As we have seen, all manmade slopes alter the natural state in some way, none more so than natural drainage patterns. To allow for proper control of water, various drains and water control features have to be provided in every manmade slope. When we create a slope, in the above example for, say, a roadway, we could simply cut so as to have one steep slope face as in Slope A or we could divide the slope face in series of smaller slopes with benches like in Slope B.
Slope A is faster to construct but troublesome in the long run. This is because rain falling on the slope Toe Drains: These are drains provided at the bottom of the slope. Toe drains accept water from the slope face directly above, from the area below the slope such as roadways, and all water discharged by the cascade drains. Toe drains channel all this water safely away to the nearest public drain or suitable water courseface has a long way to go down to the drain below. As rain water falling on the top runs down, it gathers speed and gets greater in volume as it joins water falling along the rest of the slope face. By the time all this water reaches the bottom of the slope, the energy developed can be high enough to cause the lower part of the slope to erode and in time not only cause the whole slope to fail but also to wash away the road.
Slope B consists of a series of smaller slopes such that all water falling on the smaller slope face has a much smaller distance to travel and cannot develop enough energy to be destructive. Water from each face is channeled away via drains along the benches and so volume of water never gets high enough or fast enough to be destructive. Numerous problems that may be created when using Slope A are eliminated or reduced by simply providing slope features like smaller slope faces, benches and drains.
Understanding these features and how they work and interact with each other is important if one is living in hilly areas.
FEATURES OF A MANMADE SLOPE
To understand the various features of the slope and its drainage system, we take a well-designed slope as an example. Every manmade slope will have some or all of these features depending upon the size of the slopes:
Berms: Berms are flat platforms usually about one
to three meters wide that separate two slope faces. Berms are used where the slope is very high, and they serve to break up the high slope face into a series of smaller slope faces. There is also a drain provided along this flat platform, the berm drain. Note that the slope of a berm is angled towards the drain. This is to allow all water falling on the slope face above and on the berm to be channeled into the drain. This prevents water from rushing over and eroding the slope face below.
Slope Face: The slope face is the inclined surface between the berms. Slope face is also referred to as batter. The slope face should always be well-protected against the rain by thick grass or similar vegetation or by other means such as gunite or stone pitching. Without this cover, the slope face would be exposed to rain, erode very fast and become unstable.
Cut off Drains: The drain at the very top of a slope to intercept and safely carry away all water coming from areas lying above the slope, including that from the property above. This drain is also called the interceptor drain. This drain must be large enough for the amount of water it is expected to carry. The cutoff drain channels water safely out to the nearest cascade drain and from there to some suitable water course.
Berm Drains: These drains are provided along the berms of the slope, and they catch the water coming down from the slope face immediately above and from the berm itself (blue arrows in the figure). Berm drains channel this water to cascade drains and other drains
Toe Drains: These are drains provided at the bottom of the slope. Toe drains accept water from the slope face directly above, from the area below the slope such as roadways, and all water discharged by the cascade drains. Toe drains channel all this water safely away to the nearest public drain or suitable water course
Cascade Drains: Cascade drains have a very special function. They are used when water has to be channeled from high ground to lower ground. They not only carry water coming from the cutoff drain at the top and the berm drains along the slope, but they also break up the high energy of water coming down from drains at the upper levels so that water from the top and along the slope does not damage drainage structures below. This is done by providing a series of steps or some other obstruction along the drains so that the water, instead of rushing down like one tall waterfall, comes down in a number of small waterfalls. Cascade drains are designed so that the energy of the water is dissipated considerably before it reaches the bottom of the slope.
For example, if the water is allowed to fall freely from a height, even if this height is as low as two meters, the water can do a lot of damage. This damage consists of erosion and the wearing-out of concrete at the bottom due to constant hammering by water falling from a height. Therefore it is very important that the cascade drains are properly maintained and that water is flowing properly at all times.
Broken cascade drains, missing sections of cascade drain and bad connections to other drains will allow water to cause rapid damage. The damage caused by uncontrolled water due to a damaged or broken cascade drain will cause erosion and gullies.
In particular, a blocked drain or sump at the base of a cascade drain can cause considerable damage and erosion due to water flowing out in all directions and lead to unwanted softening of area at the toe of the slope.
Variations of Cascade Drain
WORKING OF A TYPICAL SUMP
Sumps: Sumps or manholes are junctions that allow water from more than one drain to be collected and allow that water to be directed or channeled out in a specific direction, such as to a cascade drain or to an external drain. The sumps provide means for water coming from various directions to safely meet head-on, which causes a lot of mixing and considerable loss of energy. It is important that both the inlets and the outlets to a sump, and the sump itself, are in good condition, kept free of obstructions, and are able to provide free flow of water into the sumps at all times.
The diagram below shows the working of a typical sump where water from three directions enter the sump and is channeled out in one direction. Such mixing of water from different directions also helps reduce the energy of water.
DRAIN ELEMENTS WORKING TOGETHER AS A DRAINAGE SYSTEM
Now that we understand how each drain element contributes towards the control of water along the slope, let us see how they work together. Note that each drainage element has a distinct function. For the drainage system to work properly, each and every drainage element must be in a good working order. Improper function due to damage or blockage in one drain element can affect the whole drainage system and cause considerable damage.