Journal of Sustainable Water in the Built Environment

The present study proposes a new method to assess the minimum environmental flow regime of rivers in which hydrological, physical habitat, and water quality indexes are integrated in a decision tree. Conventional methods of environmental flow are not able to integrate environmental interactions, which is the main motivation of this research work. The base environmental flow was defined by a hydrological or desktop approach. However, adding ecological requirements of physical habitats and water quality should be considered for assessing needs of the river ecosystem as well. The physical habitat loss function was defined based on fuzzy physical habitat simulation. Moreover, a combined water quality index was applied to define the water quality loss model. All environmental requirements were integrated in a decision-tree model to protect and sustain the ecological status of the river ecosystem due to water abstraction. Based on case study results, the decision-tree model improved the ecological status of the river ecosystem, which means using the hydrological method, physical habitat method, or water quality index is not independently recommendable to assess minimum environmental flow regime. The decision-tree model proposed 40% of annual available flow as the minimum environmental flow. In contrast, the hydrological method, physical habitat function, and water quality index independently proposed 32%, 30%, and 38% of annual available flow, respectively. The results highlight that using a combined method such as the decision-tree method in which interactions of methods could be defined is necessary to assess minimum environmental flow regime in rivers reliably.

Low impact development (LID) practices are commonly used stormwater control measures (SCMs) designed to mimic the predevelopment hydrologic regime by restoring the natural water cycle and enhancing water quality. A full-scale LID testbed was designed and implemented to evaluate the performance of bioretention and sand filter basins under the same conditions. The LID testbed is composed of six parallel cells including (1) a regional standard bioretention soil mixture, (2) an innovative custom blend bioretention soil mixture with limestone sand, and (3) sand filter basins with limestone sand as filtration media. Each filtration media design was tested with and without an impermeable liner, to investigate the potential impacts of the direct infiltration on their water quality performance. The treatment performance was evaluated through series of water quality analyses comparing the event mean concentrations (EMCs) of the inlet and outlets. The results suggested enhanced pollutant removal efficiencies by use of limestone sand in the bioretention mixture as well as a standalone media within the sand filter basins, particularly for phosphorus. We observed 82%–93% and 97%–94% removal of dissolved and total phosphorus, respectively, and the regional bioretention soil mixture presented leaching of nutrients. No significant difference was observed between the water quality performance of sand filter and bioretention basins, or between lined and unlined systems.

Abstract

Water-quality flow (WQF) is used as the basis for designing some stormwater control measures. The conventional method for determining the WQF incorporates several assumptions that do not reflect reality. A method is presented for determining the WQF that does not require the main limiting assumptions. Specifically, the proposed method utilizes measured durations of the water-quality rainfall, local rainfall statistics, and representative infiltration characteristics of the catchment. The proposed method is demonstrated at two locations in south Florida. Defining the 90-percentile rainfall depth as the water-quality rainfall (WQR), the results show the WQR to be in the range of 3.4–3.9 cm, with corresponding average event durations in the range of 9.4–9.7 h. Most importantly, the results show that the actual WQF can be substantially greater than the WQF calculated using the conventional method, with correction factors in the range of 1.9–9.6 for a range of practical conditions.

Practical Applications

Some stormwater control measures (SCMs) such as biofiltration swales and vegetated filter strips are designed based on WQFs. The conventional method of determining the WQF includes certain assumptions that result in an underestimation of the WQF, and hence an underdesign of SCMs. The proposed method for determining the WQF does not require the limiting assumptions while maintaining the underlying physics of the runoff process. Utilization of the proposed method for calculating design WQFs should lead to more effective water-quality control by SCMs.

Design, construction, and maintenance of stormwater facilities often require field measurements of hydraulic conductivity. In recent years, the modified Philip–Dunne (MPD) infiltrometer test has been promoted as an efficient option, particularly in rain gardens and other green stormwater infrastructure. However, the literature misses some details on its performance in practice. Based on our experiences building our own equipment, executing the test in various field conditions, and determining hydraulic conductivity for the purposes of engineering design, we describe some limitations for other users to consider and improve upon. The equipment, though simple in concept, required custom metalwork, was vulnerable to damage during soil insertion, and was not always watertight. The infiltrometer was difficult to insert into hard soils, and soil piping, scour, and other complications arose during field tests. The iterative solution for hydraulic conductivity was not always straightforward; some solutions failed to converge or were unreasonable, especially in less permeable soils. Even with successful tests, values varied and necessitated careful interpretation, spatial averaging, and application of appropriate safety factors for design. Despite its limitations, the test remains useful as a minimally invasive, repeatable, and, in permeable soils, rapid method for testing infiltration rates in the field. We recommend that the ASTM standard be updated, that safety factors be developed further, and that MPD users share their data to help advance the practice.

Required compaction of roadside soil, the largest built environment in space-constrained urban areas, lowers its infiltration capacity and makes it impractical for stormwater treatment. However, roadside soil can be turned into sustainable stormwater solutions by adding a bulking agent, such as sand, that could infiltrate stormwater and recharge groundwater. Yet, the quantity of sand needed for different hydrologic soil groups is unknown. Measuring the hydraulic conductivity of compacted soils mixed with different amounts (50%–90% by volume) of sand, we estimated that 70%–85% sand was needed to achieve a threshold infiltration capacity of $127–254\mathrm{mm}{\mathrm{h}}^{-1}$ irrespective of soil type. We developed an empirical equation that could predict the hydraulic conductivity of compacted soil and sand mixture based on their effective grain size ($D_{10}$). The results inform the urban planner to transform the road infrastructure from a source of water pollution to a network of stormwater treatment solutions.