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Birmingham City Council

How the level of the land can affect the risk of flooding

Surface levels and features need to be considered carefully. For example, we have seen cases where the ground level, in the form of a patio or other hard landscape, has been raised to the point where rainwater has run off the hard surface and under door frames or in through air bricks.

In other cases, extensions have been built causing the building to be a 'dam' across the natural flow of runoff water. A series of properties, once separated by drives or access 'entries, becomes an effective dam to flow when these gaps are in filled by kitchen extensions, conservatories, granny flats or garage conversions.

A further problem arises when ground levels are changed in a garden or from one garden to the next. Terracing can result in pooling or in flows being sent in a different direction to the norm. Furthermore terracing can also result in the ground levels being reduced to the groundwater table level causing seasonal springs to emerge at the step in ground levels. These sometimes appear naturally in high groundwater areas and being natural are the responsibility of the land owner.

Improvement Measures

If flows are to be prevented from entering properties from straight forward run off it is essential that original built thresholds are maintained and this generally means maintaining a minimum of 150mm from ground level to the top of damp proof courses, airbricks, the undersides of doors etc.

If the flow path between buildings is blocked by extensions, walls etc it is essential that compensating drainage is installed. This will allow flows to pass the blockage, which the new building represents, without causing ponding against the building. If adequate drainage is not provided, the building will act as a dam and the only way for water to get past it may be through the building.

Drainage should be considered when lowering, levelling or terracing gardens. Lowering ground levels below surrounding levels will cause water to form a pond. Likewise, raising levels alongside adjacent property may cause water to form a pond on the adjacent property causing 'injury' to your neighbours. In all landscaping care must be taken not to interfere with the natural drainage paths of the land. Where natural levels and drainage paths are altered it may be necessary to install additional drainage or land drains.

3.Maintaining drainage
4.Vandalism and other illegal acts
5.Understanding watercourses
6. Financial help after severe flood damage