Nov 29

Flood Resilience and Water Quality

A practical way to keep living spaces safe and water clean

Heavy rainfall will continue to occur in Sri Lanka. Yet, storms forming elsewhere and heavy rainfalls do not need to be disasters and constant surprises that Lankans gasp at and become a victim. We already have many features that can help: urban wetlands, old paddy fields, low-lying open spaces, drains, canals, rivers and coastal wetlands. Studies on Colombo and other urban areas show that these systems can store stormwater, reduce peak flows and protect houses and infrastructure when they are kept and used properly.

This article gives practical steps Sri Lanka can take to reduce flood damage and keep water cleaner, using realistic engineering and nature-based measures that match both Sri Lankan evidence and global experience.

Overview

The idea is simple: give water safe places to go, and support that path all the way from the town to the sea.

Rainwater should soak into the ground wherever possible, so drains are not overloaded.

Trees and forests help this process by increasing how much water soil can absorb, slowing stormwater and reducing how quickly rainfall turns into damaging runoff, especially in tropical climates like Sri Lanka.

Extra water must have open spaces (sports grounds, playgrounds, old paddy fields and other low areas) that can take shallow water during storms without harming homes.

Drains and canals must guide water safely towards wetlands and larger canals so it does not stay on roads or inside houses.

Wetlands and marshlands must clean and store part of the stormwater before it enters rivers.

Rivers must carry water to the sea in a controlled way, not all at once, so downstream areas are not hit by sudden peaks.

Mangroves and coastal wetlands must protect the coastline, reducing coastal flooding and wave impact.

Modern early-warning systems based on weather forecasts, rainfall data and river-level gauges provide advance notice long before riverbank indicator boards, giving communities and local authorities time to clear trash grills, drain inlets and canal entry points before peak rainfall, reducing the risk of urban flooding.

Role of Trees in Preventing Floods

Forests play a major role in controlling floods because they increase how much water the soil can absorb and reduce how quickly rain turns into runoff. When there is more tree cover, flood damage is usually much lower, especially in medium and frequent storms. A global study across many developing countries showed that a 10% loss of natural forest caused significantly more floods and longer flood durations.

1. What the evidence shows: Forests and Floods at the Global Scale

Researchers in hydrology and land-use science consistently confirm the following:

• Forests increase infiltration and soil stability, helping stormwater enter the ground where soil and terrain are suitable.
• Forest soils store water for days or weeks.
• Forests slow down stormwater, reduce runoff speed, and reduce erosion before it reaches streams.
• Deforestation causes faster runoff and higher flood peaks.

The direction of the evidence is clear and strong: More trees result in less damaging floods. Less trees result in more damaging floods.

This relationship is especially strong in tropical climates such as in Sri Lanka.

It is important to use a mixture of native species rather than monocultures, as diverse riparian vegetation improves stability, ecological resilience and hydrological function. Because Sri Lanka-specific quantitative measurements remain limited, field validation such as pilot planting and hydrological monitoring is recommended before large-scale deployment.

2. Why Floods in Sinharaja Rarely Cause Damage

Sinharaja receives extremely high rainfall every year, yet damaging floods almost never occur inside the forest. There are four reasons:

• Soil in forests has the highest capacity to absorb water because of deep roots.
• Forest soil structure has a sponge-like nature due to high organic matter.
• Rainfall falling on tree canopies evaporates or slows down before reaching ground.
• Trees dry the soil between storms through evapotranspiration, making room for new rain.

As a result, stormwater reaches rivers slowly and gradually. Even large rain events rarely turn into disasters.

Sri Lankan disaster-risk studies confirm that loss of forests directly increases the speed and height of floods in downstream towns.

3. What Happens When Forests Are Cleared?

When forests are cleared:

• Soil becomes compacted and water cannot enter deep.
• Runoff becomes rapid and uncontrolled.
• Rivers rise quickly and suddenly.
• Flood damage increases sharply even during moderate storms.

This has already happened in parts of the Kelani Basin and Gampaha lowlands because forest and marsh buffers were replaced by concrete and filled land.

4. How Trees Improve the Soil’s Water Capacity

• Roots create macropores that guide water deep underground.
• Organic matter increases the ability of the soil to retain water.
• Roots bind soil, preventing erosion of banks during high flow.
• Tree shade protects soil from drying too hard and cracking incorrectly.

The result is simple:
Forests increase the room in the landscape to store stormwater.

5. Native Sri Lankan Trees that Minimize the Impact of Floods

• Kumbuk – Terminalia arjuna
• Mee – Madhuca longifolia
• Hora – Dipterocarpus zeylanicus, D. hispidus
• Dun (Doona group) – Shorea trapezifolia
• Karanda – Pongamia pinnata
• Nuga (Banyan) – Ficus benghalensis
• Bo – Ficus religiosa

These species:

• Grow well in wet zone and intermediate zone floodplains
• Stabilize riverbanks with strong root systems
• Improve infiltration, reduce erosion, and support soil and bank stability rather than directly absorbing large volumes of floodwater
• Support biodiversity and shade along river corridors

Additional species that reinforce soils:

• Vetiver – Chrysopogon zizanioides
• Clumping bamboo – Bambusa, Dendrocalamus

These grasses and bamboos strengthen slopes and prevent erosion during floods.

6. Key points relevant to Sri Lanka

Forests cannot stop the largest extreme flood events. However, they greatly reduce:

• How often floodwater rises high enough to cause damage.
• How fast stormwater reaches rivers.
• The severity of erosion and riverbank collapse.

In simple physical terms,

With trees, water sinks into the ground
Without trees, water rushes over the ground

Therefore, growing more trees is one of the most effective and affordable flood-mitigation actions for Sri Lanka.

Wetlands and marshlands: cleaning water and handling storms

What wetlands do

Soil and slow water in wetlands let sediment and attached pollutants settle before water reaches rivers and canals. Plant roots in wetlands absorb nutrients and other contaminants, reducing eutrophication and improving downstream water quality. Microbial processes in wetland soils and water help break down organic pollution and some industrial wastes into less harmful forms. Therefore, water leaving wetlands is usually less polluted and safer for rivers, lakes, lagoons and wells.

Wetlands as flood buffers

Wetlands in Colombo and Muthurajawela have been shown to store large volumes of stormwater, lowering peak discharge and flood depth in surrounding urban areas.

Pre-treatment before water reaches wetlands

To avoid wetlands being choked by waste:

• Trash grills must be installed where street runoff and household outflows enter main drains.
• Sand and silt traps should be used at key entry points.
• Oil and grease traps should keep petroleum products out of wetlands.

Rules for protecting wetland function

• Wetlands must not be land filled or built over with constructions.
• Wetlands must stay hydrologically connected to streams and drains (both unclogged inflows and outflows should exist).
• Wetlands must not be treated as dumping grounds (you cannot throw plastic bottles in there and expect them to be degraded).

Space for water inside towns and cities

• Sports grounds should be designed so they can safely hold shallow water during big storms without damaging nearby houses or buildings.
• Playgrounds can double as temporary water storage.
• Low-lying municipal land should remain open so water has safe areas to spread to.
• Old paddy fields and floodplains should be protected as flood-retention areas.

These areas still serve daily uses; they only become water storage during storms.

Drains and canals that guide water safely

Pipes under roads must be surveyed and repaired. In streets that flood repeatedly, drains must be widened or deepened. New roads and buildings must not cut across natural water paths without proper channels. Every catchment must have more than one drainage path. Where possible, drains should first send water to wetlands or retention ponds.

Engineering upgrades for drains with low maintenance

Must be planned with expert advice.

Plants for stabilising riverbanks and canal banks

Certain trees and grasses (Kumbuk – Terminalia arjuna, Mee – Madhuca longifolia, Hora – Dipterocarpus zeylanicus, D. hispidus, Dun (Doona group) – Shorea trapezifolia, Karanda – Pongamia pinnata, Nuga (Banyan) – Ficus benghalensis, Bo – Ficus religiosa, Vetiver – Chrysopogon zizanioides, Clumping bamboo – Bambusa & Dendrocalamus) are suitable for stabilizing banks and reducing flood impacts. At the water’s edge, bamboo species and vetiver can be planted. On the middle slope of the bank, mixed rows of native trees stabilise soil, absorb water and nutrients, and provide shade and habitat. On the upper bank and floodplain, deeper-rooted trees can be used.

Before forecasted large storms, reservoir water levels must be managed carefully.

Safe housing only in flood-risk zones

In known flood-risk zones, main living floors should be raised on short pillars or stilts above recorded flood levels. The ground level should remain open or be used only for parking and storage. Yards should not be concreted. In very dense floodplains, apartment buildings allow more people to live safely above flood level.

River mouths and coastal wetlands

Mangroves and coastal wetlands reduce wave energy, storm surge height and coastal flooding. Heavy construction should be kept away from river mouths and lagoon edges. Sand bars or blockages at outlets must be cleared periodically. Releases from upstream reservoirs must be timed carefully.

Monitoring water levels and acting early

Installing water-level gauges together with rainfall data allows early warning before floods become severe. Using weather and tide forecasts helps time releases. Before major storms, reservoirs and upstream storage must be managed to create buffer space.

How early-warning systems help in practice

• Local maintenance teams can clear trash grills before rain
• Pumps and gates can be started early so water is moved out of low-lying canals before they overflow
• Households and shops in risk areas can move vehicles, goods and important documents to higher ground before water arrives
• Schools and families can arrange for children to stay safe at home when flood risk is high

End result

If these measures are put in place:

• Water will be cleaner
• Floodwater will have safe places to go
• Economic damage will be lower
• Groundwater and wells will be safer
• Coastal flooding and erosion will be reduced
• Floods will occur, but with proper planning, they will not ruin people’s lives every time it rains.

Climate change, modelling and ecosystem-based adaptation

Hydrological and hydraulic models such as HEC–HMS have been applied in key basins to:

• Assess how changes in land use and rainfall extremes alter peak flows and flood extents
• Calibrate multi-variable models in data-scarce dry-zone river basins
• Test the effect of different combinations of reservoirs, wetlands, paddy fields and urban drainage on downstream risk

Ecosystem-based adaptation is repeatedly identified as a cost-effective and multi-benefit strategy for Sri Lanka’s water sector.

These measures:

• Reduce flood peaks and erosion
• Improve baseflows and water quality
• Support biodiversity and livelihoods

Model results and field evidence together show that ecosystem-based adaptation is most powerful when embedded in basin-scale planning, early-warning systems and clear institutional responsibilities for maintenance.