Debris Flow Characteristic along the Main Channel with
Structures in the Arenal de Mejicanos, San Salvador, ElSalvador.
Ingrid Altagracia Alfaro Lopez

Supervisor: Prof. Atsuhiro Yorozuya

MEE13626
ABSTRACTEl Salvador is a vulnerable country to water/sediment related disasters. Arenal de Mejicanos is
considered historically as one of the hazardous basin due to debris flows. In order to mitigate thedisasters induced by debris flow it is important to focuses on two main points 1) to define the debris
flow capacity of the main channel, and 2) to understand debris flow characteristic in the main channel.
This study applied a 1D numerical model for debris flow simulation based on the governing equationsof mass/momentum conservation of water/sediment mixture flow and mass conservation of bed
sediment. The simulation results indicates that channel has enough capacity to carry the sediment ofdebris flow in the upper part. In the downstream part, the capacity of the channel decreases by thestructures e.g. bridge and underpass and debris cannot pass through the channel. Subsequently,
inundation by the flood water takes place around upstream side of the bridges. Therefore the objectiveof this study is to propose a systematic countermeasure, in order to prevent damages by debris flow
event.
Keywords: Debris, capacity, management, sediment, structures.
INTRODUCTION
Arenal de Mejicanos, is one of the basin involving theSan Salvador Volcano which historically has been
vulnerable of debris flow. The largest disasters arerelated to Debris flow event due to heavy rainfall e.g.,
the first one occur in 1934 with 2,125,000 m3 ofdebris volume; the other event occur in 1982 with
425,000m3 of debris volume (Kiernan y Ladru) thatburied many houses, and caused casualty of almost
500 people. (USGS). Recently the structures likebridges and underpass constructed along the channelof the basin, but not property designed concerning
about hydraulic capacity. Actually, when the debris Figure 1 Basin Arenal de Mejicanos and theStructures along the main channel.
flow occurred, those structures (bridges) cause theclogging of the debris flow. Based on this the authors
thought magnitude of the recent debris flow diminished.
METHODOLOGY
Using a 1D numerical model based on the governing equations are employed to compute the debris
flow characteristic. Mainly applying 4 inputs: 1) the discharge calculated for 10 and 200 years returnperiod, 2) several grain size (0.1, 0.2 and 0.4 m) including mud flow 3) Natural riverbed profile, and 4)
erosion depth of 5 m.
Engineer of the Construction Department. Planning Office of Metropolitan Area of San Salvador, El Salvador.
ICHARM Research & Associate Professor of GRIPS, International Centre for Water Hazard and Risk
Management, & National Graduate Institute of Policy Studies, Japan

Mass conservation equation for debris flow is divided into water-sediment mixture, and sediment:

Where h is flow depth, ū is averaged velocity, B is the channel width, E is erosion rate or erosion
velocity, is the sediment concentration of the stationary sediment layer, is the spatial average
sediment concentration, and γ correction parameter for the sediment transport rate.
The momentum equation isAnd
H=(4)Where t is time, x is the coordinate along the flow direction, h is the flow depth, θ is the bed slope, isthe spatial average velocity, B is the flow width, is the bed elevation measured from a datum line, gis the acceleration due to gravity, E is the erosion rate or erosion velocity, is the bed shear stress,is the mass density define as(σ- ) + , σ is the mass density of sediment particle, is the massdensity of the water including fine sediment, and β is the momentum correction factor and H is theelevation of the free surface of the flow body.
The formula of only fluid type shear stress is employed, with assuming the debris flow is dominatedby turbulent flow:
Whereis the friction factor related to the fluid motion. Withand where α= Kc/σ
The procedure to obtain the channel capacity is according with the estimated slope, which depends on thecritical bed slope , assuming the sediment is in equilibrium state we can obtain the value considering
the bed slope as follow:

Where σ is the mass density of sediment particles, ρ is the mass density of water, θ is the inclination
angle of the slope, Ø is the internal friction angle of sediment particles, and c is the sedimentconcentration by volume and can be estimated asWith assuming the uniform flow following equation can be obtained from equation (3).
12+ 1 gh sin=(𝑓𝑏)15

1𝑎𝑛Ø𝑔(7)With equation (7), the H-Q relation curves can be obtained at each section as different slope angle aswell as the different shape of the section. For example, Figure 2a shows the picture in a section as anopen channel with a bed slope of 16 degrees.

Figure 2a Picture in Arenal de Mejicanosin a section with bed slope of 16 degreesFigure 2b Picture in Arenal de Mejicanosin a section with bed slope of 10 degrees

Firstly with velocity of debris flow can estimated with different h. Thereafter, area of the debris flow
can be obtained. With combining the velocity and area, debris flow discharge can be obtained withdifferent h. On the other hand, Figure 2b shows a section with the bridge. In this case, hydraulic radius
are implemented to estimate velocity for concerning about the pipe flow. Regarding of the types ofsection, the section from 1 to 4 are mostly open channel, whereas the section from 5 to 8 have bridges
and the underpass. Therefore, the calculation of the hydraulic radius are not same with the differenttypes.
RESULTS AND DISCUSSION
In order to create different scenarios of thedebris flow, simulation with several grain size
(0.1, 0.2 and 0.4 meter and mud flow) withdifferent rainfall return period has conducted.
Figure 3 shows the time series of depth anddischarge of debris flow, and river bed
elevation at the point A as shown in figure 1
which is 1,500 m downstream from the upperboundary with a bed slope of 12° and a grain
size of 0.1 m. As this figure indicates, thedebris flow reached at this point within 200
seconds. Then celerity of the debris flow can be
estimated as 0.93 m/s. After it reached to theFigure 3 Debris flow characteristic for a return
point, water depth reached to 2.5 m and debris
period of 10 years and grain size of 0.1
flow discharge 140 m3/s.
Thereafter, water depth increased to 2.5 m and gradually decreases till 1.7 m. The debris flow
discharge correspond to the same trend. Concerning about the river bed elevation, initially it is 898.7(meters above sea level, masl), and starts to decrease because of riverbed erosion take place by thedebris flow. Therefore, about 2.3 m of erosion takes place at this point by the debris flow.
Figure 4 shows the time series of depth anddischarge of debris flow, and river bed
elevation at the point B as shown in figure 1
which is 4,000 m downstream from the upperboundary with a bed slope of 8° and grain sizeof 0.1 m. In this section, debris flow reached at
this point within 1,300 seconds. Since the point
is, maximum celerity of the debris flow isabout 0.4 m/s. Thereafter, water depth
increased to 0.6 m and keeps increasing till 3.0
m. Concerning about the river bed elevation,
initially it is 711 (meters above sea level, masl),and starts to decrease because of riverbed
erosion by the debris flow. Thereafter, about 9m of deposition takes place at this point by thedebris flow.

Figure 4 Debris flow characteristic for a return
period of 10 years and grain size of 0.1

Figure 5 shows the comparison between the obtained channel capacity by the equation (7) and theresults from the numerical simulation with boundary condition of10 and 200 years. Furthermore eachpoint of the graph is representing a cross section where the river structure locates along the main
channel, each section was determine according with the river bed slope, the slopes 18°, 16°,14°, 12°,10°, 8°, 6° and 4°. Figures shows in the y direction, the value of discharge m3/s for each return period
and for the calculated channel capacity and also in the x direction shows the length and location inmeters.

Mud flow

Grain size = 0.1 m

Grain size = 0.2 m

Grain size = 0.4 m

Figure 5 Comparison of the channel capacity vs. Simulation results obtained for 10and 200 years return period.
As figures indicate, those four cases show similar trend, though sediment size is quite different. Thosecharacteristics are following. Q10 and q200 values do not show much difference. They have amaximum discharge values at the second section. The maximum values are about 2,200 m3/s. Thevalues at each point gradually decrease as it goes to downstream. Regarding to qc, they have larger
values in the upper section, whereas they have smaller values in the lower section. Compared with
q10/q200 and qc, upper sections always enough capacity, though they do not have in lower section.
The figures clearly suggests that channel does not have enough capacity for the debris flow, and river
structures is main reason for it. It should be highlighted that the clogging by the actual debris flow also
occurred at this section.
With this problem in the basin is very important and urgent to propose a Sediment ManagementSystem, that consider not only the short time solutions but also the long time, and also it has toconsider the structural and non-structural countermeasures, in order to prevent more damage in case ofhazard.
Expected Management in Structural Changes
Debris control dams
Sabo's works can stabilize sites, and are a great contribution to all problem and the disasters that occur
due to sediment transport, reducing also flooding in downstream areas. The principal objective is toretain certain volume of sediment that is transport since the mountainous area in the beginnings of theriver, achieving trapping sediment that could damage downstream infrastructure is achieved also
reduce flooding downstream. Sediment deposition of sabo dam should be larger than designed total
sediment discharge of debris flow. When the space behind a Sabo dam is not enough large to capture
all of the designed sediment discharge of debris flow, several sabo dams should be planned to capture
all of the designed sediment discharge of debris flow. In this case, total designed sediment deposition
volume of several sabo dams should be larger than the designed sediment discharge of debris flow.
(Katsuo Sasahara, 2013)
The shotcrete is one of the most useful techniques in El Salvador in order to control de Landslides
movements in the upper part of the basin which is very important and clue part to control the problems
downstream, solution for the control of sediment most include this structural countermeasures that inmost of the cases are made of concrete or even steel, which make them very expensive to construct oreven to give them the maintenance needed to create a good control of sediment.
CONCLUSIONS1) Debris flow characteristic are evaluated along the main channel Arenal de Mejicanos, assuming
mud flow and debris flow with several sizes of sediment taking place.
2) Channel capacity are realized in order to obtain information for sediment management
3) Predicted channel capacity is the largest for mud flow, and it decreases with increase of sedimentsize. The present channel capacity is lower than the expected mud flow, assuming even if sedimentdeposition does not take place.
4) It is realized that river bed aggradation takes place actively due to sediment deposition in theurban area. Correspondingly channel capacity is much lower than the capacity evaluated in termsof cross section area.
5) When we conducted the sediment management, deficiency of cross sectional area and decrease ofchannel capacity due to sediment deposition must be taken into consideration.
RECOMMENDATION
For this study, it is clearly understood that the river structures are the main cause of clogging thedebris flow, then the clogging induce the inundation. To eliminate this kind of hazard, and with theknowledge is able to propose a systematic countermeasures to decrease the effects of this natural
hazard: Determine the real size of sediment: as a first stepto make a survey in the field is necessary andcollect the necessary information to determine theactual size of sediment.
 Conduct a numerical simulation of debris flow with
the information based on the field survey.
 Design the bridge opening: next step is to make thebridge opening wider in order to let the sedimentpass through.
 Preparation of Debris Flow Hazard Map: it isnecessary to make a 2D numerical analysis in orderto obtain the affected area of the debris flow withdifferent return period.
 Design of Sabo works or Dams: is better to consider
the sabo works for the retention of the sediment in
the upper part of the basin.
ACKNOWLEDGEMENTS
I give them to God for being with me in all times. I am very grateful to my Professors Shinji Egashiraand Atsuhiro Yorozuya for sharing their knowledge, time and observations with a lot of patience formy improvement.
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