ABOUT WATER
In every landslide and erosion-related problem, water plays a very important role. This is more so in countries like ours where we have considerable rainfall almost all year round. We are used to rain, plenty of it. Our rainfall volumes are large and the large volumes of water play a far greater role in causing landslides and debris flows than in other in sub-tropical countries.
In most cases the soil in the slope would be dry or moist. In these conditions, the weight of soil in slopes can be easily estimated and slope stabilities are easy to calculate. Just enough water in the soil to keep the soil moist actually helps keep soil particles together. This is similar to building sandcastles on the beach. Sandcastles can be built only with moist sand, not dry sand or sand under water. Engineers refer to this action as "soil suction".
When too much water is allowed to get into the slopes, it adds to and increases the weight of the soil mass in the slope. More than that, it decreases soilsuction effect and increases pressure within the soil mass called pore water pressure. This decreases the cohesion between particles in the soil, and weakens the slope.
Whenever we refer to water as the culprit, we are referring to too much water, mostly uncontrolled.
The adverse effect of water is countered by allowing for drains and similar water control facilities. When these facilities are affected or the construction itself was bad, water can saturate the soil inside the slope and the combined weight of the soil and water can increase dramatically and lead to failure.
How does rainfall affect us?
Our monthly rainfall varies from 400mm to 900mm or more, depending on where we are and the time of the year. Every time it rains 10mm, we get 10kg of water for every square meter of ground area. When this water is not given proper attention, we encounter problems.
So what does 500mm of rain in one month mean?
A quarter of an acre land site has an area of roughly 405 square meters, where every 10mm of rain means 405 x 10 = 4,050kg of water. 500mm of rain would mean fifty times more or 20,250kg of water or 20 metric tons of water. A lot of water for not that much of rainfall in a month. This water has to be controlled, otherwise it can do a lot of damage. If most of this water runs down the slope or is allowed to seep into the slope, the slope can rapidly reach unstable conditions.
Water, drainage and your property
Severe erosion and landsliding typically occur during or shortly after periods of heavy rainfall. It is important to understand the magnitude of water involved. The quantity of water affecting slope areas can be quite significant. A large storm can drop over 100mm or more of rain in a relatively short period. If this water falls on the roof and driveway surfaces of a typical 7,000 square feet residential area,
it can result in over 60 metric tons of water.
Additionally, if this pattern is repeated on other similar properties which are uphill from your property, the amount of water generated can be huge. If such a huge amount of water is allowed to flow uncontrolled onto your property or slope, it can have a serious impact on the stability of your slope.
Therefore, it is important that you understand your specific drainage conditions and identify the sources directing water onto your property and slope, even if you have to get somebody else to explain this to you. If you can identify the sources of slope surface and groundwater, you can then take steps to control some of these sources either by yourself, with the help of your neighbours or with professional help. It is also important to understand that if you are living along a slope you cannot live in isolation whether you live above, below or next to someone. Whatever happens to any one of you, affects everybody: you, those living above, below and along your property.
An extreme example of how distress at one location can affect others
1. Consider two dwellings along the hillside with a road and roadside drain.
2. If the roadside drain is damaged, the fast flow of water during rain will cause the damage to increase...
3. ...and cause water to find its way down the slope, and in time weaken the slope...
4....leading to failure and collapse of the house along the weakened slope and also causing loss of toe support for the slope above.
5. If no immediate steps are taken, water coming into the now broken drain will overflow, and rain will also help erode the damaged toe support further.
6. Such damage can only increase with further rain and soon the slope and the dwelling above will collapse...
7. ...and result in a precarious situation...
8....ending in the collapse of the dwelling above.
Changes to the drainage patterns
Every development affects the natural drainage patterns. In large-scale developments, drainage is planned by taking into account every home lot during earthwork planning and landscaping.
Where individual lots along slope areas are separately developed and at different times, overall planning may not exist and it is likely that drainage planning varies from lot to lot. Therefore, coordination between neighbours becomes very important in such situations.
If your property is undeveloped, your development will significantly affect existing slope surface water and groundwater flow. Modifications to slope drainage can have impact on your property in the following ways:
Channeling of new or existing drainage sources to an existing natural drainage course, catch basin, pipe or culvert, or routing drainage directly onto a neighbour's property. Routing drainage to areas that are not able to accommodate the flow can cause considerable erosion and/or slope stability problems.
Converting permeable surfaces to impermeable surfaces such as roofs, driveways, parking areas, or any compacting or cementing over the earth areas. These development features cause an increase in storm water runoff (compared to rain that falls on native vegetation). This increases the potential sheet and concentrated flows that can cause erosion and slope stability problems.
Removal of vegetation, slope cutting or filling. Cutting and filling on a slope can impact slope stability and can also alter existing drainage paths. Removing vegetation can significantly reduce slope stability.
Slope erosion and drainage
As we have seen, uncontrolled water can cause a lot of damage to properties situated along slopes or hillsides. It is not difficult to protect your property or slope. Three basic steps can help protect your property along a slope to a great extent against accelerated erosion and landsliding.
First, understand your property. It does not take extensive effort to generally characterize your slope area and identify the water movement around your property and the slope. Take a walk around your property during rain.
Second, identify problems and plan appropriate improvements at your site. Take the opportunity during property development to include drainage control with your landscaping work. On each property, there may be site constraints that must be considered. Identifying the opportunities and constraints of your site are key goals of your planning effort.
Third, carefully construct and where already constructed, maintain your drainage system. Take the time to ensure that good materials and workmanship are used on your property. The importance of this cannot be overemphasized. Give your system periodic maintenance tune-ups.
Surface water flow can cause erosion of soil on the crest and face of your slope. Usually, the most severe erosion occurs where surface water flows are concentrated. The steeper the slope angle and higher the slope, the greater the potential is for erosion.
Some soil types erode more easily than others. For example, soils such as sand and gravel are more prone to erosion than are silt and clay.
Steep slopes are also likely to have been created by dumping soils down the slope. Such soils are lot more erodible because they are loose. Badly filled soils are always subject to rapid erosion.
Erosion can also cause soil on adjacent slope areas to become steeper to the point that they become susceptible to slips.
Surface water often causes severe erosion to the slopes where pipes or drains are allowed to discharge onto the slope face below. This situation occurs where dwellings or development are at a high level and the planning is bad.
At times such pipes do not only discharge storm water, but can include waste water as well. This is particularly bad.
Damage caused by water on constructed slopes
This same scenario is repeated for concentrated surface drainage that flows over the top of a slope.
Over time, concentrated flows can create substantial gullies.
If the volume of water is large and the velocity of water is high, deep gullies can form very quickly, even within hours. Sheet flow-water flowing like a sheet over a slope face-can also lead to substantial erosion. Sheet flow naturally tends to concentrate into small rills and channels of water. Once such channels are formed, more water will use the same path and rills can rapidly deteriorate into gullies. The small rills and channels can become larger and larger if left unattended.
It is critical that surface water flows be controlled since they can lead to rapid, severe erosion of your property. Control is particularly important for newly developed or modified properties. Something as simple as re-grading your driveway can change surface water flow patterns and cause erosion.
Often the first heavy rain locates potential problem areas. Immediate adjustments to new site development and existing site properties should be performed to avoid erosion problems. The use of erosion control methods during construction (short-term) and for long-term erosion prevention is important to reducing the effects of erosion on slopes. Some of these issues are addressed under section titled "Do's and Don'ts".
