When we discuss about flood control in Sri Lanka, The growth areas of Sri Lanka, Specially Colombo district are more often treating by natural disaster of floods even in the light rain. Recently, reasonable attention has been given by the government to mitigate disaster of flood. The construction projects of storm water drainage systems are implementing according to severity of disaster & frequency of floods occurrence. The fortune of civil engineers & peoples in Sri Lanka is least effect of natural disaster comparing other countries in South Asia & even world wide. The main reasons could be identified as unplanned urbanization, absence of soil conservative practices & bad habits of some peoples.
The population growth of the Colombo district is rapidly increased in day by day. The retention areas are greatly reducing due to construction including the illegal as well. the increment of intensity of the paved areas are effecting to more surface run off due to less infiltration that will evenly reduced due to higher ground water table. The lower latitude of Colombo district also directly influenced to flow rate of the storm water paths. The lesser gradient indirectly influences clogging of water path even piece of garbage. The construction of storm water drainage system is somewhat difficult now in these areas & development projects may get more time & cost rather originally designed and budgeted amount.
When considering of flood control in Sri Lanka, design of the storm water drainage systems are mainly based on the past rainfall details, catchment characteristics and many other considerations of engineering hydrology & environmental engineering .the open channel construction is widely used in Sri Lanka including underground conduit, pipes sections which are need covering road crossing, building when alternative plans are impossible. The open channels are earth, gabion, random rubble masonry & reinforced concrete, the concrete, gabion channels can be either rectangular or concrete which are basically employed when the spaces are limited. Drainage system may consist of tertiary, secondary, & main canals types according to their capacity. The frequency of other structures like culverts, bridges, conduits, underground pipes, maintenance road are vary according to intensity of roads & buildings etc.
The construction part of drainage system is vital task within this kind of environment, has to employ extreme knowledge of construction technology. The construction technologies may depend on
Spaces available for construction work
The construction techniques has to change according to the spaces available for the construction works such as the machinery, plants, vehicles, trucks, material has to accessible. In commonly the spaces available along channel work that most area has to construct the full width of channel. Therefore the section by sections has to complete due to limited spaces.
Water Diversion
The water path has to be diverted from construction section. The diversion channels are formed along the channel by separating sheet pile line or shoring line or otherwise polyethylene pipe. The construction of diversion channel has to change on section to section due to variation of site condition.
Traffic Diversion & Traffic intensity
The traffic diversion plan has to decide facilitating the vehicle flow. Sometimes some of the road sections have to close permanently according to alternative roots. But in main road crossing are cannot to be closed. Therefore single pile traffic has to allow & but in most cases pre cast concrete sections are providing complete such sections as soon as possible.
Soil condition
The soil conditions near the coastel area are very loose or sandy soil available. The excavations for channel construction are critical concerning safety of near building, pedestrian access, vehicle by pass etc…Therefore the excavated area has to be firmly separated by applying sheet pile or shoring will be vary according to bed level of channel
Ground water Table
The ground water table is very high near the coastal area. Therefore seepage of water has to be control by protecting cofferdam. The usages of water pumps are critical & construction schedule has to be tight minimizing the dewatering cost.
Intensity Lateral Loads
The buildings are much closed to the construction channel even in channel reservation. The construction method has to be changed considering the loose soil conditions also.
Utility Diversion
Detection of underground utility lines is important along the construction channel and has to be diverting according to the invert levels of channel & other utility poles , lines has to be relocate according to alignment of the construction channel.
Public Interference
Public interference is considerably too high because intensity of people is too high. The variety of peoples form higher level to lower level involving many kind of complains objections etc. Therefore some of the construction sections may get unexpected time & cost.
Pedestrian access
When construction side drains along the road, the access for the building, house has to be removing for the construction works. Therefore temporary access has to be constructing for pedestrian as well as the vehicles. But sometimes has to provide Vehicle Park somewhere else that temporary access impossible for vehicles.
The new construction methods has to be improved to implement storm water drainage systems in urban & sub urban areas effectively while construction company should have experience to be a contractor of this kind of projects. the construction company & government body that the improvement of low lands is belong to Sri Lanka Land Reclamation & Development Corporation (SLLRDC) are doing great job towards mitigating the disaster of flood control in Sri Lamka. The Colombo district municipal councils are doing their maximum efforts to maintain storm water drainage systems effectively & helping to maximize productivity of newly construction drainage systems.