Sediment loads in Hawke's Bay after Cyclone Gabrielle

Sentinel-3 satellite data from the European Space Agency provides some insights into the amount of sediment that washed into Hawke's Bay after cyclone Gabrielle.?Physical modification and land use change in Hawke's Bay have resulted in landscapes with hardly any remaining natural resilience to control floods and related ecosystem services. The remaining healthy soils are being lost in an alarming rate which is the key ingredient for healthy resilient ecosystems.?“It also takes around a thousand of years to create just a few centimetres of topsoil and to help land restoration” (UN, 2022).?

The Sentinel-3 TSM_NN band contain information expressed as the total suspended matter (TSM) that can be explained as the measure of the concentration of particulate material in the surface water such as mud, silt, and other fine-scale debris, including both organic and inorganic fractions. Sentinel-3 TSM concentration is measured in gram per cubic meter (g.m3). An analysis of the Sentinel-3 TSM concentration in Hawke's Bay before and after Gabrielle indicate that the TSM (sediment load) increased from an average of 2.185 g m3 on the 6th of February 2023 (already relatively high compared to November 2020 Sentinel-3 TSM_NN flood data) to an average of 5 g m3 on the 18th of February (6 days after the flood). ?Cloud cover over the area made it difficult to get more accurate readings. The northern section of the study area (black dash line) to the north of the Māhia Peninsula included data gaps due to cloud cover.?On the 22nd of February the average sediment loads decreased to 3.6 g m3 which is still a very high reading and there was little interference by clouds. ?The largest concentration of sediment after the floods can be seen at the river mouths of large rivers and especially within the bay where sediment accumulated. ?The sediment load was concentrated along the coast on the 18th of February where it became more dispersed further from the shore on the 22nd of February. The concentration of sediment was the highest on the 18th where the count of high values between 2.7 to 4 g m3 dominated.?The highest sediment reading of 311 g m3 also occurred on the 18th. An estimated >24 241 metric tonne of sediment for a 1 m depth of water was suspended between the 6th and the 18th of February within the study area (within the black dash line).?

It is evident that flooding occurred in this area spanning over 100 years and probably even longer including on a similar scale in 1938 (https://www.nzgeo.com/stories/inundated/ & https://tinyurl.com/58948xpy). Land degradation similarly happen over a long time and have been accelerated mainly through human damage of the land in this area. The UN predicts that up to 40% of the world’s land are now classified as degraded. These same soil particles, now called sediment, once formed part of fertile topsoil on land. We can't afford to keep wasting soil. Soil is our life support system. Excessive sediment in our oceans are also harmful to the marine environment as it smothers plants and animals. These sediments also carry pollutants such as nutrients, heavy metals, organic chemicals, bacteria and other pathogens along with it. These insights of sediment load into our oceans is just another reminder that New Zealand is dependent upon a stable and resilient soil system both for our economy but also for healthy ecosystems that provide natural resilience for society.

*EUMETSAT (2021) indicate that the validation of the product with in-situ measurements is still preliminary and that TSM values may be quite inaccurate.