Tanqwa

Upper Tanqwa

Tributaries, upstream from Abiy Addi, include[2]

• Tsech'i River
• May Qoqah
• Arwadito
• Adawro River

Tanqwa bridge at Abiy Addi

The runoff footprint or annual total runoff volume is 41 million m³ at the bridge in Abiy Addi and 79 million m³ at the confluence with Giba River at Barashuwa. Peak discharges up to 543 m³ per second occur in the second part of the rainy season (month of August) when there are strong rains and the soils are saturated with water in many places. The percentage of total rainfall that directly leaves the catchment as storm runoff (also called runoff coefficient) is 23%.[1] This is high, in comparison to other rivers of the Giba River basin, and has been explained by the fact that the larger part of the Tanqwa basin is composed of impervious rocks, particularly Ashangi Basalts, Alaji Basalts and Precambrian basement rocks.[3] The total amount of sediment that is transported by this river amounts to 596,000 tonnes per year. Median sediment concentration in the river water is 3.95 grammes per litre, but may go up to 68 g/L. The highest sediment concentrations occur at the beginning of the rainy season, when loose soil and dust is washed away by overland flow and ends up in the river.[4] As such water contains many nutrients (locally it is called “aygi”), farmers estimate that it strengthens their cattle, which they will bring to the river.[3] All in all, average sediment yield is 3627 tonnes per km² and per year. All measurements were done at purposively installed stations, in the years 2006-2007.[4]

Runoff mostly occurs in the form of high runoff discharge events that occur in a very short period (called flash floods). These are related to the steep topography, often little vegetation cover and intense convective rainfall. The peaks of such flash floods have often a 50 to 100 times larger discharge than the preceding baseflow. These flash floods mostly occur during the evening or night, because the convective rain showers occur in the afternoon.[3]

Evidence given by Italian aerial photographs of the catchment, taken in the 1930s show that 63% of the catchment was covered with woody vegetation (against 32% in 2014). This vegetation could slow down runoff and the runoff coefficient was smaller (13% in 1935 against 23% in 2014). As a consequence, discharges in the river were less and the river was narrower than today.[5] Up to the 1980s, there was strong pressure on the environment, and much vegetation disappeared.[6] This river had its greatest discharges and width in that period. The magnitude of floods in this river has however been decreased in recent years due to interventions in the catchment. On other steep slopes, exclosures have been established; the dense vegetation largely contributes to enhanced infiltration, less flooding and better baseflow.[7] Physical conservation structures such as stone bunds[8][9] and check dams also intercept runoff.[10][11]

Tanqwa gorge at Ruba Dirho