A flood occurs when areas of typically dry ground become flooded with water, ranging in depth from a few centimetres to several metres. Floods can take a long time to diminish because the water needs somewhere to go. Flooding commonly causes damage to personal property, buildings, and infrastructure, bridges and road closures, service disruptions, injuries, and even fatalities.
Floods can occur anywhere, but some regions are especially vulnerable. This includes sites near rivers, where vast amounts of water constantly flow, and urban areas with many paved surfaces, where water pours onto the ground from roof funnels and cannot be absorbed.
What is Flood Mitigation?
Flood mitigation refers to systematic and ongoing activities that significantly lower the risk of destruction to human life, property, and the environment. Rather than attempting to prevent floods entirely, flood mitigation involves managing and controlling flood water movements, such as directing flood discharge through floodwalls and gates. Flood mitigation minimises the overall risk of flood damage to structures and the severity of flood damage when it occurs. Flood mitigation strategies are classified into two types: structural and non-structural.
A Structured Approach to Flood Mitigation
Structured approaches of mitigation reduce harm by rebuilding landscapes. This project includes levees, dams, floodwalls, jetties, retention roads, and evacuation routes. Structural mitigation aims to redirect water away from people and towns. Non-structural flood defence measures are a group of mitigation or adaptation measures that do not rely on typical structural flood defence techniques. Non-structural measures of mitigation consist of land use regulations, flood forecasting and warning, disaster prevention, preparedness, and response mechanisms. The objective of non-structural flood mitigation is to alter people’s interactions with the flood plain and flood risk and to relocate them from flood-prone locations.
The role of Commercial Landscape Design in Flood Mitigation.
Urban flooding is the most widespread yet least understood environmental issue in cities and towns globally. Even in places that might get drier, changes in the extremes of rainfall are likely to make this threat worse in the future. One of the biggest causes of urban flooding is the development of urbanisation itself. Infrastructure like roads, pavement, and buildings are designed to withstand heavy rain and flooding. Aside from flooding, stormwater runoff is a significant source of pollution and ecological destruction in urban streams, contributing to increased flooding. According to a report, even in arid years, the average quantity of stormwater generated by a typical urban area in Melbourne is around 315,000 litres per year (Assuming a total impervious area of 250 square meters).
Under such conditions, residential and commercial gardens might behave as sponges. Plants intercept water on their leaves and canopies when it rains. Rainwater can then seep into the ground or evaporate back into the atmosphere. More trees, shrubs, and grasses planted in urban gardens would help intercept more runoff, transporting water back into the atmosphere by vegetation. Land under a tree is more equipped to absorb rainwater. This reduces the amount of water that flows over the surface after rain and the amount of water that enters rivers and streams.
The Role of Urban Green Space in Flood Mitigation
According to research, gardens constitute a substantial amount of green area in towns and cities and hence can mitigate urban flooding by lowering surface runoff and enhancing infiltration. According to another study, front gardens of urban residences can be essential in minimising rainwater runoff. Studies showed that establishing gardens in Melbourne can reduce stormwater from 83000 litres per year to 15000 litres per year, nearly 81%.
How Landscaped Gardens can help
Such gardens mitigate flooding by temporarily storing water from heavy rainfall to reduce peak runoff, measured by the peak flow delay time. For the same heavy rain, the peak flow is inversely proportional to the peak flow delay time, and the watershed’s flood mitigation function improves as the delay time increases. Urban wetlands, like the Balwyn Community Centre, play a key role in storing runoff from public spaces with the added bonus of providing habitats for wildlife.
Australia is the world’s driest populated continent. Nonetheless, large floods occur practically every year in regions of Australia. Heavy or prolonged rainfall is the primary cause of floods in Australia, causing rivers to surpass their capacity and overflow. Floods may destroy people and communities, costing millions of dollars each year. Flood mitigation is critical to avoiding this damage. Installing a garden is also a flood mitigation strategy, and the following are some landscape design ideas for flood mitigation.
Best tips for Landscape Design for Flood Mitigation
Native Plants help to reduce flooding.
Native plants are the best for flood control. Native plants are preferred because they are better adapted to the climate in which they thrive and are typically hardier and more drought-resistant. In contrast to lawn grasses and sod, native trees, shrubs, and plants have powerful and profound root systems. These root systems can store water during and after heavy rains, lowering the risk of flooding caused by extensive storm systems. Additionally, they reduce erosion and soil loss on your land by slowing water flow. Researchers discovered that soil under mixed native plants absorbs water 67 times faster than other plants. Native trees have such deep roots that they create pathways for water to travel deeper underground. The soil beneath native plants works as a sponge, a reservoir that absorbs and gently releases water.
Lilly pilly, Swamp banksia, tree ferns, bottle brushes, Eucalyptus, paperbarks, Swamp lily etc., are some Australian native plants that can absorb a lot of water and adds colours to your garden.
Retaining Walls can play a key roll.
A retaining wall is a structure that retains the soil in place behind it. Typically, they bind soils at different altitudes or support terraced gardens and fill the earth. Retaining walls are usually constructed from concrete blocks, poured concrete, coated wood, and natural stone or rocks. Retaining walls have acquired popularity quickly and are often employed in commercial and residential landscape projects. Retaining walls serve several functions. A retaining wall can perform the same purposes as a fence, including providing privacy and redirecting water flow. Still, it may be preferable to some homeowners because of its more aesthetically pleasing design. A retaining wall is a landscaping element that increases the usable space of a property. With a well designed gabion retaining wall can help control erosion alongside creeks and reduce or slow the flow of floodwaters.
Landscape architects say retaining walls serve multiple functions, including holding soil and preventing erosion. They can help deflect flood water by directing it towards the street. A retaining wall can protect your home from flood damage from water and floating debris. If you do not want to place sandbags every time there is a flood, a retaining wall is an excellent permanent solution. If your land is hilly or rolling and it rains, a retaining wall will keep the soil in place and stop mudslides.
A thick layer of Mulch can help
Mulching is a layer of various materials put on top of the soil. Mulching is used for multiple purposes, including soil moisture conservation, improved fertility and soil health, weed control, and aesthetic appeal. Mulching is a fantastic strategy to protect your yard and home against flooding. Therefore, using mulch in your yard is essential to prevent water from pouring towards your house.
How do I choose Mulch?
Choosing the type of mulch for flood-prone areas is very important because many mulches are unsuitable. You must select a wide variety of mulch that will inhibit fast-moving water flow. Lightweight mulches are problematic in such areas because they float away immediately with the flood—substitute light mulch with heavier material, such as wood chips, which may absorb more water. Wood chips also outlast most other materials.
Remember, if you wish to add mulch near your home, it should be placed 15 to 30 mm away from the slab. This will keep moisture away and keep your home’s structure from degrading.
Permeable Paving and Porous Surfaces
Unfortunately, gardens rapidly change in the modern period due to new commercial reasons and cultural values. Researchers discovered that residential garden paving increased by 13% over 33 years. As a result, the runoff from the same gardens increased by 12%. Because asphalt, concrete, and other hard surfaces stop water from draining naturally, which makes flooding much more. The research reveals that paving over a garden can have unexpected environmental effects. Paving, concrete, and asphalt are not permeable compared to parks, causing water to rapidly runoff into into street kerbs and creeks. This can result in localised flooding. Permeable concrete and permeable paving help to reduce this runoff.
Therefore, building a property with porous surfaces and substances that absorb water can be helpful in highly flooded regions. Permeable paving offers numerous advantages. This method not only helps in water diversion but also in enhancing groundwater supplies. Studies showed that permeable pavement had good performance on flood mitigation, and the effect was more significant than other technologies, including rainfall harvesting and green roofs in the USA. In another study, Huang et al. (2014) reported that implementing permeable pavements reduced about 35.6% of total runoff and 28.7% of peak flow in China. Therefore permeable pavement is highly recommended in flood mitigation.
Create a Rain Garden
A rain garden is a landscaping depression that collects and absorbs precipitation from a roof, patio, lawn, driveway, or street. Rain gardens can be a cost-effective and visually beautiful way to prevent runoff from your property when planted with grasses and flowery perennials. The purpose of a rain garden is to collect and retain rainwater so that its effects, in flood, will be beneficial to the landscape rather than destructive.
Rain gardens are also efficient at removing up to 90% of pollutants and nutrients and up to 80% of sediments from the rainwater runoff they collect. Rain gardens provide 30% greater water absorption into the soil than regular turf. Since the majority of the plants in the rain garden are native to the area, one could conclude that rain gardens help to preserve native vegetation while also providing an appealing appearance. Rain gardens limit peak storm flows, preventing stream bank erosion and reducing the probability of local floods.
How to build a Rain Garden
A rain garden can be created and planted similarly to any other perennial garden, with a few exceptions.
- The rain garden must be placed where water can flow and must be at least 3–4 metres away from the building’s foundation and sewage system.
- A shallow dip in the shape of a saucer is built in the garden to collect rainwater before it seeps into the ground. The garden should comprise 20-30% of the area from which it receives runoff.
- Sometimes, soil replacement and further preparations are required for success. A suitable rain garden soil mix is 50-60% sand, 20-30% topsoil, and 20-30% compost.
- Perennial plants and shrubs native to the region are recommended since they are adapted to the local climate and do not require additional maintenance once established—plant flood-tolerant species in the centre and drought-susceptible species around the edges.
- Your rain garden will not be complete without a layer of mulch made of shredded hardwood. It keeps the soil moist and ready to soak up rain and makes your garden low maintenance.
Tall healthy Grass areas can slow the water flow.
Soil with appropriate grass cover retains moisture better than bare soil and catches more water when it rains. Grasses make our landscape more drought-resistant and help to minimise flooding and erosion when it rains. So, if your grass is short and dried, think about adding a new species. During severe rainfall, the grass’s root system can help absorb more water, keeping it from reaching your home’s foundation. According to studies, a new grass that reduces water runoff could help to reduce increasing flooding. Remember not to trim your grass too short once it has grown in. This can weaken the roots and cause flooding in your yard.
Install a garden drainage system
Design the necessary drainage system after inspecting the landscape to avoid water collecting and flooding. There are numerous methods for achieving this, including using French drains and swales. Aside from your real garden, go over your roofs as much as possible. Install commercial gutters to divert rainfall and keep it from running into your back or front yard. Inspecting driveways and, if necessary, constructing drainage systems is also a brilliant idea.
Slope your Garden Away from your House
The easiest method to minimise significant damage is to slope your yard away from your house. To do so, note the lowest and tallest spots in your yard and make the required adjustments.
-
-
- Garden steps, mulched garden
and lawn sloping away from the house.
-
Improve the drainage of soggy soils
Flood-prone soil can be improved by digging in many organic materials, such as well-rotten garden compost or manure. Mixing grit into the bottom of planting holes will aid drainage, and planting shrubs or elevated mounds can help protect roots from water-logging. Depending on your topsoil type, an application of gypsum can make your garden less soggy. For lawn areas, regular aeration is recommended.
FAQs
Does a French drain help with flooding?
French drains help prevent flooding and reduce the risk of long-term water damage to your property. French drains divert water away from your home and yard.
How do I drain flood water from my water?
Flooded water in the yard can cause different problems. You can drain flood water from the yard through a French drain, improve the lawn’s ability to soak up water, and incorporate drainage into the patios.
More Reading on Flood Mitigation
Flood mitigation measures – Northern Territory Government
How cities can make room for water – University of New South Wales Sydney