Septic Field Bed Seeding

We are building a new house and need ground cover for the septic field bed. The health unit will issue the permit if it is seeded or sodded. As we will be on a lake shore property I don't want any grass at all – I am sick of the care involved, sick of the endless grass weeds in the flower beds and want the whole garden to be much more natural. So what type of seed would work, bearing in mind that the roots need to be shallow so as not to interfere with the drainage system and it should be covered well within the first year to avoid erosion. I am nervous of seeding from a packet of wildflower seeds, which would be lovely, as I'm afraid some of the roots would be too deep. What do you think of crown vetch for this area? I think it will be in full sun and it should be ready for seeding in August or September. by the way we are in Zone 3-4.
Also can you give me an idea of how much coverage is appropriate? We don't know the area yet.

If you are planning on having flower beds, having a meadow adjacent to it is not going to set off your efforts there. That is why a lawn looks so nice adjacent to flower beds : the low green carpet contrasts so beautifully with the various heights and textures and colours in the flower bed. Have you considered getting a plug tray or two of purple carpet lemon thyme? It would be choice!
Crown vetch is good at outcompeting some weeds, but gets quite tall over a septic tank bed.