Journal of Environmental Protection, 2010, 1, 129-135
doi:10.4236/jep.2010.12017 Published Online June 2010 (http://www.SciRP.org/journal/jep)
1
Environmental Risk Imposed by Diverted Flood Waters on
Water and Soils in Emergency Retention Basins
Karl Erich Lindenschmidt1*, Robert Harrison1, Martina Baborowski2
1Manitoba Water Stewardship, Surface Water Management, Winnipeg, Manitoba, Canada; 2Department of River Ecology , UFZ-Centre
for Environmental Research, Magdeburg, Germany.
Received December 6th, 2009; revised February 1st, 2010; accepted February 3rd, 2010.
ABSTRACT
Emergency retentio n basins (ERB) are diked enclosures alongside rivers into which water from the main river channel
is diverted during extreme floods. If the basins are operated during extreme flooding, two negative environmental im-
pacts may occur: 1) contamination of the soils due to their transport by suspended sediments to the basin and 2) deple-
tion of dissolved oxygen in the basin water. A computer-based methodology is presented which was used to assess the
environmental risk exhibited by the operation of an ERB system proposed for the Elbe River in Germany. The August
2002 extreme flood event was used as a test case. For such a flood, the results showed that there is a 77% risk that dis-
solved oxygen levels fall below 2 mg/L in the water and a 48% chance of exceeding the inspection value of 500 mg
zinc/kg in the soil.
Keywords: Environmental Risk, Inundation, Retention Basins, Water Quality, Contaminated So ils, Quasi-2D Model
1. Introduction
The European Union (EU) has passed a Floods Directive
to provide a legislative foundation for flood protection
and mitigation in Europe. Since most large river basins in
Europe extend over several countries, the directive will
help to orient and broaden flood management efforts
beyond the municipal and state au thorities to the nation al
and international levels and establish an improved and
more cost-effective flood protection scheme (see also
[1]). Interestingly, the directive includes many references
to the environment and the EU Water Framework Direc-
tive, e.g.:
 Article 4 (Preliminary flood risk assessment): …[must
consider] potential adverse consequences for human
health, the environment
 Article 9 (Flood hazard maps and flood risk
maps): … shall be coordinated with and may be in-
tegrated into the reviews provided for 2000/60/EC
[WFD]
 Article 7 (Flood risk management plans): … shall
take into account costs and benefits, flood extent and
conveyance routes, environmental objectives of
2000/60/EC [WFD], soil and water management
Hence, it will be mandatory to consider what impact
new flood defence measures will have on the environ-
ment. Also, these measures are not to conflict with the
goals of the EU WFD of achieving a good ecological
status of wa t er bodies.
After the extreme flood event along the Elbe River in
August 2002, efforts were made to revamp flood man-
agement schemes in the river basin. Hence, the construc-
tion of an emergency retention basin (ERB) system has
been proposed for the middle reaches of the Elbe River
to reduce flood risk in the area [2].
Emergency retention basins are diked enclosures
alongside rivers used to retain flood water by diverting
and storing a por tion of the r iver disch arge during floods.
The diversion from the main river channel to and from
the ERBs is controlled in order to attain maximum cap-
ping of the peak discharge volume. The floodwater di-
version reduces water levels downstream in the river and
inundated areas and alleviates stress and damages in re-
gions prone to high flood risk.
Studies on environmental risk assessment on retention
basins by flood events are sparse in the literatu re. There-
fore, the aim of the paper was to provide a possible
computer modelling methodology on how such assess-
ments can be tackled in fulfilling the proposed EU direc-
tives. Further, a preliminary assessment of the environ-
ment risk of heavy metal soil contamination and dis-
solved oxygen depletion in ERBs should be given. To
solve these goals, excerpts have been drawn from [3-6].
In detail, the following steps were in the focus of the
research:

Environmental Risk Imposed by Diverted Flood Waters on Water and Soils in Emergency Retention Basins 130
 develop and test a quasi-2D (two-dimension) model
in which the equations of motion and continuity are
calculated in 1D (one-dimension) but the discretisa-
tion scheme allows for a 2D representation of flow
and substance distribution; this minimises comput-
ing and pre-processing expenditure without com-
pensating spatial diff erentiation of flood behaviou r,
 test how effective the proposed ERB system is able
to cap the peak discharge of the hydrograph from the
August 2002 flood,
 investigate the environmental risk of heavy-metal
contamination of ERB soils after flood water reten-
tion, and
 assess the environmental risk of dissolved oxygen
depletion in the water retained in the basins.
2. Study Area
Two ERBs were investigated in this studie, P1 and P3
(see Figure 1). They are situated between Torgau and
Wittenberg. Morphological characteristics of the ERBs
are given in Table 1. Several gates are to control the
flow of water through the ERBs: an inlet gate at the
southernmost tip of P1, an outlet gate at the westernmost
point of P3 by Pretzsch and a connecting gate between
P1 and P3.
Table 1. Morphological characteristics of emergency reten-
tion basins P1 and P3
Po lderSur face ar eaVolumeHeadDepth
(km
2
)(10
6
m
3
)(m.a.s.l.) (m)
P1 24.5 85 77.53.3
P38.22075.3 2.3
Figure 1. Investigated emergency retention basins (ERB)
proposed for the Elbe River reach between Torgau and
Wittenberg, modified from [2]
3. Modeling Tool and Setup
The hydrodynamic module DYNHYD (based on St. Ve-
nant equations), the sediment and contaminant transport
module TOXI and the water-quality module EUTRO
from the WASP5 (Water quality Analysis Simulation
Program v.5) modelling package, developed by the U.S.
Environmental Protection Agency [7] was used for the
floodwater simulations. Figure 2 (left panel) shows the
discretisation of the river-ERB system domain in junc-
tions and channels. The junctions ensure volume conti-
nuity, hence all junction water volumes represent the
total water volume in the system. The channels are the
basis for computing the flow in 1D between the junctions.
Many channels may branch from a junction allowing the
discretisation to take a 2D characterisation. The segment
discretisation for the contaminant transport and wa-
ter-quality model, shown in Figure 2 (right panel), cor-
responds 1-to-1 the junction discretisation.
In TOXI, processes that describe the transport and fate
of heavy-metal contaminants include:
 longitudinal dispersion in the water column,
 vertical diffusion of dissolved substances between
the water column and the bottom sediments,
 sedimentation of suspended solids,
 re-suspension of solids from the bottom sediments to
the water column, and
 sorption of dissolved substances to suspended and
deposited sediments.
EUTRO was used to simulate water quality pertaining
to the oxygen balance in a river and ERBs using the cy-
cles of dissolved oxygen (DO) decomposition and nutri-
ent-limited phytoplankton growth. Oxygen is also con-
sumed in the sediments, which is described in the model
by the sediment oxygen demand. An important source of
oxygen into the water body is re-aeration via the water
surface from the atmosphere.
An important source and sink of DO are phytoplank-
ton photosynthesis and respiration, respectively. Phyto-
plankton growth is also light limited and its loss rate is
governed by respiration, death, settling and zooplankton
grazing.
The DYNHYD, EUTRO and TOXI modules were
structured in a simulation platform HLA (High Level
Architecture) to allow Monte-Carlo analyses (MOCA) to
be carried out [8]. The MOCA allowed uncertainty
bounds to be computed in order to assess the degree of
risk in exceeding environmental thresholds. For the en-
vironmental risk appraisal the following were investi-
gated: 1) the amount of zinc, with high contamination
potential along the Elbe [9], sorbed and deposited from
the diverted flood waters and 2) the minimum concentra-
tion of DO attained in the basin waters.
4. Results
For a comprehensive description of the development,

Environmental Risk Imposed by Diverted Flood Waters on Water and Soils in Emergency Retention Basins 131
Figure 2. Discretisation for the hydrodynamic model with junctions and channels (left panel) and the water-quality model
with segments (right panel), modified from [6] and [5]
01234567891011
Simulation da
y
72
73
74
75
76
77
78
79
Wate
r
level (m.a.s.l.)
river (no polders)
river (with polders)
north polder (P3)
south polder (P1)
G
F
E
D
C
B
A
Figure 3. Left panel: water levels in the river and ERBs during operation. Right panel: flow velocity field in the ERBs during
filling (longest vector length corresponds to 1.2 m/s), adapted from [6]
calibration and validation of the models with corre-
sponding sensitivity analyses of the parameters and
boundary conditions, the reader is referred to [6] for the
hydrodynamic modelling, [4] for contaminant transport
modelling and [5] for the eutrophication modelling. Only
the key results to support the environmental assessments
will be presented here.
4.1 Hydrodynamics
Figure 3 (left panel) shows the water level hydrographs
of the August 2002 flood simulated along the river at
Positions A, B and C (see Figure 2 for spatial reference).
The dashed line of the hydrograph crests represent water
level heights that occurred without the ERB system, the
solid line represents the lowering of the water levels that
can be obtained wh en the ERBs are included in the mod-
elling. The capping lies in the range of 35 to 40 cm and
progressively recedes as the flood wave moves down-
stream (compare Position C). This is due to flood wave
attenuation and widening of the floodplain downstream
from Pretzsch towards Wittenberg (see Figure 1). Fig-
ure 3 (right panel) shows the 2D spatial differentiation of
the flow field. Details on the development, calibration
and validation of the model and the optimisation of the
control of ERB filling are given in [4] and [6].
4.2 Soil Contamination
Figure 4 (left panel) shows the results of zinc concentra-
tions in the river and ERBs. The initial concentrations in
the ERB segments on the onset of water inflow corre-
spond to the con centrations in th e water in the immediate
upstream-lying segments. Once the ERBs are filled and
the flood gates are closed (approximately after Day 6 for
P3 and after Day 7 for P1), there is a substantial drop in
substance concentrations in the ERB water column due
to sedimentation of suspended sediment. Up to 90% of
Time = 5.25

Environmental Risk Imposed by Diverted Flood Waters on Water and Soils in Emergency Retention Basins 132
012345678910
Simulation Day
0
10
20
30
40
50
60
70
Total zinc (

g/L)
Figure 4. Total zinc (left panel) in the river at Position A (see Figure 2) and in ERBs P1 and P3. Zinc concentration (mg/kg) in
bottom sediments during flood water retention [4]
024681012
Simulation day
2
4
6
8
10
mg O
2
/L
P1 inlet
P1 extremity
P3
river
Wittenberg
saturation
max_ 5
min_ 0
DO (mg O2/L)
Figure 5. Left panel: DO concentrations in the river and ERBs during floodwater diversion. Right panel: lower 10% quartile
of minimum DO at each segment throughout the ERB system [5]
the sediment settles out of the water column to the bot-
tom sediments. Figure 4 (right panel) shows the spatial
distribution of particulate zinc concentrations in the bot-
tom sediments representing zinc deposition during the
retention stage of flood water diversion. Particulate zinc
is deposited quite quickly when water enters P1. Much of
the particulate matter has also been trapped in P1 before
the water enters P3. During flood water retention, zinc
concentrations are even ly distributed thro ughou t th e ERB
bottom sediments.
4.3 Dissolved Oxygen Depletion
Figure 5 (left panel) shows the DO simulations in the
river at Wittenberg and in the ERBs. The DO levels in
the river remain substantially below saturation levels.
Upon entering the ERBs, diverted flood water receives
more oxygen from the hydraulic re-aeration when pass-
ing through the inlet gates. The initial flow of shallow
waters during basin filling (Day 4 to 7) also contributes
to the input of oxygen into the water. There is a progres-
sive decline in DO concentrations during flood water
retention (Days 8 and 9) which progresses into the emp-
tying phase (after Day 9).
The lower decile of the minimum DO concentrations
is shown spatially in Figure 5 (right panel). The DO lev-
els are higher in P3 than in P1. The extremity (eastern
bay) of P1 is generally less oxygenated than the remain-
ing water in the ERB. It should be emphasised that the
August 2002 simulation represents a best-case scenario.
Additional factors that may impact DO negatively but
were not included in the model are 1) higher sediment
oxygen demand if surfaces are arable lands and not
grasslands and 2) an oxia in the water-soil interface caus-
ing re-dissolution of inorganic nutrients from the ERB
max=155
min=155
FRAME = 216 Da
y(
FRAME
)
= 9

Environmental Risk Imposed by Diverted Flood Waters on Water and Soils in Emergency Retention Basins 133
soils.
5. Environmental Risk Assessment
5.1 Soil Contamination
In order to make an environmental risk assessment of
contamination in the ERBs, the deposition of pollutants
onto the ERB surfaces must be placed within the legal
context of the German sewage sludge ordinance. An
example is given here using zinc as the heavy-metal
contaminant. The ordinance provides threshold values of
zinc content allowed in the soils and sediment (“sludge”)
deposited onto the land surfaces within the basins (sum-
marised in Table 2).
In accordance to the German Sewage Sludge Ordi-
nance, the background zinc concentrations in the soils,
where the ERBs are to be situated, must be determined.
The geogenic value of 150 mg zinc/kg sediment [10] for
the Elbe catchment is used here for illustration. This
value does not exceed the maximum allowable zinc con-
tent in the soils of 200 mg/kg. Hence, sediment may be
deposited from the river water diverted into the ERBs
during extreme flooding. The content of zinc within the
deposited sediment may not exceed 2500 mg/kg, if the
land surfaces are to continue to be used for agricultural
purposes. Table 3 summarises the results from the
MOCA showing that the probability of reaching this
value is minute (< 1%). A different approach based on
standards provided by the German Soil Protection and
Contaminated Sites Ordinance is given in [5].
An important question that arises is how many flood-
ings of the ERB can occur before the concentrations in
the soil increase from the geog enic level of 150 mg/kg to
the threshold value of 200 mg/kg. Figure 4 (right panel)
shows a spatial distribution of zinc bound to deposited
sediment, which corresponds to an 8% exceedence
probability extracted from the MOCA. The zinc content
in the soil increases by 5 mg/kg. Hence, approximately
ten ERB floodings may cause zinc accumulation to reach
the threshold value of 200 mg/kg.
Although the environmental risk for zinc contamina-
tion is low, it should be noted that an amendment to the
sewage sludge ordinance is currently being discussed in
which the threshold values for allowed zinc content in
soils and deposited sediment (“sludge”) are lower (see
Table 2). Careful measurements of the background zinc
levels in the soils of the ERB sites should be carried out
to determine how close these values are to the maximum
allowable content in soils.
The environmental risk analyses presented here shows
that there is minimal risk of plant contamination by zinc
deposited onto the ERBs during floods. However, the
risk may be substantially increased if the ERBs are lo-
cated further downstream. Figure 6 shows the zinc con-
tent in deposited sediment along the portion of the Elbe
River in Germany. Imbedded in the graph are the results
Table 2. Threshold values of zinc content in soils and deposited sediment (“sludge”) as given by the German ordinances for
sewage sludge
mg zinc / kg
sediment Comment
Sewage sludge ordinance
max. content in soil allowed 200soils with high clay content
max. content in sediment allowed 2500soils with high clay content
Proposed amendments to the
sewage sludge ordinance
max. content in soil allowed 150for loamy soils
max. content in sediment allowed 1500
Table 3. Summary of the MOCA giving the probability of certain values of zinc in deposited sediments exceeding 50%, 16%
and 1% in ERBs P1 and P3
50% 16%1%
(

)≈(

)≈(

)
P1mg/kg 4606421000
P3mg/kg510 626 858

- mean;  - standard deviation
Exceedence probability
Polder Units

Environmental Risk Imposed by Diverted Flood Waters on Water and Soils in Emergency Retention Basins 134
Z
n
c
o
n
t
e
n
t
(
m
g
/
k
g
)
Elbe-km
downstream from Mulde
upstream from Mulde
MOCA
results
flow direction
Saale Mulde
0200500 400300600100
0
200
400
600
800
1000
1200
1400
1600
Figure 6. Longitudinal profile of zinc loading on soils along
the Elbe River measured in 2003 after the extreme flood in
the previous year, modified from [11], p. 242
obtained from the MOCA, the range of which complies
with the range of all measurements taken upstream from
the Mulde River inflow. For the river stretch downstream
from the Mulde and Saale tributaries, the range and the
maximum of zinc content in deposited sediments is twice
as high as for the upstream reach.
5.2 Dissolved Oxygen Depletion
From the MOCA runs, probability distributions of the
lowest DO levels attained in the ERBs during the simula-
tion time frame were derived. The cumulative frequency
distributions for three locations in the ERB system are
shown in Figure 7. The threshold value of the minimum
concentration of DO in inland waters used for a risk as-
sessment is 3 mg O2/L—the minimum oxygen content
with which most fish in the Elbe River can endure for a
short term [12]. As soon as the oxygen content sinks be-
low approximately 3 mg/L, the fish begin to search for
water pockets with better oxygen supply or begin to snap
for air at the water surface [13].
For a summer time extreme flood such as the August
012345
0
0.2
0.4
0.6
0.8
1
P1 inlet
P1 extremity
P3
DO (mg O2/L)
cumulative frequency
Figure 7. Cumulative frequency distributions of the lowest
dissolved oxygen concentrations attained in the ERBs
2002 event along the Elbe, the probability of the DO
values dropping below a threshold value of 3 mg O2/L is
quite high, 77% for the inlet region of P1, 65% for the far
eastern point of the P1 extremity and 54% for P3 (see
Figure 7). In general, the DO concentrations are higher
more frequently in P3 than in P1 due to the shorter resi-
dence times of the water in this ERB and allowing less
time for de-oxygenation of the water. The generally
higher DO content in P3 is also due to its sh allower depth,
making it a more favourable aquatic environment for
phytoplankton growth. The low DO values are also a
result of low re-aeration (wind calm period) and high
nitrification.
In contrast to experiences gained from the Havel River
during the August 2002 flood, the low oxygenated water
emptying into the River Elbe does not appear to have
negative implications to the oxygen levels in the river
water. Although the water of the Elbe River was under
Figure 8. Dissolved oxygen concentrations at Wittenberg, Magdeburg, Schnackenburg and Bunthaus (last three locations ≈
105, 260 and 390 km downstream from Wittenberg) during the August 2002 flood of the Elbe River, adapted from [13]
(mg O2/L)

Environmental Risk Imposed by Diverted Flood Waters on Water and Soils in Emergency Retention Basins 135
the DO saturation level, DO was still above the threshold
value of 3 mg/l at Wittenberg (see Figure 8).
However, if these ERBs were situated further down-
stream where the DO concentrations of the river water
fall below the threshold value, the emptied basin water
could reduce the DO concentrations further and further
stress the aquatic ecosystem.
6. Conclusions
The quasi-2D approach was successful in simulating the
spatial distribution of flow and substance deposition in
ERBs and determining the effectiveness of discharge
capping. The environmental risk to heavy-metal con-
tamination is minimal but may increase due to stricter
threshold values. Risk may also be higher for ERBs con-
structed further downstream past the Saale and Mulde
river confluences where exposure to contamination is
higher. There is a high potential for DO levels in the wa-
ter retained in the basins to drop to very critical levels (<
3 mg O2/L), even after 4 days of flood water retention.
From an ecological perspective, the basins should also be
emptied as soon as possible after the flood in order to
reduce the time of de-oxygenation of the water.
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