Uniqueness Deposit of Sediment on Floodplain Resulting From Lateral Accretion on Tropical Area : Study Case at Kampar River, Indonesia

Kampar rivers has a length of 413 km with average depth of 7.7 m and width of 143 m. Sixty percent of  this rivers are meandering fluvial system which transport and deposit a mixture of suspended and bed-load (mixed load) along low energy. River channel that moving sideways by erosion is undergoing lateral migration and the top of the point bar becomes the edge of the floodplain and the fining-upward succession of the point bar will be capped by overbank deposits of Kampar River. Along the Kampar Rivers, there are more than 60% of ﬂoodplain sediments and almost all of the ﬂoodplain formed by bend migration on the suspended-load channels of Kampar watershed. This formation consist of succession of fine to medium sand and silt/mud, with root traces, that form as drapes on the prograding bank. These beds dip mostly channel wards and quickly wedge out as they grade up and onto the ﬂoodplain. The depositional model is presented showing how lateral accretion can make a signiﬁcant contribution to the preservation of ﬁne-grained within channel deposits in contemporary ﬂoodplains. The examples presented here demonstrate that analogues to ancient point-bar deposits containing alternating sandstone and shale sequences are common in the low-energy ﬂuvial environments of Riau rivers especially Kampar rivers.

1. Int r oduct i on M eanders develop by erosion of the bank closest to the thalw eg, accom panied by deposition on the opposite side of the channel w here the flow is sluggish and the bed-load can no longer be carried and river is considered to be m eandering if there is accumulation of sedim ent on the inside of bends. M eandering rivers transport and deposit a m ixture of suspended and bed-load (m ixed load) a long low energy. The bed-load is carried by flow in the channel, w ith the coarsest m aterial carried in the deepest parts of the channel. Finer bed-load is also carried in shallow er parts of the flow and it is deposited along the inner bend of a m eander loop w here friction reduce the flow velocity (Nichols, 2009). Type of sedim ent is form ed by m eanderi ng pattern are channel deposit, point bar, natural leeve, floodplain, oxbow lake, and crevasse splay.
The relative contribution of a variety of accretion deposits to the form ation of m eandering river floodplains have been the subject of prolonged discussion in the geom orphological literature (Page et al., 2003). A channel m oving sidew ays by erosion on the outer bank and deposition on the inner bank is undergoing lateral m igration and the deposit on the inner bank is point bars and it w ill show finingup from coarser m aterial at the base to finer at the top (Nichols, 2009). M igration of m eanders produces a general fining-upw ard point bar deposits and, in turn silty and m uddy floodplain deposits (Allen, 1965;Boggs, 2005). M ultiple episodes of m eander migration produce vertical stacking of fining-upw ard succession in m eandering-river deposit (Boggs, 2005).
Floodplains are dynamic feature that co-evolve w ith channel so at present no universal theory is available to predict floodplain w idth in natural rivers (as a function of drainage area, bank-full discharge, or sedim ent flux, etc) because floodplain m orphology dynamically integrates across these and the other factors, over som e unconfined tim e interval in the environment history of the w atershed (Belm ont, 2011) .In this study w e focus on floodplain deposit resulting from lateral accretion surface at river bend of Kam par Kanan River, Riau Province. Lateral accretion surfaces are m ost distinct w hen there has been an episode of low discharge allow ing a layer of finer sedim ent to be deposited on the point bar surface (Allen, 1965;Bridge, 2003;Collinson, J.D.;M ountney, N ;Thom pson, 2006;Nichols, 2009).

Over vi ew of Kam par Ri ver
One of the m eandering river system in the Riau Province, Indonesia is the Kam par River. Kam par River on the island of Sumatra in Indonesia originates in the m ountainous Bukit Barisan of W est Sumatra, and em pties into the M alacca Strait on the island's eastern coast. The river is the confluence of tw o big tributaries are Kampar Kanan River and Kam par Kiri River. The tributaries m eet in the Langgam subdistrict, Pelalaw an Regency, before flow ing into the M alacca Strait as the Kam par River. Koto Panjang, an artificial lake upstream of the river, is used to pow er a hydroelectric generating plant.
Study area is located in one of the Kam par Kanan river bends at Buluh China Village, Kam par District, Riau Province, Indonesia (Fig 1). The Ri ver has a lenght of 413 km and an average depth of 7.7 km and average w idth of 143 m . Fluvial m eander system develop typical morphology that oxbow lake w ith tropical rain forest, sand bar and river w ith fishery product. It has been used as a local attraction. This study is one of the appreciation of Buluh Cina Village com m unity that has been keeping the forest and natural conditions so this area can be used as one of t he Geo-tourism in the Riau Province.

Geom or phi c and Geologi cal Set t i ng
Buluh Cina Village is plain area w ith an elevation of 2 6 m above sea level. The landscape of study area is characterized m ainly by active channels, abandoned channels , natural leeve, backsw am ps and floodplain. The study area is the floodplain of Kam par Kanan River, so this area w ill flood if higher rainfall. Distribution of geom orphic feature throughout the tudy area suggests that the landform s w ere developed mostly during Quaternary. Along The Kam par Kanan River is deposited Young Alluvium (Qh) during Holoceneaged. Young Alluvium (Qh) are consist of gravels, sands and clays. M ost of sand deposit along Kam par Kanan River has becom e m ining site location, w hether they have perm ission or illegally. This activity not only w ill affects the deposition process that happen naturally in the river, but also affects t he river biota, tourism and w ater resources becom e dam aged due to the m ining. M orphological changes is forming in the Kam par Kanan River happen because of natural leeve (riverbank) erosion and sand mining, it m ake sliding riverbank and deepening of the riverbed.

Floods
Buluh Cina area located as floodplain area from Kam par Kanan River. This w ater are provided from large catchm ent area up to 5.321 km 2 cam e from Gadang M ountain and surroundings, w ith debit of w ater about 700 1000 m 3/s and relative slope 0,0008. In headw ater part, there are hydroelectric pow er plant at Koto Panjang that built beside for electricity, and also to control volume of w ater. If the top side area and the dam of pow er plant cannot endure it, the condition w ill becom e a big flood that w ill be affected to this area. And this event has happened in January 2016, w hich the w ater of the river rising about 3 m from the condition at dry season, and flooding all the area.

M et hodology
Data that used for this research consist of three data location of trench (TR-01, TR02, and TR-03), tw o drilling data (BC-01 and BC-02), and satellite im age from landsat.
The m ethodology for this study are consist of several steps and m easurem ent, starting from scouting field surveys to check condition and to m ark the drilling position, and then continued doing trenching at three position w hich located 2 m and 400 m aw ay from the river. Som e of them are already excavated and exposed, and revealed a 1 2 m thick sequence of sedim ent.
Drilling also conducted at tw o location up to 5 m depth using hand auger for coring, located 5 m from river and 400 m aw ay from the river. This drillng data w ill provide a better visualization of the texture of sedim ent from bottom to top of the layer. All of sedim ent that taken from trenching and drilling, w ill be analyze w ith sieve analysis by using m esh from 2.38 m m , 1.19 m m , 0.6 m m, 0.297 m m , 0.149 m m , and 0.074 m m to m easure the w eight each grain size and define the dom inate grain size in every layer collected from field.
Interpretation of satellite im age are using landsat im age from april 1989 until april 2016. This im age provide visual im age to seek the geom etrical changes of the river, and interpret the m igration of the channel, and also correlate it w ith field survey data.

Lat er al Accr et i on and Floodplai n Deposi t 4.1 Lat er al Accr et i on Deposi t on Kam par Kanan Ri ver at Buluh Ci na Vi llage
Based on the com parison of Satellite im age data in 1989 and 2016, it show s a m igration of the channel that occured in Kam par Kanan River that located near the Buluh Cina village (Fig 2). The basic dynam ics of flow around m eanders leads to erosion on the outside parts of bends and deposition on the point bars. Helical flow transport sedim ent, eroded from the cut bank, across the stream along the bottom and deposits it by lateral accretion on the point bar (Boggs, 2005). As the channel m igrate the top of the point bar becom e the edge of the floodplain and the fining-upw ard succession of the point bar w ill be capped by overbank deposits. Field observations indicate that lateral accretion consists of a point bar, oblique accretion and overbank deposit. Sedim ent of point bar deposited m edium sand w ith good sorting and upw ardsfining couplets during flood events. Som e flood channels becom e enlarged during floods, and m ay becom e the dom inant channel (W ood et al., 2008). Oblique accretion sedim ent is defined here as the lateral accumulation of fine-grained floodplain deposit by progradation of a relatively st eep convex bank in assosiation w ith channel m igration (Page et al., 2003). Oblique accretion are developed on the m eandering low energy rivers of Kam par Kanan River. The presence of oblique-accretion deposits in Buluh Cina Floodplain w as dem onstrated by prelim inary investigations at natural bank exposure, coring to a depth 5m , and trenches excavated at Kam par Kanan Rivers.
W hen the point bars occur, oblique accretion deposit occupy that part of the convex bank directly above at the point bar (Page et al., 2003). At site oblique-accretion layers consist of sand, m uddy fine sand strata dipping conform ably w it h bank surface (Fig 3). Dip angles vary from near horizontal at the top and the bottom to less than 150 w hile at the bottom , there are point bar deposit w ith dip angle m ore than 150. (Fig 3).
On trench location (TR-01 and TR-03) show s very fine to fine sand sedim ents, root plant on the top and bioturbation on the top layer. It indicates as oblique accretion deposit. The decreasing rootplant and bioturbation and sedim ent w ith m edium sand on the bottom layer and it indicates as point bar deposit (Fig 4). On TR-02 (Trench) location consist of very fine sedim ent at the bottom of the layer (125 75 cm ) w hich indicates an oblique accretion deposit, and then silt m ud w ith 75 m thick that interpret as overbank deposit ( fig  5). The boundary betw een silt and fine sand are gradual contact w hich affected from different grainsize and hardness, and also activity of organism and pedogenesis. Overbank sedim ent deposited on a floodplain w as identified in the field using standard criteria (Allen, 1965;Brow n, 1987;Guccione, 1993). It w as distinguished from channel deposits and colluvium by general lack of gravel, the lateral fining of texture w ith increasing distance from the channel and general lack of bedding , except proxim al to channel (Guccione, 1993). Based on sieve analysis from TR-01, TR02, and TR-03, considered from tw o different deposit show s that m edium sand dom inate at the bottom and very fine to fine sand sedim ent dom inate at the upper part ( fig 6).

Lat er al Accr et i on Deposi t on Kam par Kanan Ri ver at Buluh Ci na Vi llage
Floodplain is a strip of land that borders a stream channel and that is norm ally inundated during seasonal floods. Sedim ent is transported over the flooding as bed load and suspended load during floods. The sedim ent com es from the m ain channel, the valley sides and the floodplain itself (Bridge, 2003;Posam entier, Roger G.;W alker, 2006). Based on drillling data using 5 m depth of core hand auger , it show s that floodplain form ation originated from lateral accretion as sandbar, oblique accretion and overbank. Sandbar deposit located at the bottom and continued w ith oblique accretion deposit, and covered w ith overbank deposit at the top. The overbank deposits are not alw ays show n in the floodplain form ation, due to erosion process.

Di scussi on
Based on this research, it considered that lateral accretion gives a big contribution to the formation of floodplain. This form ation form tw o type of deposit w hich are sandbar and oblique accretion that form at low energy in meandering river. Floods that happen periodically in this area also bring finer sedim ent such as silt and m ud that characterize overbank deposit but not significally show n in this area. Therefore, in floodplain form ation, lateral accretion gives bigger contribution than vertical accretion. Lateral accretion deposit characterize w ith fine to m edium sand w hile overbank characterize w ith finer sedim ent (silt and clay).] The floodplain model that modified by (Page et al., 2003) state that there are three m odels of stratigraphy produced by oblique accretion for different floodplain seetings: floodplain w ith point bar, floodplain w ith no point bar and floodplain w ith point bar and scrolls. Based on the m odel, this area are include in m odel of floodplain w ith point bar (Fig 8).