Large Wood Transport Research Articles

Two‐dimensional modelling of large wood transport during flash floods

ABSTRACTLarge woody material (LWM) transported by rivers may be entrapped at critical stream geometry configurations (e.g. bridges) and therefore dramatically increase the destructive power of floods. This was the case in a Spanish mountain river where a flood event with a high degree of LWM transport took place in 1997. The aim of this study was to simulate a bridge clogging process and reconstruct the wood deposit patterns, modelling individual pieces of wood moving with the water flow and interacting among them and with the bridge. A two‐dimensional numerical model was developed to simulate the transport of LWM and its effect on hydrodynamics. Different scenarios for the wood transport rate allowed us to study the influence of inlet boundary conditions on bridge clogging. For the studied event, the scenario which best reproduced the bridge clogging effect and flood characteristics was one in which 60% of the total wood entered before the peak discharge. This dropped to 30% at the peak itself, and finally fell to 10% during the recession curve. In addition, the accumulation patterns of LWM along the reach were computed and compared with post‐event field photographs, showing that the model succeeded in predicting the deposition patterns of wood and those areas prone to form wood jams. Copyright © 2013 John Wiley & Sons, Ltd.

Reconstruction of a flash flood with large wood transport and its influence on hazard patterns in an ungauged mountain basin

AbstractThe reconstruction of past flash floods in ungauged basins leads to a high level of uncertainty, which increases if other processes are involved such as the transport of large wood material. An important flash flood occurred in 1997 in Venero Claro (Central Spain), causing significant economic losses. The wood material clogged bridge sections, raising the water level upstream. The aim of this study was to reconstruct this event, analysing the influence of woody debris transport on the flood hazard pattern. Because the reach in question was affected by backwater effects due to bridge clogging, using only high water mark or palaeostage indicators may overestimate discharges, and so other methods are required to estimate peak flows. Therefore, the peak discharge was estimated (123 ± 18 m3 s–1) using indirect methods, but one‐dimensional hydraulic simulation was also used to validate these indirect estimates through an iterative process (127 ± 33 m3 s–1) and reconstruct the bridge obstruction to obtain the blockage ratio during the 1997 event (~48%) and the bridge clogging curves. Rainfall–Runoff modelling with stochastic simulation of different rainfall field configurations also helped to confirm that a peak discharge greater than 150 m3 s–1 is very unlikely to occur and that the estimated discharge range is consistent with the estimated rainfall amount (233 ± 27 mm).It was observed that the backwater effect due to the obstruction (water level ~7 m) made the 1997 flood (~35‐year return period) equivalent to the 50‐year flood. This allowed the equivalent return period to be defined as the recurrence interval of an event of specified magnitude, which, where large woody debris is present, is equivalent in water depth and extent of flooded area to a more extreme event of greater magnitude. These results highlight the need to include obstruction phenomena in flood hazard analysis. Copyright © 2012 John Wiley & Sons, Ltd.

Large wood export regulated by the pattern and intensity of precipitation along a latitudinal gradient in the Japanese archipelago

We examined the relationships between large wood (LW) export and precipitation patterns and intensity by analyzing the data on the annual volume of LW removed from 42 reservoirs and the daily precipitation at or near the reservoir sites. We also calculated the effective precipitation by considering the antecedent precipitation. Both daily and effective precipitation data were used as explanatory variables to explain LW export. The model selection revealed that the precipitation pattern and intensity controlling LW export varied with latitude in the Japanese archipelago. In small watersheds with narrow channel widths and low discharges, mass movements, such as landslides and debris flows, are major factors in the production and transport of LW. In this case, the effective precipitation required to initiate mass movements regulated the LW export and did not vary with the latitude. In intermediate and large watersheds with wide channel widths and high stream discharges, heavy rainfall and subsequent floods regulated buoyant depth, influencing the initiation of LW movement. In southern and central Japan, intense rainfall accompanied by typhoons or localized torrential downpours causes geomorphic disturbances, which introduce abundant pieces of LW into the channels. However, these pieces continue to be removed by repeated rainfall events. Therefore, LW export is supply‐limited and potentially produces less LW accumulation. Conversely, in northern Japan, where typhoons and torrential downpours are rare, LW export is transport‐limited because LW pieces recruited by bank erosion, tree mortality, and windthrow accumulate and persist on valley floors. These pieces may be easily exported by infrequent flooding.

Distribution and characterization of in-channel large wood in relation to geomorphic patterns on a low-gradient river

A 177 river km georeferenced aerial survey of in-channel large wood (LW) on the lower Roanoke River, NC was conducted to determine LW dynamics and distributions on an eastern USA low-gradient large river. Results indicate a system with approximately 75% of the LW available for transport either as detached individual LW or as LW in log jams. There were approximately 55 individual LW per river km and another 59 pieces in log jams per river km. Individual LW is a product of bank erosion (73% is produced through erosion) and is isolated on the mid and upper banks at low flow. This LW does not appear to be important for either aquatic habitat or as a human risk. Log jams rest near or at water level making them a factor in bank complexity in an otherwise homogenous fine-grained channel. A segmentation test was performed using LW frequency by river km to detect breaks in longitudinal distribution and to define homogeneous reaches of LW frequency. Homogeneous reaches were then analyzed to determine their relationship to bank height, channel width/depth, sinuosity, and gradient. Results show that log jams are a product of LW transport and occur more frequently in areas with high snag concentrations, low to intermediate bank heights, high sinuosity, high local LW recruitment rates, and narrow channel widths. The largest concentration of log jams (21.5 log jams/km) occurs in an actively eroding reach. Log jam concentrations downstream of this reach are lower due to a loss of river competency as the channel reaches sea level and the concurrent development of unvegetated mudflats separating the active channel from the floodplain forest. Substantial LW transport occurs on this low-gradient, dam-regulated large river; this study, paired with future research on transport mechanisms should provide resource managers and policymakers with options to better manage aquatic habitat while mitigating possible negative impacts to human interests. Copyright © 2011 John Wiley & Sons, Ltd.

Watershed controls on the export of large wood from stream corridors

Large wood maintains in-channel and floodplain habitats by influencing the biophysical character of the river corridor. Large wood dynamics in a river corridor are a product of watershed wide processes and also of local recruitment, transport, and storage. This complexity of scales added to the logistical constraints in taking measurements limits our understanding of large wood dynamics through the watershed. To begin to unravel this issue, we compiled a data set of the volume of large wood deposited annually into 131 reservoirs across Japan and compared large wood export to flow discharge and watershed characteristics (watershed size, latitude, channel slope, percent forest, and forest type). We found that large wood was predominately transported during peak flow events. Large wood export increased logarithmically with watershed area. The decreasing export rate of large wood per watershed area is interpreted as a combination of annual export variability in upper watersheds, a non-significant increase in large wood recruitment along the longitudinal gradient (potentially human influenced), the increase in long-term storage on adjacent large floodplains, and significant decay/fragmentation downstream. Watersheds<10–20km2 had a highly variable large wood export pattern, conforming generally to previously published work that suggest transport limitation in smaller watersheds. The data suggest the existence of an export threshold (∼75km2) where large wood export is no longer related to watershed size. Export across all watershed sizes was controlled by watershed characteristics (slope, percent forested, etc.) and peak discharge events. The connection with upstream watersheds and laterally with the floodplain increases the net flux of large wood through downstream transport and retransport of buried logs. Identifying rates of large wood transport from watershed connectivity as a potential key input process will improve our basic understanding of geomorphic and ecological patterns within the watershed. These results highlight the importance of understanding the local- and watershed-scale dynamics of large wood in creating and maintaining more heterogeneous riparian and aquatic habitat along the river corridor.