| 翻訳と辞書 |
Log jam
A log jam is an accumulation of large wood (commonly defined as pieces of wood more than 10 cm in diameter and more than one metre long also commonly called large woody debris) that can span an entire stream or river channel.
Historically in North America, large "log rafts" were common across the continent prior to European settlement . The most famous natural wood raft is the "Great Raft" on Louisianna's Red River, which prior to removal in the 1830's affected between 390-480km of the main channel. It has been suggested that such extensive log rafts may have been common in Europe in pre-history .
==Effects on stream/river geomorphology==
Log jams alter flow hydraulics by: diverting flow towards the bed or banks, increasing flow resistance and creating upstream pools, diverting flow onto the floodplain and damming the channel causing water to spill over the structure . These altered channel hydraulics change local patterns of erosion and deposition, which can create greater variety in local geomorphology and thus create provision and variety of habitat for in stream biota. The formation of a logjam against one bank typically concentrates flow in the wood free portion of the channel, increasing velocity through this section and promoting scour of the river bed, the formation of channel spanning logjams can lead to the formation of an upstream pool, water spilling over the structure generating a "plunge pool" immediately downstream .
The hydraulic and geomorphological effects of logjams are highly dependent on the slope of the river (and thus the potential power of the stream), in steep channels logjams tend to form channel-spanning step-like structures with an associated downstream scour pool, whereas in large lowland rivers with low slopes logjams tend to be partial structures primarily acting to deflect flow with minimal geomorphological change.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
■ウィキペディアで「Log jam」の詳細全文を読む
スポンサード リンク
| 翻訳と辞書 : 翻訳のためのインターネットリソース |
Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.
|
|