Hydraulic Performance of River Crossing Structure Against Sediment Aggradation and Trash Accumulation (The Case of Arsema Bridge from German round about to Lebu Mebrate Haile)
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Date
2019-05
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Arsema Bridge was constructed on Jemo River, Jemo River is tributary of Little Akaki River which is
the sub catchment of Akaki basin and study area has an inclusive area of 73.3 Square Kilometer,
Arsema Bridge built as River crossing structure to connect the ring road from German round about
to Lebu Mebrate Haile under Addis Ababa city Administration.
It had been built just before 15 years ago and currently the Bridge faces a serious problem of sediment
aggradations both at the left and right side of the abutment, and trash accumulation on the entrance of
the Bridge. Due to this; overtopping of the Bridge and flooding of residential house were observed
frequently.
HEC- HMS modeling was used to determine peak discharge of 2, 5, 10, 25, 50 and 100 year return
periods and then 67, 97.6, 117, 136.1, 154.5 and 174.9 m
/sec results were found, subsequence HECRAS
Modeling
was used
to
determine
water
surface
profile
level,
performance of
existing
bridge
when
difference
return period peak discharge pass through it and sediment transport at the bridge location
based on the current river sectional profiles however the bridge accommodate only up to 10 year return
flow.
Then Q50 was used to analysis before and after bridge constructed, then after the water surface elevation
is 2214.36m and 2214.57m amsl respectively however existing bridge high chord is 2213.684m amsl
i.e. 0.676m and 0.886m difference before and after the bridge constructed, this show us the existing
bridge is submerged as per previously estimated flood.
After detail analysis of the result and discussion I recommend the appropriate remedial measure for
sedimentation aggradations and trash accumulation as well as flooding of the residential house, HECRAS
was
used
with
combination
of
HEC-
RAS
Mapping:
to
redesign
of
Bridge
with
Trash
Trap
Screen
and
Flood
Hazard
map,
the
result
show
us
road
surface
finishing
level
has
to
increase
at
least
0.766
m
high
at
the
bridge
and
refill
both
left
and
right
side
of
the
bridge
up
to
150
m
length
as
per
road
profile;
to
accommodate
the
redesigned
bridge
high
chord
of
2214.45m
amsl,
residential
house
upstream
to
bridge
has
to
consider
flood
protection
or
guide
wall
up
to
2.0m
high
depending
on
their
OGL
against
the
flood
hazard
map,
relocate
of
all
utilities
line
which
are
pass
through
bridge
and
period
clearing of
trash
trap
screen is
highly
recommended
as
main
mitigation
measure.
Description
Keywords
Bridge, HEC Geo HMS, HEC _HMS, flood frequency analysis, HEC_RAS, RAS mapping, trash accumulation, sediment aggradations