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Morphometric analysis and flash floods assessment were conducted for the watersheds of Ras En Naqb escarpment, south Jordan. The study area comprises of twelve small watersheds occupying the faulted-erosional slopes, and the dip slopes. The drainage network shows that dendritic and sub-dendritic patterns dominated the dip slopes, whereas trellis pattern characterized the faulted-erosional slopes. Stream orders range from fourth to sixth order. The mean bifurcation ratios vary between 4.2 and 5.38 for the dip slope basins, and between 3.5 and 5.0 for the faulted-erosional slope watersheds, indicating a noticeable influence of structural disturbances (
i.e., faulting and uplifting), and rejuvenation of drainage networks. All watersheds have short basin lengths, ranging from 23.8 km to 42.2 km for the dip slope basins, and between 15.3 km and 45.4 km for the faulted-erosional slope catchments. This is indicative of high flooding susceptibility associated with heavy rainstorms of short duration. The circularity ratios range from 0.177 to 0.704 which denote that the catchments are moderately circular on the faulted-erosional slopes, and to some extent elongated on the dip slopes. The length of overland flow values ranges from 0.854 to 0.924 for the dip slope catchments, whereas L
_{O} values for the faulted-erosional slopes vary from 0.793 to 0.945 denoting steep slopes and shorter paths on both dip slope and faulted-erosional slope watersheds. Values of stream frequency range from 1.509 to 1.692 for the dip slope, and from 1.688 to 2.0 for the faulted-erosional slope catchments. F
_{S} values are also indicative of slope steepness, low infiltration rate, and high flooding potential. The watersheds of the dip slopes show lower values of form factor varying from 0.079 to 0.364, indicating elongated shape and suggesting a relatively flat hydrograph peak for longer duration. Similarly, values of D
_{d} are high for catchments on the dip slope basins (1.709 - 1.85) and the faulted-erosional slope watersheds (1.587 - 2.0) indicating highly dissected topography, high surface runoff, low infiltration rate, and consequently high flooding potential. Furthermore, high relief values exist, ranging from 388 m to 714 m for the dip slope basins, and from 421 m to 846 m for the faulted-erosional slope catchments indicting high relief and steep slopes. Morphometric analysis, and flash flood assessment suggest that ten watersheds (83.3%) are categorized under high and intermediate flooding susceptibility, and the faulted-erosional slope catchments are more hazardous in terms of flooding. Thus the protection of Ma’an, El Jafr rural Bedouin settlements, and Amman-Aqaba highway from recurrent flooding is essential to ensure sustainable future development in Ras En Naqb-Ma’an area.

Drainage basins represent fundamental geomorphic units for hydrological management and sustainable natural resources development. Geological and morphological setting, topography, and climate constitute major physical factors controlling the geometry of fluvial system, drainage systems and density. Variations in physical conditions occasionally resulted in variations of morphometric characteristics of drainage basins and the associated fluvial systems [

The Ras En Naqb escarpment covers an area of approximately 566 km^{2}, and is located between 35˚28'E to 35˚83'E longitude, and 29˚47'N to 30˚06'N latitude (

Cambrian, Ordovisian and lower Cretaceous Kurnub sandstones and shale (

Recurrent landslide movement is characteristic of Ras En Naqb escarpment, and occasionally threatened the Amman-Aqaba highway. Oversteepening of slopes due to fluvial erosion, differential weathering, and steep cut slopes enhance rock falls and rock slides from heavily jointed limestone beds. Percolation of water from relatively abundant rainfall over Ras En Naqb (140 mm of annual rainfall/snow), and the repetitive heavy rainstorms towards impervious marl, shale and clay beds of the nodular limestone unit reduce the cohesion of the materials, thus encouraging landslide activity. The dip slope of the Ras En Naqb escarpment is composed of lower Cretaceous marl, shale and chalk, sloping 3˚ to 5˚ NNE towards Ma’an. A number of monoclinal flexures exist here instead of faulting, and graben structures dominate a considerable part of the escarpment (

morphological unit due to the presence of soft rock units, i.e. the Kurnub sandstone and shale, and the lower Cretaceous limestone, marl and shale. These rock units are heavily faulted and jointed. Along with recurrent intense rainstorms, they are deemed a major factor influencing slope instability. Old and fresh landslide scars exist, which indicate that landsliding is an active process at present, and threatens the Amman-Aqaba highway once every few years.

Morphometric analysis for Ras En Naqb watersheds was conducted using topographic maps with a scale 1:50,000 (20 m contour interval), ASTER DEM, and Arc GIS 10.1 software package. ASTER DEM is provided on line cost free to all researchers, and is available in Geo Tiff format, with geographic latitude/longitude coordinates at 1 arc-second, approximately 30 m grid cell size. The elevation error of the DEM is reported to be +7.4 m for forest land cover, and −0.7 m for bare land and complex terrain resembling the Ras En Naqb area. Furthermore, the horizontal error is stated to be an East/West shift of −0.13 arc-seconds and North/South shift of 0.19 arc-seconds when compared to the 10-m mesh DEM produced by the Geographical Survey Institute (GIS) of Japan [_{b}), perimeter (P), stream order (u), stream number (N_{u}), stream length (L_{u}), were measured directly from the DEM using GIS software. Other parameters including bifurcation ratio (R_{b}), drainage density (D_{d}), drainage frequency (F_{s}), length of overland flow (L_{o}), circularity ratio (R_{c}), elongation ratio (R_{e}), basin relief (B_{h}), relief ratio (R_{r}), form factor (R_{f}), and shape factor (B_{s}) were calculated based on mathematical equations illustrated in _{c}) and hypsometric integral (H_{i}) were prepared earlier using topo sheets at a scale 1:50,000 [

To assess flash floods potential for the Ras En Naqb watersheds, El-Shamy’s approach was adopted [_{b}) and drainage density (D_{d}) whereas the second approach employed the relationship between bifurcation ratio (R_{b}) and stream frequency (F_{s}). Drainage density (D_{d}) refers to relief dissection, runoff potential, infiltration capacity of surface materials, climate, and land cover of the watershed. Accordingly, low values of D_{d} indicate optimal conditions of infiltration, thus decreasing runoff potential, while, high stream frequency (F_{s}) represents impermeable sub-surface materials, poor vegetation cover, high relief, and low infiltration capacity, thus, increasing runoff potential [

Applying, this relationship separately to each catchment, will provide reasonable estimation on flooding risk, and groundwater aquifer recharge. The resultant illustrations for D_{d} vs. R_{b} and F_{s} vs. R_{b} have to be plotted graphically, where each illustration contains two curves dividing the area into three zones which can be described as follows:

・ Zone A represents low probability of floods and high groundwater aquifer recharge.

・ Zone B refers to catchments with intermediate possibility of floods and moderate potential for groundwater aquifer recharge.

・ Zone C indicates high possibility of floods and low recharge potential.

If a watershed has two different fields, then the appropriate classification plot has been selected.

Quantitative analysis of the 12 watersheds developed on Ras En Naqb escarpment was implemented based on 23 morphometric variables which represents drainage network, geometry, texture and relief aspects of the catchment. The drainage pattern is dendritic to sub-dendritic type on dip slopes, whereas, trellis pattern dominates the faulted-erosional slopes. In the present study, stream ordering for the Ras En Nagb watersheds has been ranked according to Strahler’s method of the hierarchical ranking system [

The calculated morphometric parameters are illustrated in

Morphometric parameters | Formula/definition | References |
---|---|---|

I. Drainage network | ||

1) Stream order (u) | Hierarchical rank | [ |

2) No. of streams (N_{u}) | N = N_{1} + N_{2} + ∙∙∙ + N_{n} | [ |

3) Stream length (L_{u}) km | Lu = L_{1} + L_{2} + ∙∙∙ + L_{n} (km) | [ |

4) Mean stream length (L_{sm}) km | L_{sm} = L_{u}/N_{u} (km) | [ |

5) Stream length ratio (R_{L}) | R_{L} = L_{u}/L_{u−1}, where L_{u} = the total stream length of order “u”, L_{u−1} = the total stream length of its next lower order | [ |

6) Bifurcation ratio (R_{b}) | R_{b} = N_{u}/N_{u+1}, where N_{u} = total no. of stream segments of order “u”, N_{u+1} = no. of segments of the next higher order | [ |

7) Mean bifurcation ratio (R_{bm}) | R_{bm} = average of bifurcation ratio of Strahler all orders | [ |

II. Basin geometry | ||

8) Basin length (L_{b}) km | Length of the basin (km) | [ |

9) Basin area (A) km^{2} | Plan area of the watershed (km^{2}) | [ |

10) Basin perimeter (P) km | Perimeter of the watershed (km) | [ |

11) Form factor (ratio) (R_{f}) | [ | |

12) Elongation ratio (R_{e}) | [ | |

13) Shape factor (B_{s}) | [ | |

14) Lemniscate ratio (k) | K = L^{2}/4A | [ |

15) Circularity ratio (R_{c}) | R_{c} = 4*π*A/P^{2} | [ |

16) Drainage texture (D_{t}) | D_{t} = N_{u}/P, where N_{u} = Total no. Streams of all orders, P = perimeter (km) | [ |

III. Drainage texture analysis | ||

17) Stream frequency (F_{s}) | F_{s} = N_{u}/A | [ |

18) Drainage density (D_{d}) km/km^{2} | D_{d} = L_{u}/A | [ |

19) Drainage intensity (D_{i}) | D_{i} = F_{s}/D_{d} | [ |

20) Length of overland flow (L_{o}) km | L_{o} = 1/2 D_{d} | [ |

IV. Relief characteristics | ||

21) Basin relief (B_{h}) or total relief (H) m | B_{h} = h − h_{1}, where, h = maximum height (m), h_{1} = minimum height (m) | [ |

22) Relief ratio (R_{r}) | R_{r} = H/L_{b}, Where H = total relief, L_{b} = basin length | [ |

23) Ruggedness number (R_{n}) | R_{n} = D_{d}*(B_{h}/1000) | [ |

24) Dissection index (D_{is}) | Dis = B_{h}/Ra, where R_{a} = absolute relief | [ |

25) Hypsometric curve (HC) | HC is achieved by plotting the proportion of the total height (h/H) against the proportion of the total area (a/A) of the basin, where H is the total relief height, a is the total area of the basin above a given line of elevation h. | [ |

26) Hypsometric integral (Hi) | [ |

order (

The total number of streams (N_{u}) for the 12 watersheds is 2096, and the first order streams account for 79.3% of the total number of streams in all basins. The details of stream characteristics are ascertained by Horton’s first law [_{u} are plotted on an ordinary graph (

1) Stream length (L_{u}) is a significant hydrological property and indicative of runoff characteristics, geomorphic development of stream segments, and tectonic instability. Generally, the higher the order, the longer the length of stream in nature. The steam length has been calculated according to the law elaborated by [

Par. No. | Morphometric parameters | W.Jamam | W.Hanout | W.Jaded | W.Ghafir | W.Hafir | W.Rabigh |
---|---|---|---|---|---|---|---|

1 | Stream order (u) | V | V | V | V | V | V |

2 | No. of streams (N_{u}) (Total) | 405 | 345 | 193 | 333 | 166 | 180 |

3 | Stream length (L_{u}) (Total) Km | 421.253 | 302.434 | 181.362 | 321.742 | 168.939 | 169.140 |

4 | Mean stream length (L_{sm}) (km) | 1.040 | 0.876 | 0.939 | 0.966 | 1.017 | 0.939 |

5 | Mean bifurcation ratio (R_{bm}) | 3.752 | 3.987 | 5.014 | 4.439 | 4.208 | 3.812 |

6 | Basin perimeter (P) (km) | 86.710 | 62.717 | 59.179 | 57.898 | 45.548 | 51.266 |

7 | Basin length (L_{b}) | 25.419 | 23.256 | 20.965 | 18.964 | 15.900 | 15.309 |

8 | Basin area (A) (km^{2}) | 222.756 | 172.291 | 114.271 | 188.000 | 97.193 | 98.410 |

9 | Basin relief (B_{h}) (m) | 825 | 846 | 786 | 730 | 560 | 421 |

10 | Relief ratio (R_{r}) | 0.032 | 0.037 | 0.037 | 0.038 | 0.035 | 0.027 |

11 | Elongation ratio (R_{e}) | 0.662 | 0.636 | 0.575 | 0.815 | 0.699 | 0.730 |

12 | Circularity ratio (R_{c}) | 0.372 | 0.550 | 0.409 | 0.704 | 0.588 | 0.470 |

13 | Lemniscate ratio (k) | 0.725 | 0.784 | 0. 961 | 0.478 | 0.650 | 0.595 |

14 | Drainage density (D_{d}) (km/km^{2}) | 1.891 | 1.755 | 1.587 | 1.711 | 1.738 | 1.718 |

15 | Stream frequency (F_{s}) | 1.818 | 2.00 | 1.688 | 1.771 | 1.707 | 1.829 |

16 | Form factor (R_{f}) | 0.344 | 0.318 | 0.259 | 0.522 | 0.313 | 0.419 |

17 | Shape factor (B_{s}) | 2.900 | 3.139 | 3.846 | 1.912 | 2.601 | 2.381 |

18 | Drainage texture (Dt) | 4.670 | 5.500 | 3.261 | 5.751 | 3.644 | 3.511 |

19 | Dissection index (D_{Is}) | 0.504 | 0.508 | 0.489 | 0.472 | 0.392 | 0.326 |

20 | Ruggedness number (R_{n}) | 1.560 | 1.484 | 1.247 | 1.249 | 0.973 | 0.723 |

21 | Drainage intensity (D_{I}) | 0.961 | 1.139 | 1.063 | 1.035 | 0.982 | 1.064 |

22 | Length of over land flow (L_{o}) Km | 0.945 | 0.877 | 0.793 | 0.855 | 0.869 | 0.859 |

23 | Hypsometric integral (H_{i})^{*} | 0.720 | 0.700 | 0.740 | 0.800 | 0.800 | 0.77 |

^{*}Source: [

Par. No. | Morphometric parameters | W.Wuheida | W.Huseinan | W.Tawayil el Hamd | W.el Batra | W.Aub Tarfa 1 | W.Aub Tarfa 2 |
---|---|---|---|---|---|---|---|

1 | Stream order (u) | VI | IV | VI | V | VI | IV |

2 | No. of streams (N_{u}) (Total) | 356 | 239 | 722 | 412 | 786 | 189 |

3 | Stream length (L_{u}) (Total) Km | 402.657 | 246.086 | 863.549 | 455.759 | 881.136 | 204.957 |

4 | Mean stream length (L_{sm}) (km) | 1.131 | 1.029 | 1.196 | 1.106 | 1.121 | 1.084 |

5 | Mean bifurcation ratio (R_{bm}) | 4.212 | 4.825 | 4.595 | 4.573 | 5.010 | 5.388 |

6 | Basin perimeter (P) (km) | 73.981 | 99.998 | 106.900 | 108.828 | 99.539 | 73.857 |

7 | Basin length (L_{b}) | 23.815 | 42.193 | 41.609 | 35.353 | 37.984 | 30.084 |

8 | Basin area (A) (km^{2}) | 235.583 | 141.246 | 466.985 | 257.098 | 491.850 | 118.284 |

9 | Basin relief (B_{h}) (m) | 514 | 714 | 682 | 673 | 532 | 388 |

10 | Relief ratio (R_{r}) | 0.021 | 0.016 | 0.016 | 0.019 | 0.014 | 0.012 |

11 | Elongation ratio (R_{e}) | 0.802 | 0.317 | 0.585 | 0.511 | 0.658 | 0.402 |

12 | Circularity ratio (R_{c}) | 0.540 | 0.177 | 0.513 | 0.272 | 0.623 | 0.272 |

13 | Lemniscate ratio (k) | 0.494 | 3.150 | 0.926 | 1.215 | 0.773 | 1.960 |

14 | Drainage density (D_{d}) (km/km^{2}) | 1.709 | 1.742 | 1.849 | 1.772 | 1.791 | 1.732 |

15 | Stream frequency (F_{s}) | 1.509 | 1.692 | 1.546 | 1.602 | 1.598 | 1.597 |

16 | Form factor (R_{f}) | 0.364 | 0.079 | 0.269 | 0.205 | 0.340 | 0.127 |

17 | Shape factor (B_{s}) | 1.977 | 120.603 | 3.707 | 4.613 | 2.933 | 7.843 |

18 | Drainage texture (Dt) | 4.812 | 2.930 | 6.753 | 3.785 | 7.896 | 2.558 |

19 | Dissection index (D_{Is}) | 0.297 | 0.427 | 0.424 | 0.411 | 0.370 | 0.286 |

20 | Ruggedness number (R_{n}) | 0.878 | 1.230 | 1.261 | 1.192 | 0.952 | 2.404 |

21 | Drainage intensity (D_{I}) | 0.882 | 0.971 | 0.836 | 0.904 | 0.892 | 0.922 |

22 | Length of over land flow (L_{o}) Km | 0.854 | 0.871 | 0.924 | 0.886 | 0.895 | 0.866 |

23 | Hypsometric integral (H_{i})^{*} | 0.900 | 0.920 | 0.850 | 0.850 | 0.860 | 0.840 |

^{*}Source: [

Wadi Jamam watershed | ||||||
---|---|---|---|---|---|---|

Par. No. | Morphometric parameters | Stream order | ||||

1 | Stream order (u) (5) | I | II | III | IV | V |

2 | No. of streams (N_{u}) (Total) (405) | 307 | 73 | 19 | 5 | 1 |

3 | Stream length (L_{u}) (Total) (421.253 km) | 200.120 | 123.388 | 62.573 | 18.140 | 17.032 |

4 | Mean stream length (L_{sm}) (1.040 km) | 0.651 | 1.690 | 3.293 | 3.628 | 17.032 |

5 | Stream length ratio(R_{L}) | 0.616 II/I | 0.507 III/II | 0.289 IV/III | 0.938 V/IV | |

6 | Bifurcation ratio(R_{b}) | 4.205 I/II | 3.842 II/III | 3.800 III/IV | 5 IV/V | |

Wadi Hanout watershed | ||||||

Par. No. | Morphometric parameters | Stream order | ||||

1 | Stream order (u) (5) | I | II | III | IV | V |

2 | No. of stream order (N_{u}) (Total) (345) | 260 | 66 | 15 | 3 | 1 |

3 | Stream length (L_{u}) (Total) (302.434 km) | 148.428 | 72.734 | 39.358 | 17.312 | 24.602 |

4 | Mean stream length (L_{sm}) (0.876 km) | 0.570 | 1.102 | 2.623 | 5.770 | 24.602 |

5 | Stream length ratio (R_{L}) | 0.490 II/I | 0.541 III/II | 0.439 IV/III | 1.421 V/IV | |

6 | Bifurcation ratio (R_{b}) | 3.939 I/II | 4.400 II/III | 5 III/IV | 3 IV/V | |

Wadi Jaded watershed | ||||||

Par. No. | Morphometric parameters | Stream order | ||||

1 | Stream order (u) (5) | I | II | III | IV | V |

2 | No. of streams (N_{u}) (Total) (193) | 154 | 29 | 7 | 2 | 1 |

3 | Stream length (L_{u}) (Total) (181.362 km) | 91.476 | 46.734 | 17.944 | 11.970 | 13.238 |

4 | Mean stream length (L_{sm}) (0.939 km) | 0.594 | 1.611 | 2.563 | 5.985 | 13.238 |

5 | Stream length ratio (R_{L}) | 0.510 II/I | 0.383 III/II | 0.667 IV/III | 1.105 V/IV | |

6 | Bifurcation ratio (R_{b}) | 5.310 I/II | 4.142 II/III | 3.500 III/IV | 2 IV/V | |

Wadi Ghafir watershed | ||||||

Par. No. | Morphometric parameters | Stream order | ||||

1 | Stream order (u) (5) | I | II | III | IV | V |

2 | No. of streams (N_{u}) (Total) (333) | 258 | 59 | 12 | 3 | 1 |

3 | Stream length (L_{u}) (Total) (321.742 km) | 166.076 | 87.640 | 29.834 | 25.548 | 12.644 |

4 | Mean stream length (L_{sm}) (0.966 km) | 0.643 | 1.485 | 2.486 | 8.516 | 12.644 |

5 | Stream length ratio (R_{L}) | 0.527 II/I | 0.340 III/II | 0.856 IV/III | 0.494 V/IV | |

6 | Bifurcation ratio (R_{b}) | 4.372 I/II | 4.916 II/III | 4 III/IV | 3 IV/V |

Wadi Hafir watershed | ||||||
---|---|---|---|---|---|---|

Par. No. | Morphometric parameters | Stream order | ||||

1 | Stream order (u) (5) | I | II | III | IV | V |

2 | No. of streams (N_{u}) (Total) (166) | 125 | 31 | 6 | 3 | 1 |

3 | Stream length (L_{u}) (Total) (168.939 km) | 80.343 | 51.763 | 13.563 | 11.123 | 12.147 |

4 | Mean stream length (L_{sm}) (1.017 km) | 0.642 | 1.669 | 2.260 | 3.707 | 12.147 |

5 | Stream length ratio (R_{L}) | 0.644 II/I | 0.262 III/II | 0.820 IV/III | 0.1.092 V/IV | |

6 | Bifurcation ratio (R_{b}) | 4.032 I/II | 5.166 II/III | 2 III/IV | 3 IV/V | |

Wadi Rabigh watershed | ||||||

Par. No. | Morphometric parameters | Stream order | ||||

1 | Stream order (u) (5) | I | II | III | IV | V |

2 | No. of streams (N_{u}) (Total) (180) | 133 | 34 | 9 | 3 | 1 |

3 | Stream length (L_{u}) (Total) (169.140 km) | 80.446 | 48.200 | 21.998 | 8.611 | 9.885 |

4 | Mean stream length (L_{sm}) (0.939 km) | 0.604 | 1.417 | 2.444 | 2.870 | 9.885 |

5 | Stream length ratio (R_{L}) | 0.599 II/I | 0.456 III/II | 0.391 IV/III | 1.147 V/Iv | |

6 | Bifurcation ratio (R_{b}) | 3.911 I/II | 3.777 II/III | 3 III/IV | 3 IV/V |

Wadi Wuheida watershed | |||||||||
---|---|---|---|---|---|---|---|---|---|

Par. No. | Morphometric parameters | Stream order | |||||||

1 | Stream order (u) (6) | I | II | III | IV | V | VI | ||

2 | No. of streams (N_{u}) (Total) (356) | 271 | 63 | 15 | 4 | 2 | 1 | ||

3 | Stream length (L_{u}) (Total) (402.657 km) | 200.860 | 104.462 | 55.948 | 23.369 | 10.835 | 7.183 | ||

4 | Mean stream length (L_{sm}) (1.131 km) | 0.741 | 1.658 | 3.729 | 5.842 | 5.417 | 7.183 | ||

5 | Stream length ratio (R_{L}) | 0.520 II/I | 0.535 III/II | 0.417 IV/III | 0.463 V/Iv | 0.662 VI/V | |||

6 | Bifurcation ratio (R_{b}) | 4.301 I/II | 4.200 II/III | 3.750 III/IV | 2 IV/V | 2 V/Vi | |||

Wadi Huseinan watershed | |||||||||

Par. No. | Morphometric parameters | Stream order | |||||||

1 | Stream order (u) (4) | I | II | III | IV | ||||

2 | No. of streams (N_{u}) (Total) (239) | 189 | 40 | 9 | 1 | ||||

3 | Stream length (L_{u}) (Total) (246.086 km) | 121.183 | 52.140 | 25.837 | 46.926 | ||||

4 | Mean stream length (L_{sm}) (1.029 km) | 0.641 | 1.303 | 1.033 | 46.926 | ||||

5 | Stream length ratio (R_{L}) | 0.430 II/I | 0.495 III/II | 1.816 IV/III | |||||

6 | Bifurcation ratio (R_{b}) | 4.725 I/II | 4.444 II/III | 9 III/IV | |||||

Wadi Tawayil el hamed watershed | ||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

Par. No. | Morphometric parameters | Stream order | ||||||||||||

1 | Stream order (u) (6) | I | II | III | IV | V | V I | |||||||

2 | No. of streams (N_{u}) (Total) (722) | 564 | 118 | 29 | 7 | 3 | 1 | |||||||

3 | Stream length (L_{u}) (Total) (863.549 km) | 400.669 | 208.872 | 124.487 | 80.538 | 41.349 | 7.634 | |||||||

4 | Mean stream length (L_{sm}) (1.196km) | 0.710 | 1.770 | 4.292 | 11.505 | 13.787 | 7.634 | |||||||

5 | Stream length ratio (R_{L}) | 0.521 II/I | 0.595 III/II | 0.646 IV/III | 0.513 V/IV | 0.184 VI/V | ||||||||

6 | Bifurcation ratio (R_{b}) | 4.779 I/II | 4.068 II/III | 4.142 III/IV | 2.333 IV/V | 3 V/Vi | ||||||||

Wadi el batra watershed | ||||||||||||||

Par. No. | Morphometric parameters | Stream order | ||||||||||||

1 | Stream order (u) (5) | I | II | III | IV | V | ||||||||

2 | No. of streams (N_{u}) (Total) (412) | 322 | 71 | 15 | 3 | 1 | ||||||||

3 | Stream length (L_{u}) (Total) (455.759 km) | 219.609 | 99.826 | 60.298 | 37.346 | 38.680 | ||||||||

4 | Mean stream length (L_{sm}) (1.106 km) | 0.682 | 1.406 | 4.019 | 12.448 | 38.680 | ||||||||

5 | Stream length ratio (R_{L}) | 0.454 II/I | 0.604 III/II | 0.619 IV/III | 1.035 V/Iv | |||||||||

6 | Bifurcation ratio (R_{b}) | 4.535 I/II | 4.733 II/III | 5 III/IV | 3 IV/V | |||||||||

Wadi Abu Tarfa 1 watershed | ||||||||||||||

Par. No. | Morphometric parameters | Stream order | ||||||||||||

1 | Stream order (u) (6) | I | II | III | IV | V | V I | |||||||

2 | No. of streams (N_{u}) (Total) (786) | 631 | 119 | 24 | 8 | 3 | 1 | |||||||

3 | Stream length (L_{u}) (Total) (881.136 km) | 412.593 | 238.258 | 91.978 | 99.210 | 26.902 | 12.195 | |||||||

4 | Mean stream length (L_{sm}) (1.121 km) | 0.653 | 1.087 | 3.832 | 12.401 | 8.967 | 12.195 | |||||||

5 | Stream length ratio (R_{L}) | 0.577 II/I | 0.386 III/II | 1.078 IV/III | 0.271 V/Iv | 0.453 VI/V | ||||||||

6 | Bifurcation ratio (R_{b}) | 5.151 I/II | 4.958 II/III | 3 III/IV | 2.666 IV/V | 3 V/Vi | ||||||||

Wadi Abu Tarfa 2 watershed | ||||||||||||||

Par. No. | Morphometric parameters | Stream order | ||||||||||||

1 | Stream order (u) (4) | I | II | III | IV | |||||||||

2 | No. of streams (N_{u}) (Total) (189) | 154 | 28 | 6 | 1 | |||||||||

3 | Stream length (L_{u}) (Total) (204.957 km) | 100.082 | 39.989 | 40.726 | 24.160 | |||||||||

4 | Mean stream length (L_{sm}) (1.084 km) | 0.649 | 1.428 | 6.787 | 24.160 | |||||||||

5 | Stream length ratio (R_{L}) | 0.399 II/I | 1.018 III/II | 0.593 IV/III | ||||||||||

6 | Bifurcation ratio (R_{b}) | 5.500 I/II | 4.666 II/III | 6 III/IV | ||||||||||

streams constitute 75.7% and 78% of the total stream length related to faulted-erosional slope watersheds and dip slope catchments respectively. The stream length characteristics of all basins verify Horton’s second law [

2) Mean stream length (L_{sm}) values for the faulted-erosional slope catchments vary from 0.837 to 6.9, while L_{sm} for the dip slope basins range from 0.87 to 6.01.

3) Stream length ratio (R_{L}) is the ratio between the mean length of streams of a given order to the mean length of streams in the next lower order. R_{L} is considered a significant factor in relation to both drainage composition and geometric development of drainage basins [_{L} values between the streams of different orders pertaining to the faulted-erosional slope catchments (0.494 - 1.421), and watersheds belong to the dip slopes (0.184 - 1.816). This variation is attributed to morphological changes in slope and relief along both dip slopes and the faulted-erosional slopes, and the youth-age stage of geomorphic development of the watersheds as verified later through hypsometric analysis (

4) Bifurcation ratio (R_{b}) is elaborated by Horton [_{b} values range between 3.66 and 6 for watersheds in which the geological structures distort the drainage pattern. By contrast, lower values of R_{b} are representative for structurally less disturbed catchments without any distortion in drainage pattern [_{bm}) for the dip slope catchments vary between 4.212 and 5.39. By contrast, the R_{bm} values for the faulted-erosional slope basins vary between 3.75 - 5.014. Such high figures denote that drainage development of the watersheds is remarkably influenced by structural disturbances such as faulting, uplifting of the Ras En Naqb escarpment, subsidence of El Jafr basin, and rejuvenation of the drainage network.

A prominent variation exists in the values of morphometric parameters which represent basins geometry (basin area, basin length and basin perimeter). The areas of the dip slope catchments are varied. It ranges from 491.85 km^{2} (W.Abu Tarfa 1) to 118.28 km^{2} (W.Abu Tarfa 2), whereas the areas of faulted-erosional watersheds range from 97.193 km^{2} (W.Hafir) to 222.76 km^{2} (W.Jamam). Basin length of the faulted-erosional slope catchments ranges from 15.31 km (W.Rabigh) to 25.42 km (W.Jamam), while the basin length of the dip slope catchments varies between 23.82 km (W.Wuheida) and 42.19 km (W.Huseinan) (

1) Form factor (R_{f}) is expressed as the ratio between the area of the catchment (A) and the square of the catchment length [_{f} parameter has been developed to predict the intensity of a basin of a defined area. For a perfectly circular basin, the value of the form factor should always be less than 0.79 [_{f} (<0.45), the more the basin will be elongated. Catchments with high R_{f} have peak flows of shorter duration, whereas elongated watersheds with low form factors have lower peak flow of longer duration [_{f} values related to four catchments are less than 0.56, while the values for two catchments are around 0.45, indicating elongated shape and suggesting a flat hydrograph peak for longer duration. Flood flows of such elongated basins are easier to manage than watersheds developed towards rectangular to circular shape such as W.Rabigh (R_{f} = 0.68) and W.Ghafir (R_{f} = 0.522) belonging to the faulted-erosional slope of Ras En Naqb escarpment. Thus, high peak flows of shorter duration are expected during flash floods [

2) Elongation ratio (R_{e}) is defined as the ratio between the diameter of the circle of the area as represented by the drainage basin to the maximum basin length [_{e} values vary generally between 0.6 to 1.0 over a wide range of climate and geological conditions. Values close to 1.0 are characteristic of regions with very low relief, whereas values in the range of 0.6 - 0.8 are normally diagnostic of watersheds with high relief and steep slopes. Where R_{e} approaches 1.0, the shape of the drainage basin approaches a circle [_{e} values for W.Rabigh and W.Ghafir which belong to the faulted-erosional slopes are greater than 0.7 (0.73 and 0.815 respectively) (_{e} values for the other four wadis are less than 0.70. Such figures indicate that W.Rabigh and W.Ghafir are approaching the circular shape, whereas the other four wadis (

3) Shape factor (B_{s}) is calculated by dividing the square of the length of a basin by the area of the basin [_{f}). The shape of the drainage basin along with the length and relief affect the rate of water and sediment yield. B_{s} values for the catchments of the dip slope range from 1.977 to 12.6 with an average of 5.596; thus, it is expected to have the shorter basin lag time, whereas; the Bs values for the watersheds of the faulted-erosional slopes vary from 1.9 to 3.85 with an average of 2.76. Therefore, it may have a longer basin lag time.

4) Lemniscate ratio (k) is a measure elaborated to describe how closely the actual drainage basin shape approaches the loop of a lemniscates [

5) Circularity ratio (R_{c}) refers to the ratio of catchment area (A) to the area of circle having the same circumference as the perimeter of the catchment [_{c} is controlled by the length and frequency of the streams, geological structures, landuse, land cover, climate, relief and slope steepness of the catchment. Drainage basins with a range of circularity ratios of 0.4 to 0.5 were described by Miller [_{c} for the watersheds developed on the faulted-erosional slope is in the range from 0.45 to 0.85 indicating that these watersheds are characterized by high relief, elongated and relatively permeable surface resulting in greater basin lag times, while catchments belonging to the dip slopes show delayed time to peak flow, and most wadis of the faulted-erosional slopes show shorter time to peak. It can be concluded that R_{f}, R_{e} and R_{c} significantly influence the hydrological response of the Ras En Naqb watersheds. Also, the combination with basin shape and the arrangement of stream segments has a direct influence on the size and shape of flood peak [

6) Drainage texture (D_{t}) denotes relative spacing of drainage lines in a fluvially dissected terrain. It is defined as the total number of stream segments of all orders per perimeter of the drainage basin [_{t} represents one of the main concepts in drainage basin geomorphology. D_{t} is influenced by several intrinsic physical factors such as: climate, rainfall, vegetation, soils, lithology, infiltration-capacity, relief and stage of basin development. Smith [_{t} values for three dip slope catchments are less than 2 and the three watersheds are greater than 3, but less than 4, whereas; D_{t} values for two catchments of the faulted-erosional slopes are <2, and four catchments have D_{t} values between 2 and 4. Thus, the Ras En Naqb watersheds exhibit a very coarse to coarse drainage texture, which indicates the presence of relatively resistant, permeable materials with moderate relief.

1) Stream frequency (F_{s}) represents the ratio of the total number of streams (N_{u}) in a basin to the basin area (A), and is defined as the number of streams per unit of area [_{s} values depend on lithology of the catchment, and reflect the texture of the drainage network. The Fs values are positively correlated with D_{d} values of a watershed, which means that the increase in stream population is connected to that of drainage density [_{s}) values range from 1.688 to 2.00 for the faulted-erosional slope catchments, and from 1.509 to 1.692 for dip slope watersheds. It is obvious that F_{s} values indicate steep slopes, with low permeability rocks, thus facilitating less infiltration and greater surface flow and high flooding potential [

2) Drainage density (D_{d}) is defined as the closeness of spacing of channels, and considered a quantitative expression of terrain dissection and runoff potential of the catchment. Drainage density is a measure of the total lengths of streams in a catchment per unit area. High drainage density of an area implies high runoff, consequently low infiltration rate, whereas; low drainage density of an area implies high runoff, and consequently low drainage density of an area refers to low runoff and high infiltration [_{d} are infiltration-capacity of the soils, and initial resistance of terrain to erosion. The poorly drained basins have a drainage density of 2.74, while the well-drained one has a density of 0.73 or one-forth as great [_{d} values for Ras En Naqb catchments vary from 1.6 - 1.9 for the faulted-erosional slope to 1.7 - 1.85 for the dip slope catchments. Relatively high D_{d} values for the faulted-erosional slopes compared to the dip slope catchments are indicative of the presence of rugged terrain especially in the middle catchments. Also, the persistence of steep slopes (25˚ - 45˚, and 60˚, denotes high runoff and low infiltration-capacity. However, the lower catchments of the faulted-erosional scarp belong to the inselberg landscape (of southern Jordan), and the upper catchments exhibit coarse texture terrain. Therefore, the presence of headward-eroding streams, and fault controlled canyons in the middle catchments are probably features inherited from pluvials recognized in the Mediterranean zone [

3) Length of overland flow (L_{O}) relates to the length of water over the ground before it becomes concentrated into definite steam channels. It is considered the most crucial independent variable affecting hydrological and geomorphological development of drainage basins. According to Horton [_{o} values for the dip slope catchments range from 0.584 to 0.921, with an average of 0.882, whereas L_{o} values for the faulted- erosional slopes vary from 0.855 to 0.945, with an average of 0.866, indicating very steep slopes and shorter flow paths on the faulted-erosional slopes, and relatively moderate-steep slopes and longer flow paths characterizing the dip slope catchments.

4) Drainage intensity (D_{i}) is defined as the ratio of the stream frequency (F_{s}) to the drainage density (D_{d}) [_{i} values of the dip slope catchments are lower than those representing the faulted-erosional slope watersheds. This denotes that the latest watersheds are more susceptible to flooding, gullying and sliding. Repetitive inundations were recorded in Qa El Jafr to the northeast, and three other playas in the southwest.

1) Basin relief (B_{h}) or “total relief” of a watershed is defined as the difference in elevation between the highest and lowest points on the basin [_{h} values for the watersheds of the faulted-erosional slopes range from 421 m (W.Rabigh) to 846 m (W.Hanout). High B_{h} values are restricted to the western catchments of the Ras En Naqb escarpment where elevations are maximum. By contrast, low B_{h} values dominate the eastern part of the escarpment, where the morphology is remarkably subdued. High B_{h} values for the faulted- erosional watersheds indicate a high potential erosional energy of the drainage system especially during flooding. As a result of the sinking base level of El Jafr depression, the dip slope catchments are relatively of high potential energy during intense rainstorms.

2) Relief ratio (R_{r}) is considered a reasonable mean to measure the overall steepness of a drainage basin. Also, it is an indicator of the intensity of erosion processes operating on the watershed slopes [_{r} is defined as the ratio between the total relief (or basin relief B_{h}) of a catchment and the longest basin length parallel to the principal drainage line. R_{r} normally increases with decreasing drainage area and size of a given catchment. Schumm [_{r} values for the dip slope catchment range from 0.012 to 0.019, whereas R_{r} values for the faulted-erosional slopes vary from 0.027 to 0.038. Low values of R_{r} normally indicate the predominance of slow erosion processes, as in the case of the inselbergs landscape in southern Jordan [_{r} in the faulted-erosional slope catchments imply that these wadis are characterized by more intense erosion as compared with wadi of the dip slope. Hence, W.Ghafir is more susceptible to erosion, and W.Abu Tarfa 2 is the least among all watersheds of the Ras En Naqb area, if this parameter alone is considered for erosion intensity evaluation.

3) Ruggedness number (R_{n}) is defined as the product of drainage density (D_{d}) and basin relief (B_{h}) divided by 1000 [_{n} values can be described by high susceptibility to soil erosion, landsliding, and a high response to an increase in peak discharge. In the present investigation the R_{n} value is minimum in W.Rabigh (R_{n} = 0.723; faulted-erosional catchment) and maximum in W.Abu Tarfa 2 (R_{n} = 2.404; dip slope catchment). This implies that W.Rabigh is the least susceptible to erosion, and W.Abu Tarfa 2 is the most susceptible to erosion among all watersheds of the Ras En Naqb escarpment.

4) Dissection index (D_{is}) has been elaborated to evaluate the degree of dissection or vertical erosion, and the stage of landform development in a given catchment [_{is} is the ratio between the total relief (or relative relief) and absolute relief of the basin, which always ranges between 0.0 (complete absence of dissection and thus the dominance of flat topography) and 1 for infrequent cases such as vertical cliff topography at the sea shore, or vertical escarpment of hillslope. Extreme values of D_{is} certainly exceed 1 such as Wadi Kerak and other rivers/ wadis draining to the rift region of Jordan as an example [_{is} values vary from a minimum 0.286 (W.Abu Tarfa 2; a dip slope catchment) to a maximum of 0.508 (W.Hanout; a faulted-erosional slope watershed). The spatial variation in dissection index (D_{is}) refers to the presence of relatively dissected terrain at the western part of the escarpment, and the subdued terrain at the eastern part. Moreover, the average values of D_{is} are 0.45 for the faulted-erosional slope catchments, and 0.37 for the dip slope basins, which clearly indicates that the faulted-erosional slopes are more dissected compared to the dip slopes of the escarpment.

5) Hypsometric curve and hypsometric integral were elaborated to understand the geomorphic evolution, type of erosion processes and relative age of landforms, along with influence of tectonic, lithology and climate on watersheds morphology. In this regard, hypsometric means the relative proportion of an area at different elevations within a watershed; therefore, it represents the distribution of area with respect to altitude [

Variations in morphometric and morphological characteristics of these watersheds have influenced the potential of flash floods occurrence. High intensity rainstorms are common in southern Jordan, and occasionally have resulted in severe flash floods [

landscape were flooded with water of 20 cm depth (

The flooding risk for the twelve watersheds were determined using El-Shamy’s diagrams as illustrated in _{b}) and drainage density (D_{d}), and then the relationship between (R_{b}) and stream frequency (F_{s}) have been utilized. The bifurcation ratios for the dip slope watersheds range between 4.2 and 5.4, with an average value of 4.72. By contrast, the R_{b} values for the faulted-erosional slope catchments vary between 3.75 and 5.01, with an average value of 4.234, which indicates a noticeable control of geological structure on drainage network development. Furthermore, the calculated stream frequency (F_{s}) values range from 1.688 to 2.0 for the faulted-erosional slope watersheds, and from 1.51 to 1.69 for dip slope basins. It is perceivable that F_{s} values denote steep slopes, with low permeability rocks, thus facilitating less infiltration, greater runoff and high flooding potential. The watersheds exposed to flash floods were demarcated and assessed in order to determine catchments of low, intermediate and high flooding potential based on the relationship of two morphometric parameters (R_{b} vs. D_{d} and R_{b} vs. F_{s}), then the final flood hazard maps were generated with the aid of Arc GIS tool. Based on the relationship between Rb and Dd, watersheds nos. 1 (W.Rabigh) and 2 (W.Hafir) of the faulted-erosional slopes are categorized as of high susceptibility to flooding (

Morphometric analysis carried out for Ras En Naqb watersheds confirms the presence of two catchment categories: the dip slope catchments and, the faulted-erosinal watersheds. A pronounced variation exists in the geomorphometric parametrers characterizing both categories. Drainage density (D_{d}), relief ratio (R_{r}), elongation ratio (R_{e}), circularity ration (R_{c}) and ruggedness number (R_{n}) vary considerably. High values of mean bifurcation ratio (R_{bm}) indicate the structural and lithological control on drainage network development across the Ras En Naqb escarpment. The variation in stream length ratio is attributed to variation in morphological characteristics of watersheds especially slope and topography. A dendritic drainage pattern dominated the dip slopes, whereas a trellised pattern characterized the faulted-erosional slopes. Channels extension by head-erosion along both terrain units of the escarpment is supported by uplifting of the scarp zone. The Ras En Nagb escarpment is well drained by the 12 watersheds which produced undulating-rolling terrain on the dip slopes, and rugged-steep topography and deeply incised wadis on the faulted-erosional slopes. The catchments of the dip slopes are more elongated, whereas the watersheds of the faulted-erosional slopes are less elongated, and approach the oval category. Values of hypsometric integral (Hi) range from 0.70 (W.Hanout, faulted-erosional slope catchment) to 0.92 (W.Huseinan, dip slope catchment), which indicates that the Ras En Naqb watersheds are at a youth-age stage of geomorphic development.

The relationship between R_{b} and D_{d} reveals that watersheds nos. 1 (W.Rabigh) and 2 (W.Hafir) of the faulted- erosional slopes are classified as with high susceptibility to flooding, whereas, four watersheds (Nos. 3, 4, 5, and 6) are categorized under intermediate flooding susceptibility. By contrast, watersheds Nos. 1 (W.Abu Tarfa 2) and 4 (W.Tawayil el Hamd) of the dip slopes are designated of high flooding potential. Similarly, watersheds Nos. 3 and 6 are denominated of intermediate susceptibility to flooding. However, the relationship between R_{b} and F_{s} provided more consistent results of flood hazard susceptibility. In this regard, all the six faulted-erosional slope watersheds are classified as catchments of high flooding susceptibility, while four dip slope watersheds are categorized under intermediate flooding liability. It can be concluded that ten watersheds (83.3%) are classified under high and intermediate flooding susceptibility, and the faulted-erosional slope watersheds are more hazardous in terms of flooding. Thus, the protection of Ma’an city, El Jafr rural Bedouin settlements, and Amman- Aqaba highway from recurrent flooding is essential to ensure future sustainable development in the area under consideration. Exceptional recurrent heavy rain storms, morphometric characteristics of drainage networks, pronounced/sharp morphology, and poor land cover, are the most important factors initiating flash floods in the Ras En Naqb area.

Yahya Farhan,Omar Anaba,Ali Salim, (2016) Morphometric Analysis and Flash Floods Assessment for Drainage Basins of the Ras En Naqb Area, South Jordan Using GIS. Journal of Geoscience and Environment Protection,04,9-33. doi: 10.4236/gep.2016.46002