Boulder Transport Research Articles

Extreme block and boulder transport along a cliffed coastline (Calicoan Island, Philippines) during Super Typhoon Haiyan

This paper presents data and analysis for block and boulder transport during Super Typhoon Haiyan along a 4.5km long, low (5–12m) cliffed coastline in Calicoan Island, Eastern Samar, Philippines. Wave runup exceeding 15.2m elevation above mean sea level drove large limestone clasts, with volumes up to ~83m3, up to ~280m inland. A few very large clasts with volumes 65–132m3 were not transported by the waves. When combined with recent transport reported in May et al. (2015), Cox et al. (2016), and other literature, it is becoming increasingly clear that the largest blocks transported by storms overlie much of the tsunami transport range, increasing the difficulty in attributing the transport source without additional evidence. Comparison of present results with a global database of storm boulder transport shows a mass-elevation envelope outside of which no transport is observed.Initiation of motion criteria were extended to include non-rectangular cross-sections, which significantly reduces inferred velocities, particularly for overturning motion. These new relations were applied to the largest observed sliding and overturning boulders while considering coefficient uncertainties, and resulting velocity uncertainty was large enough that direct inference of wave heights would be problematic. Estimates of initiation velocities for cliff-edge boulders computed using lifting/joint-bounded relations were unreasonably large when compared to those for sliding and overturning boulders, suggesting that processes other than Bernoulli lift forces dominated at cliff edges.

Testing the numerical models for boulder transport through high energy marine wave event: An example from southern Saurashtra, western India

Presence of large boulders along rocky coasts can be used as archives to reconstruct past extreme wave event history, vital for the coastal hazard assessment. Dimensions of boulders lying in supratidal zone have been used to reconstruct the origin of wave responsible for their transport and processes. There are several mathematical equations available which take into account submerged, subaerial and joint bounded block scenarios for boulder transport to estimate the wave height required to transport the boulders to their final position. However, there are limited studies which validate these equations using modeling and observational analysis. Navabandar coast, situated in western India provides an excellent opportunity for such analysis as there exists three boulders derived from shore platform during the November 1982 storm. We present the analysis of these boulders and event history of region to conclude that all the mathematical equations overestimate the wave heights required to dislodge and transport these boulders by at least 5 times the actual event. We emphasize that there is a vital need to re-evaluate and improve the existing equations considering the local control variables for better visualization of ‘palaeo’ tsunami/storm events.

Ancient high-energy storm boulder deposits on Ko Samui, Thailand, and their significance for identifying coastal hazard risk

Coastal geomorphic processes associated with high-energy storm events are difficult to estimate over recent geological history, though their frequency and magnitude are important to assess in order to understand present-day coastal vulnerability. Studying ancient coastal boulder deposits can shed light on the previous physical conditions necessary for their deposition. In this study, we estimated the physical processes required to move reef-derived coral boulders on the east coast of Ko Samui, a rapidly developing tourist island off eastern peninsular Thailand. The position and dimensions of 97 coral boulders (weight: mean 2.9t, max. 12.7t; transport distance: max. 125m) were measured at two sites and dated using uranium/thorium methods. Flow velocities of 2.3–8.6m/s were required to transport the measured boulders, with individuals deposited up to 4.7m above mean sea level. Age-dating suggests that events capable of the highest flow velocities occurred around AD 1600 and AD 1750. These were probably driven by tropical cyclones (typhoons). Boulder transport by events of similar magnitude has not been detected within the last 250years. The non-occurrence of similar events in living memory has implications for hazard perceptions at this important tourist destination. However, there is also evidence of substantial Holocene sea-level changes in the Gulf of Thailand, as observed at nearby Ko Phaluai. This potentially offers a challenge for the interpretation of older boulders dating from the mid-Holocene, as sea level may have been more than 2m higher than present. Thus, studies using coral boulders as a proxy for past storm-wave conditions must consider the broader sea-level history, and are probably best limited to the period post-2000BP in the Gulf of Thailand.

Observations and Modeling of Coastal Boulder Transport and Loading During Super Typhoon Haiyan

Boulders numbering in the high hundreds/low thousands, and with masses up to ∼30 tonnes, were transported onshore by Super Typhoon Haiyan in Calicoan Island, Philippines to maximum ground elevations that could exceed 9 m and terminal positions up to ∼180 m inland. One-dimensional Boussinesq hindcasts of coastal boulder motion showed intermittent transport initiated at the fronts of infragravity swash bores. Transport distances were found to be highly sensitive to wave-height, enough so that observations of terminal positions may be a viable method of estimating rough paleostorm magnitudes. The large accelerations at bore fronts generated significant inertial forces, particularly for larger boulders, but drag forces had greater root-mean-square magnitudes in all simulations. Widely used relations to infer fluid velocities from boulder properties were tested using modeled boulders — inferred velocities at modeled terminal boulder positions were compared to maximum computed Boussinesq fluid velocities at these locations and found to be significantly lower. This underprediction of inferred velocities was greatest for smaller boulders that were strongly mobile. Inferred drag loads compared to modeled values were somewhat more accurate for large boulders when a Froude number of unity was assumed to estimate flow depths. Although these boulders were unequivocally transported by storm waves, their large sizes and distances traveled venture into what has been considered the tsunami range. Thus, care must be taken to interpret the provenance of coastal boulder fields with unknown origin for lower to mid-latitude regions.

Geomorphological control on boulder transport and coastal erosion before, during and after an extreme extra‐tropical cyclone

AbstractExtreme wave events in coastal zones are principal drivers of geomorphic change. Evidence of boulder entrainment and erosional impact during storms is increasing. However, there is currently poor time coupling between pre‐ and post‐storm measurements of coastal boulder deposits. Importantly there are no data reporting shore platform erosion, boulder entrainment and/or boulder transport during storm events – rock coast dynamics during storm events are currently unexplored. Here, we use high‐resolution (daily) field data to measure and characterize coastal boulder transport before, during and after the extreme Northeast Atlantic extra‐tropical cyclone Johanna in March 2008. Forty‐eight limestone fine‐medium boulders (n = 46) and coarse cobbles (n = 2) were tracked daily over a 0.1 km2 intertidal area during this multi‐day storm. Boulders were repeatedly entrained, transported and deposited, and in some cases broken down (n = 1) or quarried (n = 3), during the most intense days of the storm. Eighty‐one percent (n = 39) of boulders were located at both the start and end of the storm. Of these, 92% were entrained where entrainment patterns were closely aligned to wave parameters. These data firmly demonstrate rock coasts are dynamic and vulnerable under storm conditions. No statistically significant relationship was found between boulder size (mass) and net transport distance. Graphical analyses suggest that boulder size limits the maximum longshore transport distance but that for the majority of boulders lying under this threshold, other factors influence transport distance. Paired analysis of 20 similar sized and shaped boulders in different morphogenic zones demonstrates that geomorphological control affects entrainment and transport distance – where net transport distances were up to 39 times less where geomorphological control was greatest. These results have important implications for understanding and for accurately measuring and modelling boulder entrainment and transport. Coastal managers require these data for assessing erosion risk. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Block and boulder transport in Eastern Samar (Philippines) during Supertyphoon Haiyan

Abstract. Fields of dislodged boulders and blocks record catastrophic coastal flooding during strong storms or tsunamis and play a pivotal role in coastal hazard assessment. Along the rocky carbonate coast of Eastern Samar (Philippines) we documented longshore transport of a block of 180 t and boulders (up to 23.5 t) shifted upslope to elevations of up to 10 m above mean lower low water level during Supertyphoon Haiyan on 8 November 2013. Initiation-of-motion approaches indicate that boulder dislocation occurred with flow velocities of 8.9–9.6 m s−1, which significantly exceeds depth-averaged flow velocities of a local coupled hydrodynamic and wave model (Delft3D) of the typhoon with a maximum < 1.5 m s−1. These results, in combination with recently published phase-resolving wave models, support the hypothesis that infragravity waves induced by the typhoon were responsible for the remarkable flooding pattern in Eastern Samar, which are not resolved in phase-averaged storm surge models. Our findings show that tsunamis and hydrodynamic conditions induced by tropical cyclones may shift boulders of similar size and, therefore, demand a careful re-evaluation of storm-related transport where it, based on the boulder's sheer size, has previously been ascribed to tsunamis.

Clast mobility within boulder beaches over two winters in Galicia, northwestern Spain

A micro-drone was used to make low altitude flights over boulder beaches at Laxe Brava and Oia in Galicia, northwestern Spain. Flights were made in July 2012, May 2013, and spring 2014. High resolution digital terrain models and orthophotographs, combined with GIS mapping, were used to monitor changes in the position of thousands of boulders. Maximum storm wave height was higher in the winter of 2013–2014 than in winter 2012–2013, and this was reflected in an increase in the proportion of the boulders that moved in the two winters, from 17% to almost 48% at Laxe Brava, and from 53% to almost 88% at Oia. The greater mobility of the boulders at Oia can be attributed in part to their generally smaller size, although there was considerable overlap between the size of boulders that moved and those that did not move within and between the two beaches. There were mobile boulders in areas up to several metres above the high tidal level on both beaches, and boulder transport in the shore-normal and alongshore directions triggered some changes in the beach profiles, particularly in the middle to upper parts of the beaches. Estimates of threshold transport conditions, scaled to boulder mass, breaker height, and other variables, suggested that all but the very largest boulders on the two beaches should have been mobile, even during the summer months when the waves were much lower than in winter. Model over-prediction can be attributed to a number of factors, including: constraints on movement imposed by surrounding boulders; differences in boulder size and their effect on pivoting angles and on the degree to which boulders are exposed or sheltered from wave impact; and difficulties in assigning appropriate values to model coefficients.

Boulder dislodgement and transport by solitary waves: Insights from three‐dimensional numerical simulations

AbstractThe analysis of boulder motion and dislodgement provides important insights into the physics of the causative processes, i.e., whether or not a boulder was moved during a storm or tsunami and the magnitude of the respective event. Previous studies were mainly based on simplified models and threshold considerations. We employ three‐dimensional numerical simulation of the hydrodynamics coupled with rigid body dynamics to study boulder dislodgement and transport by solitary waves. We explore the effects of three important nondimensional parameters on the boulder transport problem, the Froude number Fr, the aspect ratio in logarithmic scale α, and the submergence factor, β. Our results indicate that boulder motion is complex, and small changes in one of the nondimensional parameters result in significantly different behavior during the transport process and the final resting place of boulders. More studies are needed to determine the role of boulders in tsunami and storm hazard assessments.

Untangling boulder dislodgement in storms and tsunamis: Is it possible with simple theories?

Boulders can move during storms and tsunamis. It is difficult to find a simple method to distinguish boulders moved by tsunami waves from those moved during storms in the field. In this contribution, we explore boulder dislodgement by storm and tsunami waves by solving an adapted version of Newton's Second Law of Motion in polar coordinates and defining a critical position for boulder dislodgement. We find that the boulder dislodgement is not only a function of the causative wave, but also of the roughness in the vicinity of the boulder and the slope angle. We employ the amplitude of storm and tsunami waves to dislodge boulders of given masses to evaluate if boulder dislodgement in storms can be untangled from boulder transport in tsunamis. As the main result of our numerical experiments, we find a significant difference between storm and tsunami waves to dislodge the same boulder for large masses and large roughness values. This allows us to conclude that simple theories are applicable to answer the questions asked in the title, but we argue only if they contain a critical dislodgement condition like the one presented here.

Local paleo-tsunami size evaluation using numerical modeling for boulder transport at Ishigaki Island, Japan

In this study, we simulated the transport of a large coralline boulder on southern Ishigaki Island of the Sakishima Islands, Japan, to evaluate local paleotsunami size in comparison to a well-known historical event (the 1771 Meiwa tsunami). According to the geological evidence, the boulder was deposited at 10 m elevation by two paleo-tsunami events. We assumed two types of fault models and eight dislocations for each fault. Then we investigated whether there are any combinations of the fault models that can satisfy the movement of the boulder from its presumed initial position to the present position by two tsunamis. Results show that several combinations of tsunami source models can satisfy the geological constraints. Our results revealed that at least one tsunami event during the prehistoric age was equivalent to or even larger than the 1771 tsunami in terms of the flow depth at the southeastern coast of the Ishigaki Island. Although the accuracy of this method depends strongly on the available geological evidence, we infer that the numerical modeling for boulder transport will have great value for evaluation of the local tsunami size, which is important for local tsunami risk assessment.

What was the transport mode of large boulders in the Campine Plateau and the lower Meuse valley during the mid-Pleistocene?

The Campine Plateau in northeastern Belgium, a remnant of an alluvial fan deposited by the Meuse River during the mid-Pleistocene, is characterised by the presence of boulders with maximum dimensions of up to 2m, embedded in a gravel matrix. These boulders originated in the Ardennes region and are generally assumed to have been transported by ice-rafting processes. This paper investigates for the first time quantitatively the possibility of purely hydraulic transport of the boulders, taking into account channel and flow characteristics in the boulder provenance area during the mid-Pleistocene. Empirical transport relations that describe incipient motion thresholds in nonuniform river beds as a function of the relative grain size, or the ratio between the grain size of interest and the median grain size of the channel bed, are applied in order to calculate critical water depths for transport of boulders of various sizes. Results indicate that hydraulic transport of boulders with intermediate diameters <1m could have occurred within limited reaches of the palaeoriver, more specifically in the palaeo-Amblève tributary; whereas the small slope gradient of the palaeo-Meuse most probably inhibited boulder movement by hydraulic forces only. Although calculations of the ice volume required to lift a boulder to the water surface and comparison of the ice floe's dimensions with palaeochannel morphology do suggest that ice rafting is theoretically possible, several alternative, more probable transport mechanisms for the larger boulders of the Campine Plateau are proposed, requiring much smaller critical ice volumes and water depths than ice-rafting processes or purely hydraulic transport. These hypotheses include decreased bed friction and effective boulder density caused by a limited ice layer attached to the river bed and the boulder, hence lowering hydraulic transport thresholds, as well as the formation of ice jams and dams inducing catastrophic flooding and mechanical impacts by ice floes upon failure.

Advantages of beachrock slabs for interpreting high-energy wave transport: Evidence from Ludao Island in south-eastern Taiwan

Powerful typhoons frequently affect eastern Taiwan. On Dabaisha beach on the SW coast of Ludao Island, a small volcanic island lying SE off mainland Taiwan, the presence of numerous beachrock slabs provides evidence of the sediment transport capabilities of high-energy waves generated by these storms. With a well-defined beachrock source from which the slabs were derived, it is easy to differentiate clasts transported in run-up or backwash flow based on their current position relative to the outcrop. In order to estimate the wave-generated flow velocities from the measured beachrock slabs, the hydrodynamic equations for boulder transport by Nandasena et al. (2011) have been rearranged so that they are appropriate for this particular geomorphic setting, where exposed in situ beachrock outcrops liberate clasts which may then be transported either by run-up onto the beach or backwash flow onto adjacent reef platforms. Such settings are common on tropical coastlines with fringing coral reefs and beaches comprising mostly biogenic sediments. In the Dabaisha beach case, the lowest flow velocities required to transport all measured beachrock slabs from their source outcrop to current positions are 2.18–3.16m/s for run-up and 1.76–3.24m/s for backwash. With a discussion of advantages and limitations on the use of beachrock clasts for interpreting wave behaviour, this paper presents another proxy for studying high-energy wave events that is highly suitable for certain carbonate-rich coastlines.

Numerical models of tsunami sediment transport — Current understanding and future directions

Abstract Researchers who study tsunami deposits share common ultimate goals of their work, which are to better assess the magnitude information of paleotsunamis and to contribute to the assessment of future tsunami risks. Numerical modeling of tsunami sediment transport is an important piece of interdisciplinary research that fills the gap between geological studies and practical utilization of tsunami deposits. Forward and inverse numerical models that address tsunami transport of sand and boulders have been developed over the last two decades. Forward models are capable of delineating the time evolution of tsunami hydrodynamics, sediment transport and the resulting morphological changes associated with erosion and deposition. Inverse models estimate tsunami characteristics, such as flow speed and depth, from deposits. Numerical modeling can be used not only to quantify paleotsunamis but also to enhance our understanding of tsunami sedimentology and hydrodynamics. To make progress towards the ultimate goal of improved tsunami risk assessment, development of an in-depth mutual understanding between modelers and geologists of the advantages, limitations and uncertainties in both numerical modeling and geological records is an important challenge.

Numerical simulations to account for boulder movements on Lanyu Island, Taiwan: tsunami or storm?

Boulders that originated from the Holocene coral terrace and coral reef are distributed on the coral terrace or talus on the shore of Lanyu Island, Taiwan. We employed numerical simulation of storm waves and estimated whether the boulders could be moved by storm waves with a return period of 50 years, by larger storm waves with return periods of hundreds of years, or by tsunamis. The coral boulders are distributed between 36 and 128 m from the shoreline at elevations of 2.7 to 9.3 m. The sizes of the boulders are in the range 0.3 to 6.4 m. The boulder volume and a-axis length versus distance from the shore show weak and moderate correlation (r = 0.21 and 0.48), respectively. We reproduced the runup of waves using the equation of continuity for a two-dimensional non-compressive fluid and the Navier-Stokes formula. In order to handle the free surface of the fluid, the volume of fluid method was applied. For the flow velocity to move the boulder, we employed the theoretical formula for the transport of boulders. We used the wave height for a return period of 50 years and set the height of the input wave to 13.1 m. The results show that all the large boulders on Lanyu Island could be moved by storm waves with a return period of 50 years. Moreover, the computations show that most of the boulders could not have been transported by tsunamis generated by the Mw8.7 earthquakes. The ages of two boulders indicate that they were transported after 340 and 6,330 years ago. If these boulders were moved by tsunamis several hundred years ago, they would have been relocated subsequently by storm waves.

Boulder transport by the 2011 Great East Japan tsunami: Comprehensive field observations and whither model predictions?

Abstract Predicting the local size of a historic high-energy event from its boulders using numerical models is a challenging research topic. Modern high-energy events and their deposits are useful to validate these models; however, validating the accuracy of the results is difficult due to the scarcity of good datasets or the ambiguity of existing field data. Data on boulders transported by the 2011 Great East Japan tsunami at coastal sites (Settai, Taro, and Karakuwa) on the northeast coast of Japan were compiled. Pre-tsunami locations and settings and transport distances were found from evidence such as photographs, aerial images, and the testimony of survivors. The estimated weight of the boulders analyzed ranged from 11 to as much as 167 t, while the transport distance varied from a few to up to 600 m. Modeling results predicted that the minimum limit of maximum flow velocity of the tsunami at the pre-tsunami locations of the boulders varied from 4.2 to 6.8 m/s. The measured maximum flow depths at Settai (17–18 m) and Taro (14 m) were within the predicted range of flow depth when the Froude number = 1.0–1.5. Numerical model estimates for an older boulder (285 t) in Settai indicate that it was probably transported by a historical tsunami (1611 Keicho Sanriku event?) which may have been similar to or bigger than that of the 2011 event in the area. The maximum flow velocity could not have been less than 6.1 m/s, and if the boulder was transported to the present location by rolling, the flow velocity must have been within 7.5–23.7 m/s. Following systematic validation, the numerical modeling of boulder transport is proving promising for reconstructing the local magnitude of historical, high-energy events. Further improvements can be made with additional high quality field data from modern high-energy events.

Reprint of “Boulder transport by the 2011 Great East Japan tsunami: Comprehensive field observations and whither model predictions?”

Abstract Predicting the local size of a historic high-energy event from its boulders using numerical models is a challenging research topic. Modern high-energy events and their deposits are useful to validate these models; however, validating the accuracy of the results is difficult due to the scarcity of good datasets or the ambiguity of existing field data. Data on boulders transported by the 2011 Great East Japan tsunami at coastal sites (Settai, Taro, and Karakuwa) on the northeast coast of Japan were compiled. Pre-tsunami locations and settings and transport distances were found from evidence such as photographs, aerial images, and the testimony of survivors. The estimated weight of the boulders analyzed ranged from 11 to as much as 167 t, while the transport distance varied from a few to up to 600 m. Modeling results predicted that the minimum limit of maximum flow velocity of the tsunami at the pre-tsunami locations of the boulders varied from 4.2 to 6.8 m/s. The measured maximum flow depths at Settai (17–18 m) and Taro (14 m) were within the predicted range of flow depth when the Froude number = 1.0–1.5. Numerical model estimates for an older boulder (285 t) in Settai indicate that it was probably transported by a historical tsunami (1611 Keicho Sanriku event?) which may have been similar to or bigger than that of the 2011 event in the area. The maximum flow velocity could not have been less than 6.1 m/s, and if the boulder was transported to the present location by rolling, the flow velocity must have been within 7.5–23.7 m/s. Following systematic validation, the numerical modeling of boulder transport is proving promising for reconstructing the local magnitude of historical, high-energy events. Further improvements can be made with additional high quality field data from modern high-energy events.

Deciphering boulder mobility and erosion from cosmogenic nuclide exposure dating

Large boulders are prominent features in many geomorphic systems and are frequently targeted for cosmogenic exposure dating. Presently, there are little data or theory predicting exposure age, erosion rate, and mobilization frequency of boulders in environments such as channels, talus slopes, or moraines. Here we explore the potential for cosmogenic isotope analysis to constrain the transport and erosion history of boulders. Through a series of numerical experiments, we model the statistical evolution of nuclide concentrations around the surface of boulders. Stable boulders have distinctive radial distributions of surface concentration in comparison to those that are periodically mobile, and this can be used to establish boulder stability. Mean nuclide accumulation rates around the surface of an eroding boulder increase when the radius is smaller than approximately 1.5 e‐folding lengths (~1.2 m) of neutron flux intensity, whereupon nuclide accumulation on the underside of the boulder becomes non‐negligible (~10%). Model results for cases of no cosmogenic inheritance and uniform erosion indicate the normalized standard deviation of nuclide surface concentration systematically decreases with increasing number of boulder mobilization events. This may be used to constrain the minimum number of times a boulder has moved for up to approximately four events, or distinguish between rarely and frequently mobilized boulders. Using non‐dimensional scaling relations between surface concentration statistics, boulder size, and time, we propose methods to estimate the minimum age, frequency of movement, and erosion rate of mobile boulders with application to a range of geomorphic problems.

Numerical modeling of impact-induced tsunami and boulder transport on Mars

Numerical modeling of impact-induced tsunami and boulder transport on Mars

Sedimentary evidence against a local ice-cap on the Shetland Isles at the Last Glacial Maximum

Abstract The Shetland Isles occupy a critical position equidistant to major ice-dispersal centres of the British-Irish and Fennoscandian ice sheets during the last glaciation, and have long been the subject of controversy regarding the timing, geometry and style of glaciation on the Shetland Isles. Previous reconstructions of the last glaciation of Shetland, favour development of a local ice-cap, possibly subsequent to early inundation by Fennoscandian ice. However, this model requires complex ice shed migrations and dynamic changes to explain the evidence. Recent studies identify an extensive landform record of NW-flowing ice extending across the region from the North Sea Basin, incompatible with evidence from striae and erratic boulder transport studies. This paper presents glacigenic lithofacies descriptions and structural (clast-fabric, shear-planes) analysis of seventeen sections from around the Shetland archipelago. The sedimentary record of glaciation supports a model of inundation by ice from the North Sea Basin, and includes no sediments representing multiple phases or dynamic local ice-cap glaciation. A preliminary framework which integrates all previous datasets with the results of this study proposes: (1) an early phase of local glaciation that was subsequently overwhelmed by extension of a major ice stream flowing from the North Sea Basin at the LGM; and (2) absence of an extensive local ice-cap formed over Shetland during deglaciation. This study concludes by identifying key areas that require further investigation, most notably more detailed analysis of ice-flow indicators and sediments in the eastern islands of Whalsay, Fetlar and Bressay, and further work to establish the temporal relationships between glacially eroded bedrock and associated subglacial till emplacement.

Boulder transport by high energy: Numerical model-fitting experimental observations

A well-controlled experimental study was conducted to validate a numerical model of boulder transport by tsunamis. The model can simulate boulder transport due to sliding, rolling, and saltation. The boulders were modeled by cubic/rectangular blocks (flatness number varying from one to three) with different densities. The tsunami overland flow was simulated by the dam-break flow. The numerical model was capable of reproducing experimental observations fairly well. The difference between the modeling and experimental results was mainly attributed to secondary motions like swing and rotation due to both the variable friction and fluid forces applied to the blocks. The impact force generated when the flow front hit a block was modeled by a simple formula. Though it can increase the initial velocity of the block, its effect on the maximum transport distance was not significant. The maximum transport distance was sensitive to the lengths of axes and density of the block, density of the fluid, and coefficients of drag and friction. The pre-transport angle of a block affected the maximum transport distance. Placing the long axis of the block perpendicular to the fluid flow produced the maximum transport distance, while the minimum transport distance occurred when the long axis was parallel to the flow. An interaction effect was unavoidable if many boulders are transported together. The experimental results revealed that collisions of blocks of the same weight at the beginning of transport lead to different maximum transport distances. The higher the number of blocks, the greater the collision effect was. Even though the collision effect was very local and marginal, the effect on the maximum transport distance became more significant with an increasing number of blocks.