Low Impact Development (LID)
Background

To meet the water needs of aquatic ecosystems and maintain characteristics of the natural flow regime, we must change our approaches to land development and water management. Dr. Bradford’s research supports an approach that focuses first on using stormwater as a resource and management of rainwater near where it falls. It supports a shift from flow rate based management to volume based management (matching post-development components of the water balance to pre-development levels rather than matching peak flows) and protection of natural capital (forests, wetlands) for the water quality and quantity functions it provides.

Bradford, A. and Denich, C. 2007. Rainwater management to mitigate the effects of development on the urban hydrologic cycle. Journal of Green Building, 2(1): 37-52.

 

Rainwater Harvesting

Rainwater harvesting presents significant opportunities for rainwater management, in addition to providing benefits for water supply. The interaction of harvested rainwater with the urban hydrologic cycle depends upon the allocation of harvested water to indoor and outdoor uses. Water that is used for landscape irrigation returns to the hydrologic cycle and contributes to soil moisture or evapotranspiration. Where there is centralized wastewater treatment, water that is directed to indoor uses will bypass the local hydrologic cycle and re-enter it where the wastewater treatment plant discharges.

The strength of rainwater harvesting lies in its potential to support a water balance approach to rainwater management, with the associated benefits to the quantity and quality of baseflow and runoff discharged to receiving waters. It is a means of restoring the water storage capacity that was distributed throughout a watershed before urbanization. It is not desirable to intercept and use so much rainwater that the pre-development infiltration component of the water balance, and baseflow targets in downstream watercourses, cannot be met. However, a well-considered rainwater harvesting strategy can contribute to mitigating the host of impacts associated with the increase in runoff volume typical of post-development conditions.

A spreadsheet-based model has been created to examine appropriate extent of implementation of a rainwater harvesting system for a range of soil and development type characteristics. The output may be used in conjunction with other models to simulate subwatershed conditions and provide a more direct means of predicting receiving water response. The study demonstrates that effective integration of rainwater harvesting demands a good understanding the pre-development water balance and the effects of urbanization and other rainwater management techniques on the post-development water balance.

 

Bioretention: Cold Climate Issues

Bioretention, an important component of LID, offers potential to help sustain pre-development evapotranspiration and infiltration, while minimizing effects on groundwater quality and providing a landscaped aesthetic. While a considerable foundation of research exists on these systems, there remain gaps in our knowledge, particularly with regard to cold climate issues. These issues include the effects of snow, salt and sand loads on system performance, potential trade-offs between groundwater quantity and quality, and system lifespan, specifically media clogging and the potential effects of winter maintenance materials on the biochemical processes and removal of heavy metals.

With a view to expanding the use of bioretention areas for road and parking lot applications in Canada, one of our studies is simulating winter runoff conditions in cold climate regions where the use of chemical road deicers is common. The testing is proceeding in two stages. First, bioretention systems are being subjected to synthetic winter runoff (i.e. water mixed with salt and aggregate) during the plant’s dormant period (Jan-April). The potential risk to groundwater quality and the effects of winter runoff on infiltration rates and plant health are being evaluated. Subsequently, a “contaminant-based” synthetic runoff is being applied to all the columns during the active growing season (May-Sept). Active growing season testing is intended to confirm published removal rates and to quantify the effects of salt application on contaminant removal rates.

This stage of investigation is intended to demonstrate the potential to expand the functionality of bioretention facilities to cold climate regions and to identify potential changes to the design of the systems to enhance functionality.

Denich, C. and A. Bradford. 2008. Cold Climate Issues for Bioretention: Assessing impacts of salt and aggregate application on plant health, media clogging and groundwater quality. LID 2008. Seattle, Washington.

 

Bioretention: Contributions to Urban Hydrologic Cycle

In moving from traditional urban hydrology models that focus on runoff, it is clear that there are substantial gaps in our knowledge of groundwater recharge and evapotranspiration (ET) in urban areas. It is desirable to quantify ET and recharge separately, rather than lumping these “abstractions,” to provide better input to urban water resources models and allow better predictions of the effectiveness of low impact development to achieve management objectives. To complement ongoing research on bioretention and to begin to address these data gaps, a study of evapotranspiration and recharge from bioretention areas has been implemented.

A subsurface, weighing lysimeter with a removable, 1 m3, bioretention system, has been designed. The ground surface around the bioretention system is slightly raised and is also vegetated. The soil media is instrumented with time domain reflectometry probes; soil moisture and the weight of the bioretention system are continuously monitored. Rainfall is measured at the site and the volume of infiltrate collected (in an external tank) is recorded, allowing estimation of evapotranspiration and groundwater recharge from the system. Results at this scale will vary with urban surroundings (variability of winds, advection of heat from built surfaces), but will nevertheless contribute to the scant information available on urban ET and recharge. Installation of similar systems in various urban micro-meteorological conditions is planned. To complement the “local” scale measurements, ET measurement on a “neighbourhood” scale using the eddy correlation approach is also planned.

 

Selected Other Publications

Varangu, L., Bradford, A., Cowen, K. Farahbakhsh, K. 2007. Wet weather flow improvement opportunities for the small business sector. 42nd Central Canadian Symposium on Water Quality Research, Burlington, Ontario. February 13, 2007.

Herrera, M., Heathcote, I., James, W. and Bradford, A. 2006. Multi-Objective Calibration Of SWMM For Improved Simulation Of The Hydrologic Regime. Intelligent Modeling of Urban Water Systems, Monograph 14. James, William, 1937- , Editor in Chief, co-edited by Kim N. Irvine, Edward A. Mc Bean and Robert E. Pitt. Guelph, Ontario, Canada.

Leidl, C., Farahbakhsh, K., and Bradford, A. 2006. Investigating the feasibility of large-scale rainwater harvesting in Ontario. Canadian Water Resources Association 59th Annual Conference, Toronto, Ontario, June 4-7, 2006.

Bradford, A. and Gharabaghi, B. 2004. Evolution of Ontario’s Stormwater Management Planning and Design Guidance. Water Quality Research Journal of Canada. Theme Issue on Managing Urban Wet-Weather Flows: On the Road to Sustainability. 39(4): 343-355.

 

2008 Andrea Bradford