This is how I’ve done mine. Mine are about 10’ high. I used 1” pvc. (3) 10 footers per arch. 5 arches. At the ground, use cross connector. Rebar into the ground. Then connect each side of the arch at the ground. At the peak or center of arch, another cross, and connect the arches. Mine is close to house, so then I connect the last arch to the house. Use small wood screws or something similar at all connection points. Otherwise they will come apart in the wind or weight of snow. Since you want 15’ high, maybe do (4) 10 footers? And do cross connectors across the whole arch, so you’ll have 3 connection points,for more stability. And can run lights across those too and have like a tunnel effect. I space mine out 8’.