Deduru Oya Basin Flood Impact Analysis – November 2025
Comprehensive Assessment of Flood Hazards
The Deduru Oya Basin is the fifth-largest river basin in Sri Lanka, covering about 2683.6 km² and originating in the Matale Hills in the wet zone before flowing to the western coast near Chilaw in the Puttalam District. Rainfall is the main water source, with high rainfall during the Maha season (September–December) and low rainfall during the Yala season (March–June). The basin includes wet and intermediate climatic zones, which create uneven water distribution and seasonal water shortages in some areas.
The basin area is distributed across several administrative districts, mainly Kurunegala District, and parts of Matale, Kandy, Kegalle, Anuradhapura, and Puttalam Districts. Kurunegala covers the largest portion of the basin, while the river flows westward and finally reaches the sea in Puttalam District. This distribution across multiple districts makes water management more complex, as different administrative authorities are involved in managing land and water resources within the basin.
During late November 2025, the Deduru Oya Basin experienced a significant flood event due to intense and continuous rainfall over the upper and middle catchment areas. Heavy rainfall recorded throughout the last week of November caused a rapid increase in river water levels, leading to major flood conditions across the basin. As inflows increased substantially, the Deduru Oya Reservoir began releasing water at discharge rates of approximately 16,200 cubic feet per second around November 26–27, 2025, to manage the rising storage levels.
The Department of Irrigation issued and extended multiple “Red Alert” flood warnings between November 23 and November 29, 2025, particularly for low-lying and flood-prone areas. High-risk locations included Wariyapola, Nikaweratiya, Mahawewa, Kobeigane, Bingiriya, Pallama, Halawatha (Chilaw), Arachchikattuwa, and Rasnayakapura. By November 27–28, major flood conditions were observed, with river water levels exceeding minor flood levels at key gauging stations such as Moragaswewa. The flooding affected residential settlements, agricultural lands, and local infrastructure, highlighting the basin’s vulnerability to extreme rainfall events during the northeast monsoon period.
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Assessment of assets and infrastructure located within the flood-prone areas, roads, and land use patterns.
The flood hazard level was classified into six categories based on flood depth to represent the severity of potential impacts on people, infrastructure, and the environment. Areas with lower depth values were categorized as Low and Moderate hazard, indicating limited inundation and minor potential damage.
As flood depth increases, hazard levels progress to High and Very High, where structural damage and risks to human safety become significant. Depths exceeding critical thresholds were classified as Extreme and Catastrophic, representing areas with severe inundation, high destructive potential, and serious threats to life and property. This classification clearly identifies high-risk zones and supports effective flood risk management and planning decisions.
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The Deduru Oya flood exposure analysis reveals a substantially wider administrative impact compared to the Deduru Oya basin flood. The affected area extends across both Puttalam District and Kurunegala District, encompassing 19 Divisional Secretariat Divisions (DSDs) and more than 340 Grama Niladhari Divisions (GNDs). This highlights the broader spatial footprint and administrative complexity of the Deduru Oya flood event.
Within Puttalam District, flood exposure covers 9 DSDs and 168 GNDs. The impact stretches from coastal lowland areas such as Arachchikattuwa (32 GNDs) and Chilaw (44 GNDs) to interior floodplain divisions including Madampe, Mahawewa, Mundel, and Pallama. Notably, even fringe divisions such as Kalpitiya, Mahakumbukkadawala, and Puttalam town fall within the defined exposure boundary, demonstrating the extensive reach of inundation across both coastal and inland environments.
On the Kurunegala District side, the flood extends across 10 DSDs and 176 GNDs, primarily affecting rural and agricultural communities. The most exposed DSD is Panduwasnuwara West (34 GNDs), followed closely by Bingiriya (32 GNDs). The flood impact continues through mid-basin divisions such as Kobeigane, Nikaweratiya, Rasnayakapura, and Udubaddawa, while also touching the peripheral areas of Wariyapola, Kuliyapitiya West, and Kotawehera, indicating significant upstream and mid-basin vulnerability.
The disaster impact assessment of selected major transport segments indicates that a total of 31.91 km of road and railway infrastructure is exposed to flooding. The affected segment lengths range from 0.52 km to 4.54 km, demonstrating considerable variation in the intensity of exposure across the network. On average, each affected segment accounts for approximately 1.52 km of impacted distance.
Notably, the Peliyagoda – Puttalam Road (A003), a critical national transport corridor, records one of the highest exposure lengths at 4.53 km, highlighting its strategic vulnerability. While a few major corridors experience impacts exceeding 3 km, the majority of affected segments are less than 1 km in length, indicating that flood impacts are spatially distributed and occur in fragmented sections rather than along a single continuous route.
When considering the entire transport network by road type, the total flood-exposed distance increases to approximately 1,101 km. The largest proportion of impact is observed on Jeep/Cart Tracks (about 750 km), indicating that rural and agricultural access routes are the most vulnerable within the basin. This broader analysis shows that although major highways are strategically important, the overall flood burden is predominantly concentrated on rural and secondary transport infrastructure.
The land use analysis under Classification 01 indicates that cultivation area is the dominant land cover type, covering approximately 23,642 ha. This represents the largest proportion of the affected landscape, significantly exceeding built-up areas (around 6,557 ha). The dominance of agricultural land highlights the high exposure of productive farming zones within the flood-affected area.
A more detailed breakdown under Classification 02 further emphasises the agricultural significance of the basin, with Paddy and Coconut emerging as the two most important cultivation types. Paddy covers approximately 14,732 ha, making it the single largest land use category, while Coconut plantations account for about 7,580 ha. Together, these two categories comprise the majority of cultivated land, indicating that flood impacts are heavily concentrated on staple food production and perennial cash crops.
Other agricultural and related land uses, such as Other Cultivation (about 1,190 ha), Aquatic Farms (approximately 2,096 ha), and Homesteads/Garden areas (around 4,308 ha), also contribute notably to the cultivated landscape. However, the predominance of paddy fields and coconut plantations clearly demonstrates that the hazard area.
During the early 1900s, at the beginning of the 20th century, the department established a geodetic triangulation network with an average accuracy of 1:40,000. Following this, the entire island was mapped using the plane table method at the scale of one inch to one mile. This map series, popularly known as the “One Inch Topographic Map Series,” consists of 72 sheets covering the entire island. These maps provided the first systematic and standardized representation of Sri Lanka’s topography, forming the foundational dataset for subsequent mapping and land management activities.
An overlay analysis using the digitised One-Inch map dataset in 2020, collected by the GIS Branch of the Survey Department, identifies approximately 3,118 tank (reservoir) features within the Deduru Oya Basin. Among these, about 2,591 tanks are currently functioning, while 527 are classified as abandoned.
These tanks play a critical role in flood control by temporarily storing excess runoff during heavy rainfall events. During peak discharge periods, tanks act as decentralised retention basins, reducing downstream flood intensity by slowing surface flow, promoting infiltration, and regulating water release through spillways. For example, in upper and mid-basin areas, functioning tanks can intercept rapid monsoonal runoff before it reaches major river channels, thereby minimising flash flooding in low-lying agricultural and settlement zones. Rehabilitation of abandoned tanks could further enhance this natural flood buffering capacity, contributing to integrated basin-scale flood management and climate resilience.