7 Marco Teórico
7.5.2 Almidón
7.5.2.1 Amilosa y amilopectina
While UBCDflow was found to be useful in predicting the travel distance and event volume for torrent debris flows in Section 6.3.2.1, UBCDflow does not provide an estimate of important parameters such as flow height, flow velocity, or depositional shape. DAN-W does provide these parameters. It is possible to run a UBCDflow simulation to estimate a travel distance and volume for a torrent event, then use this data to back-calculate input parameters for the Voellmy rheology in DAN-W. The resulting simulations in DAN-W may give the investigator an idea of the velocities, discharges, and deposit area involved in a potential flow.
To test this, hypothetical starting volumes of 100 m3, 500 m3, and 1000 m3 were simulated entering reach 2 of FJ4 with UBCDflow. Regardless of the starting volume, the simulated flow always deposited in reach 12 due to lack of confinement and low slope angle.
Also, the depths of erosion remain the same for each simulation, as the entering volume is not included in the function which calculates the volume change.
The eroded depths predicted by UBCDflow in each reach were then entered into DAN-W as the maximum erodible depth in the corresponding reach. The flow was then simulated with starting volumes with 100 m3, 500 m3, and 1000 m3. Using a maximum velocity of 10.6 m/s, the best fit µ for each simulation was 0.21. The best-fit ψ was 200, 100, and 50 ms-2, respectively. Figure 6.16 plots the depths of erosion and deposition observed in the field, as well as predicted those predicted by UBCDflow for the run with 500 m3 starting volume. It also shows the shape and depth of deposited material.
In this case, UBCDflow predicted the total event volume well, to within 12% of the volume observed. However, as Figure 6.16illustrates, it did so for reasons which did not reflect field observations. The model under-predicted the erosion observed in the upper reaches, where a bedrock failure in weak, fault damaged rock contributed most of the event volume. It over-predicted entrainment in the lower part of the transport zone. In this case, the two errors cancel each other, which yields a remarkably accurate, and lucky, prediction of the event magnitude.
Figure 6.16 also illustrates some of the limitations of DAN-W. Both models, in fact, significantly overestimate the depth of material deposited on the fan and correspondingly un-derestimate the amount of material deposited as levee or lag deposition in the transport zone.
This effect must be considered when dealing with low magnitude events.
Despite these limitations, DAN-W is quite useful in visualizing the potential velocities and flow depths in the event (Figures6.17and6.18). The simulated peak velocities occurred in the mid to upper reaches of the flow path. At a point 200 m along the path, the flow velocity was 8 m/s and the flow depth was 2 m. While the actual flow velocity was unknown, inferred
flow depths from field evidence were approximately 1.5 to 2m. This information would not be provided by just UBCDflow or another empirical model for travel distance.
Figure 6.16.: Combined output for simulation using 500m3 starting volume, µ of 0.21, and ψ of 100 ms-1 Observed and predicted depth of effective erosion or deposition by UBCDflow and observed in the field. The deposition shape along the path observed and that predicted by DAN-W is also shown. The depth of deposition is exaggerated by a factor of 10.
Figure 6.17.: Velocity plot for simulation of FJ4. The line represents the velocity at the flow front.
Figure 6.18.: Flow depth and velocity for simulation of FJ4 using a 500 m3starting mass at a control point 200m along the horizontal. Flow depth is shown in red, while flow velocity is in black. DAN-W simulates a flow front 2 m deep and moving at approximately 8 m/s.
6.5. Summary and conclusions
This chapter summarizes the methods used to simulate 17 debris flows in New Zealand with DAN-W and UBCDflow. The results may be useful to debris flow investigators, especially when trying to predict the travel distance and event volume of small debris flows in one of the field areas studied.
• The back-calculated Voellmy friction coefficients for the debris flows simulated in this study were generally higher than reported elsewhere in the literature for torrent type debris flows. This is thought to be related to the relatively small magnitude of the events and the steepness of the paths. Smaller events are more likely to be effected by in-channel vegetation, channel constrictions and roughness, and small changes in confinement, in-creasing the overall frictional resistance of the channel.
• Friction parameters for hillslope flows were higher than for torrent type flows. This may be due to a lack of confinement and smaller contributing area to these flows.
• When the dataset is supplemented by data from the literature over a wide-range of vol-umes, the Voellmy friction coefficient tends to decrease with increasing flow volume. In general, flows above 50,000 m3 will have a back-calculated µ of less than 0.1. Flows smaller than 50,000 m3 will have a µ greater than 0.1, but vary considerably among flows of similar magnitude. The increased mobility of larger could be due to the decreased susceptibility discussed above, but may also be related to rheological differences between large and small magnitude events.
• UBCDflow predicted the travel distance of torrent type flows with well-defined uncon-fined reaches in 3 out of the 4 cases tested. The maximum volume passing was often over-predicted. Care should be taken to identify the mode of flow correctly (confined, unconfined, or transitional), as the model is very sensitive to this input. The model’s success is a result of the tendency for torrent, granular debris flows to begin depositing in unconfined reaches at approximately 20°, regardless of event volume.
• UBCDflow generally over-predicts the runout and entrainment for hillslope type flows.
Both UBCDflow and DAN-W over-predicted volume passing, as neither one could ad-equately simulate progressive levee deposition. Setting all reaches of hillslope flows to unconfined gave more accurate predictions of travel distance, as UBCDflow’s rules for unconfined reaches reflected the slope angles where hillslope flows tended to erode and
deposit. In some cases, however, UBCDflow dramatically over-predicted both entrain-ment and travel distances. UBCDflow is best applied when there are large accumulations of colluvium which can be entrained from the channel floor or sides.
• Modelling entrainment in DAN-W, as opposed to the simplistic method of modelling the flow as one large starting mass, did not change the back-calculated µ substantially for the flows tested, but did increase the ψ value needed to simulate higher velocities. It also provided more realistic flow heights as the flow proceeded down-slope.
• Using UBCDflow and DAN-W in combination may be a useful decision support tool in preliminary hazard analysis. UBCDflow is able to give good approximations of event vol-ume and entrainment behaviour for torrent flows, and reasonable approximations of event magnitude for hillslope events. For hillslope events, the eroded depths from UBCDflow and the back-calculated friction parameters found in this and similar studies can be used together. For torrent events, the outputs from UBCDflow can be used to back-calculate friction parameters in DAN-W.