Impact of structures on stabilising either bed level or plan form

Hydraulic structures may stabilise the plan form of a river. Probably the most common form of such a structure is a weir. This stabilises the bed level of a river immediately upstream of the weir. The same may be achieved by geological constraints on the bed of a channel. The plan form of a river may also be stabilised by the presence of

structures. River training works can control both the location of a channel and the flow direction. Other structures, though not designed to control the plan form, may inhibit channel movement at particular locations. Thus a weir, bridge or barrage may prevent lateral channel movement but may allow changes in the angle of approach. Again the same may be achieved by the presence of geological constraints.

In designing structures, particularly on meandering or braided rivers, it should not be assumed that the plan form of the river in the neighbourhood of the structure will not change during the design life of the structure. For example, a bridge was built

approximately 100 years ago and the piers were well aligned with the flow. During the intervening period the meander pattern has progressed downstream. The river is constrained to pass under the bridge but the angle of approach has altered by

the flow. Presumably in approximately 100 years time the piers may well be well aligned with the flow once more.

Plan form problems may be experienced immediately upstream of reaches whose plan form has been stabilised by river training or other means. Though the plan form of a reach of a river may have been fixed, the reach upstream may continue to adjust naturally. This may result in a short transition zone between the two where there may be extreme developments in the plan form.

In some parts of the world tectonic activity may have a significant impact on the slope of the river. This will then also significantly affect the plan form of the river.

9.4 Deposition in Reservoirs

Reservoirs on rivers are characterised by cross-sections that are larger than the average cross-section of the river. As a result the water surface slope and velocity are reduced. This means that a proportion of the sediment transported by the river will be deposited in the reservoir resulting in loss of storage and a wide range of other

environmental effects. Reservoir sedimentation can lead to: • a loss of storage,

• increased flooding risk upstream,

• reduced water depths at the head of the reservoir, this can in turn lead to: • increased evaporation losses,

• increased water levels upstream, • reduced navigation depths,

• increased vegetation in the upper part of the reservoir which can lead to: • increased transpiration

• trapping of increased amounts of sediment.

Rates of loss of storage can be high. One reservoir in the USA substantially filled with sediment within a year of construction. On average 2 to 3 % of the storage in the world’s dams is lost through sedimentation each year. This represents a large economic cost. In the UK loss rates are generally smaller but some of the older UK dams have lost significant amounts of storage.

There are a number of methods for predicting the rate of reservoir sedimentation. The simplest methods are desk based and are based on the idea of trapping efficiency. This is the proportion of sediment that is trapped by a reservoir in relation to the overall volume of sediment entering the reservoir. When first introduced the trapping

efficiency was based on figures collected over a number of years but HR Wallingford extended the concept of trapping efficiency to other time periods. One can thus consider an instantaneous trapping efficiency or a trapping efficiency derived over a month or other time period.

The amount of sediment trapped in a reservoir depends upon both the flow into the reservoir and the size of sediment. The most important flow related parameter is frequently the residence time, that is the average period of time that the water spends in the reservoir. If this is large then most of the sediment will settle in the reservoir but if it is short then a greater proportion of the sediment will be transported through the reservoir. This is affected, however, by the size of the sediment. For the same

residence time a larger proportion of coarse sediment will deposit than fine sediment because of the larger fall velocity of coarse sediment.

The trapping efficiency can be estimated using simple desk methods. Brune (1953) derived curves that described trapping efficiency as a function of the capacity of the reservoir to the annual inflow. Reservoirs for which the capacity approaches or exceeds the annual inflow effectively provide over year storage and their trapping efficiency normally approaches 100%, that is, they trap all the incoming sediment load. As the ratio of capacity to annual inflow reduces the trapping efficiency reduces. Churchill (1948) expressed the trapping efficiency as a function of a sedimentation index, which is the ratio of the period of retention to the mean transit velocity. Both these desk approaches give quick simple methods of assessing the volume of sedimentation but they ignore many important factors, such as sediment size, annual flow distribution and the shape of the reservoir. HR Wallingford introduced a desk- based computational procedure that was aimed at overcoming many of these shortcomings (HR Wallingford, 1989).

Alternatively a numerical model can be used to predict reservoir sedimentation. Such models can be used to provide information on:

• the distribution of sediment deposits within the reservoir, • the composition of the deposited sediment,

• revised stage storage curves for the reservoir, • the impact on water levels upstream.

Reservoir sedimentation models can also be used to investigate strategies for reducing the loss of storage by, for example, the use of sediment flushing. Practical experience has shown that flushing can preserve storage in reservoirs but it is only practicable in limited circumstances (White and Bettess, 1984).

9.5

Sediment Deposition on Floodplains

During floods sediment is transported in the main channel. Either by diffusion or convection this sediment is carried onto the floodplain and, as the flow velocities on the floodplain are generally low, it is deposited. The rate of sediment deposition normally reduces rapidly as one moves away from the main channel. Sediment deposition on floodplains is normally largest where there is an exchange of water between the main channel and the floodplain, commonly at bends.

Sedimentation rates of the order of 1gm/cm2_{/year have been measured on UK rivers}

(Walling et al, 1996). Sedimentation rates of this order imply increases in bed level of the order of 1cm/year. If these rates of sedimentation persist for periods of the order of centuries then significantly increased bank heights may result together with floodplains that slope away from the main channel. Floodplain archaeology indicates, in general, much lower rates of accretion. The difference may well be due to the fact that in the past a river channel did not occupy the same location on the floodplain for an extended period of time. Within the last 100 years we have constrained some river reaches and it remains to be seen what the long-term impact is on the development of the river systems.