A study of Disastrous Flash Flood in Khlong U-Tapao River Basin Utilizing aCombined Physical and Mathematical Simulation Model
Winai Sae-ChewAsst. Prof. Civil Engineering Department, Faculty of Engineering,
Prince of Songkla University, Thailand 90112
ABSTRACT: A severe flash flood, with no warning, occurred on the 21 st ofNovember 2000 in Khlong U-Tapao river basin, in the Songkhla Lake Basin insouthern Thailand, and it was four days before the waters subsided. This was thesecond such disaster, the first occurring in 1998.
The penultimate cause of the flood, which reached 2.5-3.0 meters above thenormal river bank, was a north-east monsoon rainstorm which, in one day, dropped
352-420 mm of rain. It caused a least 6 deaths, flooded over 20,000 homes, andeconomic losses of over $300 million US. In an effort to understand more fully theprocesses that resulted in this disaster and take appropriate action to prevent arecurrence, both physical and mathematical simulation models were utilized to devise
a flood warning system, which was eventually completed and known as theProtection-Mitigation Plan for the basin.
A large quantity of hydrological data were collected for the mathematicalsimulation model. It was found that the most sensitive parameters were the coefficientroughness of the channel (n), weighing factor (k) and the cross-sectional areas of theflow. These were adjusted in the model to render the best fit of the flood flowhydrographs. The models gave a better understanding and increased the predictabilityof flood events, and showed the consequences of environmental changes resulting
from human activities. Similar studies in the future can utilize the model also, as
southern Thailand is recognized as a critical flood area as the result of ENSO.
1. INTRODUCTION
Khlong U-Tapao river basin is of importance as a central area of burgeoning
economic growth in the lower southern region of Thailand. The basin has an area of
2,300 km2 with a population of more than 200,000 people. Its climate is primarily
influenced by two monsoon seasons, and result in a primarily dry and a primarily
rainy season where rainfall varies from 1,500-2,100 mm. The general topography ofthe basin is mountainous, with a mild slope from the south to the north. As the resultof heavy, but irregular, rainfall, the basin has been faced with the threat of floods
since 1959-2000, with floods lasting from 1-14 days when they occur. The mostrecent notable flood occurred from the 20-25th November 2000, peaking at between
2.0-3.5 meters above the river bank, and causing severe human and economic losses.
Following the disaster, the government launched a flood mitigation and warning planfor the protection of Hatyai municipality, which was the major center impacted by thedisaster. This study is a part of that plan, and uses a combined physical andmathematical simulation model to forecast the impact of various heavy rainfall
situations.

Figure 1. History of Serious Flood Events In Khlong U-Tapao Reiver (HatyaiMunicipality Records)
2. FLOOD OCCURRENCE SEASON AND THE FAILURE OF THE OLDFORECASTING SYSTEM
The floods in the basin, when they occur, usually take place during November, at thelatter part of the prevalent August-November monsoon season. An examination of theHatyai municipality flood records shows that the floods have become more severe inrecent years. It is believed the this is connected to uncontrolled urbanization andinfrastructure building, and the related reduction in the amount of natural wet lands
from dumping and draining. Although there were flood control systems in the area,
the November 2000 flood was so unexpected and heavy that it completely
overwhelmed this system in the area, the November 2000 flood was so unexpected
and heavy that it completely overwhelmed this system without even an early warning
the disaster was on the way. Post mortem analysis revealed that: Telemetering systems were not functioning, Rainfall data was not collected in a timely fashion due to the remoteness of thecollection
points The Channel Flow stations could not be reach during the crisis
 Hydrological technology was behind in development
 Lines of authority were unclear during the event
3. FIELD INVESTIGATION FOR PHYSICAL SIMULATION MODELCONSTRUCTION
Preliminary site surveys on both land and water were conducted with a GIS systemover the entire area of the basin, and focusing especially on the area along the canal.
Some significant and sensitive parameters of channel flow characteristics, including
channel flow rate, bed slope, side slope, vegetation and average topographic slope,were collected and/or calculated. GPS was employed in this work also in order toobtain the transactional slope of the water flow. Manning’s roughness coefficient (n)was calculated, using both the formula and an experiment in a hydraulic loboratory toobtain an average value. The physical model was constructed under a different scalein order to render an appropriate vision and workable model. A scale of 1:6,000 of theX, Y plan was selected, with a Z plan of 1:150, which enable a model channel flow inlaboratory experiments.

Figure 2. The Flood Peak In 1988 In The Upper And Lower Basins In
Khlong U-Tapao
FM =UR

UMTR =FPUP=GM LM=GP LPGR LR =LR ; GR = 1LRYR

QR = LRYR1.5
The simulated storms will be related to actual storms using the above equations fromthe physical simulation model. It was found that the storms which drop more than 250mm of rain in a short period of time, with an average of 70% of the catchment in theupper stream, will cause severe floods downstream with various inundated durations
from 2-10 days.

Figure 3. A Physical Model Of The Khong U-Tapao River Basin

4. MATHEMATICAL SIMULATION MODEL
Various mathematical simulation models were deployed to visualize a graphical flood
depth along the channel flow. The following mathematical model were utilized in thestudy:
 Flood Routing (Distributed Flow Routing) Model Finite Difference Approximation Model Depth-Area Flow rate model
4.1 Flood Routing (Distributed Flow Routing) Model
Distributed Flow Routing is composed of:∂Q∂H+B−q= 0∂X∂t(1)Momentum Equation
 Q2 ∂ βA ∂Q ∂y
++ g. A − S0 + St + Se  − β q.Vx + W f B = 0∂t
∂x
 ∂x


(2)

4.2 Finite-Difference Approximations
Numerical Solution of The Kinematic Wave with one parameter, Q:∂Q∂Q+ α .β .Q β −1 ⋅=q∂X∂t Qi j + Qi j +1 Qi j++11 − Qi j +1+ αβ 2∂X


(3)
β −1

 Qi j++11 − Qi j+1  qij++11 + qij+1
 =2
∆t



(4)

Solving Qi j++11jj +1 β −1 ∆t q j +1 + qij+1 j +1j  Qi +1 + Qi + ∆t  i +1
 Qi + αβ Qi +1 22∆x



Qi j++11 =  ∆t Qi j+1 + Qi j +1 +αβ2 ∆xNonlinear Kinematic Wave Scheme
Qi j++11 − Qi j +1 Ai j++11 − Ai j+1 qij++11 + qij+1
=+∆t
2
∆x

(5)

 q j +1 + qij+1 ββ∆t j +1∆t j +1
Qi +1 + α Qi j++11 =Qi + α Qi j+1 + ∆t  ii +1∆x∆x2


()

( )

(6)

Newton’s Method was applied, therefore; the equation will be formulated byfinite-different grid point
 q j +1 + qij+1 β∆t j +1
(7)C=Q0 + α Qi j+1 + ∆t  i +1
∆x2


( )

4.3 Depth-Area-Flow rate model
The solution will render the flow in the channel and this flow will becalculated by the Flow Depth and Velocity Theory. Thus, flood forecasting will befeasible when the flow depths are known. There are two Flows into channel to beconsidered: Overland Flow
 Channel Flow

4.3.1 Overland Flow
The Continuity Equation∫∫ vdA = f * L cos θ + V * y − i * L cos θ = 000cs

Leteral flow = q0q0 = V* y = (i – f) L0 cos θV = Average flowg * s0 * y 2v =3v
hs0 = st = t
L24 vLV 2ht =V * y * 4 y * 2g
 L  V 2   
ht = f  24 R   2 g Reynolds number Re = (4 VR/v)

If Re < 2000 Laminar Flow
fC1
ihf/L
Flow depth====C1/Re
96 + 108(I)0.4
Intensity, inch
Uniform Flow( )1 fq02  3y =  8 gs0 q0 = VY

If Re > 2000 Turbulent Flow
211.49 2 3v =Sf RnR = y
Sf = S0
q0 = VY35 nq0 Y =1 
 1.49 S 2 0 
Y = Unit in Feet

4.3.2 Channel Flow
From Manning’s equation, then
2

1

1.49 2Q =* S0 * A * R 3n
Solving the equation by Newton’s Method:f(yj) = Qj – Qactual flow
df ( y j )dy jdf ( y j )dy j=dQ jdy j−02
 1.49 12S A j R j3  / dy j=  n

2
1.49 12  2 1 dR 1 dA += S A j R j3  *  n  3 R dY A dY  j 2 1 dR 1 dA = Qj + 3 R dY A dY  j
0 − f ( y) j df = =y j +1 − y j dY  j

y j +1 = y j −f (Y j ) df   dy  j
1 − Q  Q j
y j +1 = y j −  2 1 dR 1dA    +  3 R dY Ady  

A combined flow depth will be compared with the river bank elevations at eachobserved station along the channel flow. Thus, earlier warning will be possible

Figure 4. Showing the graphically simulated flow depth in Khlong U-Tapao Canal

Figure 5. Physical simulation hydrograph in Khlong U-Tapao canal due to rainfall
intensity of 250 mm.

Figure 6. Impacted area in Khlong U-Tapao River basin following a heavy storm
which released over 250 mm of rain
5. CONCLUSION
In this study, the experimental calculation indicate that if there is a prevalent storm
releasing over 250 mm of rain, in the southern region of the river basin, with theprecipitation covering over 70% of the catchment area, it will probably cause a severe
flash flood in adjacent areas and northern regions of the basin. The flow rate in thephysical model will be measured by electrical sensor instruments mounted at focusedstations. The measured flow rates will be simultaneously calculated from the flow
depths. From the mathematical simulation model, the channel flow depth along theflow will be calculated and compared with the river bank elevations. In this study
there still are some erros arising in the mathematical calculations such as the channel
flow area characteristics, channel slope and Manning’s roughness(n). Theseparameters are sensitively significant and only average values were employed. In
general, the combination of both models seems to work rather well; unfortunately,
there were no historic flow rates recorded during the flood peak events of theNovember 2000 flood, and therefore, in the study, it was impossible to compare theactual flow depth with the simulation in models.
REFERENCES
1.Proceeding of the International Symposium on Floods and Droughts, Nanjing,
China, 18-21 October 1999, IHP-V Technical Document in hydrology No. 4,
UNESCO Jakarta office, 1999
2.Ven Te Chow, David R. Maidment, larry W. Mays, 1989, Applied Hydrology,
McGraw-Hill International Editions.
3.Richard H. French, 1987, Open-Channel Hydraulic, McGraw-Hill International
Editions 2nd Printing Sae-Chew, W. 1992. Feasibility Study of Flood Warning
System for the Sadao-Hatyai Reiver Basin, Sonkhlanagarin J. Sci. Technolo. 14(2):163-173.