Designing a Rain Garden: a technical guide
The management of rainwater in our cities: here are 3 practical examples of rain garden design to download for free
As we all know, the strong impact of human activities and the pollution they produce have significantly accelerated global warming and climate change in recent decades.
The effects of all this are clearly visible around the world: just think of the exponential increase in extreme rainfall events (floods, fires, landslides, acid rain, etc.).
Opportunities and strategies to effectively manage these risks already exist and can be developed/implemented, on a local or international scale, also thanks to new sustainable and environmentally friendly technologies.
Let’s take a look, then, in this week’s focus, at practical applications of these principles: some examples of rain garden design (a simplified project and a more complex/bioretention garden) with three practical examples to download for free created with an architectural design software.
Complex rain garden design overview
Contents
What is a rain garden?
In general, a rain garden refers to a depression or basin (natural or artificial) that collects runoff water, that is, the water collected from roofs, roads, sidewalks, and other impermeable or semi-permeable urban surfaces. This depression, along with the layers beneath the surface, allows for partial or total disposal of the water into an underground drainage system (and often also its recycling/reuse for irrigation purposes).
However, the term rain garden also refers to a type of project with a layer of “living” material that allows for the filtration of pollutants, pesticides, fertilizers, etc., carried by the runoff water before entering the drains: it has indeed been estimated that up to 70% of pollution in rivers and lakes comes from rain that “washes” urban areas.
This is why the more complex rain garden projects are often referred to as “bioretention” (ponds, areas along roads/railways, gardens, and so on): green spaces that are used to “clean” the runoff water.
Rainwater management and environmental impact
A rain garden allows for the control of stormwater runoff through natural engineering systems that enable a set of actions aimed at reducing the risks related to uncontrolled rainwater runoff.
A necessary first step in rainwater management is to understand how water interacts with the territory, its orography, stratigraphy, vegetation, and existing constructions.
It should also be noted that an effective approach to rainwater management must be based on a thorough hydrographic analysis.
For example, it is essential to anticipate the volumes involved in rainfall events. A rainfall event is often characterized by a brief initial peak of a very high volume of water. If the volume of water exceeds the capacity of the sewer system, there is a risk of flooding. Another common cause of flooding is inadequate maintenance, which can create an obstruction (i.e., blockages in the water flow due to debris) and consequently the inefficiency of the water transport system.
Urban areas are characterized by structures, buildings, roads, and generic surfaces that limit the amount of water discharged and filtered by the ground, thus resulting in a significant increase in surface water volumes. Moreover, surfaces such as roads become a preferential pathway where the flow quickly increases in speed and potential devastation.
Different infiltration vs runoff ratio between urban and natural surfaces
Response of urban drainage to climate change
As shown during the “11th International Conference on Urban Drainage Modelling”, the climate crisis could play a substantial role in changes to precipitation patterns. For example, it has been predicted that there will be an increase in the number and intensity of “extreme summer rainfall”.
Consequently, a new risk assessment must be made, particularly regarding its effects on the drinking water system and the wastewater system.
In many areas of the world, drainage engineering calculations have so far been based on the assumption that rainfall statistics will remain the same in the future, maintaining the same trends as in the past and present.
To properly consider climate change, precipitation data must be multiplied by a factor that includes a projected increase in intensity of rain. Based on these assumptions, a new plan can be developed taking into account:
- the design of new sewer systems;
- the maintenance, upgrading, and adaptation of existing systems.
The rain garden solution fits perfectly into this second (and more economical) category of works.
Possible consequences of improper stormwater management
Designing a rain garden: two different solutions
In order to mitigate these issues, several solutions have been envisioned: in this Focus, we will create a fully functional project with two options.
It is important to keep in mind that each proposed solution must be adapted to local conditions, considering climatic factors, local regulations, the availability of materials, and so on. For the objectives we have set, in this example we will assume that all preliminary investigations have already been carried out and assessed in the initial stages of design.
An example of simplified rain garden design
Simplified Design of a Rain Garden
A rain garden in its very simple form is a system designed to capture runoff water, redirecting the flow from the surface to the subsurface, triggering some reactions that treat and purify the water.
The simplicity of the design makes it applicable practically anywhere, even in private spaces, to capture very small amounts of water. For example, it can be found in home gardens right in the direction of the drainage outlet (see images below).
Area view with measurement for an easy rain garden solution
As can be observed, the rainwater coming from the roof system, after being collected, is channeled through the pebble path and then towards the rain garden.
To define a sizing criterion for a small rain garden, we need some basic information, easily obtainable:
- the drainage area of the roof;
- the number of downspouts;
- the slope of the land;
- the type of soil;
- the amount of rainfall.
In this example, let’s assume we have a roof of 200 m2 with 4 downspouts: therefore, each downspout serves an area of 50 m².
Cross-section of a small scale rain garden with excavation (green pattern) and fill (yellow pattern)
So, considering the following guidelines:
- for slopes equal to or less than 4%, the pond should be about 10 centimeters;
- for slopes between 5% and 7%, the pond should be about 15 centimeters;
- for slopes between 8% and 12%, the pond should be about 20 centimeters;
- for steeper slopes, it becomes too complex to use this type of rain garden.
Let’s assume we have a slope of 6% and a pond depth of 15 centimeters.
Soil quality will determine the speed at which water is absorbed by the subsoil: sandy soil will drain water faster than loamy soil, and clayey soil will drain water much more slowly compared to the other two categories.
In this case, we will consider loamy soil.
Regarding the amount of water, let’s consider an average of 5 centimeters of rain per event.
Given the following formula:
rain garden area = roof area x amount of rain / garden depth
in our example, we will have:
50 m² * 0.05 m = volume of water = 2.5 m³
2.5 m³ / 0.15 m = rain garden area = 16.7 m²
So our rain garden will need to be at least 16.7 m².
The corresponding BIM project of the rain garden is available for free download at the end of this article. Inside the file, there is also a volume table for calculating earthworks.
The image below shows an example of designing a small-scale rain garden in a more urbanized context. In the absence of available space, this collection system can be very useful.
Download the 3D model (.edf file) of a simplified rain garden project #1
Download the 3D model (.edf file) of a simplified rain garden project #2
Easy solution for a rain garden in urbanized area
Advanced design of a rain garden
In a highly urbanized context, it may seem difficult to apply these concepts, as the larger volumes of water involved may require a larger pond area: this obstacle can only be overcome by increasing the drainage capacity of the layers beneath the pond.
The aim of this in-depth analysis is simply to provide a guide with useful tips on rain garden design; for this reason, we will leave out all calculation aspects and the related norms and regulations (which should be considered at the local level).
However, let’s assume that the garden area is about 2% of the impervious urban area, a pond should be 10-30 cm deep and have a hydraulic conductivity of 100-300 mm/hour.
Let’s see some design features.
Rain garden, complex design: cross-section with measurement and vertical stratigraphy
In this example of a rain garden, we will start by designing a digging section of 2.75 m x 1.40 m (excluding volumes occupied by structural elements), with a waterproof lining on both the sides and the bottom.
We need to capture rainwater runoff coming from the road on one side and from the bike path and sidewalk on the opposite side.
The flow of water will be channeled into the garden through drainage holes, which also represent the highest limits that the water can reach before the “overflow” system begins to operate.
Let’s remember that the “overflow” will drain the water without any treatment.
An additional useful component may be to construct a small maintenance trench just below the drainage holes. This pre-treatment will intercept and hold sediments that could otherwise clog the biofilter, shortening its lifespan and compromising overall performance.
The first technical layer that the water will encounter is the filter area. This is where the presence of vegetation becomes important.
This layer should be chosen considering the structural properties of the materials and the hydraulic conductivity, generally between 100 and 300 mm/hour.
The main purpose must be to allow water drainage and thus contain less than 3% silt and clay (the materials are processed by washing to remove clay and silt fractions).
Clearly, it is appropriate to create a substrate to support the expansion of plant roots, and also to promote a rich microbiological environment.
Considering the root system of the variety of plants chosen, the thickness of this layer should not exceed 40 to 60 cm.
Just below the biofilter layer, a transition layer must also be implemented. This area acts as a filter that prevents the vertical migration of the materials that make up the biofilter into the drainage area; it must be composed of well-graded sand with less than 2% fine particulates.
In order to facilitate drainage, each lower layer must have greater hydraulic conductivity than the one immediately above. The minimum thickness of this layer should be 10 cm.
The layer at the bottom of the stratification is the drainage system, composed of fine aggregates (gravel size from 2 to 7 mm). The main function of this area is to collect and transport treated rainwater and, finally, in this particular layered design, to retain and store a water reserve accessible to vegetation during dry periods.
The thickness of this (temporarily) submerged zone should be between 45 and 50 cm (a minimum of 30 cm is required). Long periods without rainfall events will obviously have a negative impact on the vegetation. For this reason, it is crucial to estimate the operational period of the submerged area, and this simple formula can be considered:
water runoff time (days) = porosity of the medium x submerged zone depth (mm) / evapotranspiration rate (mm/day)
The hydraulic conductivity of the system here is higher than that of the overlying transition zone: a collection pipe can also be installed in this area as part of the raised intake.
A strongly recommended additional component in this system is the “raised outlet” which allows for the creation of a submerged basin that enables filtering and channeling excess water.
Download the 3D model (.edf file) of a complex rain garden project
Selection of Vegetation for a Rain Garden
As mentioned, plants are essential elements in bioretention systems.
It is difficult to define a variety of vegetation that can be planted in all climatic conditions around the world; for this reason, in this example, we will highlight some basic principles to consider when choosing vegetation.
The main rule: it is necessary to plant local varieties and species.
Indigenous plants are fundamentally in tune with the environment; thus, they will react better to climate variations, pest attacks, and competition from invasive weeds. In short, they are more resilient.
Another fundamental aspect to consider is the characteristics of the roots. The best performance is directly proportional to the root length, but also to the high presence of fine roots and an overall contact surface between roots and soil.
Moreover, a good practice is to use a high planting density combined with a high diversity of species and types. Different plants may vary in their performance in relation to the changes in weather during the changing seasons. The capacity to absorb nutrients, minerals, and metals can also vary considerably between different plants.
These are just a few of the most important considerations to make. A separate and localized study of the plant component is essential for a higher performance of the system.
In conclusion, we have described a couple of types of rain gardens; however, there are many other possible variants in terms of shape, size, and complexity.
For example, in the image below, you can see a large-scale rain garden that is even more efficient than the first model described earlier, with some features also used in the complex design type (the complete project can be downloaded for free below).
section of a complex rain garden design
complex rain garden design with geofabric liner
Finally, if you are interested in this topic, I recommend reading the following articles:
Resources and downloads for rain garden design
In this section, you will find links to download project files and presentation slides.
Download the 3D model (.edf file) of a simplified rain garden project #1
Download the 3D model (.edf file) of a simplified rain garden project #2
Download the 3D model (.edf file) of a complex rain garden project
Slides on how to design a rain garden
FAQ
What is a rain garden?
A rain garden is a natural or artificial depression that collects runoff water from roofs, roads, and other urban surfaces. It serves to filter and dispose of water into the ground or underground drainage systems, reducing water pollution and improving water management.
What is the role of the rain garden in stormwater management?
The rain garden controls stormwater runoff through natural engineering systems, reducing the risk of flooding and filtering pollutants before they enter natural water systems.
What are the environmental benefits of a rain garden?
Rain gardens improve water quality by reducing runoff pollution, help prevent flooding, promote urban biodiversity, and can contribute to groundwater recharge.
What are the key elements of a complex rain garden?
A complex rain garden includes several layers of filtration, an advanced drainage system, layering with specific materials for water retention and filtration, and an overflow system for managing excess water.
