異重流對粘性土捲昇之起動條件分析

The Critical Condition on Disturbing Cohesive Clay by Saline Density Current

許少華(Shao-Hua Marko Hsu) ； 林正傑(Cheng-Chieh Lin) ； 曾志民(Chih-Ming Tseng) ； 謝正倫(Chjeng-Lun Shieh)

《中華水土保持學報》 41卷2期 (2010/06) Pp. 143-152

https://doi.org/10.29417/JCSWC.201006_41(2).0008

摘要

水庫於颱洪事件後經常攔截集水區大量之細顆粒泥砂，這些剛落下的淤泥於下次颱洪事件往往較容易受異重流擾動而再次懸浮，提高水庫水體變更濁的可能性而影響到自來水的取水與淨水操作。本文以實驗觀測鹽水異重流擾動底床粘性土（高嶺土）之捲昇現象，並且提出以密度差福祿數探討分析此捲昇現象起動的水理條件。根據實驗數據顯示：(1)發生捲昇現象時，該沖刷斷面之臨界密差福祿數(Frd*)皆大於1，即該異重流皆屬於超臨界流；(2)Frd*值隨著斷面之單寛流量(qsec)增大而有漸減的趨勢，而且隨著底床粘性土之抗剪應力增加，此趨勢線僅微幅向上，此意謂高流量之異重流僅須較少的濃度，即有足夠的擾動剪應力可將粘性土捲起；不過隨著qsec減少，則趨勢線則有顯著的增幅，此意謂低流量之異重流隨著粘土的密實度增加，則須要更高的異重流濃度才能造成淤泥捲昇；(3)受異重流擾動而捲起之粘性土，大致集中於異重流頭端之底部，後來由於沿程被捲昇量逐漸增加，導致異重流頭端有顯著分層的現象，不過沿程之頭端速度並未有明顯增加或衰退的趨勢。

關鍵字

異重流 ； 粘性土 ； 密差福祿數

並列摘要

During a typhoon storm, a large portion of sediments enters a reservoir, and most of the fine sediments are transmitted to the dam site by turbidity currents. If those silting sediments deposited only for relatively short period, they might be easily disturbed or re-suspended by the turbidity current of a next storm event. This study proposes densimetric Froude number as a criterion for incipient condition of re-suspension of cohesive clay by the density current. A flume with 2% slope is employed, saline water (dyed with red color) is made for density current, and Kaolin clay is chosen for the bed material as deposited clay. Experimental results reveal that: (1) All density currents which did disturb the clay were supercritical flow. (2) The critical densimetric Froude number (Frd*) decreased as the unit-width discharge of the density current increased, and its trend line moved up as the bed stress increased, which means that the density currents of high discharge with less concentration are powerful enough to disturb and re-suspend the clay deposit. Oppositely, the density currents with smaller discharge require higher concentration to be able to disturb the clay deposit, especially when the density of deposited clay has increased. (3) Re-suspension of cohesive clay occurred on the bottom of density-current head. Most of the suspended clay aggregated near the bottom and showed stratification on the head when the density current passes certain distance. However, the velocity of the moving head retained the same even through the re-suspended clay had increased in the density-current head.

並列關鍵字

density current ； cohesive clay ； densimetric Froude number