Introduction

Trees reduce the volume of storm-water surface runoff in three steps: First, the leaves, branches, and trunks catch and intercept the raindrops. The water then trickles off into the ground or evaporates into the air. The rate of the interception, infiltration, surface runoff reduction and, therefore, the flood response increase with the forested area in relation to the catchment size. In this regard, the measure is also linked to surface water management (see Category II).

Mangrove forests are a specific type of tree for flood risk mitigation. They are multi-functional ecosystems that mainly occur along sheltered tropical and subtropical coasts. As for flood risk mitigation, mangroves reduce the wind height, incoming waves, and the level of storm surges. In addition, they protect the coastline and control its erosion.

Figure below: Example of a Miyawaki forest built by the SUGi team in Buea, Cameroon, 2.5 years after the tree planting. SUGi 2022.

Benefits & Risks

Next to mitigating the stormwater flow, trees comprise several other benefits. They reduce heat islands in built environments. Trees also absorb pollutants from the air, the groundwater and the soil, reduce high-frequency noise and, by that, tackle human health issues such as respiratory illnesses or mental distress.

With afforestation representing a form of ecological restoration, forests or single trees also support the local biodiversity. For example, the tree canopies of mangroves serve as bird nesting and resting grounds while fostering coral reefs and seagrass beds. They only can be planted in very specific contexts of natural environment and climate. Because mangroves commonly grow surrounded by half-salty water, the trees depend on adequate sea-surface and air temperature and may be fragilized by repeated flood events and other climatic effects.

Environmental Impact

Forests, particularly mangroves, act as carbon sinks and serve carbon sequestration and nutrient cycling above and below the ground. However, this function comes with the risk of high carbon release in case of loss or deforestation of trees, especially mangroves.

Good Practice

Design Example: Miyawaki Pocket Forests For Flood Risk Mitigation

The pocket forests for flood risk mitigation of the organization SUGi describe 4 to 10-meter-wide patches of trees that form a barrier between a waterbody and the infrastructure that needs protection. The SUGi forests implemented around the globe are based on the Japanese Miyawaki forest planting technique, which is a special approach to highly dense forest planting with several layers of vegetation at different heights, combining shrubs, (sub-) trees, and canopy trees. Thanks to their density and strong root system, the pocket forests build a wall-like structure based on vegetation to shield the infrastructure from flooding (SUGi 2022).

References

SUGi    (2022):  Revive waterways and biodiversity in Cameroon

UN Environment Programme    (2020):  Celebrating International Mangrove Day: spare a thought for our coastal ecosystems.

Watson, Julia  (2020):  Lo-TEK - Design by Radical Indigenism

Score Card

Criteria

Scale of Intervention

Shelter-Plot-Block Settlement Supra-settlement

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Type of Intervention

Engineered Nature-based Hybrid Non-structural

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Targeted Natural Hazard

Pluvial Flood Coastal/Riverine Flood

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Strategy Type

Relocate Reduce Hazard Magnitude Reduce Asset Vulnerability Reduce Casualties

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Implementation Time

Short (1 day ‐ 1 month) Medium (1 month ‐ 1 year) Long (> 1 year)

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Effect Duration

Short‐term ( <1 year ) Medium‐term (1 year to 10 years) Long‐term (>10 years)

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Targeted Vulnerable Assets

Buildings Transport Technical Infrastructure Land Cover

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Investment Costs

Low Medium High

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Maintenance Costs (yearly)

Low (<10% investment costs) Medium (10-50%) High (>50%)

  Next to training local personnel, the maintenance (and its cost) includes the watering, pruning, thinning, debris removal, and disease inspection.

Materials

Soil, Water, Native seedlings, Mulch, Shovels, Backhoe

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