I am peripherally involved in a small developing co-housing neighborhood on 15 acres of land in Durham county, North Carolina, USA. The land does not have steep slopes (approx 1 ft rise over 20 ft). The planning department requires a detention pond to catch the extra rain waterrunoff due to the addition of houses and roads. I am just learning about permaculture but is seems to me that swales should be able to replace the detention pond as long as the combined swales hold the same volume of water as the detention pond. The advantage is that no dam has to be built and you get an orchard which is much prettier in my opinion. Has anyone done this successfully? Are there any professionals in central North Carolina that could help us with strategies for the planning department? (I suspect the planning department will object.)
The landscape architect involved does not understand how swales work (or maybe I don't). His response to this suggestion was that they have not been successful at getting approval for vegetative mitigation of runoff. My understanding is swales do not use vegetation to detain the water but that the swale structure itself is what detains the water on the land until it can seep into the ground, i.e. swales are just a bunch of small oblong detention ponds built on contour without dams. The berm is optional for this purpose and does not need to be built to dam specs since it doesn't hold back the detained water. But with the berm the whole system gives you an orchard as a bonus.
Can you make a bunch of smaller detention ponds aka basins? Brad Lancaster discusses the use of basins instead of swales in his book Rainwater Harvesting for Drylands Volume 2. They have the advantage of being easy to fit between existing vegetation and structures, and may look more "natural" in the landscape than swales tend to. Basins can be any shape but tend to be more round or oval than swales. Basins might look enough like detention ponds to pass more easily than the typical swale.
Swales are great but need earthworks too. I woudld first have a closer look at the costs. In a cohousing project the pond could be a great feature, with ducks and some benches, if big enough you could take a dip in summer and do ice skating in winter. It is actually a great feature you can stock and raise some fish too.
Angelika Maier wrote:Swales are great but need earthworks too. I woudld first have a closer look at the costs. In a cohousing project the pond could be a great feature, with ducks and some benches, if big enough you could take a dip in summer and do ice skating in winter. It is actually a great feature you can stock and raise some fish too.
A real pond would be a great asset but I don't think detention ponds hold water for very long. They are meant to temporarily stop the runoff from a large rain event. Around here I've seen completely dry man-made depressions at new developments. They are not pretty. The code says the property can have no more runoff after all the houses/roads are built than before. So we have to detain (and the code says detain not retain) all the water that hits these new structures. It seems to me that a real pond would just overflow and not detain the water.
City of NY has a standards book for bio-swales (green infrastructure they call it). It might be helpful.
nyc standard And I think there are many similar standards that you can implement. Tucson has some. tuscon presentation Housing projects require some legal basis, so unfortunately not much liberty of thought is left I think. In many states they should be engineered.
Moreover if you do not have a city system to rely on, there is something that is critical. For such projects, one has to consider different rain events to determine the scale. For example, for my location, an annual average is 700 mm (27,5inch) per year (1/1 year event), 1 in 10 year event might be 900 mm (35 inch) , 1 in 50 year event might be 1200 mm (47 inch). 1/1 year event translates to 25 mm (10 inch) per day for my area (weather stations give this data). Making it more complicated each state/ nation has its own standards for this. Some require max intensity of 30 min rush to be considered. So which one I should consider while designing the capacities? That is always a bit of a problem.
Instead of directing all run off to a single basin, which will be a big big big project even for small areas, directing each to different sinks will help. Such as road run off to road side swales (I would go for a 1/10 year event, wouldn't want everywhere getting flooded each year), roof run off to rain barrels (1/1 event). If it overflows, water goes to the recreational pond (1/1 to 1/10 event) and then to a dry well/swale/ or a detention pond for it to overflow. And if it fails to hold all water (and it will definitely will, Murphy's law) one should consider a safety passage for the water to leave the property (1/50 year event).
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.