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Flood response time of small and medium forested watersheds in the western slopes of the cascades mountain range Pinto Gregori, Xavier Martin

Abstract

The estimation of peak flows is important from a variety of different ecological concerns, physical processes, engineering needs, and socioeconomic reasons. Perhaps the most important parameter used in deterministic models for peak flow estimation at small-ungauged watersheds is the flood response time. This measure was used as a synonym of the time of concentration of the catchment (T[sub C]). An accurate determination of the T[sub C] is significant since deterministic models assume that the maximum annual discharge or flood will occur when the whole watershed is contributing to the streamflow after the time of concentration has elapsed. The T[sub C] as a characteristic of streamflow has been analyzed less commonly than the effects of logging and road building have on peak flows. In addition, the analysis of the time of concentration can contribute to a better understanding of streamflow processes and how these processes can be affected by management practices at the watershed level. This study investigated the time of concentration of seven catchments, ranging in scale from 0.13 to 62.4 km² , located on the H.J. Andrews (HJA) Forest, in the western slopes of the northern Cascades in Oregon. The T[sub C] was determined from hydrograph analysis as the most common minimum time of rise (T[subR]) of the hydrograph. The characteristic T[sub C] for the catchments in the HJA Forest were found to be unusually long when compared to values cited in research performed in the same region. The T[sub C] for the analyzed catchments in most of the cases was in the order of days. It was hypothesized that the long T[sub C] values in the HJA basins were a result of the combination of different factors. Firstly, the response of the hillslope was dependent on the exceedance of a threshold rainfall depth (and/or intensity) during the hydrograph rising limb that converts the system from a matrix-dominated (combination of flow through the soil matrix and the micro-topographic relief at the bedrock boundary) near-system flux to a faster pipeflowdominated hillslope hollow system (through preferential flow pathways). A reverse switching back to matrix-dominated near-system flux occurs during the stream recession as soon as perched water tables on hillslopes are dissipated. Secondly, the delivery of water to the streams was influenced by rain-on-snow events when snow falling on the basins usually remains on the ground for 3 to 4 days before melting. Finally, the water delivery to the streams was also influenced by the underlying porous volcanic bedrock surface, which creates additional meandering flowpaths, therefore, delaying the delivery of runoff to the streams. In addition, the determined T[sub C] values for the different catchments were found to be poorly correlated to the area of the watershed. Poor correlations were also found between T[sub C] and main stream length and slope. The results suggested that for the analyzed catchments the T[sub C] was independent of basin scale. Hence, traditional equations that express T[sub C] as a function of some watershed physiographic descriptors, such as area, main stream length, and slope, would not yield realistic T[sub C] estimates if used in the catchments of the HJA Forest. Furthermore, most hydrologic models used for the estimation of peak flows assume linearity in the runoff response. The effects that the rainfall intensity and peak flows had on the T[sub C] were investigated, when assessing the response of the basins in the HJA Forest. The different basins analyzed did not show a nonlinear response. Finally, the implications of the use of traditional "textbook" equations for the estimation of the T[sub C] in the basins of the HJA Forest were analyzed. It was revealed that the computed T[sub C] values were drastically underestimated if compared to the T[sub C] values determined from hydrograph analysis. When T[sub C] values computed with these traditional equations were used in peak flow estimation, the resulting peak flows (QT) were consistently overestimated.

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