Flood Control in Sri LankaThe 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 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.
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
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.
The soil conditions near the coastal 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 that 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.