Nature Based Flood Mitigation: Observations Centred around Protestors Falls Day Use Area

By Jeremy Stewart

An example of a functioning upper catchment floodplain can be seen in the Protestors Falls day use area, Nightcap National Park. The forest plants in this locale are predominantly Bangalow Palms with some Blue Quandong and Red Cedar amongst the largest trees.

In the high rainfall event of Feb 2022, Bat Cave Creek running from the Protestors Falls direction would have been a raging torrent of water, sediment, rock, timber, and smaller vegetation. The closest landholder recorded 980mm of rainfall in 18 hours.

The vegetation and trees from the Creek seem to have been carried until there was a knotted jam – it was near the intersection with Terania Creek where the waters meeting each other would have created turbulence and added to the slowing effects at this point.

Woody Debris Jam on Bat Cave Creek

The jam created about a 2m weir in the main channel, slowing the water above it and letting the stone and sediment more readily drop out of the waters rather than continuing downstream.

As the cross section of Bat Cave Creek was now changed with wood, stone and deep water, the floodwaters spilled and spread through the surrounding palm forest.  

The thousands of palm and tree trunks, roots and rock forms would have created an infinitely complex pattern of micro eddies, again reducing the waters celerity as they interacted with each other.

As a result, it is estimated several thousands of tons of material became captured in a space of forest a couple of hectares in area.

New Ground Zero

It appears the plants here are well adapted to surviving in such conditions, their root systems likely to be efficient at trapping nutrient rich sediment. Some were submerged up to 1.8m in a fresh silt/rock deposit.

Each event just creates a new ground zero for the next generations of forest plants.

Event Horizon

A new incision was created in the forest floor at one point, creating a window into geological history. It looks like there is similar material to what has been freshly deposited way down in the soil horizon.

These massive events have occurred in the distant past, and you could assume the palm forest was there then.

When you look around the Northern Rivers landscapes at the floodplain shapes; even the mini ones along the inside edges of river bends, remember they were once likely densely vegetated like this one.

Extensive vegetation coverage is a critical part of a healthy natural river system and it has been largely removed from our riparian zones and floodplains to suit preferences for cattle farming, cropping, settlement, and sugarcane growing.

Vegetation slows the movement of water down the catchment decreasing flood peak heights downstream.

Vegetation additionally removes large amounts of sediment from the flows, reducing the overall volumes, the water’s turbidity and, its erosive potential.

By trapping sediment and woody debris, vegetation assists the cycling of carbon in soils and recharges floodplain landscapes with soil/fertility.

Natures First Responders

In Gibbergunyah Creek in Nightcap National Park, a new bank of deposited material has self-seeded with Bangalow Palms.

It appears that Bangalow Palms are well adapted to pioneer these areas.

This area of about 20m2, perhaps shows us what the embryonic stage of a more extensive palm forest such as at Protestors Falls may have looked like.

Sediment begat seed, seed begat plant, plant begat sediment.

And so, the fertile floodplain and its splendiferous forest form, one rainfall event after the next.

Energetically, the forest is transforming some of the transient kinetic energy of stormwater into perched landscapes and complex biodiverse ecosystems. It is slowing the rate of entropy.

The Protestors Falls palm forest example shows us that complex native vegetation communities have evolved to play a critical role in sediment capture, and even landscape formation.

Evidence of large quantities of sediment is examples such as the Bat Cave Creek palm forest prove floodplain forests effectiveness in removing volume from floodwater, and in slowing the rate of water flow through the catchment and thus reducing flood peaks.

Jeremy Stewart is a volunteer with Whian Whian Landcare, a committee member of Richmond Landcare and Richmond Riverkeeper and a past community representative on the Lismore City Council Flood Mitigation Committee.