How deep should a trench be without shoring?

What is the widest part of a river called? How much does a ligthhouse keeper make in the 1900? Where is the nearest coast from the challenger deep? Do African rivers at the coast or end the coast? What materials were used to build the pamban bridge? What kind of erosion plays a role in the Missouri River? How many disappreances have there been in the last 50 years in the Bermuda triangle? Where does an under tow take sand and other particles from the shore? How long is the Colorado river in yards? What country is the Panama Canal going through? What is the second highest peak in the US? Is slender real? How can you save the rainforest for the future? What are some reasons to visit London? How do animals adapt to sand dunes? Who was responsible for the building of the magnificent Church of Hagia Sophia? Who developed a way to make paper without cutting down a tree? What is a small part of larger group? Contribution of chanakya in building the mauryan empire? What wars did windsor castle servive?