11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Design storm selection for mixed urban and agricultural
drainage systems: A case study in the Northern delta-Vietnam
Tuan Anh NGUYEN1*, Giovanna GROSSI 1, Roberto RANZI 1
1
University of Brescia, DICATA-Department of Civil Engineering,
Architecture, Land and Environment, Brescia, Italy.
* Corresponding author, e-mail [email protected]
ABSTRACT
This paper presents a design storm definition procedure for the design of channel in mixed
urban and lowland rice drainage systems in the Northern Delta-Vietnam. By analyzing a 20
years record of rainfall data from Hanoi station, typical storm events were identified and a set
of suitable design storms was determined and recommended for the optimal design storm
selection. A procedure to select an optimal design hyetograph was presented. It is based on
the combination of the design storm and continuous simulation methods. A set of simulation
experiments for the optimal design storm selection was carried out.
KEYWORDS: Design storm, urban, agricultural, drainage system, lowland rice,
hyetograph.
INTRODUCTION
The Northern Delta, covering an area 14,806 km² wide including the Red River (Song Hong)
and the Thai Binh river deltas, is one of the most heavily populated areas in Vietnam, with
about 17.5 millions inhabitants. This delta is characterised by a humid tropical climate with
annual rainfall of about 1,800 mm falling mainly in four months from June to September.
Heavy storm events are often caused by typhoons and the tropical low pressure.
The interaction of urban areas and lowland rice fields is very frequent in this region, because
the fertile alluvial soil is heavily exploited for agricultural purposes, in competition with the
process of urbanization. This feature poses difficult problems for the design of drainage
systems.
Nguyen. T.A et al.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Urban area
Pumping station
Lowland rice area
River
Figure 1. The outline of a mixed urban and rice drainage basin in the Northern DeltaVietnam
A specific feature of lowland rice is that it requires a water layer on the field maintained
during its growing period. This is quite different from the conditions favourable for the
growth and development of other crops. In rainy season, lowland rice can resist to flooding
for several days with a given submerged level. So rice fields are used as detention ponds in
order to reduce peak discharge rate. For design purposes, storm events lasting for several days
(3, 4, 5 days) are commonly considered (Thai, 1990), while the hydrological response of
urban areas is very quick and the critical duration of the storms for those areas is often less
than six hours. For this reason the identification of an appropriate design storm for mixed
urban-agricultural basins in the delta region is not an easy task.
The definition of a design storm was investigated by several authors in different climatic
environments. For instance, Keifer and Chu (1957) proposed a synthetic hyetograph for the
design of the sewer system based on the ‘alternating block’ method. Huff (1967) developed
time distribution expressions for heavy storms in Illinois. Pilgrim and Cordery (1975)
proposed a hyetograph analysis method that is based on ranking the time intervals in a storm
by the depth of precipitation occurring in each of them. Yen and Chow (1980) proposed
triangular hyetographs for four locations: Illinois, Massachusetts, New Jersey and California.
In Italy a constant intensity design storm, based on depth-duration-frequency curves scaling
with duration is still widely used in the engineering practice (CSDU, 1997).
In the Vietnamese engineering practice, for the design purpose of drainage systems for rice
basins or mixed urban and rice basins, several types of design storm were used. For instance,
a design storm with duration of 5 days, derived from the statistical analysis of daily rainfall
data with constant rainfall intensity in each day, an assumption which might not be safe
enough is suggested by Bui (1998). An alternating block hyetograph (Chow et al., 1988) with
storm duration of 5 days was applied by Dang (1999). This storm is based on the assumption
that the maximum rainfall for any duration shorter than the total storm duration should have
the same return period, which is unrealistic and may be too conservative. A time distribution
pattern derived from a typical storm event was proposed for Hanoi by Thang (2005). This
storm may overcome the drawback of the alternating block storm. However, the selection of a
typical storm event is rather arbitrary.
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Design storm selection for mixed urban and agricultural basins: A case study in the
Northern delta- Vietnam
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
In order to calibrate the design storm parameters or to find out an optimal design storm for
urban basins a combination of the design storm and continuous or event-based simulation
approaches was investigated by several authors: for instance, Cao et al. (1993), Despotovic et
al (1996), Alfieri et al. (2007). None of those works considers the matter for mixed urban and
agricultural basins.
In this paper, the problem of the identification of an optimal design storm for mixed urban
and rice drainage basins in Hanoi region is addressed. It is based on the combination of the
design storm and continuous simulation methods. Proposed design storm is intended for the
design of drainage channels for rice fields and of combined drainage channel (i.e. the main
drainage channel for both urban and rice areas).
METHODS
Determination of suitable design storms
Storm classification. The statistical analysis of 20 years recorded rainfall data at Hanoi station
showed that the probability of occurrence of the annual maximum of 1, 3, 6, 12, 24, 48, 72,
96, 120 hours precipitation in the same event is about 28%. This means that a synthetic storm
built according to the alternate block procedure based on a unique DDF curve for such a long
duration range is not realistic, because the intensities computed for any duration shorter than
the total duration would have the same return period, a hypothesis which is not supported by
the data. The statistical analysis also indicated that the probability of occurrence of the annual
maximum of a subset of durations (0.5, 1, 3, 6, 12 hours precipitation) in the same event is
about 75% and the probability of occurrence of the annual maximum of the remaining subset
(24, 48, 72, 96, 120 hours precipitation) in the same event is about 70%. This suggests the
existence of a sort of ‘meteorological split’ of critical events for short and long durations,
meaning that there could be two typical maximum storm events: one preserving statistics of
short duration peak intensities, called ‘type A-high intensity’ and the other one preserving
long duration rainfall depths, called ‘type B-high volume’.
Depth-Duration-Frequency (DDF) curve estimate. Through analyzing duration of the typical
events with a minimum interevent period of 48 hours (2 days), the result is that, type A storm
events have the mean storm duration being about 72 hours and type B storms have the mean
storm duration being about 120 hours. To establish DDF curves, the 0.5, 1, 3, 6, 12, 24, 48, 72
hours maximum depths occurred in type A typical extreme events and the 0.5, 1, 3, 6, 12, 24,
48, 72, 96, 120 hours maximum depths occurred in type B typical events for each year were
extracted from the recorded rainfall data from Hanoi station.
The appropriate Cumulative Distribution Function (CDF) for the rainfall depth samples was
selected by applying the Pearson X2 test to four distributions: Gumbel (EV1-Extreme Value
of the 1st type), Lognormal, Pearson III and Kritski-Menken. The last two distributions are
more widely used in Vietnam (Ngo, 1998; Le, 2002). By comparing the X2 values the EV1
distribution was selected as the most appropriate.
The parameters of the EV1-two duration regimes DDF curves were estimated by using the
simple regression method. To ensure the continuity of the DDF curves, the intersection point
duration between two duration regimes, D*, was computed for each return period and storm
type. Resulting DDF curves and the respective parameters for the return period of 5 and 10
years, which are of major technical interest for design purposes, are reported in equation (1)
and (2) and in Table 1.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Hd = a1. d n1 (d ≤ D*)
Hd = a2. d n2 ( D* < d ≤ 120)
(1)
(2)
Hd : rainfall depth (mm); d: rainfall duration (hour).
Table 1. Equation parameters of the DDF curves for Hanoi
T- Return
period (years)
a1
a2
n2
D*
(hours)
119.67
140.92
161.35
0.146
0.151
0.155
61
65
67
127.93
156.93
185.02
0.167
0.156
0.148
73
71
69
n1
5
10
20
76.58
85.24
93.61
5
10
20
58.26
69.45
80.17
Type A
0.255
0.272
0.284
Type B
0.351
0.348
0.346
Time distribution of design storm. To obtain an optimal hyetograph, four suitable design
hyetographs are considered: two alternating block synthetic storms named A1 and B1 with the
advancement peak coefficient r equal to 0.5 and two modified alternating block storms
named A2 and B2 with the mean advancement coefficient r equal to 0.153 and 0.193,
respectively. These values of r were estimated on the basis of the typical storm events the
same those used for DDF calculation.
Applying the modified alternating block method with a time step of one hour, the four design
hyetographs derived from the Hanoi DDF curves were computed, as shown in Figure 2 and 3.
90
Intensity (mm/h)
80
70
60
Type A1 storm
50
Type A2 storm
40
30
20
10
0
0
10
20
30
40
50
60
70
Time (h)
Figure 2. The type A1 and A2 design 10- years return period hyetographs for Hanoi region.
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Design storm selection for mixed urban and agricultural basins: A case study in the
Northern delta- Vietnam
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
80
Intensity (mm/h)
70
60
50
Type B1 storm
Type B2 storm
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100 110 120
Time (h)
Figure 3. The type B1 and B2 design 10- years return period hyetographs for Hanoi region.
Optimal hyetograph selection
In order to evaluate the suitability of the four design hyetographs and to find out an
optimal design storm a procedure is defined, including the following steps: (a) a hypothetical
mixed drainage basin is set up; (b) a continuous simulation using a rainfall-runoff and flood
routing model forced by the ten minutes duration rainfall time series, 20 years long, results in
a runoff time series at different control sections in the basin; (c) design storm modeling: the
four design storms, defined above, are used as input to the same frainage model to generate
the design hydrographs at the outlet. The design peak flow rates Qd(T) corresponding to these
storms are then determined; (d) The T-years peak flow rates Q(T) at the control sections are
estimated from the continuous simulation results after statistical analysis. The estimated
values from the continuous simulation and the design storm modeling are compared by using
the percentage error variable εQ.
εQ =
Qd (T ) − Q(T )
.100
Q(T )
The optimal design hyetograph is the hyetograph which gives the lowest absolute value of εQ.
Basin: a hypothetical mixed urban and rice drainage system is set up as illustrated in figure 4.
There, the urban area is considered as a lumped unit. Urban storm water is drained off
through the main channel of the rice area. Stormwater falling on the diked rice fields is
released through the weir at the outlet of the field. The basin is characterized by the following
parameters:
- Urban basin: area (Au), runoff coefficient (ø), the time of concentration (Tc)
- Rice fields: uniform size, area of each rice field, Ar (ha), deep percolation and seepage
rate (dp), ground elevation (Z)
- Weir at the outlet of the rice fields: Height (Hw), Width (w)
- Detention pond: surface area (Sp), minimum water level (Zmin)
- Pumping station: number of pumps: np, pump type.
- Channel system: uniform channel density, length of channel reach (Lcr), length of main
channel (Lm), length of secondary channel (Ls), slope of bottom (s), roughness (n), trapezoid
section with scarp (m). Design flow of the channels is preliminary determined as follows:
For the drainage channels for rice fields: Qi = q. Ai/1000 (m3/s)
For the main drainage channel (combined channel): Q= q.Ar/1000 + ø.I.Au/360 (m3/s)
Where: Qi = design flow of i th channel, q=drainage coefficient of rice areas, l/s/ha.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Ai = area of the rice fields drained through i th channel, ha.
Ar = total area of the rice fields, ha.
Au = area of the urban subcatchment, ha.
I = rainfall intensity, estimated as follows:
I = Hd,T/Tc
Tc = the time of concentration.
Depth-Duration-Frequency curves are estimated assuming the constant coefficient of
variation CV, i.e. a single-scaling hypothesis (Burlando and Rosso, 1996):
Hd,T-1 = m(1-0.45CV-0.78CV Ln(Ln(T/(T-1)))). dn1
(d ≤ 1hour)
Hd,T-2 = m(1-0.45CV-0.78CV Ln(Ln(T/(T-1)))). dn2
(1 hour < d ≤ 24 hours)
Hd, T: rainfall depth (mm); d: rainfall duration (hour); T: return period (year)
For Hanoi region, m=64.303, CV=0.277, n1=0.571, n2 =0.265. q can be preliminary
determined to be 6.5 l/s/ha.
Note that the dimension of the channels will be adjusted if any overflow occurs.
Ls
Lowland rice area
Wu
4
Urban area
200m
Lu
200m
po n d
200m
200m
1
p u mp s
r iv e r
3
2
Lm
1- dike of the rice field
2- weir at the outlet of the field
3- secondary channel
4- main channel
Figure 4. The outline of the hypothetical mixed urban and rice drainage basin.
Rainfall-runoff and flood routing model: a model implemented in the computer code MUAD
developed for the present study were used. Net rainfall was computed for the urban areas
assuming a constant runoff coefficient, and an average infiltration rate of 5 mm/d for the rice
areas. A linear reservoir model was assumed to route net rainfall in urban area. A water
balance and storage routing model was used to calculate the outflow from the rice fields into
the channel system, in which irrigation and evapotranspiration are included. The kinematic
wave model was used for flow routing through the channel system to the outlet. This model is
able to perform single event and continuous simulation. The model was validated for a typical
drainage system in Hanoi on some observed events.
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Design storm selection for mixed urban and agricultural basins: A case study in the
Northern delta- Vietnam
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
RESULTS AND DISCUSSION
Reference case:
Firstly, a reference case with the basin parameters defined from typical basins in the Northern
Delta is considered:
Values of the parameters are set as follows:
Urban subcatchment: Au =100 ha; ø=0.7; Tc = 0.5 hours .
Rice fields: Area of the rice basin = 800 ha
Ar = 4 ha; Hw = 0.1 m; w =0.4 m; Elev = 4.5 m; dp=5 mm/day.
Channel system:Lm=2000 m; Ls=2000 m; slope = 0.0004; roughness n=0.025 ; m=1.5.
Pond and pump: Zminp = +0.7 m; Sp=0.5 ha; Zr = 5.6 m; np=12, pump type: 2500-4.5.
Return period of storm and flood discharge: T = 10 years.
Results are shown in Figure 5 and 6, in which type A1 storm gives the most consistent
estimate for the mixed basin and type B1 storm yields the best estimate for the rice only subbasin.
20
18
16
A1
3
Q (m /s)
14
A2
12
B1
10
B2
8
6
4
2
0
0
20
40
60
80
100
Time (h)
120
140
160
180
Figure 5. Comparison among the hydrographs at the outlet of the mixed basin (the inlet of
pond) obtained for the reference case.
0.7
0.6
A1
Q (m 3/s)
0.5
A2
0.4
B1
0.3
B2
0.2
0.1
0
0
20
40
60
80
100
Time (h)
120
140
160
180
Figure 6. Comparison between the hydrographs at the outlet of the rice-only sub-basin
obtained for the reference case.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Then, a set of experiments are carried out to assess the design hyetographs performance as
both the ratio of rice area to urban area (Ra) and the return period of storm and discharge
change.
Variations in the ratio of rice area to urban area (Ra). By changing the area of rice basin
through varying the length of the main channel and keeping the area of the urban
subcatchment equal to the value assumed in the reference case. The results in Figure 7 show
that, for Ra < 8, Type A1 seems to be the optimal design storm while for Ra > 8, Type B1
gives better results than others.
20
15
A1
10
A2
EQ(%)
5
0
B1
-5
B2
-10
-15
-20
0
1
2
3
4
5
6
7
Area-rice/Area-urban
8
9
10
11
Figure 7. Variability of the relative error εQ at the outlet of mixed basin with respect to the
ratio of rice area to urban area (Ra)
Variations in the return period (T): Different design storms are obtained from the DDF curves
corresponding to three considered values of T: 5 years, 10 years and 20 years to be used in the
procedure described above. Results in figure 8 indicate that Type A1 storm seems to be the
storm in three considered values of T, 5 years, 10 years and 20 years, which is more capable
to reproduce peaks resulting from continuous simulation.
30
20
A1
EQ(%)
10
A2
0
B1
-10
B2
-20
-30
0
5
10
15
20
25
Return period (years)
Figure 8. Variability of the relative error εQ at the outlet of mixed basin with respect to the
return period T.
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Design storm selection for mixed urban and agricultural basins: A case study in the
Northern delta- Vietnam
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
CONCLUSIONS
A design storm definition procedure was developed in this study. By analyzing the recorded
rainfall data, typical storm events at Hanoi were identified and a set of suitable design storms
was determined and recommended for the optimal design storm selection.
A framework to select optimal design hyetograph for mixed urban and rice basins was
presented. A set of simulation experiments was carried out in order to assess the influence of
the different variables involved in the framework. The obtained results in the simulation
experiments indicated that:
i) for design purpose of combined channel, type A1 storm (72 hours duration, alternating
block storm derived from high intensity events) should be used;
ii) for the design of drainage channels for rice fields, type B1 storm (120 hours duration,
alternating block storm derived from high volume events) should be applied;
This design storm selection procedure could be applied also to other regions, similar to the
Northern Delta region as both climate and land use characteristics are concerned.
ACKNOWLEDGEMENT
This study was funded by the Italian Ministry of Foreign Affairs – Directorate for
Development Cooperation, by the Italian Ministry of University and Research - PRIN
2006089189_003 and by the University of Brescia - ex 60% 2007.
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