Y SCS I
2.4 Selection of an Appropriate Design Flood Estimation Method for Small Catchments in Eritrea
The first step in flood estimation is the choice of the flood estimation method to be used.
Some subjectivity is always involved in the selection of an appropriate method (Smithers and Schulze, 2001). According to Euroconsult (1997) the selection criteria for a flood estimation method should include:
• Suitability: The method should be applicable under a wide range of environmental conditions;
• Reliability: Ithas to provide a realistic interpretation of the hydrological processes and has to be based on tried and tested methodologies from similar climatic regions;
• Flexibility: The method needs to be applicable to a wide range of spatial scales (in this study scale is restricted to that of small catchments only), climatic conditions and catchment responses; moreover, it has to be transferable to ungauged catchments or regions; and
• Practicability: It has to maximise the use of readily available rainfall and physical properties of a catchment and to rely on relatively few and easily estimated parameters.
Schulze (1995a) noted the following points of caution on modelling which are applicable also in the selection of a suitable technique for design flood estimation from small catchments, viz.
• models cannot substitute for a lack of knowledge,
• neither do they create new data or facts (although they create new understandings),
• models can only anticipate the possibility that conditions as simulated, indeed occur, and
• models are constantly being improved; users are,therefore,urged to obtain the most up- to-date versions of a model.
In light of the above, a selection of an appropriate design flood method that could be suitable for wide application on small catchments in Eritrea had to be made. Ideally, design flood estimation requires many years of good quality flow data, particularly for regions such as Eritrea where the inter-seasonal and inter-annual variability is high (Euroconsult, 1997). If observed flood data were available at a site, a choice would then have to be made between empirical, flood frequency or flood envelope models (Figure 2.1). Alternatively rainfall based
methods would have to be used, either stochastic or deterministic. However, in Eritrea flow series and flood data exist for a very limited number of gauging stations only. Moreover,the available time series of recorded flows are too short and intermittent to be used for design determinations in almost all cases. Daily rainfall information, however, is generally more widely available across the country and records are often much longer and accurate than those of flow series data.
Itis clear, therefore, that it would not be possible to employ any of the streamflow data based techniques in Eritrea at present, as the available flow database is too limited. Furthermore, transfer of model parameters from gauged to ungauged catchments would be problematic, even if calibration were possible, as a result of marked spatial and temporal variability in runoff processes in most parts of the country. A lack of experts is an additional problem, as these models are also generally relatively complex and sophisticated.
Despite the limited data currently available in Eritrea, it is likely that a rainfall-based model, ideally of the deterministic or parametric (conceptual) type,would provide a successful means of simulating design floods for ungauged areas for Eritrea, especially if model parameters could be estimated from physical properties of a small catchment and, therefore, could be transferred to ungauged catchments (Euroconsult, 1997). Of the rainfall-based models for use on small catchments the Rational, SCS, Gradex, Unit Hydrograph, Time Area, Kinematic and Runoff Routing are commonly used techniques for flood estimation around the world (Schmidt and Schulze, 1987;Smithers and Schulze,2001).Itis beyond the scope of this paper to review each of these methods. However, it is helpful to discuss the merits and limitations of some of these methods, based on experiences from other countries, in order to select an appropriate method for flood estimation for Eritrea.
The Rational Method is widely employed in many countries (Alexander, 2001). Pilgrim (1986) quotes that at that point in time, it was the dominant method employed for small catchments design all over the world, with a usage in the mid-1980s in Australia of 86%, in Canada of at least 90% and in the United Kingdom of 90%. The method has a simple and logical approach (Alexander, 1989), but its major weaknesses are the judgment required to determine the appropriate runoff coefficient and the variability of the coefficients between different hydrological regimes (Pilgrim and Cordery, 1993). The Rational Method, however, computes only flood peaks and not flood volumes,and is sensitive to the input design rainfall
intensity, its successful usage depends on the experience of the user and it should not be used on catchments>15 km2in area (Smithers and Schulze, 2001).
The Unit Hydrograph, Time Area and Kinematic techniques essentially routing methods, require considerable computational steps to determine stormflow volume, peak discharge and hydrograph shape (Schmidt and Schulze, 1987). Campbell et al. (1986) conducted an evaluation of the above methods as techniques for the estimation of deign floods from small catchment in southern Africa and found that none of these methods performed adequately against observed runoff with uncalibrated parameters. However, further statistical analysis by Schulzeet al. (1986) indicated that the SCS-based models (particularly the southern African adaptation) performed well against runoff observations when compared to the Rational Method and the more complex models such as ILLUDAS (Time Area) and WITWAT (Kinematic), and this under wide ranging environmental conditions and small catchment size categories in southern Africa.
Application of SCS-based methods has increased in many countries over the past 30 years for numerous reasons. First, inaccurate estimation of the design hydrograph from small catchments has been noted in techniques such as the Rational Method and the Unit Hydrographs (Bondelid, et al., 1982; Campbell et al., 1986; Schmidt and Schulze, 1987;
Mishra and Singh, 2004). Secondly, coarse estimates of the catchment Curve Number (CN), or response index, can be obtained using remote sensing, which can reduce the cost of data collection greatly when compared with estimating CN from field work. Thirdly, the SCS method provides a high degree of flexibility in the type of hydrological analysis,unlike many other methods such as the Rational Method (Bondelidet al.,1982).
The SCS method for design flood estimation was originally developed in the USA in the 1950s. The model has been considerably modified, both internationally and in South Africa, for the estimation of stormflow volumes, peak discharges and flood hydrograph shapes for use on small catchments (Gorgens, 2002). Amongst the many SCS-based models, the SCS model adapted for southern African conditions by Schulze and Arnold (1979), and subsequently subjected to considerable research by Schulze (1982), Schmidt and Schulze (1984),Dunsmore et al.(1986),Schmidt and Schulze (1987),Weddepohl (1988) and Topping (1992), would appear to be an appropriate method for design flood estimation from small catchments in Eritrea for the following reasons:
• Three intermediate hydrological soil groups have been added to the coarse four-fold grouping of soils identified in the original SCS model.
• Unlike many other SCS-based models, CNs are adjusted according to a daily soil water balance instead of only 5-day accumulated antecedent rainfall depths.
• Ithas two options to account for typical soil moisture condition prior to a design event, viz.the Median Condition Method (the so-called "average"LlScondition) and the Joint Association Method, both of which account the soil moisture variation between storm events.
• Ithas been tested and verified under wide environmental conditions inside and outside southern Africa countries.
• Ithas been computerised into a readily accessible and user friendly package (Schulze et al., 1992).
• It is "driven" by the ACRU daily soil water budgeting model (Schulze, 1995a) to
simulate the complex nature of soil moisture changes (LlS) in the soil profile required for adjustment of CNs.
• Above all, however, typical regionalised indices ofLlSfor a wide spectrum of climatic conditions, soil properties and land use characteristics are available in southern Africa for the adjustment of runoff coefficients (CNs), and it is hypothesised that these indices can be transferred to other regions with limited data, but similar climatic conditions, such as Eritrea.
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The background, concepts and components of the SCS stormflow modelling approach used in southern Africa, and termed the SCS-ACRU approach, are discussed in detail in the next chapter. Note that the term "SCS-ACRU'is a conceptual approach and the term should not be confused with that of 'SCS-SA", which is the title of the user manual for southern Africa produced by Schulze et al. (1992).