Storm water retention devices are very important for reducing flooding down stream of storm water source by slowing the flow of the water coming off hard surfaces. They can take on many different shapes, sizes and flow rates. I have worked on developments where dry gullies have been damned with a drainage system at the base that slows the entrance of the water onto the riparian system. I have also worked on developments which use a field like structure that is bunded, during a rain event the run off collects in the pastured field after a storm event and the water that doesn't soak in slowly drains into the riparian system. The field when dry is used for recreation etc.
Another option is a ponder system which has a free board and overflow high enough above the permanent water level with a drainage device at the permenant level. So the water required to be detained is collected and slowly released back int the system. Leaving behind a pond and a watered area. Up stream.
many local governments allow water to be harvested and take the harvested total from the required detention. Such as water tanks for individual dwellings or dams for irrigation of gardens. Swales may fall into this category however you may need to re label them as contour ponds or something similar.
My advice would be to sit down with water engineers from your local government and discuss your options.
Good luck
Sean
