Swales and drainage on urban lots?

I'm sorry if this is a question with an obvious answer... I attended an introductory PDC class a couple weeks ago which took place at a residential permaculture site in Houston. There were shallow swales along the edges of the gardener's property. He discussed how urban lots drain towards the street. My question is, does this literally mean that the land on an urban lot is sloped down towards the street? To my untrained eye, the land seems pretty much level, so I was just making sure he wasn't referring to some other unseen mechanism that drains the lots towards the street gutters, in which case I wouldn't understand how the swales play into trapping the flow of water.

Many urban lots are graded to drain water away from the house, so even if they look level, they may actually slope imperceptibly away from the house. 

Thanks for your reply, Ludi.

So, does that generally mean that the land will be highest in the center of a block of houses, sloping towards the street? It never occurred to me to think of blocks as mounds before, even if very gradual ones.

I think that's the general intention, because water pooling around foundations isn't good for houses.  Of course as you're learning the water doesn't have to be directed completely off the lot, just away from the house.  Brad Lancaster's recommendation is to not site water harvesting earthworks closer than 10 feet from the foundation of the house.  This means if there's a very narrow side yard, for instance, the water might need to be diverted to the front or back yard by means of a pipe or lined drain of some kind, and then allowed to sink into the ground in a basin or swale in the larger part of the yard.

I think the other answers are correct:  land is supposed to slope away from the houses.  I think the original intent of most blocks may have been to all slope in a gentle dome away from the center, but over time land is disrupted, settles unevenly, etc.

However, I think what your instructor meant is that water that doesn't penetrate into the ground, is drained away via the street and gutters, eventually ending up in either a water treatment facility, or discharged into a stream or waterway outside of town.  This is why swales or some kind of water retention on the property is so important in an urban setting: if you don't catch and hold it, it will be shipped away.  And with so much water being used to maintain unsustainable landscaping, catching more rainwater in town goes a long way towards solving water management issues.

A bunyip or water level provides a very accurate method of comparing elevations on different parts of a property.  A good PDC should give you surveying experience which is critical for water manipulation.  I wouldn't make assumptions.  Sites can settle after mass grading.

http://www.youtube.com/watch?v=nAcT_1T25LM - this seems like a good video.