Sandbags will not seal out water.
Sandbags deteriorate when exposed for several months to continued wetting and drying.
If bags are placed too early, they may not be effective when needed. If it is necessary that bags remain durable for a longer time, the addition of cement can increase effective life. The materials can be mixed and placed dry. After all the bags are in place, a light sprinkling of water is desirable. Protection from high flow requires a more permanent type of structure.