6.3.2 ENVIRONMENTAL

SHP are in most cases 'run-of-river', which means that any dam is quite small, usually just a weir, and generally little or no water is stored. The civil works purely serve the function of regulating the level of the water at the intake to the hydro-plant. Therefore these installations do not have the same kind of adverse effect on the local environment as large hydro. However, some environmental problems can always be detected, notably where the water is extracted some distance upstream from where it is discharged back into the river. The short stretch of bypassed river can then run dry or look unsightly unless adequate compensation flow is allowed.

In most cases, new hydro installations are designed o leave sufficient water bypassing the turbines - which is not difficult except in times of low flow. Another area that requires care is the need to avoid harming fish and riverine flora and fauna, but modern turbine installations are designed with this problem in mind. Some low head systems allow fish to pass through the turbine generally unscathed, but various forms of screening (either physical screens or even electrical and ultrasonic) are also used. Fish ladders, a set of small waterfalls set in a channel, are provided to ensure that migrating fish such as salmon can safely bypass the hydro-plant. Figure 6.11 illustrates a common fish-ladder with vertical slots and bottom orifices that yields very good results.

 

Turbines also need to be protected from all the debris that are commonly found in rivers, whether natural (e.g. leaves, branches, even tree trunks) or man-made (supermarket trolleys, plastic fertiliser bags or general garbage); this is done using screens. A major operating cost element is cleaning these screens, especially in low head situations where large flow rates pass through. The hydro-plant operators are usually prohibited by law from returning the rubbish collected on their screens back into the river. Thus, garbage collection and disposal carried out at a hydro plant can serve to clean up a river considerably for the benefit of everyone downstream, but usually at considerable expense to the operator.

There are a few other environmental impact issues relating to oxygenation of the water, disturbance of the river bed or erosion immediately downstream of the turbine draft tubes, electrical machinery noise, electrical cables, the general appearance of an installation, etc. However, all these problems can be solved by suitable design techniques and the end product is a remarkably long-lasting, reliable and potentially economical source of clean energy.