Designing drainage systems

Drainage glossary

In most cases ‘drains’ can be either pipes or open ditches. A drain can act in any of the ways shown:-

The diagrams below show two simple examples of drainage systems for paths.

The following terms are used in this book to describe various types of drains:

In many situations, a simple interceptor ditch above the path which takes the water either under or away from the path will be sufficient. For path drainage work, the aim is merely to keep water away from the path, or to pass it across the path in a controlled manner. The aim is not to generally lower the water table around the path. Most drains on hillsides and upland areas need be no more than a spit deep and wide, and about 20 metres long, or else the volume of water gathered becomes difficult to disperse without causing erosion. In the lowlands, longer and larger drains can be constructed as necessary to lead into existing streams and ditches.

Surveying the site

1. Try and find out how the water reaches the path, and decide whether it is surface or sub-surface flow.
   Surface flow in streams is usually fairly simple to deal with as these can be taken across or under the path. Surface flow which occurs as direct run-off of rainwater falling on slopes above the path is more problematical. Side or interceptor drains will be needed to divert the flow away from the path. Rainwater falling on the path itself should be shed by a camber or cross-fall.
   
   Sub-surface flow can cause persistent waterlogging throughout the year, and is often spread over a wide area making the problem difficult to deal with. Look out for places where the water comes to the surface, where flow can be intercepted and taken away from the path. This usually occurs at the foot of steep slopes or where solid rock is exposed.
   
2. Inspect the path in a variety of weather conditions if possible. Mark the places where storm-fed streams run onto the path. Use wooden pegs well hammered in at the edge of the path so they do not present a hazard. Number them for reference, and paint them a bright colour if they are likely to get lost in vegetation. Photographs will be useful for reference as conditions may be very different when work is to be done.
3. Consider where the water will go once it is drained off the path. It is not enough to simply pass the problem on to the adjacent landowner. Consult the landowner for advice as it may be possible to put the water into an existing drainage system. If not, a soakaway may be needed, either constructed from a network of perforated pipes, or by digging a large pit and filling it with rubble. Ensure you are not going to cause a similar problem lower down on the path by unintentionally diverting the water back onto it.
4. It is possible to estimate the required size of pipe by measuring the catchment area. For path drainage, this is probably only relevant for the design of large diameter culverts, or for carrier drains leading into another drainage system.

The procedure is given here.

Materials and methods

In many cases, only a simple drain will be needed, such as a cut-off or a culvert, and often these can be built with local materials. In other cases, the lie of the land and the source of the water may require a more complex drainage system.

Consider the following questions:

1. Should the drains be open ditches or pipes buried in trenches? Ditches are quicker and cheaper to construct, but may spoil the appearance of the area. Consider also whether ditches will act as barriers to keep people on the path, or whether they may be a hazard to walkers or riders. Ditches require maintenance, but this is normally fairly simple to do. Sometimes a ditch may only be needed temporarily while an eroded area is stabilising, and can then be allowed to fill in naturally. In very confined spaces there may not be room for a ditch, for example, alongside a path, and a pipe will have to be laid. Pipelaying involves more expense, but if properly done, provides an unobtrusive and low maintenance system. However, if problems do occur, they are troublesome to correct.
   French drains are ditches filled with permeable backfill, and in the right location provide a low maintenance system. They are generally successful in lowland areas, but in areas of high rainfall and peat soils, such as the Lake District and Peak District, they rapidly block with sediment.
2. Can a machine be used? For major schemes which involve the clearance of new paths, it may be worth using a machine, especially if this can also be used for clearance work. Ditch-digging by hand can be hard and monotonous work if the ground conditions are difficult, and so may not be done to the required standard.
3. In general, a series of cross drains or cut-offs should be constructed starting with the highest drain or cut-off at the top of the slope, and then proceeding downwards to the bottom. Water flowing down the path is then removed as work proceeds, and drainage work lower down may be found to be unnecessary as the problem has already been solved higher up. Although not so pleasant for working, the drainage pattern is easier to observe when working in wet weather.
   
4. Long ditches are best dug from the bottom of the slope upwards, so that you avoid working in flowing water.

Apart from simple ditches, most drainage work requires materials. These may include stone, timber or prefabricated materials such as plastic, clay tile, or concrete pipes. Synthetic membranes called geotextiles can be used to line French drains, which are then filled with rubble.

Stone has the advantage of looking attractive in almost any location where it is used skilfully for building cross drains, side drains and cut-offs. It is a versatile material which can be fitted to any site requirements, and it is very durable. If the drain fails for any reason, or it erodes, stone can be retrieved and re-used, and in the meantime, does not look too unsightly. Stone has the disadvantage of being time- consuming to use, and where unavailable in the immediate vicinity, is heavy and bulky to transport.

Even in upland areas with abundant stone, it may be necessary to import stones of a suitable size and shape for drain construction. Cut-offs and small cross drains can be made of specially cut slate, as used on parts of Snowdon. These have the advantage of quick installation, but the smoothness of cut stone can look as unnatural as an artificial material.

Many authorities in upland areas are now using only stone for drainage and other pathwork, because of the advantages given above. Use of any synthetic materials can look out of place in an upland landscape, and where they fail, leave an unsightly mess to clear up.

Sawn and preserved timber can be used for cross drains and cut-offs, and are suitable for lowland and wooded areas. Lengths of drain can be made in a workshop, and then transported to the site for rapid installation.

Clay tile pipes have been used for agricultural land drainage for centuries, but new drainage work is now almost entirely done using plastic pipes. Plastic pipes are readily available, light to transport and easy to install. As long as the system is well designed and properly installed, the system should be maintenance free. For further details see here.

Ditching procedure

1. In most types of ground ditches need sloping sides for stability. A useful rule to remember is that, even in stable soils, a ditch must always have a top width at least twice that of the bottom. In many soils, three times the bottom width will be necessary. Angles of repose of various types of soil are given here.
2. The gradient of the ditch will normally be dictated by the lie of the land. Keep the gradient of the ditch bed as even as possible, so that flow is not hindered. Avoid ditch gradients steeper than 1 in 12 or erosion of the ditch is likely to occur. Erosion of steep ditches can be reduced by building check dams of stone, timber or brushwood, at intervals across the ditch. These slow the flow of water, but require periodic cleaning out as silt accumulates. Alternatively, ditches can be lined with stone along the bottom and sides, in which case they can be constructed successfully down very steep slopes. These type of ditches are usually constructed as part of schemes to restore eroded rocky slopes, and are described further in Chapter 11 – Erosion control and vegetation restoration.
3. Generally avoid a straight line, which will look artificial. Choose a natural line by taking into account the various factors of slope, path alignment and appearance.
4. Where a subsidiary ditch joins a larger ditch, make sure it enters at a gradual angle, to prevent erosion occurring at the junction. The bottom of the subsidiary ditch should be slightly above that of the main ditch.
5. The tools required will depend on the type of ground encountered. Spades may be sufficient in loams and cultivated soils, but more often forks and picks will be needed to break up stony or compacted ground.
6. When ditchdigging, work from the lowest point of the ditch uphill, so that you are not working in flowing water.
7. Often the spoil can be used for filling holes, building up the path surface, or for restoring eroded ground. Work out the logistics carefully at the outset, so that spoil is moved the minimum number of times, and without unnecessary trampling across soft or wet ground. If the spoil is not being used, spread it on the downhill side of the ditch, or on the opposite side of the path.
8. Stand in the bottom of the ditch while digging, not on the sides.
9. For larger ditches, or for pipelaying, work as a team along a length of the ditch. For example, one person strips the turf, a second loosens and removes the top spit, and a third cleans out the bottom of the ditch. If volunteers work individually on sections these are likely to differ in depth, width and alignment, which will hinder the flow of water.

Pipes and pipelaying

Apart from culverts over 225mm diameter, plastic (uPVC) pipe can be used for most lowland path drainage work. Flexible uPVC pipe is inexpensive, light to handle and easy to install. It can be cut with a sharp knife or hacksaw, and joined with push-on connectors and junctions. Although care should be taken in laying, differential settlement is not usually a problem as the pipe is flexible enough to permit some soil movement without being damaged.

An important distinction is between perforated (permeable) or unperforated pipes. Perforated pipes must be used wherever the drain is designed to collect or intercept water, or to disperse it back into the ground through a soakaway. Unperforated pipes must be used where the drain is simply carrying the water under the path or into another drainage system. Perforated pipes in peaty soils tends to get blocked with roots.

When using plastic pipe, particularly for culverts, care should be taken that the ends of the pipe are well secured in a headwall, which holds and hides them. Plastic pipe is unsightly, and drainage schemes using it need checking to ensure that the pipes are still hidden, and functioning.

For large culverts over 225mm diameter, concrete, steel or vitrified clay pipes will normally be needed, unless very large stones are available to build a stone culvert.

Bedding material

All pipes should be laid on a firm base to minimise the amount of settlement. Most clay, silty and loamy soils give a firm enough base, but any dips or patches of soft ground should be filled with suitable bedding material, such as gravel or chippings of about 30mm down to 5mm diameter. Very rocky ground or peat will require a layer about 50mm deep of bedding material.

Backfill

In most situations, the efficiency of a permeable pipe in collecting water will be increased if it is covered with permeable backfill. This should be either washed gravel, stone chippings, slag or hard clinker of size 30mm down to 5mm. It must not be at all powdery, or it will block the holes in the pipe. If suitable material is available nearby then it is always worth using. If not, and material has to be bought and transported to the site, a decision will have to be made about whether or not it will be cost effective.

Backfill above permeable pipes should be firm but not compacted, or permeability will be reduced. The spoil from the trench should be mounded at the surface to allow for settlement. Permeable backfill can be used to the top of the trench, which then forms a French drain.

The requirement for backfill above impermeable pipes is quite different, as it should be stone-free and capable of being compacted so that it is absolutely firm. This is especially important for culverts. The suitability of the backfill can be tested as follows:

1. Fill a 250mm length of 150mm diameter pipe with a sample of the backfill. Level it off, remove the surplus, and empty it into bucket.
2. Using a quarter of the material at a time refill the pipe from the bucket, tamping down each addition to maximum compaction.
3. Measure the distance from the top of the pipe to the top of the compacted material. Divide this measurement by 250 to give the ‘compaction factor’. If this figure exceeds 0.3, the material is unsuitable.

Pipelaying

1. Follow the procedure given for ditch digging, but dig the trench as narrow and steep sided as possible. The trench width should be three times the diameter of the pipe. Make the bottom of the trench as neat and even as possible, removing rocks and tree roots and filling any holes with bedding material. Lay the pipe as soon as possible after the trench is dug.
2. Start laying the pipes from the outfall end, and work uphill.
3. Inspect the alignment of the finished pipeline and adjust if necessary.
4. For impermeable pipes, backfill enough spoil to secure the pipe in position, up to about the midpoint of the pipe’s diameter. Compact, then add more in 150mm layers, tamping down carefully. Mound to allow for settlement. Backfill permeable pipes with permeable backfill, not compacted.

Ecological effects

To cross large areas of wetland or bog, boardwalks, stone flag or ‘floating’ aggregate paths are needed (see Chapter 8 – Surfacing). However, many paths cross small patches of boggy ground, where a floating section of path may be too expensive to install, but whose ecological value would be reduced if the ground were drained to improve the path. Careful drainage work is needed to improve the path without degrading the habitat.

To minimise effects on the habitat, do not dig land drains all across the area, but instead dig a side drain leading into a culvert or cross drain. Although this will lower the water table along the edge of the bog, a peat bog will hold water and the main area should be unaffected. The side drain also discourages walkers from stepping off the path. This system can be improved by putting a board along the drain, as shown. This maintains a high water table throughout the bog.

Seepage culverts are designed to take seepage flows from small bogs and mountain flushes. A cheap, simple solution is needed that will dry out the path without draining the bog or marring the appearance of the area.

Dig out a small pit or sump, and support it with stakes and rails as shown. Lay the culvert pipe from the sump under the path, and then replace the fill and vegetation over the pipe. Some trial and error may be needed to find the size of sump to take enough water without drying out the bog, and it is best to underbuild rather than risk destroying the bog. Another sump can be dug if necessary.

In many situations in peat bogs a simple narrow ditch dug a spit deep with a narrow spade will be sufficient to intercept the water and drain it away from, under or across the path.

On braided upland paths, sections of unwanted path can be converted into interceptor ditches to protect the upper side of the main path. Close off the top end of the braid with a boulder or mound so that it does not look inviting to walk along.

Do not dig large ditches across areas of peat bog, as this may irreversibly damage the bog. Heavily trampled areas of bare peat are very susceptible to wind blow if they dry out with no protecting cover of vegetation. The only method of path construction on deep peat bogs is to build ‘floating paths’ of geotextile and aggregate, stone flags or boardwalks with small cross drains and culverts where natural watercourses occur (see Chapter 8 – Surfacing and Chapter 9 – Boardwalks and bridges).

Habitats can also be adversely affected by the raising of the water table. Do not place drains where they can cause waterlogging around trees.