Although LEGO does produce arch elements, let's face it, they're not the most interesting. That however, can not be said for this arch by Barthezz Brick on Flickr. Check out his post here: https://www.flickr.com/photos/151790575@N05/43465216502/in/dateposted/
Flex tubes are used to create this archway, which you can see just behind the other pieces. These flex tubes can be bent to any shape, and clips can be attached so that the 1x2 modified bricks with curved tops are positioned in the curve. This leads to a nice transition between the pieces, and although there are some gaps, the overall design still looks great.
The colours used by Barthezz Brick blend very well and further improve the aesthetics of the build. This technique is part of his prototyping stage for an huge Venice MOC, so follow him using the link above for updates and cool techniques as they come!