low impact development

What is Low Impact Development

It is obvious that low impact development methods is the most ideal storm water management method for developing countries like Sri Lanka. Especially areas that face flood situations.

Water plays a vital part of life. Our ancestors knew how important water is; hence they treated water as a deity. But the world is developing rapidly with increasing human needs. Within this developing world, water has become a crucial element. Most of the water resources are being polluted and the amount of usable water is reducing. Thus, it is important to protect even a drop of freshwater we have. For this, we need strategies such as low impact development.

What is the low impact development

The term Low Impact Development implies the developments that are done in an environmentally friendly manner. But in Canada and the USA, this term means an alternative site design strategy that uses natural or engineered infiltration and storage techniques to control stormwater. Low impact development method emphasizes conservation and use of on-site natural features to save the water and protect the water quality.

Low impact development is mainly focused on the fact of utilizing water as much as possible before releasing it to the general runoff system. There are a few rules of thumb that should be followed when designing for low impact development.

  • Conserving natural areas as much as possible
  • Minimizing the development impact on hydrology
  • Maintaining the runoff rate and duration (trying to keep the stormwater in the site as much as possible)
  • Implement pollution prevention, proper maintenance, and public education programs

The basic processes we can use to manage stormwater include pretreatment, filtration, infiltration, storage, and reuse. Some of the designs complete all these 4 steps and some of the designs complete only a few of them.

  • Pretreatment: removal of pollutants in the form of debris, trash, and larger sediments. This is done either mechanically or using bioengineering methods.
  • Filtration: the process of filtering the stormwater and this process removes the excess nutrients, heavy metals, and other chemicals.
  • Infiltration: allows groundwater table to recharge and vegetation plays a major role in this process.
  • Storage and reuse: capturing and reusing stormwater as a resource helps maintain a site’s predevelopment hydrology. It also creates an additional supply of water for irrigation purposes. 

So let’s have a peek at the design solution that comes under the low impact developments. The most popular low impact development design strategies are as follows.

1. Bio-retention

In this process, we remove contaminants and sediments from the stormwater runoff. In bio-retention ponds, the stormwater is collected into the treatment area (the low land area). This treatment area consists of a grass buffer strip, sand bed, ponding area, organic or a mulch layer. But this arrangement is changed according to the area and the stormwater quality. If the runoff consists of many chemicals, we can increase the growth of absorption plants. But the issue with the bio-retention ponds is that they require a large area. Hence this won’t be ideal for an area that is highly populated.

2. Cisterns and rain barrels

This simple technique does not require spatial knowledge. All you got to do is, collect the rainwater that slides through your roof using a channel. The easiest method to collect this water is using a barrel; hence the name rain barrels. This water can be used for gardening purposes. It is important to cover the tank properly to maintain the purity of the water.

3. Green roofs

Green roofs quickly became a trend that attracts public attention. This has become a popular method not only in outside countries but also in Sri Lanka (eg: Havelock City). Green roofs collect the stormwater and store a portion. Since green roofs have the capability to filter water, the excess water that can’t be stored in the garden is also filtered.

Types of green roofs (Andrew Myrthong)

The initial cost of the green roofs can be high and this is not an ideal method for middle-income houses. However, in places like shopping malls and hotels, green roofs bring not only environmental benefits but also popularity to the place. The construction process of the green roofs can be complicated and it is important to consultant proper resource persons when constructing a green roof. Furthermore, there are 3 types of green roofs: extensive, semi-intensive and intensive. Selecting the correct type to build could be a challenge since these three have different construction methods and different benefits.

4. Permeable paving

It is a simple yet very effective method, which is not new for Sri Lanka either. Permeable paving allows the earth to absorb the stormwater at the site. It is collected instead of transferring it to some other place. You might have seen the potholes on the road that are filled with water, which is an issue with non-permeable paving. They prevent the absorption process. Another important aspect of permeable paving is allowing water to flow through the earth without causing any kind of erosions.

5. Bioswales

Bioswales can remove debris and pollution of the runoff and act as temporary water storage. Other than the water purification process, bioswales help to prevent floods since they can store a considerable amount of water. There is a greenery strip at the bottom of the bioswale that enhances both filtration and infiltration. An ideal plant to grow here is the “Vetiver” plant (Savandarā). The vetiver has the ability to grow its roots really deep, more than 10 feet sometimes. This not only gives the ability to speed up the infiltration process but also to stable the soil.

The implementation of bioswales can be seen in our local roadside. These methods are especially being used under the Tech City Development project in Homagama.

6. Retention and detention ponds

Retention ponds (a.k.a wet ponds) hold water back similar to how water is held behind a dam. The retention pond has a permanent pool of water that fluctuates depending on the precipitation and runoff from the contributing areas. Maintaining a pool keeps deposited sediments at the bottom of the holding area.

Detention ponds are more common in the dry west and serve as important flood control features. They are usually dry except during or after rain. Their purpose is to slow down water flow and hold it for a short period of time (24 hours). Urban areas rely on these structures to reduce peak runoff rates associated with storms, decreasing flood damage.

These two different kinds of ponds are used for flood control and draining stormwater treatment. Both systems function to settle suspended particles and other solids present in stormwater runoff.

So it seems that low impact development really is a game-changer. Let’s have a look at both the pros and cons of low impact development.

Related article: All About Green Buildings

Advantages of Low Impact Development

There are many advantages of low impact developments, such as boosting curbside aesthetics, providing habitat, protecting against flood events, and filtering detrimental pollutants collected from impervious surfaces. Stormwater, in theory, should leave a LID project site at the same rate, quality, and quantity that existed before development.

Other benefits of installing LID:

  • Reduces initial and long-term costs
  • Offers retrofit option
  • Provides groundwater recharge
  • Accents natural landscape
  • Reduces driving hazards by diverting stormwater from streets
  • Supplies shade, absorbs noise, provides windbreaks
  • Promotes watershed education and stewardship
  • Improves air quality

Limitations of Low Impact Development

Considering the possible downsides prior to installing low Impact development is also important. There are some adverse aspects, such as low impact development projects tend to involve more ongoing maintenance than the traditional pipe-to-sewer stormwater infrastructure.

Another downside is that not all low impact development projects function the same. Understanding the site specifics is critical for selecting the proper LID. For example, vegetated filter strips can be ideal for small parking lots but not for sites with large drainage areas.

Other potential cons to installing LID:

  • Requires approval from local codes or ordinances
  • Increases maintenance and required landscaping
  • Diminishes pollutant removal if improperly designed
  • Increases liability and safety concerns
  • Reduces performance over time

But when considering the overall pros and cons of the Low impact development, it is obvious that low impact development methods are the most ideal stormwater management method for third world countries such as Sri Lanka. Especially areas such as those face flood situations. And it is really happy to mention that these low impact development projects are already in the motion within Sri Lanka. For example the road development around the Mahenwatte area in Homagama based on low impact development methods.

Implementation of most Environmental friendly stormwater management systems will not only protect you today, but it will protect the world for your kids too. Think right, take the right decision, save a healthy planet for your future generations.

You reached this far, so will your potential clients.

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S. Herath
Landscape Architect Department, University of Moratuwa

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