Maps of the lowland river Sava

Martin Baptist 1 , Marjolijn Haasnoot 1 , Guido van der Wedden 2
en Huib de Vriend 2
1 WL | Delft Hydraulics and 2 Delft University of Technology
In co-operation with IAC, Ecorys, Alterra & RIZA

• Detention areas temporarily store water during floods, to alleviate
flood risks downstream.
• To work effectively, large areas are needed, preferably embanked and
equipped with inlet and outlet structures (sluices).
• Flood detention areas are considered along many European rivers.
• Flood management in the Central Sava Basin in Croatia is (already)
largely controlled by flood detention areas.
• In this study, the hydrological and ecological functioning of one of
these detention areas, Lonjsko Polje, is studied to give advise on
optimal ecological flood management and to learn from this existing
situation.

Intake of the
SOS canal
Odransko
Polje
1972: Trebež II
outlet sluice
Prevlaka inlet sluice
Cesma river
1972 UN plan:
Palanjek inlet
weir
Trebež I
outlet sluice
Dike

3650 1:100 year flood event
(m3 2650
/s)
1000
Est. storage capacity
Present: 634·10 6 m 3
1972 plan: 915·10 6 m 3
A World Bank study concluded that
for the 1972 plan the water levels of
floods will rise, the flood duration
will double and there will be more
often high waters.
max 600 in
max 2370 in
max 500 out + 250 out

• Main objective: Setting-up guidelines for the flood management of
the Lonjsko Polje area to combine (1) flood protection, (2) ecosystem
development and (3) nutrient sink capacity. Compare different
flooding scenarios.
Expected results:
• Maps of flood inundation depths, flood duration and time for
evacuation for different flood events. Important for park
management.
• Assessment of the vegetation growth and succession for different
flooding scenarios.
• (Assessment of the nutrient trapping efficiency of Lonjsko Polje,
important for the Sava nutrient load and subsequently the Danube
load.)

• Hydrology: model the
flooding of Lonjsko Polje with
the SOBEK overland flow
module.
• Ecology: assess the
vegetation structure and
succession, related to
flooding.
• (Nutrients: assess the
nutrient balans of the Lonjsko
Polje area by coupling the
flow model to a water quality
model.)

Physical state
Hydrology
Present situation
1972 plan with
additional weir and
sluice
Mean year, 1997
1: 100 year flood

• Peak discharge 3650 m 3 /s

!
"#$!%
!

& '
• Contrary to the expectation,
the flood duration for the
1972 plan is shorter than for
the present situation.
• This is due to the fact that
water can leave Lonjsko Polje
more easily via the new
Trebež II sluice and Palanjek
weir, as soon as the flood
wave has past by.
• Calculated storage capacity:
Present: 613·10 6 m 3
1972 plan: 845·10 6 m 3

Ecohydrological
assessment
SOBEK Vegetation map
from the Park Service
Flood duration
- growing season
- remaining part of year
Knowledge rules
give ecotopes after x years,
based on flood duration,
vegetation succession and
management
ecotope maps
after x years
Ecotope map

( ' '
Flood event knowledge rules Season inundation (days) Succession in years
Ecotope code site factor flooding Nov-Mar Apr-Jun 2 10 20 50 100
dry hardwood forest 101 dry < 20 d/yr 20 10 301 201 201 101 101
hardwood forest 102 dry-moist 20 - 50 d/yr 50 20 301 201 201 102 102
aspen plantation 601 moist 50-100 d/yr 100 20 301 201 201 102 102
softwood forest 103 moist 50 - 150 d/yr 120 90 302 202 104 104 105
wet hardwood forest 105 moist-wet 50 - 150 d/yr 120 90 302 202 104 104 105
wet softwood forest 104 wet 50 - 150 d/yr 120 90 302 202 104 104 105
marsh forest 106 wet > 150 d/yr 120 90 303 303 106 106 106
hardwood shrub 201 moist-wet < 50 d/yr 50 20 301 201 201 102 102
softwood shrub 202 wet 50 - 150 d/yr 120 90 302 202 104 104 105
herbaceaous rough 301 dry < 20 d/yr 20 10 301 201 201 101 101
wet herbaceous rough 302 moist > 50 120 50 302 202 104 104 105
herbaceous swamp 303 wet > 150 d/yr 120 50 303 303 106 106 106
reed swamp 304 wet-water 365 d/yr 120 30 701 304 304 304 304
moist meadow 502 moist 20 - 50 d/yr 50 20 301 201 201 102 102
floodplain hayfield 401 moist 20 - 50 d/yr 50 20 301 201 201 102 102
wet hayfield 402 wet 50 - 150 d/yr 120 50 301 202 104 104 105
wet meadow 503 wet 50 - 150 d/yr 120 50 302 202 104 104 105
wet natural pasture 504 wet 50 - 150 d/yr 120 50 302 202 104 104 105
wet grassland 505 wet > 150 d/yr 120 50 302 302 106 106 106
meadows and agriculture 501 < 20 d/yr 20 10 301 201 201 101 101
arable floodplain moist 20 - 50 d/yr 50 20 301 201 201 102 102

Physical state
Hydrology
Current situation
1972 plan with
additional weirs and
sluices
Mean year, 1997
1: 100 year flood

& '
• The effects of a 1:100 year flood event differ mainly along
the borders of the central Lonjsko Polje region. Contrary to
the expectation, the 1972 plan is favourable. The shorter
flood duration leads to less severe effects.
• The mean, autonomous succession differs as well for both
scenarios. However, results show that nature management
is a more important factor for vegetation succession than
flood duration.

• A 1:100 year flood event will inundate the entire Lonjsko
Polje within two weeks time, therefore all lifestock has to be
evacuated in time.
• The flood duration time of floods in Lonjsko Polje is not
determined by the amount of water that goes in, but by the
amount of water that can go out.
• The ecological effects of a flood event are large,
irrespective of the execution of the 1972 plan.
• It is recommended to consider an in- and outlet structure
that can take in water during dry summer periods and can
discharge water to drain the filled detention area faster.