One of the major challenges in mitigating or predicting the impacts of disturbance on hydrologic systems is to predict changes in both hydrology and in sediment transport in channels. Typically, only one of these components is considered in detail and the effect on the other component is inferred. In particular, much of the research on the effects of forest harvesting on hydrology has focused on evaluating changes in streamflow without considering the changes in the morphology of streams in which the flow occurs. For the purposes of forest management, linking hydrology and sediment transport is of crucial importance in order to improve management practices and to move from statistical and deterministic modeling to management outcomes with observable results. This is especially true of the current era in British Columbia, where widespread changes in forest cover resulting from mountain pine beetle infestations and associated salvage logging are affecting the hydrology of streams over scales ranging from small watersheds up to large regions of the province at once. The shape and dimensions of stream channels are formed not only by the water flowing from upstream and from hillslopes, but also by the sediment flux from upstream and from adjacent hillslopes and streambanks. This is especially the case for smaller streams that are directly connected to the adjacent hillslopes. Therefore, the state of the channel, its form and dimensions can be used as an index for the streamflow, sediment flux, and antecedent conditions which have shaped the channel. Bankfull discharge and effective discharge are two characteristic parameters of the channel, which can be used to evaluate the sensitivity of watersheds to disturbance (Nolan et al, 1987; Goodwin et al, 1998). The bankfull discharge is commonly defined as that discharge in which an alluvial stream is full to the top of the bank without overtopping it (Williams, 1978), although numerous alternative methods of definition exist based on morphologic, biologic, and statistical analysis methods. In formerly glaciated terrain, such as British Columbia, the top of the bank may have been formed during a flow regime which no longer exists. Therefore the bankfull discharge, although relatively easily measured, must be interpreted with caution. In general, the bankfull discharge is assumed to be a common event. Wolman and Miller (1960) assumed the bankfull and effective discharge to be equal to the mean annual flood. However, they warned that this would not be the case for smaller upland streams. Dunne and Leopold (1978) considered the return period for bankfull discharge to be approximately 1.5 years. However, Williams (1978) examined fifty studies and found that the reported return period for bankfull discharge ranged from a few months to 200 years. Castro and Jackson (2001) reported that for large watersheds (>100 kmē) in the Pacific Northwest, the mean return period for bankfull discharge varied between 1.2 and 1.5 years, depending on climate, but that some watersheds varied significantly from the mean. In British Columbia, forest management guidelines (MoF 1999, 2001) assume that bankfull discharge has a return period of 2 years for all streams regardless of size. Researchers have attempted to relate the bankfull discharge to channel and basin characteristics in order to predict the bankfull discharge. Kilpatrick and Barnes (1964) found that as channel slope increased, so did the return period of bankfull discharge. Williams (1978) also found that as channel gradient steepened, the return period for bankfull discharge increased. Andrews (1984) found that the return periods of bankfull and effective discharge increased as watershed area decreased. Castro and Jackson (2001) found that within nested catchments, the return period generally increased when moving from a downstream to an upstream gauge. However, in low-gradient headwater streams, Dodov and Foufoula-Georgiou (2005) found that ban ...
Hassan, Marwan A., Brayshaw, Drew. 2007. Hydrologic Indicators for Watershed Sensitivity to Peak Flow Changes in Small Watersheds. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2007MR298
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), Hydrology, British, Columbia
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