The Lugeon Test: Methodology, Interpretation, and Applications in Geotechnical Engineering

As urban flooding intensifies globally due to rapid urbanization and climate change, there's a growing recognition of Nature-Based Solutions (NBS) as sustainable and cost-effective alternatives or complements to traditional "gray" infrastructure for flood risk mitigation. NBS involve actions that protect, sustainably manage, and restore natural or modified ecosystems to address societal challenges, including flood risk management. These solutions, sometimes referred to as "natural infrastructure" or "green infrastructure," aim to work with nature rather than against it, leveraging natural processes to slow, store, and filter floodwaters. Beyond flood reduction, NBS offer a range of co-benefits, such as improved water quality, enhanced biodiversity, carbon sequestration, temperature regulation, and recreational opportunities.

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Wetlands Restoration: Nature's Sponges for Flood Control

Wetlands, including marshes, swamps, and bogs, are highly effective natural systems for mitigating floods, often called "nature's sponges". Their saturated soils and dense vegetation enable them to absorb significant volumes of water during heavy rainfall and flood events, gradually releasing it over time.

Their flood routing contributions include:

• Trapping and slowly releasing surface water, rain, and floodwaters, which effectively reduces the speed and peak of flood waves.
• Holding large volumes of water over wide areas, thereby absorbing rainwater runoff and preventing sudden, destructive floods. A single acre of wetland can store up to 1.5 million gallons of floodwater.
• Reducing wave energy and coastal surges by absorbing the force of incoming water, particularly in estuarine and coastal settings.

Research indicates that restoring or creating wetlands can significantly lower flood levels upstream by as much as two meters in some cases, and reduce the extent of flood damage by over 35%. For instance, a study in the Greater Kumasi Metropolitan Area (GKMA) in Ghana showed that wetland cover declined by 59% from 1986–2023, leading to more intense, shorter-duration floods. Conversely, a combined scenario of floodplain restoration and wetland creation achieved consistent peak flow reductions of 16–19% in prolonged storms.

Beyond flood control, wetlands provide crucial ecosystem services:

• Water quality improvement by filtering pollutants like heavy metals, sediment, nitrogen, and phosphorus.
• Groundwater recharge by promoting infiltration.
• Biodiversity enhancement by providing vital habitats for diverse wildlife species, including many threatened and endangered species.
• Carbon sequestration, with some wetlands, particularly peat wetlands, storing significantly more carbon than rainforests.
• Recreational opportunities such as bird watching and educational spaces.

Despite these clear benefits, approximately 90% of the world's wetlands have been lost since 1900, contributing to increased flood vulnerability. Case studies, such as the Beddagana Wetland Park in Sri Lanka, demonstrate that wetlands can capture 39% of floodwaters during storms, and their degradation leads to significant economic losses.

Floodplain Reconnection: Giving Rivers Room to Breathe

Historically, rivers naturally connected with their adjacent floodplains during high flow events, which are the relatively flat lands next to a river that become inundated in a flood. However, channelization and urban development have often disconnected rivers from their floodplains. Floodplain reconnection involves removing or setting back levees and barriers to allow rivers to access these natural floodplains during high flows.

This strategy contributes to flood routing by:

• Providing space for floodwaters to spread out, which significantly reduces downstream flood peaks and velocities and expands the river's capacity during peak flows.
• Restoring hydrological connectivity, improving sediment distribution, and enhancing groundwater recharge as water seeps into the soil over a wider area.
• Attenuating the flood wave and extending its travel time, which is critical for protecting downstream communities.

A prominent example is the Netherlands’ “Room for the River” program, which involves projects like lowering floodplains, setting back levees, and increasing the depth of side channels to give rivers more room during floods. This program reduces urban and agricultural flood risk while restoring ecological function. Similarly, in the US, setting back levees on the Missouri River proved more cost-effective than rebuilding them after multiple failures.

Beyond flood reduction, reconnected floodplains offer:

• Replenishment of groundwater aquifers.
• Deposition of nutrient-rich sediments, enhancing agricultural productivity.
• Creation of diverse habitats for aquatic and terrestrial species.
• Improvement of aesthetic and recreational value of river corridors.

Diversion Canals and Green Buffers: Redirecting Flow Safely

Diversion canals are engineered channels used to redirect excess water from rivers or storm drains to designated storage areas, wetlands, or less vulnerable parts of the landscape. When these canals are integrated with natural floodplains or restored wetlands, they become powerful flood routing tools by actively diverting peak flows away from critical infrastructure and allowing controlled inundation of areas designed to safely store or absorb floodwaters.

Green buffers, which are strips of trees, shrubs, and grasses along rivers and streams, are another effective NBS for flood routing. These vegetated zones help by:

• Intercepting and slowing surface runoff, reducing the volume and velocity of water entering rivers.
• Stabilizing banks and reducing erosion, which maintains channel capacity and prevents sedimentation downstream.
• Enhancing infiltration, promoting groundwater recharge and reducing surface flood risk.

Green buffers also provide wildlife corridors, improve water quality by filtering pollutants, and offer recreational and aesthetic benefits.

Groundwater Recharge: Storing Floodwaters Underground

Groundwater recharge is the process of directing excess surface water into the ground to replenish aquifers. During floods, managed aquifer recharge (MAR) systems, such as infiltration basins, recharge wells, and permeable pavements, can absorb and store floodwaters, reducing surface runoff and downstream flood peaks. This approach is particularly useful in regions where water scarcity coexists with seasonal flooding, as it also increases groundwater availability for dry seasons, reduces the risk of land subsidence, and helps maintain base flows in rivers.

Quantifying Benefits and Addressing Implementation Challenges

Recent advances in hydrodynamic and hydrologic modeling, such as the TELEMAC-2D model, enable planners to quantify the flood mitigation benefits of NBS. These models can simulate how restored wetlands or reconnected floodplains attenuate flood peaks compared to traditional infrastructure, generating high-resolution flood maps to inform decision-makers.

Despite their numerous advantages, implementing NBS faces several challenges:

• Land availability and competing land uses can limit opportunities, especially in urban areas.
• Quantitative data and long-term monitoring are needed to compare NBS performance with gray infrastructure and build confidence among engineers and policymakers.
• Integration with existing infrastructure requires careful planning to ensure NBS complement, rather than conflict with, traditional flood control systems.
• Overcoming decades of familiarity with gray infrastructure and perceptions that NBS are more costly or less effective, often due to a failure to account for co-benefits or avoided costs.
• Upfront costs for land acquisition or specialized environmental engineering expertise can be significant.

However, hybrid approaches that combine NBS with engineering solutions are increasingly favored to enhance both immediate and long-term flood resilience. Such combinations can leverage the ecosystem service benefits of nature-based options while protecting existing hard infrastructure and making it more effective.

In conclusion, the trend toward Nature-Based Solutions is clear. By embracing the intrinsic capabilities of natural systems—restoring wetlands and floodplains, constructing eco-engineered diversion canals, enhancing green buffers, and promoting groundwater recharge—communities can build more resilient, sustainable, and livable landscapes in the face of intensifying climate risks and growing urban footprints.