Vegetation plays a vital role in riverine ecosystems, providing food and habitat for a diverse range of animal species, as well as stabilizing soil and purifying water. However, from a river hydraulics perspective, the presence of vegetation growing along the banks and floodplains creates additional drag forces as the water passes, contributing to an increase in the flow resistance. As a result, it is important to accurately estimate the hydraulic roughness caused by vegetation to properly conduct flood analysis and river management activities aimed at risk mitigation and protection of riverine ecosystems. This thesis examines the impact of floodplain vegetation, specifically the arboreal, on flow resistance through a combination of experimental and field studies, as well as numerical simulations. Considering the numerous vegetation roughness models that exist in the literature, the first part of this work focused on experimentally validating these models in a flume using wooden cylinders to represent the vegetation. Two diameters and two spatial distributions were tested under submerged and emergent flow conditions. The resulting analysis allowed identifying the models that best represent the experimental measurements for the given flow conditions. During fluid-structure interaction, the forces acting on the flow can cause vibrations in vegetation, which depend on the vegetation's mechanical properties and dimensions. These vibrations can result in additional energy losses, potentially modifying the roughness coefficient. Using an experimental model, this study analyzed the vibration of vegetation, represented by cylinders, and its effects on energy losses employing the theory of structural dynamics. Most experimental approaches typically investigate either rigid or flexible vegetation separately. However, since floodplains are characterized by a mixture of species such as grasses, crops, bushes, and trees, a series of experiments were conducted to examine the flow resistance due to mixed vegetation. The experimental set-up included wooden cylinders (rigid) and synthetic grasses (flexible). Roughness coefficients were determined individually for rigid, flexible, and mixed vegetation to investigate the importance of mixed vegetation in flow resistance analysis. To apply the laboratory findings to the river scale, this study evaluated roughness coefficients due to arboreal vegetation in the Piave river in Italy applying the vegetation roughness models that performed best in the laboratory study. As a result, a map with the spatial distribution of roughness coefficients was produced. Moreover, a continuous water depth measurement system installed in the field allowed determining the floodplain hydrographs for a flood event that occurred in December 2020. Finally, this thesis used a two-dimensional hydrodynamic model to perform numerical simulations and validate vegetation roughness models at the river scale. The simulations focused on the arboreal vegetation and studied the flooding event that occurred in the Piave river in October 2018. The results of the vegetation roughness models were compared with those of the conventional approach, which uses a constant Manning coefficient to define roughness. Furthermore, this study showed the effect of vegetation density on water levels and the extent of flooding. Overall, this research aims to improve the understanding of the effects of floodplain vegetation on flow resistance analysis, using a combination of experimental, field, and numerical studies at both laboratory and river scales.
La vegetazione svolge un ruolo vitale negli ecosistemi fluviali poiché fornisce alimentazione e habitat ad una varietà di specie animali, oltre a stabilizzare il suolo e purificare l'acqua. Tuttavia, dal punto di vista dell'idraulica fluviale, la presenza di vegetazione che cresce lungo le sponde e le golene crea ulteriori forze di resistenza al passaggio dell'acqua, contribuendo ad aumentare la resistenza al flusso. Di conseguenza, è importante stimare accuratamente la scabrezza idraulica causata dalla vegetazione per condurre correttamente l'analisi delle piene e le attività di gestione del fiume rivolte alla mitigazione del rischio e alla protezione degli ecosistemi fluviale. Questa tesi esamina l'impatto della vegetazione golenale, in particolare quella arborea, sulla resistenza al flusso attraverso una combinazione di studi sperimentali e sul campo, nonché simulazioni numeriche. Considerando i numerosi modelli di scabrezza della vegetazione esistenti in letteratura, la prima parte di questo lavoro si è concentrata sulla validazione sperimentali di questi modelli in un canale utilizzando bastoni di legno per rappresentare la vegetazione. Due diametri e due distribuzioni spaziali sono stati testati in condizioni di flusso sommerso ed emerso. L'analisi risultante ha permesso di identificare i modelli che meglio rappresentano le misure sperimentali per le condizioni di flusso esaminate. Le forze che agiscono sul flusso possono causare vibrazioni nella vegetazione, dovute all'interazione fluido-struttura e che dipendono dalle proprietà meccaniche e dalle dimensioni della vegetazione. Queste vibrazioni possono provocare ulteriori perdite di energia, modificando potenzialmente il coefficiente di scabrezza. Utilizzando un modello sperimentale, questo studio ha analizzato la vibrazione della vegetazione, rappresentata dai bastoni, ed i suoi effetti sulle perdite di energia impiegando la teoria della dinamica strutturale. La maggior parte degli approcci sperimentali in genere studia separatamente la vegetazione rigida o flessibile. Tuttavia, poiché le golene sono caratterizzate da una combinazione di specie come erba, colture, arbusti e alberi, è stata condotta una serie di esperimenti per esaminare la resistenza al flusso dovuta alla vegetazione mista. L'allestimento sperimentale comprendeva bastoni di legno (rigidi) e prato sintetico (flessibile). I coefficienti di scabrezza sono stati determinati individualmente per la vegetazione rigida, flessibile e mista al fine di indagare l'importanza di quest'ultima nell'analisi della resistenza al flusso. Per applicare i risultati del laboratorio alla scala fluviale, questo studio ha valutato i coefficienti di scabrezza dovuti alla vegetazione arborea nel fiume Piave in Italia applicando i modelli di scabrezza della vegetazione che hanno dato i migliori risultati nello studio di laboratorio. Di conseguenza, è stata prodotta una mappa con la distribuzione spaziale dei coefficienti di scabrezza. Inoltre, un sistema di misurazione continua della profondità dell'acqua installato in campo ha consentito di determinare gli idrogrammi delle golene per l'evento di piena di Dicembre 2020. Infine, questa tesi ha utilizzato un modello idrodinamico bidimensionale per eseguire simulazioni numeriche e validare i modelli di scabrezza della vegetazione a scala fluviale. Le simulazioni si sono concentrate sulla vegetazione arborea e hanno studiato l'evento di piena di Ottobre 2018 del fiume Piave. I risultati dei modelli di scabrezza della vegetazione sono stati confrontati con quelli dell'approccio convenzionale, che utilizza un coefficiente di Manning costante per definire la scabrezza. Inoltre, questo studio ha mostrato l'effetto della densità della vegetazione sui livelli dell'acqua e l'estensione delle aree allagate. Nel complesso, questa ricerca intende a migliorare la comprensione degli effetti della vegetazione golenale sull'analisi della resistenza al flusso, utilizzando una combinazione di studi sperimentali, sul campo e numerici sia su scala di laboratorio che su scala fluviale.
Experimental and numerical modeling of hydraulic roughness induced by floodplain vegetation
HERRERA GÓMEZ, LISDEY VERÓNICA
2022/2023
Abstract
Vegetation plays a vital role in riverine ecosystems, providing food and habitat for a diverse range of animal species, as well as stabilizing soil and purifying water. However, from a river hydraulics perspective, the presence of vegetation growing along the banks and floodplains creates additional drag forces as the water passes, contributing to an increase in the flow resistance. As a result, it is important to accurately estimate the hydraulic roughness caused by vegetation to properly conduct flood analysis and river management activities aimed at risk mitigation and protection of riverine ecosystems. This thesis examines the impact of floodplain vegetation, specifically the arboreal, on flow resistance through a combination of experimental and field studies, as well as numerical simulations. Considering the numerous vegetation roughness models that exist in the literature, the first part of this work focused on experimentally validating these models in a flume using wooden cylinders to represent the vegetation. Two diameters and two spatial distributions were tested under submerged and emergent flow conditions. The resulting analysis allowed identifying the models that best represent the experimental measurements for the given flow conditions. During fluid-structure interaction, the forces acting on the flow can cause vibrations in vegetation, which depend on the vegetation's mechanical properties and dimensions. These vibrations can result in additional energy losses, potentially modifying the roughness coefficient. Using an experimental model, this study analyzed the vibration of vegetation, represented by cylinders, and its effects on energy losses employing the theory of structural dynamics. Most experimental approaches typically investigate either rigid or flexible vegetation separately. However, since floodplains are characterized by a mixture of species such as grasses, crops, bushes, and trees, a series of experiments were conducted to examine the flow resistance due to mixed vegetation. The experimental set-up included wooden cylinders (rigid) and synthetic grasses (flexible). Roughness coefficients were determined individually for rigid, flexible, and mixed vegetation to investigate the importance of mixed vegetation in flow resistance analysis. To apply the laboratory findings to the river scale, this study evaluated roughness coefficients due to arboreal vegetation in the Piave river in Italy applying the vegetation roughness models that performed best in the laboratory study. As a result, a map with the spatial distribution of roughness coefficients was produced. Moreover, a continuous water depth measurement system installed in the field allowed determining the floodplain hydrographs for a flood event that occurred in December 2020. Finally, this thesis used a two-dimensional hydrodynamic model to perform numerical simulations and validate vegetation roughness models at the river scale. The simulations focused on the arboreal vegetation and studied the flooding event that occurred in the Piave river in October 2018. The results of the vegetation roughness models were compared with those of the conventional approach, which uses a constant Manning coefficient to define roughness. Furthermore, this study showed the effect of vegetation density on water levels and the extent of flooding. Overall, this research aims to improve the understanding of the effects of floodplain vegetation on flow resistance analysis, using a combination of experimental, field, and numerical studies at both laboratory and river scales.| File | Dimensione | Formato | |
|---|---|---|---|
|
Thesis_VHG_Finale.pdf
non accessibile
Descrizione: PhD Thesis VHG
Dimensione
292.61 MB
Formato
Adobe PDF
|
292.61 MB | Adobe PDF | Visualizza/Apri |
I documenti in POLITesi sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
https://hdl.handle.net/10589/198416