Marine Sediment

May 2022

Understanding the ways in which sedimentation is affecting the coastal environment.

Sediment in our waterways causes a number of detrimental effects; decreasing clarity, reducing visibility, and making it hard for fish and invertebrate species to find food and places to live. It can carry contaminants and other pollutants from the land, further degrading the quality of streams, rivers and eventually the marine environment.


The Hawkes Bay Marine & Coastal group is trying to understand the ways in which sedimentation is affecting the coastal environment to determine the damage being caused and how that can best be mitigated. One farming couple has taken some critical steps in their small catchment area to capture incoming sediment and control runoff into waterways.


New Zealand has a vast coastal marine estate that is used for recreation, food supply and transport. However, coastal waters are increasingly under pressure from the conflicting needs of users and the effects of land-based industries. In Hawke’s Bay, hill country erosion is a key issue. About 252,000 hectares of this land class has been identified as being at high risk, with erosion rates greater than 1,000 tonnes per km2 per year. It is estimated that this land transfers 3,730,000 tonnes of sediment (about half of the regional total) into the region’s waterways and estuaries every year. 


According to the council estimates, the region as a whole delivers 7.2 million tonnes - equivalent to about 136,000 truck and trailer loads of sediment into Hawke Bay every year. This results in high suspended sediment concentrations in runoff and impacts streams, rivers and coastal areas.


The Hawkes Bay Marine & Coastal Group has been involved with the “Sustainable Seas” National Science Challenge, studying the region’s coast as part of a national drive to ensure that competing uses are managed in a way that does not degrade the marine environment, ie. using ecosystem-based management (EBM). 


Becky Shanahan is the Senior Scientist (Marine & Coasts) with the Hawke's Bay Regional Council and is facilitator of the Marine & Coastal Group. She says that the current aim of the Group is to determine how reducing the sediment carried by waterways by different amounts will affect recovery of the coastal ecosystem. 


A model called SedNetNZ has been built to help understand how soil and sediment moves through the landscape, rivers and estuaries. Extensive water quality monitoring is undertaken, and located throughout the region, a network has been set up of 20 automated sediment (ISCO) monitors to measure sediment transport at high flows. These monitoring tools are used to estimate sediment movement and load across the entire region. SedNetNZ has shown that sediment loads have increased from pre-human to current levels by between 60-600% on hill country, depending on the catchment.

As Becky notes, erosion is a natural process, but human activity has accelerated the rate of erosion and the scale of its effects. “Sediment has always been carried by rivers to the sea but when the landscape was covered in bush and forest the amount was lower and the ecosystem could recover. Today the problem is that the changes in land use and management have increased the amount of sediment, and it's a lot more than the system can cope with. It doesn't have time to recover.” Hawkes Bay’s steep hill country is prone to erosion, and clearing it for pasture greatly exacerbates the problem. Forestry may also result in substantial erosion, particularly when forests are harvested.


Becky says that big pulses of sediment after significant rain can smother living organisms on the sea floor. “Some that live in the sediment, eat by filtering the water and sifting out particles, and so if the water's quite murky the mud can actually clog up their feeding structures and their gills and they can't feed properly. Those animals are the base of the food chain in a lot of places. Other things like flounder dig in the sediment for their food and so that can have implications further up the food chain. Then there are some fin fish that can't see as well and their feeding rates actually drop in murky water. Some animals like kina, which normally attach to the bottom. find it harder to grip the surface because there's a thin layer of sediment. Similarly, algae attach to rocks using a kind of like a suction cup, and so if there’s sediment there they can't stick as well. 


“Seaweed is particularly important because it provides habitat for many species, but if sediment makes the water cloudy continuously for a long period of time then the seaweed can't access sunlight to photosynthesise and grow, and that starts to become a problem. The ocean's very good at processing things out and a healthy ecosystem should be able to handle these occasional pulses of sediment and recover, but if the pulses keep happening over and over it’s harder for the marine system to come back.  And with climate change, increased droughts and increasing torrential rain complicate the situation and make the future it more uncertain.”


The Coastal Group was formed in 2016 because some recreational fishers were seeing a decrease in their catch. Commercial operators were also concerned at the loss of some flounder fisheries because of sedimentation. The broad membership of the Group made it a good sounding board for the Sustainable Seas project.


Although it’s early days for the project and the results of modelling are not available yet, rivers have been sampled and a baseline of sediment information established.  Large differences have been found in the fate of sediments carried by different rivers.  For example, the Mohaka River carries some of the highest levels of sediment of all rivers in the region but the Wairoa River mouth is virtually sediment free because of ocean currents, so the sediment is going somewhere else.  In contrast, other estuaries can have very murky water after rain events. 


The state of coastal waters is surveyed by the Regional Council using a remote operated vehicle (ROV), an underwater drone. Sampling of rivers is also carried out regularly and samples analysed for turbidity.  The Marine & Coastal Group is responsible for accumulating some of the data but not for making decisions around mitigation or legislation.  That’s the role of the Regional Council, guided by the Sustainable Seas challenge outputs.


Land-based mitigation measures


The Hawke's Bay Regional Council has an erosion control programme that supports farmers trying to improve land management practices. It is committed to work towards keeping fertile soil on farms, where it is most valuable from an ecological and farm management perspective. The most effective way to prevent erosion on vulnerable hill slopes is to plant trees, and the incentivised Erosion Control Scheme is a start. It includes fencing off and planting alongside rivers and streams, and also hillsides to improve slope stability. 


Chris & Jo Haynes farm a 35ha block of hill country running it as a finishing area for sheep and heifers. It has a central gulley that serves as a watercourse for a number of springs and overflow from the neighbours upstream when it rains. When they first arrived nearly 20 years ago the block was devoid of trees, and the gully was a mass of blackberry that took several years of spraying and a lot of work to eliminate.  Subsequently they began to plant willows and poplars to help stop erosion and provide shade for the livestock.


“We thought we'd like to get into conservation and sustainability, protection against soil erosion, putting in wetlands for aquatic plants and birds – things that would hold our country together because there's nothing worse than hillsides slipping and having to fix a fence every year,” says Chris. “Because of the summer dry here we focused on what species would survive best, what netting to put around the trees, and we wanted to be involved with the community so that people could learn from it.”


Jo says that the clearing of the central gulley and planting of trees changed the nature of it.

“One drought we had lambs and that's where they lived during the day because of the water and the shade, and the grass was green and kept growing because of the springs and the lambs put on weight. We had a finger that fed into the gully, we planted all of that as well. And then we started going to see how high we could get before the country got too hard and dry, and we were hitting limestone rocks.  That held and cleaned the water before it came to the creek reduced the amount of silt coming down and blocking up the creek. So we tried to eliminate as many issues as we could upstream from the creek.


“We got a digger in and improved access so we could get the poplars up the hill, we changed the course of a few fingers coming into the creek. And we flattened out an area, simply to fence off the headwaters of the creek running from our dams. Springs come out of the ground, and run into these dams, from which we pump into a tank and gravity feed troughs from there. With help from the Regional Council we started fencing off parts of the creek, and we had to take out quite a bit of a hill on one particular part, so we planted it in about 400 natives.  And then on the steepest side of the gully we planted in kanuka and totara. Most of those have actually survived even through two droughts. In a third section we planted the same types of tree, but because of the harshest drought we lost about 50 of them and that was disheartening, but we've replanted and most of them are still alive.” 


The Regional Council has supplied most of the trees along with some help with planting and supplying cages to protect the trees against stock. A farming group from the Napier Boys High will be coming out and working on taking fences down prior to their replacement. 

“We've now decided to do another block.  We're going to take a fence down put in a two-wire electric fence so sheep can go under them and graze but the cattle can't. Council staff are helping us with ideas and advice – it’s pretty much a fluid scenario. 


In other areas there’s been a lot of fertiliser put on in the past and we’ve allowed the grass to become long, cover the surface more completely and re-seed naturally.


So have they essentially been working to rehabilitate the land? Says Jo: “I suppose you could look at it like that. The land was stripped of what it had way back. And then the hot Hawke's Bay sun slowly baked the earth and it cracks open. And then you get a deluge like we did in 2008 and your hills just slip. So really, it's just trying to put back what may have been there once upon a time. Just trying to restore it to some former glory, I suppose.”


There is recognition from many that despite the positive and constructive steps undertaken by many people, massive change is still needed in how trees are planted to protect the landscape. The HBRC is scaling up its tree planting through a programme called Right Tree Right Place. The intention is to partner with private landowners, investors and forestry companies on planting trees for revenue on marginal farmland while preserving farm viability, particularly for the sheep and beef industry. There is recognition that we all inhabit occupy modified environments, but that targeted and appropriate management of vulnerable land will be a key factor in ensuring our ability to occupy those environments successfully into the future.