Experimental Investigation on Velocity Fields of Seepage Flow Inside Gravel Mount

There is little information on velocity field of seepage flows. The formation of seepage flow is significant for aquatic habitat including spawning of aquatic animals in river. To create spawning fields in river, the stability of a heaped-up gravels during floods is required for its artificial formation. The velocity measurement of seepage flows is also necessary. This paper presents velocity fields of seepage flows in a heaped-up gravels. The experimental study yields that the heaped-up gravels are stabilized by installing the consecutively assembled boulders on the gravels. Also, small mean velocity and standard deviation are recorded for different discharges. Spectrum analysis reveals that fluctuation velocity of seepage flows depends on the turbulent flow above the heaped-up gravels.


Introduction
The population of Ayu sweetfish (Plecoglossus altivelis), an amphidromous fish native to East Asia, has been reducing in Japan in recent years (Kono et al., 2017).One of the primary factors of this phenomenon is speculated to be river diversion, dam construction, and many other river regulation projects, causing the degradation and reduction of spawning habitat.Similar concerns are being raised in North America and Europe, where the decline of spawning habitat of salmonids is also a pressing issue (Wheaton and Pasternack, 2004).Spawning habitats in natural river are commonly formed where the tributary joins the main channel or at meandering river (where the velocity is relatively slow).Also, seepage flow within the gravel bed plays a crucial role in the spawning habitat as it provides fresh water to the spawn eggs and regulate the water temperature around them.There are numerous attempts to artificially construct spawning beds: gravel augmentation in riverbed (McManamay et al., 2010).McManamay introduced a mountain-like structure consisting of gravel as an ideal spawning model, while also highlighting the importance of a pool-riffle sequence in the spawning habitat.In Japan, as a fisheries project, attempts have been made to respond to the spawning season by provisionally laying gravels and cobble stones that can be used as spawning beds, but in many cases, when the spawning season coincides with a flooding event, the gravel is swept away, and the spawning environment is lost.Therefore, the stability of such an artificially constructed spawning bed is of an urgent demand.Also, clarifying the mechanism of seepage flow within gravel is beneficial for constructing a spawning-friendly riverbed.Despite the importance of elucidating seepage flow, its mechanism has rarely been examined.Previously, various approaches were taken to investigate the flow characteristics of seepage flow: tracing 'dye' within the riverbed by standpipes (Orchard, 1988), injecting artificial saline tracers underground to investigate the velocity of groundwater velocities (Schirmer et al., 2013).Additionally, Chanson and Zhuang (2015) measured the seepage bubbly flow in the cavity of a gabion stepped spillway weir by phase detection probe to determine the flow characteristics inside the gabion.Nevertheless, it is difficult to determine the actual velocity of seepage flow, as the subsurface flow is intricate and the apparatus to measure the velocity underground is not completely established.
horizontal component of velocity of seepage flows by inserting a prove of electrical-magnetic current meter in the coil.This paper presents the velocity fields of seepage flows within a gravel heap beneath assembled boulders.The installation of assembled boulders may help the stability of gravels for a large discharge.To obtain different gradients of water surface at the same position, experiments were conducted under two different discharges.The velocity fields of seepage flows were investigated through mean velocity profile, standard deviation, change of velocity with time series, and spectrum analysis.The experimental results reveals that the seepage flow with low velocity including turbulence is always formed inside the heaped-up gravels, and the indirect assessment of the seepage flow based on water surface gradient and riverbed material may be limited under the set experimental condition.

Experimental Model
The experiment was conducted in a 17 m long, 0.40 m wide, 0.60 m deep slope-changeable rectangular channel which was set in a horizontal state.The experimental model is shown in Photo 1. Gravel with an average grain diameter of 0.016 m (based on the averaged value of long length, short length, and thickness) was stacked 5.5m downstream of the sluice gate, which is located at the center of the channel, for a length of 3.2 m.The stacked gravel was heaped into a mountain like shape (hereafter referred to as gravel mount), having its peak at 1m from the upstream edge of the model.The upstream slopes of the gravel mount are adjusted as 12 % averaged gradient (0.12 m height and 1.0 m long), while the downstream slope is adjusted as 5.5 % averaged gradient (0.12 m height and 2.2 m long).To prevent the gravels from being transported downstream, sharp-edged boulders (with an average grain diameter of 0.15m) that have been assembled to maintain its stability were installed on top of the gravel for a distance of 1.14 m, starting 0.30 m downstream from the upstream edge of the model: 0.70 m long at the adverse slope region, and 0.44 m long at downward slope region.Further, cobble stones (with an average grain diameter of 0.04 m) were installed on top of the gravel for a distance of 0.65 m, further downstream, to resemble a spawning bed.

Measurement Instrumentation
To measure the velocity of seepage flow inside the gravel mount, a total of nine coils with an inner diameter of 0.02 m and a spacing of 0.01 m was embedded inside the gravel mount with the end of the coil reaching the channel bed (Photo 2 (a)).Each coil was thoroughly enamel-coated to avoid interference with the magnetic field generated by the probe of the velocity meter.Aluminum cylindrical rods filled with cement concrete shown in Photo 2 (b) were inserted into each coil to prevent the gravel from entering the coil during non-measurement.This experiment was conducted under prototype conditions because no studies have directly evaluated seepage flow inside the gravels.Two different discharges were presented to investigate the effect of water surface gradient on the velocity fields inside the gravel mount: 0.0174 m 3 /s (with a critical depth of 0.06578 m) and 0.0451 m 3 /s (with a critical depth of 0.109 m).Water depth and bed morphology were measured using a point gauge.For velocity measurements, an I-type probe electrical-magnetic current meter KENEK CO. model VM-806H/VMT2-200-04P, sampling frequency 20 Hz and measurement time 30s for a total of 601 point-values measurements was used (Beretta and Yasuda 2023).To measure the velocity inside the gravel mount, the probe of the velocity meter was positioned at the center of the coil, and the velocity measurements were taken starting from the channel bed.Measurements are taken along the x, y, and z axes.The x-axis represents the longitudinal component, the y-axis represents the cross-sectional component, and the z-axis represents the vertical component.The x-axis originates 5.5 m downstream of the sluice gate, the y-axis at the channel center, and the z-axis at the channel bed.By comparing the data collected at x = 0.75 m and x = 1.67 m, which cross-sections are located at the sharp-edged boulder installation area where the water surface slope is relatively large, no significant difference is observed in the mean velocity and disturbance inside the gravel mount.However, upon further inspection on the flow characteristics near the gravel mount, it reveals that the disturbance in the downstream cross-section (data at x = 1.67 m) is larger compared to that in the upstream cross-section (data at x = 0.75 m).This phenomenon can be attributed to a development of boundary layer generated when the flow passes over the peak of the gravel mount: at the upstream cross-section, the boundary layer is in the process of developing, while at the downstream cross-section, the boundary layer has fully developed (Ohtsu and Yasuda, 1994).
At x = 2.32 m, where the cobble stones are installed, the velocity characteristics among the cobble stones (0.05 < z (m) < 0.08) are influenced by the surface flow (z > 0.08 m) over the boulders.Nevertheless, the velocity inside the gravel mount remains small, similar to that upstream.At x = 0.75 m, where the boundary layer generated when the flow passes over the peak of the gravel mount is still growing, minor deviations are observed outside the newly boundary layer generated at z = 0.20 m, 0.22 m, and 0.24 m as shown in Figure 5 (a).While intermittent occurrences of low velocity are observed at z = 0.18 m since it is located directly beneath the boundary layer (Ohtsu and Yasuda, 1994).As for the seepage flow (inner flow region), the mean velocity remains consistently small with deviations smaller than those observed outside the boundary layer, near the water surface.
At x = 1.67 m, the measurements were taken inside the boundary layer as time-series data.Compared with the data on the upstream (x = 0.75 m), the deviations of the mean velocity at the surface of the model are larger at x = 1.67 m.In addition, at the inner flow region (Figure 5 (b)), the deviation is large near the surface of gravel mount.These characteristics are due to the unreduced velocity between the installed sharp-edged boulders.For the velocity of seepage flow, the value is kept small.This might be explained as the velocity is limited when the seepage flow goes through the space among small gravels.
At x = 2.32 m, similar velocity fluctuations are observed above the gravel mount as those at x = 1.67 m (Figure 5 (c)).In the region where the cobble stones are installed (0.05 ≦z (m)≦ 0.08), the value of the mean velocity at z = 0.08 m is larger than that in the region of 0.05 ≦z (m)≦0.07.This might be caused by a large difference of mean velocity observed between the water surface flow and the bottom flow near gravel bed.The main flow is located near the water surface, and the water surface flow has a high velocity, whereas the flow near the gravel bed is considerably slower.Furthermore, it should be noted that both Case 1 and Case 2 exhibit similar deviation trend in the time-series data of the velocity for x component.According to the magnitude of the spectrum by period due to the fluctuating velocity in the seepage flow at any position of x, as shown in Figure 6, the larger the frequency, the smaller the spectrum.
In the measurement section of x = 0.75 m, the spectrum at the position of z = 0.18 m is larger than that at other z locations, spanning from low to high frequencies.This might be caused by the intermittent velocity fluctuations that occur close to the upper edge of the boundary layer.At the locations of z = 0.20, 0.22, and 0.24 m, the spectra with periods from 1.0 to 2.0 Hz and from 0.2 to 0.3 Hz are large.This is considered to be the period included in the flow entering the mount.The spectra in the seepage flow do not differ significantly in magnitude from 0.2 to 10 Hz, except at the location of z = 0.10 m.
At the section of x = 1.67 m, the spectra in the region of z ≥ 10 cm are larger than those in the seepage flow, spanning from low to high frequencies.This might be caused by turbulence within the new boundary layer developed from the gravel mount, as indicated by the time series variation.Within the seepage flow, the spectrum is larger regardless of the position of z inside small gravels, compared to that evaluated at x = 0.75 m.This is thought to be due to the increased velocity fluctuations in the upper part of the sedimentary layer.
At the section of x = 2.32 m, the spectrum is larger than that at the section of x = 1.67 m, except for the high frequencies.This might be caused by the effect of a larger velocity gradient just above the sedimentary layer due to the further acceleration of the velocity in the upper part of the sedimentary layer.
In Case 1, the spectra in the upper part of the sedimentary layer are smaller than those in Case 2, but the spectra in the seepage flow are similar to those in Case 2. (

Discussion
The gravel mount with small gravels consisted in a heaped up shape was installed in horizontal rectangular channel in order to form the seepage flow between the small gravels.The water surface gradient is varied by the formation of flow passing through the gravel mount as shown in Figures 1 and 2. The velocity measurement of seepage flow is possible by installing enamel-coated metal coils inside the gravel mount.If the coil is not coated by an enamel liquid, the magnetic field generated by the measuring instrument interferes with the metal of the coil and normal flow velocities cannot be evaluated.According to velocity measurement, small turbulent flows are always formed between small gravels with an average diameter of 0.016 m.In this case, as the seepage flow is not regarded as a laminar flow, the principal based on Darcy law cannot be applied.Mean velocity and turbulent intensity between the small gravels are consistently small in accordance with the velocity measurement due to two-dimensional electrical magnetic current meter.Change of the velocity inside small gravels remains negligibly small in two different discharges.Furthermore, change of the mean velocity  ̅ with vertical direction is large near the peak of the gravel mount, whereas the turbulent intensity is always small.Spectrum inside the seepage flow revealed that the spectrum is consistently small spanning from low to high frequencies.The flow resistance of the flow over the gravel mount appears to depend on the formation of seepage flow, and turbulence is reduced near the bottom.

Conclusion
Gravel mount consisting of gravel with average grain diameter of 0.016 m was installed on the experimental channel to generate seepage flows.Also, the gravel mount was covered with sharp-edged boulders at the upstream section and cobble stones at the downstream section.The experiment was conducted for two different water discharges and mean velocity, standard deviation of flow velocity, time-series of velocity, and result of spectral analysis on velocity fluctuation were examined.Experimental results can be summarized as follows: a) By installing boulders on top of the gravel mount, the overall structure was stabilized, preventing any gravel from being washed downstream.Therefore, when constructing artificial spawning beds in an actual river, the combination of gravel and boulder is suggested.Additionally, when constructing spawning beds, it is significant that the main flow consistently exists along the water surface both in normal stage and flood stage to prevent the gravels from being washed away.
b) Mean velocity of seepage flow was found to be correlated with water surface slope near the peak of the gravel mount.In this case, the change of mean velocity between the small gravels remained consistently low.Furthermore, the velocity measurements reveal that the velocity of seepage flow does not change for different two discharges except for a change only when there is a difference in the water surface gradient.
c) The time-series variation of the velocity between the small gravels shows that the fluctuation always occurs, although the range of fluctuation is small.Furthermore, the velocity near the gravel bed depends on the water surface gradient of turbulent flow passing over the gravel mount.
d) The spectrum evaluated from the velocity fluctuations inside the gravel mount is always smaller than that above the gravel mount spanning from low to high frequencies.Turbulence near the gravel bottom is reduced by the formation of seepage flow even if the velocity difference increases in the vertical direction over the gravel mount.
This paper is the result of the first stage of research and shows the results of the study with a limited size of crushed stone.In the future, the size of the crushed stones will be changed to clarify the difference in seepage flow due to the difference in porosity.This will enable artificial regeneration of seepage flow that leads to spawning and temperature control according to the site of the river.
Photo 1. Experimental model Photo 2. Velocity measurement using coils and cylindrical rods

Figure 1 .
Figure 1.Water surface and ground profiles at the center of the channel Figure 3. Velocity Distributions for Case 1 and Case 2

Figure 4 .
Figure 4. Longitudinal variation of the averaged flow velocity inside the gravel mount

( 1 )
Figure 5. Time-series of longitudinal velocity for Case 2 Figure 6.Spectral analysis on the time-series data corresponding to Figure 5 (for Case 2)