The best way in my oppinion would be to influence the flow of the water. Start towards the top of the mountain from where it runs into the valleys and with concrete (or something else stable enough to not wash away like large rock [and with rock you wouldnt be risking contaminating the environment]) create ditches and collect it for efficient evacuation through larger ditches (the farther down from the mountain top you are the heavier water flow you want to anticipate thus the larger ditch, or even canal you will need).
Once the water is past the hazard area store it in a reservoir or channel it to an acceptable site for runoff into the natural waterways.
Some mountain faces are too steep and unstable to prevent landslides.
Once the water is past the hazard area store it in a reservoir or channel it to an acceptable site for runoff into the natural waterways.
Some mountain faces are too steep and unstable to prevent landslides.
