Lee, I will try again to respond to your inquiry. I am composing my response in a Word file, and will paste it to the response page of the MasterPlumbers.com Bulletin Board.
I think that you are on the “right track” with your idea. By the way, your description of the problem is excellent. It seems to me that you must direct the storm water from the downspouts away (as far away as possible) from the foundation of your house. I recommend that you design a storm water collection system consisting of the appropriate size of PVC, SDR-35 pipe, and Sewer and Drain fittings. The required pipe size is determined by the volume of water to be transported from the maximum storm event for your area. I recently installed a storm water drainage system at my home in Timnath, Colorado where our maximum storm event will produce about 3 inches of rain over a period of several hours. The annual precipitation for this area is about 16 inches. I used 4-inch PVC, SDR-35 pipe in the collection system, and used Advanced Drainage Systems, Inc. (ADS) “Bio-Diffuser” Chambers as the dispersal system. Your building contractor suggested 3-inch perforated pipe to solve the problem. I think that he does not understand the problem. His suggested solution also indicates to me that his expertise is in a field other than the design of storm water drainage systems.
Determine the collection area of the roof, and then calculate the volume of water the maximum storm event will generate. Obviously, the climate of the area in which you live will have an impact on the volume of water with which you must deal. I mean, do you live in Phoenix, Arizona, Portland, Oregon, or Wagga Wagga, New South Wales?
Install a pipeline to transport the collected stormwater to the area that you refer to as “the swale.” A “French Drain” is the appropriate technology for dispersal of the stormwater into the soil. But the State-of-the-Art materials that are available to us today would make the French engineers, who developed agricultural French Drains, indeed envious. I am referring to plastic drainage products such as the “Xpandable Chamber from Eljen.” This really neat “French Drain” product is so new that its description is not on Eljen’s website as yet. I would have used this product for my storm water system had it been in existence at the time.
If you contact the Eljen folks, they will provide you with the information, and will be able to direct you to their local distributor. They may even give you some advice regarding your proposed storm water dispersal system design. I wonder if there is an Eljen Distributor in Wagga Wagga?
Check out their website. http://www.Eljen.com
Eljen also makes a “Gravelless” perimeter drain. The use of this product may solve the siltation problem occurring in the window wells, which you described.
Well Lee, good luck with your project, and if you need further direction to proceed, post a response on the Bulletin Board.
John Aldrich (Septic Tank Yank)
Septic System Consultant
Advanced Professional Engineering, Inc.
Fort Collins, Colorado