Drainage Issues – The Importance of Ensuring Adequate Provisions
1. It is an essential element in any planting scheme, whether agricultural or horticultural, that soil must be able to ‘breathe’ with oxygen entering the top 400mm, known as ‘top-soil’ either through the plant rooting system or general openness of the soil, which is ideally comprised of friable material made up of sand/ silt, and clay, containing nitrogen, phosphorus, potassium and magnesium, along with a number of chemicals known as ‘trace elements’, essential for the production of plant material including chlorine, cobalt, copper, iron, manganese, molybdenum, sodium and zinc.
2. Topsoil is the uppermost layer of soil on most land. It has the highest concentration of organic matter and micro-organisms, and is usually around 200mm deep. It is comprised of minerals, matters, water and air. Topsoil is a mixture, varying from site to site and region to region, and it contains not only the elements mentioned above, but broken-down organic matter (humus) comprising of decomposed plants, leaves, twigs, decayed animals and animal matter, and other well-rotted material.
3. Topsoil should be friable – easily broken down into particles – and is the natural home of living things, including roots, living organisms and creatures. Some top-soils are more friable than others, which may consist of more fine particles, forming a clay soil. Clay soils will retain water which will pass quickly through sandy soils, as they contain more air within their structures.
4. The soil to be found in many gardens, especially in the South, may be classed as ‘clay’, which should be maximum of 400mm (minimum 150mm) deep before it is likely to be become aerobic, producing toxic conditions for plants, trees and shrubs.
5. Below 400mm deep, clay has a tendency to self-compact and where the biochemical oxygen demand (BOD) exceeds the rate of aeration, this will result in the development of anaerobic conditions that are detrimental to plant root functions.
6. Sub-soil, which has lower BOD should always be used to create rooting zones in excess of 400mm. It should have a neutral rating (7) on the pH scale – the measure how acidic a soil (or water) is. Below hP7 the soil will be acidic. pH is the power of Hydrogen, and is used as the scale or measure of suitability for purpose of top-soil in the growing and well-being of plants.
7. Typically, clay soil will have a pH value of 7.5, but any reading between 6.5 and 8.5 is usually suitable for all general planting purposes.
8. The first principle when assessing the type of essential drainage required for a successful project, is to assess the type of soil you will be working with, together with the nature and topography of the site. Trial holes should be excavated, or an invasive survey (using a penetrometer) made to establish the nature of the soil, usually to a depth of 600mm. These tests should be made at six metre centres around the site, and the results recorded on a site plan.
9. If possible, a hydrological survey should be commissioned, showing how ground water is likely to be moving across the site, highlighting water infalls and outfalls. Water will enter each site from the highest point, and leave the site from the lowest. By ‘proving’ the natural water movement with the survey before commencing works will avoid any issues or disputes with neighbours in the future.
10. When designing a garden, the primary factor to consider is the likely movement of water across the surface and the likely effect the design will have on the potential increase in
speed and volume of water any hard paved surfaces may have on the existing site. Any areas of new hard landscaping will move water from one part to another (reduced) area, by directing the water into flower beds (for example) that has fallen on the newly impermeable paving.
11. For this reason, it is essential to devise a drainage layout of subterranean permeable pipes, to connect one section of the garden to the other, in a natural way that takes the lay of the land into consideration, i.e. linking the site from the highest to the lowest point.
12. I suggest that a copy of a drawing or plan of the site is marked up clearly showing a suggested drainage layout, taking all areas into consideration. Please note that the system should have two separate widths of drainage ditch, designed to cope with the likely volume to be removed from each area or part of the proposed garden design.
13. All trenches should be one metre deep, to both ensure that the soil can drain to a depth below any surface rooting systems of plants, and to allow other works requiring pipework and conduits e.g. irrigation or lighting to be installed without risk to the drainage pipes.
14. Please see cross sectional drawings of both primary and secondary drainage pipe construction methods, showing depth and width of shingle and method of installing pipework and protecting the pipes with non-woven geotextile fabric.
15. Land drainage falls under BS EN 752, although this is primarily concerned with drainage systems outside of buildings, where water, operating under gravity, leaves a building or paved area, roof drainage system or paved area to where it is discharged into a wastewater treatment plant i.e. through drainage channels and pipes (movement of water, not necessarily related to surface ground drainage).
16. It is a vital principle in horticulture to ensure that the requirements of plants are taken into consideration at all times. Every plant has roots, and at the tips of their roots, they have fine ‘hairs’ called ‘feeder-roots’ which draw up water and take in nutrients from the soil to feed the plant. If these feeder roots become suffocated through excess water – effectively drowning the plant roots – or the ground becomes toxic through becoming anaerobic, with no air within the topsoil, then plants cannot survive.
17. In order to de-water each area/bed, an interconnecting subterranean land drainage system, comprising channels, with perforated flexible pipes laid into pea shingle to allow water to penetrate the pipes, each channel 1000mm below finished surface levels (this is essential to prevent damage to the pipes from other conduits and cables fitted at shallower depths during construction) x 450mm wide, filled with pipe/shingle to a depth of 600mm (with 400mm of top-soil or other surfacing material e.g. paving over the channel) should be laid out across the site, commencing at the highest point and terminating at the lowest part of the garden.
18. The basic framework or network of pipes should follow ground levels, from highest to lowest, with suitably sized soak-away crate systems – deemed large enough to cope with the likely volume of water (including from hard paved areas as well as planted/turf areas) constructed within the system to cope with the amount of water. One such soak-away would normally be installed near to the lowest point of the garden to act as the final water disbursement point before excess water leaves the site via natural outfalls
The above recommendations should resolve any issues of land drainage. The first and most crucial element is to obtain a hydrological survey, which will show the likely movement of water across the site, including the exit or lowest part where all water will naturally drain.
