More Bad Reports - Street Sweeping. Part 1

This is the second in our series on the “bad reports” coming from the City. No sooner did we finish a detailed analysis of the many flaws in the staff report on the traffic committee than the staff gave us another flawed report – this one on street sweeping.

According to the City street sweeping is important not only because it keeps our streets clean, but also because it protects the water system by reducing the amount of trash and debris that enters the sewer system and ultimately the ocean, and it can help reduce localized flooding by keeping the sewer drains unclogged. This localized flooding can be a big problem in several parts of Lake Forest. At Lake 1, for example, thousands of dollars of damage has been done to the clubhouse area as a result of flooding from overflowing storm drains.

INVALID HYPOTHESIS

The City believes that –

Water Quality = f (Urban Runoff) = f (Street Debris) = f (Blocked Streets)

In other words, water quality is impacted by urban runoff, which is impacted by street debris, which is a result of inadequate street sweeping because of streets blocked by cars which prevent the street sweepers from doing their job. Expressed in other words,

·   if there weren’t so many cars on the streets when street sweepers try to clean the streets,

·   the streets would be cleaner,

·   less trash and debris would reach the water shed,

·   and there would be less pollutants reaching the water shed,

·   and water quality  would improve.

Sounds right! Unfortunately there is almost no evidence that street sweeping improves water quality. A comprehensive review of the literature in 2006 concluded – “Although new street sweeping technology can remove more than 90% of street dirt under ideal conditions, it does not guarantee water quality improvements.”

More recently, Sutherland (2011) said that one reason for the lack of any relationship between street sweeping and improved water quality may be that “up to 50% of heavy metals and other pollutants of great concern like nutrients and toxics are attached to street dirt particles too small for most mechanical brush sweepers…”

In other words, almost all of the literature indicates that street sweeping will not reduce water pollutants. If this is the case, why does the City’s report stress improved water quality as a result of the street sweeping restrictions?

BAD THEORY

Not only is the fundamental hypothesis flawed, the theory itself has little empirical proof. The basic idea that reducing blockage will increase trash pick-up may not be a sound one. One of the few experiments to report data was conducted in Brea in 2009. After a restriction and ticketing program was instituted, the amount of debris collected actually declined, from 389 cubic yards in 2008 to 278 cubic yards in 2009 during the first month of the program, despite more than 1,000 tickets. In subsequent months, the effects of the program resulted in higher debris being collected in only 2 of 7 months.  Nor was there any indication of progress over time. Hence, this Brea study indicates that the main theory for the methodology employed by the City (studying blockage) may have no merit in fact.

Next time we’ll actually look at the City’s study and what they reported. Right now it’s sufficient to note that the hypothesis itself had no empirical justification (water quality can be improved by street sweeping) and the theory (amount of debris collected can be increased by reducing blockage) may not be correct either.

BTW – you may be thinking that the City’s hypothesis and theory make sense, so why does the data not support either one? That’s a good question, but after 40 years of scientific inquiry I’ve become accustomed to giving the benefit of the doubt to the data, not the theory. In their defense, at the City Council meeting, the City Manager admitted that street sweeping is a State requirement and offered no data to support the statements in the report that street sweeping actually results in improved water quality.