Estuary Processes and Managment - River Dynamics and Integrated River Management - page 462

Moreover, the natural avulsion from the present Mississippi River channel to the Atchefalaya channel is
stopped by human structures. Consequently, the delta has a non-round fan-shape but a longer protrusion
of river mouth.

T able 8.1 Basic features of male and female deltas

Missi-
ssippi

Rhine-
Meuse

River Yellow Yangtze Ural Luanhe Irrawaddy Pearl Volga Nile Ebro

Length
(km)

5464 6300 6021 2534 877 2714 2214 1320 3688 6670 927

Watershed
(10 3 km 2 )

752 1800 3220 231 45 410 454 224 1380 2870 86

Runoff
(10 9 m3)

31.3
(1952-05)

903.4
(1950-05)

380.0 8.0 4.5 379.0 284.9 79.0 810.3 90.9 42.0

325 Annual

load (10 6 t)

778
(1952-05)

414
(1951-05)

495 2.8 20.1 75.9 3.5 27 134 30

Load/water
(kg/m 3 )

24.856 0.458 1.303 0.350 4.514 0.858 0.266 0.044 0.033 1.474 0.714

D 50 (mm) 0.019 0.009 0.010 0.065 0.0105 0.010 0.005 0.062

Delta area
(km 2 )

5450 50000 2108 671 82 32400 11300 35000 20000 23300 330

Tidal range
(m)

1.30 2.67 1.20 0.01 1.4 2.71 1.35 2.00 0.01 0.43 0.10

Gender Male Female Male Male Male Male Female Female Female M-F Male

Figure 8.2 shows the Ural and Ebro deltas. Both exhibit similar features with the Yellow River delta. The
Ebro River is 928 km long and has a watershed area of 85,835 km 2 . The Ebro watershed was a closed
basin until its opening to the Mediterranean Sea 5.3 million years ago. In the following 1.6 million years
the river extended into the sea and formed the ancient Ebro delta about 3.5 Million years BP, which was
larger than the present Ebro delta. Canicio and Ibánez (1999) used ancient maps and reconstructed the
revolutionary sequences of the delta in the past millennium. The river mouth was located to the
southwest of the present mouth and the river flowed southeastward into the sea about 800 years ago. The
river shifted its delta course and flowed northeastward into the sea before 1580. There are two delta lobs
shown in the 1580 map because the new delta lob had been formed and the old one still existed. In 1750
the river shifted its channel to the present channel. In the following 100 years, the river extended into the
sea for about 5 km and reached the present location of the mouth. Figure 8.2(b) shows the southern
deltaic lobe and the northern deltaic lobe in the past and the present lobe and the river mouth (Maldonado,
1986). The present river mouth is receding due to sharp sediment load reduction in the past decades.
Female Delta— A female delta develops if a large river empties a huge amount of water into a sea
with relatively strong tidal currents. The Yangtze River Estuary is an irregular semidiurnal tidal estuary,
with a daily tidal range of 1.47 m between the daily mean higher high tide and the lower high tide (Shen
and Pan, 1988). The thalweg of the ebb tidal current and river flow is directed to the right bank due to the
action of the Coriolis force, forming the ebb tide channel, while the thalweg of the flood tidal current is
directed to the left forming the flood tide channel. The main tide direction is nearly 305° progressing
from the East China Sea toward the river mouth area while the ebb tide current direction is nearly
90°-115°. The ebb tide current is not in a direction opposite to the flood tide direction; there is a 10°-35°
angle between the extension line of the flood and ebb tidal currents because of the Coriolis force. ebb tidal
current is obviously diverted to the south, while the flood current is diverted to the north. Thus, between
the flood and ebb tidal currents in the river mouth area there is a slack water region where sediment rapidly