The convention in the USA for old urban centers and new suburban sprawl is to construct a street or road with a crown that drains rainwater to gutters along both sides of the road, then have storm drains to convey the water from the gutter to some nearby creek or tributary. But why?
Wouldn’t it be easier to construct the road in a roughly canal shape, so that rainwater drains towards a single V-shaped gutter at the road’s center? This would cut the number of storm drains by roughly half, prevent leaves from falling directly into a drain and clogging it, make it possible to clear a drain by driving a streetsweeper over it, and also prevent a clog from flooding adjacent properties, since the road itself can temporarily impound more water until municipal authorities can clear the blockage (whereas side gutters would invariably flood the sidewalk and carry sharp debris that would damage tires entering a driveway).
Furthermore, a center drain can be built once and then retained as-is each time a suburban arterial needs expanding – “just one more lane, bro” – whereas side gutters are regularly demolished and rebuilt to accommodate additional lanes. By routing water away from the edges of the road, sidewalks avoid freeze/thaw cycles, and the road surfacing can be continuous from the curb: no more bike lanes in the gutter. As a convenient benefit, the “drop” off at a curb-cut from a driveway to street level would cease to exist.
And where required to improve water quality due to runoff pollution, a center drain can be excavated and rebuilt as a linear stormwater retention pond, where moderate stormwater can filter into the local soil slowly, with a predefined overflow level that will drain to the existing stormdrain pipes. This is already done for both surface parking lots as well as Interstate highways, so it’s not an unproven design.
Narrow alleyways in older cities do use a central drain, so I can’t see why the idea stops making sense for larger streets and roads. The only drawbacks I can envision are aesthetic – a neighbor’s excessive lawn irrigation would draw a wet line across half the street – and that the center channel would also carry leaves and wayward soccer balls into the middle.
But even still, that doesn’t seem worse than the status quo: gutters attract all sorts of detritus, but it’s usually hidden beneath the wheels of parked cars until something punctures a tire. And at least in water-starved California, irrigation runoff deserves to be noticed and called out so that it gets fixed. There may even be some small road safety benefit from having a V-shape channel in the center, since it would unmistakably divide opposite sides of the street.
For larger arterial roads that have trees in the center, this seems like free irrigation and water pollution control. It even works when the center traffic lanes are converted for running a tram or light rail train.
What am I missing here?
I’m not a civil engineer but did some civil engineering in university and have a bit of an interest in urban planning. Still, I’m mostly guessing.
Regardless of where storm drains are placed on a street, they don’t have infinite capacity. In a heavy enough rainfall event, they will flood. The placement or frequency of drains isn’t the limiting factor, it’s the outlet capacity. In nature, rain falls on the ground and is absorbed, and finds its way into the watercourse slowly. In urban areas, storm drains gather up rainfall over a wide area and channel it towards the watercourse much more quickly, and if that outflow isn’t controlled it can lead to catastrophic flooding and erosion downstream. Look up 1952’s Hurricane Hazel and its effects on Toronto for what that looks like. If more rainfall flows into the system than the outlet can handle, there’s nowhere else for the water to go and the drains flood.
With a crowned profile and drainage at the sides, excess water will pool only up to the height of the gutter before overflowing and finding ground or another low spot away from the road, so the road will only be partially obstructed at least until the flooding is much worse (and by then you’re probably well into an evacuation). If the road were sloping towards drainage at the centre, then floodwater fills the entire road basin before overflowing, and your road is blocked, and stays blocked until the drains can take the water again, long after the rain has stopped. Blocked roads make managing an emergency much more complicated.
I agree with everything up until this point:
I’m imagining the scenario of a residential suburban neighborhood, on mostly flat land. With moderate rain and fallen leaves from the Autumn, some storm drains will clog and need manual intervention. If this neighborhood were built with conventional crowning, then the properties unlucky enough to be next to a clogged drain will see some flooding, but other homes in the neighborhood will have no issue.
If instead the entire neighborhood used center gutters with center drains, but spaced the drains closer together so the drains/km is identical to a conventional build, then the same rainfall should cause smaller impacts, because: 1) a single clogged drain will only flood a small patch of the road, until 2) reaching the next open drain, which is closer and thus the flood is a smaller area, and 3) does not disadvantage the unlucky property immediately adjacent to the clog, since the flooding is concentrated at the road center.
In both constructions, the road area is identical, the rainfall is identical, and the storm drain capacity is identical, yet the latter needs only one half the linear gutter distance and can spread out the flood risk across the whole neighborhood. Phrased another way, center gutters should stave off flood damage to any property, until such point that the drainage is simply overloaded and then every property would flood. No more “lucky” or “unlucky” neighbors: either everyone is safe or they all need to evacuate.
That’s the situation in a flat neighborhood, but in a sloped neighborhood, center gutters aren’t any worse: the most critical drains are at the bottom of the neighborhood. If those fail, it’s still game over for those adjacent homes. And that still is the case for side gutters anyway.