Measurements and simulations of energy fluxes over a high-rise and compact urban area in Hong Kong

Measuring energy fluxes in a dense and high-rise urban area is extremely challenging, thus our knowledge in such area remains limited. This study assessed the surface energy fluxes and investigated the energy balance closure (EBC) over such complex urban surface in Hong Kong. Net radiation (Q_N), sensible (Q_H) and latent (Q_E) heat fluxes were measured using an eddy covariance system from September 2018 to August 2019. Anthropogenic heat flux (Q_F) was simulated by a large-scale urban energy model (i.e., LUCY) and validated by an anthropogenic heat database (i.e., AHE_KL). Storage heat flux (Q_S) was estimated by an objective hysteresis model (OHM). Among five energy terms, Q_F showed the largest values of around 750 Wm^-2 especially in the afternoon. Whereas, Q_E varying within 150 Wm^-2 showed the smallest values. The variation range of net radiation, sensible heat flux and storage heat was respectively from -50 to 600, 0 to 450 and -30 to 300 W m^-2. EBC generally showed a negative relationship with surface heterogeneity. Best EBC was observed in the direction with a relatively constant and large aspect ratio, and with the most occurrence of unstable stratifications. The uncertainties of Q_H and Q_E fluxes were respectively estimated to be approximately 8% and 7%. The result of LUCY was consistent with that of AHE_KL. EBC was not sensitive to different coefficients estimating Q_S. This study helped to fill a gap in our understanding of surface energy and turbulent fluxes in compact cities with high-rise buildings and shed insights into the future installation of eddy covariance tower in similar areas. The required height of the eddy covariance tower in such urban sites might not be as restrictive as that in other urban areas with low-rise buildings or with low building density, thus making it more feasible to set up such observation towers.

Linking hydrological and bioecological benefits of green infrastructures across spatial scales - A literature review

Green infrastructure (GI) mitigates the negative effects of urbanization and provides hydrological and bioecological benefits. However, these benefits are highly scale-dependent because the processes involved vary at different spatial scales; there are thus additional challenges in GI planning when multiple benefits are targeted. Therefore, it is necessary to review and summarize the theoretical understandings and practical experience obtained from previous studies and projects related to the hydrological and bioecological benefits of GI practices. In this review, we elaborate the conceptual linkages between the hydrological and bioecological benefits of GI practices across different scales. Smaller-scale benefits lay the foundation for larger-scale benefits. Hydrological benefits drive bioecological benefits by providing consistent water flows and maintaining a suitable soil environment. Bioecological benefits in turn enhance hydrological benefits by increasing water uptake and filtration via more active biological processes. We next summarize the study area sizes of existing studies and categorize them according to their study approaches and targeted benefits. The study area sizes in studies that make use of laboratory experiments, numerical modeling, and remote sensing have increased in recent years and vary greatly between each type of study; the study area size in studies of bioecological benefits was larger than in studies of hydrological and water quality benefits. However, there is a research gap in studies of bioecological benefits at the catchment scale. Furthermore, we summarize the major research topics and findings of bioecological benefits of GI practices at different spatial scales. We conclude this review with recommendations for future research, which include performing more studies at the catchment scale, developing hydro-bioecological statistical relationships to simplify the quantification of bioecological benefits, and developing databases to document the bioecological benefits of GI practices.

Empirical Equations to Predict the Characteristics of Hyporheic Exchange in a Pool-Riffle Sequence

The hyporheic zone is the saturated interstitial space surrounding a stream. Water actively moves into, through, and out of the hyporheic zone, resulting in hyporheic exchange (HE), which is crucial to the physicochemical and biological processes in these systems. The HE in pool-riffle sequences is one of the most common forms of HE and has received a vast amount of attention. This study aimed to derive empirical equations to predict the scale, median residence time (RT), and flux of HE in a single pool-riffle sequence by considering stream discharge, bedform geometry, streambed hydraulic conductivity, and groundwater flow. The resulting equations, which were derived using previously published data, allow quick approximations of the mean depth, median RT, and flux of HE in a single pool-riffle sequence. Therefore, these equations can be used to conveniently and efficiently generate insights into the physicochemical and biological processes, facilitating the prediction of water quality and the restoration of HE in stream rehabilitation. The overall framework of the derived equations is also generally applicable to considering HE with additional influential factors (e.g., stream width, sinuosity, bed slope, alluvial depth) and cases with more available data.

Integrated hydro-environmental impact assessment and alternative selection of low impact development practices in small urban catchments

Attention is increasingly being paid to low impact development (LID) practices in urban stormwater management. Because LID practices offer a wide variety of hydro-environmental benefits, it is often necessary to account for these benefits collectively in cost-benefit analysis and LID alternative selection. The conventional methods of quantifying these benefits, however, can hardly incorporate the preferences of decision makers, and commonly involve tedious parameter estimations. To address these shortcomings, this study adopts a relative performance evaluation method to assess the various hydro-environmental impacts of LID alternatives in small urban catchments. This study considers several categories of hydro-environmental impacts, including water balance impact, surface pollutant load abatement, and combined sewer overflow and flood risk mitigation. Several performance indicators are used for each impact category. The system-wide effectiveness of an LID alternative is then derived by the weighted aggregation of its indicator scores, which are obtained by comparing its performance with that of all of the other alternatives. The hydro-environmental impact of green roofs and bioretention cells of varying areas in New York City, U.S. are investigated in detail. The results suggest that a green roof that covers the whole catchment is as effective as a bioretention cell that covers 3%-5% of the catchment in terms of stormwater management, and that the effectiveness of a bioretention cell doubles when its surface area increases from 2% to 10% of the catchment area. These assessment results are influenced by catchment-specific assessment criteria (e.g., the high flow threshold) and management interests, which suggests that design guidelines for different catchments should be tailored to their natural and drainage characteristics. The framework used in this study allows stakeholders' interests to be reflected in LID alternative selections and the implications of different design guidelines to be thoroughly investigated.

Impacts of Streambed Heterogeneity and Anisotropy on Residence Time of Hyporheic Zone

The hyporheic zone (HZ), which is the region beneath or alongside a streambed, plays an important role in the stream's ecology. The duration that a water molecule or a solute remains within the HZ, or residence time (RT), is one of the most common metrics used to evaluate the function of the HZ. The RT is greatly influenced by the streambed's hydraulic conductivity (K), which is intrinsically difficult to characterize due to its heterogeneity and anisotropy. Many laboratory and numerical studies of the HZ have simplified the streambed K to a constant, thus producing RT values that may differ from those gathered from the field. Some studies have considered the heterogeneity of the HZ, but very few have accounted for anisotropy or the natural K distributions typically found in real streambeds. This study developed numerical models in MODFLOW to examine the influence of heterogeneity and anisotropy, and that of the natural K distribution in a streambed, on the RT of the HZ. Heterogeneity and anisotropy were both found to shorten the mean and median RTs while increasing the range of the RTs. Moreover, heterogeneous K fields arranged in a more orderly pattern had longer RTs than those with random K distributions. These results could facilitate the design of streambed K values and distributions to achieve the desired RT during river restoration. They could also assist the translation of results from the more commonly considered homogeneous and/or isotropic conditions into heterogeneous and anisotropic field situations.

A comprehensive review of spatial allocation of LID-BMP-GI practices: Strategies and optimization tools

Low-impact development (LID), best management practice (BMP), and green infrastructure (GI) are semi-engineered stormwater management practices that have been widely studied and implemented worldwide. Implemented in the complex environment of urban areas, LID-BMP-GI practices often intertwine with a very large number of hydro-environmental and socio-economic considerations and constraints. Therefore, they need to be carefully selected, designed, and allocated within an urban area. Both planning and optimization can lead to more systematic and strategic approaches to address this multi-scale, multi-parameter problem of practice allocation. In this review, we first identify the main components of the strategic planning cycle, their scope and inter-relationships, and their corresponding mathematical representations. We then present a comprehensive review of the existing literature on spatial allocation optimization tools (SAOTs) for LID-BMP-GI practices and summarize the generic structure and the systematic typology of the existing SAOTs. We conclude with a discussion of several current research gaps in the spatial allocation of LID-BMP-GI practices. In this review, we aim to summarize the strategies and optimization tools for the spatial allocation of LID-BMP-GI practices that are beneficial to practitioners. The other aim is to provide recommendations for future research on the development of more advanced and comprehensive SAOTs.

Modeling the temporal dynamics of intertidal benthic infauna biomass with environmental factors: Impact assessment of land reclamation

Anthropogenic activities such as land reclamation are threatening tidal marshes worldwide. This study's hypothesis is that land reclamation in a semi-enclosed bay alters the seasonal dynamics of intertidal benthic infauna, which is a key component in the tidal marsh ecosystem. Mai Po Tidal Marsh, Deep Bay, Pearl River Estuary, China was used as a case study to evaluate the hypothesis. Ecological models that simulate benthic biomass dynamics with governing environmental factors were developed, and various scenario experiments were conducted to evaluate the impact of reclamations. Environmental variables, selected from the areas of hydrodynamics, meteorology, and water quality based on correlation analysis, were used to generate Bayesian regression models for biomass prediction. The best-performing model, which considered average water age (i.e., a hydrodynamic indicator of estuarine circulation) in the previous month, salinity variation (i.e., standard deviation of salinity), and the total sunny period in the current month, captured well both seasonal and yearly trends in the benthic infauna observations from 2002 to 2008. This model was then used to simulate biomass dynamics with varying inputs of water age and salinity variation from coastal numerical models of different reclamation scenarios. The simulation results suggest that the reclamation in 2007 decreased the spatial and annual average benthic infauna biomass in the tidal marsh by 20%, which agreed with the 28% biomass decrease recorded by field survey. The range of biomass seasonal variation also decreased significantly from 2.1 to 230.5g/m² (without any reclamation) to 1.2 to 131.1g/m² (after the 2007 reclamation), which further demonstrates the substantial ecological impact of reclamation. The ecological model developed in this study could simulate seasonal biomass dynamics and evaluate the ecological impact of reclamation projects. It can therefore be applied to evaluate the ecological impact of coastal engineering projects for tidal marsh management, conservation, and restoration.

Optimizing surface and contributing areas of bioretention cells for stormwater runoff quality and quantity management

Bioretention cells (BCs) have received increasing attention in stormwater quality and quantity management. Selecting a suitable implementation level of BCs to concurrently achieve multiple performance targets (e.g., first flush reduction, peak flow reduction, and runoff volume reduction) is essential and often challenging. This study proposes a method for formulating suitable sizing criteria for multi-objective stormwater management. The performance of BCs of different areas is assessed first using the Storm Water Management Model (SWMM) and then look-up curves (i.e., the performance target versus the required area of BCs) for each of the performance targets and the multi-objective cases are derived. In some cases, the available area of BCs is limited; to account for the multi-objective management interests and maximize the system-wide benefits, an optimal contributing drainage area for BCs should be selected. A method is therefore developed to solve this optimization problem. A case study of Hong Kong shows that the required area of BCs increases non-linearly with increased performance targets. With a limited area of BCs, larger contributing areas are favorable if no special emphasis is placed on the intensive control of peak flow reduction. Design standards (e.g., the intensity of the design storm), evaluation methods (e.g., depth threshold of the initial portion of runoff), and management preference all exert some influence on the resultant sizing criteria and optimization results. Carefully selecting these catchment-specific evaluation methods should lead to more appropriate sizing criteria and thus promote more efficient BC adoption.

Temporal variations in water quality in a brackish tidal pond: Implications for governing processes and management strategies

Brackish tidal ponds have been constructed along coastal areas in many parts of the world for aquaculture, including some Ramsar Sites. Such ponds are considered a sustainable, wise use of wetlands if managed properly, but they can also pose serious environmental problems if mismanaged. To understand the governing processes and to promote sustainable management strategies, this study examines the different temporal variations in water quality parameters in a brackish tidal pond located within the wetland complex of the Mai Po Ramsar Site in Hong Kong, China. The variations are compared with those of the receiving bay, and the water channel that connects the pond and the bay. Equations are then developed to link the dissolved oxygen (DO) concentrations in the pond with the governing processes, and to analyze their relative contributions to DO levels. Field data show seasonal patterns in water temperature and salinity in response to the seasonal variations in solar radiation and rainfall. For the pond and the channel, DO, chlorophyll and pH exhibit fortnightly variations due to the bi-weekly water exchange between the pond and the bay. There were also diurnal variations in water temperature and DO in response to changes in solar radiation for both locations, and the tidal flushing for the water channel. Analysis of the findings indicates that water exchange influences the DO concentration more strongly than solar radiation. The DO equation links pond water quality with the time of day, and the time in a water exchange cycle, and thus provides some guidance for determining water exchange and water sampling schedules. The study sheds light on the governing processes and management strategies related to the sustainable management of a brackish tidal pond. The results are thus beneficial in elucidating and promoting the sustainable management and wise use of wetlands in other locations.

Modeling fresh water lens damage and recovery on atolls after storm-wave washover

The principal natural source of fresh water on scattered coral atolls throughout the tropical Pacific Ocean is thin unconfined groundwater lenses within islet substrates. Although there are many threats to the viability of atoll fresh water lenses, salinization caused by large storm waves washing over individual atoll islets is poorly understood. In this study, a mathematical modeling approach is used to examine the immediate responses, longer-term behavior, and subsequent (partial) recovery of a Pacific atoll fresh water lens after saline damage caused by cyclone-generated wave washover under different scenarios. Important findings include: (1) the saline plume formed by a washover event mostly migrates downward first through the top coral sand and gravel substrate, but then exits the aquifer to the ocean laterally through the more permeable basement limestone; (2) a lower water table position before the washover event, rather than a longer duration of storm washover, causes more severe damage to the fresh water lens; (3) relatively fresher water can possibly be found as a preserved horizon in the deeper part of an aquifer after disturbance, especially if the fresh water lens extends into the limestone under normal conditions; (4) post-cyclone accumulation of sea water in the central depression (swamp) of an atoll islet prolongs the later stage of fresh water lens recovery.

Modeling sewage leakage to surrounding groundwater and stormwater drains

Underground sewage pipe systems deteriorate over time resulting in cracks and joint defects. Sewage thus leaks out and contaminates the surrounding groundwater and the surface water in stormwater drains. Many studies have investigated the problem of sewage leakage but no published studies, to the best knowledge of the authors, have examined the hydrologic interactions between leaky sewage pipes, groundwater and stormwater drains. This study numerically models such interactions using generic conditions in Singapore. It first develops accurate representations of weep holes and leaky sewage pipes, and further shows the long-term and short-term system responses to rainfall events. Some of the implications include: (1) quality of water seeping into the drains tends to be low in dry years; (2) complete contaminant attenuation after pipe rehabilitation takes several years; (3) responses to rainfall events at weep holes are immediate but the effects on sewage leakage might only show up a few days later. The simulation results allow us to better understand the local-scale migration of sewage leakage from a sewage pipe to nearby stormwater drains. With calibrations and verifications with local field data, the modeling framework would be applicable and beneficial to the sewage leakage monitoring and sewage pipe rehabilitation worldwide.

Simulating a lake as a high-conductivity variably saturated porous medium

One approach for simulating ground water-lake interactions is to incorporate the lake into the ground water solution domain as a high-conductivity region. Previous studies have developed this approach using fully saturated models. This study extends this approach to variably saturated models, so that ground water-lake interactions may be more easily simulated with commonly used or public domain variably saturated codes that do not explicitly support coupled lake-water balance modeling. General guidelines are developed for the choices of saturated hydraulic conductivity and moisture retention and relative permeability curves for the lake region. When applied to an example ground water-lake system, model results are very similar to those from a model in which the lake is represented as a specified head boundary continuously updated by a lake mass balance. The high-conductivity region approach is most suitable for relatively simple geometries and lakes with slower and smaller fluctuations when the overall flow pattern and system fluxes, rather than the detailed flow pattern around the intersection of the lake and land surfaces, are of interest.