Sarawak is located immediately north of the equator and it experiences two monsoons yearly. The North East Monsoon, which usually occurs between November to February, brings with it heavy rainstorms. The annual average rainfall is above 3,000 millimeters. During these rainstorms, flooding in the low-lying areas and natural floodplains along many rivers and even in some urban areas are common.

Over the years, intensive urban development has resulted in a large proportion of the land surface been paved or covered with impervious surfaces, roads and buildings. This leads to dramatic increase in surface run-off during rainfall. The development of floodplain of some watercourses has also reduced their flood carrying capacity and has further aggravated the flooding problem.

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Development dramatically alters the local hydrological cycle. The hydrology of a site changes during the initial clearing and grading that occur during construction. Trees, grasses, and agricultural crops that intercept and absorb rainfall are removed and natural depressions that temporarily pond water are graded to a uniform slope. Cleared and graded sites erode, are often severely compacted, and can no longer prevent rainfall from being rapidly converted into stormwater runoff.

The situation worsens after construction. Roof tops, roads, parking lots, driveways and other impervious surfaces no longer allow rainfall to soak into the ground. Consequently, most rainfall is converted directly to runoff. The increase in stormwater can be too much for the existing natural drainage system to handle. As a result, the natural drainage system is often altered to rapidly collect runoff and quickly convey it away (using curb and gutter, enclosed storm sewers, and lined channels). The stormwater runoff is subsequently discharged to downstream waters such as streams, reservoirs, lakes or estuaries.

Water Quality is affected by the accumulation of trash, oil and rubber from cars, fertilizers and pesticides applied to lawns, sediment from bare or poorly vegetated ground and other pollutants entering streams and rivers. Inflow of sediment can cloud water, blocking sunlight from submerged plants. Sediment also settles to the bottom of streams, clogging the gravel beds used by fish for laying their eggs. Nutrients, such as phosphorus and nitrogen, from fertilizers enter the water and promote unusually rapid algae growth. As this algae dies, its decomposition reduces or eliminates oxygen needed by fish, shellfish, and other aquatic life for survival.

These are all examples of nonpoint source pollution, one of the major contributors to the degradation of quality in Sg. Sarawak river. Stormwater management practices help control nonpoint source pollution through the use of nonstructural and/or structural techniques to intercept surface runoff from developed areas, filter and treat this runoff, and then discharge it at a controlled rate. The overriding condition that governs the quantity of stormwater runoff is the amount of impervious surfaces located on your property (driveways, roofs, carports, sidewalks, etc.) Stormwater quality, however, is governed by the accumulation of pollutants on the entire surface area, regardless of whether it is grassed or paved. As the use of chemicals around the home such as fertilizers, pesticides, engine oils, deicing materials, and similar products increases, the more degraded the stormwater runoff from your property will be. Although the effect of one property on the quality and quantity of stormwater runoff may seem insignificant, the cumulative impact from hundreds of thousands of yards across the State continues to be destructive to our water quality.

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Limit the amount of impervious surfaces in your landscape. Use permeable paving surfaces such as wood decks, bricks, and concrete lattice to allow water to soak into the ground. Where possible, direct runoff from impervious surfaces across vegetated areas.

Allow "thick" vegetation or "buffer strips" to grow alongside waterways to filter and slow runoff and soak up pollutants.

Plant trees, shrubs, and groundcover in your compound area.

Below are some of the pictures that describe the infiltration process and the creativity transforming the conventional method to the environment friendly type of drainage:

• The process of infiltration through permeable surfaces.
• Grassy swale with coble stone bottom.
• Brick pavement to replace impervious surfacing of the road.
• Permeable surfacing for landscaping
• Pervious surfaces to slow and reduce runoff volume and to beautify the community

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1.0 Drainage Masterplan Studies

Since 1993, the government has commissioned drainage master plan studies for Kuching, Sibu and Miri. These studies comprehensively examined the adequacy of the existing drainage systems and developed short to long term drainage improvement measures to meet the current standards and the future needs.

The drainage master plan also provides various government departments with the necessary details and tools to plan and regulate future urban development to minimise the occurrence of flash flooding.

Various drainage studies have been carried out either for a town or for a catchment area. These studies are:-

• Miri Town Drainage Master Plan Study
• Sibu Town Drainage Master Plan Study
• Sg. Sarawak Environmental Control and River Management Study
• Kuching City Drainage Master Plan Study
• Sg. Siol Catchment Drainage Master Plan
• Sg. Maong Planning & Development Study
• Sg. Kuap Catchment Drainage & Flood Mitigation Study
• Sg. Sarawak Flood Mitigation Option Study (draft final report)

2.0 Structural Measures

Structural measures are one of the fundamental components of the flood prevention strategy. DID is delivering drainage improvement works under the following categories, which are at dfferent stages of planning, design and construction.

2.1 Flood Mitigation Projects

The Department has been deploying concerted efforts to relieve the flooding problem in the State. Many flood mitigation programs were initiated. These programs include a series of major river improvement projects that provide the primary drainage network to alleviate the flooding problems. One of the example of this effort was the improvement works for Sg. Seduan in Sibu in the Seventh Malaysia Plan.

Projects under planning, design and implementation stages include Sg. Bintangor in Kuching and Sg. Miri Flood Diversion Channel in Miri.

2.2 Urban Drainage Projects

This program involves local drainage improvement to the trunk drainage system. Under the Seventh Malaysia Plan, the Department of Irrigation and Drainage has implemented 39 drainage improvement projects throughout the state. Among them are Sg. Padungan Drainage Improvement Works, Sg. Siol Catchment Drainage Improvement, etc..

Under the Eighth Malaysia plan, RM 82.198 million has been allocated for urban drainage improvement projects to mitigate the flash flooding problem.

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The Urban Stormwater Management Manual for Malaysia incorporates the control-at-source approach in stormwater management. This approach utilizes detention/retention to temporary store some of the water, infiltration to reduce the runoff and purification to improve the water quality reaching the river system. With the new approach, the impact of new development on the quality and quantity of the runoff can be minimised.

The goal of this Manual is to provide guidance to all regulators, planners and designers who are involved in stormwater management. It identifies a new direction for stormwater management in urban areas in Malaysia.

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Land Use Management is a non-structural measure that needs to be adopted to prevent the deterioration of flood risk in the drainage catchments. It involves administrative or legislative procedures rather than construction orientated approach. Structural and non-structural measures are best applied together to achieve the most optimum solution in combating flooding in a catchment.

Urban developments frequently occur spontaneously with little regard to the adverse impact on the drainage systems in the areas. The filling of low lying areas within the flood plains for development has increased surface flows and blocked the drainage paths. Land use management measures are required to ensure that new development will not adversely affect the current flooding situation.

These management measures are a significant element in allowing development in the floodplain to proceed in a proper and controlled manner whilst the long term structural measures are being implemented.

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What is Stormwater Management | Stormwater Matters: Impacts of Runoff | Activities to Minimise Stormwater Runoff | Stormwater Management Strategy | Urban Stormwater Management Strategy | Land Use Management l