13 Mar 2023
 | 13 Mar 2023
Status: this preprint is open for discussion.

Methane emissions due to reservoir flushing: a significant emission pathway?

Ole Lessmann, Jorge Encinas Fernández, Karla Martínez-Cruz, and Frank Peeters

Abstract. Reservoirs can emit substantial amounts of the greenhouse gas methane (CH4) via different emission pathways. In some reservoirs, reservoir flushing is employed as a sediment management strategy to counteract growing sediment deposits that threaten reservoir capacity. Reservoir flushing utilizes the eroding force of water currents during water level drawdown to mobilize and transport sediment deposits through the dam outlet into the downstream river. During this process, CH4 that is stored in the sediment can be released into the water and degas to the atmosphere resulting in CH4 emissions. Here, we assess the significance of this CH4 emission pathway and compare it to other CH4 emission pathways from reservoirs. We measured seasonal and spatial CH4 concentrations in the sediment of Schwarzenbach Reservoir, providing one of the largest datasets on CH4 pore water concentrations in freshwater systems. Based on this dataset we determined CH4 fluxes from the sediment and estimated potential CH4 emissions due to reservoir flushing. CH4 emissions due to one flushing operation can constitute 7–14 % of the typical annual CH4 emissions from Schwarzenbach Reservoir, whereby the amount of released CH4 depends on the timing of the flushing operation within the season. The larger the thickness of the sediment layer mobilized during the flushing operation the larger the average CH4 concentration per unit volume of flushed sediment. This suggests that regular flushing of smaller sediment layers releases less CH4 than removal of the same sediment volume in fewer flushing events of thicker sediment layers. In other reservoirs with higher sediment loadings than Schwarzenbach Reservoir, reservoir flushing could cause substantial CH4 emissions, especially when flushing operations are conducted frequently. Therefore, CH4 emissions due to reservoir flushing must be included in estimates of annual overall greenhouse gas emissions from reservoirs that are subject to regular flushing operations.

Ole Lessmann et al.

Status: open (until 27 Apr 2023)

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Ole Lessmann et al.


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Short summary
In our study, we assess the significance of methane (CH4) emissions due to reservoir flushing. We generated a large dataset of CH4 pore water concentrations in a reservoir's sediment to resolve seasonal CH4 distributions in the sediment. Our results show that in the studied reservoir, CH4 emissions caused by one flushing operation can represent 7–14 % of the annual CH4 emissions and that timing and frequency of the flushing operations affect the amount of released CH4.