5 Nile Delta - Low energy

Question 1

General information: How long is the River Nile delta?

Answer 1

One of the largest in the world - 160km from north to south, 240km from west to east and covering approx 20,000km2 of land
Longest river in the world (6650km), catch,ent area of more than 3 million km2

Question 2

General information: What is the rainfall and moisture of the Nile delta?

Answer 2

• 600mm mean annual rainfall of the catchment
• Average discharge is less than 3000 cumecs (compared to 200000+ from the Amazon)
• Much runs through arid/semi-arid regions of Sahara Desert with high evaporation rates
• Ethiopian Highlands has highest rainfall in the catchment (2000mm+ of monsoon rain). Feeds Blue Nile tributary
• White Nile tributary flows from Lake Victoria in central East Africa

Question 3

General information: How is the Nile delta evolving - what was it like before the Aswan Dam?

Answer 3

• Millennia of fluvial sediment inputs > marine sediment outputs led to steady accretion of the delta
• Accretion seasonal, corresponding with the annual flood caused by monsoon rains in the Ethiopian Highlands
• Marine processes seasonal, longshore currents strongest in winter, time of greatest reshaping of delta sediment

Question 4

General information: How is the Nile delta evolving - what was it like after the Aswan Dam?

Answer 4

Dam constructed and population growth
No seasonal flood and reduced sediment input -> year-on-year net erosion, compounded by long-term eustatic and isostatic processes
Long-term trends but single events may cause significant local change (eg particular storm causing a breach in a bar)

Question 5

Energy: What are the wind patterns in the Nile Delta?

Answer 5

Prevailing wind direction is northwesterly
Winds from the northwest are more common in summer
Gentle southwesterly winds are more common in spring
Northerly wind are more common in winter

Question 6

Energy: What is the tidal range?

Answer 6

Microtidal range (just a few cm)
Enclosed nature of the Mediterranean Sea = low tidal energy input

Question 7

Energy: What are the wave patterns?

Answer 7

55-60% of waves come from the west, northwest and north
8% from east

Question 8

Sediment: What did rapid population growth in Egypt lead to?

Answer 8

98 million and an annual growth rate of 2.5% (2017)
Leads to increased water abstraction from the Nile and its distributaries, decreasing sediment supply to the delta

Question 9

Sediment: What is the sediment load and yield of the River Nile?

Answer 9

Despite a low discharge, River Nile carries a huge sediment load
Suspended sediment load is 30% day (<0.002mm diameter), 40% silt (0.002-0.02mm) and 30% fine sand (0.02-0.2mm)
Average annual sediment yield is 4.2t/ha/year and the total is 91.3 million for the Blue Nile Basin in Ethiopia

Question 10

Sediment: How did the construction of the Aswan High Dam (1964) affect sediment?

Answer 10

Regulates discharge downstream and trapping sediment behind it
Before construction, whole delta was briefly submerged during the Nile’s annual flood - alluvial deposits depths of 4m at Aswan, 9.6m at Cairo and even deeper on the delta itself
Sediment accretion on delta has dropped from 120 million tonnes/year to negligible amounts
Annual flood of the delta no longer happens

Question 11

Geology: What is the geology of the delta?

Answer 11

Soft, unconsolidated sediments of the delta have a very weak physical lithology and are therefore vulnerable to marine erosion processes (eg abrasion, hydraulic action)

Question 12

Landforms: What are the landforms on the foreshore plain?

Answer 12

Elongated ridges running parallel to the present shoreline, with brackish lagoons, salt marshes and alluvial deposits occupying the troughs between them

Question 13

Landforms: Where are the sandbars are what is their nature?

Answer 13

Nearshore underwater sandbars off Abu Air headland and Port Said, typical of tideless seas
Rip currents create a crescentic bar shape at Abu Qie
Run parallel to shore, reflecting dominant eastward longshore currents and drift.

Question 14

Landforms: Where are the higher sediment yields and what do they cause?

Answer 14

In the two main distributaries creates two lobes that extend beyond the rest of the delta

Question 15

Landforms: What is the coastal plain?

Answer 15

Consists of the foreshore plain, the frontal plain and the sandy zone
Occupies the northern part, running parallel to the Mediterranean coastline and close to or below sea level

Question 16

Landforms: What is the frontal plain like?

Answer 16

Scattered eroded limestone outcrops and clay deposits
South of the foreshore plain

Question 17

Landforms: What does the Nile split into?

Answer 17

Two major distributaries - Rosetta and Damietta - at Cairo
160km of the coast
Multiple smaller distributaries spreading throughout the delta, each contained within levees and separated by crevasse splays of land

Question 18

Landforms: What are the sandy formations at the Nile Delta?

Answer 18

Sheets, dunes and hummocks

Question 19

Landforms: What shape is the delta and why is this?

Answer 19

Smooth, concave arcuate shape instead of bird’s foot delta
Due to reshaping of deposited material by longshore currents
Reworked sediments forms a series of curved barrier bars, closing off segments of the Mediterranean to form lagoons (eg Lake Edku, Lake Manzala) that quickly become brackish and fill with fine sediment

Question 20

Processes: How was the Nile Delta built?

Answer 20

Deposition of alluvial material has occurred at a faster rate than erosion by marine processes for 3000+ years

Question 21

Processes: What is deposition aided by?

Answer 21

Deposition is aided bu flocculation, plant colonisation (stabilising deposited sediment) and a reduction in fluvial energy as the Nile enters the sea

Question 22

Processes: What is another process which contributes to sediment?

Answer 22

Fluvial erosion - especially within the wetter areas of the Nile catchment (eg Ethiopian Highlands or the Ruwenzori Mountains of Rwanda)

Question 23

Processes: What is another significant process in the arid Sahara?

Answer 23

Aeolian erosion and transportation - little vegetation to anchor fine sediments
Weathering processes in the arid/semi-arid regions of the Sahara (eg thermal expansion) also contributes sediment

Question 24

Processes: Why and where is isostatic sea level rise occurring?

Answer 24

Northern parts of the delta
Rise of 9mm/year due to compaction of unconsolidated sediments

Question 25

Processes: What process do the prevalent NW winds enhance?

Answer 25

The constant eastern movement of water and sediment in the eastern Mediterranean
Surface current velocities vary seasonally: spring (8.4cm/sec), summer (9.26-13.5cm/sec), autumn (4.46cm/sec), winter (23.4cm/sec - very strong winds)

Question 26

Processes: How was the Aswan Dam impacted the balance of processes?

Answer 26

Imbalance between sediment accretion and erosion
Shoreline of the NW Nile Delta has seen accelerated erosion and rates of retreat as high as 148m/year

Question 27

Processes: What has also contributed to higher erosion rates?

Answer 27

Rising sea levels in the Mediterranean of 1.2mm/year
Deeper water produces larger waves with higher energy levels and the potential to reach further inland
A 1m rise in sea level would result in 20% of the delta being submerged