How to secure opening to prevent access?

Question

We have a 14" space between our garage and our neighbor's block wall.

It leads to an alley in the back of our house. It's just wide enough for someone to fit through if they want to enter from the alley.

This little opening was, I believe, originally intended to allow water drainage away from our backyard. So, our backyard I kind of graded so that water exits via this side opening, and I need the ground area to still help with this function.

How do I secure this gap so that it prevents intruders from coming in AND allow me to access this area to remove debris and weeds?

This side of the opening is just the backyard. The other far side is my alley driveway.

Answer 1

I'd put a single 4x4 fence post at the alley end (or the other end, depending on your aesthetic preference), set flush on the face with the building wall(s) or recessed slightly and centered. This essentially leaves you with 5" gaps on either side--too small for most people to pass. Paint it to coordinate.

Plan view

|             |
|    gap      |
|             |
|    ____     |    garage
|    |   |    |
|    |___|    |___________________________________
|             
|      ^ --------- post
|      
|

I'd then lay heavy landscape fabric (not plastic) down the entire length of the gap and place 2" landscape rock on top. You should have very few weeds and it'll look finished.

Answer 2

Place a gated fence door there, whether chain link or wood or otherwise. The reason for the gap might be fire code - a fire wall does not exist therefore the two are separated by 14 inches..

Answer 3

Another idea may be to plant cactuses there. It will act like barbed wire and will be a deterrent. And I think that per @isherwood water will still be able to drain around the cactuses.