Bar structure in an arid ephemeral stream

Sedimentary Geology 221 (2009) 57–70

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Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o

Bar structure in an arid ephemeral stream Marwan A. Hassan a,⁎, Philip M. Marren b, Uri Schwartz c a b c

Department of Geography, University of British Columbia, 1984 West Mall, Vancouver, British Columbia, Canada V6T 1Z2 Department of Resource Management and Geography, University of Melbourne, Victoria, 3010, Australia Mishmeret, 40 695, Israel

a r t i c l e

i n f o

Article history: Received 16 February 2009 Received in revised form 30 June 2009 Accepted 31 July 2009 Keywords: Unit bars Compound bars Bar structure Sediment transport River bed

a b s t r a c t This study describes the structure of gravel bars in Nahal Zin, an ephemeral stream in the Negev desert. The internal structure of the bars was examined along trenches and in shallow pits. Gravel sheets and unit bars form during transporting flow events in the main channel, on intra-bar channels and near bar heads. Unit bars are dominated by the Go facies. Compound bars develop from accretion around, and modification of, unit bars. Compound bars are active under the current flow regime and the average depth of the fill layer is about 35 cm. The structure of compound bars is dominated by Gm (massive), containing large amounts of sand. The second most common facies is clast-supported, openwork, and well sorted sediments of the Go (pebbles) facies. Bar formation, and the development of the range of facies evident in the bars is controlled by sediment supply, particularly the high volumes of sand-sized sediment, the passage of gravel sheets and bedforms during floods, and the lateral and vertical instability of the channel. Repeated scour and fill events have produced a diverse arrangement of facies, with numerous erosional contacts between depositional units. Lateral and downstream shifts in the pattern of scour and fill due to flow and antecedent conditions shape the channel morphology and bar internal structure. Ephemeral river bars differ from those of humid and proglacial rivers in terms of the dominant facies present, the arrangement of the facies within the bars, and the sedimentary structures developed within the depositional units and on the bar surface. © 2009 Elsevier B.V. All rights reserved.

1. Introduction A large literature exists on the origin, morphology and internal structure of river bar forms. For coarse-grained gravel bars, most studies have been in humid (Miall, 1977; Bluck, 1979; Bridge, 1993a; Lunt and Bridge, 2004; Lunt et al., 2004)) and glacial environments (Krigström, 1962; Williams and Rust, 1969; McDonald and Banerjee, 1971; Rust, 1972; Smith, 1974; Hein and Walker, 1977). There have been far fewer studies of gravel bars in arid environments. Arid ephemeral streams differ from their humid and proglacial counterparts in their hydrology, hydraulic environment and sediment transport regime (Reid and Frostick, 1997; Tooth, 2000). In many arid streams, flows are typically highly unsteady, flashy, and exhibit high magnitudes (Schick, 1988; Hassan, 1990a). Coupled with a lack of armoring and high sediment supply from bare slopes, arid streams are typically characterized by large rates of bed material transfer (Hassan, 1990b; Laronne and Reid, 1993; Laronne et al., 1994; Hassan et al., 2006). The magnitude and frequency of channel and bar forming events are different to that encountered in humid and proglacial environments (Wolman and Miller, 1960; Marren, 2005). In many arid environments, the recurrence interval of geomorphologically effective floods may be long, resulting in

⁎ Corresponding author. E-mail address: [email protected] (M.A. Hassan). 0037-0738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2009.07.012

a landscape dominated by the effects of infrequent high-magnitude flooding (Baker, 1977; Wolman and Gerson, 1978). Literature on bar formation and sedimentary bedforms in ephemeral rivers has largely focused on sand-bed rivers (Tooth, 2000). This is largely because of the importance of ancient sandstones as hydrocarbon reservoirs (Tooth, 2000), but is also partly due to the ease of excavation of sandy bar forms and channels compared to gravel deposits. Two recent investigations of the morphology and structure of coarse-grained bars in ephemeral rivers are the studies by Greenbaum and Bergman (2006) and Laronne and Shlomi (2007). In both studies, the bar forms are related to specific flood events, and are associated with considerable scour and fill. Sedimentation in both studies occurred primarily in sub-horizontal, sub-parallel packages, likely reflecting transport as a series of gravelly-sand sheets, varying somewhat in texture, deposited on top of one another. Gravel units are predominantly ungraded, with very minor inverse and normal grading occurring in both studies. Poorly sorted, clast-supported gravel with a matrix fill is the dominant facies type, but matrix-supported and openwork (matrix poor) gravels also occur. The fundamental bar form in gravel bed rivers is referred to as the ‘unit bar’ (e.g. Smith, 1974; Bridge, 2003), which refers to active gravel bars with simple histories and depositional morphologies. Sediment transport on unit bars is usually as diffuse gravel sheets, one to two clast diameters thick (Hein and Walker, 1977; Church and Jones, 1982). Continued flow and sediment transport around unit bars leads

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to the development of typical mid-channel ‘braid’ bars (Ashmore, 1991; Ashworth, 1996). Variations in flow and stage lead to repeated dissection and reworking of the bar sediment, leading to more complex ‘compound’ bar forms (e.g., Bluck, 1979). In this study, the term braid bar is used as a generic term for all mid-channel bars, irrespective of their evolutionary history. The terms unit and compound bars are used to refer to ‘nascent’ and ‘complex’ braid bars respectively. Due to the paucity of studies of unit and compound bar structure in arid ephemeral streams, the differences between bar structure in arid, humid and proglacial environments have not been examined in detail. For instance, it is possible that the repeated scour and fill events associated with arid zone bar formation will lead to complex compound bars with more internal erosion surfaces compared to bars in humid environments. This study examines the internal structure of modern active braid bars in Nahal Zin, an arid ephemeral stream in the Negev Desert. The surface morphology of the bars in Nahel Zin has been described previously by Hassan (2005). The work of Hassan (2005) showed that bars in Nahel Zin are lacking in surface structures, armoring and downstream fining compared to bars in humid regions. The research was motivated from the lack of studies of internal structure and sediment texture of bars in arid ephemeral rivers. There is a particular advantage to studying shallow active riverbed deposits in Nahel Zin, since the presence of flow gauges in the catchment makes it possible to relate present environmental conditions to the nature of the deposits. This is achieved by relating the modern hydraulic conditions to the 3-dimensional stratigraphic architecture of the channel and bars. Another advantage to studying arid zone rivers is that the riverbed is generally dry and accessible, allowing excavation and trenching of the sediment. 2. The basin and study reach The Nahal Zin drains a 1400 km2 catchment flowing northeast into the Dead Sea (Fig. 1). The basin is composed of late Cretaceous limestone and dolomite, with minor clay, marl, and chalk, and Pleistocene and Holocene alluvium. The climate is arid, with mean annual rainfall between 90 mm in the upper part and 50 mm near the Dead Sea (Greenbaum et al., 2002). Flow data are available between 1952 and 1982 at Aqrabim gauge station, between 1932 and 1947 at DSW gauge, and since 1994 at Mpl and Brg gauges (Figs. 1 and 2). Based on the Aqrabim gauge record, the annual peak flow ranges from 2 to 550 m3/s, with a mean annual flood of 22 m3/s. About 50% of the events have a peak flow equal to or less than 16 m3/s. The largest flood on record occurred on 1945 with a peak discharge of 600 m3/s, and is estimated to have a recurrence interval of 50 years (Greenbaum et al., 2002). The average number of flow events is 2.3 per year; however 3 dry years were recorded between 1952 and 1982. Flow data recorded at stations Mpl and Brg between 1994 and 1999, which are relevant to this study, are summarized in Table 1. The largest recorded event at the Mpl and Brg gauge stations is equivalent to the 10 and 5 year return period events, respectively, at the Aqrabim gauge station. The study site is a 5 km long braided reach that extends from the lower waterfall at gauge station Mpl to the Arava Road Crossing about 300 m downstream of gauge station Brg (Figs. 1 and 2). The primary sediment supplied to the study reach comes from upstream sources with some local erosion of terraces. Channel width ranges between 60 and 140 m with an average width of 90 m. The bed surface slope averages 1.0% and the average alluvium depth is about 2 m. Because of continuous lowering of the Dead Sea level in the last 70 years, the channel along this reach is deeply incised into, and confined by, floodplain sediments and alluvial terraces of Holocene age. Since the building of the Arava Road and bridge in 1960, the study reach has been disconnected from the Dead Sea, and the bridge footings became the base level for the reach. Consequently, the channel incision which was associated with the lowering of the Dead Sea has now ceased, and all

Fig. 1. Location map showing the study reach, location of the gauge stations and the rainfall isohyets of the Nahal Zin basin.

erosion and incision within the study reach are related to withinchannel flow events. During floods, the road becomes part of the stream bed and does not otherwise affect flows within the study reach. Sand is relatively abundant in the upper terraces whereas gravel is very common in the lower terraces (Grossman and Gerson, 1987). The coarse pebbles and cobbles in the terraces are massive and poorly sorted. Concentration of cobbles and small boulders in large gravel bars is common on the surface of the lower terraces and the present-day floodplain (Grossman and Gerson, 1987) but sand is the major component of the sediment found in the floodplain (Grossman and Gerson, 1987). 3. Methods A total of five bars were sampled. Four bars are named using the system of Hassan (2005) to aid comparison between the morphological description and the sedimentological descriptions presented in this paper. The fifth bar, called Bar R, has not been described before. The sites were selected to represent the range of bars and channel morphologies identified along the study reach (see Hassan, 2005). The fieldwork was carried out over two periods; summer 1994 (Bar R) and summer 1999 (the rest). At all sites, a series of trenches or pits was excavated. In Bar A of Nahal Zin (Fig. 2), three cross-bar trenches,

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Fig. 2. Air photo of the 5 km study reach in Nahal Zin. Names indicate the location of bars mentioned in the text and the gauge stations.

including parts of the adjacent channel were excavated. The three trenches, 1.5 m deep and 1 m wide, were located to reveal bar structure near the bar head, midsection, and tail. Additionally, three short trenches, perpendicular to the cross-bar ones, were excavated in order to characterize the longitudinal structure of the bar. In Bar B of Nahal Zin (Fig. 2), a series of pits between 10 to 15 m apart were dug on lines perpendicular to the main channel. Three such pit series were dug near the bar head, midsection, and tail. Because of the large downstream distance between the pits, a few additional ones were dug along the bar main axis. As in Bar B, in Bar C and Bar UB30 (Fig. 2) a series of pits was dug near the bar tail, mid bar and bar head. In Bar R, three trenches were excavated across the channel, with a 30 m spacing. All three trenches were excavated to bedrock. Trench 1

extended across the full width of the channel (70 m) whilst Trenches 2 and 3 were 30 m long, exposing only the right (south side) of the channel fill. A detailed description of the stratigraphy of each bar was made along the excavated trenches or pits. The field description was supplemented by complete photo coverage of the trenches for further interpretation. Each layer was described and classified using a faciescode scheme modified from Miall (1977) and Brierley (1989) adopted for fluvial facies found in Nahal Zin (see Table 2). Sediment geometries are described using the terminology of Friend (1983) and Bridge (1993b). The scheme combines particle texture, sedimentary structure, bedding features, and relative occurrence and includes new units not identified by Brierley (1989), whilst omitting facies not

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Table 1 Summary hydrological data for the period 1994–1998. Event number

Date

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Oct. 5, 1994 Oct 10, 1994 Nov. 2, 1994 Nov. 5, 1994 Nov. 7, 1994 Dec. 4, 1994 Feb. 14, 1995 Feb. 9, 1996 Nov. 20, 1996 Jan. 17, 1997 Jan. 23, 1997 Mar. 17, 1997 Oct. 18, 1997 Jan. 11, 1998

Peak flow (m3/s) Mpl gauge

Peak flow (m3/s) Brg gauge

Flow duration (h)

90 133 68 297 43 67 42 15 9 122 10 32 23

10 6 1.5 84 6 22 15

14.8 29.5 14.8 27.7 17.0 30.7 19.7 21.9 48.9 26.5 66.2 66.0

Notes: Two small events were recorded in 1999 (< 1.0 m3/s). The differences in the discharge between Mpl and Brg gauges are due to transmission losses.

The surface of the large bars is composed mainly of large patches of sand, pebbles and cobbles. Perennial shrubs grow on the sand patches. The head of Bar A is covered with a veneer of sand (up to 40%) while both the midsection and the tail are composed of coarse material. The coarse material is scattered at the surface with very few, poorly developed, pebble clusters. Fig. 4 presents the surface particle size distribution from Bar A, and the adjacent main channel (for more details see Hassan, 2005). Two surface particle size distributions from a smaller bar (UB30) are also shown in Fig. 4, one from the bar head and the other from the bar tail. The median size of the bed material of the small bar ranged between 3.3 and 4.5 ψ; a slight downstream decline in particle size between the bar head and tail was observed. Overall, there are no clear trends in surface grain-size distribution in Nahel Zin, although, most small bars show relatively uniform sediment without any downstream trend in the particle size. 5. Bar sedimentology 5.1. Bar R

found in the study sites. Finally each bar was mapped and samples were taken for size analyses. The grain-size classifications used to describe the bar surface sediment characteristics, and to assign deposits to particular facies classifications are based on the grain-size classification of Blair and McPherson (1999). Channel stability and depth of the active volume were determined used field methods and sediment transport formulae and channel surveys. Transverse cross-sections were surveyed after every flood (Table 1) in the vicinity of gauge stations Mpl and Trench 1 (Bar R, see Fig. 2) between 1994 and 1999. Moisture sensors were inserted along Trench 1 about 40 cm below the bed surface. In addition, three crosssections with scour chains were established along the three trenches excavated at Bar A in 1999. Net scour and fill were estimated using the cross-sections, scour chains, and the moisture sensors. 4. Bar morphology The morphology of the study reach and of the surface texture of the bars has been described in detail by Hassan (2005). The channel is narrowest at the location of Bar R and is constrained by rock walls on both banks ensuring that flood flows are confined to the channel along this sub-reach (Fig. 2). Several large elongate bars measuring up to 600 m along the downstream axis occur within the reach (Fig. 3a and b). These bars rise 0.5 to 1.0 m above the adjacent channel and are flooded, on average, once every year and are therefore considered relatively active.

5.1.1. Description The three trenches at site R reveal up to 3.2 m of sediment (Fig. 5). In all three trenches the overall stratigraphy is dominated by horizontal to sub-horizontal bedding. The alluvial architecture is therefore dominated by tabular depositional units, with lens shaped margins to the units where they pinch out. All depositional units have eroded tops. Steeply dipping (60 to 80º) erosional surfaces truncate the units on at least one side (e.g. at 22 and 6 m in Fig. 5b, and at 17 m in Fig. 5c). In some cases, one side of the depositional unit consists of a gently dipping (10 to 20°) surface (e.g. at 40 to 44 m in Fig. 5a, and 4 to 20 m in Fig. 5b). Channel-shaped scours occur locally (e.g. at 54 to 46 m in Fig. 5a). The depositional units vary in thickness between 0.2 m and 1.0 m, and where they are not erosionally truncated, they pinch out laterally. The sediments exposed in the trenches are dominated by gravel and granule sized material. Coarse gravel facies are predominantly matrix-supported (facies Gm), with a smaller component of clastsupported gravel (facies G). Granule-gravel and pebble-sized facies Go (openwork gravel) forms approximately 50% of Trench 1. Trench 2 is dominated by stacked units of G, Gm and Go, each up to 0.5 m thick (Fig. 5b). Most of the Go facies consists of granule-gravel sized material. In Trench 3 the depositional units are composed of coarse gravel poorly sorted, matrix-supported and clast-supported pebblecobble gravel (facies Gm and G) and granule-gravel sized material (facies Go). At the bottom of the succession, the granule-gravel facies

Table 2 Facies coding scheme used in the study (modified from Miall, 1978 and Brierly, 1989). Facies code

Texture

Lithofacies and bedding properties

Sorting

Occurrence

Comments

G

Gravel and coarse sand

Poorly sorted

Very common

Go

Gravel and granules

Gravel-supported, Closed-work structure, occasional horizontal bedding, Openwork structure, no bedding

Well sorted or very well sorted

Very common

Gm Su

Gravel and coarse sand Fine and coarse sand, some granules

Massive, matrix-supported gravel Upward fining sand laminations

Very common Uncommon

Sp

Medium-coarse sand and pebbles

Fm Sh

Fines Fine-coarse sand and pebbles

Sr

Medium-coarse sand

Ss

Medium-coarse sand

Alternating horizontal/gently inclined layers of pebbles and granules with layers of coarse sand Massive, with cracks Horizontal lamination with no upward fining sequences Cross-bedded, with changing angles, may be pebbly None visible

Extremely poorly sorted Well sorted, — within sub-layers Moderately sorted — within sub-layers

Occasional imbrications and lenses of open work Bottom half may be closed-work structure, resembling G Typically not imbricated

Common

Very well sorted Moderately sorted

Uncommon Uncommon

Well sorted

Fairly common

Well sorted

Fairly common

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Fig. 3. Oblique photos and surface morphology of Bar A (a) and Bar B (b).

consists of alternating fine gravel and granules, and granules and sandy gravel. 5.1.2. Interpretation The tabular, gravel-dominated depositional units which dominate the three trenches at this location are interpreted as stacked unit bar deposits. These deposits dominate the upper part of the trenches, and

closely correspond with the surface morphology of the channel at this location. The steeper erosional contacts which occur lower in the sedimentary succession are interpreted as erosional truncation and cut-banks associated with cut-and-fill processes operating during floods. Thus, the basal part of the succession is interpreted as a compound bar formed from cut, fill and accretion processes during repeated floods. The formation of these tabular but erosionally

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Shlomi, 2007). As a consequence, flood sediment concentrations in this region are amongst the highest in the world (Cohen and Laronne, 2005). The origin of facies Go (openwork gravel) is the subject of some debate, but is generally attributed to flow separation on the lee side of gravel bars (Lunt and Bridge, 2007). On unit bars, which are commonly low relief features, there may be insufficient flow separation to allow well developed openwork gravel. Longitudinal sorting and differential sediment transport rates, associated with superimposed bedforms on the bar surface may be more important in forming openwork gravel beds (Iseya and Ikeda, 1987, Lunt and Bridge, 2007). 5.2. Bar UB30

Fig. 4. Size distribution of the surface bed material along Bar A (63 and 54), main channel adjacent to Bar A (60), and Bar UB30 (A and D).

truncated depositional units is demonstrated by repeat measurements of channel surface change made during the period 1994 to 1999. These changes are discussed further in Section 6. The dipping surfaces which bound some of the depositional units in the lower part of the succession are interpreted as bar margin deposits that have not been truncated. The deposits of smaller, more recent floods occur as relatively undisturbed channel fill deposits close to the surface (e.g. at 54 to 56 m in Fig. 5a). The presence of abundant matrix-supported gravel (facies Gm), and poorly sorted clast-supported gravel (facies G) is thought to be due to the high sediment loads typical of arid zone floods (Laronne and Reid, 1993). This is due to the unarmored surface of the channels and bars in Nahel Zin (Hassan, 2005; Laronne and

5.2.1. Description Pits were dug at four locations along the length of Bar UB30. The internal structure of the bar is uniform and dominated by Go (pebbles) facies (Fig. 6a) to the full depth of the excavated pits (30 cm). The Go facies is characterized by clast-supported, openwork, and well sorted sediment (Fig. 6a). In places, a veneer of sand (∼3 cm thick) covers parts of the bar. Comparison of Bar UB30 with similar bars in Nahal Zin indicates little variability in the internal structure; the Go unit dominates the structure of all examined bars. 5.2.2. Interpretation The simple internal structure, limited downstream fining, and small size of Bar UB30 indicate that it is a unit bar, which has not undergone a complex history of reworking. The sand on the surface is interpreted as the remains of a superimposed sandy bedform (Reesink and Bridge, 2007). Openwork gravel dominates the deposit, and as is the case with Bar R, this is interpreted being largely a product of longitudinal sorting of sediment during flood flows in a manner similar to that described by Iseya and Ikeda (1987). The role of flow separation, in the manner

Fig. 5. Stratigraphy and sedimentology of Bar R in Nahal Zin. (a) Trench 1, (b) Trench 2 and (c) Trench 3.

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are longer and thicker than the other sand facies and include some pebble strata. The midsection of the bar is represented by Trench 2 (Fig. 7). Along most of the cross-section, the bedrock is overlain by a c. 20 cm thick, Sp unit which is bounded by an upper erosional contact. A large Go unit forms the core of the bar. This unit is approximately 20 m long and 60 cm thick and consists of pebbles and cobbles: the bottom half is matrix-filled. The top G unit of Trench 2 is thick (20–50 cm), and constitutes most of the bar's body above the adjacent active channels. Trench 1, located near the bar tail, has a stratigraphy similar to Trench 3. The Sp unit dominates the bar and the channels adjacent to the bar. A thin Fm unit (silt and clay) separates the Sp facies from the overlying units. Another Fm unit is found below the right side of the bar under the current active channel. An extensive well sorted, crossbedded Sr unit is a distinctive feature of the bar tail. The Sr facies dips to the right at about 30°. Horizontally stratified fine to coarse sand with scattered small pebbles (Sh unit) covers most of the top parts of the bar near Trench 1 (Figs. 7 and 8).

Fig. 6. Stratigraphy and sedimentology of unit and compound bars in Nahal Zin. (a) Bar UB30, and (b) Bar C. Numbers indicate the location of stratigraphic pits.

described by Lunt and Bridge (2007) is unclear. In the experiments by Lunt and Bridge (2007) openwork gravel deposition was most commonly associated with flow separation on the accretion face. However, the low relief of unit bars in Nahle Zin (c. 10 grains thick, Hassan, 2005) is likely to inhibit the development of a flow separation zone at the downstream end of the bar. There is no evidence of cross stratification or an accretion face at the downstream end of bar UB30. 5.3. Bar A 5.3.1. Description Fig. 7 presents the bar internal structure at the bar head, midsection, and tail. Most of the bar is composed of Gm facies units. These are up to 40 cm thick and 5–10 m wide, and have massive, poorly sorted, and matrix-supported appearance. The particle size ranges from fine sand up to small cobbles. The coarse fractions are well rounded. The mediumcoarse sand and small granule matrix comprise up to 40% of the unit's sediment texture. Overlying the bedrock are several lenses of 10–20 cm thick granule and pebble-sized Go facies. The Go facies comprise massive, loose, poorly sorted, granular and pebble size material. The clasts are well rounded and the matrix is medium to coarse sand. At the left side of the bar these lenses are thicker (20–40 cm thick) and indicate cut-and-fill activity by the erosional contact between them and the overlying deposits. The G facies is widespread near the bar head (Trench 3) and adjacent channel surfaces. The facies, dipping to the right bank at about 3°, is clast-supported with some horizontal bedding and occasional thin lenses of openwork gravel. Two sets of inclined Go beds dipping to the right bank at about 10° are found at the right side of the bar. Near the main channel (right side), thin sandy units (Su, Sh, Sr, and Ss) occur; typically about 2 to 3 m long and 10 to 20 cm thick. Beds of the Sp facies

5.3.2. Interpretation The complex pattern of erosion and deposition at the bar head attests to multiple periods of reworking of Bar A. Thick G and Gm units, associated with scour into underlying units are interpreted as the deposits of high-magnitude floods (Hassan, 1990b). The abundance of Gm facies indicates that sediment loads were high, a typical feature of ephemeral river floods (Laronne and Reid, 1993). The presence of openwork gravel in association with cross-stratified beds is interpreted as the product of flow separation on the bar accretion face (Lunt and Bridge, 2007). This is in marked contrast to the thinner unit bar deposits such as bar UB30. Openwork gravels near the bar head indicate that flow separation occurred on the bar margins, as well as the bar front. The Sp facies (sand and pebbles) is interpreted as the product of deposition from superimposed dunes (Reesink and Bridge, 2007). The thin sandy units near the main channel are interpreted as sand fill units deposited in small intra-bar channels or scour pits within the active channel. The mid-bar excavation had a relatively simple structure, except near the left bar margin. This is taken to indicate that the bar midsection is less frequently reworked than the bar head and tail. It is likely that many floods rework the bar margins, but do not reach high enough to scour into and rework the main part of the bar surface. The bar-tail trench has a more complex structure, with evidence for multiple scour events. The fine layers indicate deposition from suspension and are probably mud drapes and the infill of shallow pools formed on the falling stage of flood events. The higher proportion of sand facies in the bar tail indicates that the lower magnitude floods rework and accrete onto the bar margins and tail, whilst leaving the bar core unmodified (Bluck, 1979). The Sr units are interpreted as sand wedge deposits formed on the bar margins and front during the waning stage (cf. Bluck, 1979). Overall, Bar A is interpreted as an accreted compound bar, modified by repeated floods. 5.4. Bar B 5.4.1. Description The structure of Bar B is presented in Fig. 9. Mixed cobbles and sand extend over most of the bar's surface, and the uppermost beds are facies G. The uppermost G facies layer thins down-bar, from the head to the tail. At the bar head (pits 4, 5, and 6) 5–10 cm thick units of sand (Sh) overlie the G facies. Near the mid and bar tail, beds dip down-bar (all beds in pits 18 and 25 and some beds in pits 26 and 27). At depth, the bar is dominated by dipping beds of Go facies, ranging in size from small cobbles to pebbles. Only at the bar head (pit 10) is the Go facies a minor component, here facies G occurs to a depth of 1 m. In the bar, beds tend to be thinner than in the adjacent channel (typically 5–20 cm thick) and the structure is distinct from that of the channel,

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Fig. 7. Stratigraphy and sedimentology of Bar A of Nahal Zin. Trenches 1, 2 and 3 reveal cross-bar structure near bar tail, midsection, and head, respectively. Trenches 1A, 2A, and 3A characterize the longitudinal structure of the bar tail, midsection and head, respectively.

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dominate the adjacent channel facies. The bar head and mid bar are essentially massive Gm facies, whilst the bar tail consists of bedded G, Go and Gm. The bar tail is covered by a thin veneer of sand (Ss unit) up to 4 cm thick. 5.5.2. Interpretation Bar C is interpreted as a unit bar core with an accreting bar tail. The Gm facies bar core represents the deposit of the sediment laden 1991 flood. The bedded accretion deposits on the bar tail indicates that flow separation occurred, with the overpassing of superimposed bedforms. The simple history of Bar C provides insights into the structure of nascent compound bar forms in situations where bars are reworked only by intermittent floods. 6. Sediment mobility and thickness of active layer

Fig. 8. Detail of the sedimentology of Bar A. (a) Trench 1, between 3 and 5 m (from left to right), close to the bar margin. (b) Trench 1, between 13.4 and 15 m, close to the bar core.

with a greater proportion of facies G, and with only limited facies Go. The Fm facies is limited to one site (pit 23). Fig. 10 presents the frequency of the facies types units along Bar B. Most of the units in the bar are G and Gm; they comprised about 60% of the total bar. The frequency of unit Gm is constant downstream (20–25%) whereas unit G increases from about 10% near the bar head to over 20% near the bar tail. The sand unit Sr is not found near the bar head, but appears near the mid bar and bar tail. Facies Go dominates the bar, and there is a significant downstream increase in the frequency of the Go (pebbles) unit. 5.4.2. Interpretation Bar B represents a unit bar which has accreted by the downstream migration of a bar face. The bar core is represented by the clastsupported G facies at the upstream end of the bar. The dipping Go facies that dominate the bar represent the downstream accretion deposits. Flow separation has resulted in the deposition of openwork gravel on the bar-front slip-face (Lunt and Bridge, 2007). The G facies that occurs in the adjacent channel is the product of scour and fill from flood flows (Hassan, 1990b; Laronne et al., 1994). 5.5. Bar C 5.5.1. Description Bar C (Fig. 6b) developed during the large flood of 1991 (Greenbaum et al., 2002) and was modified during subsequent events. The bar morphology is simple. The depositional units are usually thick, up to 40 cm, and the bar consists of one to three units with little vertical variability in the sediment characteristics. The bar is dominated by gravel facies (G, Go (pebbles), and Go (granule)) while Gm units

Bar sedimentology depends largely on bar dynamics which, in turn, depend on flow and sediment transport and supply regimes. Spatial patterns of bar dynamics are controlled by sediment mobility (amounts and texture), and by the magnitude and temporal variability of scour and fill along the study reach. Relating bar stratigraphy to flow regime, channel stability and patterns of scour and fill is of significance as by understanding the bar dynamics it will be possible to determine if the sediment record in the excavated trenches was created by the current hydrological regime. Large amounts of sand on the surface, a low armoring ratio and a lack of sedimentary structures characterize the bed surface of the study reach implying that Nahal Zin bed is not armored (Hassan, 2005). Post-flood field observation revealed the almost any flow can move the sand fractions along the main channel and the bar surfaces. Additionally, between 15 and 30% of the bar surface is covered by sand which is likely to influence the entrainment conditions. Due to lack of systematic sediment transport measurements, particle entrainment, transport rate, and depth of active layer were determined from survey cross-sections and computational methods using the Wilcock and Crowe model (2003). Sediment texture and the amount of mobilized sediment depend, among other things, on flow magnitude and duration. Flow magnitudes were determined using the recorded data at gauges Mpl and Bgr and at-a-point hydraulic geometry relations for the stations. The hydraulic geometry relation shows that a flow as high as 60 m3/s is needed to mobilize sediment on bar surfaces. Such a flow occurred at Mpl and Bgr gauges 4 and 3 times, respectively between 1994 and 1998. Considering flows >30 m3/s, 7 and 3 flows were recorded at Mpl and Bgr gauges, respectively. Overall, at least one event every year mobilizes sediment along the study reach. Furthermore, a major sediment mobilizing event occurs once every two to three years. The depth of the active layer was determined using the surveyed cross-sections and empirical relations developed for ephemeral streams (Hassan, 1990b). Over the period between 1994 and 1998, 14 flow events were recorded (Table 1). Repeated surveys of the crosssections provided information on net scour and fill over this period. Net changes in the bed elevation after major events are presented in Fig. 11. The two sites are 30 m apart and describe changes in the channel due to the same flow events. The magnitude of net scour and fill changes with flow magnitude and laterally across the channel. The average net fill ranged from 1–2 cm during low flows to 35 cm during medium-large flow events. At some locations, a maximum fill of 70 cm was recorded. However, moisture sensors inserted in Trenches 2 and 3 remained in place during the period 1994 to 1998, indicating that the scour depth was less than 40 cm (moisture sensor location) during the highest recorded floods. Overall, the mean net scour and fill over the study period was 30 cm and 35, respectively. Laterally, the cross-sections revealed a variable pattern of scour and fill across the channel. For example in response to the Nov. 20, 1996 event (Table 1), the flow scoured the bar and deposited a thin

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Fig. 9. Stratigraphy and sedimentology of Bar B of Nahal Zin. Numbers indicate the location of stratigraphic pits.

layer in the deep channel at cross-section Trench 2 (Fig. 11c). In contrast, the subsequent event of Jan. 23, 1997 (Table 1) deposited a layer of sediment across the entire channel width with very minor scour at about 40 m from right bank (Fig. 11d). Although differing in magnitude and location, similar lateral trends were obtained for the Trench 3 site (see Fig. 11g and h). Comparing Trench 2 site with Trench 3 site 30 m downstream shows that scour and fill alternate between the two locations. For example, during the Nov. 20, 1996 event, the flow scoured most of the bar and the main channel at Trench 3 site (Fig. 11g). A differing pattern was revealed by the Jan. 23, 1997 event; major filling occur at Trench 2 site (Fig. 11d) while bar scouring and channel filling occurred at Trench 3 site (Fig. 11h). The subsequent events show major filling at Trench 3 site (Fig. 11i and j) whereas there were only minor changes at Trench 2 site (Fig. 11e and f). These observations show that antecedent conditions play a large part in determining the lateral and downstream patterns of scour and fill in the channel. Bars created by previous floods direct floodwaters across the channel encouraging lateral switching of scour and fill. Similarly, in a downstream direction, sites which are heavily scoured

in one flood tend to become sites of sediment accumulation in the following flood, and then become a sediment source in later floods. Using the relation developed for ephemeral streams by Hassan (1990b), a mean scour on the order of 30 cm was calculated for 10year events. For extreme events (∼20 year return period), a mean scour value of 40 cm was calculated. Thus, the calculated value is similar to the estimates from cross-section surveys and furthermore, is similar to values reported by Laronne and Shlomi (2007) for the lower reach of Nahal Zin. Overall, analysis shows that the compound bars are relatively active under the current hydrological regime and that, furthermore, most of the described sediment record can be related to the current flow and sediment regimes. Cross-section survey data were used to estimate sediment volume change along the study reach. The volume of change was estimated by calculating the disturbed areas between the section and the depth of change. During large events (Qpeak > 100 m3/s), the volume of change ranged between 200 and 450 m3. For example, for a 4–5 year event (similar to event 11, Table 1), the estimated volume of change was about 370 m3. The estimated volume of change during annual floods was 160 m3. Small floods (Qpeak < 10 m3/s) are limited to the main

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Fig. 10. Frequency of facies units at the bar head, midsection and bar tail of Bar B, Nahal Zin.

channel and the lower parts of the bars and hence the volume of sediment change was relatively low. For example, the estimated volume for event 12 (Table 1) was about 30 m3. The above calculation can be used to estimate how often the bar sediment body is replaced. A large bar contains approximately 2500 m3 of sediment on average. This implies that 20% of the sediment volume of a large bar is replaced once every 4 to 5 years, and that the sedimentary record studied here is only a few decade old. 7. Discussion The characteristics of sediment facies depend on flow conditions at the time of deposition and delivered sediment characteristics. Controls on sediment deposition are further complicated by spatial and temporal variability in flow, sediment supply and transport regimes. These controls are particularly important in arid streams because of the flashy nature of flows and high sediment transport rates. However, sediment transport models are based on equilibrium conditions and do not incorporate sediment supply, which is the major control of channel morphology. Therefore, these models are of limited use for explaining bar structures. The findings of this paper extend those of Laronne and Shlomi (2007), who describe sediments from sites downstream of the study reach of this paper. Laronne and Shlomi (2007) describe sediments from pits dug around scour chains, distributed along two crosssections in Nahel Zin (as well as sites in adjacent catchments). This study concentrates on the facies and internal structure of individual bars by systematically excavating pits and trenches along and across a range of bars within the flood channel. The structure of gravel sheets and unit bars in Nahel Zin is uniform, and dominated by the Go facies. Openwork gravel deposits are common in gravel bars, usually alternating with thin, matrix-filled layers (Smith, 1974; Lunt and Bridge, 2007). Smith (1974) associated openwork layers with deposition during high flows when most of the fines move in suspension. With declining flow, finer gravels build up and finally matrix-sized material is deposited with the gravels. Recently, Lunt and Bridge (2007) have shown in flume experiments that the mechanism proposed by Smith (1974) does not occur. Rust (1984) and Anketell and Rust (1990) describe a mechanism whereby avalanching of sorted sediment in superimposed dunes produces openwork layers. Carling

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and Glaister (1987) and Anketell and Rust (1990) suggest mechanisms related to downstream migration of accretion fronts together with vertical sorting by secondary flow during deposition. Recent experimental work (Lunt and Bridge, 2007) has shown that flow separation and avalanching of gravel down the accretion face is the dominant mechanism for producing thick openwork gravel units. However, in the Nahel Zin unit bars, where facies Go is most common, it is unlikely that the bars have enough relief to generate avalanche faces and flow separation. It is therefore most likely that openwork gravel is due to pulsations in sediment transport rate, by a mechanism similar to that described by Iseya and Ikeda (1987). In this scenario, unit bars form where unimodal gravel sized sediment is sorted into a ‘congested’ zone, encouraging deposition. The unit bars in Nahal Zin lack imbrication, surface packing and bed clustering, typical of humid bars (Wittenberg et al., 2007). The lack of surface structure is attributed to the flashy nature of flow and the high proportion of sand (Laronne et al., 1994; Hassan et al., 2006). The lack of surface armoring can, in turn, contribute to high sediment transport rates during floods (Cohen and Laronne, 2005). The most abundant coarse-grained facies in compound bars is the massive Gm facies. Matrix-supported gravels are rare in the deposits of perennial streams, where clasts-supported gravels tend to dominate (Miall, 1977, 1978). Miall (1985) asserted that Gm units are likely to be deposited from bedload moving during relatively high flows. Based on this model, the Gm units (and the G units) are formed by deposition of the traction load (sand to boulders) en masse. Our field evidence shows that this unit is thick and includes large proportions of sand (∼40%) implying that the sand fraction during sediment transport should be significantly greater than in the deposit. The dominance of matrix-supported gravel in compound bars is therefore attributed to high sediment transport rates, encouraged by a lack of surface armoring (Cohen and Laronne, 2005). The G facies is the next most abundant in compound bars. The G units in Nahal Zin lack structures such as clusters and imbrication. In proglacial and humid environments this facies is usually imbricated (e.g., Bluck, 1979; Church and Jones, 1982). The massive gravel deposits of the G facies are interpreted as being deposited during high flows over the bar and the tail. In some of the coarse gravel depositional units (G facies) in Nahal Zin the proportion of the sand reached up to 55%. Hassan (2005) classified the bars in Nahel Zin into unit and compound bars based on morphological criteria. This study has shown that the distinction on morphological grounds is reflected in the internal sedimentology and structure of the bars. In particular, there is a correlation between the age and history of reworking of the bars, and the complexity of the resulting deposits. Bar UB30 reflects the structure of a small, relatively unmodified unit bar. Bar C is an example of a bar which formed by accretion, and although it has undergone some modification since its formation, by flows which have been consistently lower in magnitude that that which formed the bar, it is still largely unchanged in its internal structure, with accretion occurring on the downstream end of the bar. Bars A and B are similar in that they represent well developed midchannel bars that have been subjected to repeated floods. Bar B is distinguished by the fact that it has been more heavily dissected by later flows (Hassan, 2005), and is less elongated in a downstream direction than the other bars. In both cases though, the internal structure of the bars consists of a heavily reworked bar head, tail and margins, with a less disturbed bar core. Downstream accretion on an advancing bar front remains the dominant process of deposition and bar growth during high flows, but evidence of scour and fill by lower magnitude flows, periods of settling of sediment from standing water, and the passage of smaller sandy bedforms over the bar surface all produce a more complex stratigraphic record. The characteristics of ephemeral river compound bars identified in this paper are distinctive from those of humid and proglacial rivers in terms of the dominant facies present, the arrangement of the facies

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Fig. 11. Channel cross-section surveys between 1994 and 1998 at Trench 2 (a–f) and Trench 2 (g–j), both at Bar R.

within the bars (the alluvial architecture), and the sedimentary structures developed within the depositional units and on the bar surface. These differences are caused by the hydrology and sediment load to the floods that form the bars in Nahel Zin, and the absence of flow in the channel between floods. The high proportions of matrixsupported gravel in the compound bars described here is due to the high sediment loads and the high availability of sand-sized sediment, along with the short lived nature of the floods, which result in poor vertical sorting, and a high proportion of sand both on the surface and in the subsurface (for more details see Hassan, 2005 and Hassan et al., 2006). This is common to many floods in arid ephemeral rivers (Laronne and Reid, 1993; Cohen and Laronne, 2005). In contrast, coarse gravel bars in humid and proglacial environments are usually characterized by clast-supported gravel, with well developed imbrication and surface armoring (e.g. Williams and Rust, 1969; Bluck, 1979). In humid and proglacial rivers, matrix-supported gravel deposits are usually attributed to hyderconcentrated flow deposition associated with extreme floods (Marren, 2005), and with debris flows

on alluvial fans and proximal proglacial rivers (Miall, 1977). Thus, arid ephemeral rivers represent an environment where almost all floods are an ‘extreme’ event, both in terms of their relative magnitude and frequency, and in terms of their sedimentary expression. Both hyperconcentrated and debris flows produce distinctive sedimentary successions, which differ markedly from the bar deposits found in Nahal Zin (Costa, 1988; Benvenuti and Martini, 2002). The absence of imbrication and surface armoring at Nahel Zin (described in detail in Hassan, 2005) is also attributed to the short duration of the floods that formed the bars, and the absence of reworking by low stage flows. The high volume of sand transported by the floods is also likely to inhibit the development of imbrication (Hassan, 2005). Observations made during the waning stage of floods in Nahel Zin indicate that scour of the bar margins largely occurs during flow recession, removing sand-sized material from the surface and flanks of the bar. Between floods, the absence of flow means that there will be little reworking of bars by low magnitude flows. Therefore, the dominant mechanism for the formation of complex compound bars is

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the scour and fill processes operating during floods, particularly where scour does not completely remove the deposits from previous floods. Many floods will flow around rather than over existing bars, particularly during the early stages of a flood, leading to erosion of bar margins, and complex accretion and bar growth. Thus, arid ephemeral bar deposits are characterized by numerous cut-and-fill structures. In Nahel Zin, the development of these complex accretion surfaces can be seen through the repeat channel surveys carried out between 1994 and 1998 (Fig. 10). Accretion surfaces at Nahel Zin are largely limited to bar front deposits, in part because bar margin deposits are largely removed by cut-and-fill sedimentation, although there is little evidence for bar migration through accretion of the bar margins. This pattern of scour/cut and accretion only during floods contrasts with the growth of bars in humid and proglacial zones, where year-round flow, or highly seasonal but perennial flows lead to constant reworking, accretion and migration of bar margins by flows with a wide range of discharges (Bluck, 1979; Bridge, 2003; Marren, 2005). The difference between humid and ephemeral rivers in terms of bar modification by cut-and-fill is that in ephemeral rivers, all bar modification is by flood events, whilst in humid rivers, bar modification occurs at in association with a wider range of discharges, producing a broad range of minor sedimentary structures (Bluck, 1979). 8. Conclusions 1. The sediment record described here has been largely formed by floods which have occurred since 1994. Repeated scour and fill events have resulted in no net change in sediment volume, but have produced a diverse arrangement of facies, with numerous erosional contacts between depositional units. Lateral and downstream shifts in the pattern of scour and fill due to flow and antecedent conditions shape the channel morphology and bar internal structure. 2. The Go facies is dominant in unit bars, and a major component of compound bars throughout the Nahel Zin, despite the large amount of sand carried by the floods. The formation of this facies implies the passage of gravel sheets and bedforms during flood flows, and within-flood sorting of the sediment. 3. Bar formation in ephemeral rivers is largely controlled by the high rates of sediment supply, particularly sand-sized sediment, and the episodic nature of the sedimentation events. Bars are thus characterized by a lack of well developed sedimentary structures and imbrication, and poor vertical sorting. In compound bars, there is a down-bar transition from clast-supported gravel (lacking imbrication and surface armoring), to matrix-supported gravel, alternating with openwork gravel. The characteristics of ephemeral river bars identified in this paper are distinctive from those of humid and proglacial rivers in terms of the dominant facies present, the arrangement of the facies within the bars (the alluvial architecture), and the sedimentary structures developed within the depositional units and on the bar surface. 4. A range of sediment facies is formed during individual flood events in Nahel Zin, emphasizing the importance of temporal and spatial patterns of sediment deposition in these environments. Thus, in ancient ephemeral river bars, changes in facies should be primarily considered in terms of fluvial processes not necessarily in terms of environmental change. Acknowledgments This work is partly based on data collected for Daniel Glickman's master's thesis at the Hebrew University of Jerusalem. Daniel Glickman and Judith Lekach helped with fieldwork in Nahal Zin. This work has benefited from discussions with Judith Lekack. The Dead Sea Works provided air photos and logistic support during the fieldwork in Nahal Zin. Mike Church kindly reviewed a draft and provided many

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suggestions and comments that greatly improved the paper. Eric Leinberger prepared the figures.

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