Design of Storm Drainage System for a new city to be developed is the part of overall infrastructure design of the project. The design will include the independent storm drainage system for development area and its integration with the future storm drainage system. Purpose of study The objective of the design of storm drainage system is to ensure that there will be no flooding nearby the project area site and discharging the run-off away from the proposed site to the nearest interface point or sea in a minimum time and in safe manner.
Pounding of storm water on roads and public areas will be minimized to allow the public access on the roads and public areas during the rain and after. Existing Conditions There is no conventional storm drainage system consisting of catch basins and pipe network within the project limit. Codes and Standards Following Codes and Standards are used for establishing design criteria: American Society of Civil Engineers (ASCE) – Manual and Report on Engineering Practice No. 37. Design and Construction of Urban Storm water Management Systems, prepared by Water Environment Federation, American Society of Civil Engineers.
(ASCE Manuals and Reports of Engineering Practice No. 77) Design Criteria The following design criteria will be used for the design of storm water drainage system Design Storm Frequency/Return Period The drainage systems in the cities are designed to handle storms having return periods ranging from nearly one year to five (or ten) years, depending on the value of the properties in the drainage area and the nuisance value of the flooding. Design for ten years is common in most cities of the world. Close Conduits 1- 50 year storm – for design of a storm drain which will be located in a natural water course.
2- 25 year storm – for design of a storm drain for a sub- division 3- 10 year storm – for all other storm drain design Open Channels 100 year storm – for design of channels located in a natural water course 25 year storm – for design of road side channels used for diverting or removing surface water from highway right of way. 10 year storm – for all other open channels design with sufficient free board to contain a storm of 50 year frequency. Methodology for Estimation of Design Discharges The Rational Method The Rational Method is based on the simplification that the intensity of rainfall is constant throughout the duration of storm.
Rainfall Intensity (Magnitude) Rainfall magnitudes corresponding to different return periods have been estimated Time of Concentration Design of primary systems, the time of concentration may be calculated by Kirpich Formula Runoff Coefficient The runoff co-efficient “C” is the ratio of the maximum rate of runoff to the average rate of rainfall in the area during the time of concentration. Roughness Coefficient of Pipes and Channels Runoff Coefficient (n) that could be used in Manning’s Equation. Design of Storm Drains The storm drains will be designed for uniform flow. The velocity will be calculated based on the Manning’s Formula.
Velocity of Flow Maximum and minimum velocity of the flow needs to be considered Design of Close Conduits Close conduits are normally designed for the conditions of flowing full. Design Depth of Flow Storm drains will be designed to flow full under peak flow conditions. Gradient of Storm Drain Minimum gradient for a section of the drain will generally be taken to be the gradient necessary to satisfy the minimum velocity requirements. Sizes of Drains Sizes of catch basin lead pipe and Main Drains in the system would be specified Manhole Dimensions Manhole size, shape and dimensions need to be specified.
Catch Basins Location of the basins on main streets would be reviewed so as to avoid flooding and submergence of the roadway during rain storm. Discharge Quality Standards The quality of storm water to be disposed of into the canal / sea will meet the requirements of Environmental Protection Standards Review of Master Plan Master Plan review gives an idea about storm drainage concept. Same criteria will be used with slight variations, on the basis of which detailed design of storm drainage system will be carried out. Proposed Concept Storm water drainage system comprises of two parts i. e.
for open areas and roads. Storm water system for open areas consists of sheet flow on land, then intercepted by Gratings, Interceptors, Catch Basins, and Curb Inlets and drained to the selected locations. Strom Drainage System is required to avoid flooding on the roadways, to provide traffic safety and to protect the road facilities. Rapid removal of storm water from the traveled way minimizes the condition which results in the phenomenon of aquaplaning. Catchments Areas The area contributing to any point under consideration in a storm drainage system will be determined by accurately measuring from the maps.
Boundaries of the sub-catchments areas are established by its proposed contours. References 1. Engman, E. T. 1986. Roughness Coefficients for Routing Surface Runoff. Journal of Irrigation and Drainage Engineering, ASCE 112 (1): 39-53. 2. Hawkins, R. H. , A. T. Hjelmfelt, and A. W. Zevenbergen, 1985. Runoff Probability, Storm Depth, and Curve Numbers, Journal of Irrigation and Drainage Engineering, ASCE 111 (4): 330-340 3. Maryland Department of the Environment. 1997. Draft: Maryland Storm water Design Manual, Volume One. Prepared by: Center for Watershed Protection, Inc.