Cause analysis for a new type of devastating flash flood

This work introduces an unprecedented flash flood that resulted in nine casualties in Shimen Valley, China, 2015. Through field survey and numerical simulation the causes of the disaster are systematically analyzed, finding that the intense storm, terrain features, and the large woody debris (LWD) played important roles. The intense storm induced fast runoff and, in turn, high discharges as a result of the steep catchment surfaces and channels. The flood flushed LWD and boulders downstream until blockage occurred in a contraction section, forming a debris lake. When the debris dam broke, a dam break wave rapidly propagated to the valley mouth, washing people away. After considering the disaster-inducing factors, measures for preventing similar floods are proposed. The analysis presented herein should help others manage flash floods in mountain areas.

Debris dams as habitat for aquatic invertebrates in forested headwater streams: a large-scale field experiment

To evaluate the effects of debris dams on aquatic invertebrate communities, we sampled benthic invertebrates in debris dams and riffles in three forested headwater streams in New Zealand. As part of a large-scale field experiment, debris dams were subsequently removed from three treatment sections in each of the streams to assess effects on invertebrate communities. Prior to debris dam removal, total invertebrate densities in debris dams were not significantly different from those in riffles. However, densities of Ephemeroptera, Plecoptera and Trichoptera taxa were significantly higher in debris dams than in riffles. Debris dams contained a higher number of less common taxa (defined as <1% of total catch) and significantly higher densities of shredders. Densities for Coleoptera, Diptera and Trichoptera taxa were significantly higher in the autumn than in the spring. Non-metric multidimensional scaling axis scores indicated that both habitat and season had a significant effect on aquatic invertebrate community composition. At the reach scale, the effects of debris dam removal on the aquatic invertebrate communities were not statistically detectable because debris dams comprised only a small proportion of total habitat. However, these data highlight the importance of debris dams in contributing to the diversity of aquatic invertebrates in forested headwater streams.

Mineralogical and geochemical variation in stream sediments impacted by acid mine drainage is related to hydro-geomorphic setting

Acid mine drainage (AMD) discharge has severe, long lasting impacts on water quality and stream ecology in affected watersheds due in part to the dynamic relationship between toxic metals (e.g. Al, Mn, and Cu) and Fe(III) oxy-hydroxides. Localized areas of biogeochemical activity that can mediate mineralogical transformation changes and cause metal release are potentially linked to stream geomorphology. This relationship has not been previously considered with respect to potential longitudinal variation within an impacted stream. The current work aims to determine how Fe(III) (oxy)-hydroxide speciation and distribution, and pore water chemistry in an AMD-impacted streambed, are affected by the presence of two geomorphic structures (a debris dam and step-pool sequence) in an Ohio watershed impacted by historical coal mining. In terms of solid phase mineralogy and geochemistry, in both the tributary and main stem, goethite was the dominant Fe-bearing phase throughout the AMD deposit depth in cores taken upstream of the geomorphic structures, whereas poorly-crystalline phases dominated downstream of the structures, despite the presence of Fe in the reducible fraction. The concentrations and distribution of extractable Al, Mn, and Cu were also different upstream versus downstream of each structure. Pore water Fe and Mn concentrations were higher downstream of the structures than upstream. Strong downward hydraulic gradients were present above the debris dam and in step-pool 1, whereas weaker upward hydraulic gradients were present below the debris dam and in step-pool 2. This work highlights that AMD deposit speciation and distribution, and pore water chemistry, are not spatially uniform within stream reaches, potentially as a result of groundwater-stream exchange-facilitated interactions in the presence of AMD-derived materials.

The Influence of an EPS Concrete Buffer Layer Thickness on Debris Dams Impacted by Massive Stones in the Debris Flow

The failure of debris dams impacted by the massive stones in a debris flow represents a difficult design problem. Reasonable materials selection and structural design can effectively improve the resistance impact performance of debris dams. Based on the cushioning properties of expanded polystyrene (EPS) concrete, EPS concrete as a buffer layer poured on the surface of a rigid debris dam was proposed. A three-dimensional numerical calculation model of an EPS concrete buffer layer/rigid debris dam was established. The single-factor theory revealed change rules for the thickness of the buffer layer concerning the maximal impact force of the rigid debris dam surface through numerical simulation. Moreover, the impact force-time/history curves under different calculation conditions for the rigid debris dam surface were compared. Simulation results showed that the EPS concrete buffer layer can not only effectively extend the impact time of massive stones affecting the debris dam but also reduce the impact force of the rigid debris dam caused by massive stones in the debris flow. The research results provide theoretical guidance for transferring the energy of the massive stone impact, creating a structural design and optimizing debris dams.

Ice storm impacts on woody debris and debris dam formation in northeastern U.S. streams

In January 1998, an ice storm damaged forests in northeastern United States and eastern Canada, causing coarse woody debris (CWD) deposition in riparian areas and associated streams. During 1999 and 2000, tree canopy damage, stream physical habitat, and wood deposition were evaluated within 51 first-, second-, and third-order streams located within five eastern Adirondack Mountain watersheds (New York, U.S.A.). In first- through third-order streams, the number and volume of stream debris dams increased in response to streamside trees with canopy damage. Tree canopy damage was not a significant predictor for individual pieces of stream CWD but was correlated with CWD >10 cm in diameter in third-order, but not first-order, streams. At debris dam locations, bankfull width was greater and stream substrates consisted of increased fines. Woody debris resulting from the 1998 ice storm was not associated with increased pool formation; instead, boulders and rocky substrate were the dominant pool-forming elements. CWD length in first-order streams generally exceeded bankfull width, but in third-order streams, CWD length was shorter than bankfull width and therefore was subject to greater transport and accumulation into debris dams. Our results indicate that ice storm disturbances can increase wood inputs to first- through third-order forested stream ecosystems.