Silt Trap - Permaculture Design Course Handbook

Written by Doug Crouch

Introduction

This earthwork, while simple in its nature, is very worthwhile in several different contexts to achieve higher water quality and prevent sedimentation in other earthworks. This helps to save time and money and allow other earthworks to be more energy efficient through working with slope dynamics. The pattern

beaver dam, pic from https://www.theodysseyonline.com/beaver-dam-natural-hoover-dam

behind it is more edge; that in the past beavers probably did this work but we now use machines and hand tools to slow the descent of sediments from entering into other earthworks. They are located for ease of access and the subsequent cleaning so their sediment trapping capability stays high. In disturbed landscapes, which most are these days, it is truly amazing to see just how much sediment moves and subsequently caught.

Types

I have seen or implemented several different types of silt traps in my years of work and travel. While above I say these earthworks, they can also be man-made structures from concrete given the right context. In one such case at Terra Mae, our original development project in Portugal, there was a diversion drain across the road leading to a pipe. Instead of this pipe going straight to its final destination, a 16,000 gallon or 64,000 liter tank, it instead was piped in and out of a silt trap first. Thus the immense rain water runoff from the road laden with all sorts of sediment, but mostly sand and silts, was led from the diversion drain into a silt trap concrete box then to the water tank. Without it sedimentation would occur at a higher rate and necessitate cleaning of the tank more often. This is a very big process as we have gone through it at many sites there in Portugal. Also without the silt trap you run the risk of clogging pipes as the outlet is located towards the bottom of the tank. Furthermore it also diminishes volume capacity, which is paramount in these locations of brittle climates where rain can be sparse or non existent for five to seven months. This applies both to tanks and also earthen structures say a dam created for irrigation.

Diversion drain catching runoff from road

16,000 gallon or 64,000 liter tank, note pipe in bottom left corner entering

silt trap full showing cloudy water from clay particles which are harder to trap because of their suspension quality due to size

empty silt trap showing entry and exit

cleaning up the diversion drain, Terra Mae, Sintra, Portugal 2009

Just down the road at Terra Alta, our second development project that also harvested abundant road runoff, silt traps were put in place as well. This time they were earthen, which led to a series of rain gardens and diversion drains known there as the serpent garden. It was a unique creation containing lots of edge and silt traps were rightfully employed to filter the water as it entered the site. It was simply a hole in the ground where runoff flowed into and similar in shape to the other rain gardens of the earthworks complex. This trapped sediments before it cascaded further into the landscape.

: Terra Alta, shows the suspended clay particles but silts and sands were captured above in silt traps

: Terra Alta

Location and Design

Locating these design elements in the landscape is simple. You put them just above your intended earthworks or tanks where they are easily accessible for cleaning them. They will do their job of trapping sediments and in the case of leading to a pond, you don’t want all those sediments in a pond because its volume is decreased and succession occurs much more rapidly in terms of the pond eventually morphing into a wetlands. They are located within the valley flow that fills a pond normally. They are cleaned either by hand or by machine depending on the size of catchment and how many sediments are moving. Furthermore one way to turn this problem of movement of sediments into a solution is when you harvest them put them in potting mix. This further reduces our needs for outside inputs thus saving, time, money and resources.

Again they are simply a hole in the ground and their size depends on the amount of water flow and sediments moving. They are built in the shape of a pond in general but do not have compacted bottoms making them water proof. They will also have a spillway, just as the concrete box example from above having an inlet and outlet. This allows for them to fill up, swirl around dropping the heavier sand and silts, and then the water flow going to its intended destination. Again they will have suspended clay particles but the size of sands and silts is what really fills them up.

To finish our valley dam at Treasure Lake, Petersburg, Kentucky, USA has no silt traps. When ourselves and many of the neighbors did some light logging (2012), roads were implemented to harvest the timber, which is a big cause for sediments to move in more forested situations. As you will see in the pictures below the lakes feeder streams have been carrying sediments and making the lakes coves and back shallow part silt up and succession to occur. Especially in the back, this could have been prevented in part by not removing the beaver from the landscape as my grandfather in his old school thinking way did. Additionally one rock dams and gabbions serve as sort of silt traps and that article should referenced as well for maintaining higher water quality in streams in general especially if they are leading to an earthwork. In our valley dam each spring the water muddies diminishing the length of the spring fishing season as that is the business on the land. Thus you must create more edge in the landscape to deal with these disturbed ecosystems higher in the landscape as we employ the idea of accelerating succession and evolution. In the past it was again beavers or simply non-incised waterways having large woody debris fall across them and trap these sediments. This not only traps silts and sands but also nutrients, which through this edge also improves water quality lower in the landscape. Try it out and leave feedback. Thanks for reading.