The surface water drainage ditch over flows it's banks 2

[frenchdrain]

Hello,
Any comments, would be greatly appreciated. My questions are;
Can this ditch be saved?

If the ditch can not be saved does any one have comments or suggestions?

The house is on a small lot with a 30" wide X about 24" deep surface water drainage ditch filled with drain rock along the back fence, it narrows to 24" wide X about 12" deep from the corner of the house to the street. In addition to needing a functioning drainage method for surface water, water from the downspouts are in a solid 3 inch pipe. The base of the fence is 2 1/2 feet above the floor of the patio, house, garage.


An aerial view is attached the letter "A" is on the roof. The 3 red spots represent the beginning of the ditch and where the 3 neighbors surface water enters the yard. The blue line is the path of the ditch taking the water from the back of the house to the street. At the beginning of the ditch and where it turns around the house soil was added to the base and sides to keep the grade consistent. Water seeps out of the ditch in these areas. At the corner of the house water over flows the banks of the ditch all the way to the street.


Thanks in advance,
FD

 

[cvg]

sounds like you need a bigger ditch or a steeper ditch, or perhaps you have some shallow areas in the ditch. Not sure what you mean by "saving it". This seems to be an odd arrangement, I would not accept water from your neighbors yards. Maybe that is the solution, tell them to dispose of their own water. And what about the downspouts? What is the question?

Given the lack of information provided, not sure you will get a better answer.

 

[frenchdrain]

Thank you for responding CVG.

I like your idea of making the ditch deeper and or wider, currently a running garden hose causes the side yard to flood. It is on a hillside, is there a way to keep the water from seeping through the ditch side wall and traveling to the foundation?


All the lots on this side of the street have been accepting surface water from the neighbors since the mid 70's, very odd arrangement,,

Down spouts are not a question, just a heads up they don't drain into the surface water ditch.

 

[cvg]

seepage through the ditch can be eliminated by concrete lining, by using relatively impermeable soils such as clays, there are HDPE ditch liners that can be used or even install a pipe to replace the ditch.

Seepage should not be a problem if the ditch drains properly. If water is sitting in the ditch then you need to regrade it to eliminate that problem - make it steeper.

 

[frenchdrain]

Thanks CVG.

The ditch is about 1 week old. It flooded the first time the garden hose was run over the drain rock. Would bentonite work as a sealer in this situation?

 

[beej67]

CVG:

Depending on where you're at in the country, you are legally bound to accept the neighbor's surface water presuming it was going into your lot originally. There may be regulations limiting how much they can drain into your lot. There may be complete prohibitions on them draining into your lot. You may be bound to take everything they can throw at your lot. Regulations vary widely.

frenchdrain:

Sounds like this is a surface water drainage issue, and should be handled with hydrology. First identify the drainage area to the ditch, then characterize the drainage area using the rational (C) approach or the SCS (CN) approach, then find out what your local rainfall is like, then establish a design discharge, then use Manning's equation to design the ditch to carry that discharge. Any less engineering than that gets you sued. Manning's equation dictates that the ditch capacity will increase with smoother liners, will increase with higher conveyance area (wider ditch), and will increase with steeper longitudinal slopes.

If you're not comfortable with any of these concepts, you probably need to hire someone to do it for you. The first alarm bell I hear is "this is on a hill" .. which says to me you may not have an accurate idea what the watershed to the back yard is.

Having done some subdivision design in the past, your side setbacks look like they're maybe 5 feet tops from the photo, which says to me that the lots themselves are graded to be very flat, which will reduce the capacity of the ditch.

Your client may have been screwed by poor site engineering.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

if a garden hose is flooding the ditch, then hydrology study is the least of your worries. It sounds like the ditch doesn't run down hill, ponds and overflows...

yes, I am aware that adjoining lots may be allowed or required to drain across the property. However, if that is the case there should be a drainage easement and properly constructed drain and you may want to ask for their help. Who designed and built the ditch? If a contractor, call them back to redo it.

When you say "drain rock" what do you really mean? For a 2 foot wide ditch, with apparently very little longitudinal slope, why is it rock lined? Is it large rock or gravel? How deep is the actual ditch and does the rock fill up the ditch? Is there filter fabric under the rock? Is it supposed to convey the water to the street, or let it soak into the ground? What kind of soil do you have in the yard? What is the slope and cross section of the ditch? You really need to determine why the ditch is "leaking" before you speculate on how to fix it. No, I don't think bentonite is warranted at this point, but perhaps I really don't understand what you are trying to "seal"

 

[Zambo]

Nice arial view. I prefer metric measurements, but still is it possible to flood a 30" wide by 24" deep drain with a garden hose? I guess I'm confused but it sounds like your "ditch" is elevated above the garden, is that right?

 

[frenchdrain]

Again, many thanks cvg,

My sincere apology for previously stating the ditch is 24" deep, 14" deep is correct!
There is no fabric or pipe in the ditch. The purpose of the ditch is to carry water to the street, the soil is clay.
3/4 to 1 1/2 drain rock fills the 159 foot ditch with a pitch of 1/4 inch per foot. The ditch/trench has a flat bottom and straight vertical sides. The ditch is filled with rock to prevent the walls of the ditch from caving in and to create an ornamental dry river bed through the out door living area of the yard. ( My sincere apology for previously stating the ditch is 24" deep, it is 14" deep is correct!) The beginning of the ditch is 30" wide X 14" deep it narrows to 24" wide X 12" deep from the corner of the house to the street. At the beginning of the ditch and where it turns around the house soil was added to the base and sides to keep the grade consistent. Water seeps out of the ditch in these areas toward the foundation.

The newly added soil is eroding near the corner of the house thereby raising the floor of the ditch and causing flooding to the street.

 

[frenchdrain]

Thanks for responding Zambo,

No I don't think it is possible to flood a 30X 24 drain or a 30X14 inch drain with a garden hose when either is functioning properly.

The flow of water in this drain has eroded the newly added soil causing the floor of the drain to rise, and flooding.
You are correct the drain is about 60 centimetres above the garden and house which are about 60 centimetres above the street.
Additional photo doesn't upload :>[
Ariel view is complements of Google maps. Appreciate your preference to metric, sorry it isn't my strength, but I will try to answer any questions. Above I corrected my original post of 76.2 centimeters [30inch] wide by 60 centimeters [24inch] deep to 76.2 centimetres wide by 35.56 centimetres [14 inch] deep, I am very sorry for the error.

 

[cvg]

some details are starting to come out. 1/4" per foot is approximately a 2% longitudinal slope. That is quite adequate for a ditch, maybe even a bit steep. If you had a lot of water dump into this ditch from your neighbors, perhaps there has been some erosion and then deposition of soil near the end which has filled the ditch and reduced the slope to less than 2%? Question - why was the grade inconsistent so that soil needed to be added? Do you mean the ditch was "cut" into the yard, except that at one location it was "filled"? Was the fill soil compacted? Was it filled enough to eliminate the low spot, or is there still a low spot that fills up with water and doesn't drain, but then starts seeping out?

I question whether adding 3/4 inch rock can retain a vertical ditch side. Your sides should be at 2:1 or 3:1 if possible. Also, filter fabric should have been used under the rock to prevent erosion. With clay soil, you should not have much seepage through the soil, unless you have standing water. It sounds like the ditch is overflowing. You should berm up at the low spots to prevent the overflowing.

Also, why does the ditch get smaller towards the outfall? Most ditches get larger as the flow tends to increase as you go downstream. Your ditch is far too small.

 

[frenchdrain]

Hello cvg,
The ditch was dug a week ago, there has been no rain or water from any neighbors, it was tested using a garden hose at the uppermost end.

There were 2 low spots in the backyard, they were built up to created a consistent slope, there was compaction, obviously not enough, the seepage is in the side wall in the area where the ditch floor was raised to match the pitch of the ditch floor.
I agree the sides should slope.

I don't know why the ditch gets smaller towards the out fall, it isn't my design.

The ditch will receive some water from four 1/4 acre lots
what size would you suggest the ditch should be?

Thanks in advance cvg you have been very informative!

 

[frenchdrain]

Thanks for responding beej67,

Yes, this issue is only about surface water, average rain fall is 23' a year.

True some home work on Manning's equation is required.

The 4 lots draining on to this one total an acre. They all sit below 'their' street. The 4 lots are flat. The lot they drain on to is level with these lots at the fence extending about 5 to 10 feet. Then slopes toward the house dropping 2',levels of and slopes at the corner of the garage 2' to the street.

Poor site engineering, that seems to be a yes.

 

[beej67]

The ditch characteristics you describe (2.5 ft wide, 2% slope, gravel bottom, 14 inch flow depth) should handle around 13 cubic feet per second of flow before the ditch overtops, at a flow velocity of over four feet per second, and that's presuming no sideslope ratio. No garden hose can produce that. Are you sure you made your measurements, particularly the slope, correctly? Eyeballing it from your map, a 2% slope would mean the invert of the upstream end of the ditch is about two feet higher than the invert of the downstream end.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[frenchdrain]

Thanks much for contributing beej69.

I agree, the numbers are confusing, the flooding began the first time water traveled through the ditch.

The ditch is filled with 1 1/2 -3/4 drain rock. The beginning of the drain is 48 inches higher than the side walk. The area where the flooding begins is at the corner of the house. At that corner the ditch narrows to 2' wide and 1'deep.

This is also where soil was added to the floor and sidewall to maintain a consistent pitch of 1/4" per ft.. Soil has eroded filling the gaps between the drain rocks and raising the floor of the ditch, causing the water to rise over the banks of the ditch and flood the side yard when a garden hose is running in the ditch.
Was the soil compacted, if so not enough.

 

[cvg]

13 cfs is the max capacity without any freeboard. I usually like a foot of freeboard, which is tough because your ditch is only a foot deep. However, in your case - some freeboard is essential because of the risk of flooding to your house. I would say you can only rely on a capacity of about 5 or 6 cfs in your ditch. Especially because the bottom part is only 12 inches deep by 24 wide, it doesn't matter what the upstream capacity is, you have a bottlenect at the bottom. If you have a flatter section, than your capacity will be severely limited. Sounds like this is the case.

Regarding the fill, was the fill gravel, sand or clay? If gravel or sand, than that would explain the leaking. Obvously if you see soil in the rocks already and you have only run a garden hose, then you have erosion coming from somewhere. Suggest you confirm the location of the erosion and fix it. And as I suggested before, without filter fabric, the gravel may do little to prevent erosion. You should always place a filter beneath riprap in a ditch.

Also, please answer carefully as I am still unclear - you say the ditch is "filled with drain rock". My original question is it filled to the top with the rock or just lined with a thin layer of gravel? In other words, is this some sort of gravel subdrain or just a gravel lined open ditch? If this is truly "filled" with rock, the capacity is much less than 13 cfs.

 

[frenchdrain]

Thanks for responding cvg,

To help clarify;

1. Regarding the fill, was the fill gravel, sand or clay?

*The fill is soil from excavation of the ditch, it is clay.

2. You should always place a filter beneath riprap in a ditch.

*Absolutely! But there is no filter...yet

3. I am still unclear - you say the ditch is "filled with drain rock". My original question is it filled to the top with the rock or just lined with a thin layer of gravel?


* The ditch is filled to the top of the side walls with 1 1/2 -3/4 drain rock.

4. If you have a flatter section, than your capacity will be severely limited. Sounds like this is the case.

* Yes there is have a flatter section due to erosion

5. It doesn't matter what the upstream capacity is, you have a bottlenect at the bottom.

* Yes, the bottle neck is accessible to trenching equipment. It can be cleared of eroded soil and widened.

 

[beej67]

13 cfs is also way more than he probably needs for a few acres of residential runoff, so I wouldn't sweat the freeboard as much.

This sounds like your problem:

***
"This is also where soil was added to the floor and sidewall to maintain a consistent pitch of 1/4" per ft.. Soil has eroded filling the gaps between the drain rocks and raising the floor of the ditch, causing the water to rise over the banks of the ditch and flood the side yard when a garden hose is running in the ditch. Was the soil compacted, if so not enough. "
***

I read this to mean the ditch was in fill instead of cut as it rounded the corner of the house. Is that right? If so, then lower the ditch invert to below the finished floor elevation of the house there at the corner, if at all possible. Positive drainage away from building footprints is a fundamental principle in site engineering.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[frenchdrain]

Thanks beej69,
The ditch is for control of 1 acre of storm water.

You are correct the ditch was dug in silty clay fill at the corner of the house. Lowering the ditch invert below the finished floor elevation of the floor of the house is not possible. This is in the area of the 2' rise from the level of the house to the back yard fence. Also the slope would put the ditch below the sidewalk.

What are your thoughts about using bentonite to seal the ditch?

 

[cvg]

a garden hose has a maximum capacity of 5 - 10 gallons per minute. this equals perhaps 0.01 - 0.02 cubic feet per second - not even close to 13 cfs. This ditch overflows at 0.02 cfs, it does not have any capacity. Bentonite will not solve your problem, your ditch has insufficient slope. There apparently is a low spot with zero slope. First re-grade the ditch and compact better - you need posative slope everywhere.