The purpose of this section is to review some aspects of dam engineering for the information of fishery biologists.
Regardless of individual functions, most dams are designed to form reservoirs. The objective of dam operation is to alter (= regulate) the shape of the discharge curve of the river downstream from the dam (Fig. 4). This amounts to a redistribution of total annual discharge over time. Two basic types of regulations and reservoirs can be distinguished (Peter et al., 1977):
These are designed to increase downstream river discharge during the dry season. Accordingly a portion of the rainy season flood water mass is stored behind the dam and then released more or less uniformly during the duration of the dry season to provide a reliable and adequate year round water supply for downstream users (i.e. hydroelectric powerplants, irrigation, municipal water supply, navigation). Water level of the reservoir is usually kept close to USL. Annual variations in reservoir surface area are in most cases moderate.
Fig. 4. Hypothetical annual discharge curve of an African river to illustrate the effects of the two basic types of dams/reservoirs on downstream flow
Flood control reservoirs
These are designed to decrease the magnitude of the peak flood discharge during the rainy season in order to protect downstream areas from flood damage. They fill rapidly during the rainy season. Once the flood wave has passed, the stored water is released. Water level and surface area experience a large annual fluctuation. The reservoir as such may be in existence for only a part of the year.
Because storage reservoirs also exert a partial control effect, dams/reservoirs designed to perform both functions simultaneously have become common. This third type is ‘multipurpose’ and virtually all larger reservoirs are of this type. Either the control section of the reservoir occupies a separate section overlying the storage section, or the storage section is used for control by emptying it (= drawdown) before the inflow of new flood water takes place. The resultant downstream flow from a multipurpose dam/reservoir is illustrated in Fig. 5. It combines the desired features of increased dry season flow and decreased flood peak flow.
Fig. 5. Hypothetical annual discharge curve of an African river to illustrate the effect of a multipurpose dam/reservoir on downstream flow
It can be seen that the desired effect of dam function is to manufacture a novel downstream hydrological environment with certain specified characteristics. Direct users of the upstream reservoir (fisheries, transport) are usually given second priority. Their interests may be sacrificed in the event of a shortage or surplus 1 of affluent water in favour of maintaining the required discharge to downstream users.
Crest elevation is the principal design parameter determining the range of possible functions of a dam at a particular dam site. For most storage and control functions crest elevation (with a freeboard 2 of about 3.0 m or more) is set to provide a certain impounded reservoir water volume. To reduce construction costs the minimum crest height capable of performing the desired function(s), within a suitable safety margin, is usually selected. Dams feeding hydroelectric powerplants however need to provide hydraulic head 3 to the turbines. Increasing the reservoir water level (as a result of increasing the crest elevation) increases the head to the turbines and in turn the potential electrical output of the powerplant. Crest elevation will usually be maximized within the upper limits imposed by site geomorphology, economics, and other factors (for example, to prevent the reservoir from extending into neighbouring countries as was the case for Cahora Bassa). Both turbine efficiency and absolute power output would drop significantly if the hydraulic head is allowed to decrease. Thus, in theory, maximum electricity production requires the maintenance of the maximum possible reservoir water level at all times. In multipurpose reservoirs this conflicts with the flood control function since maintenance of a constant level would require at any point in time releasing volumes of water almost equivalent 1 to those entering the reservoir. There would in effect be no flood control downstream. Thus flood control requires a drawdown. To overcome this problem the design head of the turbines is set at a lower reservoir water level elevation than that which will be maintained in practice, and flow to the turbines is controlled by valves. However if reservoir water level is drawn down low enough to require fully opening these valves, any further drop will result in a loss of electrical output. For example, at Cahora Bassa valve regulation of flow to the turbines is necessary between reservoir water levels of 310 m a.s.l. and the USL of 326 m a.s.l. to ensure a constant optimal flow of 452 m3/sec to each turbine which then produces its maximum design output of 415 MW. If the reservoir water level falls below 310 m a.s.l. the valves will be fully opened, but inspite of this a decrease in flow to the turbines will take place due to loss of hydraulic head. At a reservoir water level of 295 m a.s.l. (the upper edge of the turbine intake) flow decreases to 408 m3/sec and electrical output to 321.5 MW (Spencer, 1977).
1 If a flood larger than the normal storage capacity of a multipurpose reservoir is expected, or dam discharge capability is impaired, the volume of the reservoir must be reduced considerably to accommodate the anticipated flood. This results in a large fluctuation in reservoir water level.
2 Freeboard is the vertical distance between dam crest and USL (Fig. 6).
3 Hydraulic head is the vertical distance between reservoir water surface elevation and tailwater discharge elevation to the river downstream. This ‘drop’ produces the hydraulic working pressure across the turbine blades which are situated at an optimum intermediate elevation.
1 Somewhat less than affluent inflow due to evaporation loss from the reservoir surface.
Fig. 6. Schematic diagram of various operational water levels for a hypothetical multipurpose dam/reservoir
Water mass in a reservoir can be visualized as consisting of a series of layers overlying each other (Fig. 6). As will be shown below it is the selection of the position of the boundary levels, especially those closer to the crest, which can have important effects on fisheries in a reservoir.
It should however be clear to non-engineers that operation of a dam requires that a delicate balance be maintained between several sometimes conflicting constraints to produce a variety of outputs. It can be appreciated that dam engineers and operators may be reluctant to add to these constraints the special operational considerations needed to optimize fisheries from the new artificial aquatic environments created upstream and downstream by the dam. Indeed, for some existing dams even minor alterations of operating rules may be highly impractical. For dams still in the planning stage the possibility for fisheries optimization is greater, but would still be hampered if the client government gives river basin fisheries a low priority rating. While the occasional dam may become optimized for fisheries by the dam designers or operators at their own initiative, in general government intervention is required to ensure that dam designs and operating rules incorporate a fishery component.