The sensational headlines started before the rain stopped, blaming Houston’s disastrous flooding on its lack of a zoning code.
This was about stormwater management design standards and the unintended consequences of federal policies to drain swamps. No political jurisdiction anywhere in the United States asks its engineers to design stormwater systems to handle a year’s worth of rain in a single storm without flooding. And what used to be called swamps have been targeted by society for dewatering since the 1600s. Three centuries later, over half of the wetlands in the lower 48 states are gone. The function of natural wetlands in absorbing stormwater to mitigate flooding was poorly understood by scientists or appreciated by the public.
Swamps were unproductive, a barrier to transportation and a source of disease bearing insects. The first of several Swamp Land Acts was passed by Congress in 1849, granting wetlands to the states for the purpose of conversion.
According to a history of wetlands from the US Geological Survey, “this legislation clearly set the tone that the Federal Government promoted wetland drainage and reclamation for settlement and development. This tone pervaded policy and land-use trends for the next century.”
Federal policy slowly started to turn in 1977, eventually leading to the Emergency Wetlands Protection Act of 1986. Requirements for mitigating loss of wetlands and stormwater management design standards were not widely adopted until the 1990s.
The original focus was on water quality, with the goal of removing pollutants from runoff before it reached underground aquifers. In eastern Washington, this was the beginning of the ubiquitous grassy swale in every new parking lot.
It’s a misconception that handling stormwater is the responsibility of the city or county, according to Mike Nilsson of the City of Spokane. Each property owner is required to retain and manage normal precipitation on their own lot. The city designs structures for water draining off streets. Individual parcels, roads and public paved areas are now designed to handle a 10-year storm event as the most likely risk.
Channels where water is concentrated are designed to a fifty year standard, FEMA regulated floodplains to a hundred year standard.
“Houston flooded because they got a year’s worth of rain in three days,” agreed Nilsson, an event that has been called a 500 or a 1000 year event.
Stormwater engineer Henry Allen with the City of Spokane Valley said the challenges with stormwater design are volume, time and topography. Allen said Houston’s biggest problem is it’s flat, and there’s nowhere for water to go rapidly “even if you have the best flood works in place.”
He noted Houston’s added challenges of concentrated rainfall and slow draining soil have made it a leader in stormwater modeling by necessity. But modeling just identifies the risk. Stormwater management balances risk against the cost of preparation.
All jurisdictions in this region use the “Stormwater Management Manual for Eastern Washington,” adopted in 2004 after a three-year process to tailor the Western Washington Manual for eastern Washington conditions. Those conditions vary between specific sites even if undeveloped.
Marianne Barrentine at Spokane County said “some areas of the County can keep up with the heavier rainfall events (sands), and others are over-saturated right away and the water runs off like asphalt within minutes (clay).”
Or in flat areas, water has no place to go and builds up into Houston scale floods as stormwater management design standards are exceeded. It’s all about physics.
For every best design practice, there’s an overwhelming worst case scenario. Catastrophic flooding in the Pacific Northwest won’t come from the remains of a hurricane stalled for three days. The engineers agreed our vulnerability is frozen soils with rain falling on accumulated snow, and the resulting rapid runoff overwhelming both designed and natural wetland systems.
In February 2014, decidedly un-urban Edwall experienced a hundred-year flood event under precisely those conditions. I used to watch reports of people who’d driven into a flash flood and comfortingly tell myself I’d never do that. Until I came around a corner and drove into water where there’d never been water before.
Edwall has zoning. It doesn’t have development. Sometimes a flood is just a flood.
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