Summary
Study of the subject of storm water drainage begins with the consideration of methods to estimate the peak surface runoff due to the storm of a given frequency that occurs over a given catchment. The estimation is mostly done by Rational Method, use of empirical formulae (relevant for particular catchments only), or special procedures applicable to specific areas (such as, Soil Conservation Service Method being used in USA) that may be developed for ease in computations.
The need for estimation of runoff basically arises once an area is proposed to be developed with a view to urbanisation, and the normal peak flow is expected to increase due to increase in the coefficient of runoff of the area because of loss of natural seepage through paved patches of land.Storm sewer systems, for carrying the flow, comprise suitable inlets in the streets, minor and trunk sewer pipes. The design of the system involves fixing the inlet locations, slope of sewer lines, and working out their diameters. Sewers and drains are designed with all the considerations and formulae applicable for the design of open channels.
Use of stormwater detention basins allows a temporary storage of water that is, later on, released; and it later on helps reduce the peak flow, and so lesser diameter of sewer lines can be adapted to effect economy in the capital cost. Such basins can be designed to cater to many storms falling in the desired range of frequency of occurrence. Hydraulic design of these structures involves the use of flood routing procedures; and a simplified procedure; namely, simplified storage factor method comes handy in this context.
In this unit, we have discussed the concepts related with the design of sewer systems. The design of sewer network involves the selection of proper pipe size and slope of pipes so as to transport the sewage to its disposal point. It was pointed out that while designing, it should be ensured that minimum flow velocity is maintained in sewers even during minimum flow conditions, at the same time velocity should not be too excessive to cause erosion in sewers.
Appurtenances are an integral part of a sewer network, and are necessary for proper functioning of the system. These include manholes through catch basins, etc. Manholes are there to permit inspection, cleaning and repair of sewer lines. Their dimensions should be sufficient to allow carrying out these operations. They are provided to accommodate change in pipeline diameter, or slope or alignment. They are also provided at all pipe intersections, uppermost end of each lateral.
Other appurtenances like terminal cleanout structures meant to provide a means for inserting cleaning tools, etc., junction chambers to allow flows from various sewers to be combined smoothly, flush tanks to clean off laterals, house sewers, lampholes to allow inserting light, street inlets, catch basins and sill basins to arrest suspended materials in the flowing sewage.
Stormwater standby tank is also an appurtenance whose function is to serve as a storage basin to even out the excess storm flow or retain the flow during high tides in the receiving water body. Measurement of sewage flow through a given system is an important activity to monitor its functioning. Sewer construction and management of the system is indeed a specialized discipline. Excavation of trenches, preparation of bedding, and laying of sewer pipes to designed alignment and slope are the main jobs to be attended to in the construction of sewers. Management of the system requires inspection and application of appropriate rehabilitation methods wherever required. Leaks need to sealed, cracks and corrosions repaired, and entry of infiltration water into the system (particularly into manholes) prevented.