"U" shaped swales vs "V" shaped - which is better

Hi All,

Was wondering if any of you have experience in which swale is better over all? A "U" shaped swale with steep walls, that would be filled with organic matter? (Think dug out with excavator) Or a broader "V" shaped swale that would not necessarily be filled, but rather could / would be planted? (Think ditch carved out with bulldozer)
I have seen both but have not implemented swales on my property as yet and now that Im in the planning stage, would love to hear feedback from personal experience on pros and cons of one verse the other.
My Thinking is that the broader "V" shape would capture more water, however, the steeper walled "U" shape with organic matter would help feed the plants on the berm and retain moisture longer.

Any feedback would be appreciated.

Thanks

I think a "U" shape filled with organic matter will greatly reduce the ability of the swale to spread the excess water across the swale. Filling it with organic matter would at least slow down the spread of that water across the swale. Depending on what you have uphill from the swale this may make little difference to you. However if you had a high concentration of water that you know is going to hit a small specific  part of the swale and you intend for that water to be spread the length of the swale or you want that swale to overflow/fill a damn than you might want to go with a V shape so that organic matter does not obstruct the flow of water.

If water infilitrates really quickly in the bottom of your swale and you're not planning to reduce the infiltration your choice in swale type may make little difference as the water might never make it to your overflow with either style.

A V shape will concentrate any flow there is in the center and if there is frequent dampness, you may get a soggy line which would make ruts with any traffic. A U shape, especially filled with rich soil, will spread the flow and let the ground stay relatively firmer than the center of a V. Whether either will let water flow laterally depends on the width and depth relative to the rainfall and distance between swales on the slope. In other words "it depends". Do you want planting areas along the swale? How permeable is the subsoil? How moist is the soil and the climate generally?

I have a lot clay in my subsoil. Re planting, I would like to plant the berms however, in the actual swale, i would probably seed with a cover like clover in a broad v shaped swale. If the swale was more narrow / steeper 'U' shaped, I dont know.
Havent figured that out yet, hence the post looking for advise

It seems you are using V and U more to describe the character of the sides and width of the swale than the center. I can not imagine any benefit to making a swale with steep sides distinct from the ground on either side; I would always make them gradually sloping from the existing hillside so a vehicle or tools could go anywhere without danger or hindrance. How wide you make each swale will depend on how much water you want to be able to accommodate.

A couple of things come to mind. What is the slope of the land where you want to dig the swale(s)? A U-shaped swale like you describe might be all right for low-percentage grades less than 5-7%. Steeper grades (7-20%) would cause the back wall to degrade during rains. You might be thinking: "Well . . . then I eventually wind up with a V-shaped swale." However, you would be wrong. Erosion does not work  in such a nice and even way. Instead, you would have a mess.

Personally, I would not dig a U-shaped swale even in low-percentage grades, because I would be worried about maintaining the integrity of the back wall over time. If you kept it full to overflowing with organic matter like a long stretched-out banana circle it might keep the back wall intact, but that would require a lot of work and a huge amount of organic matter.

As someone else already has pointed out, the primary function of a swale is water harvesting - slowing down its exit from the land and promoting a plume of deep soakage in areas below it. V-shaped swales accomplish this (they are not actually V-shaped) because the back cut is graded specifically to prevent erosion and to guide as much water as possible into the swale. That is why they are dug on contour. In fact, V-shaped swales are not recommended for grades 20% and above because it is next to impossible to get the back cut right and avoid erosion.

It seems to me that you would be better off with a hugle strategy if you want to go the organic matter route.

Swales are part of the water control protocol laid out along the lands contour with a 1% down grade travel.
They are meant to move water from one area to another and so will behave slightly like rivers.
Swales are supposed to be shallow with a downhill berm formed by the soil removed to form the swale, as such, they are neither U nor V shaped they are slight depressions that are wide between the 'edges and no more than 1/3 the width deep at the deepest point. If you look at a saucer, the profile would be very similar.

As an example, my land is currently a slope that varies from 45 degrees up hill slope at the bottom of the valley and changes gradually at 100 feet above the valley floor to a grade of 22 degrees with the top of the ridge having about a 1-3 degree slope.
To build U or V shaped swales would create severe issues with erosion, the very thing swales are supposed to help control. My swales are 3 feet wide front to back and 6 inches deep at the 1.5 foot mark from the uphill edge.
They are on a 1% grade downhill toward the edges of the property (900 feet east west) there are ponds at each end since these are terminal swales because of the severity of the slope.
Once a rain event has filled the swales and ponds, the water sheets over the pond downhill edge (only about 5-7 feet dependent on the grade between the swales), flowing to the next swale and pond, and this goes on swale by swale all the way to the valley (200 foot elevation ridge to valley).
This is being carried out on both the south slope and the north slope.
The end effect is terraces with a swale and berm, the water soaks into the soil and so far there is no runoff creating gullies, there is also no water plume effect since the swales are not moving the water through connected swales as the normal method would call for.
The berms are wide at their bases so they only stand 6-8 inches above the original surface of the soil, this means the swale/berm overall depth is less one foot, the top edge of the berms are created level so any overflow will sheet over, thus reducing the ability of the water to create gullies.

Anytime you dig deep swales you are actually not making a swale but rather a ditch which acts very differently than what a swale is supposed to.


Redhawk

Bryant RedHawk wrote:
They are meant to move water from one area to another and so will behave slightly like rivers.

"The essentials of swale construction are simple; they are all built on contour or dead level survey lines, and are neither intended nor permitted for water flow.  Their function is just to hold water."  Permaculture a designers manual, Chapter 7 page 167, Bill Mollison

This thread is about contour swales. Bryant is referencing diversion swales which are useful in their own context, including keyline design.