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Development and sensitivity analysis of a model for assessing stratification and safety of Lake Nyos during artificial degassing

机译：人工除气过程中评估尼约斯湖分层和安全性模型的开发和敏感性分析

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摘要

To prevent the recurrence of a disastrous eruption of carbon dioxide (CO_2) from Lake Nyos, a degassing plan has been set up for the lake. Since there are concerns that the degassing of the lake may reduce the stability of the density stratification, there is an urgent need for a simulation tool to predict the evolution of the lake stratification in different scenarios. This paper describes the development of a numerical model to predict the CO_2 and dissolved solids concentrations, and the temperature structure as well as the stability of the water column of Lake Nyos. The model is tested with profiles of CO_2 concentrations and temperature taken in the years 1986 to 1996. It reproduces well the general mixing patterns observed in the lake. However, the intensity of the mixing tends to be overestimated in the epilimnion and underestimated in the monimolimnion. The overes-timation of the mixing depth in the epilimnion is caused either by the parameterization of the k-epsilon model, or by the uncertainty in the calculation of the surface heat fluxes. The simulated mixing depth is highly sensitive to the surface heat fluxes, and errors in the mixing depth propagate from one year to the following. A precise simulation of the mixolimnion deepening therefore requires high accuracy in the meteorological forcing and the parameterization of the heat fluxes. Neither the meteorological data nor the formulae for the calculation of the heat fluxes are available with the necessary precision. Consequently, it will be indispensable to consider different forcing scenarios in the safety analysis in order to obtain robust boundary conditions for safe degassing. The input of temperature and CO_2 to the lake bottom can be adequately simulated for the years 1986 to 1996 with a constant sublacustrine source of 18 l s~(-1) with a CO_2 concentration of 0.395 mol l~(-1) and a temperature of 26℃. The results of this study indicate that the model needs to be calibrated with more detailed field data before using it for its final purpose: the prediction of the stability and the safety of Lake Nyos during the degassing process.
机译：为防止再次发生来自尼奥斯湖的灾难性喷发二氧化碳（CO_2），已为该湖制定了脱气计划。由于担心湖泊的脱气会降低密度分层的稳定性，因此迫切需要一种仿真工具来预测不同情况下湖泊分层的演变。本文描述了一个预测CO_2和溶解固体浓度，温度结构以及尼约斯湖水柱稳定性的数值模型的开发。该模型使用1986年至1996年的CO_2浓度和温度曲线进行了测试。该模型很好地再现了湖中观察到的一般混合模式。但是，混合的强度倾向于在上级中被高估，而在单级中被低估。上扬中混合深度的过高估计是由于kε模型的参数化或表面热通量计算的不确定性引起的。模拟的混合深度对表面热通量高度敏感，并且混合深度的误差从一年传播到第二年。因此，精确模拟混合料的加深需要气象强迫和热通量参数化方面的高精度。无论是气象数据还是计算热通量的公式都没有必要的精度。因此，在安全性分析中必须考虑不同的强制情况，以获得可靠的脱气安全边界条件。在1986年至1996年期间，恒定的湖相下源18 ls〜（-1），CO_2浓度为0.395 mol l〜（-1），温度为200℃，可以充分模拟到湖底的温度和CO_2的输入。 26℃。这项研究的结果表明，在将该模型用于最终目的之前，需要先对其进行更详细的现场数据校准：对除气过程中尼约斯湖的稳定性和安全性进行预测。

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