Ecological effect of two-stage flood relief channels on River Roding, Essex, England
The River Roding near Abridge, Essex was monitored during 1979-82 to assess the short-term ecological effects of recent and contemporary river engineering works on a small, rural watercourse. Increased environmental awareness by Thames Water Authority river engineers and scientists resulted in implementation of an ecologically-sensitive flood alleviation scheme which provided an ideal opportunity for a pioneer case study. Sampling procedure was designed to describe aquatic and riparian habitats, flora and fauna along the watercourse. An extensive, 27 Jan survey revealed that the middle Roding was a typical, highly modified clay river, with limited conservation value. Intensive, continuous monitoring was confined to a 5km stretch; with reference to annual variations recorded in a control site, ecological change produced by three phases of engineering works between 1974 and 1980, each involving two-stage flood relief channels, are described. The magnitude of disturbance to the original habitat determined ecological impact. Retention of in-channel and waterside habitats ensured normal vegetation growth the following Spring, benefitting
dragonflies and fish; furthermore, the subsequent development of a richer 'channel-margin' flora produced an increase in waterbird territories. By contrast, excavation below water level severely retarded waterside vegetation recovery, while construction of a trapezoidal, concrete-lined channel produced an impoverished environment which greatly reduced habitat diversity. Silting, plus greater aquatic plant growth in response to increased light levels caused by berm excavation, reduced channel discharge capacity, accentuated by dense Phalaris arundinacea stands on unmanaged berms subject to regular summer inundation. An hydraulically efficient two-stage profile which improves riverine wildlife potential could be produced by retention of the original dry-weather channel; excavation, from one bark, of shallow flood berms which remain dry throughout the summer; tree-planting to counter the effects of increased light; sowing low-profile grasses; and regular grazing or cutting of berm vegetation.