A Case Study of Dissolved Oxygen Characteristics in a Wind-Induced Flow Dominated Shallow Stormwater Pond Subject to Hydrogen Sulfide Production
Creators: name: Chen, Liyu
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2017
Subjects: Stormwater ponds
Dissolved oxygen
Wind-induced hydraulics
Mike 3
Chlorophyll

description: Stormwater ponds (SWPs) are becoming increasingly important due to the negative impacts on flood mitigation and water quality control that results from rapid urbanization. These ponds are not only designed to control the discharge of large precipitation and snow melt events, but also to mitigate the water quality of the retained stormwater. Consequently, improper design and maintenance may lead to hypoxic conditions in SWPs, which result in poor water quality and generation of noxious gases. Riverside South Stormwater Pond II (RSPII) in Ottawa periodically experiences low dissolved oxygen (DO) concentrations and subsequently hypoxic conditions at depth in the pond, especially during summer days with less precipitation and winter ice covered periods. Hydrogen sulfide gas (H2S) has been generated and released into the ambient atmosphere during these periods of lesser water quality. Hence, there is a need to understand how DO spatial distribution and seasonal change trigger and affect H2S production. The conventional shallow design criteria of SWPs likely cause these systems to be susceptible to wind conditions. Very few research has demonstrated the correlation between wind-driven hydraulic performance and detained stormwater quality. Hence an understanding of pond-scale mixing generated by wind-induced flow and the subsequent correlation to DO concentrations and stratification in SWPs are important to understanding the water quality and performance of these systems, especially in a wind-induced flow dominated SWPs. The overall research objective is to develop a comprehensive understanding of hypoxic conditions of SWPs and to investigate the impact of wind induced hydraulics on DO seasonal characteristics and the subsequent production of H2S. RSPII was shown to experience lower DO and longer hypoxic conditions than an adjacent reference pond (RSPI) at both non-ice covered and ice covered months. In addition, hypoxia was shown to be initiated at the outlet of RSPII where the depth of the pond was a maximum. Interestingly, chlorophyll-α blooms were observed during ice covered conditions in the study, with synurids, tabellaria, and asterionella being identified as the dominant species. A bottom-mount acoustic Doppler current profiler (aDcp) was used to collect small wind-generated currents in RSPII. The three-dimensional current and DO model produced by MIKE 3 (DHI software) suggests a conclusive result of pond scale mixing produced by wind-driven flow as well as countercurrents near the bed opposite to wind direction. A wind dominated circulation was shown to be generated even with moderate wind speed, and with a higher wind condition pond-scale complete DO mixing was created. The MIKE 3 simulation further provided a comprehensive understanding of the correlation between wind-induced hydraulics and DO concentrations distribution in a shallow stormwater pond. Therefore, this research demonstrates that wind is an essential hydraulic driver in shallow ponds, which also likely affects water quality by initiating pond mixing.