sophistication of dam operation for flood control by use … management and dam division, river...
Post on 24-Apr-2018
221 Views
Preview:
TRANSCRIPT
Water Management and Dam Division, River DepartmentWater Management and Dam Division, River DepartmentJuly 2009July 2009
Sophistication of Dam Operation for Flood Control by Sophistication of Dam Operation for Flood Control by Use of Rainfall Prediction TechnologyUse of Rainfall Prediction Technology
Climate Change Adaptation Research GroupClimate Change Adaptation Research Group
National Institute for Land and National Institute for Land and Infrastructure ManagementInfrastructure Management
Functions of multipurpose dam洪水調節方式(国土交通省ダム、水資源機構ダム)
その他方式, 11
一定量放流方式,
27
一定率一定量放流方式, 46
自然調節方式, 26
Flood control methods used at multipurpose dams can be broadly categorized into four types: ungated control method, constant volume discharge method, constant-rate constant-volume discharge method and others (e.g., bucket bottom cut method). The method most frequently used by the Ministry of Land, Infrastructure, Transport and Tourism and Japan Water Agency is the constant- rate constant-volume discharge method.
Flood control methods
What is the "constant-rate constant-volume discharge method"?
In the constant-rate constant-volume discharge method, when inflow has exceeded the flood control action level discharge (discharge below which downstream areas and facilities in river areas do not suffer major damage; the dam operating rules define refers to it as "flood discharge"; also known as "harmless discharge"), inflow is reduced at a constant rate until the peak inflow. After the peak inflow, water is released by constant volume.
In this method, the flood control effect of dams can be expected even in cases of medium or small floods. This method, therefore, is suitable for rivers whose downstream channels have not been improved substantially through river improvement projects. Compared with the constant-volume discharge method, however, this method requires a larger flood control reservoir capacity.
Constant-volume discharge method TIme
Discharge
Outflow discharge
Inflow
Discharge cutoff
Constant-rate constant-volume discharge method
Time
Flood control action level discharge =
flood discharge
Flood control methods (MLIT and JWA dams)
Discharge Discharge cutoff
Outflow discharge
Inflow
Other methods
Ungated control method
Constant-volume discharge method
Constant-rate constant-volume
discharge method
Functions of multipurpose dam
If the reservoir level is expected to exceed the surcharge level (upper limit level of flood control capacity) (i.e., if an extreme flood is expected), when the reservoir level has exceeded the 80% level of the reservoir capacity, dam operation procedures are taken so as to make the outflow discharge close to the inflow in order to prevent the reservoir level from exceeding the surcharge level.
=
These are called particular operating rules.
Besides the flood control methods as mentioned earlier, methods for other purposes such as water supply are stipulated in the operating rules for each dam.
When the occurrence of a flood exceeding the design basis flood for the dam is expected, the following action is taken.…
Note: In this case, the outflow discharge will never exceed the inflow.
Note: The outflow discharge will never exceed the inflow at or around the flood peak discharge.
Functions of multipurpose damExample of operation under the particular operating rulesIf a great flood exceeding the design flood for the flood control plan occurs, the discharge-reducing effect of the dam decreases.
無害放流量
600 [m3/s]
計画最大放流量
2400 [m3/s]
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
7/17 7/18 7/19 7/20 7/21 7/22 7/23 7/24 7/25 7/26 7/27 7/28 7/29
流量
[m3/s]
0
20
40
60
80
100
120
140
160
180
200
雨量
[mm/hr]
実績流域平均雨量[mm] 実績ダム流入量[m3/s]
実績ダム放流量[m3/s]
有効貯水容量
77,500 [千m3]
制限水位容量
5,500 [千m3]予備放流水位容量
3,000 [千m3]0
20,000
40,000
60,000
80,000
100,000
120,000
7/17 7/18 7/19 7/20 7/21 7/22 7/23 7/24 7/25 7/26 7/27 7/28 7/29
ダム
容量
[千m3]
実績ダム容量[千m3]
T Dam: Example during the July 2006 flood
Maximum inflowMaximum outflow discharge
Almost full
Design maximum outflow discharge
Harmless outflow dischargeDis
char
ge
Rai
nfal
l
Observed basin average rainfall [mm]
Actual outflow discharge [m3/s]
Actual reservoir inflow [m3/s]
Res
ervo
ir ca
paci
ty [
1,00
0 m
3] Effective storage capacity
Flood control level capacityFlood-season limited water level capacity
Actual reservoir capacity [1,000 m3 ]
Problems of current dam operating methods(1) In the event of an extreme flood, a outflow discharge higher than the design
maximum outflow discharge is needed under the so-called "particular operating rules." As a result, the damage-mitigating effect is reduced.
→ It is hoped that operation under the particular operating rules is avoided.(2) Even if an extreme flood is expected, preliminary release is not necessarily
carried out.→ There are no established criteria based on rainfall prediction for deciding whether to
carry out preliminary releases. (3) In the constant-rate constant-volume discharge method (a method which is
currently in use at many dams and which does not utilize rainfall prediction), even when a medium or small flood is expected and the reservoir has sufficient flood control capacity, downstream damage might be caused by a outflow discharge exceeding the harmless discharge.
→ If it can predicted that a medium or small flood is coming, flood control can be done by carrying out a outflow discharge within the limit of the harmless discharge.
(4) Since water supply capacity is not currently used for flood control, the effectiveness of the flood control function relying solely on flood control capacity is limited.
→ Water supply capacity can also be utilized by making effective use of rainfall prediction.
(5) Information related to rainfall prediction is lacking, and rainfall predictions are not currently used for flood control operation.
→ The accuracy of rainfall prediction is not clearly known. Operating methods using rainfall predictions have not been clearly indicated.
Purpose of researchFurthermore . . .The frequency of damaging floods has increased in recent years, and flood risk is expected to increase in the coming years under the influence of climate change. River channel improvement has not progressed as expected because of the reduced budget for public works projects. The construction of a new dam takes considerable time and funding.
Therefore . . .It is hoped, in order to make effective use of existing dams, that a large quantity of water is stored in ordinary times and when a major flood is expected, the required capacity for flood control is created by making a preliminary release.
Therefore . . .It is hoped, in order to make effective use of existing dams, that a large quantity of water is stored in ordinary times and when a major flood is expected, the required capacity for flood control is created by making a preliminary release.
○○
Social needsSocial needs
• Mitigation of flood damage in the event of an extreme flood• Flood damage prevention in the event of a medium or small flood• Efficient flood control made possible by reconsidering conventional dam operation• Rational dam operation in coordination with water supply
Merits of dam operation methods using rainfall prediction
Use of rainfall prediction makes the following possible:Use of rainfall prediction makes the following possible:
Flood control capacity is increased by making preliminary releases, and damage is minimized by reducing the maximum outflow discharge by the constant-volume discharge method.
流量In case of extreme In case of extreme floodflood
If prediction is possible, water supply capacity can be used as part of flood control capacity.
New operation method
Outflow discharge by new operation
method
Outflow discharge
under current
operating rules
時間
Inflow
Preliminary release
If flood control capacity is thought to be larger than necessary for flood control, part of flood control capacity can be used to store flood water to eliminate downstream damage.
In case of medium In case of medium or small floodor small flood
Outflow discharge by new operation
method
流量
時間
Inflow
Discharge
Time
Outflow discharge
under current
operating rules
Discharge
Time
Previous study results and bottlenecks・Improvement of rainfall forecasting techniques (FY2007/2008)
A prediction study has been conducted by using a new forecasting model (WRF) which uses JMA predictions as initial values and boundary values. The study has confirmed that rainfall forecasting accuracy can be enhanced.→
There is still a certain degree of uncertainty because there are cases where accuracy is not improved. Quantitative evaluation of accuracy is needed.
・Effective rainfall calculation method considering the water-holding capacity of soil (FY2007/2008)The relationship between the duration of dry weather and the recovery of the water- holding capacity of soil has been formulated, and a runoff prediction method considering the initial rainfall loss has been developed.→
There are still uncertainty factors such as interception by trees and depressions
・Establishing a dam operating method using rainfall prediction (FY2008)Up-to-48-hour rainfall predictions and the reservoir capacity at the time of prediction were compared, and a method for planning releases including preliminary releases has been established.
→ If rainfall prediction errors are large, there are cases where a large quantity of water is released even though there is still storage capacity or where because storage capacity is not large enough, the reservoir becomes full so that particular operating rules are applied. There is still need to consider actions to be taken when rainfall predictions have turned out to be inaccurate.
Rainfall forecasting technique (What is WRF?)
WRF Weather Research and Forecasting Model
Mesoscale weather model developed by NACR/NCEP (USA) for forecasting and research applications
Though developed from a single project, there are two types, one for forecasting applications and the other for research applications.
Based on the lessons learned from the emergence of many mesoscale models in the United States, the new model has been developed through a concerted effort.
What is a mesoscale model?
Non-hydrostatic model dealing with meteorological phenomena with horizontal scales ranging from 10 to 100 km, such as localized heavy rains and thunderstorms
Capable of 1-kilometer mesh computation
Hydrostatic model dealing with global scale weather phenomena such as typhoons and Bai-u (rainy season) fronts
10-km mesh is the limit because of hydrostatic approximation
Used for forecasting applications because computation can be done quickly
What kind of weather model is used for weather
forecasting?
Computational flow of mesoscale weather model• Define computation domain (nesting)• Enter information in computational domain(automatic downloading from USGS, etc.)
Terrain dataLand use dataVegetation dataSoil data
Download data and prepare initial and boundary value data• Global and regional objective analysis data• Computation results from global and regional weather models
PreprocessingDefine computation domain
Enter initial values and boundary values
Forecasting computation
PostprocessingDisplay computation results
Data assimilation
Specify physical process models• Rainfall models (multiple models)• Radiation models (multiple models)• Boundary layer models (multiple models)
• Although rainfall forecasting accuracy is dependent on the kind of physical process models used, but it is determined mainly by the degree of closeness of the initial values and boundary values to the realities.
Digital national land information is also available.
3D data on wind direction, wind speed, air pressure, air temperature and humidity
Displayed computation results (examples)
3-hour forecasting 12-hour forecasting
Computation results obtained by using RSM data as initial and boundary values (a case in which accuracy is improved)
Forecasting computation by WRF Forecasting computation by JMA's RSM
•Forecasting accuracy of RSM has been improved by using WRF•RSM was fairly accurate in predicting the start and end of rainfall although the predictions were not accurate quantitatively.
Basin of Sameura Dam (August 30, 2004)
Calculation start time differs.
radar-AMeDAS
ground-based observation
radar-AMeDAS
ground-based observation
Flowchart for simulation of dam-based flood control using rainfall prediction
As
the
pred
icte
d in
flow
vol
ume
beco
mes
incr
easi
ngly
gre
ater
than
the
rem
aini
ng
rese
rvoi
r cap
acity
, the
met
hod
of re
leas
e fr
om th
e da
m is
upg
rade
d fr
om (1
) to
(5).
Predicted rainfall data(time horizon: up to 48
hours)
Calculate cumulative rainfall
Predicted total inflow
Rainfall-runoff process model
Total runoff
Predicted total
inflow
Water- holding capacity
Total rainfall
Predicted total rainfall(cumulative rainfall within up to 48 hours)
Remaining reservoir capacity ≥
Total inflowRemaining reservoir capacity is greater than total inflow.
↓(1) All inflow could be intercepted, but water is released at the safe outflow discharge only when the inflow is higher
than the safe outflow discharge.
Volume to be controlled in excess of safe outflow discharge = Total inflow −
Volume that can be released at safe outflow discharge
Calculated volume that can be released at safe outflow discharge = Duration of predicted
effective rainfall × Safe outflow discharge
Remaining reservoir capacity ≥
Volume to be controlled in excess of safe outflow discharge
(2) Constant-volume interception at safe outflow discharge
Calculate preliminary outflow discharge and
volume that can be released in advance.
Preliminary release period = Predicted time of start of flood −
Present time −
Preparation time
Conditions for preliminary release• Outflow discharge restrictions for downstream safety• Discharge capacity of outlet facilities
Volume to be controlled in excess of safe outflow discharge ≤
Volume that can be released in advance Constant-volume interception at the safe outflow discharge can be made by combining it with preliminary release.
↓(3) Constant-volume interception not exceeding safe
outflow discharge during flood period + Preliminary release
Calculate volume that cannot be controlled by
preliminary release
Calculate the constant volume outflow discharge through
convergence analysis so that the remaining reservoir
capacity is used up.
Required outflow discharge ≤
Design maximum outflow discharge
(4) Constant-volume interception at a rate higher than the safe outflow discharge + Preliminary release
(5) Constant-volume interception at a rate higher than the design maximum outflow discharge +
Preliminary release (for protection from beyond- design-basis flood). The outflow discharge,
however, must not be excessively high.
Evaluating rainfall prediction errors in simulationThe prediction accuracy of JMA's VSRF, MSM and RSM at 214 rain gauge stations in the catchments of dams in 7 river systems in Japan has been evaluated.
Study on usability of weather forecast data (NILIM)
Regression coefficient = Regression coefficient = Predicted rainfall / Measured rainfallPredicted rainfall / Measured rainfall
Expectable maximum measured Expectable maximum measured rainfall = Predicted rainfall / 0.7 = rainfall = Predicted rainfall / 0.7 = 1.43 1.43 **
Predicted rainfallPredicted rainfall
Expectable minimum measured Expectable minimum measured rainfall = Predicted rainfall / 1.4 = rainfall = Predicted rainfall / 1.4 = 0.71 0.71 **
Predicted rainfallPredicted rainfallFig. 1 Upper and lower limits of the regression coefficient for cumulative rainfall determined from past study results
Use of past study research (analysis using JMA forecasts)It is necessary to make forecasting by using the newly developed WRF to accumulate
data and evaluate accuracy.
Use of past study research (analysis using JMA forecasts)It is necessary to make forecasting by using the newly developed WRF to accumulate
data and evaluate accuracy.
Upper limit of regression coefficient = 0.4 / 48 × Rainfall duration + 1.0
Reg
ress
ion
coef
ficie
nt fo
r con
tinuo
us ra
infa
ll Regression coefficient: 0.8
Lower limit of regression coefficient = 0.7
predicted rainfall
Rainfall duration
Measured rainfall = Predicted rainfall / 0.8
measured rainfall
Surface soil moisture condition RG = Surface soil moisture condition RG = Maximum waterMaximum water--holding capacity holding capacity **
expexpααTT
WaterWater--holding capacity during the rainfall holding capacity during the rainfall = Maximum water= Maximum water--holding capacity holding capacity −−
RR GG
Calculating effective rainfallInitial rainfall loss, primary runoff ratio, saturation rainfall and maximum rainfall loss are determined on the basis of total runoff and total rainfall data for single rainfall events at each dam.
The recovery of the maximum water- holding capacity is determined, by assuming it to be an exponential function, from the discharge reduction condition after the second inflection point during the discharge reduction period during a flood.
Effective rainfall at K DamEffective rainfall at K Dam
初期損失雨量 40 [mm]
一次流出率 0,4
飽和雨量 210 [mm]
最大表土保水
能力
102 [mm]
Fig. 2 Total rainfall and direct runoff at K Dam
(colored according to time from initial stage of rainfall)
Primary runoff ratio
Saturation rainfall
Maximum water- holding capacity of surface soil
Initial rainfall loss
3-5 days
5-10 days
10-20 days20 days or more
Less than 3 days
Dire
ct ru
noff
[mm
]
Total rainfall [mm]
Direct runoff = Surface runoff + Early intermediate runoff Intermediate runoff +
Groundwater runoffGroundwater runoff
1st inflection point
Inflow at
K Dam[mm/ hr]
Rainfall[mm]
2nd inflection point
Aug
. 30,
198
8
18:0
0
Aug
. 31
06:0
0
12:0
0
18:0
0
Sep
t. 1
06:0
0
12:0
0
18:0
0
Fig. 3
Calculation of reduction factor (example)
06:0
0
12:0
0
18:0
0
Sep
t.2
12:0
0
In (total inflow) [mm/ hr]
cumulative rainfall
• River use downstream and the rate of water level rise
• Time required for river patrol and the scope of patrol
• Determination of release pattern taking into consideration river users' ability to escape
Proposed procedure for safety checks for preliminary release -draft-
10cm
30cm
50cm
70cm
90cm
流速 V (m/s)
水深 H
0.5 1.0 1.5 2.0 2.5 3.0
成人男性
成人女性
子供・高齢者
対象
断面
のH-V
● cm
■ cm
▲ cm
安全性を考慮した放流パターン
0
2
4
6
8
10
12
14
0:00 1:00 2:00 3:00 4:00 5:00
豊平
峡ダ
ム放
流量
(m3/s)
経過時間
本検討パターン
現行の放流の原則
90min
10min
現行のパターンに戻る
初期流量は水深0.05m時の流量
0.00m3/sを設定)
安全性を考慮した放流パターン
1時間30分経過まで0.00m3/s放流
1時間40分経過まで0.03m3/s放流
1時間50分経過まで0.11m3/s放流
2時間00分経過まで0.23m3/s放流
以後2m3/sまで0.7m3/s/10min
0.7m3/s以上8m3/sまで2.1m3/s/10min
8m3/s以上18m3/sまで3.5m3/s/10min
18m3/s以上30m3/sまで5.0m3/s/10min
30m3/s以上50m3/sまで6.2m3/s/10min
60m3/s上限
< Important considerations>
Critical depth Critical flow velocity
Maximum velocity
Adult males
Adult females
Children/ the elderly
Source: Suga et al.: Experiment on safe evacuation behavior (wading through water) in case of flood
Water depthH
H-V
for c
ross
se
ctio
n of
in
tere
st
Adult males
Adult femalesChildren/the elderly
Flow velocity V
Release pattern chosen taking safety into consideration0.00 m3/s until 90 minutes pass0.03 m3/s until 100 minutes pass0.11 m3/s until 110 minutes pass0.23 m3/s until 120 minutes passThereafter 0.7 m3/s/10 min until 2 m3/s2.1 m3/s/10 min at 0.7 m3/s–8 m3/s3.5 m3/s/10 min at 8 m3/s–18 m3/s5.0 m3/s/10 min at 18 m3/s–30 m3/s6.2 m3/s/10 min at 30 m3/s–50 m3/s60 m3/s: upper limit
Out
flow
dis
char
ge a
t H
ohei
kyo
Dam
Return to currently used pattern
Discharge (0.00 m3/s) corresponding to a water depth of 0.05 m is used as the initial discharge.
Release pattern chosen taking safety into consideration
pattern under consideration
currently applied release principle
Time
Adult males
Adult females
Children/the elderly
Range in which safe evacuation is difficult
Range in which safe evacuation
is possible
Result of simulation of new operation method
In 22 out of the 30 flood cases in which water was actually released at out higher than harmless discharge, it would be possible to reduce water at outflow discharge lower than harmless discharge.
In 13 out of the 30 flood cases in which water was actually released at outflow discharge higher than harmless discharge, preliminary releases would be carried out.
In 8 out of these cases, it would be possible to release water outflow discharge lower than harmless discharge. In the remaining 5 cases, peak discharge would be reduced.
Results obtained in 64 flood cases at 7 dams最大放流量別ケース数の比較
26
6
4
2
34
56
0
10
20
30
40
50
60
実績 降雨予測を
活用した操作
ケー
ス数
無害放流量以下の放流ケース
ただし書き操作ケース
無害放流量以上の放流ケース
実績で無害流量以上の放流ケースにおけるシミュレーションでの事前放流実施状況
4
211
8
14
0
5
10
15
20
25
30
降雨予測を活用した操作
ケー
ス数
事前放流無しで無害放流量以下
事前放流により無害放流量以下
事前放流無しで計画最大放流量以上(流量は低減)事前放流により計画最大放流量以上(流量は低減)事前放流無しで計画最大以下
事前放流により計画最大放流量以下
Comparison of the number of cases by maximum discharge
Num
ber o
f cas
es
below harmless discharge case
particular operating rule case
above harmless discharge case
Actual resultsOperation using rainfall prediction
Below harmless discharge case without preliminary releaseBelow harmless discharge case with preliminary release
Above design maximum outflow discharge case without preliminary release (discharge reduced)
Above design maximum outflow discharge case with preliminary release (discharge reduced)
Below design maximum outflow discharge without preliminary release
Below design maximum outflow discharge with preliminary release
Num
ber o
f cas
es
Operation using rainfall prediction
Simulated preliminary releases in past above harmless discharge cases
It has been confirmed that preliminary releasing is effective to a certain degree against an extreme flood. A preliminary release, however, should be carried out in view of such factors as prediction errors and the possibility of the recovery of water supply capacity and the exceeding of flood control capacity.
At a dam where the constant-rate constant-volume discharge method is used, the decision to narrow down options to constant-volume discharge at the harmless discharge, which uses the flood control capacity more, should be made carefully. →
The relationship between forecasting accuracy and flood
control needs to be determined.
In predicting reservoir inflow, it is desirable to use a high-accuracy runoff analysis model, such as a distributed runoff model.
There are dams incapable of carrying out effective preliminary releases because of the lack of capacity of outlet facilities where preliminary releases would in theory be effective. → There is a need to reconsider existing reservoir outlet facilities.
EvaluationEvaluation Dam operation using WRF
Determining outflow discharges in cases where flood control capacity is not large enough even after a preliminary release is carried out
Policies needs to be determined as to how to use reservoir capacity in view of WRF errors
Problems to be addressedProblems to be addressed
Confirming WRF errors in order to optimize flood control so that downstream damage is minimized
Conducting runoff analyses using distributed runoff models, etc., to forecast reservoir inflow more accurately and perform more appropriate flood control operation taking into consideration the safety of downstream areas
< Improving operating methods>
< Improving forecasting techniques>
It is necessary to develop operating rules or operating manuals for new flood control methods and conduct studies on where the responsibility for operation lies and on actions to be taken dealing with risks.
< Redesigning institutional systems>
Research scheduleFY2008
Verifying the accuracy of WRF-based rainfall forecasting applied to historical floods and evaluating its usefulness in flood control
Verifying the accuracy of WRFVerifying the accuracy of WRF--based rainfall based rainfall forecasting applied to historical floods and forecasting applied to historical floods and evaluating its usefulness in flood controlevaluating its usefulness in flood control
Studies on criteria for carrying out preliminary releases taking into consideration the magnitudes of rainfall events, forecasting errors and characteristics of dams
Studies on criteria for carrying out preliminary Studies on criteria for carrying out preliminary releases taking into consideration the releases taking into consideration the magnitudes of rainfall events, forecasting magnitudes of rainfall events, forecasting errors and characteristics of damserrors and characteristics of dams
Forecasting reservoir inflow by making combined use of a distributed runoff model and rainfall forecasting, and evaluating the accuracy of forecasting
Forecasting reservoir inflow by making combined use Forecasting reservoir inflow by making combined use of a distributed runoff model and rainfall forecasting, of a distributed runoff model and rainfall forecasting, and evaluating the accuracy of forecastingand evaluating the accuracy of forecasting
Predicting rainfall by using an improved WRF (accumulation of data)
Verifying prediction accuracy
Predicting rainfall by Predicting rainfall by using an improved using an improved WRF (accumulation of WRF (accumulation of data)data)
Verifying prediction Verifying prediction accuracyaccuracyStudies on the type and accuracy of rainfall prediction
information necessary for dam operation Studies on the type and accuracy of rainfall prediction Studies on the type and accuracy of rainfall prediction information necessary for dam operationinformation necessary for dam operation
Studies on risk management using rainfall prediction and studies on dam operating rules using rainfall prediction by WRF Studies on risk management using rainfall prediction and studiesStudies on risk management using rainfall prediction and studies on dam on dam operating rules using rainfall prediction by WRFoperating rules using rainfall prediction by WRF
Improving WRF by using GSM20, precipitable water, etc.
Improving WRF by Improving WRF by using GSM20, using GSM20, precipitableprecipitable water, etc.water, etc.
Establishing a concept for dam operation methods taking rainfall prediction into consideration
Establishing a concept for dam operation Establishing a concept for dam operation methods taking rainfall prediction into methods taking rainfall prediction into considerationconsideration
Validating dam operation methods using rainfall predictionValidating dam operation methods using rainfall predictionValidating dam operation methods using rainfall prediction
FY2009
FY2010
FY2011
top related