Effective implementation of green stormwater infrastructure (GSI) requires an understanding of the hydrological processes that occur within landscapes that have been altered by human activities and the resulting impact to the water cycle. This chapter introduces the reader to urban hydrology concepts and rainfall characterization. As precipitation is a key driver of stormwater GSI analysis, design, and performance, changes to precipitation depth, intensity and frequencies will alter all of these GSI considerations. Precipitation depth is characterized based on a specified time frame of interest, such as a year, season, individual storm event, or peak rate within that storm event. Patterns of rainfall vary greatly over the course of a year. While the rainfall drives the GSI design, to be useful for design, the rainfall must be converted to the runoff generated. In general, the quality of rainwater is good.
Permeable pavement is exactly what it sounds like, pavement that allows water to pass through it rather than shed off. Five types of permeable pavements are commonly used, Permeable Asphalt, Permeable Concrete, Permeable Interlocking Concrete Pavers, Concrete Grid Pavers, and Plastic Grid Pavers. For all permeable pavements, proper installation is critical for effective performance and to ensure long-term life. Permeable pavements, especially PC, require experienced installers. Permeable pavements can be specifically designed to optimize infiltration and percolation into the surrounding soils. Permeable pavements act as a stormwater control measure primarily by reducing the creation of runoff. Permeable pavements frequently show improvement to stormwater runoff quality, but not always. Permeable friction course is an overlay of permeable asphalt on a layer of standard asphalt.
As urbanization continues to encroach upon coldwater fisheries, the effect of heated stormwater runoff on the coldwater aquatic ecosystem has been realized; however, the effect of stormwater BMPs on runoff temperature is largely unknown. There are indications that bioretention could discharge cooler effluent than stormwater wetlands and wet ponds, since the outlet is located at the bottom of the system where soil is insulated from elevated temperatures near the surface. Infiltration through soil media is generally considered to reduce runoff temperatures, with many coldwater streams able to maintain their cooler temperatures through influxes of groundwater. A monitoring study was conducted at 4 bioretention areas in western North Carolina, along the southeastern extent of United States trout populations, to determine the effect of bioretention on runoff temperature and identify any design criteria that affect effluent temperatures.
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Stormwater managers are increasingly faced with the need to address many potentially-conflicting issues as part of their activities. First, they need to control the potential flooding waters generated after urbanization. This has led to the use of detention facilities and control of peak flow rates to pre-development levels. However, these designs that focused on flow control for larger storms have been providing inconsistent performance for the control of other pollutants. Many other techniques, in addition to detention/sedimentation, have been proposed to alleviate these pollutant concerns. Stormwater ordinances/regulations and the development of TMDLs (with its subsequent need to control loadings to a stream) has driven the improvement of existing treatment technologies (often referred to as structural best management practices [BMPs]) and the development of new ones. Assumptions have made regarding the range of performance of these devices, based on the field data that currently exists, and as new data is submitted for publication, these new insights are added into the body of knowledge on BMP design and performance, specifically on the design needed to provide a specific performance.
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.
With increasing development in areas of trout sensitive waters, the effect of urban storm-water runoff temperature on the aquatic ecosystem has become a concern. A study was conducted in western North Carolina, along the southeastern extent of U.S. trout populations, to determine the effect of storm-water wetlands and wet ponds on the temperature of urban storm-water runoff. Measurements included temperature at the inlets, outlets, and at several depths within the best management practices (BMPs). Parking lot runoff temperatures were significantly higher than the 21°C temperature threshold for trout during peak summer months and water temperatures consistently increased from the inlet to the outlet in the storm-water wetland and wet pond, implicating these BMPs as sources of thermal pollution. Despite similar inflow temperatures, effluent temperatures from the wet pond were significantly warmer than those from the storm-water wetland for the period from June to September. Substantial cooling was observed as runoff was conveyed from the parking surface to the BMPs through buried pipes, which could be incorporated into BMP design to achieve thermal pollution mitigation goals. Temperatures at the bottom of the water columns were cooler than water leaving the current outlet structures, providing support for the installation of modified outlet structures in regions with cold water fisheries.
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