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(CWs) are essential NBS infrastructures in urban water management in the sponge city concept, in order to control the growing menace of urban flooding. With green spaces, rooftop gardens and permeable streets, CWs both “buffer” water flows and reduce flood during storms and purify the pollution carried by rainwater. In fact, they act like a sponge to absorb and treat water before a flood occurs and limit urban heat islands in city (Nguyen et al., 2019b).

Storm water retention systems in urban areas are mostly dedicated to the capture of particulate pollution and to infiltration. Many micro-pollutants are fixed on particles and therefore retained. It is the case of alkylphenols and bisphenol A, as shown by Wiest et al. (2018) in a storm water detention basin of an industrial area or polycyclic aromatic hydrocarbons (PAHs) and metals (such as Zn or Cu) in a urban case (Walaszek et al., 2018). The contamination of the sediments implies their careful management. Dissolved micropollutants can, however, be transferred to groundwater and surface water. During winter, deicing salts, containing sodium and chloride, are of concern in regions where they are spread on roads. Deicing salt leads to a degradation of the quality of runoff water. Some NBS filtering structures, such as swales or ditches, significantly reduce the runoff of deicing salt to surface water, by retention and infiltration. However, while filter structures can buffer the salt content of surface water, they are also a means of loading salt into groundwater (Burgis et al., 2020; Lam et al., 2020).

      Nowadays, eco-engineering practices attempting to improve bioremediation in rivers receiving untreated or treated municipal and industrial wastewater as well as agricultural or urban runoff focus on physical actions through hydro-geomorphological modifications as constructed swales or riffles across urban creeks (Kasahara et al., 2006; Mendoza-Lera and Datry, 2017). Hydrogeomorphic manipulation of riverbed aims to increase pollution interception and bioprocessing inside the bed and to restore and enhance the river self-purification capacity.

      In running waters, redox processes are not so gradually structured according to depth as described in lake sediments (Bertrand et al., 2011) where a stratified redox gradient, from oxic condition in surface, to suboxic and anoxic conditions is formed (Figure 1.1). Preliminary results show a high sediment functioning heterogeneity according to water flow, geomorphology, hyporheic, and nutrient fluxes due to hydraulic conductivity and hydraulic gradient (Namour et al., 2015). The redox cycling of organic C and N not only drives the micro- and macro-biological communities, but also has implications for global nutrient balances and climate change.

      Structures harboring purifying capacities must selectively trap dissolved pollutants in order to concentrate them and make them available to biomass. Therefore, the zones with the highest assimilation and purification capacities are the zones where the immobilization of pollutants is the highest; this pollutant trapping is a prerequisite at any biodegradation and assimilation. Inside these filtering structures where a large heterotrophic biomass will develop and will be the most active self-purification zones, Sensu lato, we can define four main types of filtering structures:

      1 Mechanical filters, where steric obstruction retains particulates and associated pollutants;

      2 Physical filters which retain dissolved chemical compounds by adsorption;

      3 Physical-chemical filters, in which changes in pH or rH, or even UV irradiation or presence of redox compounds induce a chemical modification of pollutant;

      4 Biochemical filters, where aquatic flora and microflora degrade and metabolize the pollutant.

      In the natural environment, these four types of filters act together and their interactions are very strong. Thus, the precipitated pollutant (physical-chemical filter) is stopped in the interstices of the sediment (mechanical filter) where microbial biomass degrades it (biochemical filter). Mechanical and physical-chemical filters are not, strictly speaking, structures for the biodegradation of pollutants, but through their retention capacity, they participate by storing and concentrating the OM and the associated pollutants, thus allowing the development of microbial activities and pollutant biodegradation and leading to a real self-purification (Namour, 1999). The formation of physical-chemical gradients in sediments is the result of the balance between hydraulic conductivity pattern and retention, due to the mechanical filtering effect of pore size (Naganna et al., 2017). Retention allows the transient storage of pollutants and their biochemical transformations by the microbial flora, which depends on the physical-chemical conditions of the environment (Brunke and Gonser, 1997). Bioremediation setups based on NBSs should mimic, as much as possible, these natural filters in the construction of self-purifying nature-based systems.

1.4 Constructed Porous Ramps

      The self-purifying capacity of rivers is a valuable natural ecosystem service. During the transit of water along an infiltration/exfiltration sequence, the composition of water changes due to oxygen consumption, immobilization of organic carbon and nitrate production (Vidon et al., 2010; Peyrard et al., 2011). Moreover, the decomposition of OM by interstitial biofilms can induce the degradation of organic nitrogen by mineralization followed by nitrification (Breil et al., 2007b; Peyrard et al., 2011). It has been shown that the higher the energy of the current, the more efficient the metabolic processes are (Schmitt et al., 2011). In particular, a body of water can transform OM and nitrogenous mineral compounds into biomass or mineral nutrients which can be used by autotrophs.

Figure 1.2 The OTHU self-purification device based on a porous wood weir.

A pilot site was setup in 2000, as part of OTHU’s long-term outdoor observatory project (http://www.graie.org/othu/), to assess the impact of a self-purification device for combined sewer overflow discharges on a seasonal urban watercourse, with the aim of understanding the articulation between geomorphic, hydrological, and biotic components and identifying levers to modulate the impact of the pollutant discharge (Figure 1.2). Urban development increases the frequency of small floods (Braud et al., 2013) and riverbed erosion (Navratil et al., 2013), leading to biodiversity losses due to both poor water quality and poor geomorphic diversity.

      Constructed porous ramps (groynes) aim at the induction of oxic/ anoxic sequences conducive to biodegradation. Optimization key points have been identified:

      1 Trapping capacity of the porous groynes. It depends on the particle size distribution of the pollution (70% to 90% of the pollution in combined sewer overflows (CSOs)), the Stockes law, the downstream hydraulic gradient, and the dissipation of turbulence energy at the water-sediment interface. The trapping capacity will condition the length of the porous sediment and the height of the groyne;

      2 Minimum residence time to allow biodegradation during the circulation of the hyporheic flux and oxygen consumption (oxic to anoxic conditions). The required residence time depends on the considered biodegradation kinetics considered. The volume and shape of the porous sediment influence the upwelling and down-welling flows which can be used to control and adjust the residence time;

      3 Efficient operating system design at low flow rates. It functions as a hydraulic control point to maintain a critical water depth (by maximizing the down-welling flow of pollutants and dissolved oxygen); and when submerged at high flow rates, it does not increase the height of water during floods;

      4 Frequency of self-regeneration of porous sediments over several decimetres. It depends mainly on the granulometry and inclination of the surface of the porous sediments. Computational Fluid Dynamics (CFD) tools help us in this task before developing a model of the compartmental type that is easy to be used by engineering offices.

      As part of the ANR-EPEC research project,

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