I ran across this website that is really handy if you want to figure out sizes for your tunnels. You put in the radius (which would be the height, and also double the radius to get the width of your tunnel) and it will tell you the circumference. If you half the circumference, that will tell you how long your pipe would need to be to get that height and width on your tunnel. You can also change it to calculate the height and width you would get from a certain length pipe. For example if you had a 20 foot pipe, you would double that to get circumference of a full circle (since a tunnel is a half circle) Enter 40, and it will tell you how tall (about 6 foot four) and wide (double your 6 foot 4) your tunnel will be. Kind of cool. I haven't find a website yet that will calculate it from the point of an arc. For example, if I wanted a tunnel 10 foot hight and 16 feet wide, what length would that arc be. Maybe somebody smarter than me knows the formula for that or has found a site.

Here is a link that might be useful: calculating tunnel height and width