Abstract
Utility trenches are an integral part of a sewer system. When situated in an aquitard, their coarse granular bedding material potentially forms a preferential flow path. The hydrologic functioning of the utility trenches and their bedding has been analyzed using water level data from a unique district-wide investigation of the interaction between a storm sewer system, the associated utility trench network and an aquitard. Utility trench water level recessions have been examined using analytical results based on recession analysis with the one-dimensional (1-D) Boussinesq equation, as well as deterministic 1-D and 2-D numerical groundwater models of utility trenches. The results show that water level rises occur after cessation of exfiltration recharge. These rises are associated with nonequilibrium recharge and occur when branch utility trench flows exceed the downslope trunk utility trench flows. A near-linear recession occurs when the branch utility trenches transition into a quasi-exponential recession on cessation of these water level rises. The magnitude of water level rises and duration of the near-linear recession are increased by the convergence of flow from branch utility trenches to the trunk utility trench. A utility trench network situated in an aquitard could be considered an analog for some natural aquifers with preferential flow paths. It is proposed that significant post-recharge storage increases associated with nonequilibrium recharge events could occur in such aquifers, which may be indicated by near-linear recessions preceding quasi-exponential recessions.
Résumé
Les tranchées de service public font partie intégrante d’un système d’égouts. Lorsqu’elles sont situées dans un aquitard, leur matériau d’assise granulaire grossier forme potentiellement un chemin d’écoulement préférentiel. Le fonctionnement hydrologique des tranchées de service public et du matériel sur lesquelles elles reposent a été analysé à l’aide de données sur les niveaux d’eau provenant d’une étude unique menée à l’échelle du district sur l’interaction entre un réseau d’eaux pluviales, le réseau de tranchées associé et un aquitard. Les récessions du niveau d’eau des tranchées ont été examinées à l’aide de résultats analytiques basés sur l’analyse des récessions avec l’équation de Boussinesq unidimensionnelle (1-D), ainsi que des modèles numériques hydrogéologiques déterministes 1-D et 2-D des tranchées. Les résultats montrent que des hausses de niveau d’eau se produisent après l’arrêt de la recharge par exfiltration. Ces hausses sont associées à une recharge non équilibrée et se produisent lorsque les débits de sections du réseau de tranchées dépassent les débits des tranchées situées en aval. Une récession quasi-linéaire se produit lorsque les sections des tranchées passent à une récession quasi-exponentielle à la fin de ces hausses de niveau d’eau. L’ampleur de la montée des eaux et la durée de la décrue quasi-linéaire sont accrues par la convergence de l’écoulement des sections de tranchées vers la tranchée principale. Un réseau de tranchées de services publics situé dans un aquitard pourrait être considéré comme un analogue de certains aquifères naturels avec des voies d’écoulement préférentielles. Il est proposé que des augmentations significatives du stockage post-recharge associées à des événements de recharge non équilibrés puissent se produire dans de tels aquifères, ce qui peut être indiqué par des récessions quasi-linéaires précédant des récessions quasi-exponentielles.
Resumen
Las trincheras de saneamiento son una parte integral de un sistema de alcantarillado. Cuando están situadas en un acuitardo, su material de lecho granular grueso forma potencialmente una vía de flujo preferente. El funcionamiento hidrológico de las trincheras y su lecho se ha analizado utilizando los datos de nivel de agua de una investigación realizada en todo el distrito sobre la interacción entre un sistema de alcantarillado de aguas pluviales, la red de trincheras de saneamiento asociada y un acuitardo. Las recesiones del nivel del agua en las trincheras de saneamiento se han examinado utilizando resultados analíticos basados en el análisis de recesión con la ecuación unidimensional (1-D) de Boussinesq, así como modelos numéricos deterministas 1-D y 2-D de aguas subterráneas de las trincheras de saneamiento. Los resultados muestran que se producen ascensos del nivel del agua tras el cese de la recarga por exfiltración. Estos ascensos se asocian a una recarga no equilibrada y se producen cuando los flujos de las trincheras auxiliares superan a los flujos de las trincheras principales situadas aguas abajo. La recesión casi lineal se produce cuando las trincheras auxiliares pasan a una recesión casi exponencial al cesar estos ascensos del nivel del agua. La magnitud de los ascensos del nivel del agua y la duración de la recesión casi lineal aumentan por la convergencia del flujo de las trincheras auxiliares hacia la truncal. Una red de trincheras situada en un acuitardo podría considerarse un análogo de algunos acuíferos naturales con trayectorias de flujo preferenciales. Se propone que en dichos acuíferos podrían producirse aumentos significativos del almacenamiento posterior a la recarga asociados a eventos de recarga de no equilibrio, que podrían estar indicados por recesiones casi lineales que preceden a otras cuasi-exponenciales.
摘要
公用设施沟渠是下水道系统的一个组成部分。当位于弱透水层时,它们的粗粒状垫层材料可能会形成优先流动路径。公用设施沟渠的水文功能及其垫层已使用来自对雨水下水道系统、相关公用设施沟渠网络和弱透水层之间相互作用的独特全区调查的水位数据进行了分析。使用基于一维(1-D)Boussinesq 方程的下降分析的解析结果以及公用设施沟渠的确定性一维和二维数值地下水模型,对公用设施沟渠水位下降进行了检查。结果表明,渗出补给停止后水位上升。这些上升与非平衡补给有关,并且发生在分支公用设施沟渠流量超过下坡主干公用设施沟渠流量时。当分支公用设施沟渠在这些水位停止上升时转变为准指数下降时,就会发生近线性下降。水位上升幅度和近线性下降的持续时间通过从分支公用设施沟渠到主干公用设施沟渠的汇流而增加。位于含水层的公用沟渠网络可被视为具有优先流动路径的某些天然含水层的模拟物。有人提出,与非平衡补给事件相关的补给后蓄水量的显着增加可能发生在此类含水层中,这可以通过准指数下降之前的近线性下降来表明。
Resumo
As valas de serviço público são parte integrante de um sistema de esgoto. Quando situado em um aquitardo, seu material granular grosseiro de leito potencialmente forma um caminho de fluxo preferencial. O funcionamento hidrológico das valas de serviço público e seus leitos foram analisados usando dados de nível de água de uma investigação distrital única da interação entre um sistema de esgoto pluvial, a rede de valas de serviço público associada e um aquitardo. As recessões do nível de água das valas de serviço público foram examinadas usando resultados analíticos baseados na análise de recessão com a equação de Boussinesq unidimensional (1-D), bem como modelos numéricos determinísticos 1-D e 2-D de águas subterrâneas de valas de serviço público. Os resultados mostram que as elevações do nível da água ocorrem após a cessação da recarga de exfiltração. Esses aumentos estão associados à recarga fora do equilíbrio e ocorrem quando os fluxos da vala da concessionária de ramal excedem os fluxos da vala da concessionária do tronco em declive. A recessão quase linear ocorre quando as valas da concessionária de ramais transitam para uma recessão quase exponencial na cessação dessas elevações do nível da água. A magnitude das elevações do nível da água e a duração da recessão quase linear são aumentadas pela convergência do fluxo das valas da concessionária de ramal para a vala da concessionária de tronco. Uma rede de valas de utilidade situada em um aquitardo pode ser considerada um análogo para alguns aquíferos naturais com caminhos de fluxo preferenciais. Propõe-se que aumentos significativos de armazenamento pós-recarga associados a eventos de recarga fora de equilíbrio podem ocorrer em tais aquíferos, o que pode ser indicado por recessões quase lineares que precedem recessões quase exponenciais.
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Acknowledgements
Data from the Lisgar District Basement Water Infiltration Investigation have been used with the permission of the City of Mississauga. Keenan Lamb is thanked for providing thoughtful comments on the pre-submission manuscript. Two anonymous reviewers are thanked for providing helpful comments that have improved the report.
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Shepley, M.G., Schmidt, N. Utility trench water level recessions in an aquitard: findings from analytical and numerical analyses. Hydrogeol J 30, 2495–2507 (2022). https://doi.org/10.1007/s10040-022-02550-0
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DOI: https://doi.org/10.1007/s10040-022-02550-0