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 the most fundamental thing I can think of is heavy rain would sooner cause road flooding with a central low spot, while have two sides of drainage provides double the drainage, plus any overcapacity will first flood a sidewalk.
Curious to hear what a civil engineer has to say.
That and to clean those central channels the crew would likely block the whole street instead of one side at a time.
I did consider this possiblity, but I figure that such a center drained road would necessarily have excavated more material from the road than a conventional crowned road, so the road itself sits lower than the nearby properties, and lower than a crowned road.
If done like this, the road necessarily has more “storage” capacity during floods than whatever could fit into the shallow side gutters of a crowned road. If the issue instead is clogged drains, then the center drains could be placed 2x more closely than side gutters, such that the total number of storm drains per km is identical.
IMO, flooding the sidewalk even slightly renders that facility unusable for its primary users. Whereas covering the middle of a center drained street with four inches (100 mm) of water that has a 2% cross slope (1/4-inch per ft) means that the water covers 16 ft (4.8 m) of the street, which on a typical 30 ft (9 m) residential street leaves two narrow but just usable strips on each side. Whereas on a conventional crowned street, only 3.75 inch of water would cause water on one side of the crown to spill to the other side, so 4 inches will have covered the whole street, edge to edge, impounded only by maybe the 6 inch tall curbs. I haven’t done the volumetric calculations, but I think the same curb on the center drained steet would impound much more water before encroaching in the sidewalk or adjacent properties.
This feels like the difference between cars vs pedestrians as the more important user. Having drains on each side means that even if there’s some good buildup of water, the crowned center means you’re more likely to be able to fit one car through, but you totally wipe out the sidewalks. I’m all for /c/fuckcars and would prefer sidewalk use, but I can see the other way winning in the US as roads are primarily designed for cars. Maybe dense city centers could prioritize sidewalks, but more rural stuff and stroads could prioritize roads.
The premise of my question is that it is a false dichotomy: there need not be tension between pedestrian and roadway users if the drainage system spread out the water over a larger area of already-paved surface. Everyone wins!
In a rural area where roads are already narrow, this wouldn’t work and I acknowledged that in the title, limiting the inquiry to urban/suburban. I agree 100% that rural country roads built with ditches are entirely appropriate, cheap, and allow natural absorption into the soil. But urban streets aren’t just souped-up country roads and need to be constructed for the built environment.
Whereas rural areas prioritize land, livestock, and the great outdoors, urban areas prioritize people. And that means pedestrian facilities are non-negotiable in my book.
Just my educated guesses as a layman, but I agree with what others are saying in that a center channel would lead to more flooding in the streets. Side drains won’t flood streets or homes (unless its an actual flood) as both are elevated above said drains. Side drains also give 2x the holding capacity which further helps reduce water pooling on the road.
Additionally, having it in the center of the road means any maintenance done on said drains is going to block both lanes of travel, and even if it doesnt, would put workers in a very dangerous working position.
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:
If the road were sloping towards drainage at the centre, then floodwater fills the entire road basin before overflowing, and your road is blocked
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.
People would run into the oncoming traffic on the other side more often if the lanes are slanted inwards. And you have a single point out failure for the drain in the middle (e.g. blockage). Leaves will travel. I’m not an engineer but I play one on Lemmy: you don’t want your road to get washed away in a storm. If the edges aren’t fortified with a predetermined escape for water away from the road’s foundation, you run a higher risk of erosion.
I’m not sure I understand the issue with fortifying the edges – urban roads are generally sealed asphalt, continuous from curb to curb, and I’m not suggesting any sort of exposed soil (except for storm water impoundment).
For the inward slope, I’m struggling to understand the issue, since road crowning is already very slight. A center drain would not require a noticeably visible slope, since the physics of rainwater are the same, just that it’s sloping toward the center instead of the edges.
The concern about single point of failure is noted, but I think it should be possible to halve the distance between center drains, since this would have an identical number of storm drains per km of road. Plus, unlike side gutters that are interrupted at every intersection due to crossroad crowning, a center gutter could spill over to a cross street’s center gutters; now a flood would have to clog all the drains in an area, which would be catastrophic no matter if center or side gutters.
I’m not sure I understand the issue with fortifying the edges
Imagine a stack of papers on your desk. Clumsy you knocks over your coffee cup. Coffee hits all the papers, gets between them, everything is ruined. Now imagine it’s got spine binding. The coffee will not get into all the pages. Now imagine, somewhat unrealistically, attached under the spine binding is a little halfpipe sunken in the desk. Most of the coffee now flows away on the pipe. The drainage acts a bit like an anchor keeping stuff in place on the side.
During heavy rainfall, intersections would become lakes in the center, where most traffic is going straight, at higher speed than turning, and nearest to opposing traffic.
The drainage needs to go into something hollow, having a hollow space beneath the part of the road that needs to support the most weight of vehicles means needing to create more load bearing hollow spaces than would otherwise be necessary.
I would question why road traffic is driving very fast in “heavy rainfall”, but the solution is the same: truncate the center gutter before the intersection, so that the intersection itself drains conventionally, into the center gutters on all approaches to the intersection. Alternatively, where it makes sense, use a center drained roundabout.
having a hollow space beneath the part of the road that needs to support the most weight of vehicles
My understanding is that there is only one storm drain pipe underneath a street, somewhere near the center, and each side of the road has laterals that connect to this pipe. There are not two pipes, one for each side of the street directly under the edges of the paved surface. If it were literally under the curb, service manholes would be impossible. See https://www.amwua.org/blog/only-rain-should-go-down-the-storm-drain (relevant image only shows up on desktop mode)
Storm drains, aqueducts, and swimming pools are typically made out of concrete because it has the right properties to deal with lots of water. Roads are typically paved with asphalt. Unlike concrete, asphalt is far more subject to damage from water.
One of the main purposes of crowned roads is to ensure that water flows off the asphalt quickly in thin sheets that don’t cause erosion. A heavy flow of water on asphalt causes too much erosion. Also, pooling or standing water seeps into asphalt and causes damage and structural problems. Especially in areas where the temperature gets low enough to freeze.
There are also other problems with V shaped roads, but avoiding water damage from pooling and erosion is the big one.
Slightly V shaped roads are used where cars don’t go, like in pedestrian shopping centers with outside walkways.
That would drastically increase the frequency of aquaplaning, the convention is to drain the water from the road surface, not collect it on the road.
Central draining would also cause more floods as the drain would need to be cleaned more often, as any clog would immediately result in a flood covering the entire road surface.
Why would a single clog immediately flood the entire road surface? Sure, the water might accumulate spanwise, but no street is perfectly flat, so water also flows longitudinally along the road. Water would pool until the next drain, no different than a side gutter being clogged. And at least there’s the possible benefit of unclogging the drain by driving directly over the drain.
First, if the road floods, that would push drain clogging debris into the road, not away from it, and while many roads can operate a slight clog at the side, it becomes more problematic at the center.
Secondly, if it does clog, regardless if it was from a flood or just litter flowing into it with normal rain, some one has to go unclog it.
That someone would then either have to shut down both directions or be at risk of being hit from both directions.
Third, with the way roads are constructed, it would be a lot more expensive to design the sewers to either tolerate the loads (imagine a big heavy truck,) or burry it deep enough that the load is distributed around it anyways.
Fourth, maintenance. If something happens where you need to dig up your sewer, putting it to the side means you’re not also digging up the road. The reverse is also true.
Fifth, in freezing conditions, you don’t really want the water pooling in the center… you want it to the side, at least before it freeze back into ice.
it becomes more problematic at the center.
This is precisely what I’m trying to understand: what gets more problematic? The driving? The civil engineering? What is the exact complexity that a center drain would introduce?
some one has to go unclog it.
Conventional drains along the curb also need to be unclogged manually, except that the public works dept needs to get people to move their parked cars, which can also hide the problem from being easily noticed in the first place.
either have to shut down both directions or be at risk of being hit from both directions.
From seeing how my town accesses manholes located in the middle of a two-way road, they arrange two heavy trucks in a row, one before and one after the manhole. On a multilane arterial, this is a minor traffic disruption of one lane. On a quiet residential street, people just go around slowly.
with the way roads are constructed, it would be a lot more expensive to design the sewers to either tolerate the loads (imagine a big heavy truck,)
I believe that roads are infact designed with the sewer and storm drain pipes directly below the traffic lanes. See my other comment. A cursory review of my town’s planning documents for a new road extension shows a cross section that has all longitudinal piping underneath the lanes, so that the manholes aren’t installed under sidewalks or the curb. I am open to seeing plans for other jurisdictions that build their pipes differently.
One of the reasons is flow control. You want to be able to move a lot of water slowly until it’s where you want it. Many small inlets distributed along the surface is a simple and effective method. I’m sure achieving the design you’re speculating on is possible but it would be more complex and expensive for that design to reliably achieve similar goals/metrics.
Permeable pavements like pervious concrete and porous asphalt allow some of what you’re interested in without requiring a reinvention of the wheel.
Wouldn’t it be easier to
I don’t think easiness is the main goal.
The thing is that it must work in all conditions. Always. If your idea fails only once a year, then your neighborhood is flooded every year. It fails twice or three times a year…
prevent leaves from falling directly into a drain and clogging it
Leaves that fall directly onto the drainage grate aren’t really the concern. The vast majority of leaves that end up there were washed there by the flowing water … and that won’t change much by changing the location of the grate. Once the water picks up a leaf, it will end up there sooner or later.
This is true. At the same time though, I’ve never seen a pile of leaves remain stationary in the middle of a street for very long, as every passing car or bike inevitably disturbs the leaves. Maybe delivery drivers could be encouraged to drive over the center drains so that piles of leaves are dislodged from around the drains.
Civil engineering is a very complex thing, which also has been around. A lot of the things they due is because of lessons learnt in the past. If you have questions like “Why don’t they just?”, I’m sure an actual civil engineer with knowledge of the situation can tell you exactly why. It’s all much more complex than you could imagine.
I’d suggest watching some of Grady on Practical Engineering on YouTube to get a basic idea of how much goes into projects such as designing and building a road. In my experience a road contains a lot of infrastructure around and underneath, way more than you’d assume.
By no means am I smart, but my thought goes to those who text and drive, drive sleepy, or drive intoxicated. At least the road slopes oncoming traffic away from you if they’re not paying attention. Would of course depend on of their car is aligned right or they don’t jerk the wheel.
