MORPHOLOGICAL IMPACT OF BRIDGE CONSTRUCTION ON SUNSARI MORANG IRRIGATION INTAKEALESSANDRO CATTAPAN
Lecturer/Researcher in River Engineering, Institute of Hydraulic Engineering,
Delft, Netherlands

SANJAY GIRI
Senior Advisor, Deltares, Delft,
Netherlands

BIJAY SHRESTHA
Engineer at Department of Irrigation,
Nepal

ABSTRACTIn this paper, we present the results of a morphological study revealing the impacts of riverintervention during bridge construction phase in Koshi River near Chatara (Nepal). We
demonstrate how such intervention in the river reach resulted in morphological changes that
subsequently had an impact on the intake of Sunsari-Morang Irrigation Project (SMIP), locateddownstream of the bridge. We carried out image analysis to detect the problem andattempted to replicate it using a two-dimensional morphological model (Delft3D). The study
revealed how flow and geomorphological feature and processes as well as intervention onthese processes lead to adverse impacts on the river system on one hand, and safety andfunctionality of water infrastructures on the other. The paper demonstrates how a processbased modelling tool can be useful for a rapid assessment of morphological impacts caused
by interventions in highly dynamic and sediment-laden river like Koshi.

Figure2.
Modeldomainwith
computational grid (left) and a flatbed
initial bathymetry (right)MÁRIO J. FRANCA
Professor of Hydraulic Engineering for River Basin Development Institute of HydraulicEngineering, Delft, Netherlands

Figure 6. Simulation scenario withthe groyne at the right bank d/s ofthe bridge (cofferdam at the leftbank removed) showing erosion of
the mid-channel bar and shift ofsandbar towards the right bank anddeeper channel along the left bank

Right cofferdam

Left cofferdam
Rightcofferdam

Mid channelbar
IntakeSandbarLeftcofferdam

Figure 3. Comparison between the river bed
after spin-up simulation (from initial flatbedcondition) and Google earth image before theconstruction of the bridge (left), and simulation
result showing impact of construction of rightand left cofferdams and eventually formation ofa mid-channel bar (right) as shown in Figure 1

IntakeFigure 1. Google earth images showing the evolution of mid-channel bar as an effect ofcofferdam

Figure 7. Discharge distribution at the bifurcation after formation of the mid-channel bar showing lessdischarge towards the left branch where intake is located (indicated by red color line in left plot), and after theerosion of the mid-channel bar showing almost similar amount of discharge flowing through both thebranches (right)
u/s groyne

Leftcofferdam

Chatara bridge

Google earth images as depicted in Figure 1, which clearly shows the formation of midchannel bar after the construction of the cofferdams. It can also be seen that the bank andsandbar at the right side was eroded, apparently as an impact of the left cofferdam. In fact,this was a major reason for the formation of the mid-channel bar.

d/sgroyne

IntakeBrief description of different scenarios
S.no.
1

Scenarios
Simulation of reference
case2
Reproducing
morphological effect ofcofferdams

3

Scenario after formationof mid channel bar

leftcofferdam

Groyne d/s ofbridge

Conditions
Initial falt bed

Remarks
Fig 3(left)a) Right bank cofferdam
was placed initially.
b) Then, left bankcofferdam was placed.

Fig 3(right)

Left bank cofferdam notremovedFig 4(left)

Groyne at u/s and d/s of
Chatara bridge at rightbank and left bankcofferdam not removed.

Fig 4(right)

Groyne at u/s of Chatara
bridge at right bank andleft bank cofferdam
removed.

Fig 5

Groyne d/s of Chatara
bridge at right bank andleft bank cofferdam
removed

Fig 6

Figure 4. Simulation result for the scenario when left cofferdam was not removed (left) and thescenario showing erosion of mid-channel bar with newly constructed groyne downstream of chatar
bridge and comparing to Google Earth image (right) {Google earth images; left: Feb 2016 and right Dec
2018}

Groyne u/s ofbridge

Figure 5. Simulation scenario
with the groyne at right bankshowing erosion of mid-channel
bar and shift of sandbar towards
the right bank and the deeper
channel along the left bankFigure 8. Simulated changes of bed level,water level and sill level at the intake for thescenario shown in Figure 5
Bed level of the river near the intake is 104 m,
which stayed below the sill level for two years
up to pre-monsoon of 2017 with sufficientwater level at the intake. The worst case iswhen the bed level at the intake increases
every year after the monsoon of 2018.

Figure 9. Simulated changes of bed level,water level and sill level at the intake forthe scenario shown in Figure 6
When the groyne was constructed at the right bankbelow the bridge (as shown in fig 6) shows that thebed level has stayed below the sill level for numberof consecutive years with sufficient water level at
the intake. The bed level rises during the flood
period above the sill level of the intake and goes
down below the sill level after the flood, thus
facilitating the flow of water into the intake.
CONCLUSION AND RECOMMENDATIONThe study shows how a morphological model can be useful to simulate hydraulic and morphological impacts ofthe intervention in a river reach (in this case, the effect of cofferdams during a bridge construction). It is very
important to assess such impact before constructing any kind of interventions on a river.
REFERENCES
Baral, B. R. (2013). Effects of Koshi River Damming on Sedimentation and Channel Avulsion. Mater thesis,
Institute of Hydraulic Engineering (IHE-Delft), The Netherlands.
Devkota, L., Giri, S., Crosato, A. and Baral, B.R. (2018). Impact of the Koshi Barrage and Embankments on River
Morphology and Dynamics. ASIA 2018 –Seventh International Conference on Water Resources and Renewable
Energy Development in Asia, Danang, Vietnam.
Durbar, Singha 2012 National Planning Commission Secretariat M &E Division, SMES 2: 132.