The Hydrology-Geomorphology
Interface: Rainfall, Floods, Sedimentation,
Jerusalem Conference, May 1999). IAHS Publ. no. 2 6 1 , 2000.
Land Use (Proceedings of the
185
Episodes of flash floods and boulder transport in
the Upper Jordan River
MOSHE INBAR
Department of Geography, University of Haifa, Haifa 31905, Israel
e-mail: inbar(S),geo.haifa.ac.il
Abstract Because direct measurement of boulder transport in natural rivers is
difficult, there are very few field studies on the subject. Long-term field studies,
1969-1999, were conducted in the Upper Jordan channel to evaluate the
geomorphic impact and persistence of a 1:100 year catastrophic flood, and to
analyse the relationship between the river's hydrological regime and boulder
transport behaviour. A channel reach of 210 m length and 25-35 m width,
with 260 boulders larger than 500 mm b-axis, was resurveyed periodically
over 30 years. Detailed field surveys, photographs, and painted rocks were
used to assess the bed load transport. During the study period, four major flood
events had the ability to transport boulders of sizes from D 1000 mm up to
the largest, namely 1700 mm b-axis size. Stream power values increase where
the depth-slope product is maximized. In the wide sections of the channel with
lower slope values, the deposition of big boulders determines the formation of
bar structures. The catastrophic flood has a decisive effect on the modification
of the fluvial landform, being a geomorphologically effective event.
50
Key words bed load; boulder transport; flash floods; hydraulic geometry; Jordan River;
sediment bars; stream power
INTRODUCTION
Analysis of bed load transport is useful for channel morphology engineering and
palaeohydrological reconstruction. For small sediment sizes, empirical and theoretical
equations have been developed based on flume and river experiments. For large coarse
material, the difficulties in studying entraimnent and transport in flumes have been a
major limiting factor. Not surprisingly there are very few studies on direct
measurements of boulder transport in natural rivers.
Major floods cause adjustments to the fluvial system after exceeding the limiting
threshold of the former geomorphic system, and hence new long-term equilibrium
conditions develop (Schumm, 1973). The effect of a rare, large-magnitude flood may
last a long time depending on its geomorphic effectiveness and relaxation of recovery
times (Werrity, 1997). River basins affected by "rare great" floods exhibit a range of
threshold phenomena (Newson, 1992), and catastrophic events have received wide
attention as major factors in fluvial system adjustment (Wolman & Miller, 1960;
Costa, 1974; Baker, 1977; Graf, 1983; Kochel, 1988; McEwen & Werrity, 1988). The
catastrophic January 1969 flood in the Upper Jordan River was caused by a rare
climatic event that was made worse due to previous human activities in the area.
The Jordan River channel is composed of gravel and boulders and therefore a high
shear threshold is needed for bed load transport (Baker & Ritter, 1975). The large
boulders are 1500 mm b-axis and even reach 2000 mm, among the biggest sizes
known in fluvial bed material. In Mediterranean and semiarid climates catastrophic
186
Moshe
Inbar
floods play an essential role in modifying the fluvial environment. The effectiveness of
a flood in shaping the fluvial environment "depends upon the force exerted, the return
period of the event, and upon the magnitude of the constructive or restorative
processes which occur in the intervening intervals" (Wolman & Gerson, 1978). In this
paper the short- and medium-term temporal variation in transport rates of the largest
size boulders was estimated from field observations made over a period of 30 years,
from 1969 to 1999.
The aims of this paper are: (a) to evaluate the changes in the planimetric form and
hydraulic geometry of the Jordan boulder channel after the 1969 catastrophic flood;
(b) to analyse the relationship between the hydrological regime and sediment transport
behaviour of the river; and (c) to study the geomorphic impact and long-term
persistence of the 1969 extreme flood in the northern Jordan Valley.
The variability of the channel pattern over a period of 30 years is discussed. After
the main catastrophic event, no major changes were detected during that period. The
1969 flood was a formative event that changed the planimetric pattern of the channels,
but this has remained stable since then. Major annual floods are able to transport
almost all sizes of bed load material. It seems that the effects of the catastrophic event
may last for a long period, decades or probably centuries.
THE PHYSICAL SETTING
The Upper Jordan basin (Fig. 1) is an elongated valley, 90 km in length and between
15 and 30 km in width, covering an area of 1592 km . Its general outline is determined
by the tectonic features of the northern Jordan rift valley, which is part of the Dead Sea
transform. Elevations range from 2814 m at the Mount Hermon summit to -210 m at
Lake Kinneret level, with an average altitude of 823 m. The mean annual precipitation
for the Upper Jordan watershed is 800 mm, varying from 1600 mm in the upper area to
400 mm at the river mouth into the lake. The entire amount falls as winter rainfall,
with about 70% in the December-February period. Snow represents 11% of the total
precipitation but only 7% of total runoff. The runoff/rainfall ratio is 40% and one third
of the total runoff is in the form of floods caused by rainstorms in the northern part of
the catchment area and the Hula Valley. Summer baseflow is low, about 8 m s" , and
somewhat higher during the winter with an average of three to five floods having a
peak discharge of 50 m s" to 150 m s" (Hydrological Service of Israel, 1975). Mean
annual discharge values exceeding 60 m s" occur for only 3% of the year, but in rainy
years they may occur for up to 10% of the year. In dry years even the maximum
discharge value is less than 60m s" , meaning that during those seasons there is no
movement of coarse bed load (Fig. 2).
The mean annual suspended sediment discharge is 70 000 t, or 441 km year" .
Before entering Lake Kinneret, the Jordan River flows in a narrow and deep canyon
entrenched in basalt, which provides boulders and coarse particles for the channel bed
load. Between the canyon and the lake area the channel divides into a number of
permanent and ephemeral channels forming a braided floodplain about 2 km long and
300 m wide (Figs 3 and 4). The braided channel is composed mainly of boulders and
cobbles (Fig. 5). In the lower reach entering Lake Kinneret, the Jordan formed a new
delta after the 1969 flood (Inbar, 1987).
2
3
3
1
3
1
3
3
1
1
1
2
1
Episodes
of flash floods
and boulder
transport
in the Upper Jordan
River
187
Fig. 1 Upper Jordan watershed and location m a p . Inserts are three representative rainfall
stations showing the mean annual rain dishibution. Isohyets are for the January 1969 flood.
THE HISTORY OF FLOODING IN THE UPPER JORDAN RIVER
Rainfall data for the oldest meteorological station, Kefar Gil'adi, covers a period of
80 years starting in 1918, and proxy stations at Beirut and Jerusalem cover almost
150 years. The measured period of 40 years at the Hydrological Station of the Upper
Jordan started in 1959, but an earlier record of about 25 years is available from a nearby
Fig. 3 U p p e r J o r d a n River planimetric pattern from the c a n y o n to the delta area.
WÊBÊÊÊÊtmÊPmKm
WÊÊÊÊÊÊÊKBÊB
F i g . 4 A e r i a l v i e w of the J o r d a n c h a n n e l from the c a n y o n t h r o u g h the b r a i d e d pattern
r e a c h to the delta area.
Moshe
190
Inbar
site, before the drainage of the Hula Lake and swamps. After the drainage, peak discharges
increased. The attenuation effect of the Hula Lake and valley in the downstream trans­
lation of flood waves has been considerably reduced; before the drainage the average
maximum discharge values were 57 m s'\ against 91 m s" for the 1959-1998 period. In
the past, large floods in the Jordan canyon were probably produced by sudden breaching of
slope-landslide damming of floods, probably many centuries ago (Shroder & Inbar, 1995).
The flooding record gives the recurrence interval of the yearly maximum floods (Fig. 6).
3
3
!
1000
w~
1
10
Recurrence Interval - Years
F i g . 6 R e c u r r e n c e interval of yearly m a x i m u m discharges: U p p e r J o r d a n station.
(Station: J o r d a n R i v e r S o u t h e r n St. 1 9 5 9 - 1 9 6 0 - 1997/98).
The measured period includes the 1969 catastrophic flood, which obtained a
recurrence interval of 1:120 to 1:200 years. An analysis of a synthetic series of yearly
volumes (Simon & Yatir, 1981) based on rainfall data and the rainfall-runoff
relationship gives the same order of recurrence interval.
In the 40 years of measurement there were 13 years with peaks above 100 m s ,
and only four events with maximum discharges of 140 m s'\ Water velocities of 3 to
4 ms" were above the threshold movement of 1000 mm boulders (Table 1). The
effective floods for transport of boulders of up to 300 mm b-axis size are those
exceeding 60 m s" . Floods usually took place over a period of 48 hours (Fig. 7).
3
_1
3
1
3
1
HYDRAULIC GEOMETRY CHANGES
A study station downstream of the canyon was established in the Jordan channel in
1973 and monitored since then after major floods. The total channel length is 210 m
and the active channel is 35 m wide near the established hydrometric station, and 50 m
downstream it is 25 m wide. Figure 8 shows the transverse cross sections along a 50 m
channel width, between permanent benchmarks surveyed at intervals of 1 m or less.
The re-surveys of cross sections determined changes in channel width, bed elevations
and thalweg positions, as well as depositional bar changes. The channel bed and
elevated floodplain are composed of basaltic boulder and gravel material. The highest
Episodes
of flash floods and boulder
transport
in the Upper Jordan
River
191
T a b l e 1 H y d r o l o g i c a l data for the J o r d a n River, southern station. S t r e a m p o w e r values are calculated for
the h y d r o m e t r i c station site and 100 m d o w n s t r e a m in a n a r r o w e r a n d steeper channel. Qmax =
m a x i m u m a n n u a l flood.
Year
D a t e o f Qmax
Qmax
( m s" )
3
1
Unit s t r e a m p o w e r
(kg m- s" )
1
1
Unit stream power
d o w n s t t e a m (kg m" s" )
1
1959/60
24 M a r c h 160
39
41.30
82.33
1960/61
1961/62
16 F e b r u a r y 1961
91
79.63
151.67
23 D e c e m b e r 1961
2 6 January 1963
97
84.88
161.67
1962/63
125
109.38
208.33
1963/64
4 A p r i l 1964
110
96.25
183.33
1964/65
9 F e b r u a r y 1965
88
90.59
176.00
1965/66
3 F e b r u a r y 1966
70
94
72.06
140.00
1966/67
2 7 M a r c h 1967
82.25
156.67
1967/68
n o data
130
113.75
216.67
1968/69
23 January 1969
214
174.19
313.36
1969/70
11 M a r c h 1970
112
98.00
186.67
1970/71
18 A p r i l 1971
133
116.38
221.67
1971/72
6 F e b r u a r y 1972
77
79.26
154.00
1972/73
1973/74
8 M a r c h 1973
2 1 January 1974
47
49.84
99.22
97.6
85.40
162.27
1974/75
2 2 F e b r u a r y 1975
85.8
88.32
171.60
1975/76
14 M a r c h 1976
71.1
73.19
142.20
1976/77
8 F e b r u a r y 1977
93.2
81.55
155.33
1977/78
2 7 January 1978
99.2
86.80
165.33
1978/79
9 J a n u a r y 1979
33.6
35.63
70.90
1979/80
1 M a r c h 1980
111
97.13
185.00
1980/81
11 January 1981
110
96.25
183.33
1981/82
2 F e b r u a r y 1982
115
100.63
191.67
1982/83
5 M a r c h 1983
140
113.95
205.00
1983/84
16 April 1984
75.66
147.00
1984/85
5 F e b r u a r y 1985
146
118.84
213.79
1985/86
15 F e b r u a r y 1986
2 6 January 1987
89
91.62
178.00
1986/87
95
83.13
158.33
1987/88
2 F e b r u a r y 1988
125
109.38
208.33
1988/89
16 M a r c h 1989
41.1
43.59
86.76
1989/90
2 M a r c h 1990
37.3
39.56
78.74
1990/91
1991/92
24 M a r c h 1991
73.5
36
38.18
76.00
5 F e b r u a r y 1992
138
112.33
202.07
1992/93
11 J a n u a r y 1993
104
91.00
173.33
1993/94
1994/95
14 F e b r u a r y 1994
2 D e c e m b e r 1994
48.1
91
49.50
93.68
182.00
1995/96
24 January 1996
41
43.48
86.55
1996/97
17 J a n u a r y 1997
15
16.40
30.00
1997/98
31 M a r c h 1998
72
74.12
144.00
Average
90.65
1
96.20
levels of flood peaks were determined from flood marks after flood events or by a
stage level recorder during the experimental years 1973-1976.
Cross section surveys were conducted in the summer low discharge periods of
1974, 1975, 1985, 1986, 1989, 1993, 1994 and 1996, and monitoring included
continuous field observations several times a year in different seasons between 1969
and 1999. The flow during low and medium discharges concentrated at the edges of
192
Moshe
Inbar
Vs
3
Qmax • 134 m /s
il
it
i
r
i
©
Limnigram at Braid
Channel Station
H
m
10.1
\
\
p
r
- 10.0
3
\Qmax*85.7m /s
-
9.9
9.8
9.7
l/r
9.6
u
J'
/
"I
0
1
1
12
20
1—
1
24
Southern Slation
Sede Nehemya
Braided Area St. Limnigra
1
1
12
1
1
1
24
1
12
~1
I
24
1
1
T
1
12
24
1
1
12
1
I
24
h
2!
22
23
24
day
F i g . 7 U p p e r Jordan flood, 2 0 - 2 4 F e b r u a r y 1975. T h e high p e a k b e l o n g s to the u p p e r
station, before entering the H u l a Valley area. T h e h y d r o g r a p h shape for the s o u t h e r n
station and the site 13 k m d o w n s t r e a m of the c a n y o n are similar.
the channel, leaving a high bar or boulder concentration in the middle. During high
flows the entire central bar as well as the left bank—about 6 m width—was covered by
the flow. The right bank was not flooded for the entire period except for the major
1969 flood, but it was on this side that major accretion and avulsion processes were
detected. After 1975 the right bank aggraded about 4-5 m and the left channel was
incised 1 m, moving the thalweg from the right to the left channel. The 1985 flood
flushed the central bar but no major lateral changes were detected. The 1988 flood
eroded the right bank for the same distance, the small channel was filled by 1 m, and
the cross section returned to a shape similar to that 15 years before. The major 1992
flood, with a peak of 138 mV , vs a peak of 125 nrV in 1988, did not change the
channel's hydraulic geometry, which remained the same until 1998, and is similar to
the values determined by the 1969 formative event.
During the 30 year period four events had the ability to transport 99% of the boulder
sizes, but only after the 1985 and 1988 floods were morphological channel changes
noticed. No aggradation or incision occurred during the same period, indicating that
the channel has been in dynamic equilibrium conditions since the 1969 major flood.
1
1
ENERGY SLOPE
A detailed field longitudinal section was measured in 1974, a few years after the 1969
formative event, reflecting the established energy slope since then (Fig. 9). The
2
1-1
0
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
Fig. 9 Longitudinal section along the J o r d a n R i v e r channel thalweg, (a) Longitudinal
section for the channel from the c a n y o n to 870 m d o w n s t r e a m ; slope values range
from 0.025 to 0.038. (b) Longitudinal section and water level during major floods at
the study site; the water surface level increases to 0.046 at high stage b u t decreases to
0.043 with further increases of stage.
average slope along a 900 m distance was 0.0264 but, along reaches of 50 m, slopes of
0.06 were measured. This slope value is exceptionally high for a drainage basin
exceeding 1500 km (Baker & Costa, 1987). Along a distance of 55 m the bed slope
2
,
210m
Episodes
of flash-floods
and boulder
transport
in the Upper Jordan
River
195
was 0.029 and at high discharge the water surface energy slope was 0.046. At higher
stages the water energy slope decreased to 0.043, meaning that there was no linear
relationship between water discharge and water energy slope. Local slopes varied from
1974-1998, but the overall slope did not change and remained at the value of 0.026. At
the hydrometric station the slope value decreased owing to a slight widening of the
channel. It seems that the chamiel and water surface slope adjusts to the width-depth
ratio and boundary resistance in order to provide the power per unit area required at
maximum floods for the efficient transport of all sizes of sediment material (Inbar &
Schick, 1979). During the flood the discharge increase reduced the chamiel roughness,
and then the velocity threshold for the boulder movement was reached.
BOULDER DISTRIBUTION
Detailed sampling was performed in 1974 in the 4 km chamiel reach between the
canyon and the delta, and revealed that there was a rapid distal decrease of sediments.
In the upper area the median size was 300 mm with the largest boulders being over
1500 mm b-axis (Fig. 5). A few hundred metres downstream the median size decreased
to 100 mm and the largest were about 500 mm. No boulders attained the lower reach
and delta area, which is mostly sand and silt. During an almost dry flow in August
1994, 260 boulders, i.e. all visible in the channel with >500 mm b-axis, were measured
in a 200 m reach (Fig. 10). There was no significant change in the boulder size
distribution during both periods. A median size of 1000 mm among all boulders
>500 mm was found (Fig. 11).
500
600
700
800
900
1000
1100
B-axis (mm)
1200
1300
1400
1500
F i g . 10 Distribution of boulders > 5 0 0 m m b-axis size in the c h a n n e l b e d (n = 2 6 0 ) .
1600
196
Moshe
Inbar
Fig. 11 B e d material at the Jordan channel d o w n s t r e a m of the s t u d y site, 1994.
STREAM POWER AND CHANGES IN SEDIMENTARY STRUCTURES
Changes in stream power were examined at two cross sections: along the hydrometric
station transverse section, and 100 m downstream in a narrower and steeper channel
(Table 1). Shear stress and stream power are maximized where the depth-slope
product is maximized, hence in the narrower section there is no deposition of boulders
or bar formation. In the wider cross section a boulder bar was formed during the 1969
event; it persisted for 16 years until the February 1985 flood when trees were flushed,
all sizes of boulders moved, and the bar disappeared.
The main macro form structures developed after the 1969 flood included four units:
longitudinal bars, transverse or megabars, point bars, and diagonal bars. Only the megabars,
which cover a large portion of the flood plain, have not changed during the last 30 years.
A longitudinal bar deposited during the 1969 flood was monitored from its
formation until its removal in 1985 (Fig. 12). During the 16 year period it served as a
sediment trap and a dense willow vegetation covered it (Fig. 13). The 1985 flood, with
a recurrence interval of 1:10 years, moved all sizes of material.
The point bars increased or decreased during the years without a definite trend.
The megabars were affected by weathering processes and vegetation development.
Only their edges were covered by water during major floods, and it seems that only a
very extreme event may change their morphology.
Bed load transport is not limited by sediment availability. Although the depth of
bed load channel layer does not exceed 3-4 m, in the canyon valley there is an active
floodplain of 50 to 100 m width and the total storage exceeds the estimated annual bed
load sediment yield by three orders of magnitude. The slopes of the canyon are active
F i g . 12 Longitudinal b a r with tree vegetation in 1975. N u m b e r e d b o u l d e r s w e r e p a i n t e d a n d m o n i t o r e d for a 4 year period.
F i g . 1 3 Longitudinal bar at study site in 1975. T h e b a r was r e m o v e d in 1985.
Fig. 14 V e g e t a t i o n d e v e l o p m e n t along c h a n n e l b a n k s 20 years later.
Episodes
of flash floods
and boulder
transport
in the Upper Jordan
River
199
and supply most of the sediment evacuated by the river flows. No slope-landslides are
stabilized, and the material reaching the channel is evacuated by the average yearly
floods. Since the 1969 flood, which removed all vegetation and transported a large
amount of the stored sediment, large lateral bars on the canyon floor have become
stabilized and a tree vegetation has developed on them (Fig. 14).
CONCLUSIONS
The January 1969 flood in the Upper Jordan River caused the removal of the sediment
layer in the braided floodplain of the river and the deposition of gravels, boulders, and
a new sand delta at the river mouth into Lake Kinneret. At the study site morphological
changes occurred in the braided pattern during the 30 year period but only in the active
chamiel and did not affect the higher terrace banks. No significant vertical erosion
occurred and lateral accretion and avulsion was measured for a few metres, indicating
a stable adjusted morphology with minor temporary changes. The energy gradient on
the study reach was 0.04 during high discharge floods and did not change with
increasing discharge. Almost all bed load sizes, including boulders of more than
1000 mm in diameter, moved in events with a 1:7 year frequency. An inner bar, which
seemed stabilized by tree vegetation was also removed after a period of 16 years,
indicating that high stream power was achieved, sufficient to transport all bed load
sizes, at a low frequency size of flood. The Jordan channel exemplifies rivers with high
energy, and probably with the largest size of bed load material described in fluvial
morphology.
The 1969 Jordan catastrophic flood had a decisive role in the modification of the
fluvial landform, being a geomorphologically effective event as defined by Wolman &
Gerson(1978).
REFERENCES
Baker, V. R. (1977) Stream channel response to floods with examples from central Texas. Geol. Soc. Am. Bull. 88,
1057-1071.
Baker, V. R. & Costa, E. (1987) Flood power. In: Catastrophic Flooding (ed. by L. Mayer & D. Nash), 1-21. Allen &
Unwin, London, UK.
Baker, V. R. & Ritter, D. F. (1975) Competence of rivers to transport coarse bed-load material. Geol. Soc. Am. Bull. 86,
975-978.
Costa, J. E. (1974) Response and recovery of a Piedmont watershed from tropical storm Agnes. Wat. Resour. Res. 10,
106-112.
Hydrological Service of Israel (1967-1997) Hydrological Yearbooks of Israel. Jerusalem, Israel.
Graf, W. L. (1983) The arroyo problem: paleohydrology and paleohydraulics in the short term. In: Background to
Paleohydrology (ed. by K. J. Gregory), 280-302. John Wiley, Chichester, UK.
Inbar, M. (1987) Effects of a high magnitude flood in a Mediterranean climate: a case study in the Jordan River basin. In:
Catastrophic Flooding (ed. by L. Mayer & D. Nash), 333-353. Allen & Unwin, London, UK.
Inbar, M. & Schick, A. P. (1979) Bedload transport associated with high stream power, Jordan River, Israel. Proc. Nat.
Acad. Sci.. Wash. 76(6), 2515-2717.
Kochel, R. C. (1988) Geomorphic impact of large floods: review and new perspectives on magnitude and frequency. In:
Flood Geomorpholog)' (cd. by V. R. Baker, R. C. Cochel & P. C. Patton), 169-187. John Wiley, Chichester, UK.
McEwen, L. J. & Werrity, A. (1988) The hydrology and long-term geomorphic significance of a flash flood in the
Cairngorm mountains, Scotland. Catena J5, 361-377.
Newson, M. D. (1992) Land, Water and Development. Routledge, London, UK.
Schumm, S. A. (1973) Geomorphic thresholds and the complex response of drainage systems. In: Fluvial Geomorphology
(ed. by M. Morisawa), 299-310. State University of New York, Binghampton, New York, USA.
200
Moshe
Inbar
Shroder, J. F. Jr & Inbar, M. (1995) Geologic and geographic background to the Bethsaida excavations. In: Belhsaida: A
City by the North Shore of the Sea of Galilee (ed. by R. Arav & R. Freund), 65-98.^Thomas Jefferson University
Press, Kirksville, Missouri, USA.
Simon, E. & Yatir, Y. (1981) Synthetic scries of flows in the Upper Jordan catchment (in Hebrew). Tahal Report 1/8I/I02,
Tel Aviv, Israel.
Werrity, A. (1997) Short-term changes in channel stability. In: Applied Fluvial Geomorphology for River Engineering and
Management (ed. by C. R. Thome. R. D. Hey & M. D. Newson), 47-65. John Wiley, Chichester, UK.
Wolman, M. G. & Miller, J. P. (1960) Magnitude and frequency of forces in geomorphic processes. J. Geol. 68, 54-74.
Wolman, M. G. & Gerson, R. (1978) Relative scales of time and effectiveness in watershed geomorphology. Earth Surf
Processes 3, 189-208.