I am assuming that your planned 2' land for gardening would be tufted and not concreted or tiled.

You could always make the garden area lower than the rest of the car porch (concrete or tile surface). So, when it rains the water from the garden would not flow to the tiled/ concrete area.

As for drainage for the garden, you could use underground drainage.

You need to establish what are the water that will be getting onto the lawn. Is it just the rain, lawn watering plus some water flowing from the higher car porch floor only? Will there be any roof water flowing onto the lawn?
I think a picture speaks a thousand words

Basically an underground drainage is to drain away surplus water so that the lawn is not soggy. There are various methods used depending on the situation.