Why are "Broadchurch" and "Gracepoint" so similar?

Why are the shows Broadchurch and Gracepoint so similar?

Lemme point some of the similarities:

-boy found dead on beach

-lead role played by David Tennant

-D.T. plays a detective

-in both shows, his co-star plays a detective named "Ellie Miller", however they are played by two different actresses

-you can match up people from one show to people from the other

-both of David's roles even dress similarly!

Why??

Now, of course I'm going to watch both shows

(I <3 David), but there are just so many noticeable similarities before even watching them. Is Gracepoint just the "American version" of Broadchurch? (I say it in that order because Broadchurch came first). But then, in B.C. David plays as Alec Hardy, while in G.P. he plays as Emmet Carver. AND the two shows are owned by different companies. Please explain. :/