Abstract
ABSTRACT Due to the relative ease of application, the CN and the Triangular Unit Hydrograph (TUH) methods are commonly used in hydraulic facilities design. This work adapts these methods to incorporate the spatial variability of the physical characteristics of the catchment. The flow travel time (tv) is computed from the DEM and the CN and TUH methods are adapted to produce results on a pixel basis. This methodology is applied to the Cascata I catchment (Porto Alegre - RS), considering two variants: i) TUHout - tv calculated from each pixel up to the catchment outlet; ii) TUHdown - tv computed from one pixel to the immediate downstream. The proposed methodology showed potential to be used for hydrologic modeling of rainfall events in small catchments, as an alternative for providing information useful for urban drainage management, as the characterization of catchment areas with distinct behavior regarding velocities and potential for runoff generation. On the other side, the results showed an underestimation of peak discharges and time to peak. Initial abstraction losses of the CN method lower than the one originally prescribed provided more realistic results, in agreement to recent studies. There is the need for an improvement of the methodology for the characteristics of the Brazilian catchments besides the simple adaptation of the classical equations of the CN method to its distributed form.
Highlights
One of the biggest problems faced by urban drainage engineers and technicians is the lack of hydrological information
It is an interesting aspect of this method because is able to capture the distinct flow travel times between events that is expected to occur in reality
The results showed clear patterns as follows: i) reductions in Manning lead to an increase in Qp, and that the variations of Manning of 20% resulted in variations of Qp about 5 to 10%; (ii) reduction(increase) in Manning results in a much lower decrease of Tp; (iii) the variation of the Manning resulted in differences negligible in the runoff volume; iv) the variant TUHdown presented results more differentiated between events than the variant TUHout
Summary
One of the biggest problems faced by urban drainage engineers and technicians is the lack of hydrological information In this scarce data situation, where intensity-duration-frequency (IDF) curves and watershed characteristics (relief and land use/land cover) are normally the only information available, the most commonly used models for rainfall-runoff simulation are the Rational method in the micro drainage and the CN (Curve Number) method proposed by the Natural Resource Conservation Service – NRCS or formally SCS (SCS, 1972; USDA, 2007) together with the synthetic triangular unit hydrograph method (TUH) (USDA, 1986, 2007) for macrodrainage. The methodologies received strong critics in the literature about its validity, due to simulating the basin in a lumped form, by fixing the initial abstractions as 20% of the maximum basin storage (in turn, estimated from the mean value of the CN), and the own values of the CN parameters suggested in the original formulation (e.g., OGDEN et al, 2017; AJMAL; KIM, 2014; CHRISTIANSON; HUTCHINSON; BROWN, 2016; D’ASARO et al, 2014; RUTKOWSKA et al, 2015). Some studies have suggested adaptations such as using alternative values instead of the 20% of initial abstractions (AJMAL; KIM, 2014; CHRISTIANSON; HUTCHINSON; BROWN, 2016), the calibration of the CN parameter (D’ASARO et al, 2014; RUTKOWSKA et al, 2015), and the spatial variability of the physical characteristics of the watershed (BARTLETT et al, 2016; GRIMALDI; PETROSELLI; NARDI, 2012; ZHANG et al, 2014)
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