Piped Swale Crossings
Permaculture is a connecting system between disciplines and elements in a matrix of design, and swales are a mainframe element. The efficiency of swales is that they can interrupt water surface flow high in a landscape where it is then infiltrated relatively quickly, on contour, and moves incredibly slowly through the landscape soil and subsoil profiles. This becomes a great advantage to the potential productivity of any property, especially a property that is designed to be diverse and interactive with many ecosystem elements. When you design a property this way, a mainframe approach as a consultant designer is:
- pattern water
- pattern access
- positioning of structures.
This is almost like a mantra of approach to lay out your mainframe, and those are some of your initial selection criteria to then position your main elements, features, zones, sector enhancement and protection.
The earthworks of swales fit exactly in a landscape where the most advantage can be taken from water, so access points must subsequently be made to harmonise with the contour patterns of the swales. If access is able to follow contour just above swales, then the driveway becomes a hard surface run-off to help supply the swales in rain events. This is a great advantage to swales, but that is not always possible and often, the next best option is as close to contour as possible or directly down the center ridge lines. If you take access directly down the centre of a ridge, the road water run-off is shared on either side of the road and no extra water catchment features need to be installed — but that means they are going to cross the contour lines of the swales diagonally. So, crossing points have to be made, and this often becomes more obvious as you develop a property as more situations occur where it is beneficial to have well-placed , beneficial swale crossings.
You can casually cross a swale by slightly widening the trench and the mound and compacting that section. It appears then as a very large speed-bump. However this has some disadvantages because in really wet times it is inevitable that it will become muddy and difficult to cross. So, one of the ideal features and assets to a well designed permaculture property, which has extensive water harvesting and very good access, are piped swale crossings.
Piped swale crossings
We need to assess the amount of water that is going to overflow our swale and the right sized pipes must be installed so that there is no choke point at the swale crossing and a full discharge of water can occur if those swales are going to overflow. Not all swales do overflow, some can completely absorb all the water that they interrupt, hold momentarily before percolating into the ground, but there are occasions when large catchments are connected to swales that are not long enough to take the full downflow from your catchment in big rain events. Therefore, some sort of discharge point needs to be installed. That can be at the end of dam spill-ways, so a dam connected to a swale then has the spill-way discharge for the swale as well as the dam catchment, or a level-sill spillway can be installed, simply, in the middle of the swale. An unwanted choke point is inevitable if you don’t have large enough pipes. So a good assessment of the volume of water that overflows the swale needs to be made, then pipe size needs to be chosen carefully and the height of the pipe crossing is going to vary accordingly, as will the number of pipes you install.
You can’t just install one large pipe, then expect the water to rise higher than the swale mound, that will cause a potential catastrophe or too small a pipe will create a choke point which can create an overflow of the swale mound and then an erosive event. Although, if swales break, it is nothing like a dam breaking, it is always a very small amount of damage and erosion and can be very easily repaired, because it is not a compacted element, a very small "V" will cut into the swale mound and can be easily replaced with top soil or any other soil element that can fill the cut. We’ve seen this happen quite a few times and have gained experience in buffering the event so that it happens less and less as we install swales into larger and larger catchment properties. Often, multiple pipes are installed that keep the height of the crossing, and the top height of the pipes below the spill-point of the swale or at least well below the top height of the soft swale mound on the lower side of the trench. This makes for a very convenient crossing point because it levels the swale and a certain amount of material needs to be put on the down hill side of the mound, all of which is compacted.
These crossings become wonderful assets to a property, giving you convenient access. There is also no reason why this ‘bridge’ needs to stop you from driving inside the swales at dry times, as ramps can be set up on either side of the head wall crossings next to the pipe entrance and exit points so that vehicles can transit along the swale trench and then up a ramp, over the swale pipe crossing at right angles to the normal track and back down into the swale at the other side. This is quite simple design but extremely effective.