A two-dimensional hydro-morphological model for river hydraulics and morphology with vegetation

This work develops a two-dimensional hydro-morphological model which can be used to simulate river hydraulics and morphology with various vegetation covers. The model system consists of five modules, including a hydrodynamic model, a sediment transport model, a vegetation model, a bank failure model and a bed deformation model. The secondary flow effects are incorporated through additional dispersion terms. The core components of the model system solve the full shallow water equations; this is coupled with a non-equilibrium sediment transport model. The new integrated model system is validated against a number of laboratory-scale test cases and then applied to a natural river. The satisfactory simulation results confirm the model's capability in reproducing both stream hydraulics and channel morphological changes with vegetation. Several hypothetical simulations indicate that the model can be used not only to predict flooding and morphological evolution with vegetation, but also to assess river restoration involving vegetation

Deciphering controls for the impact of geophysical flows on a flexible barrier: Insights from coupled CFD-DEM modeling

Geophysical flows impacting a flexible barrier can create complex flows and solid-fluid-structure interactions, which are challenging to quantify and characterize towards a unified description. Here, we examine the common physical laws of multiphase, multiway interactions during debris flows, debris avalanches and rock avalanches against a flexible barrier system using a coupled computational fluid dynamics and discrete element (CFD-DEM) method. This model captures essential physics observed in experiments and fields. The bi-linear, positive correlations are found between peak impact load and Fr or maximum barrier deflection, with inflection points due to the transitions from trapezoid- to triangle-shaped dead zones. Our findings quantitatively elucidate how flow materials (wet versus dry) and impact dynamics (slow versus fast) control the patterns of the identified bi-linear correlations. This work offers a physics-based reference and insights for improving widely-used impact solutions for geophysical flows against flexible barriers

NutSpaFHy : A Distributed Nutrient Balance Model to Predict Nutrient Export from Managed Boreal Headwater Catchments

Responsible forest management requires accounting for adverse environmental effects, such as increased nutrient export to water courses. We constructed a spatially-distributed nutrient balance model NutSpaFHy that extends the hydrological model SpaFHy by introducing a grid-based nutrient balance sub-model and a conceptual solute transport routine to approximate total nitrogen (N) and phosphorus (P) export to streams. NutSpaFHy uses openly-available Multi-Source National Forest Inventory data, soil maps, topographic databases, location of water bodies, and meteorological variables as input, and computes nutrient processes in monthly time-steps. NutSpaFHy contains two calibrated parameters both for N and P, which were optimized against measured N and P concentrations in runoff from twelve forested catchments distributed across Finland. NutSpaFHy was independently tested against six catchments. The model produced realistic nutrient exports. For one catchment, we simulated 25 scenarios, where clear-cuts were located differently with respect to distance to water body, location on mineral or peat soil, and on sites with different fertility. Results indicate that NutSpaFHy can be used to identify current and future nutrient export hot spots, allowing comparison of logging scenarios with variable harvesting area, location and harvest techniques, and to identify acceptable scenarios that preserve the wood supply whilst maintaining acceptable level of nutrient export

Piloting a real-time surface water flood nowcasting system for enhancing operational resilience of emergency responders [Abstract]

Piloting a real-time surface water flood nowcasting system for enhancing operational resilience of emergency responders [Abstract

Assessment of LID practices for restoring pre-development runoff regime in an urbanized catchment in southern Finland

This study quantifies the effects of common stormwater management techniques on urban runoff generation. Simulated flow rates for different low impact development (LID) scenarios were compared with observed flow rates during different urban construction phases in a catchment (12.3 ha) that was developed from natural forest to residential area over a monitoring period of five years. The Stormwater Management Model (SWMM) was calibrated and validated against the observed flow rates in the fully developed catchment conditions, and it was then applied to parameterize the LID measures and produce scenarios of their hydrological impacts. The results from the LID scenarios were compared with the observed flow rates in the pre-development and the partially developed catchment conditions. The results show that LID controls reduce urban runoff towards the flow conditions in the partially developed catchment, but the reduction effect diminishes during large rainfall events. The hydrographs with LID are still clearly different from the observed pre-development levels. Although the full restoration of pre-development flow conditions was not feasible, a combination of several measures controlling both volumes and retention times of storm runoff appeared to be effective for managing the stormwater runoff and mitigating the negative impacts of urban development

Urbanization Further Intensifies Short-Duration Rainfall Extremes in a Warmer Climate

Abstract Intensification of short-duration rainfall extremes contributes to increased urban flood risk. Yet, it remains unclear how upper-tail rainfall statistics could change with regional warming. Here, we characterize the non-stationarity of rainfall extremes over durations of 1?24 hr for the rapidly developing coastal megalopolis of the Greater Bay Area, China. Using high-resolution, multi-source, merged and gridded data we observe greater increases in rainfall intensities over the north-central part of the region compared with the southern coastal region. Our results show, for the first time, that urbanization nonlinearly increases rainfall intensities at different durations and return periods. Over short durations (≤3-hr) and short return periods (2-yr), urban areas have the greatest scaling rates (≥19.9%/°C). However, over longer durations (≥9-hr) rural areas have greater scaling rates, with a lower degree of dependency on both durations and return periods

Numerical modelling of hydro-morphological processes dominated by fine suspended sediment in a stormwater pond

Fine sediment plays crucial and multiple roles in the hydrological, ecological and geomorphological functioning of river systems. This study employs a two-dimensional (2D) numerical model to track the hydro-morphological processes dominated by fine suspended sediment, including the prediction of sediment concentration in flow bodies, and erosion and deposition caused by sediment transport. The model is governed by 2D full shallow water equations with which an advection-diffusion equation for fine sediment is coupled. Bed erosion and sedimentation are updated by a bed deformation model based on local sediment entrainment and settling flux in flow bodies. The model is initially validated with the three laboratory-scale experimental events where suspended load plays a dominant role. Satisfactory simulation results confirm the model’s capability in capturing hydro-morphodynamic processes dominated by fine suspended sediment at laboratory-scale. Applications to sedimentation in a stormwater pond are conducted to develop the process-based understanding of fine sediment dynamics over a variety of flow conditions. Urban flows with 5-year, 30-year and 100-year return period and the extreme flood event in 2012 are simulated. The modelled results deliver a step change in understanding fine sediment dynamics in stormwater ponds. The model is capable of quantitatively simulating and qualitatively assessing the performance of a stormwater pond in managing urban water quantity and quality

Quantifying the combined effects of multiple extreme floods on river channel geometry and on flood hazards

Effects of flood-induced bed elevation and channel geometry changes on flood hazards are largely unexplored, especially in the case of multiple floods from the same site. This study quantified the evolution of river channel and floodplain geometry during a repeated series of hypothetical extreme floods using a 2D full hydro-morphodynamic model (LHMM). These experiments were designed to examine the consequences of channel geometry changes on channel conveyance capacity and subsequent flood dynamics. Our results revealed that extreme floods play an important role in adjusting a river channel to become more efficient for subsequent propagation of floods, and that in-channel scour and sediment re-distribution can greatly improve the conveyance capacity of a channel for subsequent floods. In our hypothetical sequence of floods the response of bed elevation was of net degradation, and sediment transport successively weakened even with floods of the same magnitude. Changes in river channel geometry led to significant impact on flood hydraulics and thereby flood hazards. We found that flood-induced in-channel erosion can disconnect the channel from its floodplain resulting in a reduction of floodwater storage. Thus, the frequency and extent of subsequent overbank flows and floodplain inundation decreased, which reduced downstream flood attenuation and increased downstream flood hazard. In combination and in summary, these results suggest that changes in channel capacity due to extreme floods may drive changes in flood hazard. The assumption of unchanging of river morphology during inundation modelling should therefore be open to question for flood risk management

Geomorphic impacts of rapid sediment-laden flows through computational modelling

Outburst floods are one of the most catastrophic natural hazards for populations and infrastructure. They are usually generated from storm runoff, rapid melting of glacial ice or man-made and natural dam breaks, such as river dikes, volcanic debris dams and landslide damns etc. Such high-magnitude sudden onset floods generally comprise of an advancing intense kinematic water wave that can induce considerable sediment transport. Therefore, the exploration and investigation of sediment-laden outburst floods cannot be limited solely to water flow but must also include the flood-induced sediment transport. Understanding the complex flow-bed interaction process in large (field) scale outburst floods is still limited, not least due to a lack of well-constrained field data, but also because consensus on appropriate modelling schemes has yet to be decided. In recent years, attention has focussed on the numerical models capable of describing the process of erosion, transport and deposition in such flows and they are now at a point at which they provide useful quantitative data. Although the "exact" measure of bed change is still unattainable the numerical models enhance and improve insights into large outburst flood events. In order to model and better understand heavily sediment-laden flows and resulting geomorphic impacts, this thesis adopts a layer-based conceptual model which separates the system into an active bed layer, a water sediment mixed sheet flow layer and a suspension layer. Correspondingly, a layer-based hydro-morphodynamic model is proposed fully considering both bedload and suspended load based on shallow water theory. The model system is primarily composed of a combination of three modules: (1) a hydrodynamic module; (2) a sediment transport module; and (3) a morphological evolution module. In the thesis, firstly, a robust hydrodynamic model is proposed and tested including addressing the source terms and wetting/drying issues for application to irregular beds. Then based on the robust hydrodynamic model, a layer-based morphodynamic model is developed and solved numerically by an advanced second-order Godunov-type finite volume method. A series of theoretical and experimental tests are applied to validate the model in terms of both hydrodynamic and morphodynamic aspects. The results of these tests show that the developed models can predict the hydrodynamic and morphodynamic process effectively with good agreement with theoretical and experimental results. To demonstrate a real application, a full-scale volcano-induced jökulhlaup or glacial outburst flood (GLOF) at Sólheimajökull, Iceland is reproduced by the proposed model. The simulation of the sediment-laden outburst flood is shown to perform well, with further insights into the flow-bed interaction obtained from the modelling output. These results are beneficial to flood risk management and hazard prevention and mitigation