13.3 LEVEES AND POLDERS

Polders

Not all flow regulation in rivers involves dams across the channel. Other engineering structures - especially levees and polders - change flow patterns as well as the extent and duration of floodplain inundation.

Levees--
Artificial levees are constructed along the banks of rivers for several purposes, including flood control and river regulation for navigation. Levees act as linear dams that interrupt the lateral connectivity of the river. This means that the floodplain becomes increasingly isolated from the main channel as levee construction becomes more general and, eventually, the plain may be completely cut-off. At the same time, where navigation is a major interest, the main channel is deepened and meander bends are eliminated to shorten the navigable channel. These extensive river works are protected by stabilizing banks with hard protection such as riprap or gabions. The total package of leveeing, channel shortening and bank stabilization is also known as channelization or river training.

The secondary effect of levee construction and channelization is to concentrate the discharge of the river, which would normally have spread over the floodplain, into the main channel. This accelerates the current and increases the erosive capacity of the river resulting in the cutting down of the riverbed. The current is further increased by channel straightening. Leveeing of the river channel upstream transfers what may be a local problem to sites further down the river, which may be forced to adopt even more extreme defensive measures with higher levees.

Biologically channelization of the river through levee construction means that the floodplain no longer becomes inundated, producing the same changes to living communities as those resulting from dam construction. In general, the vegetation structure of the former floodplain changes, and invertebrates and fish can no longer access the floodplain from the main channel. Many species depend on the floodplain for spawning and successful survival and growth of the young. These may be able to find alternative sites in backwaters and slack areas of the channel but such sites are less abundant than the area of the floodplain, and the species will often decline. Species that pass their whole lives on the floodplain are also adversely affected because the annual flood event needed to maintain the floodplain water bodies no longer occurs.

Many invertebrate migrate downstream with the drift, and fish in tropical rivers have eggs, larvae and young that also drift with the current. The timing of the development of the eggs and larvae is closely linked to the current speed so that the young fish arrive at the floodplains at the right time in their life cycle. Acceleration of the current accelerates the speeds at which insects drift downstream and they may be carried well past suitable sites for colonization when they settle. Similarly the young stages of fish can be carried past appropriate nursery areas and even out to sea. In either case the young fail to develop and die.


Polders--
Polders are levees that have been extended to completely enclose an area of floodable land. Polders are generally constructed to reclaim land for agriculture although they may also be used to protect urban sites. In rice growing areas such as the Mekong, the function of the polder is not to completely exclude water but to gain better control of the depth and duration of flooding to maximize yields of rice and other water dependent crops. Therefore, polders are equipped with sluice gates that can control the rate and volume of water entering the polder.

Polders may not affect insect population significantly, since a type of flooding continues to occur, but fish are affected in different ways. They tend to exclude whitefish whose floating larvae only pass the sluice during a very limited period. On the other hand, they can have resident populations of blackfishes that colonize any permanent water bodies remaining within the polder. To overcome these difficulties, polders should be managed with the interests of fish in mind as well as those of the major crop. At present, polders are managed uniquely for rice, and the resulting regime for opening and closing the sluices does not correspond to the needs of fish. There are two solutions to this. Firstly the polder is managed mainly for the blackfish crops and the permanent water bodies and rice fields are stocked with fish accordingly. Stocking may be supplemented by setting aside some reserves within the polder where fish can survive through the dry season. The second solution is to manage the sluice so that the young and breeding adults of fish of interest to the fishery are admitted to the polder. These two solutions are not exclusive and may be used together to add a valuable fish crop to the rice.