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Fig. 3. Typical cross-sections (a) just downstream of the breached dike toe and (b) 50 m downstream of the dam showing the pre-flood and post flood channel morphology and the peak water level. Bed and bank erosion from the dam-break flood increased the cross-sections by 60% on average. Rectified aerial photographs at a scale of 1:5000 aided in locating specific areas that experienced significant morphologic change (e.g. considerable scour and deposition) that were of special interest and in delineating the flooded areas (Fig. 1b). Eye-witness reports of local res- idents and professional engineering personnel completed the real-time visual scenario which was later verified in the field by ground surveying and additional photography. In the laboratory, the samples were oven-dried at 105 °C, mechani- cally disaggregated and sieved using sieves with meshes of 0.063, 0.106, 0.145, 0.250, 0.354, 0.5, 0.6, 1.0, 2.0, 4.75 and 8.0 mm for the bank loess, present channel bed and pre-flood soil. For the alluvial fan deposit the sieves used were 0.063, 0.075, 0.180, 0.425, 0.850, 2.0, 4.75 and 8 mm. For all samples grain size distributions (GSDs) were produced using

Figure 3 Typical cross-sections (a) just downstream of the breached dike toe and (b) 50 m downstream of the dam showing the pre-flood and post flood channel morphology and the peak water level. Bed and bank erosion from the dam-break flood increased the cross-sections by 60% on average. Rectified aerial photographs at a scale of 1:5000 aided in locating specific areas that experienced significant morphologic change (e.g. considerable scour and deposition) that were of special interest and in delineating the flooded areas (Fig. 1b). Eye-witness reports of local res- idents and professional engineering personnel completed the real-time visual scenario which was later verified in the field by ground surveying and additional photography. In the laboratory, the samples were oven-dried at 105 °C, mechani- cally disaggregated and sieved using sieves with meshes of 0.063, 0.106, 0.145, 0.250, 0.354, 0.5, 0.6, 1.0, 2.0, 4.75 and 8.0 mm for the bank loess, present channel bed and pre-flood soil. For the alluvial fan deposit the sieves used were 0.063, 0.075, 0.180, 0.425, 0.850, 2.0, 4.75 and 8 mm. For all samples grain size distributions (GSDs) were produced using

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Fig. 2. The breach of Nahal Oz Reservoir and the consequent flood surge into the 1st order channel of Nahal Yare'akh.
Fig. 2. The breach of Nahal Oz Reservoir and the consequent flood surge into the 1st order channel of Nahal Yare'akh.
Fig. 4. (a) Aerial view of the Nahal Oz Reservoir dam failure and the consequent scoured reach. Note the distinct flood marks that shaped the erodible loessial landscape. Cars parking on the white trail to the right below the dike serve for scale. (b) A fresh, vertical 5 mhigh loess bank of the scoured segment of Nahal Yare'akh channel. Note the exposure of eucalypt tree roots hanging in the air and the two debris aprons (view is downstream).
Fig. 4. (a) Aerial view of the Nahal Oz Reservoir dam failure and the consequent scoured reach. Note the distinct flood marks that shaped the erodible loessial landscape. Cars parking on the white trail to the right below the dike serve for scale. (b) A fresh, vertical 5 mhigh loess bank of the scoured segment of Nahal Yare'akh channel. Note the exposure of eucalypt tree roots hanging in the air and the two debris aprons (view is downstream).
Fig. 5. Longitudinal profile of Nahal Yare'akh channel bed before and after the dam-break flood and the water surface profile according to high water marks. Note the substantial irregularities of the water surface slope in comparison to pre-flood bed elevation. XS is the abbreviation for cross-section. GRADISTAT (Blott and Pye, 2001). We averaged the thickness of the flood alluvial fan deposits from the pits and multiplied it by the fan area extracted from the post-flood 1/2500 map to get the fan's sedimen- tary volume. The floodwater uprooted the seasonal grasses, shrub vegetation and young eucalypt trees (Fig. 4a). Only thick-trunked and deeply rooted eucalypt trees were capable of withstanding the water rush (Fig. 4b). Some of them served as major flow obstructions where masses of
Fig. 5. Longitudinal profile of Nahal Yare'akh channel bed before and after the dam-break flood and the water surface profile according to high water marks. Note the substantial irregularities of the water surface slope in comparison to pre-flood bed elevation. XS is the abbreviation for cross-section. GRADISTAT (Blott and Pye, 2001). We averaged the thickness of the flood alluvial fan deposits from the pits and multiplied it by the fan area extracted from the post-flood 1/2500 map to get the fan's sedimen- tary volume. The floodwater uprooted the seasonal grasses, shrub vegetation and young eucalypt trees (Fig. 4a). Only thick-trunked and deeply rooted eucalypt trees were capable of withstanding the water rush (Fig. 4b). Some of them served as major flow obstructions where masses of
Fig. 6. (a) The new braided alluvial fan of Nahal Yare'akh. The fan was created due to a change of gradient of the channel and a local road which hindered the passage of the floodwater. (b) Longitudinal profile along the alluvial fan showing the net sediment aggradation after the flood in relation to pre-flood topography and the location of the stratigraphy pits. Detailed stratigraphy and description of the sediments in the pits are presented in Fig. 7b and Table 1. In contrast to natural semiarid and arid alluvial fans with either al- ternating varved, lake sediments or fining downstream alluvial fan se- quences (Blissenbach, 1954), the stratigraphy of the present fan is somewhat chaotic (Fig. 7a, b). This is probably related to the single peak of the flood and deposition by an individual event by distributary channels and also because the flow pattern induced a strong jet current into constantly draining water body (see Laronne, 2000 for the strati- graphical difference between a regular operating reservoir in a semiarid environment and a constantly draining one). The continuous draining of the water body below the alluvial fan is a consequence of the small square concrete box culvert (1 m) (Fig. 1b) below the paved road An abrupt change in water elevation occurred some 1300 m down- stream from the reservoir due to backwater ponding of the floodwater downstream within the alluvial fan. This was caused by the blockage of the channel by a perpendicular, 1.5 m high paved road which caused flow expansion creating a large pond and alluvial fan upstream (Fig. 1b). The triangular alluvial fan (Fig. Ga) spans over an area of 0.2 km?, with a radial length of 0.55 km and maximal width of 0.7 km. Gradients were determined wherever there was a break in slope. The average
Fig. 6. (a) The new braided alluvial fan of Nahal Yare'akh. The fan was created due to a change of gradient of the channel and a local road which hindered the passage of the floodwater. (b) Longitudinal profile along the alluvial fan showing the net sediment aggradation after the flood in relation to pre-flood topography and the location of the stratigraphy pits. Detailed stratigraphy and description of the sediments in the pits are presented in Fig. 7b and Table 1. In contrast to natural semiarid and arid alluvial fans with either al- ternating varved, lake sediments or fining downstream alluvial fan se- quences (Blissenbach, 1954), the stratigraphy of the present fan is somewhat chaotic (Fig. 7a, b). This is probably related to the single peak of the flood and deposition by an individual event by distributary channels and also because the flow pattern induced a strong jet current into constantly draining water body (see Laronne, 2000 for the strati- graphical difference between a regular operating reservoir in a semiarid environment and a constantly draining one). The continuous draining of the water body below the alluvial fan is a consequence of the small square concrete box culvert (1 m) (Fig. 1b) below the paved road An abrupt change in water elevation occurred some 1300 m down- stream from the reservoir due to backwater ponding of the floodwater downstream within the alluvial fan. This was caused by the blockage of the channel by a perpendicular, 1.5 m high paved road which caused flow expansion creating a large pond and alluvial fan upstream (Fig. 1b). The triangular alluvial fan (Fig. Ga) spans over an area of 0.2 km?, with a radial length of 0.55 km and maximal width of 0.7 km. Gradients were determined wherever there was a break in slope. The average
Fig. 7. (a) An example of one of the pits (#3) dug in the alluvial fan. Pen for scale. (b) The sedimentary structure of the alluvial fan units as represented at 6 stratigraphy pits located along the axis of the fan. Detailed description of the sedimentary units is presented in Table 1.
Fig. 7. (a) An example of one of the pits (#3) dug in the alluvial fan. Pen for scale. (b) The sedimentary structure of the alluvial fan units as represented at 6 stratigraphy pits located along the axis of the fan. Detailed description of the sedimentary units is presented in Table 1.
Description of sedimentary units exposed in the 6 stratigraphy pits (Figs. 6b, 7a,b) dug along the alluvial fan. Table 1
Description of sedimentary units exposed in the 6 stratigraphy pits (Figs. 6b, 7a,b) dug along the alluvial fan. Table 1
Fig. 8. An aerial view of the flood within Nahal Hanoun, 9 km downstream of the dam breach. Although the flood at that location was still above bankfull discharge it had already lost about 90% of its peak discharge.
Fig. 8. An aerial view of the flood within Nahal Hanoun, 9 km downstream of the dam breach. Although the flood at that location was still above bankfull discharge it had already lost about 90% of its peak discharge.
Fig. 9. Grain size distributions (GSDs) of the various channel components of Nahal Yare'akh. These fine-textured, friable sediments pose only a minor resistance to fluvial erosion. this semiarid environment. Jerolmack and Paola (2010) modeled the dampening of the geomorphic signal as ‘morphodynamic turbulence’ when two different time-dependent regimes converge and one smears the other. Furthermore, Van De Wiel and Coulthard (2010) assert from a The variable fluvio-sedimentary response casts doubt on the ability to recognize catastrophic floods in the stratigraphic fluvial record of
Fig. 9. Grain size distributions (GSDs) of the various channel components of Nahal Yare'akh. These fine-textured, friable sediments pose only a minor resistance to fluvial erosion. this semiarid environment. Jerolmack and Paola (2010) modeled the dampening of the geomorphic signal as ‘morphodynamic turbulence’ when two different time-dependent regimes converge and one smears the other. Furthermore, Van De Wiel and Coulthard (2010) assert from a The variable fluvio-sedimentary response casts doubt on the ability to recognize catastrophic floods in the stratigraphic fluvial record of
Fig. 10. The recovered state of the scoured reach, 8 years after the dam-break flood. (a) The active “inner channel” flows within a 50-100 m wide valley which is the actual channel geometry of the dam-break flood. The vertical loess wall on the left bank marks the border of the peak discharge. (b) A close-up view of the new, straight, slightly incised, sandy channel adjusted to the natural flow regime.
Fig. 10. The recovered state of the scoured reach, 8 years after the dam-break flood. (a) The active “inner channel” flows within a 50-100 m wide valley which is the actual channel geometry of the dam-break flood. The vertical loess wall on the left bank marks the border of the peak discharge. (b) A close-up view of the new, straight, slightly incised, sandy channel adjusted to the natural flow regime.
Fig. 11. (a) Comparison of flood peak attenuation for selected dam-break flood studies. It is evident that an ephemeral stream hydrograph such as Nahal Oz dam-break wanes much quicker than a perennial river with a saturated alluvium and floodplain. Data from: Gleno dam-break (Pilotti et al., 2011), Lawn Lake dam-break (Jarrett and Costa, 1986), Soda Butte Creek (Marcus et al., 2001), Buffalo Creek dam-break (Costa, 1988); Kelly Barnes dam-break (Sanders and Sauer, 1979; Costa, 1988); Schaeffer dam-break (Costa, 1988), Castlewood dam-break (Costa, 1988), Little Deer Creek dam-break (Costa, 1988), Teton dam-break (Scott, 1977; Costa, 1988), Quail Creek dam-break (Enzel et al., 1994) and Big Bay dam-break (Yochum et al., 2008).(b) Comparison of two dam-break envelope attenuation curves (Costa, 1988; Washington State Department of Ecology, 2007) to measured transmission losses of two Australian ephemeral streams using the peak outflow discharge (Dunkerley, 1992; Dunkerley and Brown, 1999) and the current case study of Nahal Oz dam-break flood. All four curves underestimate the high transmission losses. The equations used are described in the text.
Fig. 11. (a) Comparison of flood peak attenuation for selected dam-break flood studies. It is evident that an ephemeral stream hydrograph such as Nahal Oz dam-break wanes much quicker than a perennial river with a saturated alluvium and floodplain. Data from: Gleno dam-break (Pilotti et al., 2011), Lawn Lake dam-break (Jarrett and Costa, 1986), Soda Butte Creek (Marcus et al., 2001), Buffalo Creek dam-break (Costa, 1988); Kelly Barnes dam-break (Sanders and Sauer, 1979; Costa, 1988); Schaeffer dam-break (Costa, 1988), Castlewood dam-break (Costa, 1988), Little Deer Creek dam-break (Costa, 1988), Teton dam-break (Scott, 1977; Costa, 1988), Quail Creek dam-break (Enzel et al., 1994) and Big Bay dam-break (Yochum et al., 2008).(b) Comparison of two dam-break envelope attenuation curves (Costa, 1988; Washington State Department of Ecology, 2007) to measured transmission losses of two Australian ephemeral streams using the peak outflow discharge (Dunkerley, 1992; Dunkerley and Brown, 1999) and the current case study of Nahal Oz dam-break flood. All four curves underestimate the high transmission losses. The equations used are described in the text.
* Visual estimation from an aerial photograph immediately after the dam-break flood. > Exposed loess surfaces. Recovery of the vegetation along the scoured reach and alluvial fan based on aerial photography (Fig. 12).
* Visual estimation from an aerial photograph immediately after the dam-break flood. > Exposed loess surfaces. Recovery of the vegetation along the scoured reach and alluvial fan based on aerial photography (Fig. 12).
Katz et al. (2005) have shown that unless a large flood occurs during the lifetime of a dam its effect on the riparian vegetation downstream is minor. The great effect of large floods on the riparian vegetation such as the Nahal Oz dam-break flood is explained by the aridity and the small size of the watershed which controls the age, structure and spatial dis- tribution of the disturbed riparian vegetation (Friedman and Lee, 2002). The older, planted eucalypt trees were the only vegetation survi- vors from the dam-break flood and their seedlings became quickly established on the moist loess soil. The lower vegetation (grasses and shrubs) were slower in recovering until taking over most of the wadi floor. Only the narrow, active sandy channel bed remained Extensive studies about transmission losses along the larger ephem- eral streams of the Negev Desert and Arava were conducted by Lange et al. (1998, 1999, 2000) and Shentsis et al. (1999, 2001a,b)Schwartz, 2001, but these are irrelevant to our study not only because of the differ- ent basin lithologies, climate, and land use, but also because they were correlated to a rainfall-runoff model. Earthen embankment breaching is a relatively slow-developing process (unlike instantaneous concrete dams failure) until the breach reaches its final form (Task Committee on Dam/Levee Breaching Processes, 2011). The most common hydrograph fitted to this type of flood, therefore, is a triangular hydrograph shape with a rather short rising limb reaching a high peak discharge followed by a longer recession limb. This type of hydrograph Fig. 12. Aerial view of the Nahal Oz Reservoir and the downstream Nahal Yare'akh channel 2 years, 5 years and 7 years after the catastrophic dam-break flood. It is evident, especially in th latter two photos, that vegetation covered most of the eroded valley floor and only the wadi bed remains bare.
Katz et al. (2005) have shown that unless a large flood occurs during the lifetime of a dam its effect on the riparian vegetation downstream is minor. The great effect of large floods on the riparian vegetation such as the Nahal Oz dam-break flood is explained by the aridity and the small size of the watershed which controls the age, structure and spatial dis- tribution of the disturbed riparian vegetation (Friedman and Lee, 2002). The older, planted eucalypt trees were the only vegetation survi- vors from the dam-break flood and their seedlings became quickly established on the moist loess soil. The lower vegetation (grasses and shrubs) were slower in recovering until taking over most of the wadi floor. Only the narrow, active sandy channel bed remained Extensive studies about transmission losses along the larger ephem- eral streams of the Negev Desert and Arava were conducted by Lange et al. (1998, 1999, 2000) and Shentsis et al. (1999, 2001a,b)Schwartz, 2001, but these are irrelevant to our study not only because of the differ- ent basin lithologies, climate, and land use, but also because they were correlated to a rainfall-runoff model. Earthen embankment breaching is a relatively slow-developing process (unlike instantaneous concrete dams failure) until the breach reaches its final form (Task Committee on Dam/Levee Breaching Processes, 2011). The most common hydrograph fitted to this type of flood, therefore, is a triangular hydrograph shape with a rather short rising limb reaching a high peak discharge followed by a longer recession limb. This type of hydrograph Fig. 12. Aerial view of the Nahal Oz Reservoir and the downstream Nahal Yare'akh channel 2 years, 5 years and 7 years after the catastrophic dam-break flood. It is evident, especially in th latter two photos, that vegetation covered most of the eroded valley floor and only the wadi bed remains bare.
Please cite this article as: Bergman, N., et al., The Nahal Oz Reservoir dam-break flood: Geomorphic impact on a small ephemeral loess-channe the semi-arid Negev Desert, Israel, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2013.12.024
Please cite this article as: Bergman, N., et al., The Nahal Oz Reservoir dam-break flood: Geomorphic impact on a small ephemeral loess-channe the semi-arid Negev Desert, Israel, Geomorphology (2014), http://dx.doi.org/10.1016/j.geomorph.2013.12.024
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Quaternary GeologyAeolian GeomorphologyAeolian sedimentology
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