We have designed a Water Retention Landscape Plan for a 17 hectare property on the San Jose burn area, and are now implementing it. We are including the project report here, if you are curious to learn more. Mirador Design [pdf]
Narrow swales, with a slope within the swale of .3-.6 % to help rainwater in a strong storm move off the mountain safely without furthering erosion. The vetiver grass forms a barrier subteraneally and above ground. Roots grow 4-5m deep and form a thick web.