Optimal Design of LIDs in Urban Stormwater Systems Using a Harmony-Search Decision Support System

During the last years, climate changes and urbanization are causing huge urban pluvial flood events in many countries in the world, driving to both develop and apply effective and innovative approaches for the design and management of urban stormwater systems. The gradual urbanization is provoking the increase of impervious surfaces and, consequently, of surface runoff and velocity and the reduction of concentration times of watersheds, both increasing soil erosion and worsening the water quality as a consequence of the intensive contamination. In this field, Low Impact Development (LID) practices for urban runoff control can be intended as an effective approach to both improve the urban resilience against the flooding risk and assure environmental interventions to adequate the urban stormwater systems to both climate and land use changes. In this paper, a Decision Support System (DSS) for the optimal design of LIDs in urban watershed is presented and discussed. The procedure is tested on Fuorigrotta (IT) and Ponticelli (IT) urban watersheds, with the aim of assessing the effectiveness of LIDs application in reducing both the flooded and conveyed volumes, at the expense of cost-effective solutions.     

Stream nutrient and agricultural land-use trends from 1971 to 2010 in Lake Ontario tributaries

Urbanization is generally recognized as the most widespread form of landuse/landcover change (LULC) within populated regions, including southern Ontario, and is often at the expense of surrounding agricultural land. However, changes in agricultural LULC within these peripheral regions should be considered when interpreting water quality changes in watersheds containing mixed LULC. The objectives of this study were to first, quantify changes in LULC within twelve Lake Ontario tributaries between 1971 and 2010, and secondly, to determine whether these changes co-occurred with changes in total phosphorus (TP) and nitrate‑nitrogen (NO₃-N) concentrations in streams. Water quality data were obtained from the Ontario Provincial Water Quality Monitoring Network while historical land use was reconstructed from agricultural census reports, historical land cover maps, and modern remotely sensed datasets. Urban cover increased, although percent increases in urban cover were small in the most agriculturally dominated watersheds (3–8%). The area of agriculture declined across all watersheds, yet the proportion of agricultural land dedicated to crop (corn, soybean, and wheat) production increased, including in the most urbanized watersheds (e.g. Mimico 89% urban; 2009–11). Total P concentrations in streams were highest at the urbanized watersheds, particularly in the 1970s, before declining in recent decades. In contrast, NO₃-N concentrations were highest (>1.5 mg/L; 2000–10) within the most agricultural watersheds (e.g. Gages 71% agriculture) and have increased over the same period of row crop expansion. Further research is needed to determine the mechanisms behind the potential relationship between expanding row crop cover and stream NO₃-N concentrations in southern Ontario.     

A Diagnostic Decision Support System for BMP Selection in Small Urban Watershed

Best Management Practices (BMPs) have become the most effective way to mitigate non-point source pollution (NPS) issues. Much attention has been paid to NPS in rural areas, where agricultural activities increase nutrients, toxics, and sediments in surface water. Stormwater from urban areas is also a major contributor to NPS pollution. For watersheds bearing various soil types and land uses, a single type of BMP cannot be the panacea to all stormwater problems. To solve these problems, a Diagnostic Decision Support System (DDSS) was developed in this research. The DDSS can identify and locate the most critical NPS areas (hotspots) within a watershed in high spatial resolution. The DDSS can provide a series of spatially distributed small-scale BMPs which are effective in treating the NPS and are suitable for the physical environment. The BMPs, varying in types and locations, are recommended at HRU (Hydrologic Response Unit) level. The DDSS was tested in Watts Branch, a small urban watershed of the Anacostia River in metropolitan Washington D.C., USA. The process-based hydrologic model, Soil and Water Assessment Tool (SWAT), was used to simulate watershed responses. The simulation results were then used by the DDSS for BMP recommendation. Hotspots of different NPS were successfully located and prescribed with spatially distributed BMPs. The DDSS serves as a useful tool to better understand urban watersheds and to make proper stormwater management plans.     

巢湖流域土地利用变化对雨洪调蓄能力的影响

为探究土地利用结构变化对雨洪调蓄能力的影响,以1980—2020年巢湖流域的土地利用数据和土壤数据为基础,采用GIS和SCS模型相结合的方法,分析了不同时期不同降雨尺度下巢湖流域径流产生及分布情况。结果表明：1980—2020年巢湖流域土地利用变化主要表现为建设用地的增加及耕地和林地的减少;流域土地利用综合指数持续增长,其中高强度土地利用区面积从127 km²增长到了1 624 km²。1980—2020年渗透性差的地区从1 706.0 km²增加到了2 398.1 km²,以城市区域扩张为主。2020年相较于1980年,在不同土地利用条件下,相同降雨量时径流量明显增大,且主要在巢湖流域的合肥地区,而在增加降雨量时,径流量的增加幅度逐渐减小,在不同降雨量条件下(日降雨量为50、100、250 mm)径流量分别增大了6.11%、2.61%和0.96%。因此可以得出土地利用变化是导致径流量变化的主要因素,降雨量是次要因素。研究结果可为巢湖流域城市规划及城市雨洪风险管理提供参考。     

Widespread microplastic pollution in Indiana,USA, rivers

Microplastics (i.e., plastic particles <5 mm in size) are aquatic contaminants of emerging concern but are poorly quantified in flowing waters of the midwestern USA. Microplastics enter streams and rivers through a variety of pathways (e.g., wastewater effluent, breakdown of larger plastic debris, atmospheric deposition) and can potentially harm aquatic organisms through both direct consumption and indirect contamination from sorbed toxins. In this study, we quantified microplastic concentrations and types (i.e., beads, fibers, films, foams, fragments) in nine Indiana watersheds representing a gradient of dominant land use (i.e., agricultural, urban, and forested). We predicted that microplastic concentration would be higher in watersheds with higher percentages of urban and agricultural land use than in forested watersheds. Our results revealed measurable quantities of microplastics in samples from all watersheds, but microplastic concentration did not vary significantly with land use or longitudinally within watersheds. Fibers were the dominant form of microplastic at all sites, suggesting that fibers may be transported primarily through atmospheric deposition rather than via direct runoff from the surrounding landscape. We conclude that rivers have a different microplastic “signature” than large lakes, likely due to retention characteristics of flowing water ecosystems, unique microplastic sources, and a shorter legacy of microplastic pollution.     

Micropollutants in stormwater runoff and combined sewer overflow in the Copenhagen area, Denmark

Stormwater runoff contains a broad range of micropollutants. In Europe a number of these substances are regulated through the Water Framework Directive, which establishes Environmental Quality Standards (EQSs) for surface waters. Knowledge about discharge of these substances through stormwater runoff and combined sewer overflows (CSOs) is essential to ensure compliance with the EQSs. Results from a screening campaign including more than 50 substances at four stormwater discharge locations and one CSO in Copenhagen are reported here. Heavy metal concentrations were detected at levels similar to earlier findings, e.g., with copper found at concentrations up to 13 times greater than the Danish standard for surface waters. The concentration of polyaromatic hydrocarbons (PAHs) exceeded the EQSs by factors up to 500 times for stormwater and 2,000 times for the CSO. Glyphosate was found in all samples whilst diuron, isoproturon, terbutylazine and MCPA were found only in some of the samples. Diethylhexylphthalate (DEHP) was also found at all five locations in concentrations exceeding the EQS. The results give a valuable background for designing further monitoring programmes focusing on the chemical status of surface waters in urban areas.     

Climate change will both exacerbate and attenuate urbanization impacts on streamflow regimes in southern Willamette Valley,Oregon

Interactions between climate change and urbanization may affect stream ecosystems in unexpected ways. With an integrated modelling framework, we assessed the combined hydrological impacts of climate change and urbanization under historical and future climate regimes across varied development scenarios in three watersheds in the Willamette Valley, Oregon. First, through an agent‐based land use change model Envision, we created four development scenarios that consisted of 2 × 2 combinations of regional growth (compact population growth in urban cores vs. dispersed growth into rural areas) and stormwater management scenarios (with vs. without integrated stormwater management, ISM). ISM was defined as the integration of strategic organization of land uses with site‐scale stormwater best management practices. Next, two future climate regimes were developed by statistically downscaling projections from two general circulation models (CanESM2 and CNRM‐CM5) that performed well in replicating historical climate. The hydrological assessment of these scenarios was then conducted with the Soil and Water Assessment Tool. Using 10 ecologically significant flow metrics, we evaluated each scenario based on the magnitude of change in each metric and the degree to which such changes could be mitigated. Climate change alone led to a drying trend in flow regimes under both future climates. Combined with urbanization, it magnified changes in six of 10 metrics but attenuated impacts for three other measures of flashiness in at least one basin. The combination of compact growth and ISM effectively mitigated alterations for seven (out of nine) metrics sensitive to the combined impacts in at least one basin, with ISM being more effective than compact growth. The modelling framework teased out both nuanced differences and generalizable trends in hydrological impacts of urbanization and climate change and offers key methodological innovations towards an integrated framework capable of linking landscape planning mechanisms with the goal of sustaining stream ecosystem health.     

Inadequacy of Land Use and Impervious Area Fraction for Determining Urban Stormwater Quality

Urban stormwater quality is multifaceted and the use of a limited number of factors to represent catchment characteristics may not be adequate to explain the complexity of water quality response to a rainfall event or site-to-site differences in stormwater quality modelling. This paper presents the outcomes of a research study which investigated the adequacy of using land use and impervious area fraction only, to represent catchment characteristics in urban stormwater quality modelling. The research outcomes confirmed the inadequacy of the use of these two parameters alone to represent urban catchment characteristics in stormwater quality prediction. Urban form also needs to be taken into consideration as it was found have an important impact on stormwater quality by influencing pollutant generation, build-up and wash-off. Urban form refers to characteristics related to an urban development such as road layout, spatial distribution of urban areas and urban design features.     

Forecasting and assessing the impact of urban sprawl in coastal watersheds along eastern Lake Michigan

The Land Transformation Model (LTM), which has been developed to forecast urban‐use changes in a grid‐based geographical information system, was used to explore the consequences of future urban changes to the years 2020 and 2040 using non‐urban sprawl and urban‐sprawl trends. The model was executed over a large area containing nine of the major coastal watersheds of eastern Lake Michigan. We found that the Black‐Macatawa and Lower Grand watersheds will experience the most urban change in the next 20–40 years. These changes will likely impact the hydrological budget, might reduce the amount of nitrogen exported to these watersheds, result in a significant loss of prime agricultural land and reduce the amount of forest cover along the streams in many of these watersheds. The results of this work have significant implications to the Lake Michigan Lake Area Management Plan (LaMP) that was recently developed by the United States Environmental Protection Agency.     

Seasonal nutrient export dynamics in a mixed land use subwatershed of the Grand River,Ontario, Canada

Algal blooms in the Great Lakes are a concern due to excess nutrient loading from non-point sources; however, there is uncertainty over the relative contributions of various non-point sources under different types of land use in rural watersheds, particularly over annual time scales. Four nested subwatersheds in Southern Ontario, Canada (one natural woodlot, two agricultural and one mixed agricultural and urban) were monitored over one year to identify peak periods (‘hot moments’) and areas (‘hot spots’) of nutrient (dissolved reactive phosphorus, DRP; total phosphorus, TP; and nitrate, NO₃⁻) export and discharge. Annual nutrient export was small at the natural site (0.001 kg DRP ha⁻¹; 0.004 kg TP ha⁻¹; 0.04 kg NO₃^—N ha⁻¹) compared to the agricultural and mixed-use sites (0.10–0.15 kg DRP ha⁻¹; 0.70–0.94 kg TP ha⁻¹; 9.15–11.55 kg NO₃^—N ha⁻¹). Temporal patterns in P concentrations were similar throughout the sites, where spring was the dominant season for P export, irrespective of land use. Within the Hopewell Creek watershed, P and N hot spots existed that were consistently hot spots across all events with the location of these hot spots driven by local land use patterns, where there was elevated P export from a dairy-dominated sub-watershed and elevated N export from both of the two agricultural sub-watersheds. These estimates of seasonal- and event-based nutrient loads and discharge across nested sub-watersheds contribute to the growing body of evidence demonstrating the importance of identifying critical areas and periods in which to emphasize management efforts.     

Evaluation of stormwater and snowmelt inputs,land use and seasonality on nutrient dynamics in the watersheds of Hamilton Harbour,Ontario, Canada

Between July 2010 and May 2012, 87 24-hour level-weighted composite samples were collected from a variety of catchment states (rain, snowmelt, baseflow) from all four major tributaries to Hamilton Harbour, Ontario, Canada. Samples were analyzed for phosphorus- and nitrogen-based nutrients, and concentrations were examined for trends with catchment state, land use, and seasonality. Total phosphorus (TP) and phosphate concentrations were consistently higher during rain/melt events relative to baseflow. Nitrogen parameters, however, exhibited either concentrating behavior or little change in concentration across a range in flows (chemostasis) depending on the parameter and catchment. Despite differences in land use among the four watersheds, TP concentrations during rain/melt events did not vary among stations; however, spatial variability was observed for other parameters, especially nitrate which was elevated in watersheds on the north shore of the Harbour. Seasonal variability was generally not observed for TP concentrations, mirroring the lack of temporal trends for TSS. In contrast, elevated concentrations of nitrate and phosphate were observed during the fall and/or winter period, except in the primarily agricultural watershed where concentrations were elevated during the summer growing season. Highly elevated concentrations of ammonia and nitrate were observed in some watersheds during the unseasonably cold winter of 2010–2011 but not in the comparatively warm winter of 2011–2012. Implications of the study are discussed including the inferred potential impacts of climate change on nutrient dynamics given the strong contrasts in weather patterns observed between years, and exploration of the feasibility of mitigation measures given the data trends.     

基于SWMM的不同降水量对城市降雨径流TSS的影响分析

受物理、化学、生物等多重因素的影响,城市降雨径流水质的模拟研究比较模糊。以TSS(总悬浮固体量)为例,利用SWMM的污染物模块及其土地利用模块、污染物累积模块和污染物冲刷模块进行城市雨水径流水质模拟;考虑了不同土地利用情况对污染物的累积及冲刷的影响,针对不同的降水量,利用EMC法(平均浓度法)以及指数方程法进行污染物的冲刷模拟,指数方程法的模拟精度大于EMC法的模拟精度,与监测值更接近。指数方程法的模拟结果表明:当降水量小于2.5mm时,其TSS冲刷量增长幅度较缓;而当降水量大于3.5mm时,其TSS冲刷量呈显著线性增加关系,从而表明,降水量较大的降水事件对于TSS的冲刷量影响更为显著。     

Evaluation of AMC-Dependent SCS-CN-Based Models Using Watershed Characteristics

This paper presents a quantitative evaluation of the existing Soil Conservation Service Curve Number (SCS-CN) model, its variants, and the modified Mishra and Singh (MS) models for their suitability to particular land use, soil type and combination thereof using a large set of rainfall-runoff data from small to large watersheds of the U.S.A. The analysis reveals that the existing SCS-CN model is more suitable for high runoff producing agricultural watersheds than to watersheds showing pasture/range land use and sandy soils. On the other hand, the two different versions of the Mishra-Singh model are more suitable for both high and low runoff producing watersheds, but with mixed land use.     

Dynamics of wet‐season turbidity in relation to precipitation,discharge, and land cover in three urbanizing watersheds,Oregon

Frequent intense precipitation events can mobilize and carry sediment and pollutants into rivers, degrading water quality. However, how seasonal rainfall and land cover affect the complex relationship between discharge and turbidity in urban watersheds is still under investigation. Using hourly discharge, rainfall, and turbidity data collected from six stations in three adjacent watersheds between 2008 and 2017, we examined the temporal variability of the discharge–turbidity relationship along an urban–rural gradient. We quantified hysteresis between normalized discharge and turbidity by a hysteresis index and classified hysteresis loops during 377 storm events in early, mid, and late wet season. Hysteresis loop index and direction varied by site land cover type and season. Turbidity values peaked quicker in the watersheds with higher degrees of urban development than in a less urbanized watershed. The positive relation between discharge and turbidity was highest in two downstream stations in the mid wet season, whereas it was highest in two upstream stations in the early wet season. Correlation and regression analysis showed that maximum turbidity was best explained by discharge range, and the sensitivity of turbidity to discharge change was higher in the larger downstream watershed than in the small upstream watersheds. A flashiness index was negatively associated with the slope of turbidity versus discharge, suggesting that turbidity is difficult to predict solely on the basis of discharge in flashy urban streams. This paper contributes to a deeper understanding of the spatial and temporal variation of discharge–concentration relationship in urbanizing watersheds, which can help water managers increase the resiliency of water‐related ecosystem services to impacts of climate change.     

Is stormwater harvesting beneficial to urban waterway environmental flows?

Urbanization degrades the hydrology and water quality of waterways. Changes to flow regimes include increased frequency of surface runoff, increased peak flows and an increase in total runoff. At the same time, water use in many cities is approaching, and in some cases exceeding, sustainable limits. Stormwater harvesting has the potential to mitigate a number of these detrimental impacts. However, excessive harvesting of stormwater could also be detrimental to stream health. Therefore, a study was undertaken to test whether typical stormwater harvesting scenarios could meet the dual objectives of (i) supplying urban water requirements, and (ii) restoring the flow regime as close as possible to 'natural' (pre-developed). Melbourne and Brisbane, which have different climates, were used along with three land use scenarios (low, medium and high density). Modelling was undertaken for a range of flow and water quality indicators. The results show that using these typical harvesting scenarios helped to bring flow and water quality back towards their pre-developed levels. In some cases, however, harvesting resulted in an over-extraction of flow, demonstrating the need for optimizing the harvesting strategy to meet both supply and environmental flow objectives. The results show that urban stormwater harvesting is a potential strategy for achieving both water conservation and environmental flows.     

中国海绵城市开发与加拿大综合雨洪管理对比研究:以多伦多为例

介绍了加拿大城市流域雨洪规划管理的发展进程以及加拿大最大城市——多伦多市雨洪管理的政策、原则和标准,与中国的海绵城市建设技术指南进行对比,对指南中存在的一些问题,如指南未给出LID设计的检验和校核方案,也未充分考虑降雨等问题进行讨论,旨在为中国的雨洪管理提供新的研究思路和参考。     

High-rate stormwater clarification with polymeric flocculant addition.

Treatment of urban stormwater by clarification, with flocculant addition, was studied in Toronto, Canada using a pilot-scale clarifier with removable lamellar plates. Almost 90 stormwater runoff events were characterised at the study site and found fairly polluted. The previous research phase indicated good treatability of this stormwater by lamellar clarification with flocculant addition (total suspended solids, TSS, removal of 84%, at a surface load of 15 m/h), but there were concerns about cleaning plates after storm events. With the aid of numerical modelling, hydraulic improvements to the clarifier inlet zone were retrofitted in 2004 and permitted the removal of the lamellar pack without a loss in treatment efficiency. In the modified clarifier, a cationic polymeric flocculant dosage of 4 mg/L with conventional clarification provided a TSS removal of 77%, at surface loads up to 43 m/h. The use of the polymer did not increase the acute toxicity of the treated effluent. The clarifier sludge was severely polluted by several heavy metals and would require special disposal. The treatment process tested could be well applied in projects requiring intensive stormwater treatment at compact sites.     

小流域氮流失特征及其影响因素

由非点源流失的氮是造成水体污染的主要因子之一，而土地利用方式往往对氮的流失产生重要影响。本课题在福建省九龙江流域遴选了4个不同土地利用方式的小流域，通过在自然降雨条件下现场采样与室内分析相结合，研究不同土地利用结构的小流域氮的流失特征和影响因素。研究结果表明：总氮、可溶态氮和泥沙结合态氮的浓度通常比径流先达到峰值，然后开始降低；氮的流失以可溶态氮为主(占73%～98.3％)，因此其流失量主要受径流量的影响与雨强关系不大；土地利用方式对氮的流失有明显影响，一般植被覆盖度提高可以降低流失，而农业活动会加剧流失；此外，在高植被覆盖度的天然林地，氮的流失呈现出以泥沙结合态为主。通过本研究可以看出土地利用结构的调整和合理布置是九龙江流域非点源污染控制的重点。     

Microbial challenge-testing of treatment processes for quantifying stormwater recycling risks and management.

Pathogenic microorganisms have been identified as the main human health risks associated with the reuse of treated urban stormwater (runoff from paved and unpaved urban areas). As part of the Smart Water initiative (Victorian Government, Australia), a collaborative evaluation of three existing integrated stormwater recycling systems, and the risks involved in non-potable reuse of treated urban stormwater is being undertaken. Three stormwater recycling systems were selected at urban locations to provide a range of barriers including biofiltration, storage tanks, UV disinfection, a constructed wetland, and retention ponds. Recycled water from each of the systems is used for open space irrigation. In order to adequately undertake exposure assessments, it was necessary to quantify the efficacy of key barriers in each exposure pathway. Given that none of the selected treatment systems had previously been evaluated for their treatment efficiency, experimental work was carried out comprising dry and wet weather monitoring of each system (for a period of 12 months), as well as challenging the barriers with model microbes (for viruses, bacteria and parasitic protozoa) to provide input data for use in Quantitative Microbial Risk Assessment.     

Evaluating Retention Capacity of Infiltration Rain Gardens and Their Potential Effect on Urban Stormwater Management in the Sub-Humid Loess Region of China

Recognized as an effective low impact development (LID) practice, rain gardens have been widely advocated to be built with urban landscaping for stormwater runoff reduction through the retention and infiltration processes; but the field performance and regional effect of rain gardens have not been thoroughly investigated. In this paper, we presented a four-year monitoring study on the performance of a rain garden on stormwater retention; hydrological models were proposed to predict the potential effect of rain gardens on runoff reduction under different storms and the future urban development scenarios. The experimental rain garden was constructed in a sub-humid loess region in Xi’an, China; it has a contributing area ratio of 20:1 and depth of 15 cm. During the study period, we observed 28 large storm events, but only 5 of them caused overflow from the rain garden. The flow reduction rate for the overflow events ranged from 77 to 94 %. The runoff coefficient from the contributing area (RC) was reduced to less than 0.02 on annual basis, and 0.008 over the four years average. Field observations also showed that infiltration rate remained stable during the operation period. The predictions based on the future landuse and storm variability of the study area showed that by converting a small fraction of the city land area into rain gardens, the negative hydrological effect from expansion of impervious area can be reduced significantly. The challenge, however, lies in how to plan and build rain gardens as an integral part of the urban landscape.