09 - flood estimation in small and urbanised catchments in ireland

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National Hydrology Conference 2012 Gebre Table 6: Outputs from regression analysis of 38 stations PCDs Parameters Standard Error t-Stat p-value Lower 95% Upper 95% Intercept -10.7733 2.6551 -4.0576 0.0003 -16.204 -5.343 Ln[AREA] 0.9245 0.1796 5.1469 0.0000 0.557 1.292 Ln[SAAR] 1.2695 0.4013 3.1637 0.0036 0.449 2.090 Ln[BFI soil ] -0.9030 0.4390 -2.0569 0.0488 -1.801 -0.005 Ln[FARL] 2.3163 0.9497 2.4390 0.0211 0.374 4.259 Ln[S1085] 0.2513 0.1007 2.4965 0.0185 0.045 0.457 The FSU4.2a regression equation redeveloped with the above parameters used on 38 stations has R 2 = 0.758, standard error of estimate (see) = 0.521, df = 5 and fse = 1.684. The final form of the small catchment regression equation is as follows: QMED = 2.3848*10 -5 AREA 0.9245 SAAR 1.2695 BFI -0.9030 FARL 2.3163 S1085 0.2513 The exponents applied to AREA, SAAR, BFI and S1085 are quite similar to those for the FSU 7-variable equation. The performance of this new equation was compared with that of the FEH statistical and the FSU 7-variable equations, and the results are as outlined in Table 7 below: Table 7: Comparing the new regression equation with FEH and FSU-3v applied to 38 stations. Flow (QMED) Coeff. of Std. dev of RMSE NSE Mean FSE estimation methods (≤30km 2 & ≥7yrs) determination (R 2 ) errors Bias FEH Statistical 0.713 3.98 0.250 0.698 0.042 1.846 FSU 7-variable 0.791 4.16 0.245 0.672 0.141 1.879 New small catchments equation 0.802 3.27 0.219 0.808 -0.305 1.674 The equation has an R 2 value of 0.80, and fse of less than 2.00 for the 38 stations. The new equation compared well to the FEH –Statistical and FSU 7-variable methods. Thus, while it takes more PCDs, it performs relatively well, with a factorial standard error of 1.674. As there were an insufficient number of urban gauging stations available for the small catchment analysis, the adjustment for urbanisation is the same as that used for the FSU 7- variable equation: QMED urban = QMED (1 + URBEXT) 1.482 The decision to use the adjustment for urbanisation as quoted above is backed up in part by research that was carried out in Work Package 4.3 of the FSU, where the proposed adjustment for urbanisation was estimated using hydrological modelling applied to three Irish urbanised catchments. This found that the urban adjustment closely followed that put forward by Naden and Polarski (1990) which proposed an adjustment factor of (1+URBEXT) 2 . The performance of the model across the 38 stations used in the analysis is shown in Figure 4 below: 97
National Hydrology Conference 2012 Gebre 38stns: FSU4.2, y = 0.9051x + 1.2966 R 2 = 0.802 30.0 25.0 Qmed_predicted 20.0 15.0 10.0 5.0 0.0 0 5 10 15 20 25 30 Qmed_amax Figure 4: Plot of observed QMED Vs estimated QMED using the new small catchment equation. While the new small catchment equation slightly outperforms the FSU 7-variable equation, the dataset on which it was based is still very small in terms of the number of catchments that would be required to develop a robust method that could be firmly recommended. There would also be the possibility that by using two different methods for catchments with areas in the region of 30km 2 that very different QMED estimates could be obtained for two adjacent points on the same river that had respective contributing areas of 29km 2 and 31km 2 . It is for these reasons that the FSU 7-variable equation is preferred for all catchments with an area of greater than 5km 2 . The benefit of using a single equation outweighs those obtained by using a second equation for catchments between 5 and 30km 2 . 5. CONCLUSIONS AND RECOMMENDATIONS Ten existing methods were investigated. Some of the methods were found to overestimate significantly, the most notable being the IH124 method which is widely used in Ireland. The FEH-statistical method slightly overestimates and the FSU 7-variable equation slightly underestimates, however both of these methods perform comparatively well according to this research. As a further option a new regression equation was developed using five variables, AREA, SAAR, BFI, FARL and S1085 to attempt to improve on the estimates obtained from the FEH and FSU methods. The results from the new method are encouraging. However it is only based on a set of 38 catchments and would need to be tested rigorously at more gauging stations with good quality data before it may be designated the preferred option. In the Irish context, the FSU 7-variable equation is preferred over the FEH statistical method for flood estimation in small catchments, mainly because the physical catchment descriptors used in the 7-variable equation have already been derived for approximately 134,000 ungauged locations in Ireland. 98
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