There are several plans for gutter downspout drainage. French drains are excellent for water drainage and can be done by most homeowners. They are simply trenches that are lined with crushed stone and perforated drain pipe. Another alternative is to drain the gutter downspout into the sewer or storm drain system. This is more plumbing than just trench digging and not exactly for the weekend
. do-it-yourselfer. More information can be found here http://www.askthebuilder.com/112_Downspout_Drain_Lines_-_Dodging_the_Deluge.shtml.