Floodplain ecosystems are among the most productive landscapes globally
They offer various indirect and direct services and goods to humans and supporting various ecological communities. In most areas, the regulation of rivers to support energy production and agriculture and prevent flood-related damage has erased the very floods upon which the ecosystems and their biological communities depend.
The last ten years have seen increased efforts to reverse the historical impacts of flow regulation. As a result, the concept of environmental flows has now become a key component in defining the future of water globally.
However, in several river basins, water stress implies that both the environmental and human needs can’t be satisfied, giving rise to difficult trade-offs when allocating finite water resources. As a result, alternative approaches are being sought to enhance environmental benefits while at the same time minimizing environmental water allocations. Some of these approaches include overrunning floodplains also known as artificial flooding through pumping diversion canals. It then follows with the control of water movement on floodplains with weirs, regulators, and levees.
There is no argument that the restoration of floodplain ecosystems via the reinstatement of floods is often slowed down by insufficient water due to competing human demands. That is why an alternative approach relies on floodplain infrastructures such as pumps and regulators. However, although the approach can achieve some ecological targets and is water efficient, it doesn’t imitate the hydrodynamics, hydraulics, and lateral connectivity of natural floods.
The engineering approaches to restoring floodplain ecosystems may risk harmful ecological outcomes such as reductions in river-floodplain productivity, water quality, and biotic connectivity. It may therefore fail to support the various ecological processes required to sustain healthy river-floodplain systems.
Here, we review and focus on the potential risks, mitigation options, and benefits of engineered artificial flooding. We particularly discuss possible risk mitigation options and identify critical design requirements, operational knowledge requirements, and research needs that can help to prevent annoying ecological outcomes that result from engineered floods.
The Hydrological Case for Engineered Artificial Flooding
One of the major effects of river regulation is the reduction in floodplain overwhelming from small-to-medium floods. This leads to indirect and direct effects in rivers and floodplains, including loss of floodplain wetlands, increased mortality of floodplain vegetation, loss of access to rearing & breeding habits for fish, and reduced river-floodplain productivity. Therefore, environmental flow programs for major rivers have a key goal of restoring the natural flood pulses that are considered to have the largest geomorphological and ecological effects.
For instance, managed floods of the Colorado River in the Southern US, are used to target geomorphic processes and maintain and restore the population of key species of fish. However, in many low-gradient rivers with extensive floodplains, large reservoirs are located in the headways.
Augmented flooding using run-of-river environmental flows is difficult without exceeding the design capacity of outflow pipes for upstream storages, exceeding volumes of available water, exceeding natural geomorphic flow-regulating features of the river, and posing a flood risk to associated infrastructure and nearby human populations. These are, therefore, systems where engineering approaches are in use or are proposed.
The main aim of engineering approaches is to manage critical environmental flow targets using less water. The easiest way to achieve this is to pump water actively or trap and divert water passively within certain wetlands or floodplains through levees, flow regulators, channels, and weirs.
Even though the engineering approaches have a high water efficiency, they have proven expensive to operate, maintain, construct and have limited spatial coverage.
Engineered Floods: Risks and Benefits
There are at least three major benefits of engineered artificial flooding: Those relevant to floodplain specialist biota, those that affect the fluxes of energy and materials between the rivers and their floodplains, and those that affect the movement of biota between the floodplain and river habitats.
Floodplain Specialist Biota
For instance, the initial objective of engineered flooding programs in the MDB was to prevent the mortality of floodplain river Eucalyptus Camaldulensis (red gum) forests that were at risk from rising saline water tables and prolonged drought. After implementation, the approach was effective and promoted tree germination, recruitment, and survival. Like natural flooding, the artificial watering of floodplains and wetlands improves the persistence of floodplain wetlands and increases the amount of standing water. This forms an important habitat for waterbirds, fish, aquatic plants, and amphibians.
Therefore, engineered floods are the best option in cases where bird breeding has declined or ceased, natural floods no longer occur, and vegetation condition is poor. The application of the engineered floods, however, presents some risks to floodplain biota. For instance, the increasing floodplain inundation in areas with shallow saline water tables offers only a partial solution to poor floodplain tree health if it isn’t accompanied by lowering the water table.
Material and Energy Exchanges
The hydrodynamics of flooding is very important for many riverine processes. These hydrodynamic characteristics have a major bearing on the patterns of energy, nutrient, sediment, and biotic exchanges. This goes as far as what leaves the river, what is exchanged on the floodplains, and what is returned to the river.
Artificial flooding, therefore, brings one of the greatest environmental risks due to loss or alteration of the above exchanges. Studies on the influence of engineered floods and their impact on the c fluxes have been carried out well. It shows that prolonged floodplain inundation with a limited water exchange from the river can cause hypoxic events.
These events are driven by subsequent microbial respiration and the leaching of C from terrestrial leaf litter. These blackwater events can lead to fish kills that have severe short-term and long-term effects on fish populations. Furthermore, the negative effects of such events can extend downstream in case sufficient hypoxic blackwater enters the river channel.
Floodplain drive C is an essential source of energy that supports production in floodplain rivers. However, for well-regulated rivers, the loss of floodplain inundation events creates an energy bottleneck limiting the carrying capacity of higher trophic levels. This can present strong economic and social implications as far as lost food production is concerned.
To overcome such bottlenecks, you must restore the food webs and their associated energy pathways. The maintenance of energy fluxes and avoidance of blackwater events depend on the hydrodynamics of floodplain inundation, such as water residence time and the duration of inundation.
Biotic Movements and Connections
Apart from floodplain processes, several in-channel ecological processes are triggered by flood events. For instance, a rise in river levels helps in triggering germination and dispersing plant propagules, promoting fish movement, promoting fish recruitment and spawning, prompting some colonial waterbirds to breed.
Certain fish species move out of the river channels to breed, but such movements can be halted and hindered by water control structures. Invertebrates and fish that survive being passed through pumps risk being trapped on the floodplain and not returning to the river channel. The result is the substantial modification of diversity and assemblage structure, thus encouraging the dominance of exotic species and altering the river-floodplain function and structure.
Making Engineered Floods Work
Relying on engineered floods to inundate floodplains comes with a mix of potential risks and benefits. First, some processes are incompatible with some inundation methods because the processes depend on how rivers and their floodplains are connected and the hydrodynamics of flooding. For instance, pumping isn’t compatible with the movement of adult fish into floodplains, even though this can favor small-bodied fish and reduce the spread of exotic species.
Appropriate management of water infrastructure could mitigate some of the risks by simply replicating natural hydrodynamic variability. For instance, blackwater events are relatively well understood and managed better by ensuring that water isn’t ponded on floodplains for long periods.
Although much attention has been paid to effects within areas targeted for water delivery, another major consequence of engineered floods is that higher flood frequencies are restored only to isolated sections of the floodplain. A potential benefit of engineered floods is the ability to target flooding toward high-value ecosystems while reducing the likelihood of inundating human infrastructure and agricultural land. This offers direct benefits to the ecosystem, for instance, by reducing the export of toxic chemicals from agriculture.
Modification of hydraulic parameters of artificial floods such as magnitude, residency, and velocity depends on floodplain topology and priority conservation values and can vary from time to time. Engineered floods and their related infrastructure requirements must be implemented and viewed in the context of floodplain restoration requirements at larger spatial scales. Their benefits should be considered within a temporal context.
A New Conceptual Framework for evaluating Environmental Flows
Engineered floods have proven to offer a practical solution for achieving environmental flow targets in water-stressed systems. However, throughout history, targets for floodplain inundation have been defined in terms of hydrologic characteristics such as frequency, duration, and timing of flood events rather than in terms of hydrodynamics such as velocity profiles, residency time, and variable flow paths.
In most cases, however, these hydrodynamic aspects of floods and their related mosaic of inundation durations, flow velocities, and depths drive essential biological and physical responses. The existing conceptual framework has proven effective when floods are delivered as run-of-river events (either as augmented or natural floods).
The growing use of engineered floods warranted additional hydrodynamic descriptors within the conceptual framework used to set and inform environmental flows, more so where targets require floodplain reconnection.
Conclusion
Engineered floods present an efficient approach to watering floodplains to achieve environmental objectives with limited water volumes. As a result, their application is growing rapidly in some areas to support environmental flow delivery. However, engineered floodplains pose ecological risks associated with the disruption of the upstream-downstream linkages and natural river-floodplain.
Adopting engineered floods involves a trade-off in terms of specific environmental benefits and water savings. There is, therefore, a need to do more research on the risks and effectiveness of this approach.