Modeling the Influences of Weir on Habitat Condition in Da-Han River, Taiwan
WEN Po-Wen1, MAO Chen-Tai1, WU Ray-Shyan1
1 Department of Civil Engineering, National Central University, Taoyuan 320, Taiwan
ABSTRACT: In this study the relationship between hydraulic conditions and ecological habitats is discussed
and compared with the use of habitat quantization assessment technology. This is done by selecting
Acrossocheilus paradoxus as target species and using a hydraulic model HES-RAS and a habitat model
RHABSIM to calculate the ecological instream flow of the habitat in several cases of weir construction proposed
in Da-Han River, Taiwan. The influences in habitat condition are quantified with “weighted usable area” (WUA).
The habitat condition is improved by introducing weirs in the river. Furthermore, a series of low-weirs generally
provide better habitat condition than a high-weir instead.
1.
INTRODUCTION
Under the consideration of water supply and flood mediation, many hydraulic structures were constructed in
Taiwan to protect people’s lives and property. However, these structures may cause low flow condition and
result in inadequate instream flow for supporting the species in the rivers. From the ecological perspective, rivers
provide much more than what human beings need.
Recently, there are many habitat flow analyzes methods developed for the rising needs of hydraulic
engineering for quantifying environmental considerations. It is important to know how low level water flow
influences the distribution of fish’s survival spaces. Hydraulic model HES-RAS 3.0 and habitat model
RHABSIM 2.2 are used in this study to calculate the ecological instream flow, and then to analyze the
relationship between the instream flow and the distribution of fish’s survival space. Moreover, for the goal of
understanding how and what kinds of the effective distribution of fish’s survival space for a given low flow
condition, different flows of Da-Han River were studied. Furthermore, this study evaluates the influence of weirs
on the nearby habitats, and focuses on what the economic benefits of building a series of low-weirs in the
research area in stead of a high-weir.
2.
LITERATURE REVIEW
2.1
The ecological basic flow
Stalnker defined “instream flow requirements (IFRs)” means maintaining the natural resources and some
indicated-limited flows. In the 1950s, IFRs was developed and used in western USA, and made a more formal
method to evaluate the needs of flow. Jowett (1997) considered that the propose of the ecological basic flow is to
continuing a stable ecological river system.
Recently, empirical methods are easy and common methods, including New England method etc. Base on
considering the low water flow season within a year, New England method emphasizes that there should be at
least 0.55cms ecological basic flow for every 100 kilometer square in water shed for ecological protecting
purpose.
Further more, according to the flows over the history, historic flow methods are designed to predict the
local ecological basic flow. Based on the mean annual flow (MAF), Tennant studied how the ecological
environment would be, in order to make different ecological protecting standards. As for the daily
flow-postpone-line method, it is recommended that taking 96% ( Q96 ) as the ecological basic flow.
2.2
Habitat methods
The basic idea of hydraulic methods is the relationship between ecological function and the instream flow.
Wetted Perimeter approach is one of the common hydraulic methods. Bartschi(1976) suggested that an
acceptable flow is the flow can maintain 80% of the Wetted Perimeter provided by the mean annual flow.
Tennant (1976), however, proposed a flow rate at the inflection point of the wetted perimeter-flow curve, which
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often is the key point while the water dramatically decreasing. Therefore, this flow can be seen as the basic flow
maintaining stable ecological status.
In order to protect the fish habitat and its grouping structure, the U.S. Fish and Wildlife Service (USFWS)
developed “Instream Flow Incremental Methodology” (IFIM) to identify flow requirement. The most significant
character of IFIM is changing the idea of the least flow fishes need to the relationship between flow and habitats.
Bovee (1982) considered IFIM is a linking concept, method and a program which combines the type of channel,
the characteristic of flow and the index-species through different simulating flows to predict whether the size of
habitats increases or decreases. In the field of ecology, IFIM develops a Habitat Suitability Curves (HSC).
2.3
Habitat improving engineering methods
Gore (1996) simulates Brushy Branch with a series of 3 low-weirs, and then finds out that the area of habitat
increases 5 times under the condition of low flow. Therefore, he points out that low weirs play a very important
role of expending the size of habitat. Wu (2000), however, tries to analyze habitat through Ho-Don-Con River
habitat-space-developing project which is for the improving of the environment of habitat by past human made
revetment region as an ecological engineering practice. Performs ecological engineering practice to improve the
habitat, then predicts sand start condition by the start critical diameter, and then considers critical diameter to
represent minimum diameter on river bed. By analyzing the target fish habitat suitability index in this stream,
Wu gets a finding about how the size of habitat enlarges a lot because of the change of suitability index for a
given the character of riverbed send diameter.
3.
THEORY
In this research, a hydraulic model HEC-RAS is employed to predict the water level data from the already
known sectional flow. Subsequently, the speed, weighted usable area and the ecological basic flow are computed
by using RHABSIM’s hydraulic and habitat sub-models. The explanations of HEC-RAS model and RHABSIM
model are as followed.
3.1
HEC-RAC model
HEC-RAS model is developed by Hydrologic Engineering Center, U.S. Army Coors of Engineerers. HEC-RAS
model can be used to simulate how hydraulic structure influences flow. In a word, HEC-RAS model is based on
‘standard step method’ which predicts next section data from the former section data. This research only utilizes
the predicted average water level as the input condition for RHABSIM model. Regarding to predicting the flow
velocity, this study employs RHABSIM model instead. This is mainly because when analyzing habitat, the total
situation of the habitat section cannot be presented by using the average water depth and the average flow
velocity in that section. As a result, RHABSIM model is utilized for predicting water depth and flow velocity in
every single point.
3.2
RHABSIM model
In 1970s, U.S. Fish and Wildlife Service proposed Internal Flow Increase Methods (IFIM) which requires
PHABSIM model developed by USFWS for data-analyzing. PHABSIM model is mainly composed by two parts,
one is the hydraulic sub-model, and the other is the habitat sub-model. In fact, the predicting data which comes
from PHABSIM hydraulic sub-model is input and then simulated through the habitat sub-model to obtain the
result so called “weighted usable area” (WUA). The system uses Riverine Habitat Simulation system
(RHABSIM) which originally comes from PHABSIM model.
The main function of the hydraulic model is to calculate the flow field, both velocity and depth for a given
flow rate in a given section area. RHABSIM hydraulic model offers 3 different water level forecasts for
evaluating water depths. These are Log-Log Regression (L-L), Channel Conveyance (C-C), and Step-Backwater
(S-B) method. Though RHABSIM provides 3 methods for evaluating water level, the section of L-L needs at
least 2 sets of flow-level data while C-C has to have one set. If S-B is used, it can not deal with the situation
when the upstream lowest base is even lower than that in downstream. Due to the difficulties in collecting data
and there exist some deep water area in this research, the water-level data estimated from HEC-RAS model
under some flow situation is used, instead of the estimation from RHABSIM model.
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Firstly, distribution of depth and velocity with any flow are obtained. Then by introducing the parameter of
the target species habitat suit curve (HSC), the corresponding HSC index, so called “weighted usable area”
(WUA), for each of the cell sections is obtained.. The formula of WUA is:
WUA   F  f (Vi ), f ( Di ), f (Ci) Ai
(1)
i
Where F   is the combined suitability factor (CSF). Ai is the water surface area in ith cell section in
study area. f Vi  , f Di  and f Ci  are the suitability index in ith cell section for flow velocity, depth and
the base components of river bed.
4.
CASE STUDY
4.1
Set up acrossocheilus paradoxus suitability curve
Acrossocheilus paradoxus is chosen as the characteristic fish in Taiwan, as proposed in Zhan (1996). The
Council of Agriculture (COA) Taiwan built up wildlife distribution database by GIS for the whole island of
Taiwan and it is noted that the spread of Acrossocheilus paradoxus suffuse in middle of western Taiwan river
field (1999).
Selecting the Acrossocheilus paradoxus suitability curve produced by Taiwan Endemic Sepecies Reesearch
Institute, COA, which is based on the study region in upstream of Zhuo-Shui river located in Central Taiwan in
year 2000. Taking into account that the fish can not still living in extreme fast velocity region and the size of the
fish and its accepting pressure, the original suitability curve is modified by smoothing and regressing. This is
done based on two assumptions. One is that the suitability reaches zero when velocity is fast than 1.3m/s. The
other is that the suitability is also set to zero if water depth is shallower than 0.05m or deeper than 1.30m.
(Zhang, 2002). There results are shown in Figure 1 and 2.
Figure 1. Acrossocheilus paradoxus suitability curve
of depth (modify)
4.2
Figure 2. Acrossocheilus paradoxus suitability curve
of velocity (modify)
Case study
This case study region is between Xi-Zhou bridge to Yong-Fu stream in the middle Da-Han river, Northern
Taiwan. The river of interested is 10.24 kilometers long, divided into 19 cross sections. These sections were
survived by Water Resource Agency, Ministry of Economic Affairs Taiwan in Dec. 2000. The Manning’s rough
parameter is estimated to be from 0.055 to 0.06. The daily flow rates are recorded from 1997 to 2001, which
show the low water season being from Nov. to Apr. with average daily flow for a 10 days period is 15.34 cms.
For understanding the ecological in-stream flow of this river, the ecological in-stream flow by habitat
method is estimated and is compared with that from the historic method and that from the empirical method.
First, by applying the history method, the 95 percent daily flow is suggested as the ecological instream flow of
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the river in Taiwan. In this case, the suggested ecological in-stream flow is 4.41cms. On the other hand, Tennant
method suggests the ecological in-stream flow to be 10 percent of mean annual daily flow. In this case, the
suggested ecological in-stream flow 3.47cms. Second, the empirical method, so called New England method,
suggests ecological in-stream flow is 4.20cms, estimated based on the 763.4 kilometer square of drainage area.
Finally, by applying the habitat method with a range of flow rates from 1cms to 45cms, given the maximum flow
being 43.64cms in low water season. Furthermore, it is assumed that each section influence extent is the same for
the up stream or down stream habitat, as shown in Figure 3.
It is a matter of choice in selecting an adequate ecological instream flow with respect to different water
resource management practices. The maximum WUA should be selected when developing ecological habitat is
the major concern. A common approach is to search for the inflection points in flow-WUA diagram and choose
the correspondence flow as the ecological instream flow. Two inflection points, corresponding to 3cms and
16cms are observed, as in Figure 4. Clearly, if the ecology is the primary target, the instream flow can be set to
16cms. However, the water resource efficiency requires considering in this case because a major reservoir is
built upstream. Therefore, the better choice is suggested to be 3 cms in this case.
Figure 3. Influence range in each section
4.3
Figure 4. Discharge verses WUA for Acrossocheilus
paradoxus .
The hydro-structure influence for habitat and its effect
The county government of Taoyuan Taiwan proposes to build a dam that locates in 130 meters downstream of
Wu-Ling bridge. The purpose of this dam is to protect the bridge’s safety when flood scours river bed. For the
Acrossocheilus paradoxus, analysis is done for identify in the influence on the environment by using same
approach described above.. The influence range should be modified because the influence changes when dam is
built. The answer shows that habitat area increases after the dam built in low water flow simulation, as show in
Figure 5. The reason is that the dam causes higher water level and benefits fish living.
On the other hand, the influences of different heights of dam are studied. Several different heights and their
relating WUA relationship are analyzed for three flow rates, namely, 3cms, 16cms and 45cms. Furthermore, it is
assumed that the building cost for each additional unit height of dam is same. However, as show as Figure 6, the
WUA remains merely unchanged when a dam lower than 2 meter is built. But the WUA varies when the height
added more than 3 meters. The maximum WUA is found at 5 meters height of dam given a flow of 45 cms. In
addition, the WUA decreases when dam becomes higher than the 5 meters. It can be found that when water level
rising causes velocity falling, the WUA decrease. For the flow at 16 cms or 45 cms, these relationships have
somehow slightly different.
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Figure 5. Relationships of WUA with height influence, Figure 6. Relationships of WUA with height influence
in different flow rate.
given dam height 7.5m for low-water season flow.
Let it be noted that the WUA in the case of 5 meter height of dam with 3 cms flow is greater than that in
nature section with 45 cms flow. This demonstrates the effect of ecological habitat by setting up the dam.
However, a relatively high dam may cause negative impact on fish living space and limit the local ecological
system. In spite of the habitat area that benefit from the dam, the localized ecological system’s effect can indirect
damage the ecological balance. Thus, the effect of a series of low-weir dams for habitat is studied. By analyzing
the WUA resulting from different heights of dams, one can determine which is better for the ecological system, a
single high dam and a series of low-weir dams.
Assume the building cost for unit height of a dam is same. Alternative way is that besides to build a
low-weir dam in the same location where the single high dam is, another low-weir dam is built upstream. The
heights of the two low-weir dams are denoted by a and b, as listed in Figure 7. The WUAs estimated by
RHABSIM model are compared with that from a single high dam, as shown in Figure 8.
Figure 7. Location of multi low-weir dam section
Figure 8. Relationships of WUA with height influence
in 3cms
For the same height in total, the WUAs of a series of low-dams present diverse results, compared with that
from a single high dam in low flow rate (Q=3cms). The reason is that a high dam can increase habitat area in tail
water area, but in the other ways decrease habitat in near dam area. Summary of the WUA for a high dam shows
some decreasing as height increasing. At the same time, the equivalent series of low-weirs may increase the
WUA at near dam region for some specially composing.
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Construct a dam can increase habitats. To obtain same habitat area condition, i.e., the same WUA, as listed
in Table 1, building a 5 meter height dam can decrease flow requirement by 1.90cms, based on a nature flow of
15cms. Thus this flow can be diverted for other purpose, such as water supply.
Table 1. Different flow requirements with building dams or without building dams
Nature
Flow rate
Habitat
Q1(cms)
(WUA)
15
16173
20
16627
30
16849
40
17229
5.
Habitat
(WUA)
17231
17777
18324
18737
Single dam(5 m)
As same habitat area condition, single
dam need flow rate Q2(cms)
13.10
13.78
14.17
15.00
The effect of dam
Q1-Q2(cms)
1.90
6.22
15.83
25.00
SUMMARY AND CONCLUSION
In this study, combine Acrossocheilus paradoxus with HEC-RAS hydraulic model and RHABSIM model to
estimate the ecological Instream flow requirement. Discussion on the influence on habitat for hydro-structure is
also provided. The following conclusions may be drawn:
1. For Da-Han river middle stream area, if the ecology is the primary target, the instream flow can be set to
16cms. However, the water resource efficiency requires considering in this case because a major reservoir is
built upstream. Therefore, the better choice is suggested to be 3 cms in this case.
2. The WUA remains merely unchanged when a dam lower than 2 meter is built. But the WUA varies when the
height added more than 3 meters. The maximum WUA is found at 5 meters height of dam given a flow of 45
cms. In addition, the WUA decreases when dam becomes higher than the 5 meters. It can be found that when
water level rising causes velocity falling, the WUA decrease. For the flow at 16 cms or 45 cms, these
relationships have somehow slightly different.
3. For the same height in total, compared with that from a single high dam in low flow rate (Q=3cms). Summary
of the WUA for a high dam shows some decreasing as height increasing. At the same time, the equivalent series
of low-weirs may increase the WUA at near dam region for some specially composing.
6.
REFERENCE
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Bovee, K. D. (1982), ”A guide to stream habitat analysis using the instream flow incremental methodology”, US
Fish and Wildlife Service Biological Services Program, FWS/OBS-82/26.
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of
agriculture
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Web
site ：
http://wagner.zo.ntu.edu.tw/wildlife/
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