RIVER BANK INSTABILITY, CHANNEL FORMATION ANDRIVER BANK INSTABILITY, CHANNEL FORMATION ANDRATES OF INCISION IN MICRORATES OF INCISION IN MICRO--CANYONSCANYONS

OF THE EXPOSED DEAD SEAOF THE EXPOSED DEAD SEA

Itamar Shapira, Shlomo DaboraItamar Shapira, Shlomo DaboraDan Bowman, Tal Svoray and Jonathan B. LaronneDan Bowman, Tal Svoray and Jonathan B. Laronne

Dept. of Geography and Environmental DevelopmentDept. of Geography and Environmental DevelopmentBen Ben GurionGurion University of the NegevUniversity of the Negev

- 4 2 0

- 4 1 5

- 4 1 0

- 4 0 5

- 4 0 0

- 3 9 5

- 3 9 0

- 3 8 51 8 0 0 1 8 5 0 1 9 0 0 1 9 5 0 2 0 0 0 2 0 5 0

y e a r

altit

ude,

m

2 0 0 3- 4 1 6 . 4

1 9 4 5- 3 9 2 . 4

recession recession of the of the

Dead SeaDead Sea

Ein-feshca

OutlineOutline

11rate of entrenchmentrate of entrenchment--widening widening WadiWadi ArugotArugot

22bank instability & channel bank instability & channel formation in the formation in the springspring--fed fed EinEin FeschaFescha &&

33flash flood flash flood WadiWadi ZeelimZeelim

the the ArugotArugot Fan near Fan near EinEin GediGedi

basin area 232 kmbasin area 232 km22

entrenchment initiated 1963/64entrenchment initiated 1963/64deltaic fan surface slope 1.2deltaic fan surface slope 1.2oo

11.811.8o o on delta front, channel longitudinal slope: 1.1on delta front, channel longitudinal slope: 1.1--3.73.7oo

max entrenchment 16 mmax entrenchment 16 m

measurement of entrenchment and wideningmeasurement of entrenchment and widening

based on EDM, GPS & digital elevation based on EDM, GPS & digital elevation models using the models using the ErdasErdas Imagine Imagine orthobaseorthobase

on stereo aerial photoson stereo aerial photos

photo year

pixel size

control points

pixel RMSE

vertical error*

m xy m 2002 0.16 16 1.05 0.46 1999 0.16 10 1.20 0.65 1995 0.25 10 1.10 0.78 1990 0.25 6 1.10 0.77

Pixel RMSE is given in pixel units Compared to total station; separately also known for GPS

5 x5 x--sectional locations, exemplified by sectional locations, exemplified by

near shore near shore

near bridgenear bridge

change detection maps change detection maps

19901990--1995 1995

19991999--2002 2002

thus:thus:magnitudes of entrenchment and widening are magnitudes of entrenchment and widening are

related not only to flood peaks and volumes, related not only to flood peaks and volumes, but also to their number and timingbut also to their number and timing

period period

length*

peak flow

flow volume

flow duration

vertical incision

widening sea drop*

yr m3/s 106 m3 hr m/yr m/yr m 1990-95 7.7 160 12.2 700 0.59 2.6 3.9 1995-99 2.4 137 4.6 458 1999-02 3.1 418 3.5 179 1995-02 5.5 418 8.1 637 0.65 2.3 5.6 * between flow events

Summary of Summary of WadiWadi ArugotArugot measurements:measurements:

rates of incision are excessively high, rates of incision are excessively high, almost as those of the receding Dead Seaalmost as those of the receding Dead Sea

rates of widening are also very highrates of widening are also very high

these are fundamentally fast rates at this these are fundamentally fast rates at this particular narrow location with respect to particular narrow location with respect to stability of a future bridge. A safe bridge stability of a future bridge. A safe bridge is apparently prohibitively unfeasible.is apparently prohibitively unfeasible.

500m

Ein Fescha – Zukim Nature Reserve

Spring fed constant discharge ~ 75 106 m3/yr

- using total station & stability tables

N S1

2

3

45

the exposed areathe exposed arealongitudinal profile

0

4

8

12

0 100 200 300 400 500

horizontal distance, m

heig

ht. m

014.1

longitudinal profiles oflongitudinal profiles ofexposed area exposed area && streamstream

0

4

8

12

0 100 200 300 400 500

horizontal distance, m

heig

ht, m

stream

exposed area

014.1

052.0

area 1: incision near the shorearea 1: incision near the shore

0

1

20 1 2 3 4

distance, m

heig

ht, m

0

1

2

3

40 2 4 6 8 10 12 14

distance, m

heig

ht, m

area 2: inception of bank failuresarea 2: inception of bank failures

0

2

4

60 5 10 15 20 25 30

distance, m

heig

ht, m

area 3: massive bank failuresarea 3: massive bank failures

0

2

4

60 10 20 30 40

distance, m

heig

ht, marea 4: meandersarea 4: meanders

area 5: vegetation & cementationarea 5: vegetation & cementation

0

2

4

60 2 4 6 8 10 12 14

distance, m

heig

ht, m

types of failurestypes of failures

types of failurestypes of failures

frequency of failuresfrequency of failuresbased on 12 measured xbased on 12 measured x--sectionssections

rotaional slide 41%

slides46%

m.r.s 41%

translation 18%

flow 12%

fall 42%

fall flow translation rotaional slide m.r.s

y = 1.5337 l(x) - 4.9315R2 = 0.93

0

1

2

3

4

5

0 100 200 300 400 500 600horizontal distance, m

entre

nchm

ent,

mmagnitude of entrenchmentmagnitude of entrenchment

with distance from shorewith distance from shore

number of slides vs bank height

0

2

4

6

8

10

0 1 2 3 4 5bank height, m

num

ber o

f slid

es

number of slides vs bank height

0

2

4

6

8

0 1 2 3 4 5

bank height, m

num

ber o

f slid

es

width of deflation zone vs distance from shore

0

10

20

30

40

0 100 200 300 400 500 600

horizontal distance, m

wid

th, m

width of deflation zone vs distance from shore

0

10

20

30

40

0 100 200 300 400 500 600horizontal distance,m

wid

th, m

EinEin FeschaFescha -- conclusionsconclusions!! Failure development is unrelated to direct shear stress of Failure development is unrelated to direct shear stress of

the flowing stream. It is influenced by the lack of the flowing stream. It is influenced by the lack of flushing of most of the failed material in wider sections.flushing of most of the failed material in wider sections.

!! The number of failures depends on bank height and the The number of failures depends on bank height and the width of the deflation zone.width of the deflation zone.

!! Depth of entrenchment increases with distance from the Depth of entrenchment increases with distance from the receding base level; but decreases in proximity to the receding base level; but decreases in proximity to the reserve due to stability, in part regained by vegetation reserve due to stability, in part regained by vegetation and cementation at this point in time.and cementation at this point in time.

!! The question is not whether the nature reserve will be The question is not whether the nature reserve will be entrenched, but when.entrenched, but when.

03≈

Zeelim fan - flash flood regimeZeelim Fan - flash flood regime

- based on a series of cartoons

first mechanism of lateral widening: undercutting

1st stage:

exposure of the sea bed and divergent flow in channels

2nd stage:

entrenchment with widening by preference

3rd stage:

continued entrenchment

4th stage:

initial mechanism of lateral widening, tension cracks occur, continued vertical entrenchment and initial undercutting

5th stage:

advanced undercutting, widening of tensile cracks

and formation of overhanging cliffs

6th stage:

block failure and inclination

second mechanism of lateral widening: sliding

1st stage:

entrenchment and appearance of tensile cracks

2nd stage:

deep entrenchment and widening of tensile cracks

3rd stage:

development of planar failure planes and sliding

third mechanism of lateral widening: toppling

1st stage:

develoment of tensile cracks in high banks

2nd stage:

collapse of bank

toppling

toppling

10.5.2005

11.2.2004

6.1.2004flushing of failedflushing of failedbank materialbank material

6.1.2004

31.3.2005

flushing of failedflushing of failedbank materialbank material

Ze-elim entrenchment takes place on an identical lacustrine deposit to that in Ein Fescha, though the Ze-elim bed is covered by a thin veneer of coarse alluvium that limits faster incision rates

ZeZe--elimelim -- conclusionsconclusions!! Failure development is related to shear stress of the Failure development is related to shear stress of the

flowing stream: undercutting and transport of failed flowing stream: undercutting and transport of failed material. Rotational slides are infrequent material. Rotational slides are infrequent ––unlike at unlike at EinEin FeschaFescha..

!! Bank height at Bank height at ZeZe--elimelim is more than 2is more than 2--fold compared to fold compared to EinEin FeschaFescha, due to large discharges and undercutting , due to large discharges and undercutting availed by flushing of failed material.availed by flushing of failed material.

!! Depth of entrenchment increases with distance from the Depth of entrenchment increases with distance from the receding base level; but decreases in proximity to the receding base level; but decreases in proximity to the upper fan area due to large input of coarse sediment.upper fan area due to large input of coarse sediment.

03≈

width of flow width of flow vsvs distance between banksdistance between banks

R2 = 0.90

R2 = 0.070

20

40

60

80

100

0 20 40 60 80 100 120

distance between banks, m

wid

th o

f flo

w, m

Ein Fescha Ze'elim

distance between banks distance between banks vsvs distance from shoredistance from shore

0

20

40

60

80

100

0 100 200 300 400 500 600

distance from shore line, m

dist

ance

bet

wee

n ba

nks,

m

Ze'elimEin Fescha

bank stability* bank stability* vsvs distance from shoredistance from shore

* Based on the Thorne* Based on the Thorne--SimonSimon--RinaldiRinaldi stability criteriastability criteria

0 100 200 300 400 500 600

distance from shore line, m

stab

ility

con

ditio

n Ze'elimEin Fescha

stable

unstable

Wadi Ogflash flood regime supplemented by

treated sewage base flow

thank you