Rain garden design: the ultimate guide

Rain garden design: the ultimate guide to designing a bioretention garden and managing rain water in our towns and cities

Global climate change is detectable and it is already producing visible effects on the world. The Earth is warming up, extreme rainfall events are becoming more frequent and sea levels are rising, increasing the risk of heatwaves, floods and other climate-related disasters that often have a a destructive impact on our eco-system. Opportunities and strategies for effectively managing these risks already exist or can be developed at a local or international scale involving new sustainable technologies.

This week’s focus insight will address some aspects regarding rain garden design. Furthermore, you’ll be finding some practical examples that are ready for you to download:

a complex rain garden design
a small-scale rain garden for a detached house
a small-scale rain garden for urban contexts.

In order to open the file, you can even download Edificius for free.

Complex rain garden design overview

What is a rain garden?

Generally speaking, a rain garden is a depressed structure (natural or artificial) that collects the superficial water flows from roof tops, streets, sidewalks and other impermeable or semi-permeable urban surfaces after torrential rain events. This depressed area and the layers under the surface allow a partial or total water soak in an underground drainage system. It is also a design type with a “living” layer that allows the filtration of pollutants, pesticides, fertilizers, etc., brought by the runoff water before they wash down storm drains. More complex rain garden projects are often referred to as bioretention ponds, areas, gardens and so on.

Stormwater runoff in urbanized areas are much more conspicuous than in natural environments, as the semipermeable materials and the road pattern convey water and minimize infiltration.

Animated GIF showing the difference between urban and natural surfaces

Different infiltration vs runoff ratio between urban and natural surfaces

The urban expansion constantly subtracts areas to the natural environment which makes the local water cycle less effective. It is also clear that, the urbanization rate is faster than the adaptation capacity of the natural environment. In addition, the climate change is even faster than normal sewer system capacity.

The runoff flow also washes away urban surfaces carrying huge amounts of debris (called nonpoint source pollution). It has been estimated that up to 70% of pollution in rivers and lakes comes from the rain washing off urban areas.

Precipitation events and stormwater management

Stormwater management involves the control of runoff water from precipitation through designed plans and systems including a set of actions to reduce risks relating to rainwater runoff.

A necessary first step of stormwater management is to understand the way water interacts with the system being studied. A stormwater event is often characterised by an initial short peak of a very high volume of water. In case the volume of the water exceeds the storm sewer system capacity, there will be a risk of flooding. Another common flooding cause is the inadequate maintenance that can create an obstruction (i.e. water flow blockages due to debris) and consequently the inefficiency of the water conveying system.

Possible consequences of improper stormwater management

An effective approach to stormwater management needs to take in consideration a deep hydrographic analysis. For example, an early prediction of the volumes involved in rainfall events is essential.

Urban areas are characterised by structures, buildings, streets and generic surfaces that limit the amount of water drained and filtered by the ground, thus resulting in a significant raise of the superficial water volumes. In addition, surfaces like streets become preferential path in which the flow quickly increases the speed and the devastation potential.

Urban drainage response to climate changes

As shown during the “11^th international conference on Urban Drainage Modelling”, the climate crisis may have a substantial role in precipitation pattern changes. For example, it has been predicted that there will be an increase in the number and intensity of “extreme summer rainfall events”. Consequently, a new risk evaluation must be accounted for in particular for its effects on the drinking water system and wastewater system.

In many areas worldwide, the drainage engineering calculation has so far based on the assumption that rainfall events statistics will remain the same in the future and keep the same trends of past and present times. To properly consider climate changes, the precipitation data need to be multiplied with a factor that includes an expected rise in rain intensity.

Based on these assumptions, a new plan may be developed taking in account:

the design of new sewer systems
maintenance, upgrade and adaptation of the existing systems.

The raingarden solution perfectly fits in this second (and cheaper) category of works.

The surface washout of urban pollutants

While water floods surfaces, the flow created picks up different types of substances and debris of various dimensions. Here a list of the most impacting categories:

sediments: at a first impression, it may seem that the concentration of sediments in rural areas runoff is much higher as compared to the quantity in the runoff in urban areas. But, due to cities surface characteristics, the presence of construction sites (characterised by a high erosion rate) and the occasional urban waste crumbling, the total amount of sediments in the two environments can be considered comparable. A conspicuous part of debris is composed by flakes of metal (produced from rusting vehicles, and metal structures), particles from tires and brakes consumption, fragments of construction material, small particles from residential and industrial chimneys;
nutrients: chemicals like nitrogen and phosphorus are already present in excess in urban runoff water (these elements in runoff water is estimated to increase between 10% and 70% by 2050). The main issues relating to Nitrogen are that compounds such as ammonia and nitrates can have an adverse impact on humans, animals and plants. Nitrification of water can subtract excessive amount of oxygen and kill aquatic life and can contaminate drinking water. Phosphorus compounds, on the other hand, can exceedingly impact weeds and algae’s growth (thus raising maintenance costs);
organic materials: decomposing processes of organic waste (leaves, grass, pet waste and so on) can subtract a significative amount of oxygen from water and consequently kill aquatic life;
bacteria: bacterial pressure in runoff water can be 20 to 40 times higher than the minimum standards allowed for a swimming pool;
trace Metals: lead, zinc, copper, cadmium and chromium are just the major elements of a long list of toxic metals that contaminate runoff water;
insecticides herbicides, and other toxic chemicals have also been observed in high quantities.

Rain garden design solutions

In order to attenuate the above-mentioned issues, some solutions have been studied. In this article, we’ll design a fully functional project with two levels of solution:

simplified rain garden design;
complex type of rain garden design.

Please keep in mind that any proposed solution must be readapted to your local conditions, considering climate factors, local regulations, materials availability and so on. According to the objectives to achieve in this example, we will assume that all kinds of prior investigations have been already conducted and considered in the early stages of the design process.

The simplified rain garden design

A rain garden in its very basic form is a system designed to capture runoff water, redirecting the flow from the surface to the underground, while activating some reactions that process water and, eventually, taking care of this water. Because of the simplicity of the design, it is applicable almost everywhere even in private spaces to capture very small amounts of water. For example, it can be found in the backyards just in the direction of the downspout exit (as you can see in the images below).

An example of simplified rain garden design

As you can see, the rainwater that comes from the roof system, after being collected, is then conveyed across the cobblestoned pathway and finally towards the rain-garden.

In order to define a sizing criteria for a small rain garden, we need to take into account some basic information which is also quite easy to get hold of:

the drainage area of the roof
the number of downspouts
the ground surface gradient/slope
the type of soil
the amount of rainfall.

In this example we suppose that we have a roof of 200m² with 4 downspouts and each downspout will serve an area of 50m².

Then, considering the following guidelines:

for slopes less than 4%, the pond should be approximately 10 cm
for slopes between 5% and 7% the pond should be around 15cm
for slopes between 8% and 12% the pond should be around 20cm
for slopes greater than 12% a pond system becomes too complex for using this typology of rain garden.

Let’s assume we have a slope of 6% and a pond 15cm deep.

The soil quality will determine how quickly the water is absorbed under the ground: a sandy soil will drain the water quicker than a silty soil and a clayey soil will drain the water very slowly compared to the other two categories. In this case we will consider a silty soil for the sake of this example.

Regarding the water quantity, we have an average of 5cm of rainfall per rain event.

Here we have the formula:

Roof area multiplied by the rainfall amount and divided by the garden depth (all measures must be converted in the proper measurement units).

50 m² * 0.05 m = volume of water = 2.5 m³ 2.5 m³/0.15 = raingarden extension = 16.7 m²

with this information we should build a rain garden of at least 16.7 m².

The rain garden BIM project calculated as in the example above is ready for you to download for free at the bottom of this article. Within the file you’ll also find the table of volumes for the earthworks calculation.

Area view with measurement for an easy rain garden solution

Cross-section of a small scale rain garden with escavation (green pattern) and fill (yellow pattern)

The image below also shows a small scale rain garden design in a more urbanised context. Where there is lack of space available, this collecting system can result very useful.

Easy solution for a rain garden in urbanized area

Download the complete EDF project of these two rain garden models for free

Download the 3D BIM model (.edf file) of the small-scale rain garden project 1

Download the 3D BIM model (.edf file) of the small-scale rain garden project 2

A more complex type of rain garden design

In a highly urbanized context, it may seem difficult to apply these concepts, because the higher volumes of water involved may require also a wider pond surface, but this obstacle can be overcome just by increasing the draining capacity of the layers underneath the pond.

The objective of this focus insight is to simply provide a guide with useful tips regarding rain garden design, for this reason we will leave out all the calculation aspects and relating standards and regulations (having to consider them at your local level). However, we assume that the garden surface should be around 2% of the impervious urban area, a pond should be 10-30cm deep and a hydraulic conductivity 100-300 mm/hr.

Let’s see some design characteristics.

In this rain garden example, we’ll start off by designing an excavation section of 2.75m x 1.40m (excluding the volumes occupied by structural elements), with an impermeable liner both on the sides and at the bottom. We need to capture the runoff stormwater that arrives from the street on one side and the bicycle lane plus the sidewalk on the opposite side.

The stream of water will be conveyed in the garden through the weep holes which are also the higher limits that the water can reach before the overflow system starts to work. Keep in mind that the overflow will drain out the water without any treatment.

A useful add-on can be to build a small maintenance trench just under the weep holes. This pre-treatment will intercept and retain sediments that otherwise will clog up the biofilter, shortening its lifespan and compromising the overall performance.

The first technical layer that the water will encounter is the filter area. Here is where the presence of vegetation becomes important. This specific layer should be chosen considering the structural properties of materials and the hydraulic conductivity typically between 100 and 300 mm/hr. It’s main purpose must be to allow water drainage and therefore should contain less than 3% of silt and clay (materials are processed by washing so to remove clay and silt fractions). Clearly, a substrate must be created to support plants maximizing the root system expansion, and also to promote a rich microbiological environment. Considering the root system of the plants variety chosen, the thickness of this layer shouldn’t be greater than 40 and 60 cm.

Just under the biofilter layer, a transition layer needs to be implemented too. This area works as a filter preventing the vertical migration of materials that compose the biofilter into the drainage area. It should be composed of well graded sand with less than 2% of fine particlate. In order to promote the drainage, the hydraulic conductivity here should be higher than the overlaid filter media. The minimum thickness of this layer should be 10cm.

The layer at the bottom of the stratification is the drainage system, made of fine aggregates (gravel dimension 2 to 7mm). The main function of this zone is to collect and convey treated stormwater and eventually, in this particular lined design, to retain and store a reserve of water accessible to the vegetation during dry times. The thickness of this submerged zone should be between 45 and 50 cm (a minimum of 30cm is required). Long periods without rain events will obviously have a negative impact on vegetation. For this reason, it is fundamental to estimate the operational period of the submerged zone and the following easy formula can be taken in consideration:

Drawdown period (days)= porosity of the medium multiplied by the submerged zone depth(mm) divided by evapo-transpiration rate (mm/day).

The Hydraulic conductivity of the system here is higher than the overlaid transition zone. In this area, it can also be installed a collection pipe as part of the raised outlet.

A strongly recommended add-on to this system is the raised outlet that allows the creation of a submerged basin that still allows the water in excess to be filtered and conveyed away.

cross-section of a complex rain garden design

Rain garden, complex design: cross-section with measurment and verticall stratigraphy

Download the complete EDF project of the complex rain garden model for free

Download the 3D BIM model (.edf file) of the complex rain garden project

Rain garden design: vegetation

As mentioned before, plants are essential elements in the bioretention systems. It’s difficult to define a variety of vegetation that can be planted in all climatic conditions all around the world, therefore, for the sake of this example we’ll just highlight some basic principles to take in account when choosing vegetation.

rain garden design, plant effects and root environment

some phisics and chemical reaction around the plant and their roots

First and most important rule: you need to plant local varieties and species. Native Plants are basically attuned with the environment; hence they will better react to climate variations, pest attacks and the competition against weeds. Generally speaking, they are more resilient.

Another fundamental aspect to be considered is the root characteristics. Higher performances are strictly proportional to the total length of the roots, but also a high presence of fine roots and general surface of contact between roots and soil.

Also, a good practice is to use a high planting density combined to a high diversity of species and types. Different plants can vary in their performances in relation to the variation of weather during the change of seasons. Even the capacity to subtract nutrients, minerals and metals can vary considerably between different plants.

These are just some of the most important considerations to make. A separate and localised study of the vegetation component is essential for a higher performance of the system.

In conclusion, we have described just a couple of rain garden typologies but there are many more possible variants in terms of shape, dimension and complexity. For example, in the following image you can see a large-scale rain garden that is even more efficient than our first model that we have described earlier, with some features used also in the complex project type (the complete project can be downloaded below for free).