How to handle water runoff on a road sloped in two directions?
October 3, 2023 4:29 PM Subscribe
Help us solve an engineering problem. My neighbors and I share use of a 25’ wide private road that is sloped in two directions. Here’s a handy diagram. How do we mitigate water erosion and icing when the two slopes cause water to flow all the way across and all the way down the road?
The road is higher on the north side and slopes down to the south side. The road is also higher on the west end and slopes down to the east end. On its way to the city drain, water from the gutters and sump pumps of the north-side houses crosses both the width of the road and some length of it. Water from houses at the west-end crosses the length of the road and some of the width. Being in New Hampshire, this means the entire road is covered with ice a few months a year because of the freeze-thaw-freeze cycle.
The road needs to be repaved soon and it’s an ideal time to figure out how to run this water to the drain effectively. How to do this?
Some constraints:
- We all use the road on a daily basis because there is no overnight city parking allowed. We can’t just not use the road.
- All water needs to reach that city drain from above ground. We can’t tie into the city drain below ground.
- Each house’s runoff has nowhere else to go but onto our road. We can’t tie into a city sewer below ground.
- Each house already has a dry well and mitigates some water on its property. No one’s being a dick about their water.
- Water is allowed to flow across city property (the sidewalk between our road and the drain) but it cannot be pumped across city property.
- Gravel is not an option because the road needs to be snow-plowable.
- Permeable pavers seem great (recycled plastic, permeable surface) and possibly affordable because we’re a group. However, there are gas, water, and sewer lines below the road. In the past year all of those have been worked on at one time or another. They pave asphalt over their work. My understanding is that permeable pavers would basically get destroyed by utility work.
- We have called six civil engineers and cannot get call backs from any. We’ll have final plans reviewed somehow, but we want to start kicking around ideas.
We are also a friendly group, so we’re open to sharing the cost of solutions that accommodate one house on paper but actually benefit all. Some ideas we’ve floated:
- Permeable pavers (but see above)
- Dry wells beneath the road itself. But how does that water get from below ground to the city drain?
- Driveway channels/drains. But then all that water still needs to cross either the width of the road or the city sidewalk (or both) to reach the drain.
- Grading the road more steeply in both directions, hoping the water will flow faster off of the road and not have time to freeze. (None of us think that’s realistic).
One paving company that looked at the job said they can grade the road to eliminate icing, but they couldn’t really explain how. Would grading alone handle this? If so, how exactly? I guess we’re skeptical.
The road is higher on the north side and slopes down to the south side. The road is also higher on the west end and slopes down to the east end. On its way to the city drain, water from the gutters and sump pumps of the north-side houses crosses both the width of the road and some length of it. Water from houses at the west-end crosses the length of the road and some of the width. Being in New Hampshire, this means the entire road is covered with ice a few months a year because of the freeze-thaw-freeze cycle.
The road needs to be repaved soon and it’s an ideal time to figure out how to run this water to the drain effectively. How to do this?
Some constraints:
- We all use the road on a daily basis because there is no overnight city parking allowed. We can’t just not use the road.
- All water needs to reach that city drain from above ground. We can’t tie into the city drain below ground.
- Each house’s runoff has nowhere else to go but onto our road. We can’t tie into a city sewer below ground.
- Each house already has a dry well and mitigates some water on its property. No one’s being a dick about their water.
- Water is allowed to flow across city property (the sidewalk between our road and the drain) but it cannot be pumped across city property.
- Gravel is not an option because the road needs to be snow-plowable.
- Permeable pavers seem great (recycled plastic, permeable surface) and possibly affordable because we’re a group. However, there are gas, water, and sewer lines below the road. In the past year all of those have been worked on at one time or another. They pave asphalt over their work. My understanding is that permeable pavers would basically get destroyed by utility work.
- We have called six civil engineers and cannot get call backs from any. We’ll have final plans reviewed somehow, but we want to start kicking around ideas.
We are also a friendly group, so we’re open to sharing the cost of solutions that accommodate one house on paper but actually benefit all. Some ideas we’ve floated:
- Permeable pavers (but see above)
- Dry wells beneath the road itself. But how does that water get from below ground to the city drain?
- Driveway channels/drains. But then all that water still needs to cross either the width of the road or the city sidewalk (or both) to reach the drain.
- Grading the road more steeply in both directions, hoping the water will flow faster off of the road and not have time to freeze. (None of us think that’s realistic).
One paving company that looked at the job said they can grade the road to eliminate icing, but they couldn’t really explain how. Would grading alone handle this? If so, how exactly? I guess we’re skeptical.
The usual way this would be handled is with ditches/swales on either side of the road and pipes under the road to carry the collected water across to the other side. If you can't dig down enough you raise the road. The drainage pipes keep the road from becoming an inadvertent dam and flooding the properties that currently drain across the road.
Depending on how cold it actually gets it may be possible for regrading the road to reduce the amount of ice that forms, but you'll probably still get at least some ice some of the time.
posted by wierdo at 4:36 PM on October 3, 2023 [5 favorites]
Depending on how cold it actually gets it may be possible for regrading the road to reduce the amount of ice that forms, but you'll probably still get at least some ice some of the time.
posted by wierdo at 4:36 PM on October 3, 2023 [5 favorites]
Yes, you absolutely should have ditches/swales on at least the south side, preferably both sides of the road. From your drawing, though, it seems you may not have room for that.
The options that will work for you are going to depend on factors like the slope in both directions, the contributing watershed area, and the precipitation patterns (e.g. what's the most rain that you get in a 24 hour period, in the average year?).
We could give more suggestions with more data, but there are going to be lots of contractors in your area that are familiar with local conditions, and have suggested solutions. Knowing what is customary in the area will also tell you what local contractors know how to build. It's worth talking with some local GCs and/or landscape architects to get their suggestions.
posted by agentofselection at 5:49 PM on October 3, 2023 [3 favorites]
The options that will work for you are going to depend on factors like the slope in both directions, the contributing watershed area, and the precipitation patterns (e.g. what's the most rain that you get in a 24 hour period, in the average year?).
We could give more suggestions with more data, but there are going to be lots of contractors in your area that are familiar with local conditions, and have suggested solutions. Knowing what is customary in the area will also tell you what local contractors know how to build. It's worth talking with some local GCs and/or landscape architects to get their suggestions.
posted by agentofselection at 5:49 PM on October 3, 2023 [3 favorites]
Our city is re-doing a lot of streets for water management, and as I understand it, that includes reordering the water, gas and electricity lines and constructing swales. Some of these swales are "underground" which means they are open but covered with gridirons, and others are open, meaning they are green, managed with plants that can handle the seasons. Finally some have a narrow above-ground concrete swale, supported by a green area that can handle extreme events. I think you are right that the permeable pavers can't handle the stress, but old-fashioned cobblestones would be good, as would grass reinforcement blocks.
In your case, I think you could do with a swale/ditch on just the south side, because of the slope that way, since you don't have a lot of space. It would be very pretty with a green solution, but could you fit all the cars on the north side?
The water finally just goes under the city sidewalk in a tube, no problem.
Since the engineers aren't replying, could you find a landscape designer? They are the real experts here, because they do cross-disciplinary work and should know a lot of different solutions.
Also, that diagram is very good.
posted by mumimor at 6:14 PM on October 3, 2023 [1 favorite]
In your case, I think you could do with a swale/ditch on just the south side, because of the slope that way, since you don't have a lot of space. It would be very pretty with a green solution, but could you fit all the cars on the north side?
The water finally just goes under the city sidewalk in a tube, no problem.
Since the engineers aren't replying, could you find a landscape designer? They are the real experts here, because they do cross-disciplinary work and should know a lot of different solutions.
Also, that diagram is very good.
posted by mumimor at 6:14 PM on October 3, 2023 [1 favorite]
We installed permeable pavers in our puddle-prone back garden, and they have corrected the problem beautifully, but I'm not sure they would alone be up to the task of absorbing large-volume run-off from several houses. They could reduce the volume of run-off that reaches the street, but for an integrated solution that solves your drainage issues I, too, recommend a landscape architect, preferably one that works with water drainage.
When we installed our pavers we received a partial subsidy from our city, totaling $2000 per property, which was aimed at multi-pronged efforts to control run-off to the city sewers, frequently overwhelmed during hard rains. Maybe there's something like that in your area?
posted by citygirl at 7:49 PM on October 3, 2023 [1 favorite]
When we installed our pavers we received a partial subsidy from our city, totaling $2000 per property, which was aimed at multi-pronged efforts to control run-off to the city sewers, frequently overwhelmed during hard rains. Maybe there's something like that in your area?
posted by citygirl at 7:49 PM on October 3, 2023 [1 favorite]
Best answer: If the overall topography is downhill toward the southeast, the road might work better insloped with a gutter and embankment on the north (upslope) side. Curb and gutter on the downslope side tends to create runoff from the street through driveway aprons into front yards. You will want a cross-slope of 3-5% to drain water from the travel lanes into the gutter, ideally just one as 25' is a bit narrow to include gutters on both sides. Existing underground utility infrastructure may impede regrading the cross-slope.
If the overall grade was at least 4% downhill toward the east, you would just have to smooth out the flat spots to keep water flowing toward the drain. If the grade is merely 1-2%, yes, a film of water will stick to the surface and freeze. A coarse chip seal would work better than a fine slurry seal here to maintain winter traction.
Porous asphalt or pervious concrete would also help keep the road surface dry, but I question whether they would work well in conjunction with your underground utilties. My understanding is that the weaker surface layer requires an entirely different roadbed underlain by geotextile. Unless your road is so destroyed that you need to grind up whatever's left and reconstruct the subgrade, you are probably better off adding another layer on top of the existing impervious surface. The EPA has permeable pavement resources specific to New England, though some of the links are stale.
Bioswales will require both more width than you have available alongside your road and an underground connection to the storm sewer that your city would not approve. If you have sump pumps, attempting to infiltrate that water back into the ground instead of channeling it away above ground seems counterproductive.
On reflection, if "downhill" means something like 0.1%, you can slope your own road ~4% toward the gutter then use some kind of permeable paving within the gutter trough itself without compromising the structural integrity of the remainder of the road. Whether this still requires a membrane to prevent water from undermining the road is a question for a New Hampshire-licensed engineer.
I may be misinterpreting something, such as whether you are literally driving across the sidewalk to get to the public street or whether there's an at-grade intersection or barricade. My visualization and interpretation of your situation may be inaccurate. If so, my apologies.
posted by backwoods at 1:29 AM on October 4, 2023 [1 favorite]
If the overall grade was at least 4% downhill toward the east, you would just have to smooth out the flat spots to keep water flowing toward the drain. If the grade is merely 1-2%, yes, a film of water will stick to the surface and freeze. A coarse chip seal would work better than a fine slurry seal here to maintain winter traction.
Porous asphalt or pervious concrete would also help keep the road surface dry, but I question whether they would work well in conjunction with your underground utilties. My understanding is that the weaker surface layer requires an entirely different roadbed underlain by geotextile. Unless your road is so destroyed that you need to grind up whatever's left and reconstruct the subgrade, you are probably better off adding another layer on top of the existing impervious surface. The EPA has permeable pavement resources specific to New England, though some of the links are stale.
Bioswales will require both more width than you have available alongside your road and an underground connection to the storm sewer that your city would not approve. If you have sump pumps, attempting to infiltrate that water back into the ground instead of channeling it away above ground seems counterproductive.
On reflection, if "downhill" means something like 0.1%, you can slope your own road ~4% toward the gutter then use some kind of permeable paving within the gutter trough itself without compromising the structural integrity of the remainder of the road. Whether this still requires a membrane to prevent water from undermining the road is a question for a New Hampshire-licensed engineer.
I may be misinterpreting something, such as whether you are literally driving across the sidewalk to get to the public street or whether there's an at-grade intersection or barricade. My visualization and interpretation of your situation may be inaccurate. If so, my apologies.
posted by backwoods at 1:29 AM on October 4, 2023 [1 favorite]
Mod note: One comment removed. Just a gentle reminder that the OP does mention that they're in the state of New Hampshire, in the United States.
posted by Brandon Blatcher (staff) at 6:03 AM on October 4, 2023
posted by Brandon Blatcher (staff) at 6:03 AM on October 4, 2023
Could you grade the road so that the lowest point is in the center, thus creating a channel for the water to run in meaning the water a) doesn't pool on the South side and b) moves faster therefore is less likely to freeze?
(the cross section would look something like an obtuse checkmark)
posted by The 10th Regiment of Foot at 6:28 AM on October 4, 2023
(the cross section would look something like an obtuse checkmark)
posted by The 10th Regiment of Foot at 6:28 AM on October 4, 2023
Can you create a ditch that runs between the houses and the road, parallel to the road, preventing the water from going on the road. Then when it needs to cross the bottom of your road can you put a shallow ditch with a metal cover, like this
You would likely also need to have the same kind of trench grate over the ditch for each driveway but the rest of the ditch could be open, and possibly planted with Siberian Iris, or other water loving plants.
Your diagram doesn't show enough space for a feature like that, but I assume the houses don't actually front directly onto the street. If they do, then can you move the road back two feet or narrow it to make room for the drainage ditch?
posted by Jane the Brown at 8:56 AM on October 4, 2023
You would likely also need to have the same kind of trench grate over the ditch for each driveway but the rest of the ditch could be open, and possibly planted with Siberian Iris, or other water loving plants.
Your diagram doesn't show enough space for a feature like that, but I assume the houses don't actually front directly onto the street. If they do, then can you move the road back two feet or narrow it to make room for the drainage ditch?
posted by Jane the Brown at 8:56 AM on October 4, 2023
Response by poster: Thanks for all the thoughtful input. To answer some questions that might clarify options... (plus a new diagram for those that like them):
- 24 hr. rain totals - Since last November (the start of icing season) there's been at least one day a month until April (the end of icing season) where the total is over an inch and more often 1.2, 1.4". We also get an occasional +3" rainfall in summer that these ideas would help with.
- We hadn't considered permeable or pervious solutions for just the long (north and/or south) sides, which is an idea. If we could align those outside of the diggable area for utilities, that could certainly alleviate some flow.
- There actually might be enough room for a swale between the north-side houses and the road. There are not formal sidewalks, but there are skirtings on each property that we might be able to work into one coherent swale or drainage area. However, for reference one house already pumps their overflow onto a gravel area about 36 sf. and it's not enough to accommodate all their water, but that's more a problem with hard summer rains.
- I'm not sure of the current grades, but having those suggestions of 4%, 5%, are helpful to envision or float with paving companies. Any utility patching would not respect the grading, but with the added swales and drainage channels, maybe it would all work enough.
- Consulting a landscape designer never crossed our minds so that's a great suggestion
- We did consider a center channel (the "obtuse checkmark") because it's one option that doesn't just send water down to the south-side houses faster. But that's over the utilities and would probably get dug up for some reason or another within a year or two.
- Still wondering if there's a better way to get the water over the public sidewalk. We can't run a pipe under it, or underground to join the city drain.
- The new diagram/current envisioning of the various proposals, not including the grading %s. (This is quickly looking like one of the nicest and most expensive little roads in our town.)
posted by KneeHiSocks at 11:10 AM on October 4, 2023 [2 favorites]
- 24 hr. rain totals - Since last November (the start of icing season) there's been at least one day a month until April (the end of icing season) where the total is over an inch and more often 1.2, 1.4". We also get an occasional +3" rainfall in summer that these ideas would help with.
- We hadn't considered permeable or pervious solutions for just the long (north and/or south) sides, which is an idea. If we could align those outside of the diggable area for utilities, that could certainly alleviate some flow.
- There actually might be enough room for a swale between the north-side houses and the road. There are not formal sidewalks, but there are skirtings on each property that we might be able to work into one coherent swale or drainage area. However, for reference one house already pumps their overflow onto a gravel area about 36 sf. and it's not enough to accommodate all their water, but that's more a problem with hard summer rains.
- I'm not sure of the current grades, but having those suggestions of 4%, 5%, are helpful to envision or float with paving companies. Any utility patching would not respect the grading, but with the added swales and drainage channels, maybe it would all work enough.
- Consulting a landscape designer never crossed our minds so that's a great suggestion
- We did consider a center channel (the "obtuse checkmark") because it's one option that doesn't just send water down to the south-side houses faster. But that's over the utilities and would probably get dug up for some reason or another within a year or two.
- Still wondering if there's a better way to get the water over the public sidewalk. We can't run a pipe under it, or underground to join the city drain.
- The new diagram/current envisioning of the various proposals, not including the grading %s. (This is quickly looking like one of the nicest and most expensive little roads in our town.)
posted by KneeHiSocks at 11:10 AM on October 4, 2023 [2 favorites]
OH MY GOD YOU MAKE THE BEST DIAGRAMS
posted by kbanas at 4:45 PM on October 4, 2023 [2 favorites]
posted by kbanas at 4:45 PM on October 4, 2023 [2 favorites]
Okay, one question for you. You live in New Hampshire, yes? And you get snow? Has any of the problem been caused by accumulated snow directing the water onto the road? Is this idea still going to work if you end up with a rampart of snow in front of the houses, when the snow gets ploughed?
It definitely could get expensive... but that may be necessary in order to prevent the ice. If it looks good too, that is a bonus.
posted by Jane the Brown at 5:07 PM on October 4, 2023
It definitely could get expensive... but that may be necessary in order to prevent the ice. If it looks good too, that is a bonus.
posted by Jane the Brown at 5:07 PM on October 4, 2023
Can you rip up channels where your gutters drain, put them underground with permeable pipe (french drain) with gravel underneath? Utilities are generally buried far deeper than you would need to dig - in the 2-3 foot range. People where I live dig undergound piping under the sidewalk to the curb, which the city don't complain about, but we've got the grass strip thing that makes digging under a sidewalk pretty easy. If the driveway area is asphalt instead of poured concrete, this wouldn't be that expensive to do.
The other option would be to install a heated mat under your driveway space, if you are not concerned about water but rather about ice potential. They are somewhat expensive, but you have the advantage of several neighbors to split the costs with.
posted by The_Vegetables at 8:56 AM on October 5, 2023
The other option would be to install a heated mat under your driveway space, if you are not concerned about water but rather about ice potential. They are somewhat expensive, but you have the advantage of several neighbors to split the costs with.
posted by The_Vegetables at 8:56 AM on October 5, 2023
Forgot to mention, the only drain for the french drain system would be in the bottom corner, adjacent to the city drain, where water would overflow if the rainfall was heavier than what could be collected and retained in the underground french drains.
posted by The_Vegetables at 9:10 AM on October 5, 2023
posted by The_Vegetables at 9:10 AM on October 5, 2023
Best answer: Snowmelt atop frozen or saturated ground necessitates a gutter/ditch/swale on the upslope side of the road. If the other side of the road is downslope, it will not need a drainage ditch if you cross-slope the road against the natural grade into the ditch on the upslope side. If the topography is gentle and the road is crowned, you will want drainage channels on both sides. It's possible to cram 30" gutters and 10' travel lanes into a 25' private road easement, just not ideal.
If the gradient along the length of your road down to the city drain is less than 0.5%, this sort of drainage channel may be insufficient, and you might prefer something like a bioswale designed to infiltrate rather than convey stormwater. I don't think this can be designed effectively within that narrow of an easement. If the property owners on the north side want to collectively give up part of their front yards to reduce the impacts on the entire block, that's great, but it will require legal agreements to make sure whoever moves in next will cooperate.
I approached the initial question as a civil engineering challenge to transport water from the cul-de-sac into the city drain. So I had grandiose solutions like a jet nozzle spraying parabolic arcs over the sidewalk into the grate. Later I realized the city's objection to underground interconnections probably related to its MS4 permit for discharge into WOTUS. Your city engineer/public works department can clarify if the constraint indeed derives from the Clean Water Act and what sort of workaround would be permissible, given that they probably don't want stormwater scouring their sidewalk any more than you do.
Stormwater design is based on rainfall intensity over a period that depends on topography. If you were on the side of a mountain, there would be no difficulty conveying water down toward the river, and the challenge would be preventing flash floods. If you were in a basin, one inch in half an hour might just soak into the ground, while sustained rain over a longer period might turn lawns into ponds rather than running off toward the stream. Calculating the rise/run of your road with a laser level or just eyeballing it with a spirit level from below would make it easier to give specific advice. My suggestions influenced by my own landscape may not pertain.
posted by backwoods at 11:56 AM on October 5, 2023 [1 favorite]
If the gradient along the length of your road down to the city drain is less than 0.5%, this sort of drainage channel may be insufficient, and you might prefer something like a bioswale designed to infiltrate rather than convey stormwater. I don't think this can be designed effectively within that narrow of an easement. If the property owners on the north side want to collectively give up part of their front yards to reduce the impacts on the entire block, that's great, but it will require legal agreements to make sure whoever moves in next will cooperate.
I approached the initial question as a civil engineering challenge to transport water from the cul-de-sac into the city drain. So I had grandiose solutions like a jet nozzle spraying parabolic arcs over the sidewalk into the grate. Later I realized the city's objection to underground interconnections probably related to its MS4 permit for discharge into WOTUS. Your city engineer/public works department can clarify if the constraint indeed derives from the Clean Water Act and what sort of workaround would be permissible, given that they probably don't want stormwater scouring their sidewalk any more than you do.
Stormwater design is based on rainfall intensity over a period that depends on topography. If you were on the side of a mountain, there would be no difficulty conveying water down toward the river, and the challenge would be preventing flash floods. If you were in a basin, one inch in half an hour might just soak into the ground, while sustained rain over a longer period might turn lawns into ponds rather than running off toward the stream. Calculating the rise/run of your road with a laser level or just eyeballing it with a spirit level from below would make it easier to give specific advice. My suggestions influenced by my own landscape may not pertain.
posted by backwoods at 11:56 AM on October 5, 2023 [1 favorite]
If you don't have access to a transit or laser level, a line level and a hundred feet of string will let you fake it pretty effectively for a couple bucks.
posted by agentofselection at 8:21 AM on October 6, 2023
posted by agentofselection at 8:21 AM on October 6, 2023
You are not logged in, either login or create an account to post comments
posted by kbanas at 4:31 PM on October 3, 2023 [12 favorites]