To install drainage and mitigate flood can be more complicated than what a lot of people first recognise.
Flooding is increasing in many parts of the world; which is leading to a rapidly growing need for people who understand and are skilled to deal with drainage. If you want to work in this field; there are growing opportunities. Even if you just want to better manage your own property, there is a greater need than ever before to understand how to manage drainage.
Why is Drainage important?
Drainage is a technique for managing water on the surface and beneath the ground.
Ground water drainage can be either natural or artificial.
Natural drainage occurs in soils with a deep hydrological profile and hydraulic conductivity values that are either constant or rise with depth. As a result, impermeable soil layers are either non-existent or positioned deep in the soil profile. The ground water table cannot be identified within 6 feet of the ground surface in such situations, therefore artificial drainage is not necessary.
A lack of natural drainage frequently results in a high water table and, in certain cases, water ponding on the surface. These conditions are the result of a shallow hydrological profile. When the rate of rainfall or the rate at which water is added exceeds the permeability of an impermeable layer, a high water table and waterlogging in the soil are common. This problem can easily be recognised in early dry season when the area are fairly dry but the affected area is wet. The wet spot will most likely be in the middle of a hill rather than at the bottom. Artificial drainage systems can be set up to remove surplus water and keep the water table at a healthy level.
Managing Water at the Source
Excessive water has to come from somewhere. This may be rainfall, a rising water table, excessive irrigation, a burst pipe, concentration in one part of a property, or a flow of water from outside the property (eg. Rising river levels). The best way to prevent excessive water is to understand where it is coming from, then catch it and move it away before it reached unwanted levels in an inconvenient location.
Many people do not appreciate and consider the way water can be concentrated. A simple example is the way rainfall is displaced from roofs or paved surfaces.
• In its undeveloped, natural state, 1000 square metres, without any shrubs or trees, has 1000 square metres of ground and low plants that can catch the quantity of rainfall that falls.
• If you cover half of that 1000 sq metres with roofing and concrete paving;, almost all of the rain falling on these built areas will divert to the remaining 500 sq metres. This doubles the quantity of water which that the site needs to cope with.
• The problem can be more complex, because the runoff tends to be concentrated in specific places, rather than spread over the entire area. At the edges of a path, the adjacent soil will receive far more water than the ground further away from the path edge. This is why we will often find seeds germinate, or grass grows, better along the edge of a path after rainfall.
• If you plant lots of trees and shrubs in the 500 metres that is not built on, this will provide a massive quantity of leaf surface which can catch and hold water in a rain event. This reduces the quantity of rain on the ground. This offset is only effective to a point. Once the plant leaves are saturated; the excess water then begins to fall on the ground and the rate of water build up can approach what it may have been without any planting.
WHAT TO DO?
The first thing is to observe how and where the water collects in your property.
• Does the water flow from higher parts of the property and collect in low spots?
• Does water come from outside your property or inside it? If outside,; where is it entering your property?.
• Does rainwater soak into the soil or does it just lay on top?
• Are some parts of the property more prone to waterlogging than others?
Once you have an understanding of where water is coming from and moving to, you can then set about considering how it might be either drained away, or prevented from building up.
It is a good idea to dig a hole in the problem areas to find out about the properties of the soil. In some cases, the reason the water doesn’t drain away is because the topsoil is a thick clay that can only absorb small amounts of water at a time. In other places, the subsoil might not be able to absorb water as quickly as the topsoil.
When you have identified where and why the problems are occurring, you can start remedying the situation.
Learn More: Study Drainage and Flood Mitigation