2.2 Stone Lett
The stone lett is a development of a simple drainage channel cut through the turf at the path edge; a technique often used on upland footpaths. A stone structure prevents the channel from becoming overgrown, and is therefore easier to maintain. It should hardly be noticeable, as it is built on the edge of the path using only a few well placed stones.
The purpose of the lett is to create a short open channel at the path edge to allow small amounts of standing or running water to drain through, and away from the path surface.
Bill of Quantities (example)
Use local weathered stone to construct a stone lett with the lining flush with the path surface to collect standing water, and angled to allow water to drain away freely.
POSITIONING OF STONE LETTS
Positions for letts will be apparent if the path is assessed immediately after heavy rainfall. Letts will be required where standing water collects; but only if this is causing erosion of the path edge or braiding where users avoid the puddle. Being at the path edge the lett should not have to withstand the pressure of path use, only water flowing through it. This should never be excessive. If it is, a crossdrain is probably required.
The stone lett should be positioned at the lowest point of the standing water, where the amount of water released from the path edge can be maximised. Further positioning of letts may be required to remedy bad puddling following new path surfacing.
The lett is a simple design consisting of only three or four stones. Face stones create the side walls for the water to flow between with a liner stone, or stones, placed between them to provide a smooth surface for the water to flow over.
- the width of the lett should be at least a spade width, for ease of maintenance
- the liner surface should be flush with the path surface at the path edge
- the liner should have a draining run of approximately 5
- the top surface of the face stones should be approximately 100mm above the path surface, and at the same height as the vegetation on the path edge
Block stone does not need to be substantial in size, as letts are subjected to less pressure than cross-drains. Local stone should be used in its natural form, preferably weathered.
- the side stones should be at least 300mm deep
- the faces that form the sides should be without any protrusions
- the liner stone should be at least 200mm deep; if it is too thin it may be undermined by water and wash out of position
- the liner stone should have a smooth top surface and fit the side face stones as tightly as possible.
Method of Construction
Excavate a shallow trench
- excavate wide and deep enough to accommodate the selected stone and achieve the required dimensions and draining levels
Position the side face stones
- set to the height required above the liner and so that half their depth will be below the top surface of the liner stone
- set to accommodate the width of the liner stone, and at least a spade’s width apart, to allow easy cleaning out off debris and silt
- the top surfaces should be at the same height as the vegetation on the path edge
Position the liner stone
- set to fit tightly between the side stones
- set the liner surface flush with the path edge for water to flow freely off the path; if it is too high it will block the out-flow
- set at approximately 5 to ensure a drainage run for the water to flow out of the lett
- wedge and pack gaps with smaller stones so that the whole structure is solid and immovable when completed
Depending on the stone available one larger liner stone may be adequate. If not it will be necessary to extend the liner using a splash plate stone, set slightly lower than the liner. A short length of ditching may also be required to ensure that the water is dispersed from the path and soaks away into the surrounding ground.
Once the lett construction is complete, re-instate vegetation by turfing up the path edges behind the side stones. Additional turfing may be required if the path edge has been eroded or trampled by walkers avoiding the puddle.
Key points to watch out for:
- set the liner stone at the correct level - too high will block water draining from the path; too low can create a weak path edge and erosion of the path surface
- avoid making the lett too big - it is only needed to channel small amounts of water; minimal impact is essential
Where there is no suitable stone available, or the drainage problem is not serious, a simple drainage channel cut into the path edge can be used.
Letts can also be used to extend simple drainage channels formed in the surface across the path.
Stone letts should only require minimal maintenance. However it is essential to keep them free from silt as they become blocked.
- clear out debris and silt blocking the lett or the out-flow
- check the stability of stonework; re-pack where there is any movement or gaps
- re-pack path edge surfacing in front of the lett if there has been any compaction or erosion
- top up the path level if the puddle is not draining away fully
- silt or gravel cleared from the lett can be reused in packing and re-surfacing
- use natural looking weathered stone, that will blend in with the surrounding landscape turf over damaged or eroded areas around the lett to look as natural as possible
HEALTH AND SAFETY HAZARDS
- use safe lifting techniques when moving and positioning stone for the lett
- do not be tempted to use a lett to solve every standing water problem. They may have minimal effect; or cross-drains may be more effective in dealing with the problem; one drain higher up the path may reduce the formation of several puddles further down. A proper assessment of the site will ensure that the correct decision is made.