|Stormwater Drainage and Land Reclamation for Urban Development (HABITAT, 1991, 94 p.)|
|III. TECHNICAL OPTIONS FOR STORMWATER DRAINAGE AND LAND RECLAMATION|
Factors affecting storm flows
The two principal factors affecting storm flows are rainfall and run-off. It is necessary, in designing storm drainage systems, to use rainfall data for the catchment under consideration. This leads to an estimation of the various stream flow characteristics and the run-off quantities to be coped with in the drainage system. It therefore follows that, for satisfactory drainage design, adequate rainfall data are essential. However the forms in which rainfall data are normally retained vary considerably, depending on the country and the purpose for which the rainfall data were originally measured. The commonly used forms of data for hydrological analysis include estimates of basin rainfall depth, rainfall intensity (the amount of rain falling at a certain point over a given period of time), rainfall duration and storm pattern characteristics.
It is common to try to classify rainfall information for drainage design into intensity-frequency-duration data and storm pattern data. In developed countries the meteorological agency which collects and records rainfall data at many locations throughout the country is able to produce rainfall intensity-frequency-duration curves for a particular location. However in developing countries the availability of data is often limited to daily rainfall information and a great deal of effort is needed to convert this data into a useful and reliable basis for storm drainage design. Further details are given in the annex.
Run-off is that measure of the quantity of water which occurs from the rainfall events. The volume measurement of its estimation is the basis for drainage design. It is unusual to find sufficient information available in developing countries for the measurement of run-off in urban catchments, and thus estimating procedures must be used based on the rainfall intensity-duration-frequency curves generated from rainfall data. Run-off depends on a number of factors apart from rainfall, such as the rate at which the water flows across the surface of land and reaches the drain being designed. The nature of the ground will affect the eventual quantity of run-off. More porous catchment areas will provide more infiltration. Flat slopes will provide more areas for storage in low-lying areas in the catchment and consequently opportunities for transpiration, evaporation and infiltration. Catchments which are already wet (antecedent wetness) will have higher quantities of run-off for a particular storm.
The more of this rainfall which is lost during the run-off process, the lower will be the requirement for drainage capacity.
Estimation of flood flows
The processes involved in the transition from rainfall to run-off can be summarized as follows:
(a) Rain falls at a rate which varies with time and with the extent of area covered;
(b) Water runs off the different surfaces in the catchment, the latter flow being affected by retention on the surfaces by their relative distribution by infiltration and by the hydraulic characteristics of the surface channels and inlets;
(c) Once in the drain, the water flows at varying rates and the volume of water contained in the drain also varies with time.
Because of the inadequate records available for run-off, methods for designing storm drains have generally been based on rainfall records which are available at many more locations and for substantially longer periods than any run-off records.
Figure 3.5. Effectiveness of retarding basin in an urbanised catchment
Figure 3.6. Possible accumulation of hydrographs
Run-off must be less than rainfall by an amount depending on the physical characteristics of the catchment.
The simplest and most widely used example of this approach is the rational formula, which is based primarily upon the assumption that the maximum rate of run-off will be produced by the most intense rainfall that can be expected to occur over the basin for the period of time required for surface flow from the furthest part of the basin to reach the point at which the peak flow is being computed.
The simplicity of the formula and the clearly evident logic on which it is based have led to its widespread and continued use. Except in the case of urban areas for which it was originally developed, however, a number of important secondary assumptions implicit in the formula are seldom satisfied. The rational method is an adequate method of approximating the peak rate of run-off from a rainstorm in a given catchment. Its greatest drawback is that it normally provides only one point on the run-off hydrograph. When the catchments become complex and where subcatchments join, the rational method tends to overestimate the actual flow, so resulting in oversizing of the drainage components. The rational method provides no direct data to route hydrographs through the drainage facilities - that is, it takes no account of the storage and retention within the drainage system itself and consequently will tend to over-estimate both the run-off and peak flows.
One reason the rational method is normally limited to small areas is that good design practice requires routing of the flows for large catchment areas to achieve an economic design. There is, however, no agreement on the size of the catchment which should limit the use of the rational formula. Nonetheless it is reasonable to take 20 ha as a maximum area, and where urban catchments are larger they should be broken into sub-catchments of about this size.
One of the basic assumptions underlying the rational method is that run-off is a function of the average rainfall rate during the time required for water to flow from the most remote part of the drainage area under consideration to the point or design flow being calculated. However, in practice, the time of concentration can be an empirical barrier.
The TRRL method
This approach to the estimation of run-off was developed by the Transport and Road Research Laboratory in the United Kingdom to overcome the problems inherent in the rational method. The TRRL method is a hydrograph technique, which in its present form is implemented as follows:
(a) A hydrograph is constructed for a given pipe, using an area-time graph for the catchment area directly connected to the pipe and a rainfall profile;
(b) This hydrograph is added to the outfall (i.e., exit) hydrograph of the pipe immediately upstream of the one under consideration, displacing it by a time equal to the flow-time in the pipe being considered;
(c) The combined hydrograph is now routed through the reservoir formed by the pipe being considered, assuming instantaneous uniform proportional depth in the pipe under consideration to the given outfall hydrograph for it;
(d) This outfall hydrograph becomes the upstream (i.e., inflow or entry) hydrograph for the next pipe downstream, and so on.
The procedure is illustrated in figure 3.7, and the process is equally applicable to open channels, as well as to pipes. The method, which has been computerized and for which programs are commercially available, has been modified for tropical conditions where some account has to be made of the stormflow from unpaved areas.