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Description of the Hyporheic zone of a river
The hyporheic zone is an important hydrological, chemical, biological interface and between the watercourse and groundwater. It was shown that the most important changes are within a few centimeters below the sediment / water interface. Using a device capable of sampling at the centimeter scale, it has been observed profound changes in pH and concentrations of COD (Chemical Oxygen Demand) and CH4 (methane) water that flows at the hyporheic zone. There has also been an increase in COD in its superficial part accompanied by a decrease of the average molecular weight, which led to speculation that this part is an area of intense biochemical change (oxidation by benthic organisms) for water in transit.
This transformation process thus makes available many chemical forms which provide a fundamental bioenergy source for watercourse plants. However, the “Chaudanne” remains a relatively oligotrophic medium in so far as it is a sandy substrate.
In addition, there is a link between the distribution of interstitial fauna and hydrogeological structure of a given sector. Marmonier & Dole (1986) and then Marmonier (1988) have clearly shown the differences of population that exist upstream, at the apex and downstream of the gravel banks of the canal Miribel (Rhône) depending on whether these zones are fed by the water surface or, conversely, by gushing of groundwater. In all cases, the water that passes through the hyporheic zone can undergo significant chemical changes over small distances.
It seems therefore that the hydrological exchanges between surface water and hyporheic water influence the distribution and composition of benthic species. Since they are responsible for the degradation of organic matter, we see the advantage of knowing the direction and intensity of these exchanges in the context of the assessment of the assimilative capacity of the Chaudanne. (5)
Under flow at the hyporheic zone
The need to better understand the nature of the flows within the sediments of rivers, and also, to better define the hydrodynamic characteristics, is one of the consistent findings during the last decade to the ecology of the environment.
Through the literature, one can find a certain amount of information on the study of the hyporheic flow in a river. However, they are mainly about the continuous exchange between the river and the sub-surface.
It is also known that, depending on the exchange surface, the hydraulic gradient, the longitudinal profile and the permeability of the bed of the river, the phenomenon of penetration into the hyporheic zone varies widely throughout its course. Not only have variations on the horizontal can be observed but also on the vertical scale. For example, hyporheic fluxes which are due to sand bumps penetrate sediments of a depth approximately equal to the distance between the ridges when they reach a much greater depth in the presence of a coarser substrate.
It can be observed two phenomena upwelling and downwelling at this preferential exchange area. The water plunges (downwelling) provides the hyporheic zone with substances like oxygen while water that rises (upwelling) can be rich in nutrients needed to biotic surface system.
Moreover, at the downwelling area, it was shown that the benthic fauna penetration was deeper. We can observe at the study area the “Grézieu la Varenne” site a series of riffles comparable to a series of steps with a sequential arrangement of sandy zone which ends in a gravel riffles lens. Presumably the type of continuous exchanges described above is largely disrupted at a high rainfall event. Indeed, the sudden increase in the flow rate has consequently the extraction of 2 to 3 cm of sand in the upper portion of the substrate that includes, according to the biologists, much of the biological activity in undisturbed period.
Stony areas of the raft remain in some measures relatively more stable, they provide sustainability and they retain the role of hyporheic filter.
Thus we see that the underflow at the raft has some importance in the performance of sewer overflows to the extent that it appears to facilitate the penetration of water and nutrients in the substrate. It remains to be seen in what hydraulic context these zones promote this type of exchange.
Analysis of disturbed functioning of the hyporheic zone
In the context of our metrological site, the receiving environment is a small river in which flows rejected by the storm overflow can reach frequently the average flow of high water which is about 30 liters / second .This causes a rapid rise in water level over periods ranging from minutes to hours.
The watershed upstream at discharge point has a response time to rain of about 30 min while the network produce spills after 10 minutes for fairly average rainfall (monthly frequency) In the case of substantial rainfall, subsurface runoff and Intumescence of the water originating from the natural watershed can produce an increase in the river flow with a certain lag compared to the effect of rejection.
These features of the site may be favorable to the intensification of hyporheic flows, as shown in the figure below.
We will assume a two times operation with inversion of flow direction in the hyporeheic zone. This inversion is due to the gradient of hydraulic head between the water and the river. (5).
Effect of a natural meander
During low flows the nitrification would be located just near the riffle. Longer water trajectories are presumed to take place from the run to the pool with the nitrificationdenitrification sequence. During flood events, stream water invades mainly the external bank of meanders/sinuosities what supplies ground water with oxygen and would reverse local denitrification process. However, the general biochemical scheme of internal bank of meanders/sinuosities is not expected to change a lot considering the low hydraulic conductivity of these banks in comparison to substrates hydraulic conductivity. Internal meanders/sinuosities water pathways would hence have a more stable biochemical process with flow stage variations thanaccompanying stream substrates. (11) Main degradation process should take place at internal bank of meanders or sinuosities.
Stratification reactions in an artificial structure
The expected effect of an artificial structure on biodegradation in saturated porous medium in natural stream is that during average flow from the storm spillway (origin of organic nitrogen) associated with oxygen O2 resulting from atmospheric exchange, the nitrification process is favoured on the surface (blue area) while the denitrification process in present in the hyporheic zone and formation of mineral NO3.
During low flow, we can observe the impregnation of organic matter due to the fact that the medium is non saturated with high OM content which favours the vertical cycle of nitrification denitrification at the upper part of the substrate while at the bottom there is a saturated medium with high Norg content (due to summer releases) which may favours denitrification over nitrification.
Water flow time series
The Chaudanne creek is an experimental site which belongs to the long term field observatory for urban storm waters management (see http://www.graie.org/othu/). In this frame water flows are recorded at several points including sewer system, overflow outlet and stream system for more than 10 years. These data are used to figure the hydraulic functioning of the structure which is the core of my project. The sand-body is located right downstream a combined sewer overflow outlet which deliver a mixture of sewage and rainfall waters during storm events. In reason of the naturally intermittent flow regime of the creek, the percentage of water loaded with pollution can represent at least 100% of the water flowing through the Chaudanne creek during low flows periods. This is well illustrated in figure IV-7 which represents the creek flow rate variations along one year from August 2011 to November 2012 at the entrance of the structure.
Table of contents :
Chapter I: General context
I-1-Presentation of the host company
I-2-Context of the internship and problematic
I-2-1-The EPEC Project
I-2-2-Internship context:
Chapter II: Hyporheic zone
II-1-Definition of hyporheic zone
II-2-Description of the Hyporheic zone of a river
II-3-Under flow at the hyporheic zone
II-4-Analysis of disturbed functioning of the hyporheic zone
II-5-The existing processes in the river
II-5-1-The detailed processes of nitrogen degradation
II-5-2- Effect of Natural structure taking into account the effect of hydrological regime
II-5-3-Effect of a natural meander
II-5-4- Stratification reactions in an artificial structure
Chapter III: On site measurement and procedures
III-1-topographic campaign:
III-2-Depth measurement:
III-3-Measurement of hydraulic conductivity in the substrate in situ
III-4-Temperature measurements
III-5-Conclusion:
Chapter IV: Field investigations and data analysis
IV-1-Water flow time series
IV-2-Water quality data
IV-3-Geometrical and hydraulic features
IV-4-Conclusion:
Chapter V: Introduction to Hydrus 2D/3D
V-1-Description
V-2-Modeling
V-2-1-Modeling principles:
V-2-2-General characteristics of the HYDRUS-2D code:
V-2-3-Constructed Wetland Module
V-2-4-Heat transport
Chapter VI: Simulation using Hydrus 2D/3D
VI-1-Hydrus 2D/3D interface
VI-2-Geometry
VI-3-water flow
VI-3-1-Initial and boundary conditions
VI-3-2-Results of the simulation
VI-4-Observation points
VI-5- Tracking of particles in the soil according to the hydraulic gradient under the water flow condition only:
VI-5-1-low hydraulic gradient
VI-5-2-medium hydraulic gradient
VI-6-Solute transport
VI-6-1-Initial conditions
VI-6-2-Boundary conditions
VI-6-3-Parameters
VI-6-4-Results of the simulation
VI-6-5-Conclusion:
VI-7-Heat transport
VI-7-1-Modeling heat transport with Hydrus 2D/3D
VI-7-4-Results
VI-7-5-Conclusion
Chapter VII: conclusion
References
