Las figuras tributarias cedidas a las Comunidades Autónomas

The mixing model was able to derive mean hourly sediment source contribution estimates throughout the storm hydrograph. This showed a number of fluctuations occurring throughout the flow hydrograph. Mean Mudstone source proportions fluctuated between ≈ 20 – 60 % throughout the storm duration while Hill Subsurface and Hill Surface both approached 0 % minimum, but upper limits of ≈ 23 % and ≈ 24 %. The Channel Bank sediment had the lowest proportions, remaining effectively 0.0 % throughout the storm. The Mountain Range sediment source fluctuated from ≈ 24 – 46 %. Gravel Terrace and Loess both have estimated contributions near 0 % but display maximums as much as ≈ 6 % and ≈ 0 % respectively. Limestone shows low sediment source contribution which range between ≈ 1 – 3 %. These hourly observations exhibit considerable variability, appearing as high frequency variations (Fig. 44). This variability could arise from uncertainties inherent in the current sediment fingerprinting technique. Several studies (Pulley et al., 2015, Belmont et al., 2014, e.g.Koiter et al., 2013b) have identified issues arising from methodological assumptions and unconstrained factors connected to tracer and model selections. This limits the ability to interpret changes in suspended sediment source estimates between hourly observations, especially when source contribution changes are small.

Sediment transport mechanisms may also provide a geomorphological explanation for the observed variability. Bedload moves through the channel in pulses and waves (Nicholas et al., 1995), and each pulse or wave of sediment can represent a distinct or unique combination of source sediments which may also apply to suspended sediment patterns. This provides different geochemical signatures, which incline the sediment source signals towards sporadic patterns. This has implications for achieving a desired temporal resolution, as not only is the sampling interval important, but the actual timing of the sample is important too. This study employed a one hour sample interval, which is relatively frequent considering the storm duration and the number of sediment samples collected as a result, but still may not fully capture the sediment dynamics occurring within the catchment. Sediment waves occur in different scales and require different lengths of time to move through a fluvial system, meaning that multiple superimposed patterns of variability occur within a single event, and the higher frequency sediment source contribution variations may in fact be missed altogether if the sediment waves are moving past the sample location faster than the sampling interval. The optimal time intervals required for adequate sampling warrant further consideration to reveal any patterns within the source variability and may require considerably more auxiliary information to provide better insights into sediment movement.

13 3 Fi g. 44: Mean hour ly s ediment so ur ce c o ntr ibutions dur ing t h e stor m event; per centage (top), s ediment load (bottom)

Fig 45: Mean proportions of each sediment process throughout the storm event (dashed line) and a 5 point moving mean (bold line)

Despite the high variability experienced between adjacent hourly samples, a moving average identifies some low-frequency patterns and switches in sediment source contributions. Five of the sources show a distinct shift occurring from the period between 2:00 and 8:00 hrs, 28th Nov, compared with the rest of the storm duration (Fig 45). Mean Mudstone source proportions were as much as ≈ 50 % between 2:00 – 8:00 hrs compared to contributions averaging closer to ≈ 40 % for the remaining duration. Similar elevated contributions between 2:00 – 8:00 hrs were also observed for Mountain Range and Loess derived sediment sources. The Mountain Range sediment exhibited contributions of ≈ 40 % in this heightened period compared to an average of ≈ 30 %, while Loess sediment almost reaches ≈ 0.4 %, compared to an average of ≈ 0.1 % throughout the remainder of the storm flow.

The inverse of this pattern is observed for Hill Subsurface, Hill Surface and Gravel Terrace, showing relatively low sediment contributions between 2:00 and 8:00 hrs, 28th Nov. Both the Hill Subsurface and Hill Surface contributions average ≈ 2.5 % compared to ≈ 13.5 % and ≈ 10 % for the remaining storm duration respectively. The Gravel Terrace contributes ≈ 1.0 % during the 2:00 – 8:00 hrs section and closer to ≈ 3.5 % sediment source contributions for the remainder of the storm duration. The Channel Bank and Limestone sediment sources do not display this pattern. In the case of the Channel Bank sediment, this is likely due to the contributions being effectively zero throughout (Fig 45).

Explanations for the difference between the 2:00 - 8:00 hrs phase and the rest of storm duration come from two factors. The first is the beginning of the shift between dominant sub- catchment flows, particularly the introduction and increase of the Upper Manawatu flow contribution (Fig. 43). This would lead to a relative decrease in flow contribution originating from the Pohangina sub-catchment and a shift in geochemical signature. However, the sedigraph shows that despite increasing flow contribution from the Upper Manawatu, the sediment load is almost exclusively still sediment originating from the Pohangina sub- catchment (Fig. 43). This indicates the changes here are less related to switches in sub- catchment flow and more likely reflect changes in suspended sediment either side of peak flow in the Pohangina sub-catchment occurring at approximately 1:00 hrs, Nov 28th (Fig. 43).

Fig 46: Suspended Sediment Concentration (SSC) – Discharge hysteresis patterns for Manawatu River at Teachers college (top) and Pohangina at Mais Reach (bottom)

Suspended sediment – discharge hysteresis for the Pohangina flow peak demonstrated an overall clockwise loop followed by a second counter-clockwise loop to reach the flood peak (Fig 46). Clockwise hysteresis patterns can reflect early sediment exhaustion which is often an indication of sediment sources occurring from within-channel causing an initial flushing of sediment or a significant sediment source close by (Lenzi and Marchi, 2000, Smith and Dragovich, 2009, Brasington and Richards, 2000, Klein, 1984). Alternatively, counter-clockwise hysteresis shows peak sediment concentration lags behind the flow peak. This potentially indicates a distal sediment source or the temporal difference in flood-wave celerity and mean flow velocity (Brasington and Richards, 2000, Klein, 1984, Smith and Dragovich, 2009, Lenzi and Marchi, 2000).

Thus, the initial clockwise loop in this storm indicates exhaustion of available sediment for transportation which coincides with the reduced influence of Hill Surface and Hill Subsurface sediment sources from the Pohangina sub-catchment (Fig 45, Fig. 47). It would appear that material from these two sediment sources is may be in storage within the channel, but are flushed through in the initial peak flow, while Mudstone and Mountain Range sources provide a more sustained and continuous supply of sediment throughout the storm. The Hill Surface and Hill Subsurface sediment sources reappear in the second peak flow, but this time a counter-clockwise hysteresis (Fig 46) is exhibited by a delayed sediment source arrival, reflecting the distal location and subsequent travel times of these sediment sources.

There is also an apparent shift in sediment contributions occurring towards the end of the storm event where an increase in Mudstone derived sediment occurs in the last few hours of the sampled storm event. This could be caused by similar hysteresis effects as observed for the Pohangina sub-catchment displayed in the final few hours of the Upper Manawatu sediment flow (Fig. 47), or it could be due to an increasing flow contribution from the Tiraumea River (Fig. 43) which is the last sub-catchment flow to reach the T-Col sampling station, at the end of the receding limb. The Tiraumea is strongly associated with a high mudstone composition as well as high suspended sediment concentrations (Basher et al., 2012, Vale et al., 2016a). In addition to the increase in Tiraumea flow reaching the T-Col site, the extent of the Mudstone proportion increase is likely enhanced by the recession of all the other major sub-catchment flows, particularly from the Upper Manawatu, allowing the Tiraumea sub-catchment sediment load to influence the source proportions more influentially.

Regression analysis carried out in the sub-catchment sediment loads and the source proportions (Table 27). The strongest partial correlations were attributed to the Upper Manawatu and the Pohangina as these were the two most dominant sediment loads during this event. Despite controlling for each of the remaining sub-catchment sediment loads, the Mangahao, Mangatainoka and Pohangina do not display any strong partial correlations with any of the sediment sources, possibly reflecting the significantly lower sediment loads in those sub-catchments and an inability for the regression model to detect these correlations. The Upper Manawatu correlates high for all the sources except Channel Bank (0.282) and Loess (0.231), while exhibiting the greatest correlation with Mountain Range sediment (0.838). The proportion of Channel Bank sediment is effectively zero, removing any observable correlations. In addition, Vale et al. (2016b) identified Channel Bank sediment was the most difficult to

Fig. 47: Differentiation of sediment sources from the two dominant sub-catchment flows; A) Upper Manawatu; B) Pohangina

clearly distinguish geochemically because it is derived from other primary sediment sources, thus complicating any sediment correlations. Remobilization of in-channel fine-grained sediment also likely complicates the ability to associate storm related sediment source erosion with the suspended sediment sampled throughout the storm. The temporal and spatial pattern of sediment storage is dependent on previous storm events and processes. However an assumption is made that this stored sediment will still retain the signature of the primary erosion source during short time periods and still reflect original source locations which relate to their sub-catchments but may not be directly related to erosion from that storm. The higher partial correlations with Mountain Range sediment possibly indicate that the sources of sediment within that sub-catchment are relatively more sensitive to storm events. This would indicate the Mountain Range, Gravel Terrace, and Hill Subsurface are influenced more in the Upper Manawatu compared to the Pohangina sub-catchment where Mudstone (0.401) and Mountain Range (0.359) provide the largest partial correlation relative to Hill Surface (0.044) and Hill Subsurface (0.286), supporting the hysteresis effects observed in the Pohangina sub- catchment (Fig. 47).

Table 27: Partial Correlations controlling for sub-catchment sediment loads derived from Regression analysis Sub-catchment Sediment Load Upper Manawatu

Mangahao Mangatainoka Pohangina Tiraumea Corr. p-value Corr. p-value Corr. p-value Corr. p-value Corr. p-value Mudstone 0.693 0.001 -0.199 0.184 0.187 0.214 0.401 0.006 0.092 0.544 Hill Subsurface 0.72 0.001 -0.284 0.056 0.133 0.378 0.286 0.054 0.18 0.233 Hill Surface 0.57 0.001 0.034 0.825 -0.109 0.469 0.044 0.773 0.001 0.997 Channel-bank 0.282 0.057 -0.169 0.262 0.096 0.525 0.246 0.1 0.189 0.207 Mountain Range 0.838 0.001 -0.146 0.333 0.053 0.726 0.359 0.014 -0.106 0.484 Gravel Terrace 0.748 0.001 -0.122 0.418 0.014 0.926 0.282 0.058 0.186 0.217 Loess 0.231 0.122 0.148 0.327 0.088 0.561 0.238 0.111 0.042 0.782 Limestone 0.633 0.001 -0.024 0.875 -0.101 0.503 0.042 0.782 -0.059 0.695

The differences in sediment source proportions between the Upper Manawatu and Pohangina sub-catchment are possibly attributed to disparate transport pathways between the Hill Surface/Hill Subsurface from the other remaining sediments sources. The Hillslope terrains are typically distal to the active channel, and therefore likely affected by intermittent and impeded

transport pathways. Kasai et al. (2005) noted, in a similar New Zealand terrain, decoupling of the channel and slopes subjected to shallow landslides can occur within 10 years, continually diminishing the average sediment yield. In contrast, sediment derived sources from Mountain Range and Mudstone have a much more direct transport pathway to enter the active channel. Additionally, a large influx of sediment from Mountain Range or Mudstone sources due to a storm event would also cause a relative reduction in contributions from the remaining source groups, specifically that of Hill Subsurface and Hill Surface sediment sources. A significant limitation is that only one storm was sampled in this study due to resource constraints, however the storm does represent a typical (but not extreme) flood event within the catchment displaying a return period of 1.3 years and was the 4th largest flood event in the Manawatu River in 2013.

5.7

Conclusion

This investigation explored the capacity for sediment fingerprinting techniques to be applied at a high temporal resolution for a large catchment. Suspended sediment samples were collected at hourly intervals for 53 hr of a storm event. High variability is observed between adjacent hourly samples which either relates to the highly variable nature of the sediment transport dynamics or the high levels of uncertainty encountered in sediment fingerprinting studies. However, general changes in the sediment source contributions were observed occurring in relation to the switching of dominant flows from the Pohangina to the Upper Manawatu sub- catchment, as well hysteresis patterns experienced within the Pohangina sub-catchment. This caused relatively higher contributions of Mountain Range and Mudstone, and lower contributions of Hill Subsurface, Hill Surface and Gravel Terrace sediment sources to occur within the first twelve hours of sampling. This may give some indication of different processes occurring in each of the two dominant sub-catchments and how they respond differently to high rainfall events. Channel Bank and Limestone sediment contributions remained low throughout the storm duration. Source contributions to each sub-catchment were modelled in order to provide information of relative inputs but are difficult to confirm actual measurements to critically compare between. Future sediment analysis needs to consider the interval at which sediment waves move downstream throughout a storm event.

5.8

Summary

Temporal fluctuations of suspended sediment sources taken at 1 hour intervals during a 53- hour storm event were investigated. Changes in sediment sources were observed throughout the event which relate to changes in sub-catchment flow and as well as storm flow hysteresis effects. This was despite encountering a high level of sporadic variability between adjacent hourly sediment samples. This high variability could arise from uncertainties and a high margin of error in the source estimation, which raises a concern regarding the individual source estimates and the level of confidence which can placed in them. The next chapter addresses the levels of uncertainty and geochemical variability encountered in the sediment sources.