Dredging the Clays of the Nile: Potential Challenges and Opportunities on the Shores of the Aswan High Dam Reservoir and the Nile Valley in Egypt

Abstract

The Aswan High Dam (AHD) Reservoir has become a major storage for sediments over the last 50 years. The southern part in Sudan called Lake Nubia is heavily silted and is developing a new delta. The construction of the Great Ethiopian Renaissance Dam (GERD) is expected to retain an important volume of silt and lower the levels of the water in the AHD Reservoir during fill‐up. The impact is a reduction of the storage capacity in Sudan and Egypt, but the silver lining is an opportunity for development of small communities through dredging and the construction of onshore sediment ponds that can be turned into farm land.

The large accumulation of clays requires a special approach for dredging. Lake Nasser, clay fractions can range from 30 to 90% of the sediments in Egypt, after that the coarse material had deposited in Lake Nubia in Sudan. For the fraction smaller than 0.42 mm, its plasticity index is used as measure of the tendency to form balls of clay – which would slow greatly dredging efforts. There is a dearth of data on the plasticity index of clays from the AHD Reservoir, but comparative studies can be done at Kalabsha, 50 km south of Aswan, at Aswan, Toshka, Qena, as 2% of the sediments mostly clays manage to pass through the AHD. At AHD Reservoir, they are composed of a large portion of smectite (~70%), kaolinite (<25%), and illite (<10%) particularly in Lake Nubia. Illite is typically of volcanic origin. The new delta forming within the Aswan High Dam Reservoir contains high portions of kaolinite through the erosion of the shores and from wind-transported material. Kaolinite-rich sediments reach 50% of the central and northern sections of Lake Nasser in Egypt.

While high plasticity index is a challenge to dredging, it is a positive property for building dykes around sediment ponds, quick formation into impermeable layers to prevent seepage of water, and the manufacture of bricks and for a local ceramic industry. Balls of clays sediment much faster than particles of clay and would cut down the size of settling ponds for farming.

To simulate the potential problems in Egypt, samples of natural clay sediments were dredged in the USA and sent to the slurry lab of Splitvane Engineers. Samples with a plasticity index of 32% were tested at a different velocity from 1.4 to 5.8 m/s and over different periods of time to simulate pumping over 8 km. No similar ablation wheel was found in Egypt or Europe at the time of writing this chapter. The results of lab tests are presented to understand the potential challenge of dredging the clays of the AHD Reservoir and should be repeated on samples from the Nile sediments by building a dedicated testing facility in an Egyptian Research Institute. These natural clays showed a different degradation rate than previous tests conducted by the USACE on synthetically composed clays. The degradation was dependent on properties of the sample, the tangential velocity of the drum, and the distance of pumping. These tests should be repeated on samples from the AHD Reservoir for better planning the development of new ceramic industries and small communities.