Analysis of Ribb River channel migration

Abstract

The Ribb
River is one of the components of the  Blue
Nile River system located in the North  Western
part of Ethiopia. It drains to Lake  Tana,
the source of the Blue Nile River. The  Ribb
has a length of 130 km, with a catchment 
area of 1,812 km2. The average yearly rainfall  of the catchment is 1300 mm, with 80 %  occurring between the months of June and  September. The average and daily maximum  discharge of the river are 15 m3/s and 220  m3/s, respectively. A large dam and a
diversion  weir 30 km downstream of the
dam are under  construction to irrigate
15,000 ha of Fogera  flood plain (WWDSE
and TAHAL, 2007).  Downstream of the dam
location, the Ribb is a  meandering river
with slope ranging from  0.18% to 0.03%.
The river bed material is  dominated by
sand with a gravel component in  its
upper reaches. Intensive agriculture without 
any natural resources conservation, 
deforestation, dike construction, pump 
irrigation and sand mining are the most 
impactful activities in the Ribb watershed  (Tarekegn et al., 2010; Garede and
Minale,  2014). The Lake Tana level is regulated since  1995 for hydropower production, which  enhances flooding along the lower river
reach.  During the 2006 event, 45 people
died, 30,000  persons were displaced and
5371 ha of  agricultural land were
inundated (ENTRO,  2010). To prevent
flooding, dikes have been  constructed in
the lower reach of the river.  This study
aims to describe current river  morphodynamic
trends, including planimetric  changes
for the definition of the pre-dam  conditions
of the river. The first part of the work 
is presented here with some preliminary 
results, focusing on the river planimetric  changes. This paper describes the initial
state  of the study.