by Ranger Kidwell-Ross
The goals for the study generated by the City of Ellensburg, WA, were to:
- investigate whether the frequency of street sweeping significantly influenced sediment accumulation in catch basins (and transport from catch basins) during the dry season in a semiarid location,
- recommend a combination of street sweeping and catch basin cleaning procedures for achieving permit requirements for catch basin cleaning, and
- use the study results to justify a condition in the next permit for O&M procedures that allows street sweeping practices to offset the frequency of catch basin cleanings.
These goals were achieved by conducting a two-year study in the City of Ellensburg. The study area was a 1,065-feet long section of SR 97 which included four catch basins located on each side of the road that each discharge runoff to a swale.
During year one (2018), one side of the road (test site) was swept and the catch basins were cleaned every other month starting in April and ending in October. On the
other side of the road (thr control site), the catch basins were cleaned at the same time as the test site, however the street was swept in April and October.
During year two (2019), the test site and control site switched to the other side of the road and the catch basin cleaning and street sweeping only occurred once in October on the control side. Throughout the study, the City of Ellensburg
followed their typical practices for street sweeping and catch basin cleaning and used the same equipment they typically used: a 2016 Elgin Crosswind J Regenerative Air Sweeper and a 2012 VacCon V311/1000 Combination Vactor Truck.
The significance of street sweeping frequency compared sediment accumulation in catch basins (and transport from catch basins) was evaluated by comparing the sediment accumulation rates during years one and two.
In semi-arid regions that have distinct wet and dry seasons, more frequent street sweeping during the dry season should reduce sediment transport to catch basins during the wet season. This is because rainfall events are less frequent and may not produce enough runoff energy to transfer a significant amount of sediment to catch basins. As such more frequent sweeping throughout the drier season can reduce the sediment accumulation on roads, which should reduce the sediment that could be transferred catch basins during the wet season.
While there was generally a decreasing trend in the sediment accumulation rates in catch basins and catch basin socks (washout) as the frequency of street sweeping between the test side and control side were statistically insignificant (p=0.95 and p=0.96 respectively). These results suggest that frequency of street sweeping performed within this study does not significantly reduce the rate of sediment accumulation in catch basins (and transported from catch basins). However, that does not mean that more frequent street sweeping does not have a significant effect on catch basin sediment accumulation rates.
Conclusions:
More frequent street sweeping during the dry season should be more effective in semi-arid regions that have distinct wet and dry seasons (Caraco, 2000). Low annual rainfall, droughts, high evaporation rates, and high intensity rainfall events are all characteristics of semi-arid locations. Ellensburg’s mean annual precipitation is 9-inches of which 25% is from snow (22-inches) and 67% of the precipitation occurs between October and March.
During the dry season (April to October) in a semi-arid location such as Ellensburg, rainfall events are less frequent and may not
produce enough runoff energy to transfer a significant amount of sediment to catch basins unless there is a high intensity rainfall event. As such more frequent sweeping throughout the drier season can reduce the sediment accumulation on roads, which will reduce the sediment that could be transferred catch basins during the wet season (Caraco, 2000).
A recommended schedule for catch basin cleaning based on the frequency of street sweeping was developed using the data collected from this study. Use the button below to access the PDF file of the entire study, including a host of supporting data and documents.
In the event you have questions or comments about this study, contact the City of Ellensburg Public Works Department. He may be reached via email sent to morrowj@ci.ellensburg.wa.us, or call 509.962.7201.
To reach Kidwell-Ross send to editor@
WorldSweeper.com.
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