Rainwater harvesting has a long history. However, in many places around the world, the practice of rainwater harvesting has diminished significantly. Admittedly, it is much easier to obtain water by turning on the tap. However, as water consumption outpaces the rate of groundwater recharge, water supplies are becoming increasingly stressed. This problem is exacerbated by the fact that new freshwater supplies are difficult to find. For this reason, it is important that people and communities implement strategies to improve water efficiency. Rainwater harvesting is a great example of one of these strategies. After all, it does not make sense to allow rainfall to flow away to the storm drain. That’s free water literally going down the drain! Some advantages of rainwater harvesting include:
- Using harvested rainwater to offset potable water demand. This minimizes the stress put on aging municipal infrastructure.
- Harvested rainwater can be used to recharge groundwater.
- Rainwater harvesting can help restore watershed health.
- Rainwater harvesting systems capture stormwater runoff, which reduces flooding.
- Lower water bill.
- Increased resilience and reduce reliance on municipal water supply as the world continues to face challenges related to drought and climate change.
So, what does a rainwater harvesting system entail? Many people associate rainwater harvesting with large tanks or cisterns. In reality, rainwater harvesting is a much broader term. Rainwater harvesting systems are classified as either active or passive. Active rainwater harvesting systems involve capturing rainfall and storing it in a tank. In contrast, passive rainwater harvesting systems capture rainfall and store water in the soil.
The following blog post will discuss each of these strategies in more detail.
Active Rainwater Harvesting
Active rainwater harvesting techniques typically involve capturing precipitation in a tank or cistern. These systems are what people typically associate with rainwater harvesting. A typical active rainwater harvesting system includes components that deliver runoff captured from roofs or other impermeable surfaces to a water tank. Active rainwater harvesting systems can be scaled, which means systems can be designed for residential-scale uses (e.g., watering the landscape) or large tanks can be installed in agricultural settings for the purpose of livestock washing.
In general, active rainwater harvesting systems are designed for outdoor nonpotable end uses such as irrigation or vehicle washing. However, there is a growing movement to use harvested rainwater for indoor nonpotable uses such as toilet flushing and clothes washing. In some places, active rainwater harvesting is also used for indoor potable uses such as drinking, bathing, and cooking. These types of systems require water treatment to ensure that potable water standards are met. For this reason, it is important to consult a rainwater harvesting design professional before installing a system that involves indoor water use.
There are a number of local, state, and national codes that govern the design and installation of an active rainwater harvesting system. In the United States, one of the national codes is ARCSA/ASPE/ANSI 63-2013: Rainwater Catchment Systems. This standard was developed by the American Rainwater Catchment Systems Association (ARCSA), the American Society of Plumbing Engineers (ASPE), and the American National Standards Institute (ANSI). In addition to Standard 63, a rainwater harvesting practitioner may consult ARCSA/ASPE/ANSI 78-2015: Stormwater Harvesting Design for Direct End-Use Applications.
Passive Rainwater Harvesting
As previously mentioned, passive rainwater harvest involves the collection and infiltration of precipitation into the soil. Rainwater does not enter a storage tank in passive rainwater harvesting. In general, passive rainwater harvesting systems can collect more water than an active rainwater harvesting system and are cheaper to install if you consider $/gallon of water captured. In addition, passive rainwater harvesting systems improve watershed health by controlling erosion, mitigating flooding, reducing pollutant loading to receiving waters, and improving soil health.
As a stormwater engineer, passive rainwater harvesting holds a special place in my heart. In the stormwater world, passive rainwater harvesting features are often called structural stormwater best management practices (BMPs). Some examples of structural stormwater BMPs include: bioretention cells, grassed swales, infiltration trenches, infiltration basins, sand filters, detention ponds, and retention ponds. In general, these structures are designed to capture and treat stormwater runoff in urban areas. In many areas of the United States, new developments are required to install stormwater BMPs to reduce the effects of the new development on watershed hydrology.
Engineered passive rainwater harvesting systems are designed to capture a specific amount of water or flow. They also include overflow structures that protect the surrounding areas from flooding. Stormwater BMP design requirements differ by geographic location and are specified in local or state regulations. In addition, engineered systems may include piping and/or riser structures that control water levels and ultimately discharge treated water into the storm drain system.
Many passive rainwater harvesting systems are designed as landscape features rather than engineered features that are connected to the storm drain system. Some examples of such systems include rain gardens and bioswales. Landscaped passive rainwater harvesting systems are designed to achieve the same goal as an engineered stormwater BMP – capture, slow, and treat stormwater runoff. However, the design is not governed by the quantitative requirements associated with an engineered stormwater BMP.
Examples of Passive Rainwater Harvesting Systems
The following sections discuss some of the most common examples of passive rainwater harvesting systems. However, it is important to note that there are many more examples of passive rainwater harvesting systems.
Rain Gardens
Rain gardens are landscaped depressions that collect stormwater runoff from the surrounding area and allow that water to infiltrate through the soil. In simple terms, they can be described as a “leaky bowl.” The term “rain garden” is often used interchangeably with passive rainwater harvesting because these structures are so common. Technically speaking, rain gardens should also contain soil amendments to increase the soil’s water-holding capacity and make it more “sponge-like.” This makes water stored in the soil more accessible to plants and minimizes the amount of water lost to evaporation.
Bioswales
Bioswales are similar to rain gardens in that they are landscaped features that slow and infiltrate runoff. However, bioswales are conveyance structures which means their primary purpose is to direct flow to another place. Bioswales are often used as overflow features for rainwater harvesting tanks.
French Drains
French drains area holes or trenches filled with rock, gravel, or rubble that allow water to drain and soak into adjacent soils. The water entering a French drain should contain as little fine-grained sediment as possible because these structures are prone to clogging.
Contour Swales
A contour swale is a curvilinear depression that runs along lines of equal elevation, often referred to as contours. Contour swales intercept water from the area above them and allow water to infiltrate into the soil.
Curb Cuts
Curb cuts are simply openings in the curb that allow stormwater runoff to enter a vegetated or landscaped area rather than allowing the water to continue being conveyed to the storm drain system. The term “curb cut” refers to both the cut in the curb and the adjacent vegetated structure.
Brad Lancaster, the author of Rainwater Harvesting for Drylands and Beyond and a well-known rainwater harvesting practitioner, popularized curb cuts. He famously helped legalize curb cuts in the city of Tucson, Arizona, after installing then-illegal water-harvesting curb cuts at his and his brother’s home. Since then, curb cuts have become increasingly popular throughout the United States. My company is currently working on the planning phase for a curb cut project in the City of Ojai, California.
While Brad Lancaster’s story ultimately worked out, it is still illegal to unlawful in many areas for any person to cut, break or remove any curbing or sidewalk without the appropriate permits. For this reason, it is important to contact the appropriate local authorities before modifying publicly owned infrastructure in any way!
Regenerative Stormwater Conveyance (RSC)
Regenerative stormwater conveyance (RSC) systems are in-channel stormwater structures that function like grade control structures. They are comprised of a series of riffles and pools. However, unlike conventional grade control structures, the downstream end of each boulder weir is a pool that allows water to infiltrate into the ground. This type of structure minimizes channel degradation and recharges stream baseflow.
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