Design and Engineering Considerations for Surface Drainage Systems

By Ankit Sehgal

When it comes to developing and executing an effective surface drainage plan, a number of factors have to be taken into account. The basic purpose of a surface drainage system is to shape and mold the land into a watershed that can convey the runoff into an organized drainage system comprising of catch basins, trench drains, and storm sewers. If your surface drainage is below par, it will have a detrimental impact upon the subsurface drainage.

Surface drainage has to do with any manner of regulated removal of surface runoff on account of the spring thaw, irrigation, precipitation, irrigation, or anything else that leads to surface water accumulation.  If you don’t deal with overflow in time, it will have an adverse impact on the land and surrounding structures. Improper planning may lead to erosion and sedimentation. When one talks about a comprehensive drainage system, surface drainage is at the forefront. A good surface drainage system is the only way to cope with heavy volumes of surface water accumulation after bouts of intense rainfall. Not doing so will quickly saturate and overwhelm the capacity of the soil to absorb water, which could lead to extensive flooding and changes in landscape structure.

Let’s look at what some of the major surface drainage systems are:

1. Open Drains

These can be of varying dimensions and are principally of three types:

Shallow- These are only up to 0.3 meters deep and can be created with the help of a hand shovel and can help remove water from shallow depressions and direct them to a larger drain or a stream. These are not really suited for draining a large expanse of land and are more in the shape of a temporary arrangement.

Medium Depth- Such drains are between 0.3 to a meter deep and are created with the help of an excavator and are typically V-shaped and flat at the bottom and a gradient that is steep enough to help water flow through quickly without damaging the drain walls or bottom. These are best suited for flat areas.

Large- Large open drains can be several meters deep and wide and are created with the help of dragline excavators, bulldozers or scrapers and are capable of evacuating large volumes of water.

2. Humps and Hollows

This is a system where a surface in question is shaped into parallel humps separated by hollows. This allows the humped shapes to shed excess moisture into the hollows which double up as shallow surface drains.  This type of surface drainage is ideal for areas where tile or mole drainage is not possible on account of inadequate depth or fall of the soil.

The humps and hollows system creates a series of lateral surface drains that can help discharge water into headland drains. One can use formulae to calculate the size of the drain taking into account the amount of water needed to be removed. The spacing between the humps may vary between 10 to 20 meters depending upon the speed with which the water needs to be removed. The greater the space between the humps, the slower will be the rate of discharge of water.

3. Levees

Levees are surface channels usually created on land with a gradient in such a manner that the soil removed to create these forms a levee on the downslope of these channels.  This helps the surface runoff not build so much velocity while flowing down a slope that it starts eroding the land. The banks or levees have to follow the lay of the slope and make the water flow down gently and not create make the lane below susceptible to landslips.

Ideally, such levies should have a spacing of 30 to 50 meters for slopes that possess a gradient of five to twelve degrees. Every levee can cater to an area of about 3 hectares with a total length that is less than 400 meters. Care must be taken to establish a grass cover immediately after creating a levee or the channel could quickly erode.

4. Grassed Waterways

A grassed waterway is typically quite shallow and may be narrow to a few meters in width. These can be used to regulate drain outflows going down slopes, so as to cheaply prevent erosion.  The ratio between the vertical and horizontal sections of the waterway needs to be in 1:4 proportion. A necessary prerequisite of such a waterway is a dense expanse of grass. The quantity of water to be evacuated as well as the steepness of the slope will have a bearing on both the size and the shape of the waterway.

The steeper the slope, the wider should be the waterway. It is important that the bottom of the waterway be as horizontal as is possible to enable the water to spread out evenly.  It is important that such waterways not be constructed in areas prone to erosion or they will fail.

Final Thoughts

Surface drainage systems are an essential part of any holistic drainage system. Not having a well designed surface drainage system can wreak havoc with a landscape unable to permeate excessive runoff. It, therefore, makes eminent sense to keep it at the forefront in site planning, design and engineering. These systems are used in conjunction with other drainage applications such as catch basins and heavy duty trench drain systems (https://swiftdrain.com/heavy-duty-trench-drain/).

The methods of surface water evacuation described are some of the more popular sustainable methods used to deal with a myriad of potential waterlogging issues. A well-designed surface drainage system ensures that the duration of ponded water is never long enough to cause any damage on a particular landscape. A free-flowing surface drainage system enables free flowing of water thereby preventing flooding of a specific area. That apart, such systems decrease the health implications associated with stagnant water buildup such as those caused by mosquitos.

Managing surface drainage can often be done with simple but effective designs that don’t have a relatively large capital expenditure and impact to the natural topography.

Ankit Sehgal is the President and Head of Engineering at Swiftdrain Inc., a global drainage and infrastructure products manufacturing company. He oversees all product design and development at the firm, including engineering, materials sourcing and production. https://swiftdrain.com