Yvan Rossier

Numerical simulation as a tool for checking the interpretation of karst spring hydrographs

Abstract A schematic representation of karst aquifers may be that of a high hydraulic conductivity channel network with kilometre-wide intervals, surrounded by a low hydraulic conductivity fractured limestone volume and connected to a local discharge area, the karst spring. The behaviour of the karst spring (hydrographs, chemical or isotopic composition, etc.) represents the global response of the karst aquifer to input events. The available data on karst aquifer hydraulic parameters are limited. Global response is therefore more easily obtained and is commonly used to make inferences on the recharge and groundwater flow processes, as well as on the hydraulic parameter fields. Direct verification of these interpretations is, obviously, very difficult. We have used an indirect method of verification, consisting of introducing well-defined theoretical karst structures into a finite element model and then analysing the simulated global response according to presently accepted interpretation schemes. As we know what we put into the numerical model, the validity of any interpretation may be checked. The first results indicate that some of the generally accepted interpretations are not necessarily true. In particular: (i) separation of simulated recession hydrographs into several components shows that different exponential components do not necessarily correspond to aquifer volumes with different hydraulic conductivities; (ii) non-exponential parts of recession hydrographs do not always give information about the infiltration process; and (iii) the recession coefficient of the baseflow (i.e. the last, nearly exponential part of the recession hydrograph) depends on the global configuration of the whole karst aquifer, not just on the hydraulic properties of the low hydraulic conductivity volumes.

Numerical versus statistical modelling of natural response of a karst hydrogeological system

Abstract Structural and hydrodynamic characteristics of karst aquifers are mostly deduced from studies of global responses of karst springs (hydrographs, chemical or isotopic composition). In this case, global response is often used to make inferences with respect to infiltration and ground water flow processes as well as on the hydrodynamic parameters. Obviously, the direct verification of these inferences is very difficult. We have used an indirect method of verification, introducing well defined theoretical karst structures into a finite element model and then analysing the simulated global response according to the currently accepted interpretation schemes. As we know what we are introducing into the numeric model, the consistency of the interpretation may be checked immediately. The results obtained in the hydrogeological study of two karst basins in the Swiss Jura and from 2-D and 3-D numerical simulations show the difficulty of finding structural parameters and hydrodynamic behaviour from statistical methods alone, i.e. correlation analyses discharge–discharge and precipitation–discharge. In effect, our first results show that the form of the correlograms depends on several factors besides the structure of the karst aquifer: (i) on the form of the floods, in other words the contrast between quick flow and base flow, (ii) on the frequency of hydrological events during the period analysed and (iii) on the type of infiltration processes, in other words the ratio of diffuse infiltration to concentrated infiltration. Obviously, the variability of a karst hydrograph is a result of a combination of these factors. Distinction between them is not always possible on hydrographs, and therefore on correlations (discharge–discharge and precipitation–discharge).

Deterministic modeling of the impact of underground structures on urban groundwater temperature

Underground structures have a major influence on groundwater temperature and have a major contribution on the anthropogenic heat fluxes into urban aquifers. Groundwater temperature is crucial for resource management as it can provide operational sustainability indicators for groundwater quality and geothermal energy. Here, a three dimensional heat transport modeling approach was conducted to quantify the thermally affected zone (TAZ, i.e. increase in temperature of more than +0.5°C) caused by two common underground structures: (1) an impervious structure and (2) a draining structure. These design techniques consist in (1) ballasting the underground structure in order to resist hydrostatic pressure, or (2) draining the groundwater under the structure in order to remove the hydrostatic pressure. The volume of the TAZ caused by these underground structures was shown to range from 14 to 20 times the volume of the underground structure. Additionally, the cumulative impact of underground structures was assessed under average thermal conditions at the scale of the greater Lyon area (France). The heat island effect caused by underground structures was highlighted in the business center of the city. Increase in temperature of more than +4.5°C were locally put in evidence. The annual heat flow from underground structures to the urban aquifer was computed deterministically and represents 4.5GW·h. Considering these impacts, the TAZ of deep underground structures should be taken into account in the geothermal potential mapping. Finally, the amount of heat energy provided should be used as an indicator of heating potential in these areas.

Deterministic modelling of the cumulative impacts of underground structures on urban groundwater flow and the definition of a potential state of urban groundwater flow: example of Lyon, France

Underground structures have been shown to have a great influence on subsoil resources in urban aquifers. A methodology to assess the actual and the potential state of the groundwater flow in an urban area is proposed. The study develops a three-dimensional modeling approach to understand the cumulative impacts of underground infrastructures on urban groundwater flow, using a case in the city of Lyon (France). All known underground structures were integrated in the numerical model. Several simulations were run: the actual state of groundwater flow, the potential state of groundwater flow (without underground structures), an intermediate state (without impervious structures), and a transient simulation of the actual state of groundwater flow. The results show that underground structures fragment groundwater flow systems leading to a modification of the aquifer regime. For the case studied, the flow systems are shown to be stable over time with a transient simulation. Structures with drainage systems are shown to have a major impact on flow systems. The barrier effect of impervious structures was negligible because of the small hydraulic gradient of the area. The study demonstrates that the definition of a potential urban groundwater flow and the depiction of urban flow systems, which involves understanding the impact of underground structures, are important issues with respect to urban underground planning.RésuméLes structures souterraines ont montré avoir une grande influence sur les ressources du sous-sol dans les aquifères urbains. Une méthodologie d’évaluation de l’état actuel et potentiel de l’écoulement des eaux souterraines dans une zone urbaine est proposée. L’étude développe une approche de modélisation en trois dimensions pour comprendre les impacts cumulés des infrastructures souterraines sur les écoulements des eaux souterraines en milieu urbain, en utilisant une étude de cas dans la ville de Lyon (France). Toutes les structures souterraines connues ont été intégrées dans un modèle numérique. Plusieurs simulations ont été effectuées : l’état actuel de l’écoulement d’eaux souterraines, l’état potentiel de l’écoulement d’eaux souterraines (sans structures souterraines), un état intermédiaire (sans structures imperméables) et une simulation en transitoire de l’état actuel des écoulements d’eaux souterraines. Les résultats montrent que les structures souterraines fragmentent les systèmes d’écoulement d’eaux souterraines conduisant à une modification du régime de l’aquifère. Pour le cas étudié, les systèmes d’écoulement sont stables au cours du temps avec une simulation en transitoire. Les structures avec des systèmes de drainage ont un impact majeur sur les systèmes d’écoulement. L’effet de barrière des structures imperméables est négligeable à cause du faible gradient hydraulique de la zone. L’étude démontre que la définition d’un écoulement potentiel d’eaux souterraines en milieu urbain et la représentation des systèmes des flux en milieu urbain, qui impliquent la compréhension de l’impact des structures souterraines, sont des sujets importants en matière de planification urbaine souterraine.ResumenLas estructuras del subsuelo han demostrado tener una gran influencia sobre los recursos subterráneos en los acuíferos urbanos. Se propone una metodología para evaluar el estado actual y potencial del flujo de agua subterránea en una zona urbana. El estudio desarrolla un enfoque de modelado tridimensional para entender los impactos acumulativos de las infraestructuras del subsuelo en el flujo de las aguas subterráneas urbanas, usando un caso de la ciudad de Lyon (Francia). Todas las estructuras conocidas del subsuelo fueron integradas en el modelo numérico. Se llevaron a cabo varias simulaciones: el estado actual del flujo de agua subterránea, el estado potencial del flujo de agua subterránea (sin estructuras en el subsuelo), un estado intermedio (sin estructuras impermeables), y una simulación transitoria del estado actual del flujo de agua subterránea. Los resultados muestran que las estructuras en el subsuelo fragmentan los sistemas de flujo del agua subterránea conduciendo a una modificación del régimen del acuífero. Para el caso estudiado, se demuestra que los sistemas de flujo son estables a través del tiempo con una simulación transitoria. Se muestra que las estructuras con sistemas de drenaje tienen un impacto mayor en los sistemas de flujo. El efecto de barrera de las estructuras impermeables fue despreciable debido al pequeño gradiente hidráulico de la zona. El estudio demuestra que la definición de un flujo potencial de agua subterránea urbana y la esquematización de los sistemas de flujo urbano, que implica entender el impacto de las estructuras del subsuelo, son cuestiones importantes con respecto a la planificación de las estructuras del subsuelo urbano.摘要地下建筑物显示出对城市含水层下层土壤有很大的影响。本文论述了评价城区地下水流实际和潜在的状态的方法。本研究利用(法国)里昂一个实例开发了三维模拟方法以了解地下建筑对城市地下水流的累积影响。所有已知的地下建筑物都并归到数值模型中。进行了几个模拟 :地下水流的实际状态,地下水流的潜在状态(无地下建筑物),中间状态(没有不透水的建筑物),地下水实际状态的瞬时模拟。结果显示,地下建筑物使地下水流系统四分五裂,致使含水层发生改变。在研究的实例中,水流系统在瞬时模拟中显示随着时间的过去趋于稳定。有排水系统的建筑物显示对水流系统有重要影响。无透水建筑物的屏障效应可以忽略,因为该地区水力梯度太小。研究显示,潜在城市地下水流的定义及城市水流系统的描述涉及到对地下建筑物影响的了解,是城市地下规划的重要课题。ResumoEstruturas subterrâneas comprovadamente apresentam uma grande influência sobre os recursos subterrâneos de aquíferos urbanos. Propõe-se neste estudo uma metodologia para avaliar a situação real e potencial do escoamento de água subterrânea em uma área urbana. O estudo desenvolve uma modelagem tridimensional para entender os impactos cumulativos das infraestruturas subterrâneas sobre o escoamento de água subterrânea urbana utilizando-se o caso da cidade de Lyon (França). Todas as estruturas subterrâneas conhecidas foram integradas ao modelo. Foram realizadas diversas simulações: a situação real do escoamento de água subterrânea, a situação potencial do escoamento de água subterrânea (sem estruturas subterrâneas) e uma simulação de transitório da situação real do escoamento de água subterrânea. Os resultados mostram que as estruturas subterrâneas segregam o sistema de escoamento alterando o regime de escoamento do aquífero. O efeito de barreira das estruturas impermeáveis foi desprezível devido ao gradiente hidráulico reduzido na área. O estudo demonstra que a definição de uma situação potencial de escoamento de água subterrânea urbana e o mapeamento do escoamento de sistemas aquíferos urbanos, que envolvem o entendimento do impacto de infraestruturas, são discussões importantes com respeito ao planejamento do ambiente subterrâneo urbano.

Urban Underground Space Management: An Approach by Indicators for Groundwater Protection

Urban growth is often limited by available horizontal space. Lots of reasons can explain this horizontal limitation: coastal or mountainous areas, areas submitted to natural hazards, protection of agricultural areas, etc. Human activities, will favour vertical development of the city: by soaring up buildings above the soil surface, but also by underground constructions at increasing depths. This vertical urban development is not without consequence on subsoil resources. In particular, groundwater resource can be vulnerable when operating in this space. This article deals with the development of a new tool useful for urban underground space management and dedicated to groundwater resource. The approach is based on three dimensional modelling of the hydrogeological context of a living area. Thus, the influence and hydrogeological interactions of deep buildings will be interpreted to develop indicators. These indicators are intended to integrate vertical dimension of the city into planning documents. They will enable to assess the disturbance liable to be induced by underground constructions on groundwater flow.