Thermal Impacts

Thermal Impacts, Stormwater Management, and Surface Waters

arial view of parking lotfish in streamwetlandsfield

The research presented here examines thermal characteristics as they relate to surface waters and runoff in the built and natural environment. As a watershed is developed, and impervious surface area increases, stormwater runoff can be a significant, or even the primary source of water to a surface water body. Small streams are highly sensitive to changes in temperature. Increased temperature damages cold water fisheries, and altered temperature regimes interfere with spawning and migration patterns. This research 1) characterizes the thermal signature of stormwater runoff in relation to Best Management Practices normally installed to address stormwater volume and pollutants other than temperature, and 2) explores the thermal regimes of streams throughout NH and MA. Results of this research are needed to inform the appropriate choice and design of stormwater BMPs in sensitive, threatened, or impaired watersheds or ecological areas.

The Problem

As urbanization and build-out occurs, the thermal regime of the surrounding environment is altered. Heated stormwater runoff flows into receiving waters where it mixes, and potentially increases the base temperature of the surface water in lakes, streams, and estuaries. The amount of heat transferred, and the degree of thermal pollution is of great importance for fisheries management and the ecological integrity of receiving waters. Coldwater fisheries in particular are most sensitive to thermal pollution.

The increase in thermal energy in stormwater runoff is primarily a product of the increase in impervious cover of the surrounding area. Impervious surfaces can be generalized as any constructed surface that inhibits the infiltration of stormwater runoff (e.g. building rooftops, roads, parking lots, and sidewalks). Impervious cover (IC) absorbs and emits heat, creating air and surface temperatures that are significantly higher than those of rural areas. An increase in IC also results in additional surface runoff. The combination of these two phenomena creates a larger volume of runoff with increased temperatures.

Examination of Thermal Impacts from Stormwater Best Management Practices

Robert M. Roseen, Nicolas Digennaro, Thomas P. Ballestero, Alison W. Watts, James J. Houle

This study examines 4 years of runoff temperature data for a range of stormwater best management practices (BMPs) in relation to established environmental indicators for a study in Durham, NH. Stormwater BMPs examined include conventional, Low Impact Development, and manufactured treatment designs. Surface systems that are exposed to direct sunlight have been shown to increase already elevated summer runoff temperatures, while other systems that provide treatment by infiltration and filtration can moderate runoff temperatures by thermal exchange with cool subsurface materials. The examination of BMPs indicates that outflow from the larger surface systems exhibit greater thermal variations than water typical of drained parking lot areas and larger subsurface systems exhibit greater thermal buffering, with outflows consistent with groundwater temperature.

Thermal Regimes of Northeast Streams

Dr. Jennifer Jacobs (ERG)

In 2009 EPA awarded a grant to the University of New Hampshire for a project entitled, Temperature Regime Characteristics of High-Quality Coldwater Streams in New England. This talk presents the temperature regime characteristics results from several years of hourly data from sites in New Hampshire and Massachusetts. All datasets from MADEP, MDFW, NHDES, and NHFG dataset were analyzed if they met the newly established QAPP criteria. Because most sites had observations during the summer months, results are presented primarily for June to October. Monthly data summaries conducted for approximately 100 sites that support coldwater species (present) and approximately 40 sites that do not support coldwater species (absent) are compared. The data were analyzed to better understand the magnitude, frequency, timing and duration, ranges of variation and rate of temperature change across the sample sites.

Effect of conventional and LID strategies on the statistical characteristics of site runoff quantity and quality: which technologies return runoff to pre-development characteristics?

Thomas P. Ballestero, Alison W. Watts, Robert M. Roseen, James J. Houle

The lofty goal of modern stormwater management is to return runoff from developed lands to receiving water with the same water quality and quantity characteristics as had there been no development. Very often the comparison metrics include the volume of runoff, the peak flow for design storms, average concentration, and temperature. These metrics can be based on event data or real time data. With real time data, a very common metric in hydrologic studies is the “flow duration curve”, which is nothing more than the probability distribution of observed flows. When comparing flow duration curves (probability distributions) between pre-development and post-development site conditions, this powerful graphical device can be employed to identify and discriminate undesirable consequences. This probability distribution includes the maximum, median, and minimum flow as well as other statistics such as variance and skew. However what such comparisons ignore is system “memory”: the relationship between future values and past values. System memory is an important component of any ecosystem. For example rapidly changing water quality adds to environmental stress on organisms, potentially affecting survival.

Managing Stormwater to Protect the Thermal Regime in Streams

Alison W. Watts, Robert M. Roseen, Nicolas Digenaro, Thomas P. Ballestero, James J. Houle

The thermal impact of stormwater flow to a stream is a function of both the volume of flow, and the temperature differential between the runoff and the stream. A simple mixing model can be used to estimate the thermal impact of a predicted effluent flow to a stream with a given thermal regime. The impact of elevated runoff temperatures to streams can be mitigated by some stormwater management choices; systems which retain runoff in subsurface storage have both lower peak temperatures, and generally less extreme thermal ranges. Thermal inputs to a stormwater system include influent and effluent temperature, solar radiation, longwave radiation, convection and diffusion from both the atmosphere and subsurface, and infiltration. A simple thermal model is being developed to calculate the impact of effluent from a stormwater pond to a small stream. Ultimately the goal is to develop design models for standard stormwater devices that can be used to meet specific effluent temperature standards and to maintain the required thermal regime in a receiving stream.

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