Should I leave a treeline to mitigate flooding in my backyard?

Question

I have a 1942 colonial near Hartford, CT. It is a corner lot, and one of the streets I am on recently experienced a flash flood. Our house stayed dry, but we saw water up to three feet in the street and our yard flooded up to a foot or two. The water stopped closer to the house than I'd like. It drained within a few hours, but it was pretty crazy to see.

This was a fixer house, and one of our many projects was clearing out a treeline in our yard near the edge of the property to make more room before we put a fence in. We started this ourselves, but after the flood, we are reconsidering it and have halted the work.

I'd like to know if leaving the trees makes sense or not given that the area can flood. Some information is below:

• The trees are mostly smaller and younger with the exception of one, large, red oak.
• I'm not really sure of the density of the soil. It's pretty "good" soil to the best of my knowledge (in that plants grow very well in it), but I don't know what this says about it, exactly.
• The treeline is maybe 50-75 yards from my nearest structure (attached sunroom) on the side that floods.
• Flooding frequency is unknown. The neighbors said they hadn't seen it get to the point of flooding our yard in the two years they've lived there, but others have told us stories of cars being washed away on that street.
• We plan to put a fence in behind this treeline whether we take it down or not plus a raised bed garden in front of the treeline in the yard.
  • There is poison ivy in the treeline, which is one reason we were clearing it, although mostly we were clearing it for aesthetic reasons and for the fence.

Please let me know any other information that would help!

Answer 1

Poison ivy

To get rid of the poison ivy, 1-2x/year spray all the (undesired) broadleaf ground cover in the area with 2,4-D herbicide according to instructions. 2,4-D only harms broadleaf plants, so overspray won't hurt your lawn. It won't hurt established trees (unless you use way too much). It is a very old, trusted and well-proven herbicide that hasn't had troubles like Roundup has. Buy it pre-diluted (pink) not farm-grade (deep, opaque crimson) because the latter is strong enough acid to blind you. keep it away from areas you plan to garden/eat becuase organic is better :)

Flood water

This question can only be answered with a deep understanding of the lay of the land. This is engineering - not particularly hard engineering, well within the reach of a serious "jack of all trades" - but it answers the question so conclusively that it's the only science you need on the subject.

Note that "swift moving water" type floods, like you get on hillsides, are very different in nature than "water slowly rising" type floods like you see in Houston or the Mississippi River valley, and I assume you are talking about the latter.

Water will very reliably seek level. So if you know the comparative altitudes of each part of your property, you can predict where the water will go at different flood stages. Very likely the city has that information. The insurance company also clearly has some data, or they wouldn't have written flood insurance, and you wouldnt have gotten a mortgage.

The number you will typically see on maps is "feet above mean sea level". And a contour map will show lines across the property, each lone is at a certain altitude. Suppose you have a 25' contour line at the base of your property, and if you're lucky the map has them every 1'. There'll be a 26' contour line a few feet up your yard, then a 27' line, etc. etc.

If water rises to 25', everything below the 25' contour line will be flooded. If it rises to 26', then the area between the 25' and 26' lines get flooded. Etc.

If you can get a contour map and look at it, then walk around your property, it will make a lot more sense. So talk to City Hall or your county GIS department, i.e. The guys who do the tax maps, and see if they'll print you one out for a few bucks.

Answer 2

To a first approximation, when it floods, the water is flat - completely horizontal. The really interesting question is "how high above the treeline is your property"? Note that in low-lying areas, very small differences in height make a huge difference to the amount of water involved in the flood†.

Now, I said "to a first approximation". If the water is flowing really fast, then there is a slope (which is driving the water flow). In that situation, a close-boarded fence, reinforced with sand-bags, can probably divert a flood of water in the road that is 6" or perhaps 12" deep. Such a fence will need to extend far enough that the water is diverted somewhere away from your house. And if there is a hole in the fence (to let cars in for example), it might as well not be there. Obviously an open fence will do nothing.

A tree-line will be much like an open fence, unless there is so much rubbish brought in by the flood that the trees + rubbish start to act like a dam - but if there is that much rubbish, you really want to be somewhere else.

If you are worried about a bigger storm flooding your basement, you need to look into flood-proofing your basement. It is possible to fit flood barriers over windows for example.

† The market place in Cambridge is called "Market Hill" because it is slightly raised above the surrounding (former) marsh. It is about 25' above sea level - and the sea is 30 miles away.