Higashino Makoto, Stefan Heinz G
Departmant of Civil Engineering, Oita National College of Technology, 1666 Maki, Oita 870-0152, Japan.
Water Res. 2005 Sep;39(14):3153-66. doi: 10.1016/j.watres.2005.05.032.
Dead organic material accumulated on the bed of a lake, reservoir or wetland often provides the substrate for substantial microbial activity as well as chemical processes that withdraw dissolved oxygen (DO) from the water column. A model to estimate the actual DO profile and the "sedimentary oxygen demand (SOD)" must specify the rate of microbial or chemical activity in the sediment as well as the diffusive supply of DO from the water column through the diffusive boundary layer into the sediment. Most previous experimental and field studies have considered this problem with the assumptions that the diffusive boundary layer is (a) turbulent and (b) fully developed. These assumptions require that (a) the flow velocity above the sediment bed is fast enough to produce turbulent mixing in the boundary layer, and (b) the sediment bed is long. In this paper a model for laminar flow and SOD over a sediment bed of finite length is presented and the results are compared with those for turbulent flow. Laminar flow near a sediment bed is encountered in quiescent water bodies such as lakes, reservoirs, river backwaters, wetlands and ponds under calm wind conditions. The diffusive oxygen transfer through the laminar diffusive boundary layer above the sediment surface can restrict the microbial or chemical oxygen uptake inside the sediment significantly. The developing laminar diffusive boundary layer above the sediment/water interface is modeled based on the analogy with heat transfer, and DO uptake inside the sediment is modeled by Michaelis-Menten microbial growth kinetics. The model predicts that the rate of SOD at the beginning of the reactive sediment bed is solely dependent on microbial density in the sediment regardless of flow velocity and type. The rate of SOD, and the DO penetration depth into the sediment decrease in stream-wise direction over the length of the sediment bed, as the diffusive boundary layer above the sediment/water interface thickens. With increasing length of the sediment bed both SOD rate and DO penetration depth into the sediment tend towards zero if the flow is laminar, but tend towards a finite value if the flow is turbulent. That value can be determined as a function of both flow velocity and microbial density. The effect of the developing laminar boundary layer on SOD is strongest at the very lowest flow velocity and/or highest microbial density inside the sediment. Under quiescent conditions, the effective SOD exerted by a reactive sediment bed of a lake or wetland approaches zero, i.e. no or very little oxygen demand is exerted on the overlying water column, except at the leading edge.
湖泊、水库或湿地底部积累的死亡有机物质通常为大量微生物活动以及从水柱中消耗溶解氧（DO）的化学过程提供了底物。一个用于估算实际溶解氧分布和“沉积氧需求（SOD）”的模型必须指定沉积物中微生物或化学活动的速率，以及溶解氧通过扩散边界层从水柱扩散进入沉积物的供应情况。以前的大多数实验和现场研究在考虑这个问题时都假设扩散边界层是（a）湍流的且（b）充分发展的。这些假设要求（a）沉积物床上方的流速足够快，以在边界层中产生湍流混合，并且（b）沉积物床足够长。本文提出了一个关于有限长度沉积物床上层流和沉积氧需求的模型，并将结果与湍流情况进行了比较。在平静风况下，在诸如湖泊、水库、河流回水、湿地和池塘等静止水体中会遇到沉积物床附近的层流。通过沉积物表面上方的层流扩散边界层的扩散氧传输会显著限制沉积物内部微生物或化学对氧的吸收。基于与热传递的类比，对沉积物/水界面上方发展的层流扩散边界层进行建模，并通过米氏微生物生长动力学对沉积物内部的氧吸收进行建模。该模型预测，在反应性沉积物床开始处的沉积氧需求速率仅取决于沉积物中的微生物密度，而与流速和类型无关。随着沉积物/水界面上方的扩散边界层变厚，在沉积物床长度方向上，沉积氧需求速率和溶解氧渗透到沉积物中的深度都会降低。如果水流是层流，随着沉积物床长度的增加，沉积氧需求速率和溶解氧渗透到沉积物中的深度都趋于零，但如果水流是湍流，则趋于一个有限值。该值可以根据流速和微生物密度来确定。发展中的层流边界层对沉积氧需求的影响在沉积物内部流速非常低和/或微生物密度非常高时最为强烈。在静止条件下，湖泊或湿地的反应性沉积物床施加的有效沉积氧需求接近零，即除了前沿外，对上覆水柱没有或几乎没有氧需求。
