Noel A. Trustrum

Gully erosion in Mangatu Forest, New Zealand, estimated from digital elevation models

The methodology and errors involved in determining the amount of sediment produced during two (19·5 and 33·2 year) periods by 11 (c. 0·01 − >0·20 km2) gullies within a 4 km2 area in the headwaters of the Waipaoa River basin, New Zealand, using sequential digital elevation models are described. Sediment production from all gullies within the study area was 0·99 ± 0·03 × 106 t a−1 (2480 ± 80 t ha−1 a−1) during the period from 1939 to 1958. It declined to 0·62 ± 0·02 × 106 t a−1 (1550 ± 50 t ha−1 a−1) during the period from 1958 to 1992, when many of the smaller gullies were stabilized by a programme of afforestation, which commenced in 1960. Both figures are very high by global standards. The two largest (the Tarndale and Mangatu) gully complexes together generated 73 and 95 per cent of the sediment in the specified time periods, but the latter amount is equivalent to only c. 5 per cent of the total annual sediment load of the Waipaoa River.

Event Suspended Sediment Characteristics and the Generation of Hyperpycnal Plumes at River Mouths: East Coast Continental Margin, North Island, New Zealand

Steepland rivers draining small, coastal watersheds often have very high suspended sediment loads, but the event characteristics of suspended sediment concentration and yield in this class of river is not well documented. Continuous monitoring at four sites in the Waipaoa River basin, New Zealand, demonstrates that during individual and composite events, suspended sediment concentration versus water discharge relations typically show clockwise hysteresis and that event maximum concentration is poorly related to event peak discharge. The signature of different erosion processes is also imprinted on the event yield magnitude frequency distributions. Gully‐dominated tributary basins produce relatively high yields at all frequencies, reflecting greater sediment availability, whereas in tributary basins, where shallow landsliding is the dominant erosion process, there is a steep increase in yields in relation to return period. We estimate that flood discharges from the Waipaoa River approach or exceed the critical suspended sediment concentration (∼40,000 mg L−1) for hyperpycnal plume generation (because of negative buoyancy) at the river mouth once every ∼40 yr, but in the neighboring Waiapu and Uawa Rivers, the threshold concentration may be exceeded once a year and two to three times a year, respectively.

Source, sea level and circulation effects on the sediment flux to the deep ocean over the past 15 ka off eastern New Zealand

The last post-glacial transgression and present highstand of sea level were accompanied by a reduction in the terrigenous flux to the deep ocean bordering the active convergent margin off the eastern North Island of New Zealand. Although in accord with long-established models of highstand shelf deposition, new data from giant piston core MD97 2121 (2314 m depth) reveal that the flux also varied with terrigenous supply and palaeocirculation. Between 15 and 9.5 ka, the flux reduced from 33 to 20 g/cm2/ka as supply declined with an expanding vegetation cover, and mud depocentres became established on the continental shelf. An increase from 20 to 27 g/cm2/ka during 9.5–3.5 ka coincided with a strengthened East Cape Current which probably introduced sediment from fluvial and shelf sources in the north. The flux profile shows no immediate response to the establishment of modern sea level ∼7 ka. However, accumulation decreased from 3.5 to 1 ka as more sediments were retained on the shelf, possibly under wind-strengthened, along-shelf currents. Over the last 1 ka, the flux decline halted under increased terrigenous supply during anthropogenic development of the land. Despite the proximity of the North Islands Central Volcanic Region, major eruptions caused only brief increases (centuries duration) in the terrigenous flux through direct deposition of airfall and possibly fluvial redistribution of onshore volcanic deposits. Frequent earthquakes also had little short-term effect on accumulation although such events, along with volcanism, probably contribute to the long-term high flux of the region. The other measured flux component, biogenic carbonate, reached maxima of 6 g/cm2/ka between 11 and 8.5 ka when nutrient-bearing waters of the East Cape Current dominated the palaeoceanography. After these peaks, carbonate accumulation declined gradually to modern levels of ∼3 g/cm2/ka.

A high resolution record of storm-induced erosion from lake sediments, New Zealand

The sustainable management of erodible pastoral hill country is a major focus of land use research in New Zealand. A multi-disciplinary study, using a high resolution lake sedimentation record, is being conducted to determine the role that cyclonic storms and natural and human-induced vegetation changes play in the erosion history of a landslide-prone hill country watershed.Sediment cores from Lakes Tutira and Waikopiro in northern Hawkes Bay were analysed to construct the magnitude-frequency history of storm-induced erosion since European settlement. Pulses of sediment representing individual storms can be clearly identified and are correlated to a storm history derived from analysis of a 93 year daily rainfall record. Correlation and dating are confirmed by pollen and diatom analysis,137Cs distribution, tephrochronology and reference to a well documented land use history. Annually laminated, organic rich deposits, which occur in the uppermost sediments and represent the annual decomposition of biogenic material associated with eutrophication, are also used to confirm the chronology.A high correlation was found between storm sediment thickness and total storm rainfall (R2=0.8). Although sediment producing storms (>150 mm) occur on a near annual basis, the two largest storms (>600 mm) contributed 54% of the total sediment thickness.The presence of well defined ‘storm sediment pulses’ has enabled the lake storage component of a sediment budget to be calculated for Cyclone Bola (1988), the most recent and largest rainstorm on record. The integration of this budget with the storm-magnitude-frequency history will be used to develop watershed-based models to predict the impacts of land use changes and the erosion response to climate scenarios.

El Niño–Southern Oscillation signal associated with middle Holocene climate change in intercorrelated terrestrial and marine sediment cores, North Island, New Zealand

A synchronous textural variation in intercorrelated, high-resolution sediment records from floodplain, continental-shelf, and continental-slope settings of the eastern North Island, New Zealand, provides evidence of increased storminess after ca. 4 ka. An upcore change in sediment texture reflects the transition to landsliding, which supplanted fluvial incision as the dominant mode of sediment production in the middle Holocene. This signal, which appears in all three records, indicates a regional response to external forcing and records the impact of an intensified atmospheric circulation marking the establishment of the contemporary climate that is strongly influenced by the El Nino–Southern Oscillation. The change in climate was a hemispheric event, and in the Southern Hemisphere its timing is confirmed by independent proxy records from elsewhere in New Zealand and the circum–South Pacific region.

Gully erosion and sediment production: Te Weraroa Stream, New Zealand

conversion to pasture early in the twentieth century, was ameliorated by reforestation that commenced in 1962. Estimates of sediment production were made using the change in gully area observed in sequential aerial photographs. Channel storage was assessed from stream cross-section surveys. At its peak, gully erosion affected � 6% of the total catchment area. The amount of sediment contributed from gullies declined by 62% as the forest became established, but of the 28.7 Mt of sediment generated by gully erosion between 1950 and 1988, 48% was stored in the channel along the lower 8 km of Te Weraroa Stream. Even if the amount of sediment generated by gully erosion continues to decline, it likely will be many decades before the gravel is released from storage. INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1824 Hydrology: Geomorphology (1625); 1878 Hydrology: Water/ energy interactions; KEYWORDS: channel storage, gully erosion, sediment production Citation: Gomez, B., K. Banbury, M. Marden, N. A. Trustrum, D. H. Peacock, and P. J. Hoskin, Gully erosion and sediment production: Te Weraroa Stream, New Zealand, Water Resour. Res., 39(7), 1187, doi:10.1029/2002WR001342, 2003.

Soil depth-age relationship of landslides on deforested hillslopes, Taranaki, New Zealand

Abstract A soil chronosequence was examined on landslide scars of different ages in the Taranaki hill country. This area, underlain by Tertiary silty sandstone, was deforested 90 years ago. Sequential aerial photographs, historical terrestrial photographs and documented erosion events were used to date landslide scars formed since forest removal and establish age control for the chronosequence. Soil depth increased with landslide age and was used as an index of soil formation. Mean soil depth increased from 5 cm on 15 year old scars to 20 cm on 82 year old scars. Measures soil depths were attributed to rafted soil, colluvium and bedrock weathering. A chronofunction was derived by regressing mean soil depth against the logarithm of known scar age. Similarly, a second chronofunction was derived by excluding depths of rafted soil from the calculation of mean soil depth to describe soil accumulation on exposed bedrock within the landslide scar. This chronofunction showed a better correlation (r2 = 0.92 compared with r2 = 0.79) and can give an estimate of the age of other landslide scars to within ±27% for ages up to 90 years. The rate of soil depth increase averaged 3.5 mm yr−1 over the first 40 years after slipping but dropped to 1.2 mm yr−1 over the following 50 years. The logarithmic chronofunction suggests that the rate of soil formation further decreases beyond 90 years. Soil formation is primarily a result of bedrock weathering and accumulation of colluvium derived from surface fragmentation of exposed bedrock and crumbling scar margins.

Influence of storm-related sediment storage on the sediment delivery from tributary catchments in the upper Waipaoa River, New Zealand

Although much is known about overall sediment delivery ratios for catchments as components of sediment production and sediment yield, little is known about the component of temporary sediment storage. Sediment delivery ratios focused on the influence of storm-related sediment storage are measured at Matakonekone and Oil Springs tributaries of the Waipaoa River basin, east coast of New Zealand. The terrace deposits of both tributaries show abundant evidence of storm-related sedimentation, especially sediment delivered from Cyclone Bola, a 50 year return rainfall event which occurred in 1988. The sediment delivery ratio is calculated by dividing the volume of sediment transported from a tributary to the main stream by the volume of sediment generated at erosion sites in the tributary catchment. Because the sediment delivery volume is unknown, it can be calculated as the difference between sediment generation volume and sediment storage volume in the channel reach of the tributary. The volume of sediment generated from erosion sites in each tributary catchment was calculated from measurements made on aerial photographs dating from 1960 (1:44 000) and 1988 (1:27 000). The volume of sediment stored in the tributary can be calculated from measurements of cross-sections located along the tributary channel, which are accompanied by terrace deposits dated by counting annual growth rings of trees on terrace surfaces. Sediment delivery ratios are 0·93 for both Matakonekone catchment and Oil Springs catchment. Results indicate that Oil Springs catchment has contributed more than twice the volume of sediment to the Waipaoa River than the Matakonekone catchment (2·75 × 106 m3 vs 1·22 × 106 m3). Although large volumes of sediment are initially deposited during floods, subsequent smaller flows scour away much of these deposits. The sediment scouring rate from storage is 1·25 × 104 m3 a−1 for Matakonekone stream and 0·83 × 104 m3 a−1 for Oil Springs stream. Matakonekone and Oil Springs channels respond to extreme storms by instantaneously aggrading, then gradually excavating the temporarily stored sediment. Results from Matakonekone and Oil Springs streams suggest a mechanism by which event recurrence interval can strongly influence the magnitude of a geomorphic change. Matakonekone stream with its higher stream power is expected to excavate sediment deposits more rapidly and allow more rapid re-establishment of storage capacity. Copyright

Contribution of floodplain sequestration to the sediment budget of the Waipaoa River, New Zealand

Abstract Rapid vertical accretion on the Waipaoa River floodplain is conditioned by the river’s high suspended sediment load (30 000–40 000 mg l−1 at flood stage). Cumulative sediment accumulation curves derived from three cores suggest an average (post-1850) rate of vertical accretion of c. 60 mm a−1, though a 15 year lacuna in flood activity has depressed the post-1948 rate to c. 40 mm a−1. Rates of aggradation during floods are several orders of magnitude larger than the time-averaged rate. Within a 44 km long reach, cross-section surveys indicate that 0.2–0.8 m of sediment was deposited between 1979 and 1990. Over this period floodplain storage accounted for 5% of the total suspended sediment load, and 16% of the suspended sediment load transported during events that exceeded bankfull stage. The Waipaoa River floodplain may be representative of floodplains bordering rivers with high suspended sediment loads, produced by rapid, episodic vertical accretion, on which overbank deposition occurs across the entire floodplain, and is complemented by channel aggradation. Such rivers are able to construct high banks. Thus channel capacities are greater and the incidence of overbank flows is less than in rivers where overbank deposition is slow relative to the rate of floodplain destruction by lateral migration. The difference between our time-averaged estimate for sequestration on the Waipaoa River floodplain and comparable estimates for actively meandering rivers, and meandering rivers with low sediment loads, reinforces the notion that there is a link between the sediment transport regime of a river and its sedimentary record. To elucidate this link it is necessary to view vertical accretion in the context of the flood events that generated it, rather than in the context of a time-averaged sediment budget.

Estimation of temporally averaged sediment delivery ratio using aggradational terraces in headwater catchments of the Waipaoa River, North Island, New Zealand

The sediment delivery ratio was estimated for two periods (28 years and eight years) following reforestation of seven tributary catchments (0·33 to 0·49 km2) in the headwaters of the Waipaoa River basin, North Island, New Zealand. In these catchments, gully erosion, which largely resulted from clearance of the natural forest between 1880 and 1920, is the main source of sediment to streams. Reforestation commenced in the early 1960s in an attempt to stabilize hillslopes and reduce sediment supply. Efforts have been partially successful and channels are now degrading, though gully erosion continues to supply sediment at accelerated rates in parts of the catchment. Data from the area indicate that the sediment delivery ratio (SDR) can be estimated as a function of two variables, ψ (the product of catchment area and channel slope) and Ag (the temporally averaged gully area for the period). Sediment input from gullies was determined from a well defined relationship between sediment yield and gully area. Sediment scoured from channels was estimated from dated terrace remnants and the current channel bed. Terrace remnants represent aggradation during major floods. This technique provides estimates of SDR averaged over periods between large magnitude terrace-forming events and with the present channel bed. The technique averages out short-term variability in sediment flux. Comparison of gully area and sediment transport between two periods (1960–1988 and 1988–1996) indicates that the annual rate of sediment yield from gullies for the later period has decreased by 77 per cent, sediment scouring in channels has increased by 124 per cent, and sediment delivered from catchments has decreased by 78 per cent. However, average SDR for the tributaries was found to be not significantly different between these periods. This may reflect the small number of catchments examined. It is also due to the fact that the volume of sediment scoured from channels was very small relative to that produced by gullies. According to the equation for SDR determined for the Waipaoa headwaters, SDR increases with increasing catchment area in the case where Ag and channel slope are fixed. This is because the amount of sediment produced from a channel by scouring increases with increasing catchment area. However, this relationship does not hold for the main stem of the study catchments, because sediment delivered from its tributaries still continues to accumulate in the channel. Higher order channels are, in effect, at a different stage in the aggradation/degradation cycle and it will take some time until a main channel reflects the effects of reforestation and its bed adjusts to net degradation. Results demonstrate significant differences among even low order catchments, and such differences will need to be taken into consideration when using SDR to estimate sediment yields. Copyright