Transport Processes and Optimization Strategies in Wetland Design

Transitional areas, between inland and coastal environments, represent an important habitat for their environmental and natural value. They act as a natural buffer for all those chemicals which are produced by diffused sources of pollutants (run-off rain water from agriculture) or from hidden sources (sewers not connected to a wastewater treatment plant). Pollutants produced by this type of sources can lead, if not conveniently treated, to eutrophication and to other water quality problems along coastal areas. Traditional wastewater treatment methods appear to be not effective in these conditions because of the big volumes of water and the relatively low concentration of dissolved pollutants to be treated. Since traditional wastewater treatment plants can not be used, it becomes important to better understand transport phenomena in transitional environments (rivers and wetlands) and all the removal processes in such zones in order to manage them to treat all the chemicals before they arrive to the coastal areas. Particular attention must be therefore stressed on retention processes and on the formulation of predictive models which allow scientists and engineers to better manage and design these buffer areas. In Chapter 1, the role of different transport processes is analyzed focusing the attention on different spatial and temporal scales. Principal modeling approaches are discussed underlining the role of each term on the mass balance equation and the most classical model closures are described in this chapter. In Chapter 2, retention characteristics of three different rivers are analyzed, relating different model closures with planimetric features of the rivers, their vegetational cover and bottom permeability. The analysis is carried on using STIR (Solute Transport In Rivers) model, a one-dimensional solute transport model that describes concentration breakthrough curves implementing a wide set of retention phenomena characterized by different time scales, represented by a specific residence time distribution in each retention domain. Comparison of modeling results and experimental data shows the capability of the model to characterize, with an inverse analysis, retention processes that occur in a river. In Chapter 3 a two-dimensional schematic wetland is studied with a numerical model that solves, with a shallow water approach, hydrodynamic and mass transport equations. A specific processing of the numerical results is used to determine numerical residence time distributions of the wetland as a function of a particular vegetation distribution that reproduces a central channel delimited by two lateral, more densely vegetated, banks. To each different density ratio it corresponds a specific shape of the residence time distribution, that present a clear bimodality below a critical value. To model this specific phenomenon, typical in natural environments, a simple and a more easy to use one-dimensional model approach is implemented in the former STIR model. The new version is called STIR-DTD. In Chapter 4 a new innovative optimization approach to wetland design is defined. Numerical solution of a two-dimensional shallow water model using the open-source suite TELEMAC2D, is integrated with an evolutionary optimization algorithm. At the initial stage of the evolution strategy, the removal efficiency of a random population of individuals (each individual represents a specific distribution of vegetated patches over the wetland domain) is evaluated numerically solving a shallow water hydrodynamic model coupled with a solute transport model. Once the removal efficiency is known, the evolutionary algorithm, using a wide range of selection operators that mimic natural evolution, evolve the initial population to an individual that maximizes the pollutant mass removal. Performed tests show how the optimized distribution tends to cover the maximum wetland available area or, if a maximum vegetated area is kept fixed, how the distribution tends to lengthen the flow paths between the inlet and the outlet section of the wetland. Chapter 5 shows results of a preliminary analysis on the removal efficiency of randomly distributed vegetation characterized by a Gaussian spatial probability density function. Vegetation density is treated as a random variable characterized by a mean, a variance and an homogeneous correlation length. The effect of each distribution on the removal efficiency is numerically evaluated by a coupled hydrodynamic and solute transport that accounts for the pollutant decay. Results show how removal efficiency is correlated with the statistical parameters of the space probability density function used to generate the random filed

AN INTEGRATED APPROACH TO PREVENT THE EROSION OF SALT MARSHES IN THE LAGOON OF VENICE

The loss of coastal habitats is a widespread problem in Europe. To protect the intertidal salt marshes of the lagoon of Venice from the erosion due to natural and human causes which is diffusely and intensely impacting them, the European Commission has funded the demonstrative project LIFE VIMINE. LIFE VIMINE aims to protect the most interior, hard-to-access salt marshes in the northern lagoon of Venice through an integrated approach, whose core is the prevention of erosion through numerous, small but spatially-diffuse soil-bioengineering protections works, mainly placed through semi-manual labour and with low impact on the environment and the landscape. The effectiveness of protection works in the long term is ensured through routine, temporally-continuous and spatially-diffuse actions of monitoring and maintenance. This method contrasts the common approach to managing hydraulic risk and erosion in Italy which is based on large, one-off and irreversible protection actions. The sustainability of the LIFE VIMINE approach is ensured by the participatory involvement of stakeholders and the recognition that protecting salt marshes means defending the benefits they provide to society through their ecological functions, as well as protecting the jobs linked to the existence or conservation of this habitat

Transport Processes and Optimization Strategies in Wetland Design

Transitional areas, between inland and coastal environments, represent an important habitat for their environmental and natural value. They act as a natural buffer for all those chemicals which are produced by diffused sources of pollutants (run-off rain water from agriculture) or from hidden sources (sewers not connected to a wastewater treatment plant). Pollutants produced by this type of sources can lead, if not conveniently treated, to eutrophication and to other water quality problems along coastal areas. Traditional wastewater treatment methods appear to be not effective in these conditions because of the big volumes of water and the relatively low concentration of dissolved pollutants to be treated. Since traditional wastewater treatment plants can not be used, it becomes important to better understand transport phenomena in transitional environments (rivers and wetlands) and all the removal processes in such zones in order to manage them to treat all the chemicals before they arrive to the coastal areas. Particular attention must be therefore stressed on retention processes and on the formulation of predictive models which allow scientists and engineers to better manage and design these buffer areas. In Chapter 1, the role of different transport processes is analyzed focusing the attention on different spatial and temporal scales. Principal modeling approaches are discussed underlining the role of each term on the mass balance equation and the most classical model closures are described in this chapter. In Chapter 2, retention characteristics of three different rivers are analyzed, relating different model closures with planimetric features of the rivers, their vegetational cover and bottom permeability. The analysis is carried on using STIR (Solute Transport In Rivers) model, a one-dimensional solute transport model that describes concentration breakthrough curves implementing a wide set of retention phenomena characterized by different time scales, represented by a specific residence time distribution in each retention domain. Comparison of modeling results and experimental data shows the capability of the model to characterize, with an inverse analysis, retention processes that occur in a river. In Chapter 3 a two-dimensional schematic wetland is studied with a numerical model that solves, with a shallow water approach, hydrodynamic and mass transport equations. A specific processing of the numerical results is used to determine numerical residence time distributions of the wetland as a function of a particular vegetation distribution that reproduces a central channel delimited by two lateral, more densely vegetated, banks. To each different density ratio it corresponds a specific shape of the residence time distribution, that present a clear bimodality below a critical value. To model this specific phenomenon, typical in natural environments, a simple and a more easy to use one-dimensional model approach is implemented in the former STIR model. The new version is called STIR-DTD. In Chapter 4 a new innovative optimization approach to wetland design is defined. Numerical solution of a two-dimensional shallow water model using the open-source suite TELEMAC2D, is integrated with an evolutionary optimization algorithm. At the initial stage of the evolution strategy, the removal efficiency of a random population of individuals (each individual represents a specific distribution of vegetated patches over the wetland domain) is evaluated numerically solving a shallow water hydrodynamic model coupled with a solute transport model. Once the removal efficiency is known, the evolutionary algorithm, using a wide range of selection operators that mimic natural evolution, evolve the initial population to an individual that maximizes the pollutant mass removal. Performed tests show how the optimized distribution tends to cover the maximum wetland available area or, if a maximum vegetated area is kept fixed, how the distribution tends to lengthen the flow paths between the inlet and the outlet section of the wetland. Chapter 5 shows results of a preliminary analysis on the removal efficiency of randomly distributed vegetation characterized by a Gaussian spatial probability density function. Vegetation density is treated as a random variable characterized by a mean, a variance and an homogeneous correlation length. The effect of each distribution on the removal efficiency is numerically evaluated by a coupled hydrodynamic and solute transport that accounts for the pollutant decay. Results show how removal efficiency is correlated with the statistical parameters of the space probability density function used to generate the random filed.Le zone di transizione tra entroterra e mare costituiscono una porzione di territorio molto importante dal punto di vista ambientale e naturalistico. Esse rappresentano un naturale filtro per tutte quelle specie chimiche che sono prodotte da fonti di inquinamento diffuse (dilavamento di suoli agricoli) o occulte (scarichi non collettati o irregolari) che possono creare, se non opportunamente trattate, problemi di eutrofizzazione e di qualità delle acque lungo le coste. I tradizionali metodi di depurazione si rivelano poco efficaci nel trattare questo tipo di effluenti, per le grandi portate da gestire e per le relativamente basse concentrazioni di inquinanti. Risulta importante quindi, nell'impossibilità di impiegare i tradizionali impianti di depurazione, comprendere le dinamiche di trasporto negli ambienti naturali (fiumi e aree umide) e i meccanismi di rimozione degli inquinanti in tali zone, in modo da poterle utilizzare per riassorbire, in modo sostenibile e naturale, il carico di inquinanti che altrimenti raggiungerebbe direttamente le coste. A questo scopo è necessario focalizzare l'attenzione sui processi di ritenzione e sulla formulazione di appropriati strumenti modellistici che consentano ai tecnici e ai modellisti una comprensione sufficientemente ampia dei fenomeni e forniscano loro degli strumenti pratici che aiutino nella gestione e riprogettazione di queste aree tampone. Nel Capitolo 1 viene analizzato il ruolo di differenti processi di trasporto focalizzando l'attenzione su diverse scale spaziali e temporali di analisi e descrivendo i principali approcci modellistici utilizzati per trattare ciascun fenomeno. E' evidenziato il contributo di ciascun termine al bilancio di massa e sono prese in considerazione le chiusure modellistiche più classiche oggi adottate. Nel Capitolo 2 si analizzano le caratteristiche dei processi di ritenzione in tre diversi corsi d'acqua mettendo in relazione le diverse chiusure modellistiche adottate in funzione delle caratteristiche planimetriche degli alvei, della loro composizione vegetazionale e delle caratteristiche di permeabilità del fondo. L'analisi \'e eseguita utilizzando il modello di trasporto monodimensionale STIR (Solute Transport In Rivers) che si presta a descrivere le curve di concentrazione implementando una vasta gamma di fenomeni di ritenzione a diverse scale temporali, descritte da specifiche distribuzioni dei tempi di residenza del soluto in ciascun comparto di ritenzione. L'accordo dei dati sperimentali con le curve di concentrazione mostra come si possa, tramite analisi inversa, caratterizzare un fiume dal punto di vista della ritenzione. Il Capitolo 3 prende in considerazione un'area umida bidimensionale di cui si risolvono, con un approccio modellistico alle acque basse, l'idrodinamica e il trasporto di massa. Una opportuna procedura di analisi dei risultati numerici è utilizzata per determinare le distribuzioni dei tempi di residenza dell'area umida in funzione di una particolare distribuzione di vegetazione che riproduce un canale principale delimitato da due zone laterali a maggiore densità di vegetazione. A diversi rapporti di densità corrisponde una specifica forma della distribuzione che presenta, al di sotto di uno specifico valore di soglia, una evidente bimodalità. Per rappresentare opportunamente tale fenomeno, comune negli ambienti naturali, con un approccio modellistico mono-dimensionale di più semplice utilizzo, è proposta in questo capitolo, una nuova versione del modello STIR denominata STIR-DTD. Il Capitolo 4 presenta un approccio innovativo di ottimizzazione alla progettazione di un'area umida. La risoluzione numerica di un modello bidimensionale alle acque basse tramite il modello TELEMAC2D è integrata infatti con un algoritmo evolutivo di ottimizzazione. Allo stadio iniziale dell'evoluzione, è definita, in modo casuale, una popolazione di individui (ciascun individuo rappresenta una specifica distribuzione di zone vegetate) di cui il modello valuta l'efficienza depurativa. A partire dal livello di efficienza depurativa dimostrata da ciascuna distribuzione, l'algoritmo evolutivo, tramite specifici operatori genetici che mimano i processi di selezione naturali, evolve la popolazione verso la distribuzione di vegetazione che massimizza l'abbattimento di inquinanti. I test effettuati mostrano come la distribuzione ottimale evolva verso configurazioni che tendono a coprire tutta l'area vegetata disponibile o, qualora questa sia fissata, a prolungare il più possibile i percorsi di flusso all'interno delle aree vegetate. Il Capitolo 5 riporta i risultati di una prima analisi eseguita su campi random di vegetazione, descritti da una opportuna funzione densità di probabilità spaziale (Gaussiana). La risoluzione tramite un modello bidimensionale accoppiato ad uno di trasporto e decadimento mostra come l'efficienza depurativa e la portata siano correlabili con i parametri (densità media, varianza e lunghezza di correlazione) che caratterizzano la particolare distribuzione statistica di vegetazione adottata

Evidence of distinct contaminant transport patterns in rivers using tracer tests and a multiple domain retention model

Solute transport in rivers is controlled by surface hydrodynamics and by mass exchanges with distinct retention zones. Surface and hyporheic retention processes can be accounted for separately in solute transport models with multiple storage compartments. In the simplest two component model, short term storage can be associated to in-channel transient retention, e.g. produced by riparian vegetation or surface dead zones, and the long-term storage can be associated to hyporheic exchange. The STIR (Solute Transport In Rivers) multiple domain transport model is applied here to tracer test data from three very different Mediterranean streams with distinctive characteristics in terms of flow discharge, vegetation and substrate material. The model is used with an exponential residence time distribution (RTD) to represent surface storage processes and two distinct modeling closures are tested to simulate hyporheic retention: a second exponential RTD and a power-law distribution approximating a known solution for bedform-induced hyporheic exchange. Each stream shows distinct retention patterns characterized by different timescales of the storage time distribution. Both modeling closures lead to very good approximations of the observed breakthrough curves in the two rivers with permeable bed exposed to the flow, where hyporheic flows are expected to occur. In the one case where the occurrence of hyporheic flows is inhibited by bottom vegetation, only the two exponential RTD model is acceptable and the time scales of the two components are of the same magnitude. The significant finding of this work is the recognition of a strong signature of the river properties on tracer data and the evidence of the ability of multiple-component models to describe individual stream responses. This evidence may open a new perspective in river contamination studies, where rivers could possibly be classified based on their ability to trap and release pollutants