It's unusual for anything capable of causing a blockage to get any further than the u bend. If it is light enough to be washed through the bend, then it is light enough to be carried away completely. More often than not, it is an obstruction (such as a small piece of slate or stone) that gets lodged just around the other side of the bend. Other lighter debris then collects up behind it. If you have cleared right the way down to the bottom of the u bend, you'll probably find the problem just around the bend. As I say, scooping this out will generally clear it. It can be tricky as you can generally only get your finger tips around the bend, but it is worth the extra effort.
Another possibility is that the pipe run itself has collapsed. If you have access to the inspection chamber, you should be able to tell if your rainwater gulleys flow into this. Just look at the pipes comming into the chamber, and look at the direction thay are coming from. If they are not eminating from the direction of your rainwater gulleys then you haven't found them (drain pipes do not have bends below ground level, therefore they run in straight lines).
Alternatively, dig down to see which direction the pipes run out of the gulley.
Once you have established the path taken by the pipes, look at the ground above to see if there is any sign of subsidence or heave of the surface. It's possible that someone in the past parked a vehicle off the side of the drive (and unwittingly, caused a break in the pipe below). Another possibility is that a very large tree root has continued to grow beneath the pipe causing heave, and therefore collapse of the pipe.
If you can find their exit in the inspection chamber, Once you have pushed your rods in as far as they will go, check this length along the surface, then dig, that's where the pipe has caved in.
I have also encountered a situation where all the rainwater gulleys for the entire street, empty into a pipe running parallel with the building line (just 2 ft away from it) This was just 6 inches below the surface!. The neighbour of my customer had broken this pipe while digging out footings for a porch, and thought it was a discarded piece of pipe in the ground. They then filled it in with concrete and built around it. As they had built the porch during a long dry spell, their folly didn't come to light untill the autumn brought some rain.
Another possibility is that the pipe run itself has collapsed. If you have access to the inspection chamber, you should be able to tell if your rainwater gulleys flow into this. Just look at the pipes comming into the chamber, and look at the direction thay are coming from. If they are not eminating from the direction of your rainwater gulleys then you haven't found them (drain pipes do not have bends below ground level, therefore they run in straight lines).
Alternatively, dig down to see which direction the pipes run out of the gulley.
Once you have established the path taken by the pipes, look at the ground above to see if there is any sign of subsidence or heave of the surface. It's possible that someone in the past parked a vehicle off the side of the drive (and unwittingly, caused a break in the pipe below). Another possibility is that a very large tree root has continued to grow beneath the pipe causing heave, and therefore collapse of the pipe.
If you can find their exit in the inspection chamber, Once you have pushed your rods in as far as they will go, check this length along the surface, then dig, that's where the pipe has caved in.
I have also encountered a situation where all the rainwater gulleys for the entire street, empty into a pipe running parallel with the building line (just 2 ft away from it) This was just 6 inches below the surface!. The neighbour of my customer had broken this pipe while digging out footings for a porch, and thought it was a discarded piece of pipe in the ground. They then filled it in with concrete and built around it. As they had built the porch during a long dry spell, their folly didn't come to light untill the autumn brought some rain.
