U. Bethers, J.Seņņikovs -...
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A flood risk assessment and mapping for Riga city U. Bethers, J.Seņņikovs Laboratory for Mathematical Modelling of Environmental and Technological Processes, Faculty of Physics and Mathematics, University of Latvia Riga (population ca. 800,000) is the capital of Latvia. The city is located at the southern coast of Gulf of Riga along the lower stretch of the River Daugava. The water bodies adjacent or within the territory of the city are the River Daugava itself, its connection (Bullupe) to another major river – Lielupe, several minor tributaries of the River Daugava, as well as an interconnected lake system of estuarine origin. The flow in the lower stretch of the River Daugava is regulated by Riga hydropower plant which is situated approx. 25 kilometers upstream the river mouth. The lowest 12 kilometers of the river channel is dredged to ensure the operation of the Riga Freeport. The aims of the study were (1) the identification of the flood risk situations, (2) the quantification of the flooding scenarious of different return periods, (3) the building and calibration of the hydrodynamical mathematical model for the domain potentially vulnerable for flooding, (4) the calculation of flood events with different return period, and (5) the detailed (horizontal accuracy around 10 m) mappping of the potentially flooded areas. 1. Scenarious The combination of storm surges in the southern part of Gulf of Riga with unfavorable regime of hydropower plant operation was found as the most dangerous flooding situation. The time series of water level at the mouth of the River Daugava was analysed for more than 130 year long time period. The significant trend was found in the annual peak water level. Five significant storm events were found in time period 2001-2007 which roughly correspond to storm surges with return period once in 5, 10, 20, 50 and 100 years. The model storm events were created by scaling waterlevel and meteorological conditions during these selected events, and superposing them with hydropowerplant operation regime. 2. Model The finite-element based shallow water model was built for the area, potentially vulnerable for flooding. Heterogeneous depth/terrain information from various sources was integrated in the model. The linear objects (watercourses, dams, etc.) of hydraulic importance were included in the model. The typical spatial resolution of approx. 50-100 m was reached with total number of finite elements around 250,000. 501000 501500 502000 502500 503000 503500 322600 322800 323000 323200 323400 323600 323800 324000 324200 324400 324600 324800 325000 -22.48 -18.77 -15.06 -11.35 -7.636 -3.926 -0.216 3.494 7.204 10.91 14.62 18.33 3. Calibration The hydrodynamical model was calibrated on the basis of water level observations in 5 different locations during 5 selected real storm events. Model forcing was hourly observations of 2 sea levels (Lielupe, Daugavgrīva), Daugava (at HPP) and Jugla river discharges, and wind speed. Calibration results are illlustrated for the storm of Jan-2005 comparing hourly observed and calculated water levels 0,5 1 1,5 2 2,5 3 2005.01.08 9:00 2005.01.08 11:00 2005.01.08 13:00 2005.01.08 15:00 2005.01.08 17:00 2005.01.08 19:00 2005.01.08 21:00 2005.01.08 23:00 2005.01.09 1:00 2005.01.09 3:00 2005.01.09 5:00 2005.01.09 7:00 2005.01.09 9:00 2005.01.09 11:00 2005.01.09 13:00 2005.01.09 15:00 2005.01.09 17:00 2005.01.09 19:00 Waterlevel, m Andrejos ta - obs erved K īšezers - observed Rīgas HES - observed Andrejosta - calculated Ķīšezers - calculated Rīgas HES - calculated The hydrodynamics of the flood scenarios were calculated for the model storm situations. The importance of the dynamical modeling of flooded areas was shown for the domain with a complex channel system and typical length of storm event below 18 hours. Figure: water level time series for storm with return period 10 years. 4. Scenario calculation Time W a t e r l e v e l , m 5. Flood mapping The method for the mapping of the results of hydrodynamical calculations on the digital terrain map of much higher (10 m) spatial resolution was proposed and applied. See illustration of flooded area correction. 6. Examples of flooded areas Traffic junction (left above), river delta (right above), northern part of Ķīšezers lake (below). Return period 10 years (green), 100 years (red).
Transcript of U. Bethers, J.Seņņikovs -...
A flood risk assessment and mapping for Riga city
U. Bethers, J.SeņņikovsLaboratory for Mathematical Modelling of Environmental and Technological Processes, Faculty of Physics and Mathematics, University of Latvia
Riga (population ca. 800,000) is the capital of Latvia. The city is located at the
southern coast of Gulf of Riga along the lower stretch of the River Daugava.
The water bodies adjacent or within the territory of the city are the River
Daugava itself, its connection (Bullupe) to another major river – Lielupe,
several minor tributaries of the River Daugava, as well as an interconnected
lake system of estuarine origin. The flow in the lower stretch of the River
Daugava is regulated by Riga hydropower plant which is situated approx. 25
kilometers upstream the river mouth. The lowest 12 kilometers of the river
channel is dredged to ensure the operation of the Riga Freeport.
The aims of the study were (1) the identification of the flood risk situations, (2) the
quantification of the flooding scenarious of different return periods, (3) the building
and calibration of the hydrodynamical mathematical model for the domain potentially
vulnerable for flooding, (4) the calculation of flood events with different return period,
and (5) the detailed (horizontal accuracy around 10 m) mappping of the potentially
flooded areas.
1. Scenarious
The combination of storm surges in the southern part of Gulf of Riga with
unfavorable regime of hydropower plant operation was found as the most
dangerous flooding situation. The time series of water level at the mouth of the
River Daugava was analysed for more than 130 year long time period. The
significant trend was found in the annual peak water level. Five significant
storm events were found in time period 2001-2007 which roughly correspond to
storm surges with return period once in 5, 10, 20, 50 and 100 years. The model
storm events were created by scaling waterlevel and meteorological conditions
during these selected events, and superposing them with hydropowerplant
operation regime.
2. Model
The finite-element based
shallow water model was
built for the area,
potentially vulnerable for
flooding. Heterogeneous
depth/terrain information
from various sources was
integrated in the model.
The linear objects
(watercourses, dams,
etc.) of hydraulic
importance were included
in the model. The typical
spatial resolution of
approx. 50-100 m was
reached with total number
of finite elements around
250,000.
501000 501500 502000 502500 503000 503500
322600
322800
323000
323200
323400
323600
323800
324000
324200
324400
324600
324800
325000
-22.48
-18.77
-15.06
-11.35
-7.636
-3.926
-0.216
3.494
7.204
10.91
14.62
18.33
3. Calibration
The hydrodynamical model was calibrated on the basis of water level
observations in 5 different locations during 5 selected real storm events. Model
forcing was hourly observations of 2 sea levels (Lielupe, Daugavgrīva),
Daugava (at HPP) and Jugla river discharges, and wind speed. Calibration
results are illlustrated for the storm of Jan-2005 comparing hourly observed and
calculated water levels
0,5
1
1,5
2
2,5
3
20
05
.01
.08
9:0
0
20
05
.01
.08
11
:00
20
05
.01
.08
13
:00
20
05
.01
.08
15
:00
20
05
.01
.08
17
:00
20
05
.01
.08
19
:00
20
05
.01
.08
21
:00
20
05
.01
.08
23
:00
20
05
.01
.09
1:0
0
20
05
.01
.09
3:0
0
20
05
.01
.09
5:0
0
20
05
.01
.09
7:0
0
20
05
.01
.09
9:0
0
20
05
.01
.09
11
:00
20
05
.01
.09
13
:00
20
05
.01
.09
15
:00
20
05
.01
.09
17
:00
20
05
.01
.09
19
:00
Wa
terl
ev
el,
m
A ndrejos ta - obs erved
K īš ez ers - obs erved
R īgas HE S - obs erved
A ndrejos ta - c alc ulated
Ķ īš ez ers - c alc ulated
R īgas HE S - c alc ulated
The hydrodynamics of the flood
scenarios were calculated for the
model storm situations. The
importance of the dynamical
modeling of flooded areas was
shown for the domain with a
complex channel system and
typical length of storm event
below 18 hours. Figure: water
level time series for storm with
return period 10 years.
4. Scenario calculation
Time
W
a
t
e
r
l
e
v
e
l
,
m
5. Flood mapping
The method for the mapping of the results of
hydrodynamical calculations on the digital
terrain map of much higher (10 m) spatial
resolution was proposed and applied. See
illustration of flooded area correction.
6. Examples of flooded areas
Traffic junction (left above), river delta
(right above), northern part of Ķīšezers
lake (below). Return period 10 years
(green), 100 years (red).
