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    Multi-scale morphodynamics of unconfined coarse-bedded rivers in the Ruamāhanga catchment : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2023) Conley Jr, William C.
    Riverine floods are the most frequent global natural disaster and a fundamental behaviour of alluvial systems. Flood protection management tends to focus on constraining the spatial extent, frequency, and magnitude of inundation, but often neglects a second intrinsic alluvial behaviour: changes to the channels themselves. This omission can be particularly problematic for gravel-bed rivers, which are well-known for their change propensity across spatiotemporal scales due to complex sediment dynamics. Changes often vary asynchronously on timescales far greater than individual flow events with controls and processes that nest across spatial scales. Further, differential feedbacks mean that similar water discharge may result in two or more very different channel responses. This complex array of riverscape responses and biophysical feedbacks in space and time encapsulates riverscape dynamics, which lie at the heart of this thesis. The applied research of this thesis addresses important gaps regarding 1) suitability assessment of geospatial time-series data, 2) river avulsion hazard screening, 3) tectonic forcing of alluvial rivers, and 4) channel response to human forcing. The gravel-bed rivers of New Zealand’s Ruamāhanga catchment provide an excellent real-world setting to explore these gaps. The short, steep rivers draining the high-relief Tararua Range experience frequent, intense rain-driven hydrology and transit the low-relief of an active forearc basin after emerging from the mountain front. The rivers cross numerous active geologic structures including the Wairarapa Fault, known for the world’s largest single-event land-based horizontal displacement (18.7 m) in 1855. Rivers draining the Tararua Range are known to have filled and expanded into the late-1860s as landslide sediments introduced to rivers during the 1855 shaking gradually worked downstream. Associated changes in river channel and floodplain forms would have generated more severe flood responses given a comparable pre-quake precipitation event. While the rivers eventually adjusted to a more relaxed state of response, their high intrinsic dynamism continues to challenge human habitation. With extensive riparian agriculture and 26% of all buildings occurring on Holocene alluvium, humans have exerted considerable control over the past sixty years using frequent (annual- to sub-annual) earth-moving to incrementally train multithreaded, wandering rivers into the narrower, straighter, and steeper forms seen today. Confidence in localizing hazards and characterising river dynamics rests on certainty that the same locations is/are being compared through time and expressed as coregistration error for members of a time-series. I developed a numerical model to illustrate how root-mean-squared-error (RMSE) values, the customary form of error expression, from intercomparison of randomly-varied and systematically-biased datasets are substantially greater than RMSE values derived from comparison to a common reference. A case study from the Ruamāhanga catchment spanning a wide quality spectrum of archival aerial photomosaics (n = 5) indicates assumptions of residual normality are tenuous and RMSE consistently represents only 65-75% of error frequency and 30-36% of the maximum observed error magnitude. My results suggest that an empirically determined 95% confidence value is better suited to address simulated and real-world limitations. After reprocessing a subset of the historic data, some prior interpretations of channel movement were found erroneous. I propose a conceptual framework to aid fitness-for-purpose determination for different types of geospatial data quality, errors, and analyses to promote more suitable interpretations. Globally, the direct observational record of major river relocations (avulsions) is highly limited given their infrequent occurrence on timescales from decades to millennia. Field- and model-based investigations over the last 45 years have identified a diverse array of contributing factors. These include sedimentation rates, conveyance capacities, and erodibility of existing channels relative to potential receiving areas, although to varying degrees across landscapes. My review identifies topographic advantage as a common denominator across landscape settings and develop a relative digital elevation model (rDEM) approach for rapid, landscape-scale screening in GIS. I propose revision of the traditional two-phase avulsion model to be consistent with other threshold phenomena (e.g., landslides) that divides factors of the set-up phase into static and dynamic components. I present a simple, dichotomous conceptual framework along a gradient of sensitivity (threshold proximity) to aid resource prioritisation for follow-on investigation and/or mitigation. Explicit inclusion of coseismic displacement adds novelty, particularly as adjacent streams appear to be out-of-phase. Control of rivers by tectonic processes is traditionally investigated at millennial and orogen scales though some recent studies have explored annual and reach scales. Nonetheless, fluviotectonic dynamics operating between these timescales are relatively unexplored, especially regarding gravel bed rivers. I relate changes from a time-series of benchmarked cross-sections to surface deformation interpreted from a high-resolution DEM for a 16-kilometre study segment that crosses four active oblique strike-slip faults and several folds. Net and total bed change within and between cross-sections exhibit a high-degree of noise and lack reach-scale patterns. In contrast, patterns of total change accumulated over the time-series show strong spatial partitioning by intersecting geologic structures. The least dynamic cross-sections are generally in proximity to uplifted axes while the most dynamic cross-sections are generally downstream of such intersections and/or coincide with inferred back-tilting. This is the first analysis to suggest morphological forcing of an alluvial river by active geologic structures is detectable at decadal-scale and persistent during an interseismic period. Human river management is a critically important control, but seldom addressed by research across spatiotemporal scales. I address this gap by evaluating a mix of short- and long-term records to assess congruence between two common management aims: increased channel stability and reduced active footprint. Active belt width at the riverscape scale (~16 km) interpreted from a 69-year aerial photo record shows decreasing trend (-48% mean) and increasing uniformity (-62% SD) that converges on the width of the contemporary design corridor (fairway). By contrast, ultrahigh-resolution, reach-scale morphological budgeting over a series of sub-annual events finds roughly sixfold greater volumetric changes in sub-reaches with recent in-channel flood protection earthworks than adjacent untreated reaches. Treated subreaches experienced up to 16 metres of lateral bank erosion, had more instances of increased activity outside the fairway, and propagated changes into untreated areas upstream and downstream. While management actions appear collectively successful in long-term constraint of the riverscape’s active belt, the same actions amplify bed changes from common flow events. I call this anti-pattern the “fairway paradox”. Increased magnitude and frequency of reach-scale movements over short time periods creates maintenance dependencies and makes river responses to flooding less certain. Such scale-dependent responses mean action-effectiveness can only be assessed if aims explicitly identify spatiotemporal scale. Greater certainty regarding a river’s location at any point in time may be gained by less frequent mechanical interventions. This study is the first rigorous evaluation of the effectiveness of NZ river fairway management with broad implications given the widespread application of similar river management throughout NZ. Collectively, this thesis highlights importance of identifying different controls on river behaviour and the scales on which they operate. Substantive doubt is cast on broad application of theoretical or averaged design conditions over alluvial riverscapes with diverse and comingled controls. This is particularly important considering management has occurred under best-case conditions and a large sediment-generating earthquake is expected regionally every ~150 years. As the first work of its kind in NZ and possibly globally, this thesis provides a robust example for future multiscale investigations.
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    Seasonal and year to year variation in the macroinvertebrate communities of New Zealand forest streams : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Ecology at Massey University, Palmerston North, New Zealand
    (Massey University, 2002) Minchin, Stephen Mark
    The bed movement of 42 streams in the Ruahine Forest Park, Urewera National Park, and Cass-Craigiebum region was predicted from each stream's channel and catchment characteristics. While a stepwise regression was relatively unsuccessful in predicting tracer particle movement, an artificial neural network analysis achieved strong correlations with measured tracer particle data. Forty-three streams in the Ruahine and Tararua Forest Parks were sampled in the summers of 1996 and 2001, and the macroinvertebrate communities compared. Changes in community structure between the two surveys did not correlate with any measured environmental characteristics including stream bed movement and change in periphyton biomass. MCI scores changed by a mean of 12.8 points between the two surveys, and the number of sites attaining an MCI score indicative of a 'pristine' stream dropped from 40 to 29. This appears to be related to a change in stream temperature, with streams that were cooler in 2001 than in 1996 showing an increase in MCI, while those which were warmer showed a decrease. Changes such as these could have a marked effect on biomonitoring programmes that use reference sites similar to these streams. In both 1996 and 2001, a greater number of taxa were collected from sites with more periphyton - taxon richness appears to asymptote at chlorophyll a concentrations greater than 5 μg/cm² Twelve streams within the Ruahine Forest Park were sampled every three months between June 2000 and May 2001. Both periphyton biomass and macroinvertebrate taxon richness tended to decrease with bed movement. While macroinvertebrate community structure showed marked changes over the study period, these changes were not linked to bed movement or variation in periphyton level. The seasonal changes observed in these streams are not significantly different to the changes seen between the summers of 1996 and 2001 - community structure was no more stable between two summers separated by five years than it was between the seasons of a single year. Eight artificial channels were laid on the bed of the Turitea Stream. At the onset of the experiment, half of the channels contained neither invertebrates nor periphyton cover, while the other half had no invertebrates but an initial periphyton layer. Drift samples indicate that approximately one in four drifting invertebrates colonised the channels during the 14 day study period, with benthic taxon richness reaching a peak after only four days. Colonisation was not affected by periphyton biomass. Some of the less common taxa that were present in the water column did not colonise the channels within 14 days.
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    Downstream fining in the Waipaoa River : an aggrading, gravel-bed river, East Coast, New Zealand : thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Quaternary Geology at Massey University
    (Massey University, 1997) Rosser, Brenda J
    The Waipaoa River, East Cape, New Zealand, drains a catchment from the Raukumara Ranges into Poverty Bay, near Gisborne. Conversion of the catchment from indigenous forest to pasture, between 1880-1920, initiated a phase of intense erosion in the hill country. The underlying geology consists of crushed and sheared sandstone, siltstone, argillite, and mudstone of Cretaceous and Tertiary age. Channel aggradation occurred in response to the influx of bed material load. Suspended sediment yields in headwater catchments are as high as 7 000 – 17 000 t km. For the period 1948 to 1988, aggradation in the upper reaches was > 5 m, while in the lower reaches it was ~0.5 m. The Waipaoa River is a gravel-bed river. Its morphology changes from a braided to a meandering configuration in the downstream direction. A bed material survey of the Waipaoa River in 1995/6 investigated the fluvial transfer of coarse bed material through the river system. Bed material samples were collected at 1 km intervals along the mainstem, as well as from major tributaries, near their confluence with the Waipaoa River. Surface and subsurface samples were systematically collected between the coast and 104 km upstream. The results of this survey were compared with earlier bed material surveys undertaken in 1950, 1956, and 1960. Results of the 1996 bed material survey indicate that the bed material in the Waipaoa River is polymodal. The gravel-sand transition occurs approximately 8 km upriver from the coast. Over the remaining 96 km reach, the median particle size declined from 5 mm in the headwaters, to 2 mm near the coast. The coarser particle size fractions exhibited a greater rate of downstream fining, and, over the same distance, the coarsest 10% declined from 48 mm to 6 mm. The bed material is dominated by fine sediment, which is illustrated by the fine median particle size over the length of the river, as well as the low fining coefficients for the finer particle size fractions . No downstream change in the proportion of each main pebble lithology was observed, and each pebble lithology exhibited a similar rate of downstream fining. No downstream alteration in particle shape was observed, although particle roundness did increase downstream. Close relationships were observed between the bed slope and particle size. The highest degree of correlation was observed between slope and the coarsest particle size fractions, representing the limiting condition of channel competence. Selective transport is the dominant process that produces downstream fining in the Waipaoa River, however, particle fragmentation, sediment supply or abrasion may be important processes within specific reaches. The rate of downstream fining was consistent for the period 1948 to 1996.