INTRODUCTION
Flooding is a serious natural disaster which has many socioeconomic and
environmental consequences for all activities and infrastructure within
an affected floodplain. Regarding to the continually-growing population
of the world, it is certain that without proper guidance, a large number
of homes will be built in areas that place them at the mercy of flood
events (Walker and Maidment, 2006). Therefore, Accurate delineation of
flood extents and depths within the floodplain is necessary for flood
management and to make accurate decisions regarding construction and urban
development, insurance and other regulated practices on land and property
potentially affected by flooding (Noman et al., 2003).
Introducing of advance computer technology to hydraulic simulation provided
greater flexibility for the purpose of floodplain mapping. Recent advances
through the combination of hydraulic simulation model, HEC-RAS within
Arcview GIS environment have the potential to further that flexibility
to create geometric representations, simple import and export capabilities
and displaying the results in spatial format in a more cost-effective
manner. As Werner (2001) stated the end result of the process is not only
quicker floodplain delineation with greater accuracy than traditional
methods, but also a flow depth grid could be extracted, indicating the
level of inundation in the floodplain. It is evident that GIS has a great
role to play in floodplain mapping and other disaster management because
natural hazards are multi dimensional and the spatial component is inherent
(Coppock, 1995).
GIS greatly facilitates the operation of floodplain mapping and flood
risk assessment. The main advantage of using GIS for flood management
is that it generates a visualization of flooding that could be very useful
in flood mitigation planning process. Floodplain mapping have been conducted
by some researchers on several regions using integration of HEC-RAS hydraulic
simulation model and GIS.
Earles et al. (2004) demonstrated the utility of the HEC-geoRAS
model for floodplain delineation and determination of key hydraulic parameters
and also, HEC-RAS capability of producing hydraulic results in Los Alamos,
New Mexico, USA. Abdalla et al. (2006) introduced hybrid approach
for flood risk assessment through GIS. The developed approach was based
on the integration of hydraulic simulation and GIS analysis, which allows
spatial-based visualization and prediction of flood disaster. The results
indicated that the developed methodology was efficient in modeling and
visualizing the spatial extent of different flood scenarios and in determining
flooded areas at risk. Williams (2006) carried out a study on Santee River.
It was outlined the advantages of integration to model the impact of the
Santee River rediversion, which was completed in 1941.
This resulted in rapid silting of the Charleston Harbor and in 1984 flow
was rediverted into the Santee valley about 30 km downstream of the dam.
Flooding pattern within the Santee floodplain has been altered by rediversion
and operation of the hydroelectric plant on the rediversion canal. HEC-RAS
3.2 and the HEC-geoRAS extension for Arc-View were used to examine the
flooding regime prior to and subsequent to rediversion operations. Yang
et al. (2006) developed a direct-processing approach to river system
floodplain delineation. Floodplain zones of the South Nation River system,
located east of Ottawa, Ontario, were mapped in two dimensions and three
dimensions by integrating the hydraulic model with GIS. HEC-RAS simulations
were performed to generate water surface profiles throughout the system
for six different design storm events. The in-channel spatial data were
mapped in the GIS domain and floodplain zones for the six design storms
were reproduced in three dimensions by overlaying the integrated terrain
model for the region with the corresponding water surface Triangulated
Irregular Network (TIN). Chuan and Jing (2006) explored the methodology
for compiling the torrent hazard and risk zonation map by means of GIS
technique for the Red River Basin in Yunnan province of China, where was
prone to torrent. Six different factors were analyzed and superimposed
to create the torrent risk evaluation map based on a 1:250,000 scale digital
map. Alho et al. (2007) investigated Jökulhlaups that are
the consequence of a sudden and significant release of meltwater from
the edge of a glacier. Such floods are sourced commonly from ice-dammed
lakes, but occasional volcanic eruptions beneath ice can produce intense
jökulhlaups due to prodigious rates of meltwater release. It was
presented the results of one-dimensional hydraulic modeling of the inundation
area of a massive, hypothetical jökulhlaup on the Jökulsá
á Fjöllum River in Northeast Iceland. Remotely sensed data
were used to derive a digital elevation model and to assign surface-roughness
parameters. Also it was used a HEC-RAS/HEC-geoRAS system to host the hydraulic
model; to calculate the steady water-surface elevation; to visualize the
flooded area; and to assess flood hazards.
In this research, 3 km end of Zaremroud River, upstream of the Tajan
River, was selected. The main objective of this study is to accurate delineation
of flood extents and depths within the floodplain based on the integration
of hydraulic simulation model, HEC-RAS and GIS analysis. The results of
this research could be used for flood mitigation planning purposes, regarding
to the historical flood threatens in the study area.
MATERIALS AND METHODS
Study area: Zaremroud River lies in about 15 km southeast of Sari,
Mazandaran, Iran between 36° 26` 15" to 36° 26` 44" N latitude
and 53° 8` 23" to 53° 9` 52" E longitude. The length of river
is approximately 95 km that it flows eastward to westward to join Tajan
River and ultimately draining in to the Caspian Sea. The area has a supper
humid climate and the maximum precipitation occurs in autumn and minimum
precipitation in summer. The maximum and minimum temperature in Zaremroud
Catchment is 20.4 and 8.95°C, respectively; and the mean annual temperature
is about 14.5°C. The length of river reach selected in this study
is about 3 km of Zaremroud River. The village, Garmroud is located in
the adjacent to this reach of river. According to a report from Mazandaran
Jahad Agricultural organization the dangerous floods of this river caused
lots of losses such as breaking down two bridge openings of river and
destruction of Garmroud hydrometric station and some similar cases during
recent years. In addition, the river stream causes bank undercutting that
may cause land sliding some parts of village in the long term.
Datasets: The analyses of this research relied on two types of
data; annual peak flow, GIS data including cross sections, elevation points
and topographic map. Peak flow data recorded at Garmroud hydrometric station,
located in upstream of the selected reach, was used for this study. The
length of data set is 24 years from 1986-87-2000-01 water years. After
evaluation of the accuracy of the data, flood frequency analysis could
be conducted using different statistical distribution. The most commonly
used statistical distribution for flood frequency analysis is included
Log-Normal, Log-Normal III, Pearson III, Log-Pearson III and Gumbel. Estimated
peak flow in 2 to 100 years return periods can be used as input for the
hydraulic simulation of the river reach.
Topographic map of area with scale of 1:25000 and river plan with scale
of 1:1000 was applied for TIN generation, using 3D analyst capability
of ArcView. TIN is used for preparation of required data for hydraulic
simulation in HEC-RAS. Whereas the surveyed points of the Zaremroud River
plan have relative elevation, GPS have been used for determining absolute
and correct positioning of all cross sections and elevation points. The
HEC-geoRAS extension is used in conjunction with 3D analyst for interpolation
of digital terrain data and Spatial Analyst for proper display of the
output flow depth grids and velocity grids.
Hydraulic simulation: In this research HEC-RAS was used which
is a numerical model that designed for hydraulic simulation. This model
could be used to perform one-dimensional steady flow, unsteady flow calculations.
The system is comprised of graphical user interface, separate hydraulic
analysis components, data storage and management capabilities, graphical
and reporting facilities. The steady-flow version of the model solves
one-dimensional step-backwater calculations. To use this version for the
natural river, it is assumed that flow is comparatively steady along the
whole reach because time-dependent variables are not included in the energy
equation; flow varies gradually between cross-sections due to the energy
equation having a postulated hydrostatic pressure distribution at each
cross-section; flow is one-dimensional and therefore the calculation is
based on the premise that the total energy head is the same at every point
in a cross section; the bed-slope of the channel is less than 10% because
the pressure head is represented by water depth, which is measured vertically
in the energy equation; and the energy slope is constant over the cross-section
(Hydrologic Engineering Center, 2005).
Steady flow analysis is applied to calculate water surface profiles for
steady gradually varied flow condition. Additionally the steady flow component
is capable of modeling subcritical, supercritical and mixed flow regime
water surface profiles (Snead, 2000). The basic computational procedure
in HEC-RAS model is based on the solution of the one-dimensional energy
equation. The energy equation is written as Eq. 1. Energy losses are evaluated
by friction (i.e., Manning`s equation) and contraction/expansion coefficient
multiplied by the change in velocity head.
Where:
| y1, y2 |
= |
Depth of water at cross sections |
| z1, z2 |
= |
Elevation of the main channel inverts |
| v1, v2 |
= |
Average velocities (total discharge/total flow area) |
| a1, a2 |
= |
Velocity weighting coefficients |
| g |
= |
Gravitational acceleration |
| he |
= |
Energy head loss |
The momentum equation is utilized in situations where the water surface
profile is rapidly varied. These situations include mixed flow regime
calculations, hydraulics of bridges and evaluating profiles at river confluences
in stream junctions. Water surface profiles are computed from one cross
section to the next by solving the energy equation with an interactive
procedure called the standard step method.
The basic data requirements for simulation are included: geometric data,
study limit determination, river system schematic, cross section geometry,
ineffective flow areas, reach lengths, energy loss coefficients, Manning`s
n, Equivalent Roughness ‘k`, contraction and expansion coefficients,
steady flow data, boundary condition, flow regime. Selection of a suitable
value for Manning`s n is very significant to the accuracy of the computed
water surface profiles.
Boundary conditions are another part of model that must be completed.
Boundary conditions are necessary to establish the starting water surface
at the ends of the river system. In a subcritical flow regime, boundary
conditions are only required at the downstream ends of the river system.
If a supercritical flow regime is going to be calculated, boundary conditions
are only necessary at the upstream ends of the river system. If a mixed
flow regime calculation is going to be made, then boundary conditions
must be entered at all open ends of the river system. Ultimately after
completing of all essential data, model could be run.
RESULTS AND DISCUSSION
In this research, steady flow was simulated along 3 km end of Zaremroud
River, upstream of the Tajan River in North of Iran. HEC-RAS simulation
model in combination with GIS capabilities was used for this purpose.
After preparing the project file, a TIN theme was extracted based on georeferenced
field cross sections and topographical data in order to prepare required
data to be processed. Topographic map with scale of 1:25000 and river
plan with scale of 1:1000 were applied for TIN generation using 3D analyst
capability of ArcView. The HEC-geoRAS extension is used in conjunction
with 3D analyst for interpolation of digital terrain data and Spatial
Analyst for proper display of the cross sections. The stream centerline
and left and right channel banks, flowpath and cross section cut lines
themes have prepared and then generate RAS GIS import file for hydraulic
simulation in HEC-RAS model.
Frequency analysis of peak flow data was conducted to select the most
accurate input for the hydraulic simulation of the river reach. It has
shown that Log-Pearson III is the best distribution to estimate peak flow
in different return periods, regarding to the least differences between
observed and estimated peak flow. Table 1 has shown magnitude of peak
flow in 2-100 years return periods. Peak flow estimated using flood frequency
analysis (Table 1) was used as the steady flow data for simulation. Normal
depth for upstream and critical depth for downstream was considered as
boundary conditions for this analysis. Other inputs such as Manning`s
n value, river system schematic, contraction and expansion coefficients,
flow regime entered to model and HEC-RAS model has run for steady flow
and mixed flow regime.
| Table 1: |
Peak flow rates for 2-100 years return periods |
 |
|
| Fig. 1: |
Flood level for 2 years peak flow rate in one of the cross sections |
|
| Fig. 2: |
Flood level for 100 years peak flow rate in one of the cross sections |
Flood levels in one of the analyzed cross sections can be shown in
Fig.
1 and
2 for 2 and 100 return periods, respectively.
There is more than 1.5 meter difference between flood levels in two mentioned
return periods (
Fig. 1,
2). One of the
most important results of HEC-RAS simulation is preparing different water
surface profiles of different T-year floods. In the next step, the results
of hydraulic simulation within HEC-RAS model were exported to GIS for floodplain
delineation and further analysis. Delineation of flood extents and depths
within the floodplain of Zaremroud River was conducted in different return
periods based on the integration of hydraulic simulation results and GIS
analysis using the HEC-geoRAS extension of ArcView.
Figure
3 and
4 have shown flood affected area for the 2 and
100 years flood events, as a sample in the study area.
Critical flooding area along the river could be distinguished based on
the grid layer of flood depths. As can be shown in Fig. 3 and 4,
flood affected area for 2 and 100 years flood events were compared. Flood
affected area for 100 years event is much larger, as it can be very close
to the residential area of Garmroud village. As an implication result
of this study, flood affected area in the upstream reach of the river
were evaluated. As can be shown in Fig. 4, two sample critical area were
specified using A and B remarks. It has shown that in point A, flood extents
are affected some area near the village. To prevent the flood hazard to
the village (section A), structural measures including flood walls and
levees construction and also channel excavation and modification, should
be considered for future flood mitigation plans.
 |
| Fig. 3: |
Flood affected area for 2 years flood events |
 |
| Fig. 4: |
Flood affected area for 100 years flood events |
Application of hydraulic modeling in GIS environment provides the capability to
simulate flood depth in different part of the floodplain. Delineation of flood
depths within the floodplain have shown in
Fig. 3 and
4
for 2 and 100 years return periods, respectively. Hydraulic levels can be seen
very variable, which is depends on geometry of the channel and the hydraulics
condition of the river system. As can be seen in
Fig. 4, flood
depths are critical just before second meander (section B), about 300 m after
the location of the village, which is one other sample point in the reach that
have critical flood hazard. In this section, transition of the flow condition
from subcritical to supercritical and vise versa could be analyzed. Structural
measures including flood walls and levees construction and also channel excavation
and modification should be considered for future flood mitigation plans at the
section B. Structural measures could prevent floodwaters from inundating surrounding
farmland and residential areas.
Hydraulics simulation for floodplain mapping could be beneficiary in several
aspects for land and water resources management and also engineering purposes.
It can be applied to prevent unwise land use in floodprone areas and flood insurance
studies, based on modeling of water surface elevations for design flood events.
The design of bridge and culvert openings for roadway crossings of streams and
consequences of flood reduction measures such as dams, levees and channel modifications
could be predicated on proper floodplain hydraulic analysis. Increasing the
size, slope, or depth of the channel or decreasing its roughness can lead to
a reduction in flood levels because of the additional channel capacity. On the
other hand, channel modifications can also have negative effects, such as increasing
in flow velocity, which could be simulated using hydraulic model.
CONCLUSION
This study focused on integrating of hydraulic simulation with GIS analysis.
Results of this study can reasonably separate high-hazard from low-hazard
areas in the floodplain to minimize future flood losses. The evaluation
of floodplain delineation are rather complex and demanding activities,
which require a comprehensive approach to hydraulic floodplain simulation
and can be largely enhanced by using GIS capabilities. Flash floods cause
serious inundation hazards in area under urban development. Structural
and non-structural flood mitigation measures are necessary for flood hazard
control in critical area, where was specified in this research. As Correia
et al. (1998) stated GIS is a valuable tool for addressing urban
growth modeling and defining possible floodplain management measures.
Also one of the most important conclusions made from this study is that
use of GIS for the undertaking of a hydraulic simulation has the potential
to be both an accuracy improving and cost-saving for floodplain and flood
hazard mapping.
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