MITIGATING STORM AND FLOOD DAMAGE

The risk of damage to gardens in severe weather events is increasing due to effects of climate change. That increased risk can, however, be reduced by:

🔸Understanding how extreme weather can cause damage, and

🔸Developing landscapes in a way that both softens the severity of extreme weather and reinforces capacity to resist and recover from damage.

Whilst everyone is familiar with the increased risk of heat and drought associated with climate change, the other side of the coin is the greater likelihood of severe storms and flooding events.

Properties that have taken years to establish, can be devastated by a single so-called ‘freak’ weather event.

🔸Trees can lose branches or be ripped from the ground completely.

🔸Other plants can be weakened, broken or killed by mechanical damage or suffocation of roots via floodwaters.

🔸Garden soil, paths and driveways can be eroded.

🔸Buildings, furniture, fences, walls and other hard landscape features can be destroyed or damaged.

Good design and management to reduce the risk of damage from storms and floods on properties such as homes, commercial buildings and farms, can be achieved in many ways. It just requires a change in how people plan and use green spaces.

PREPARING FOR EXTREME WEATHER

When it comes to flooding, some areas are more prone than others. However, when water cannot escape, flood zones expand. The ideal solution to flooding is to slow down the flow of water, allow the water to spread out and let it sink into the ground where it can be stored and used later. Excess water that cannot be held in the ground can go into other storage places like tanks, ponds, creeks or dams.

Some practical ways to prepare gardens for flood events include:

🔸Use furrows, diversion drains or swales to catch, slow and redirect water away from plants.

🔸Shift containers, ornaments and statuary to higher ground or place them on top of solid walls and garden structures.

🔸Place expensive garden equipment or machinery on solid wooden pallets, gabion walls, stone benches or other heavy objects.

🔸Weigh down items that cannot easily be moved or if there is nowhere to store them, e.g., use bricks or rocks to weigh down furniture.

🔸Get all loose materials, tools, etc., off the ground.

🔸Place garden chemicals on high shelves.

🔸Construct solid diversion structures to capture and redirect excess run-off from existing buildings or hard surfaces.

🔸Harvest any produce that is ready.

🔸Secure ponds with strong wire netting to avoid losing fish.

🔸Place sandbags over manholes or other ground coverings that could become dislodged.

🔸Turn off any irrigation systems or outdoor water supply.

🔸Unplug any electrical equipment in sheds, on patios, in ponds, etc.

Flooding may or may not be accompanied by stormy weather. Regardless, there are also some practical measures that can be adopted to mitigate the potential damage of powerful storms:

🔸Shift pots to more protected areas of the garden or inside buildings.

🔸Stow away any lightweight garden furniture and ornaments.

🔸Reinforce shade cloth, remove or fold away hammocks and loose materials like shade sails and outdoor blinds.

🔸Ensure doors and windows on outdoor buildings are closed and locked where possible to avoid them being blown open.

🔸Prune taller plants so that they have less resistance in windy conditions.

🔸Support young plants with stakes and tie climbers and delicate plantings to sturdy supports like solid trellis.

🔸Harvest any fruits and vegetables.

DESIGNING TO SOFTEN THE SEVERITY OF EXTREME WEATHER

In terms of garden design, nothing is going to stop flooding or storms, but much can be done at the planning stage to soften the severity of the impacts of severe weather.

Plantings

Certain types of vegetation offer greater protection against storm and flood damage than others. When planning plantings, consider the plant’s form, height and density, as each influences how well it can buffer wind and water. Plants that have grown in exposed, windy environments usually develop stronger, deeper root systems that help anchor them more effectively. Even so, species choice still plays a major role in how resilient a landscape will be.

🔸Some plants are brittle, e.g., Melaleucas, Willows, Poplars, Robinias; while others will bend with the wind, e.g., Cedars, Firs, Spruces, Olives.

🔸Some have stronger, deeper root systems that are more likely to hold in a storm, e.g., Cypress, Pines, Hornbeam, and some Eucalypts.

🔸Some plants have a natural tendency to drop branches under stress, e.g., some, if not all, Eucalypts will shed large lower branches as the tree gets taller (as do Oaks, Beeches and Elms).

🔸A clump of different sized trees or shrubs will support each other and slow the wind. Whereas a single, exposed tree or shrub will bear the full force of wind and be more susceptible to damage. Windbreaks also soak up excess water using their deep root systems. Some of the many options include varieties of Hornbeam, Willow, Holly, Beech, Acacia, Eremophila, Westringia, and Leptospermum.

🔸A dense root system helps bind the soil, while a thick canopy absorbs and temporarily holds water, slowing the amount that reaches the ground. However, if foliage becomes overly saturated, the excess weight can damage the plant.

🔸Some shrubs, low-laying plants and groundcovers will create a dense and relatively deep mat of roots and foliage across the ground. This holds soil together, reduces erosion and slows the rate atw hich water flows over surfaces. Some plants which create dense groundcovers include Mondo Grass, Pachysandra, Juniperus horizontalis, Cotoneaster horizontalis, Agapanthus, Dietes, Dianella and Lomandra.

It isn’t just about the plant varieties you use though. Also, think about where you plant them and how you care for them in the months and years before a storm, e.g., deep, extensive, root systems do not pull out of the ground so readily.

The way that we develop and treat a garden can impact the way plants resist storms.

If a tree gets most of its moisture from near to the ground surface, it may have a tendency for roots to grow shallower, thus, it may not be anchored as well. You are better to water trees deeply, e.g., via a pipe set deep into the subsoil. In drier conditions, the roots go deeper in search of water creating a better anchoring root system.

Land Use

Be careful about creating too much hard surface footprint. Consider that a 500 square metre property with a 300 square metre home built on it. If you then add 100 square metres of concrete drive way and paths; that property only has 100 square metres of ground surface left to soak up water in a heavy storm. When this property receives the same amount of rain as it did before building, that rain is diverted from roofs and paving i.e., five times the amount of water is going onto one fifth of the ground that remains uncovered. This increases the potential for both erosion and flooding.

To reduce runoff, use more permeable surfaces like gravel, or use bricks and paving stones that are not set into concrete. Permeable pavers are also widely available which permit the passage of water into a specially designed porous substrate below them. Where a solid surface is needed, permeable concrete will take up some moisture.

Other ways to deal with excess water include:

🔸Install French drains to distribute water to other parts of the garden e.g., ponds.

🔸Collect runoff from roofs in storage tanks and barrels.

🔸Apply mulch to bare soil to improve its permeability over time.

🔸Grow plants on raised beds and mounds that drain well so they can withstand occasional flooding.

🔸Consider having a green roof or green walls to increase the amount of plants available to take up water.

🔸On sloping sites, install swales or terracing to slow the rate of runoff and erosion.

🔸On sites that flood regularly consider installing a rain garden i.e., using reeds and other bog plants that withstand flooding.

RESISTING DAMAGE

Look at where trees are located. Consider where they could fall if they do fall. Control the direction they can fall, e.g., attach cables so that they cannot fall onto the roof of a building or hard landscaping that is difficult to replace. Establish other tall plants around trees to soften the blow from dropping branches or falling trees, e.g., clumps of tall bamboos, palms or medium sized shrubs.

Erosion control measures may be geared toward directing water flow, perhaps using swales to slow speed of water. Improving surface and subsurface drainage is always helpful, but drains not only need to be in place, but also maintained. Check drainage before any impending storms and clean them out to ensure water flow is good.  Remove any loose litter from low lying areas of a garden before a storm, so there is minimal material that can be caught by fast moving water.

Be aware that low lying or steeply sloped areas are particularly susceptible to being affected by fast moving water. When those areas are mulched by loose materials such as small stones or wood mulch; that material is likely to be swept away and build up causing further problems where it settles. If a susceptible area is covered with large rocks and big stones though; the chances are that the stone will largely hold in place, reducing erosion.

The other way to reduce erosion is to cover susceptible slopes and low-lying areas with deep rooted, matting plants that will allow water to flow but hold the soil from eroding.  A mown grass cover can do the job; but choose the grass varieties well. Some grasses have much deeper root systems that are more likely to hold firm in severe weather events.

Be careful not to create a loose surface on a steep slope that will slide down in heavy rain. Consider installing a sheet of biodegradable mulch, e.g., jute matting and peg it deeply into the subsurface.

A water-absorbent surface will soak up moisture. Organic matter absorbs and holds more water. Deep topsoil and free-draining subsoil are better. Don’t just build up the topsoil in a garden – improve the subsoil too. Use soil chemicals like gypsum, lime, etc. to increase water penetration. 

RECOVERING FROM DAMAGE

After a storm or flood, soil is wetter and the water table is higher; so subsequent storms can reach a point of causing damage faster. Weed seeds can be dispersed from far away and unless controlled immediately, new types of weeds can invade areas previously not contaminated by those weeds. It can be important to re-establish desirable plants as soon as possible to compete with invading weeds. If a damaged garden is left bare for too long, weed growth will inevitably become a problem.

Cleaning up after a storm can be easier if the garden is designed with storms in mind, there is good access, and you use the right techniques.

Tips

Avoid removing biomass. Vegetation that falls or dies during or after a storm may need to be removed from where it fell for either aesthetic or practical reasons. It may be clogging drains, or stopping flow of water through parts of a property. To remove it from the property though will potentially diminish nutrition and organic content in soil. It’s better to compost or chip and mulch material than take it off the property.

🔸Inspect plants for signs of damage. Prune dead and damaged stems.

🔸For large-established plants, try digging a trench around them and backfill with porous material to aid drainage. Fork the ground to increase aeration.

🔸Severe erosion may displace, degrade or remove soil – it may be necessary to replace what is lost.

🔸Low lying flooded areas often collect a layer of silt after heavy storms or flooding; and that silt can become a fertile place for weeds to thrive – even on top of a mulch mat.

🔸Silt build-up can also change drainage patterns, and clog up drains.

🔸Don’t forget to inspect and attend to water features such as ponds, pools or lakes. Silt from a flood can build up and be largely unnoticed. A balanced water ecosystem can become unbalanced. Water quality and depth can change. Health issues can emerge – anything from mosquitos to a build-up of dangerous microorganisms.

🔸Avoid standing on waterlogged soil. It will become compacted and starved of oxygen. If you must gain access, stand on boards to spread your weight.