La sombra del boom es alargada

The presence, size and shape of bed features affects the hydraulic roughness of a channel. This has a direct impact on the velocity and depth of flow. If the hydraulic roughness of the channel is low then the flow will be relatively shallow and fast. As the hydraulic roughness increases the flow becomes deeper and slower. As the flow characteristics in terms of depth and velocity are an integral part of the riverine environment changes in the flow conditions will have an immediate impact on the environment. Many fauna exhibit preferences for particular flow conditions. Changes in the flow conditions will have an effect on the available area of suitable habitat for different species and will hence influence both the range and abundance of species available.

Large bed features significantly modify the near bed velocity and shear stress distribution and so can have a significant impact on the ecology of a channel. Under certain flow conditions the flow can separate from the crest of a dune. This

generates a turbulent mixing zone downstream from the crest, with a re-circulation zone near the bed. This has a significant effect on the velocities adjacent to the bed and the shear stress on the bed and modifies them considerably from the conditions without bed features. The impact of the bed feature is thus to modify the potential habitats and so to have an effect on the ecology of the stream.

An important factor for the ecology of the stream is the speed of movement of the bed feature. Where the speed of movement is large then organisms do not get an opportunity to colonise the bed but the slower the rate of change then the greater the opportunity for colonisation. The movement of some bed features depends upon the flow conditions. Particularly in gravel rivers, movement can be episodic and may only take place during large flows.

In certain river systems it seems that it is possible for large bed features to develop and propagate at small speeds. In the Meghna River in Bangladesh a large sand dune with an overall length of approximately 1km has been observed to be moving slowly downstream while on the River Lochy in Scotland a gravel bar approximately 30m long and 500 mm high appeared to be progressing downstream at a low rate. The gravel bar was observed to be moving over a surface that was supporting a range of plant and animal life. The material of the bar itself seemed sterile. In such cases the movement of large bed features can destroy existing habitats which may take some time to recover.

10.3 Water Quality

Where bed features are large enough and the flow is appropriate then the flow may separate from the crest of the feature. A re-circulation zone will then form in the lee of the crest. Such ‘dead zones’ can be important in the movement of pollutants through a river system. Dead zones act as temporary stores for pollutants, gradually releasing material after the peak of the pollution passes. This mechanism can attenuate and lengthen pollution concentration profiles downstream from where pollution has entered a river.

The presence of bed features on the bed of a channel will alter the pressure distribution on the bed of the channel, causing local variations. This may locally cause flow into or out of the bed. This will enhance the interchange of water between the channel flow and the water in the interstices of the bed sediment. In the

presence of pollution this may enhance the movement of pollutants between the channel flow and the water in the bed of the river.

In many instances pollutants can become attached to the surfaces of sediments. If the sediments are then incorporated into bed features then this may temporarily remove them from contact with the flow in the river channel. As the bed feature moves, however, the polluted sediments may later be once again exposed to the flow. In this way bed material can act as a temporary store of pollutants. This can lead to the attenuation of pollutant plumes. The incorporation of polluted sediments into bed features may also make the clear up after pollution incidents difficult and reduce its effectiveness.

Within a river system there are many sources and sinks of sediment. These are of varying extent and the duration for which sediment is stored can vary significantly. Storage location Duration of storage

Bed features Minutes to days Deposition on bed of channel Minutes to millions of years Deposition on floodplain Days to millions of years

Such stored sediment may be introduced back into the river system by: • bank erosion or

In many river systems the transfer of sediment on to the floodplain can represent an important ‘sink’ for suspended sediments. Lambert and Walling (1987) measured the suspended sediment load entering and leaving an 11 km reach of the Lower River Culm in Devon, where the floodplain is regularly inundated during flood events. They estimated that approximately 28% of the suspended sediment entering the reach was deposited on the floodplain within the reach. Walling and Quine (1993) estimated that 23% of the total suspended sediment transported through the main channel system of the River Severn during the period 1986 to 1989 was deposited on the floodplain. Trimble considered the sediment budget for a 200 km2 _{catchment of}

Coon Creek in the USA. He demonstrated that more than 50% of the sediment mobilized from the slopes of the basin during the period from 1850 to 1938 was deposited on the floodplain of the lower and middle valley and its tributaries. Thus significant amounts of sediment may be deposited on floodplains during out of bank flows.

Sediment deposition on the floodplain can act as a sediment sink and thus store contaminants. If agriculture is taking place on the floodplain then this may make the pollutant available for incorporation into the foodchain. At a later date these

floodplain sediments may be released back into the river system by the process of bank erosion. Leenaers and Schouten (1989) have shown that recent streambank erosion on the river Geul in the Netherlands is mobilising substantial quantities of floodplain sediments contaminated with lead, zinc and cadmium which were mostly deposited during the peak of ore extraction in the catchment in the nineteenth

century. 66, 47 and 39% of the lead, zinc and cadmium, respectively, that enters the channel is supplied by river bank erosion. Gold mining around Lead, South Dakota, USA, during the period 1870s to 1978, produced approximately 100 million tonnes of mine tailings contaminated with arsenic. It has been estimated that up to 15% of these tailings are still stored on the floodplain of a 164 km reach of river downstream (Marron 1987,1989). Thus floodplain deposits can be an important factor in the movement of pollutants through fluvial systems.

Bank erosion is the most important factor in the release of these pollutants stored on the floodplain back into the river system. Large and rapid plan form change can ensure that large quantities of floodplain material are released into the river. Thus the potential impact of excessive bank erosion on pollutant release and movement should be considered. The bank material of the Red River in Cornwall has a very high concentrations of arsenic as the result of industrial processes associated with extensive lead mining in the past. This means that any bank erosion has the potential to release large quantities of arsenic into the river downstream. Under these circumstances, the prevention of bank erosion may become more important than allowing the plan form of the river to develop naturally.

10.4 Habitat Hydraulics

The size of substrate can interact with the habitat in a number of ways. The size of substrate affects the hydraulic roughness of the channel and hence affects flow velocity and depth, which are major factors in the habitat. As discussed above, the size of substrate can also affect flora and fauna directly. This influence is

considered, if somewhat crudely, in habitat models such as PHABSIM. The

PHABSIM model simulates a relationship between stream flow and physical habitat for various life stages of a species of fish, benthic invertebrates or for a recreational

activity such as canoeing. In PHABSIM the nature of the substrate is one factor which is considered explicitly.

11 Methods of Study of