Wow, that looks like the beginning of a potentially large and expensive problem. I just don't understand builders that would have left behind a situation like that. Hopefully some of the engineering types on this forum will help you, however if it were me I would consult a landscape or civil engineer close to home.

Patty

Wow, I agree with the trench at the top and the bottom. Just a nice wide, gentle swale at the bottom, nothing too noticeable or unmowable. Then at the top a swale to collect and direct the flow. I would take some of that gorgeous rock and create a "dry" waterfall/stream on the slope where you want it to come down. Make it bigger than you think you need it, because you will always get more rain than you think, and with a swale to collect it, you could get some serious water through there. As for the slope, it is up to your preference, shrubs, flowers, even trees, just get them in this fall. Also, mulch that slope with 3or 4 inches of woodchips, it will hold under all but the worst storms. Free woodchips from a tree service, or maybe your county waste center are a good place to start. Not really any aesthetic or design points, just practical advice from another slope dweller. Also look at the plants for difficult spaces forum.
NN

A bit of information, please. Is the soil glacial till composed of gravel, sand and clay? And, I am seeing birches on the hill? Also, is the water free to run where ever or is it draining onto adjacent properties?

Th at sounds about like the soil, it is a glacial area. The property has white birches on it. A lot of the water runs towards the neighboor, there is a spot where it kind of pools up into a little pond with heavy rains (this was naturally formed) Most of the water then goes towards the street where it goes into the storm drains.

This was a 100yr rain so I would imagine this will be about the worst I will see, but would still like to fix this to handle this amount of water.

The minute that I opened your pictures the haunting voice of an old friend and dean of civil engineering instruction came roaring out of the past...."Always design and build for the 100 year flood." We worked together over the years plus doing presentations to planning board groups.

He would bellow, "When you are working with a builder who builds 'off the shelf' house designs don't let them shape
the earth to fit the design. You will quite often run into land/erosion problems. Make that builder alter the foundation or whatever to fit the land."

You have a problem. If I were designing a fix for this situation I would call in a civil engineer for a consultation. The unstable nature of the glacial till makes me think that trenching at the top of the slope is going to exasperate the situation. I wouldn't touch this without an engineer's imput. You have a neighbor to consider. They can get touchy when water is diverted to their property. A civil engineer can help you to make the right repairs.

"This was a 100yr rain so I would imagine this will be about the worst I will see, but would still like to fix this to handle this amount of water."

Your post reminds me of one we saw here from "Prairie_Love" just a few months ago. Her property is on a 100 year flood plain. Of the top 10 floods ever recorded in her area, six of them occurred in the last 20 years. The biggest recorded flood there was in 1997. The second biggest was 2006. With changing global weather patterns, we have to be prepared for more frequent floods.

For many of us, our home and property is our largest single investment, and that investment is worth protecting (not to mention human lives). Asking friends, neighbors and internet forum members what to do is an okay start, but to develop a plan that will best address the problem, you need to call in a local expert.

I have a great site about Slope Erosion Control, where you can find all of the latest and best techniques on slope erosion control and if you like you can comment or help other people with your ideas

Here is a link that might be useful: Slope Erosion Control Site

There is a really great site about erosion control techniques! We are trying to start a forum and get the best/most up to date information.

Here is a link that might be useful: erosion control techniques