Stormwater management in nutrient-sensitive watersheds: a case study investigating impervious cover limits and pollutant-load regulations
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Abstract The objective of this research project was to compare two stormwater management strategies within a nutrient-sensitive watershed: impervious cover limits versus pollutant-load regulations. A case study was conducted in the nutrient-sensitive Falls Lake watershed in North Carolina, USA, where a commercial fitness complex was constructed in a zone previously restricted to low-density housing. The Falls Lake watershed has a stormwater regulation that limits total nitrogen and total phosphorus export loads to 2.47 kg/ha/yr and 0.37 kg/ha/yr, respectively. Hydrology and water quality were monitored pre- and post-development to quantify changes to stormwater volumes, pollutant concentrations, and annual export loading rates. On-site stormwater control measures (SCMs) reduced nutrient export loading rates below the regulatory standard. However, increased stormwater volumes and nutrient export loading rates were observed from pervious surfaces that were disturbed during construction (total nitrogen increased from 2.06 to 4.24 kg/ha/yr, total phosphorus increased from 0.41 to 0.73 kg/ha/yr). Results from this case study suggest that (1) impervious cover limits do not adequately account for a parcel's nutrient export loads and (2) SCMs that reduce volume and treat pollutants can reduce nutrient export loads below regulatory levels in the Falls Lake watershed.Keywords:
Impervious surface
Low-impact development
The concept of low impact development is to develop land with minimal impact to the local environment, with a focus on stormwater. Preventing pollutants from being placed, deposited, or mobilized in the urban landscape will result in higher runoff quality and reduced need for treatment. Street trees can be a valuable part of the green stormwater infrastructure. Trees can provide multiple benefits for stormwater management, similar to other, more formalized stormwater control measures. A disconnected impervious surface is a built-upon area (usually a roof or a paved surface) that discharges runoff to a vegetated area that is sized and graded to reduce runoff and pollutants. Compromised water quality in urban runoff results from the wash-off of pollutants built into or deposited onto the drainage areas. Education is and will continue to be a critical component of stormwater management.
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Low-impact development
Urban runoff
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Rapid urbanization has altered the hydrologic cycle, causing increased runoff rates and peak flows in the drainage system.Cities are now facing serious problems relating to stormwater management such as water scarcity, degraded waterways, and increased flooding.Under such circumstances, green roofs present numerous benefits including the retention of rainwater for a longer time and a delay in the peak discharge.Using data from various storm events, this study examined the performance of retrofitted green roofs for stormwater management in the eThekwini region of South Africa.The study also used the Personal Computer Storm Water Management Model (PCSWMM) to investigate the effect that the best performing green roof would have on stormwater flow rates and volumes for the region.The results concluded that the green roof systems proved to significantly reduce stormwater runoff flow rates and volumes, and retention largely depended on the intensity and duration of the rain events.The PCSWMM model further confirmed that when compared to the base model, peak flow rates from the green roof model decreased by over 40% for all storm intensities.It can therefore be concluded that the implementation of green roofs within the eThekwini Central Business District (CBD) will be highly effective in reducing peak stormwater flow rates.
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Green roofs, which have been used for several decades in Europe, offer a unique approach to stormwater management. This paper presents hydrologic data for a specifically designed irrigated green roof stormwater treatment system. The major hypothesis is that green roof stormwater treatment systems can effectively reduce runoff volume and pollutant mass. A computer model is introduced to help with the design of these green roof stormwater treatment systems to achieve a specific stormwater retention yearly volume. This model is developed and discussed within this work as well as compared to experimental results.
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As cities expand and encroach onto rural lands, large impervious surfaces in the form of buildings, parking lots and roads cover the landscape. These impervious surfaces generate increased quantities of stormwater runoff which flow rapidly into municipal conveyance systems bypassing the natural infiltration processes resulting in polluted water and altered natural hydrologic systems. By replacing the footprint area of a building with a rooftop garden, greenroofs may offer an opportunity to control runoff and remove pollutants from rainwater. In this report we examined two greenroofs, one in Petaluma, California and the other in Portland, Oregon to determine their hydrological characteristics and how they perform as stormwater management techniques. We found that both roofs reduce the volume of stormwater runoff by an average greater than 60% while also reducing peak flows and increasing runoff time. Our results show that greenroofs are viable on-site stormwater control devices.
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Bioretention
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<p>Urban-growth increases impervious-surface cover and ‘flashy’ streamflow responses. Research has suggested that flashy streamflow occurs when total impervious area (TIA) approaches and crosses 10% of a watershed. This study examines the spatiotemporal variability in river flashiness of rural/urbanizing watersheds in southwestern Ontario. This research addresses relationships between river flashiness and watershed TIA across 37 watersheds as they approach and/or cross 10% TIA between 1990 and 2017. The Richards-Baker Flashiness Index (RBFI) were calculated using hydrometric-data for each watershed. Watershed TIA was estimated for the years 1990/2000/2010/2017 using land-use-data. Strength and direction of the RBFI-TIA relationship is compared to other watershed characteristics (area, soil type). Results show that spatial variability in RBFI is best explained by soil type rather than TIA. Mann-Kendall trend-analysis revealed 10/37 watersheds exhibited significantly increasing RBFI trends. Watersheds with larger increases in RBFI over the study period had lower urban-growth suggesting factors beyond TIA influencing </p> <p>watershed flashiness. </p>
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<p>Urban-growth increases impervious-surface cover and ‘flashy’ streamflow responses. Research has suggested that flashy streamflow occurs when total impervious area (TIA) approaches and crosses 10% of a watershed. This study examines the spatiotemporal variability in river flashiness of rural/urbanizing watersheds in southwestern Ontario. This research addresses relationships between river flashiness and watershed TIA across 37 watersheds as they approach and/or cross 10% TIA between 1990 and 2017. The Richards-Baker Flashiness Index (RBFI) were calculated using hydrometric-data for each watershed. Watershed TIA was estimated for the years 1990/2000/2010/2017 using land-use-data. Strength and direction of the RBFI-TIA relationship is compared to other watershed characteristics (area, soil type). Results show that spatial variability in RBFI is best explained by soil type rather than TIA. Mann-Kendall trend-analysis revealed 10/37 watersheds exhibited significantly increasing RBFI trends. Watersheds with larger increases in RBFI over the study period had lower urban-growth suggesting factors beyond TIA influencing </p> <p>watershed flashiness. </p>
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An impervious cover in the watershed management has been used as effective indicators. It is a very useful barometer to measure the impacts of watershed development on aquatic systems. Hence, it is necessary to survey the impervious cover of a watershed and to develop an impervious cover model (ICM) for supporting best management practices. The main objectives of this study were to investigate the spatial patterns of the impervious cover, to calculate landscape indices using FRAGSTATS, and to develop an ICM in the Gap-stream watershed and its six sub-watersheds. The results showed that the impervious cover of the Gap-stream watershed increased from 4.9 % in 1975 to more than 11.2 % in 2000, the number of impervious cover fragments increased from 662 to 3,578, and the landscape shape index increased from 27.0796 to 91.1982. Fragmentation was severe within the Yudeungcheon downstream and the Gapcheon downstream of six sub-watersheds. This paper presented the results derived landscape indices to define landscape patterns and structure for the Gap-stream watershed. Our results indicate that altered land use might be influenced changes in landscape structure.
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Fragmentation
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Environmental flow assessment frameworks have begun to consider changes to flow regimes resulting from land-use change. Urban stormwater runoff, which degrades streams through altered volume, pattern and quality of flow, presents a problem that challenges dominant approaches to stormwater and water resource management, and to environmental flow assessment. We used evidence of ecological response to different stormwater drainage systems to develop methods for input to environmental flow assessment. We identified the nature of hydrologic change resulting from conventional urban stormwater runoff, and the mechanisms by which such hydrologic change is prevented in streams where ecological condition has been protected. We also quantified the increase in total volume resulting from urban stormwater runoff, by comparing annual streamflow volumes from undeveloped catchments with the volumes that would run off impervious surfaces under the same rainfall regimes. In catchments with as little as 5–10% total imperviousness, conventional stormwater drainage, associated with poor in-stream ecological condition, reduces contributions to baseflows and increases the frequency and magnitude of storm flows, but in similarly impervious catchments in which streams retain good ecological condition, informal drainage to forested hillslopes, without a direct piped discharge to the stream, results in little such hydrologic change. In urbanized catchments, dispersed urban stormwater retention measures can potentially protect urban stream ecosystems by mimicking the hydrologic effects of informal drainage, if sufficient water is harvested and kept out of the stream, and if discharged water is treated to a suitable quality. Urban stormwater is a new class of environmental flow problem: one that requires reduction of a large excess volume of water to maintain riverine ecological integrity. It is the best type of problem, because solving it provides an opportunity to solve other problems such as the provision of water for human use.
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