Aspiring sand castle architects will get a chance to show off their skills in two contests this week in the South Bay. If the past is any guide, the fertile imagination of youngsters will produce a myriad of castles, dragons, mermaids, dinosaurs, octopuses and sea monsters. “This year we’re expecting to see Teen-Age Mutant Ninja Turtles,” said Jim Graham, an organizer of Redondo Beach’s eighth annual Sand Castle & Design Competition, to be held Thursday. Today, Manhattan Beach sponsors its annual sand castle contest just south of Manhattan Beach pier, an event coordinated with the International Surf Contest also taking place this weekend. The sand castle contest, which begins at 9 a.m., will be divided by age groups, starting with a category for 5- through 8-year-olds. Teams that include participants of all ages will also be competing in separate divisions. The Redondo Beach competition will start at 10 a.m. Thursday at Seaside Lagoon. Children up to age 14 will compete in free-form and castle-building categories. Although other contests along the California shoreline often encourage more grandiose projects--some even using bulldozers, cranes and backhoes--the Redondo event is more in the spirit of an old-fashioned, low-tech family outing, Graham said. Admission for both contests is free.