Hi there
Strange as it sounds, unlike the swimwagon in the river dio,I think in this case that you can get away with the grass in the straight line on the road level. This is because you have a definite, and constant line formed between the structure of the road make with its sub base and the stonework of the retaining wall construction.
The swimwagon dio had a wider river bank of grass that the reeds or long grass could migrate through with greater variation.
The height of the grass is a different matter but might be very suttle differences over the short length of the dio. Any grass that might get inbetween the cobbles of the road would get quickly trodden down so would only be as moss or the like in the joints of the cobbles with no real height.
The long grass that is within the small section of the river I think is perfect and the weathering on the tank turret with water marks is smashing.
JMPOV
By the way I think the figure positions are great too.
Nige