E How has a very simple model using "Duplo" blocks.
http://www.ehow.com/how_6548446_make-dam-model-science-project.html
This may be too simplistic for a 14 year old. However, it can inspire you to build one of the more complex versions, such as a buttress dam. You can look at various dam designs here.
http://www.pbs.org/wgbh/buildingbig/dam/basics.html#butt
By adding information about the forces, you can bring the project up to the level expected.
