Research shows that tree stormwater control measures (SCMs) provide excellent stormwater benefits. With trees, however, come leaves – and leaf litter that falls on impervious street surfaces can actually add unwanted nutrients to stormwater runoff (leaf litter provides many benefits on pervious surfaces). The contribution of leaf litter to nutrients in stormwater runoff is more than offset by the nutrient removal of the tree SCM as a whole. Moreover, research now shows that nutrients contributed to stormwater by leaf litter can be cost effectively mitigated by street sweeping.
Several past articles have addressed research on stormwater nutrient reduction of tree stormwater control measures. Page et al’s research monitoring stormwater benefits of trees with bioretention soil under suspended pavement found excellent water quality results. Total Phosphorus removal, for example, was greater than 70% (Page et al 2014).
Research Quantifying Nutrient and Solids Removal of Street Sweeping
Researchers at the University of Minnesota, in partnership with the City of Prior Lake, MN, monitored total amount of sediment and nutrients removed by street sweeping in 392 street sweeping events over a period of two years. Here are the details of what they tracked:
- Nutrients and solids in sweepings from nine sweeping routes
- Sweeping route tree canopy cover ranged from low, to medium, to high year round (except when there was snow) – this allowed them to study seasonal influences on sweeping efficiency.
- Different sweeping frequencies (1x, 2x, and 4x/month) on each canopy cover class. To analyze cost effectiveness of the varying street sweeping frequencies, they also tracked street sweeping costs, including driver wages and benefits, fuel, vehicle wear and tear (Kalinosky et al, 2014).
Researchers found that percent canopy cover, sweeping frequency, and month of sweeping all influenced nutrient removal by sweeping, as well as cost effectiveness. Not surprisingly, recovered loads were higher in routes with higher canopy cover, and in the fall and spring.
Spring (March and April) was the best time for cleaning up solids, including road salt, sand, and fines left behind from soil and debris entrained in snow after the snow melts (Kalinosky et al, 2014). Fall sweeping, after fall leaf drop, was the most important for nutrient recovery, following by spring (Kalinosky et al, 2014).
Compared to other Stormwater Control Measures (SCM)s, street sweeping was found to be a very cost effective tool for nutrient reduction, especially for the high canopy routes in spring and fall, with costs as low as $18/lb P removed (Kalinosky 2013). Estimated cost per pound of P removed for other SCMs (such as bioretention, stormwater ponds), is typically orders of magnitude higher, ranging from hundreds to thousands of dollars per pound of phosphorus removed!
Translating Research into Action
Based on their results, these researchers developed a planning calculator tool to estimate nutrient and solids load recovery through street sweeping based on tree canopy cover, frequency, and the timing of sweeping.
To use the calculator, the user needs to enter the following information:
- Route ID (user-defined)
- Number of curb miles swept
- Average percent canopy cover
- Default cost per curb mile (only needed if user wants to evaluate cost effectiveness)
Once this information is entered, the user can choose the number of sweeping events each month. Based on the route’s canopy cover and the chosen sweeping frequency and timing, the calculator provides the following information for each month:
- pounds of wet solids removed
- pounds of dry solids removed
- pounds of phosphorus removed
- nitrogen removed
- cost of sweeping
- cost per pound of phosphorus removed
So what? Potential Uses of the Calculator Output
The output provided by the calculator can be used to plan the timing and frequency of street sweeping operations to meet sweeping goals as cost effectively as possible.
The calculator can also be used to compare sweeping effectiveness (loads removed) in different months of the year, and to analyze how much sweeping effectiveness changes with more or fewer street sweeping events per month or per year.
Quantification of loads removed is especially useful for TMDL (total maximum daily load) plans. If a lake is impaired for phosphorus, for example, a TMDL plan must be prepared that estimates current P loading to the lake and P load reduction that is required. Results from the street sweeping calculator can be used to quantify how much street sweeping can contribute to the phosphorus load reduction goals under varying street sweeping scenarios (timing and frequencies).
Where can the calculator be used?
The calculator is calibrated to conditions in Prior Lake, MN, and is therefore recommended for regions with comparable climate and vegetation: cold climates with primarily deciduous street trees. Additionally, because the Prior Lake study used a regenerative air sweeper, this calculator is recommended primarily for routes that will be using a regenerative air sweeper or other high efficiency sweepers. Recovery of fines will likely be lower with older, less efficient, mechanical broom sweepers (Kalinosky et al, 2014).
Street sweeping is an underestimated tool in the stormwater control measures toolbox. Research by Kalinosky et al (Kalinosky et al, 2014) shows that with optimal street sweeper technology, sweeping timing, and sweeping frequency, street sweeping can be very cost effective for nutrient and solids removal, especially in sweeping routes with high canopy cover.
Using this ground breaking new calculator, it is now possible to quantify solid and nutrient load reductions achieved by street sweeping for varying canopy cover, sweeping frequency, and month of sweeping, in regions with cold climates and canopy dominated by deciduous trees.
The calculator, user manual, and supporting research can be accessed here. Please try it out and let us know how it works for you!
Nathalie Shanstrom is a landscape architect specializing in sustainable design with the Kestrel Design Group.
Page, J.L., R.J. Winston, and W.F. Hunt, III. 2014. Field Monitoring of Two Silva Cell™ Installations in Wilmington, North Carolina: Final Monitoring Report.
Kalinosky, P., L. Baker, S. Hobbie, R. Bintner. 2014. User Support Manual: Estimating Nutrient Removal by Enhanced Street Sweeping. Report to the Minnesota Pollution Control Agency and the U.S. EPA.
Kalinosky, P. 2013. Quantifying Nutrient Removal through Targeted Intensive Street Sweeping. St. Anthony Falls Laboratory and the University of Minnesota Updates Newsletter 8:3. Accessed fromhttp://stormwater.safl.umn.edu/updates-march-2013