Uncategorized

California Pavement: Drought, Flood Threats and Green Solutions

By January 2, 2015 No Comments

The heavy rains on the West Coast in mid-December 2014 presented a mixed bag for the millions of people who live and work there. On one hand, the ample precipitation may signal the end of a devastating three-year drought. But the damage incurred by flooding and mudslides – with incidences of pavement ripped up by flows of water and earth – means it was a destructive event as well.

Facts are that floods and droughts alike can stress and destroy pavement. But some new methods of road building might mitigate some of the effects of both.

The pre-Christmas storm system (called a “pineapple express,” as it is characterized by a stream of warm, moist air originating from the Hawaiian tropics) also caused a small tornado in downtown Los Angeles and blizzard-like conditions in the Sierra Nevadas. What eluded news coverage – but will nonetheless affect beleaguered motorists on the Left Coast for months to come – will be potholes. Millions and millions of potholes.

Yes, potholes in California. Freezing conditions and snowplows are not the only things that damage pavement. Moisture is the culprit here, either through flash floods or lingering puddles, both of which can undermine the asphalt sub-pavement. Some of America’s most potholed highways, roads and streets are in semi-tropical areas due to the other factors that damage asphalt and concrete: moisture, traffic, time and sometimes an extreme lack of moisture.

Indeed, drought itself can damage pavement. Where roads are constructed over fine-grain clay, aridity along with a drop in groundwater and desiccation by tree roots can result in soils pulling apart (shrinkage). When moisture is reintroduced, as with heavy rains, the soils swell in volume. The result can be pavement heave, which a 1973 study (Jones and Holtz) identified as costing $2.3 billion in the U.S. (considering inflation, that number is likely much higher today). Civil engineers design to account for this, but it nonetheless is a factor made worse by extreme conditions.

On roads climbing the foothills of the California mountain ranges, where temperatures drop below freezing, there will be potholes as well. Moisture in freeze-thaw temperature zones inevitably pries apart pavement cracks, allowing water to enter the sub-pavement. The areas where temperatures frequently fluctuate above and below 32 degrees (F) can be particularly affected.

California roads are ranked among the worst in the nation, based on 2011 survey of data compiled by the Federal Highway Administration. Among cities with a population of 500,000 or more, the Los Angeles-Long Beach-Santa Ana region was found to have the highest percentage (64%) of roads nationwide judged to be in poor condition.

Smart roads to mitigate floods and droughts?

But there is at least one point of hope where weather, climate and road building converge. That is where the design of roads can reduce flooding and actually capture stormwater for later use.

Think of it as the emergence of “smart streets.” For example, a $2.7 million retrofit in 2010 of an entire city block in Sun Valley, California was done to test a neighborhood-wide program that turns stormwater into an asset. It involved excavation to a depth of six feet under road pavement, which was then filled with gravel, leaving enough porosity to absorb water flowing from adjoining properties during rainstorms. Those properties were also reconfigured with rain gardens and rain barrels, which absorb initial rainfall off of roofs and other hard (non-permeable) surfaces such as sidewalks and driveways. Through these techniques, storm water that would otherwise flow via culverts to the city’s arroyo, a concrete-lined river bed, is absorbed into the ground onsite. Previously, it was allowed to wash into the Pacific, carrying oils and other pollutants to the Santa Monica Bay.

When water soaks into the ground it recharges the aquifer. Natural processes in the soil involving microorganisms and plants help remove pollutants such that the groundwater is cleansed and usable for various purposes (landscaping, industrial use and agriculture). The streets in the Sun Valley test project were formerly a site of frequent storm flooding. Studies on the use of such techniques show they work and can be cost-effective for both water capture and sometimes less expensive at installation than to increase the grey infrastructure (use of pipes).

Bioswales and rain gardens are still considered novel but are increasingly used across the country in flood-prone areas. Very often they line roadways or are positioned alongside sidewalks and parking lots, where the topography is typically formed to channel rainfall to roadside ditches and storm sewer systems. Instead, much more of that water is stopped short of the pavement and left to sink in naturally – as it had done for eons, before the hard surfaces of roads, parking lots and structures were built.

So California’s dual problems – droughts alternating with heavy rain – have a pavement connection. And the solutions might involve a somewhat more sophisticated approach to infrastructure that recreates what nature had devised in the first place.