets within a geographic information system (GIS) framework, together with the necessary consultation with all the stakeholders in the catchment.

Recommendations

This research has identified a number of ways in which the modelling of 'natural flood attenuation' and the assessment of the impacts of any enhanced inundation on floodplain assets can be improved. These include:

1. Further development of the modelling work is needed with a wider range of flood scenarios, floodplain roughness, floodplain definition and channel representation.
2. The broad-scale generic assessment of the 'natural floodplain' should be piloted on a small catchment in Scotland (<150km ²) for which a new flood alleviation scheme is being planned.
3. The MDSF-based analysis of the economic impact of flooding on agricultural land covers should be revised to specifically incorporate and implement current Scottish agricultural and rural conditions.
4. Investigations are required to identify what scale and combination of catchment land management changes (e.g. upland grips, modification to hydrological connectivity) coupled to floodplain management (improved 'natural attenuation' or larger engineered schemes) would be required to alter sub-catchment hydrographs sufficiently to reduce peak flows and offset the peak hydrograph at the downstream risk location. This requires the further development of linked hydrological and floodplain modelling and analysis tools.

Research Output

The following research output has been produced by the project:

R&D Final Report - ENV/1/03/12 - Natural Flood Storage and Extreme Flood Events