Environmental Engineering Reference
In-Depth Information
rainfall excess of a uniform rain with duration equal to the time of concentration, and
reflects the level of impermeability of the drained area.
4.3.2
The unit hydrograph method
A hydrograph is a plot of flow rate versus time at a point of interest on the drainage
system along a man-made conduit or along a natural stream or river. The development of
the unit hydrograph theory serves to represent in mathematical form the transformation of
rainfall excess into runoff. The unit hydrograph could be defined as the division of the
excess rainfall into small pulses of the same duration as the unit rainfall input, and
applied uniformly over the whole catchment basin. The unit hydrograph is the multiplier
determining individual response hydrographs from the basin with the same duration. All
individual hydrographs, based on the unit hydrograph applied for the specific conditions
within the catchment area, are summed in order to obtain the final hydrograph,
representing the total flow variation during a specified rainfall event for this specific
basin.
The NRCS method for rainfall excess determination provides a methodology for the
overland routing of excess rainfall. It proposes a triangular shaped dimensionless
hydrograph, which allows for the peak flow determination. The time at which the peak
flow will occur (peak time) would determine the shape of the hydrograph and would be
different from the time of concentration. The value of the peak flow of the unit
hydrograph will be determined by the watershed's area, storage and average slope. There
is provision for adjustment of the result to incorporate the effect of imperviousness of the
different types of areas. After the unit hydrograph for the area under consideration is
constructed, it is used to convolute the excess rainfall into the runoff of the real storm
event. The application of the method requires the determination of the storm hyetograph.
Urbanization leads to an increase in the impervious surfaces and the construction of
drainage structures results in a fast transport of the surface runoff. Both of these reflect a
considerable increase in the runoff peak flow, resulting in a changed urban streams
morphology. Higher runoff peak flows lead to a higher erosion of the streambed and
stream banks, lower base flow conditions and generally, causes damage to the natural
habitat. In terms of diffuse pollution aspects, higher peak flows result in a higher
possibility for pollution transport and change in the background water quality. In order to
prevent this, in the USA, developers are obliged to provide measures and keep the peak
runoff flows (or runoff volumes, or both), in relation to a predetermined design storm, at
the pre-development levels (Novotny 2003). This could be achieved by different
abatement measures and by careful urban planning practices, such as the reduction of
impervious surfaces, provision of retention structures, infiltration facilities, etc. More
details on the different types of abatement measures, with respect to diffuse pollution are
presented in Chapter 2.
4.4
Rainfall-runoff models
The methods described above are event orientated. They allow for the determination of
peak flow rates from selected storm events, defining specified conditions, which usually
are connected with the design of flood protection structures. In addition, they assume
