I really like your idea of removing water using that water control structure. Here's a company in the USA that does something like that:
http://www.agridrain.com/watercontrolproductsinline.asp


www.hoosierpondpros.com


http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).