Gullies not working the problem
As detailed above ground levels and development can have an impact on the adequacy of drainage. It is essential when looking at the adequacy of drainage that the whole catchment to a point that floods is inspected. For example a large driveway or communal parking area gully that is blocked or thought to be not taking the flow may simply be trying to take more flow than can reasonably be expected due to problems elsewhere. The gullies upstream of a gully in a low spot may contribute to a problem on larger areas. For example, on more than one occasion, a problem with a gully at the bottom of a cul-de-sac actually proved to be due to gullies at the bell mouth, which should have intercepted flows from the main road, being blocked and themselves not taking flow. Indeed in some cases the steepness of the uphill area is such that the heavy flows run straight across the gully grids.
Making sure that Gullies work
Road gullies and those on private land and hardstand areas such as car parks should be checked and cleansed frequently and their connections checked to ensure that they carry flow (are not blocked or collapsed). The gully grids should be cleaned more frequently to prevent matting of leaves and debris that will hamper flows into them. Where there is a large area falling toward a low spot this area should be checked to ensure that there are adequate gullies and that they are effectively taking the water.
In cases where the area falling to a low spot is very steep and flows run across the top of the gully grids without much actually going in it may be necessary to install different types of road etc. drainage or at least to double up on gullies. An investigative approach rather than isolated problem solving approach is required at all times whether it be drainage to large formal or smaller informal areas.