Climate & Cities
Snowmelt Arrives Faster Than The Drains Expect
A warm rain falling on accumulated snow releases stored water all at once, producing urban flooding from a storm whose rainfall total looks entirely unremarkable.
Urban drainage is designed against rainfall intensity, but the most damaging winter events combine modest rain with a snowpack already sitting on the ground. The stored water is what overwhelms the system.
Snow is a reservoir on the surface
Accumulated snow holds water that has not yet reached a drain. Depending on density, a snowpack can hold the equivalent of a significant rainfall in suspended form.
Cold weather keeps that reservoir locked. As long as temperatures stay below freezing, the water is effectively removed from the drainage system's workload.
A rapid thaw releases it. The drainage network then handles the melt and the rain simultaneously, which is a total the design storm never contemplated.
Frozen ground removes the soil's role
Green space normally absorbs a share of any storm. When the ground is frozen, infiltration falls sharply and lawns and parks behave more like pavement.
That means the city's effective impervious area rises in winter without anything being built. Drainage models based on annual averages understate the winter condition.
Rain-on-snow events are consequently sensitive to sequence. The same rainfall produces very different results depending on what happened in the preceding week.
Ice blocks the openings
Storm drain inlets are frequently obstructed during a thaw by ice, by plowed snow piled over them or by debris carried in the first flush of melt.
A blocked inlet sends water down the gutter to the next low point, concentrating flow in one place rather than distributing it across the network as designed.
Public works departments respond by clearing inlets ahead of a forecast thaw, which is inexpensive but depends on knowing where the critical ones are.
Salt travels with the melt
Deicing chemicals applied over a winter accumulate on surfaces and in snow piles. A thaw carries them into storm drains and from there into creeks and rivers.
Chloride does not break down in the way organic pollutants do, so it persists in receiving waters and accumulates in groundwater over long periods.
This has pushed some agencies toward reducing application rates, using brine before storms rather than salt after them, and calibrating spreaders more carefully.
Where the design assumption is shifting
Warming winters change the mix of precipitation in ways that are not simply less snow. More events fall near the freezing threshold, and that is exactly where rain-on-snow occurs.
Systems designed for a climate with a long stable freeze and a single spring melt face a pattern with repeated partial thaws instead.
Design storms are derived from historical records, so updating them requires agencies to formally revise the standards engineers work to. That process is under way in many places and moves slowly.
Questions readers ask
Do green roofs save energy?
Modestly, and mostly on poorly insulated buildings. On a well-insulated roof the thermal effect is small. Run-off attenuation is the benefit that reliably justifies them.
Are green roofs maintenance free?
No. They need outlet clearance, occasional weeding and inspection, and they need access to do it. Most failed installations were neglected rather than badly built.
