Modeling of Deposition Process of Particulate Organic Matter ( POM ) with Sand on Vegetated Area in a River

The transport and deposition of particulate organic matter (POM) in river streams has recently received much attention as one of important ecological processes in rivers. We focused on interacted behaviors of sand particles in bed load and POM in vegetated area on sand bars. The purpose of this study is to clarify the characteristics of deposition of POM with bed load on sandbars with the riparian vegetation. A basic experiment on POM transport and deposition with vegetation is conducted in a laboratory flume. It demonstrates that several issues still remain to be future investigated. In particular, the shear due to the bed roughness in the vegetated area and the transport and deposition process of sand particles and POM are required to be described by the proper modeling which will be introduced into a simulation model of various fluvial processes. The main results of this study are that ripples are formed by bed load in riparian vegetation and POM deposition is promoted by ripple behavior. Based on these results, the POM deposition with ripples in vegetated area is described by a conceptual model which will affect various aspects in ecosystem management based on fluvial processes.


Introduction
River landscapes are characterized by an interrelating system of flow, sediment transport, morphology and vegetation.It is known that particulate organic matter (POM) with sand particles in bed load and suspended load transported by flood are captured and deposited on sandbar with riparian vegetation.Capture of POM there must be significant in ecosystem (Vannote et al. [1], Cummins [2]) and it is important to understand how POM drifts are different from sediment behavior in flow over the vegetated area, because it reversely influences the vegetation productivity and supports the diversity through riverine bio-geochemical processes.The purpose of this study is to clarify the deposition mechanism of POM in consideration of the influence of sediment transport in the riparian vegetation, and to develop the model of POM behavior based on the deposition mechanism.Thus, field observation and laboratory experiment were conducted, and then a modeling necessary in numerical calculation was also conducted.

Characteristics of POM Based on Field Observation
An investigation was conducted on the Yahagi River (Chubu region, Japan) during normal flow stage.To evaluate the deposition characteristics of various POM with sand on sand-bar with riparian vegetation in a field, we selected a conspicuous island sandbar covered with vegetation.The longitudinal length of sandbar was around 300 m, the cross sectional length was around 100 m and the average bed slope was 1/150.The bed was composed mainly cobbles and boulders.An herbaceous plant occupied the whole sandbar.
Deposition characteristics of POM with sand particles was estimated by measuring the topography of sandbar and the thickness of sediment deposition layer, observing the spatial distribution of vegetation, and analyzing the grain size distribution of deposited sediments and the quantity of POM contained the deposited sediments.The thickness of deposition layer was measured as the height from the top of basement rock to the top of fine sand layer by using a soil auger at 15 locations in the target sandbar (see Figure 1).
POM content ratio to the total samples included in the sediment was distributed from 0.5% to 1.5% at each pointthrough longitudinal direction of sandbar.In addition, specific gravity of POM was observed in a laboratory.POM is classified into CPOM and FPOM according to their sizes.The criterion is 1 mm.We here focused on POM finer, which have less than 5 mm of particle diameter such as vegetation seeds and litters.Figure 2 shows that the specific weight is distributed from 1.02 to 1.26 for POM while it is around 2.65 for sand.The movement of POM in settling process is completely different from sediment.Sediment was settled rapidly in a group however POM moved flutteringly and settled slowly from top to the bottom of water.It is clear that the settling process of sediment and POM is differed even if they have same diameter.

Flume Experiment
Flume experiment was conducted to understand the deposition mechanism of POM, especially CPOM such as vegetation seeds, litters was selected as our target in this study.In the laboratory, a model vegetation made by a group of cylinders made of bamboo arranged in staggered pattern ( 0.25 mm D = Firstly, the flow measurements ( U and h ) were conducted along the centerline of the flume (Obana et al. [3]).Shear velocity ( ) u * is also important factor for an analysis of sediment transport.The form drag is predominant and velocity profile is uniform along the depth in flow with non-submerged vegetation sonly except the thin layer near the bed where the boundary layer is developed to bring a shear flow.Thus, after measuring the velocity near the bed with and without vegetated area respectively, shear velocity of vegetated area ( ) was deduced based on the assumption under the assumption of the similarity in the ratio of the velocity near the bed ( ) b u to the shear velocity between with and without vegetated area (see Table 1).In this table, numerical subscripts 0 and v of shear velocity u * , velocity near the bed b u , indicates a value of non-vegetated area and vegetated area respectively.The result of this method is confirmed with the theoretical discussion on bed roughness boundary layer (Jeon et al. [4]).
Then, the condition of sand and coarse POM (CPOM) was selected as shown in Figure 4 where diameter d = , σ , mass ρ = density of sand and water.The presumed amounts of them were fed at 1m upstream of the vegetated area with constant volume along the width.As for CPOM model, we selected PVC (polyvinyl chloride) controlled the specific weight ( 0.15 cm d = , 1.26 σ ρ = ).Each supplied sand and POM rate was 0.047 cm 2 /s and 0.06 cm 2 .At 20 minutes after sediment supply, water is stopped and then deposition of sand and POM in the vegetation area was measured along the centerline in the vegetated zone.Two kinds of experimental cases were run to observe respective fluvial processes of sand and POM.In Case-1, only sand was supplied, while in Case-2 both sand and CPOM were supplied.Movement condition of respective sand and CPOM is changed with/without vegetation; however, they were transported as bed load under this hydraulic condition.

Deposition Process of POM Transported with Sand
Figure 5 shows comparison of deposition thickness of sand and CPOM between Case-1 and Case-2 at 1200 seconds after the experiment starts.We observed that sand deposition was occurred even upstream of the beginning of vegetation area in both of cases, and sand was deposited into wavelike fashion along to longitudinal direction.It was judged "ripples" by the figure in the book of Yalin (1977, [5]) which are prepared by overlaying the formation area of ripples and dunes on Shields diagram.Ripples were formed by sediment transport and propagated to downstream with time progress.In contrast, CPOM had never deposited when it was transported by itself in vegetated area.However it was captured and deposited with the progress of ripples due to the both of interactions.The 77% of all CPOM supply was deposited in vegetated area with sand deposition.
The CPOM deposition mechanism with sand is as follows; 1) firstly fine sand forms ripples; 2) CPOM is deposited behind the crest of ripple, and then, fine sand is deposited and covered on CPOM deposition; 3) finally new CPOM is coming and deposited behind shifted crest of ripple.These steps are then repeated.In Figure 5, we can confirm that CPOM deposition is increasing and propagating with the development of ripples.CPOM deposition amount was measured at representative point in vegetated area as shown in Table 2.By measuring each parameter of ripples at measuring section ( ) , the averaged wave length of ripple was about 9 -10 cm ( ) , and wave height was 0.45 -0.65 cm.And it was clear that each parameter of Case-2 was decreased because of CPOM which was reduced the development process of ripples by disturbing sand supply from upstream as shown in Table 3.

Model Concept
The flume experiment made it clear that CPOM was captured behind the crest of ripple by separation vortex, then they were deposited and buried in ripples with time progress.Bed load transport can be described by the formula proposed by Ashida & Michiue [6], and written as follows.
( ) where B q , B q * = bed load transport rate and its dimensionless expression; ( )  Shields number; and c t * = dimensionless critical tractive force.This equation has a dimensionless form, and it may be applied for the various particles which have each different size and relative density ( ) σ ρ .The Equation ( 1) is consisted of two kinds of important factors of bed load motion such as sediment number density g ν and particle velocity g u as follows (Ashida & Michiue [6]).
( ) Bed load transport rate has the following relationship with the above two kinds of factors.
where 3 A = geometrical coefficient of sand; R µ = friction coefficient of sand ( ) (constant); t * = dimensionless tractive force; and c t * = critical tractive force.The CPOM particles transported to the lee side of crest of ripple can be described by using the accumulation of supplied number density g ν , particle velocity g u and capture ratio ψ During the 1 st step of time progress      2).The figure shows the comparison of the evaluated results at 1200 seconds with the calculated results with time progress.The calculated value agrees well with the evaluated value.Therefore, our conceptual model is effective to understand the general tendency of temporal and spatial change of CPOM deposition as shown in Figure 8.

Conclusion
In this study, our efforts have highlighted evaluation of POM deposition on sandbar which is characterized by vegetated area and fine sand deposition there.We investigated the deposition mechanisms of sediment and POM through a field observation and a laboratory experiment.After the basic deposition characteristics were extracted by the field observation, we observed the deposition mechanisms by the laboratory experiment.The conclusions of this study are summarized that ripples are formed by the bed load in the riparian vegetation.And CPOM is

Figure 1 .
Figure 1.Site location and spatial distribution of POM content ratio according to grain size.

Figure 2 .
Figure 2. Measurement of specific weight of POM.
the interval of 5 m in a flume 20 m long and 0.5 m wide with the constant slope as shown in Figure 3.The bed was rigid.

Figure 3 .
Figure 3. Plan view of experimental channel.

Figure 4 .
Figure 4. Detail of each experimental sample.

Figure 5 .
Figure 5.Comparison of deposition state between Case-1 (Sand) and Case-2 (Sand + CP-OM) at 1200 seconds after the experiment starts.
k : number of ripple) according to the sand deposition.They are buried and deposited along the longitudinal direction ( )x ∆while the ripple migrate one wave length.Thus, CPOM is deposited on a straight line which is connected with the trough of ripple (see Figure6).And then, temporal change of CPOM deposition amount can be calculated at representative point of vegetated area time step.Accumulated deposition of CPOM (in number) kj N from 1x to j x along longitudinal direction according to the temporal change k t could be calculated as shown in Figure7.When a first ripple is formed ( ) for the second step ( )2 k = , two ripples are formed.Accumulated amount of CPOM 21N * is described with sand re-entrainment according to ripplemigration presented as the capture ratio ψ and the residual rate β written as follows mentioned, we can calculate the temporal and spatial distribution of CPOM deposition with interaction of ripple by using the conceptual model.

Figure 8
Figure8shows the comparison between calculated and evaluated results of CPOM deposition by using the conceptual model we proposed.The temporal and spatial change of CPOM deposition amount was calculated by using the proposed model on the basis of assumption of 0.7 ψ = , 0.4 β = .Ripples were formed at 500

Figure 6 .
Figure 6.Concept of deposition process of CPOM with ripple.

Figure 7 .
Figure 7. Model concept of CPOM deposition process with ripple.

Figure 8 .
Figure 8.Comparison between calculated and evaluated results of CPOM deposition.deposition amount at each point is increased with the development of ripples, and propagated to the downstream.These characteristics of CPOM deposition were represented well by our calculation as shown in Figure 8. X marks in the figure represent the evaluated CPOM deposition amount by using the measured experimen- tal data of CPOM deposition amount at 1200 seconds after the flume experiment starts.They are evaluated by using the relationship between several factors of ripple migration and bed load transport rate based on the following equation cm 2 ) N*=N/[v g (u g /Uw)]x j =x 0 +jL (x 0 =30cm, L=10cm, x=0：starting point of vegetated area) Calculated Evaluated

Table 3 .
Comparison of wave length, height and propagation velocity of ripple (x = 40 -60 cm).