Water – do about water runoff that is damaging soil around the house

This page says clay soil absorbs water at the rate of 1/2 inch per hour, 4 times slower than sandy soil or loam.

http://www.ornamentaledibles.com/tips/soil.htm

Let's assume you get 5-10 gallons per minute or 300-600 gallons per hour. A gallon is 231 cubic inches or 462 square inches 1/2 inch high. So 600 gallons/hour needs 277200 square inches or 1925 square feet. Assuming your trench is 12 inches wide, you need 1925 linear feet of pipe under steady state conditions.

Let's consider another situation where you store all the rain water and let it absorb as slowly as it wants. Assume you get 600 gallons which requires 80 cubic feet. So if your trench were 12" by 12" you would need 80 linear feet of pipe.

I had to draw a small diagram to see how the area was around the house. I drew a swale in to show what is there already, or hopefully something that can be created

What looks troubling here is the concentration of runoff heading to the house IF there is no swale there, that is why I hope there is one there, if not you really need to get the flow away form the house. It is bad enough that it is sandy, the water can still perk into the foundation, though it will be minimized with a swale. The clay base on the north side, sounds exactly as you describe, a bowl.

You need to do whatever you can to minimize the water infiltration, to start. It may not require to pipe the downspouts away from the house to a drywell or into a French drain, well not a French drain as you may plan around the house. French drains return the water to the ground, and in turn the crawlspace. What I mean is terminating the downspouts into an in ground pipe that daylights downhill of the home. A drywell would work too, but it is much more than may be needed.

This is the minimum I would suggest, although it is still a lot to do. I would also consider waterproofing the foundation, it may work on most of the foundation, but I think the runoff would still migrate under the foundation on the south side since it is still on sand/ clay mix presumably.

All of this is still tempered by the symptoms created by the water in the crawlspace. If there is no evidence of black marks (mildew) or white fuzzy stuff growing in places from the excess moisture in the crawlspace, if none of these symptoms exist, al least extreme measures are not needed. Regrade if needed, would be the minimum, downspouts to daylight away from the house the next active remedy, drywell addition the next, and if you are doing all that digging, waterproof the foundation, all these measures combined are the extreme I mean.

Edit 1/26/13

Aside from the shape of the house and other extensions from it, it shows the same thing happening except your swale that exists is more of a collection point. It appears the rain runoff takes the same path, but the house is in the way. The swale in the above sketch I posted would divert the rain water before it gets too close to your home. Is this a possibility to happen? It can be done by lowering the grade or if the house is high enough above grade, the grade can be raised, but that has implications too. The water in the swale may be of no concern. I would draw more concern to the south and west side where the rain looks like it is path is unencumbered to the foundation.

I will edit your drawing to show a more accurate idea and post it.

Back at the downspouts that I mentioned earlier, if the grade looks like it can daylight a underground lead would help reduce the amount of water at the foundation. Yet the addition of the swale will redirect the downspout water as well.