11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Field Observations of Discharge and Runoff Characteristics
in Urban Catchments Area
S. Kure1*, A.Watanabe1, Y. Akabane1 and T. Yamada2
1
Graduate School, Chuo University, Japan
1-13-27 Kasuga Bunkyo-ku Tokyo 112-8851 JAPAN
2
Dept. of Civil Engineering, Faculty of Science and Engineering,
Chuo University, Japan
1-13-27 Kasuga Bunkyo-ku Tokyo 112-8851 JAPAN
*Corresponding author, e-mail [email protected]
ABSTRACT
In order to clarify the runoff mechanism in urban catchments area, field observation of
discharge and application of runoff analysis to the urban catchments are reported in this paper.
Land-use in urban area has become more complicated by urbanization during the last several
decades. As a result of urbanization, the peak discharge of flood increases and the
concentration time of flood decreases in urban rivers. In addition, large amount of the
population and property are concentrated in floodplains which have the large potential of
inundation in Japan. It makes flood disasters in urban area more serious problem. However,
discharge data in urban catchments in Japan are very few. In this paper, field observation of
discharge are carried out and runoff analysis method for land surface area based on
morphological and geological properties is applied several urban catchments to evaluate the
effects of urbanization on runoff in urban area.
KEYWORDS
Field observations of discharge; runoff characteristics; runoff analysis; subsurface flow
INTRODUCTION
Recently, the flood of urban rivers has become serious problem due to heavy rainfalls in the
urban catchment areas in Japan. Especially, the heavy rain caused the storm water in drain
pipe to overflow the ground and urban flood increases. The extension of urbanization areas by
development of residential sites, road areas and so on and the improvement of sewer system
make an influence on drainage abilities in urban areas. As in the past, flood control plan and
drainage plan in Japan carried out with runoff analysis using rational method in urban
catchment areas, but integrative flood routing method which deal with surface, sewer system
and river network flow at the same time is needed on runoff analysis in urban areas, because
flood runoff are affected by land use and abilities of drainage interdependently. Land use
information play important role in surface runoff. In order to investigate the effects of
urbanization on runoff, it is necessary to take accounts land surface change into the runoff
simulation. And the sewer system ability makes significant effects on drainage ability in
urban area. For the precise flood prediction in an urban catchment area, we have to consider
land surface and sewer information in the runoff simulation.
By the way, there are many types of urban cathcment area from fully urbanized catchment to
catchment with forested area. For the fully urbanized area hortonian overland flow is main
Akabane et al.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Table 1. List of field observation of discharge in urban catchment area
Observation site Observation period
Observation method
Observation
item
Itachi river basin Three flush flood from 2000 to
We observed velocity Velocity，
Minakamibashi 2001
and water depth of five Water depth
point
(R=195.5mm,Rmax=6mm/10min points inthe cross
（1.0km upstream R=140mm,Rmax=12.5mm/10min section direction
point from end of R=89.5mm,Rmax=7mm/10min)
the river basin）
Area=13.9km2
O city basin
Three months from September to We put a weir in the
Rinfall，
commerce area
December,1996
outlet and observed
Velocity，
Outlet
Ten months from June, 1998 to
velocity and water
Water depth
Area=10.27ha
April, 1999
depth
(R=43.5mm,Rmax=3mm/10min
R=34.5mm,Rmax=1.5mm/10min
R=59.5mm,Rmax=4mm/10min
R=50mm,Rmax=3.5mm/10min)
Tsurumi river
basin
Sueyoshibashi
point
（5.9km upstream
point from
the river mouth）
Area=235km2
Two flush flood from 2003 to
2006
(R=52mm,Rmax=38mm/hr
R=189mm,Rmax=11mm/hr)
We put the H-ADCP at Velocity，
the river dike and
Water depth
observed velocity
and discharge
We gave a ADCP a lift
in a river boat and
observed velocity and
discharge at a bridge
Knada river basin
Ikkyubashi point
（10.8km
upstream point
from
the Tokyo bay）
Area=105km2
Five flush flood from 2002 to
2007
(R=29.5mm,Rmax=11mm/hr
R=95mm,Rmax=14.5mm/hr
R=102mm,Rmax=14.5mm/hr
R=81mm,Rmax=12mm/hr
R=36mm,Rmax=7mm/hr)
・We observed velocity Discharge，
and water
Velocity，
depth of three points in Water depth
the cross
section direction with
using price current
meter
・We gave a ADCP a
lift in a river
boat and observed
velocity and discharge
at a bridge
component of direct runoff. On the other hands, subsurface flow will be generated from
forested area. From these, we have to use the runoff model which can express over land flow
and subsurface flow for the runoff simulation in urban catchments.
In the large scale river in Japan, a lot of hydrologic date is available, because the Ministry of
Land, Infrastructure and Transport and Tourism collects and manages hydrologic data. But,
hydrologic data in small catchments in Japan are not sufficient. Especially, the flow discharge
and velocity data in sewer network is very few.
The purpose of the present paper is to clarify runoff characteristics in urban catchment areas
where hydrologic data is insufficient from the observation of discharge data in urban
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Field Observations of Discharge and Runoff Characteristics
in Urban Catchments Area
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
catchment areas and to investigate the effects of urbanization on flood runoff in urban
catchments area. For the runoff simulation in land surface, the runoff analysis method based
on morphological and geophysical properties are proposed and applied for several urban
catchments. Finally, we compared before and after urbanization in Itach river catchment
which have forested area in upstream area from the point view of flood runoff.
RUNOFF ANALYSIS METHOD FOR LAND SURFACE AREA BASED
ON MORPHOLOGICAL PROPERTIES
The runoff simulation carried out in this paper is composed of two mail parts. One is for the
land surface part and the other is for the sewer system and river channel network part. Runoff
results from surface part are used as lateral or point source boundary conditions for unsteady
flow calculation in sewer and river channel network.
Catchments are divided into many sub-catchments according to the location of manhole point
using the division of thiessen. For each sub-catchment runoff parameters are determined from
land use information and the runoff model written in bellow is applied. Calculated discharge
from each sub-catchments are used as point source boundary condition to flood routing in
sewer network system.
Multilayer flow analysis method considering the morphological properties
The rainfall runoff analysis model which can express multilayer flow related to overland,
vertical infiltration, saturated and unsaturated seepage flow from the relationship between
morphological and geological characteristics and rainfall intensity are proposed by Kure et al.
(2004). The multilayer runoff calculation is carried out by the use of 4 simultaneous ordinary
equations expressed as follows. This equation is composed of overland flow, subsurface flow,
vertical infiltration flow and water depth of overland flow as follows,
⎧ dqs
βs
⎪ dt = as qs (r (t ) − q0 − qs ) ・・・・・Surface flow
⎪
⎪ dq∗ = a q β (q − q ) ・・・・・・・・Subsurface flow
0 ∗
0
∗
⎪ dt
⎪
(q0 − k s )2 q − ks
q0
⎪ dq0
−
= (r (t ) − q0 ) 0
⎨
hs + hk (θ s − θ i ) k s (hs + hk )
⎪ dt
⎪・・・
Vertical inf iltration flow from surface flow
⎪
to
subsurface flow
⎪
⎪ dhs
= r (t ) − q0 − q∗ ・・・・・Surface water depth
⎪
⎩ dt
⎧⎪(h > D ):
⎨
:
⎪⎩(0 ≤ h ≤ D ) , r (t ) < k s qT = q s + q ∗
q∗ = q 0 = q∗
q0 = r (t )
h=D
(1),
(2).
where, q0 is vertical infiltration rate[mm/h], qs is runoff rate from surface part[mm/h], q* is
runoff rate from subsurface part[mm/h], r(t) is rainftall intensity[mm/h], hs is water depth of
overland flow[mm], ks is saturated hydraulic conductivity[cm/s], hk is capillary negative
pressure in wet line[cm], a0,as,β,βs are runoff parameters. In here, runoff parameters are
determined by geophysical and morphological quantities. This theory is applicable to urban
catchments area and mountainous basins because it is based on geological and morphological
properties. All of the basic equations are ordinary equations, so calculation is done
immediately. In addition, the both type of Hortonian and Dunne overland flow are represented
Akabane et al.
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
by the use of the model. With regards to
sewer network system and river channel
network, one-dimensional unsteady flow
calculation written in bellow was
conducted for flood routing.
Japan
Flood routing in river channel
network
Flood routing in pipe flow and river
channel is calculated by using
continuous equation and momentum
equation of unsteady flow (Saint-Venant
equation).
These
equations
are
respectively shown in Eq.(3) and Eq.(4)
as follows,
∂ A ∂Q
+
=q
∂t
∂t
⎛ Q2
∂⎜⎜ α
A
∂Q
+ ⎝
t
x
∂
∂
Kanda river basin
Itachi river basin
Tsurumi river basin
Figure 1. Location of Itachi river basin,
Tsurumi river basin and Kanda river
basin in Japan
(3),
⎞
⎟
2
⎟
⎠ + gA ∂h + n gQ Q = 0
4
∂x
AR 3
(4).
where, A is the flow area [m2], Q is the discharge [m3/s], q is the latelal flow [m/s], α is the
energy correction coefficient, h is the water level [m], g is the gravitational acceleration
[m/s2], n is the manning roughness coefficient, R is the hydraulic radius [m].
FIELD OBSERVATION OF DISCHARGE IN URBAN AREA
We carried out field observations of velocity, discharge and water depth in several urban
catchments. A list of field observation is shown in Table 1. In this paper 4 catchments of
Itachi river basin, O city basin, Tsurumi river basin and Kanda river basin are focused on. The
outlines and results of field observation of discharge in the urban catchment areas are shown
in following sections.
Outlines and results of field observation of discharge in urban catchment area
Urban catchment area with forested part in upstream area; Itachi river basin, is a part of the
Kanagawa prefecture in Japan, is focused as urban catchment area which forested area exists
in upstream area in this paper. Schematic drawing of river channel network and sewer
network of Itachi river basin are shown in Figure 2. The area of the Itachi river basin is 13.9
km2 with a total length of 9.0 km. The upstream area of Itachi river basin occupied mainly
forested area and downstream area occupied mainly urbanized area. In the proportion in land
use on Itachi river basin, the urbanized area is 67 %, the field area is 9 %, the paddy field area
is 1 % and the forested area is 23 %. The velocity and the water depth are observed using a
price current meter in two points where located stations of 1.0 km upstream and 2.2 km
upstream from the end of the Itachi river basin. As an example, Figure 3 shows observed
water depth, discharge and velocity in 10 September 2001. From the result, the concentration
time of flood in Itachi river basin is about 20 minutes. It is found that the concentration time
of flood in urban area is very short. In this paper, the concentration time of flood is defined as
the interval time between peak discharge time and peak rainfall time.
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Field Observations of Discharge and Runoff Characteristics
in Urban Catchments Area
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
1km
△
○：Manhole
□：River
：Outlet
：Drainpipe
Figure 2. Schematic drawing of river
channel network and pipe network in Itachi
river basin
Velocity[m/s]
3
Discharge[m /s]
5
3.0
Runoff coefficient：0.34
Concentration time of flood：
20[minute]
2.0
10
1.0
0.0
40.0
30.0
：Upper stream side
：Down stream side
Raingfall[mm/10min]
Water depth[m]
0
Total rainfall=89.5[mm]
Peak rainfall=
7.0[mm/10min]
10:00∼11:30
Missing value
20.0
10.0
0.0
2.0
1.5
1.0
0.5
0.0
18:00
0:00
6:00 12:00
10 Sept.
Time
18:00
0:00
11 Sept.
Fully urbanized catchment area; O city Figure 3. Observed water depth, discharge and
basin is focused as fully urbanized
velocity at stations of 1.0 km upstream and 2.2
catchment area in this paper. Schematic
km upstream from the end of the Itachi river
drawing of river channel network and
sewer network of O city basin are shown
in Figure 4. The area of O city basin is 10.27 ha and holds about 867 inhabitants. The O city
is a fully urbanized catchment area. In the proportion in land use on O city basin, the house
area is 67 %, the road area is 29 %, and the field area is 4 %. We observed velocity and water
depth for three months from September to December, 1996 at the outlet. Figure 5 shows
observed water depth, discharge and velocity in 9 September 1996. From the result, the
concentration time of flood in O city basin is about 10 minutes. It is found that the
concentration time of flood in O city basin is very short more than Itachi river basin which
have forested area in upper side.
Urban tidal river; Tsurumi river basin, is a part of the Tokyo metropolitan and Kanagawa
prefecture in Japan, is focused as urban tidal river in this paper. The observation site is located
on the Sueyoshibashi point of 5.9 km upstream from the Tsurumi river mouth. The area of the
Tsurumi river basin is 235 km2 with a total length of 42.5 km. The point of 13.8 km upstream
from the river mouth is tidal area and is received influence of a tidal variation. We observed
water level and velocity distribution every ten minutes using H-ADCP located at the
Sueyoshibashi point. And, we observed velocity and discharge every five minutes by using
workhorse ADCP (RD instruments, 1200 kHz) on the boat in the section of about 50m in the
upstream in the H-ADCP observation point. We carried out field observation of discharge in
the Tsurumi river basin from 2003 to 2006. Figure 6 shows observed water level and
discharge in 5 October 2006. The behavior of the water level of Kamenokobashi point
(13.8km upstream from the river mouth) corresponds to variation of the H-ADCP discharge.
But, the water level of the Sueyoshibashi point has strongly received the influence of tide
variation more than flood effects. Therefore, we can say that velocity and water level in tidal
river changes according to the timing of the tide on even the same scale flood. And, it is found
that H-ADCP observation discharge and ADCP observation discharge represents almost same
characteristics quantitatively and qualitatively through the ADCP observation period, by
comparing the observation discharge changes.
Akabane et al.
5
[m]
Water depth[m]
Standard
○：Manhole
：Drainpipe
12160.0
12140.0
12120.0
12100.0
12080.0
12060.0
12040.0
12020.0
9/Sept./1996
0.4
○：observation data
0
5
0.2
10
12000.0
11940.0
Flo
w
11920.0
11900.0
11880.0
11860.0
11840.0
0
100
200[m]
11820.0
11800.0
-110900.0
-110850.0
-110800.0
-110750.0
-110700.0
-110650.0
-110600.0
-110550.0
-110500.0
-110450.0
-110400.0
0
3
11960.0
Discharge[m /s]
Flo
w
11980.0
-110350.0
Outlet(Obserbation point)
0.2
Rainfall[mm/10min]
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
15
Total rainfall=43.5[mm]
Peak rainfall=3.0[mm/10min]
0.1
[m]
0
Velocity[m/s]
Figure 4. Schematic drawing of pipe
network in O city basin
Runoff coefficient：0.53
Concentration time of flood：10分
1
Small urban river catchment area; Target
basin is Kanda river basin, is a part of the
Tokyo metropolitan in Japan. As the
0
6:00
12:00
18:00
24:00
6:00
characteristics in this river basin, the land use
Time
9 Sept.
10 Sept.
form occupies urbanization areas about 90
Figure 5. Observed water depth, discharge
percent, and the percentage of sewer system
and velocity at the outlet in O city basin
maintenance is 100 percent. The area of
Kanda river basin is 105 km2 with a total
length of 24.6 km. We observed velocity and the discharge using a price current meter and
ADCP in Ikkyubashi point where located stations of 10.8 km upstream from the Tokyo bay.
We carried out the field observation of discharge from 2002 to 2007. Figure 7 shows observed
discharge in 25 August 2005. By comparing the observed discharges variation of a price
current meter and ADCP, both values are almost same quantitatively and qualitatively
through the observation period. But, the peak value of the observed discharge using a price
current meter is a little small compared with the peak value of the ADCP observed discharge.
And, the concentration time of flood in Kanda river basin is about 15 minutes. It is found that
the concentration time of flood in Kanda river basin where maintained sewer system 100
percent is very short.
Relationship between runoff characteristics and basin characteristics
In this section, we compare and evaluate the relationship between runoff characteristics and
basin characteristics by the use of runoff coefficient from the results of observed discharge in
several urban catchment areas. In this paper, the runoff coefficient is defined as the ratio of
peak rainfall and the peak runoff rate. The runoff coefficient in Itachi river basin was 0.34, the
runoff coefficient in O city basin was 0.53 and the runoff coefficient in Tsurumi river basin
was 0.58. We could not calculate the runoff coefficient in Kanda river basin, because we were
not able to specify exact location of the outlet and drainage area. And, the concentration time
of flood in Itachi river basin was 20 minutes, the concentration time of flood in O city basin
was 10 minutes, the concentration time of flood in Tsurumi river basin was 40 minutes and
the concentration time of flood in Kanda river basin was 15 minutes. The target flood is a
flood that was observed the peak discharges, and was large scale comparatively. And, in the
flood where two or more peak discharges exist at a flood period, we calculated runoff
coefficient and concentration time of flood using the maximum peak discharge and the peak
rainfall at time corresponding to the maximum peak discharge. From the results, the runoff
coefficients in O city basin and Tsurumi river basin were high value from 0.5 to 0.6. On the
other hands, the runoff coefficient in Itachi river basin was 0.34. This reason is an influence
of water holding capacity of a forested area exists in upstream area. The concentration time of
6
Field Observations of Discharge and Runoff Characteristics
in Urban Catchments Area
3
2
：Water level(13.8km from
river mouse)
：ADCP observation
discharge
：H–ADCP observation
discharge
20
400
300
200
1
100
0
0
–1
–100
0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 24:00
5/Oct.
6/Oct.
7/Oct.
Figure 6. Observed water level at stations of 2.0 km upstream, 5.9 km upstream and 13.8
km upstream from Tsurumi river mouth and
Observed discharge at stations of 5.9 km
upstream from the Tsurumi river mouth
0
10
200
：ADCP observation discharge
：Propeller meter observation discharge
（average）
Total rainfall：102[mm]
Peak rainfall：
14.5[mm/hr]
Concentration time of
flood：15[min]
20
100
0
6:00
12:00
25 Aug.
18:00
0:00
6:00
26 Aug.
Time
12:00
18:00
0:00
17 Aug.
Figure 7. Observed discharge at station of 10.8
km upstream from the Tokyo bay
flood in three basins except Tsurumi river
basin is very short as from about 10 minutes
to 20 minutes. But, the concentration time of
flood in Tsurumi river basin was long comparatively as 40 minutes, because Tsurumi river
basin area is large comparatively compared with other basins, and Tsurumi river basin is tidal
river. In this way, the runoff coefficient is high value, and the concentration time of flood is
very short in urban catchment area. We can say that it is necessary to carry out measures to do
the action of prevent floods and the behaviour of evacuation for a short time at the flood event
more efficiently in urban catchments area. And, because the runoff coefficient in O city basin
and Tsurumi river basin where impervious area rate is comparatively high is high value as
from about 0.5 to 0.6 and the concentration time of flood in Itachi river basin where
impervious area rate is comparatively low is low value as 0.34. Therefore, it is found that the
impervious area rate plays very important role in runoff process and simulation.
APPLICATION OF THE RUNOFF MODEL TO URBAN CATCHMENTS
AREA
In order to clarify the effects of runoff characteristics in urban area, we applied the proposed
rainfall runoff model to several urban catchments. Itachi river basin is focused as urban
catchment area with forested part in upstream area and O city basin is focused as fully
urbanized catchment area.
Urban catchment area which forested area exists in upstream area.
The inflow from sub-catchments to each storm outfall is calculated by surface runoff
calculation considering the land use information of each sub-catchments. We determined the
runoff parameters as follows. The thickness of surface soil layer is 15 cm, the effective
porosity is 0.25, the slope gradient is 1/1000, the slope length in impervious area is 50 m, the
slope length in field area is 25 m, the slope length in forested area is 40 m, the saturated
hydraulic conductivity in impervious area is 0.00001 cm/s, the saturated hydraulic
conductivity in field area is 0.001 cm/s, the saturated hydraulic conductivity in forested area is
0.01 cm/s, the manning roughness coefficient in impervious area is 0.03 m-1/3s, the manning
roughness coefficient in field area is 0.04 m-1/3s and the manning roughness coefficient in
forested area is 0.05 m-1/3s. The values of morphological parameters generally used were set,
because it is difficult to decide the manning roughness coefficient in surface area from a
physical viewpoint. The calculated and observed discharge hydrograph in surface land are
shown in Figure 8. From the result, it was found that the Hortonian overland flow is mainly
generated from impervious area because rainfall intensity exceeded water holding capacity of
Akabane et al.
Rainfall[mm/hr]
4
10
3
Water depth(m)
：Water level(5.9km from river mouse)
25/Aug./2005
300
Discharge[m /s]
：Water level(–2.0km from river mouth)
Total rainfall：189[mm]
Peak rainfall：11[mm/h]
Runoff coefficient：0.58
Concentration time of flood
：40分
3
5
0
Discharge(m /s)
6
Rainfall(mm/h)
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
7
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
0
9/Sept./2001
50
：Observed
10
25
：Observed
：Calculated(Include pipe flow 5
and channel flow)
3
Discharge[m /s]
Total ranifall：88.5[mm]
Peak rainfall：7.0[mm/10min]
：Calculated
（Impervious area＋Pervious area）
5
：Calculated（Pervious area）
：Calculated（Field）
：Calculated（Forest）
Rainfall[mm/10min]
3
Discharge[m /s]
50
Total rainfall：88.5[mm]
Peak rainfall：7.0[mm/10min]
25
10
15
0
0:00 10/Sept.
12:00 10/Sept.
Time
0:00 11/Sept.
Figure 8. Calculated and observed discharge
of land surface at stations of1.0 km upstream
from the end of Itachi river basin
15
0
0:00 10/Sept.
12:00 10/Sept.
Time
0:00 11/Sept.
Figure 9. Calculated and observed discharge
at station of 1.0 km upstream from the end of
Itachi river
soil. Meanwhile subsurface flow is mainly generated in pervious area because water holding
capacity of soil exceeded rainfall intensity. Using the results of surface runoff analysis as the
point source boundary conditions, we have carried out flood routing calculation to the sewer
system or river channel network. The results of the discharge hydrograph including pipe flow
and channel flow calculation at observation station where located 1.0 km upstream from end
of the Itachi river basin is shown in Figure 9 as a black line. The calculated discharge is match
well with the observed discharge. It can be concluded that the rainfall runoff model for urban
area based on morphological and geological properties proposed in this paper can express
runoff phenomenon in urban catchments area effectively.
Fully urbanized catchment area.
In O city basin, we defined the parameters as follows, the thickness of surface soil layer is 15
cm, the effective porosity is 0.25, the slope gradient is 1/1000, the slope length in house area
is 50 m, the slope length in load area is 40 m, the slope length in field area is 30 m, the
saturated hydraulic conductivity in house area is 0.00001 cm/s, the saturated hydraulic
conductivity in load area is 0.00001 cm/s, the saturated hydraulic conductivity in field area is
0.001 cm/s, the manning roughness coefficient in house area is 0.03 m-1/3s, the manning
roughness coefficient in load area is 0.35 m-1/3s and the manning roughness coefficient in
field area is 0.04 m-1/3s. The calculated and observed of discharge hydrograph in surface
runoff analysis in O city basin are shown in Figure 10. The characteristic in O city basin is a
fully urbanized catchment area. From the result, it was found that the calculated discharge is
mainly composed of the Hortonian overland flow, the calculated discharge from field area as
pervious area is not generated. Using the results of surface runoff calculation for boundary
conditions, flood routing calculation in sewer network system was carried out. The result of
the calculated discharge hydrograph including pipe flow calculation at outlet in O city basin is
shown in Figure 11 as a black line. It was found that the calculated peak discharge is larger
than the observed peak discharge, but, calculated discharge hydrograph is match with the
observed discharge hydrograph except peak discharge.
LAND CHANGE IMPACT DUE TO URBANIZATION ON RUNOFF
CHARACTERISTICS
Finally, the calculation was carried out by the change of urbanization rate in urban area with
forested part in upstream area for evaluating the land change impact due to urbanization on
runoff characteristics. We carried out runoff simulations in case of present land use condition
and case that forested area change an urbanized area to the condition of the urbanization rate
8
0
Field Observations of Discharge and Runoff Characteristics
in Urban Catchments Area
Rainfall[mm/10min]
9/Sept./2001
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
0
6:00
12:00
9/Sept.
18:00
24:00
6:00
10/Sept.
Time
Figure 10. Calculated and observed discharge
of land surface at the outlet in O city basin for
surface flow
0
Total rainfall=43.5[mm]
3
Peak rainfall=3.0[mm/10min]
○ ：observed
0.2
：Calculated
5
0
6:00
12:00
9/Sept.
18:00
24:00
Rainfall[mm/10min]
5
9/Sept. 1996
0.4
Discharge[m /s]
3
Discharge[m /s]
0.2
Total rainfall=43.5[mm]
Peak rainfall=
：Calculated（All discharge） 3.0[mm/10min]
：Calculated（House）
：計算値（Road）
：計算値（Field）
○：Observed（Outlet）
0
Rainfall[mm/10min]
9/Sept./1996
0.4
6:00
10/Sept.
Time
Figure 11. Calculated and observed
discharge at the outlet in O city basin
70 %, 75 %, 80 %, 85 % and 90 %. Here, we defined the impervious area rate as urbanization
rate. We defined the morphological parameters as follows. The thickness of surface soil layer
is 15 cm, the effective porosity is 0.25, the slope gradient is 1/1000, the slope length in
impervious area is 50 m, the slope length in field area is 25 m, the slope length in forested
area is 40 m, the saturated hydraulic conductivity in impervious area is 0.00001 cm/s, the
saturated hydraulic conductivity in field area is 0.001 cm/s, the saturated hydraulic
conductivity in forested area is 0.01 cm/s, the manning roughness coefficient in impervious
area is 0.03 m-1/3s, the manning roughness coefficient in field area is 0.04 m-1/3s and the
manning roughness coefficient in forested area is 0.05 m-1/3s. Urbanization of forested area
was expressed by the change of land use, therefore, the change is expressed the change of
saturated hydraulic conductivity, slope length and manning roughness coefficient. The
calculated discharge hydrograph is shown in Figure 12. It was found that the calculated
overland flow discharge from impervious area increase with increasing the urbanization rate.
And, it was found that the discharge of subsurface flow generated from pervious area
decreases with increasing the urbanization rate. And peak discharge value is almost twice as
the value before urbanization. The relationship between urbanization rate and runoff
coefficient is shown in Figure 13. The runoff coefficient is defined as the ratio of the peak
rainfall and the peak runoff rate. From the result, it was found that the runoff coefficient, in a
word, the peak discharge almost rises linearly with the increase of urbanization rate. The
existence forested area make large influence on peak discharge in the area where the forested
area occupied with about 23 % like an Itachi river basin. And, it was shown that the effect of
forested area, in other word, urbanization on flood runoff was large.
CONCLUSIONS
The purpose of present paper is to clarify the runoff phenomenon in urban catchments area
based on field observations of discharge and application of the runoff model for land surface
area based on morphological properties in order to evaluate the urbanization on flood runoff.
The conclusions obtained in the present paper are as follows.
•
Field observation of velocity and the water depth are carried out in several urban areas
and it is found that the concentration time of flood in urban area is very short. And, it
Akabane et al.
9
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
0
25
Discharge from Forest
10
：Present land use
(Urbanization rate 67％)
：Urbanization rate 70％
：Urbanization rate 80％
：Urbanization rate 90％
15
0.6
0.55
Runoff coefficient
Total rainfall：88.5[mm]
50 Peak rainfall：7.0[mm/10min]
Surface runoff
：Present land use
(Urbanization rate 67％)
：Urbanization rate 70％ 5
：Urbanization rate 80％
：Urbanization rate 90％
Rainfall[mm/10min]
3
Discharge[m /s]
75
0.5
0
0:00 10/Sept.
12:00 10/Sept.
Time
0:00 11/Sept.
Figure 12. Land change impact due to
urbanization
•
•
•
0.45
0.4
50
60
70
80
90
100
is found that the impervious area rate
Urbanization rate[％]
plays very important role in runoff Figure 13. Relationship between
process and simulation.
urbanization rate and runoff coefficient
The proposed rainfall runoff analysis
method for land surface area based on
morphological properties can apply universally in fully urbanized catchment area and
urban catchment area which forested area exists in upstream area.
From the results of application of the runoff model to the several urban catchments
area, it was found that Overland flow is mainly generated from impervious area when
rainfall intensity exceeded water holding capacity of soil. Meanwhile subsurface flow
is mainly generated from pervious area when water holding capacity of soil exceeded
rainfall intensity.
The existence of forested area decreases peak discharge in the area where the forested
area exists by about 23 % like an Itachi river basin. Therefore, it was shown that the
effect that the forested area gave to the runoff phenomenon was large.
REFERENCES
Akabane Y., S. Kure, R. Ebana and T. Yamada (2006), Effects of urbanization on flood
runoff characteristics in an urban catchment area, The 3rd Asia Pacific Association
of Hydrology and Water Resources Conference, CD: ST1-12-A13-414.
Kure S. and T. Yamada (2004), Nonlinearity of runoff and estimation of effective rainfall in a
slope, The 2nd Asia Pacific Association of Hydrology and Water Resources
Conference, 2, 76-85.
Tsuchiya S., M. Dohi, S. Unno and T. Yamada (2002), Studies on flood runoff characteristics
of urban river using a physically based model of sewage networks, Annual Journal
of Hydraulic Engineering, JSCE, Vol.46, pp 259-264. (In Japanese)
Kure S., T. Yamada and H. Kikkawa (2004), A study on multilayer runoff analysis method
considering the generation of overland flow, Annual Journal of Hydraulic
Engineering, JSCE, Vol.49, pp 169-174. (In Japanese)
Kure S. and T. Yamada (2006), A study on the effects of slope and river in runoff, Annual
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Akabane Y., S. Kure and T. Yamada (2008), Field observation of discharge in urban
catchments and effects of urbanization on runoff characteristics, Annual Journal of
Hydraulic Engineering, JSCE, Vol.52, pp 481-486. (In Japanese)
Kure S., Tomizawa, R. Ebana and T. Yamada (2007): The effects of river and slope on runoff
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10
Field Observations of Discharge and Runoff Characteristics
in Urban Catchments Area