Floodplains and wetlands: making space for water

Sustainable floodplains through large-scale reconnection to rivers, Opperman et al. 2009

The area of floodplains allowed to perform the natural function of storing and conveying floodwaters must be expanded by strategically removing levees or setting them back from the river. Floodplain reconnection will accomplish three primary objectives: flood-risk reduction, an increase in floodplain goods and services, and resiliency to potential climate change impacts [Opperman 2009: 1487].

Floodplain reconnection reduces flood risk by: (1) replacing vulnerable land uses with flood-tolerant uses, thereby reducing damages, and (2) giving the water somewhere to go, thereby sparing downstream and other nearby communities. Furthermore, by storing and conveying water, floodplains alleviate pressure on upstream dams/reservoirs for flood control and water supply, increasing the resilience of this infrastructure. Finally, by restoring biological activity and diversity, floodplain restoration activates ecosystem services, including carbon sequestration and water quality improvement and groundwater recharge.

The authors note that agricultural lands would be less expensive than densely populated residential areas to reconnect and should be prioritized. Furthermore, agricultural land could remain as such by switching to production of flood-tolerant crops, such as timber and pasture. Furthermore, floodplain reconnection has proven popular among farmers, who requested more than ten times the amount of land be enrolled in a one-time floodplain easement program than the USDA could afford to support with American Recovery and Reinvestment Act funds.

Multifunctionality of floodplain landscapes: relating management options to ecosystem services, Schindler et al. 2014

Human societies tend to value the potential benefits that a landscape might provide in a limited way, adjusting management practices towards desired outputs by maximizing the benefits gained from one or some of the services (often the provision of goods) leading to the loss of multifunctionality and the degradation of natural capital at the expense of human welfare. As a result of this biased valuation, the opportunity costs of biodiversity conservation have been perceived as too high [Schindler 2014: 230].

Using a lens of landscape multifunctionality, this study evaluates 38 potential interventions (ranging from mining, agriculture and residential development to dam removal, natural habitat creation and hiking trail maintenance) in European floodplain ecosystems for their potential to provide multiple ecosystem services (ESS). “Most ESS arise from living organisms and the interaction of biotic and abiotic processes, and refer specifically to the ‘final’ outputs from landscapes that provide benefits to humans”  [Schindler 2014: 230].

Each intervention was evaluated to determine whether its effect on a given ecosystem service was positive, negative or neutral. The more ecosystem services an intervention was considered to positively affect (such as pollination, water purification, flood mitigation, providing for farming, fishing, drinking water, or recreation), the greater its contribution to landscape multifunctionality.

Interventions with the most positive effects were related to the creation of natural habitat, dike relocation, lateral floodplain reconnection, creation of channels, oxbows and ponds, whereas the interventions [related to] terrestrial settlement and transportation infrastructure, navigational infrastructure, and intensive forms of agriculture, forestry and fisheries are rather problematic when preserving multifunctionality in floodplains [Schindler 2014: 238].

Thus the authors found that:

Restoration and rehabilitation measures strongly improved the multifunctionality of the landscape and caused win–win situations for enhancing overall ESS supply for all regulation/maintenance and cultural services, but also for provisioning services [Schindler 2014: 242].

In short, a multifunctional approach allows for ecosystem services and goods that we depend on yet often take for granted, such as clean, abundant drinking water, clean air, pollination, and productive wild fish populations, for example, to be considered in economic evaluations of sites and landscapes such as floodplains.

Need for ecosystem management of large rivers and their floodplains: these phenomenally productive ecosystems produce fish and wildlife and preserve species, Sparks 1995

In their natural state, rivers are not separate or separable from surrounding lands. Rather, a river channel is just one integral part of a larger river-floodplain ecosystem. Annual flood pulses and larger flooding events connect river channels to their floodplains, driving the cycles of life for the particularly diverse ensemble of species that live in floodplain ecosystems. For example, fish use floodplain lakes and backwaters for spawning, shelter, feeding and nurseries. Plants on the floodplain depend on nutrients supplied by sediment deposited during flooding. Due to their geological age, size, habitat complexity, and variability, large river ecosystems – such as the Amazon basin – are among the more biodiverse ecosystems on Earth.

Building levees to contain river water eliminates annual flood pulses, thereby fracturing an ecosystem dependent on these processes. Therefore, for example, “in both tropical and temperate rivers, fish yield per acre is considerably greater in rivers with flood pulses and floodplains than in nearby impoundments where flood pulses are reduced or absent” [Sparks 1995: 172]. In addition,

On land, the natural nutrient-replenishment system once provided by the flood must be replaced with commercial fertilizer. Some societies practice a flood-adapted form of agriculture or harvest both fish and a compatible crop, such as rice, but intensive, high-yield agriculture often conflicts with fisheries, particularly if pesticides are used that can contaminate fish through biomagnification [Sparks 1995: 172].

To at least partially reconnect rivers with floodplains, the author recommends modifying existing structures to divert some flow to create or maintain side channels into the floodplains and restore the annual flood pulse.

The Value of Coastal Wetlands for Flood Damage Reduction in the Northeastern USA, Narayan et al. 2017

The authors address the lack of high-resolution, large-scale assessments of the value of coastal wetlands for reducing property damages from flooding. In the first part of this paper, they assess Hurricane Sandy-induced damages to wetlands. The second part examines the risk reduction benefits of salt marshes in Ocean County, NJ, in terms of average annual economic flood losses. This study involved over 2000 synthetic storm events in Ocean County. The storm events were matched in frequency with actual storms that occurred between 1900 and 2011.  

Wetland extent was positively correlated with damage reduction in all but one of 12 states impacted by Hurricane Sandy. The authors used a hydrodynamic model that calculated the propagation of storm surges from the coastal shelf on to land.  The average amount of damage reduction was slightly over 1%; however, four states with extensive wetlands experienced flood damage reduction of 20-30%.

Losses were less for areas with salt marshes than for those without. On average, salt marshes reduced flood-related damages by 18%. Higher elevations were also correlated with damage reductions.

The authors noted that damage reduction was also apparent at locations several kilometers upstream of affected wetlands. A few areas, however, showed increased storm damage because of their proximity to wetlands. These areas often were dammed, or had their stream channel redirected. Based on their findings, the authors advocate for the increased use of flood risk models  by the insurance industry and small businesses.

The second warning to humanity – providing a context for wetland management and policy, Finlayson et al. 2018

The authors of this article note that prior agreements to halt wetland degradation, such as the Ramsar Convention of 1971, have been largely unsuccessful. They advocate for both a re-emphasis on how wetlands help mitigate climate change, and how to protect existing wetlands from the damaging effects of climate change. They had previously authored the Second Warning to Humanity and Wetlands, which urged the Society for Wetland Scientists (SS) and other organizations to raise the profile of wetlands. Doing so can lead to policy changes which would attenuate the deleterious actions that humans currently apply to wetlands.

The authors then provide 11 recommendations for preserving and renewing wetlands. These recommendations include halting the conversion of wetlands to other land uses, rewilding wetlands with native species, and reducing the wastage of wetland-derived foods. Other recommendations are increasing wetland education, adopting renewable energy sources that don’t impact wetlands, prioritizing the enactment of connected, well-funded and well-managed networks of protected wetland areas, and supporting ecologically sound financial investments.  

Wetlands in a changing climate: science, policy and management, Moomaw et al. 2018

This article emphasizes the global importance of protecting and restoring wetlands in the context of climate change and outlines policy strategies for wetland protection and restoration.

Wetlands play a major though under-appreciated role in climate change mitigation and adaptation. Wetlands enhance local resilience to climate change by providing: “flood storage, buffering of storm damage, protecting water quality by filtering pollutants and sediment out of runoff generated by severe storm events, groundwater recharge and provision of water supply during drought, provision of wildlife refuges and corridors and maintenance of biodiversity” [Moomaw 2018: 192], as well as “direct harvests of fish, animals, and plants” [Moomaw 2018: 188]. Furthermore, wetlands/peatlands store massive amounts of carbon, drawing it out of the atmosphere.  

Peatlands and vegetated coastal wetlands are among the most carbon rich sinks on the planet sequestering approximately as much carbon as do global forest ecosystems [Moormaw 2018: 183].

 

Wetland conditions are critical for C accumulation and storage since decomposition in these systems is limited by a lack of oxygen due to water saturation. Therefore, when plant productivity exceeds decomposition there is an accumulation of soil C. This process eventually develops deep peat deposits, which may accumulate for thousands of years [Moomaw 2018: 187].

By the same token wetlands can become major GHG sources when damaged or destroyed by land use change, fire or climate change.

Altering wetlands can increase the vulnerability of the organic C pool by weakening the self-regulating feedbacks that exist in many peatland systems. Land use change that affects wetland hydrology has had substantial impacts on wetland structure and function. Draining wetlands decreases CO2 uptake and increases rates of microbial decomposition and CO2 release. Soil C is also lost by peat extraction, drainage and other disturbance. The hydrologic changes can be so large that they result in massive losses of C to the atmosphere, such as occurred during the fires in tropical peatlands in Southeast Asia [Moomaw 2018: 187].

 

Many land-use practices in or near wetlands reduce wetlands’ resilience to any further stress, such as hotter, drier weather wrought by climate change.

Unfortunately, many of the world’s freshwater wetlands are already stressed by increased land-use pressure, so that additional hydrological alteration can contribute to an overall decrease in resilience to climate change. Human alteration is commonplace throughout river corridors, challenging management as the impacts of upstream alterations accumulate along the waterway. As demands for river resources increase, such problems are expected to worsen. Flowing water is compromised by river re-engineering practices, even though moving water generally improves oxygenation and plant health. Also, upriver freshwater extraction in tidal freshwater wetlands coupled with sea level rise can cause the salinification of surface and ground water, with accompanying stress and even the collapse of tidal vegetation in the freshwater reaches of estuaries [Moomaw 2018: 188].

On the other hand, wetland resilience can be bolstered through proper land management.

The effects of climate changes on wetland C storage will be determined largely by the extent to which the wetlands have been modified through land-use change [Moomaw 2018: 187].

 

One opportunity to decrease the amount of saltmarsh loss that is likely to occur with sea level rise is to actively plan for future inland marsh migration now [Moomaw 2018: 191].

The authors express concern that wetlands are overlooked in policy discussions on climate change, noting that climate scientists tend to sideline the role of wetlands, while wetlands science and management have often failed to acknowledge the outsized role of wetlands as a carbon sink. Thus:

To play a more effective role in climate change mitigation and adaptation/resiliency, wetland scientists need to clearly communicate the significance of wetlands to the wellbeing of society and the economy. Communicating with policy makers and the public requires aligning wetland science and specific climate mitigation and adaptation/resiliency ecosystem services with the concerns and mindset of the audience [Moomaw 2018: 198].

A handful of policy structures at international, national and subnational levels aim to better account for and protect wetlands. For example, the International Panel on Climate Change (IPCC) has since 2013 provided guidance (through the Wetlands Supplement) to countries about including wetlands in national GHG inventories, thus moving “closer to requiring countries to account for the substantial emissions from these ecosystems when they are disturbed or destroyed” [193]. The 1975 Ramsar Agreement establishes an international framework for wetland management, but lacks adequate guidance on how to best protect wetlands from the stressors of climate change. At the local level, decisions about wetlands are often made by land managers.

Thinking globally and acting locally, wetland managers can incorporate carbon management and climate resiliency science into project-level work (including developing a body of climate-related Best Management Practices), whether or not governing policies and regulations exist. As noted earlier in this article, avoidance of impacts to wetlands, and associated carbon stocks and processes, is likely to be the most effective management practice for preventing increases in GHG emissions from wetlands, protecting climate resiliency functions, and protecting traditional wetland ecosystem services, and it is therefore important for managers to understand the underlying science [Moomaw 2018: 197].

Future response of global coastal wetlands to sea-level rise, Schuerch et al. 2018

The vulnerability of coastal wetlands to sea-level rise is disputed, with some researchers predicting most will be flooded out of existence by the end of the 21st Century. Coastal wetlands provide critical ecosystem services, including protection from storm surges, water quality improvement, fisheries habitat and carbon sequestration. By accounting for the enhancement of sediment build-up when storms are more frequent and more severe and for the possibility of “accommodation space” for coastal wetlands to move inland, however, these authors reach a more optimistic conclusion. (Sediment build-up, or accretion, allows coastal wetlands to grow vertically, potentially remaining at a higher elevation than sea-level.) They estimate that:

Rather than losses, wetland gains of up to 60 per cent of the current area are possible, if more than 37 per cent (our upper estimate for current accommodation space) of coastal wetlands have sufficient accommodation space, and sediment supply remains at present levels [Schuerch 2018: 231].  

This is an important ecosystems restoration message because it means humans can directly influence the persistence of coastal wetlands, and thus the continuation of the essential ecosystem services they provide.

This is an important ecosystems restoration message because it means humans can directly influence the persistence of coastal wetlands, and thus the continuation of the essential ecosystem services they provide.

Our simulations suggest that the resilience of global wetlands is primarily driven by the availability of accommodation space, which is strongly influenced by the building of anthropogenic infrastructure in the coastal zone and such infrastructure is expected to change over the twenty-first century. Rather than being an inevitable consequence of global sea-level rise, our findings indicate that large-scale loss of coastal wetlands might be avoidable, if sufficient additional accommodation space can be created through careful nature-based adaptation solutions to coastal management [Schuerch 2018: 231].

The authors describe specific solutions to protect coastal wetlands, which they recommend be implemented at a large, regional or landscape scale.

Existing nature-based adaptation solutions that allow coastal wetlands to migrate inland include the inland displacement of coastal flood defenses (typically along highly engineered coastlines) or the designation of nature reserve buffers in upland areas surrounding coastal wetlands. These schemes, however, are currently implemented as local-scale projects only; strategically upscaling such projects, for example, as suggested by the shoreline management plans in England and Wales or the coastal master plan in Louisiana, may help coastal wetlands adapt to SLR [sea level rise] at the landscape scale and protect rapidly increasing global coastal populations [Schuerch 2018: 234].