Thursday, April 30, 2015

Stationarity Is Dead

By Kathleen Sayce

When civil engineers, architects and planners design buildings, roads, bridges, levees, dams, drainage canals and other structures, they use the principle of stationarity to decide how high, how strong, how wind resistant, this structure has to be to withstand a typical 50-year, 100-year, 500-year or 1000-year event.

All construction balances on a line between built 'strong enough' and 'over-built too much' to keep the cost as low as possible. The stationarity principle has historically ensured that the structure will last for its planned life, which may be anywhere from twenty years to several hundred years.

To settle on the event standards to design to, professionals refer back to applicable weather metrics and disaster occurrence histories, including high and low temperatures, rainfall, stream flows, floods, snow falls, wind storms, tornadoes, droughts, and earthquakes.

A recent rain burst in the west Willapa Hills overloaded a culvert on Peter's Creek that runs under Highway 4 near Naselle. In the background you can see flagging on the washout edges; the highway pavement is at the very top of the image. Photo by Kathleen Sayce.

One of the reasons modern cultures measure weather events is to provide metrics for infrastructure and building designers, planners and insurance agents. Building codes also emerged, to set minimum standards that ensure a building will not flood, catch fire or blow down during normal events, and will stay in good condition for its design life.

Several years ago a national science magazine ran an editorial which stated that the concept of stationarity was dead [I did not think of this title, I borrowed it from that article]. The authors are engineers, who explained that when a river community had three thousand-year flood events in five years, it was time to redefine a one-thousand-year event. That it was past time to reevaluate appropriate event standards with a new, broader measure of caution. Five percent (higher, wider, stronger) might not be enough anymore. Twenty-five percent might be better, or in some instances, fifty percent. [1 February 2008, P.D.C Milly et al, access via]

This washout on Peter's Creek occurred when blockage in the culvert due to debris coincided with a rain burst. Water built up behind the highway levee and pushed through, washing out the soil and road surface above the old culvert. Photo by Kathleen Sayce. 

Change goes on around us all the time, both in our culture and in the natural world. In the past three decades, local air temperature measurements changed. Plant growing zones are defined by winter low temperatures, and have been shifting steadily warmer for many locations. Fifty years ago, the South Coast of Washington was defined as a region 7 growing area, with winter low temperatures between 0 and 10° F. Today this same geographic area is considered zone 8, with lows between 10 and 20 °F. Similar changes have happened for many areas.

Along with warming winter temperatures, we’ve seen higher summer temperatures. In 2012, in just one hot spell, over one thousand high temperature records were broken in the Midwest. Many locations set new records day after day, until the heat wave finally quit. New records for consecutive days over 100 °F were also set.

If you are an engineer working on cooling systems, you have to design for increased cooling capacity. Otherwise, the cooling system will never work properly. Ditto on insulation and heating standards, stormwater, and roof snow loads.

The Astoria-Megler Bridge was designed in the 1960s. At the time, the design standards based on then-current stationarity guidelines looked pretty good. But now, knowing about local earthquakes and tsunamis, with ships four to five times larger and ten times heavier, with longer, heavier commercial trucks, and heavier passenger vehicles, the bridge is woefully under-designed. A new bridge in this location today would be designed to a new standard.

No one thinks this bridge is in eminent danger of collapse––that is not the point. The point is that data about traffic loads, weather extremes, wind loading, and seismic events has changed. The degree of uncertainty about event severity that can be expected has also changed. Stationarity has changed.

For a house, this means more insulation, stronger framing, a tougher roof, a higher foundation or a location on higher ground. For a road crossing a river, it may mean a larger culvert or stronger, higher bridge, along with higher road levels and deeper ditches to each side. For a stormwater system, it means more capacity.

Last winter, a rain burst in the west Willapa Hills flooded South Bend, overloaded culverts that drained west to the bay from there south to Naselle, and blew out a culvert on Peter's Creek in Naselle, taking out a section of Highway 4. Part of the flooding was due to blockages in culverts, and part to culverts that were faced with water flows well beyond their design capacity.

The new box culvert on Peter's Creek will look much like this one––a box culvert under Highway 101 that drains Chinook Marsh to Baker's Bay. This is a small bridge, and the new one is being designed now. Note where the dirty concrete begins; this is how high the water gets in this culvert on a regular basis. Photo by Kathleen Sayce

The culvert that formerly ran under Highway 4 will be replaced with an open box culvert, which means that there will now be a small bridge where there once was a corrugated pipe. Engineers are designing it now. In South Bend, storm drains were cleaned out, and their capacity will probably be reviewed.

A change in stationarity means, when constructing anything––a road, a culvert or bridge, a home, or other structures––it's time to let go of thinking that we know what might happen based on the past, and design instead for the next increment stronger, windier, colder, hotter, wetter, to be appropriate for that structure and location. The problem with our time is that the weather is not what it used to be, and our old stationarity standards need to be reset.

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