Not that you shouldn't have an interest in addressing a possible run-off issue, but in addition...
It's possible and not even difficult to greatly, drastically improve the drainage in any beds by adding organic matter (anything that will decompose) to the surface periodically. Doing such adds fertility and humus to the soil, improves the tilth, and helps to both increase drainage in spots that aren't draining well, and hold moisture in spots that dry too quickly, and makes weeds less numerous (assuming they aren't coming from neglect,) easier to pull, no downside to doing it. The details of how and why are well explained in this 15-minute video:
http://permaculturenews.org/20...
