The Next Frontier: Representing Groundwater Interactions in InfoWorks ICM
Adam Cambridge, Senior Consultant, Atkins
William Rust, Assistant Hydrologist, Atkins
1
*This paper was presented at CIWEM’s “Surface Water Management 2014”
“Groundwater Extremes and Surface Water Impacts” conferences.
and the BHS’s
Abstract
Infiltration into sewers can add significant cost to sewerage undertakers’ operations (for
instance, impacts of conveying and treating / discharging additional flows) and exacerbate
flood risk for Lead Local Flood Authorities (LLFAs) by reducing the available headroom in
the sewers. The exceptional rainfall events over the winter of 2013-2014 have highlighted
this and showed that groundwater infiltration is a significant contributory factor in surface
flooding issues and needs to be managed.
This paper outlines an approach that was used to integrate the dynamics of a groundwater
system into an InfoWorks ICM model, and a basis for simulating design storms with
appropriate antecedent conditions. The approach was developed as part of a Surface Water
Management Plan (SWMP) in Southern England to address time varying and rainfall
induced groundwater infiltration that has long been known to be the primary flood risk
mechanism in the catchment (Grey, 1998). This aspect is currently not generally considered
in SWMPs because:


of the technical complexities of integrating groundwater systems with river, sewer,
above ground, and coastal models; and
SWMPs have generated a legacy of focusing on improving the understanding
between sewers, open channels, and above ground systems, but not necessarily
groundwater.
The InfoWorks ICM model was developed to represent rivers, sewers (foul and storm),
interactions with the coast, as well as groundwater infiltration. Such holistic ability underlines
how InfoWorks ICM has revolutionised hydrodynamic modelling in making the prospect of
incorporating an explicit representation of groundwater dynamics achievable. Such a move
would introduce a positive step change in modelling and managing sewer infiltration, as this
would allow calibrations to be physically based and allow sewerage undertakers, as well as
LLFAs, to make informed long term and least cost investment decisions.
The physically based approach to groundwater infiltration undertaken for the SWMP
demonstrates that better water management decisions will be made if a step change is
pursued because the work:


questioned the suitability of using traditional design storms and antecedent
conditions for assessing river and sewer performance, as this approach highlighted
that this may be significantly underestimated;
questioned whether the sewer system should have been constructed in the manner
they have been because from a design standard perspective a high groundwater
level would stipulate the sewers being constructed:
o above the design groundwater level; or
1
http://www.ciwem.org/media/1287101/3.4_adam_cambridge_will_rust_ciwem_sw14__the_next_frontier_-_integrating_groundwater_systems.pdf
1
o

using construction techniques that would be highly resilient to deterioration if
constructed below the design groundwater level.
highlighted that strategic surface flood risk mapping, or indeed other SWMPs, should
account for groundwater dynamics within sewers that are in infiltration prone
catchments, as it can have a marked impact on the predicted risk.
Conclusions from the study have steered the SWMP towards sustainable drainage
techniques, allowed authorities to meaningfully understand flows arriving at the treatment
works and discharges from CSOs, as well as prepare mitigation measures.
2
The Infiltration and Groundwater Flooding Problem
Infiltration into sewers has been an area of interest for sewerage undertakers and modellers
of sewer systems for some time, as this additional flow can result in:
 excessive volumes being stored and/or pumped;
 capacity and treatment problems at the wastewater treatment works;
 structural deterioration of pipes due to bed washout;
 compliance issues with consented outfalls; and
 insufficient hydraulic capacity.
Groundwater flooding, as well as surface water induced flooding due to insufficient hydraulic
capacity, has been a responsibility for Lead Local Flood Authorities (LLFAs) since the
enactment of the Floods & Water Management Act in 2010. This area of responsibility has
not necessarily been fully accounted for in the preparation of Surface Water Management
Plans (SWMPs) due to:
 of the technical complexities of integrating groundwater systems with river, sewer,
above ground, and coastal models; and
 SWMPs generating a legacy of focusing on improving the understanding between
sewers, open channels, and above ground systems, but not necessarily
groundwater.
In most cases, flooding during the 2007 event was caused by the intensity of the rainfall
bursts, but in other areas it was the result of a combination of the rainfall bursts and
unusually saturated ground conditions for the summer. These ground conditions were once
again seen in the winter of 2013/2014 when prolonged rainfall saturated catchments and
highlighted that the sheer volume of rainfall could be just as influential as the intensity of the
rainfall burst.
A key piece in the urban flood management jigsaw has always been to properly understand
the sources and mechanisms of flooding, as without this understanding you cannot plan,
design, or enable effective designs. InfoWorks ICM has revolutionised these aspects, as it is
now possible for modellers to develop fully integrated river, sewer, above ground, and
coastal models. For its complexity, InfoWorks ICM is a platform that is numerically stable,
computationally efficient, reliable, and holistic. The remaining piece of the “hydrological
jigsaw” that needs to be included is the representation of groundwater systems and their
interaction with the sewers.
It has been estimated that infiltration into sewers can typically represent 40% of the domestic
flow (Ofwat, 2011). This, combined with the 2013/2014 floods, highlights that groundwater
interactions are risks that need to be understood and managed. There is then a need to
complete the “hydrological jigsaw” and especially if you consider that the future outlook of
higher density urban centres, urban expansion, and changes to our climate system, will
require more efficient and sustainable water management techniques.
In this paper the authors outline an approach that was used to integrate the dynamics of a
groundwater system into an InfoWorks ICM model, as well as how antecedent conditions
were established for simulating design storms. It is an approach that could be used in other
catchments, but moreover, it is work that:
 reiterates the deficiencies of using design storms to evaluate river and sewer
performance;
 marks the beginnings of what could provide a basis for future research into the
integration of the groundwater system in InfoWorks ICM; and
3

can potentially lead to a step change in the way the industry assesses groundwater
flood risk, manages sewer infiltration, and completes Pitt’s vision of the “integrate and
infiltrate” approach.
Approaches to Integrating Groundwater Systems
To overcome the technical complexities, but still represent the mass dynamics of
groundwater infiltration in sewers and/or baseflow in rivers, modellers (and the underpinning
approaches) have often resorted to simplifications and assumptions. In SWMPs where both
river and sewer systems have been considered in developing urban flood management
strategies, modellers have developed models on the basis of:
 the sewers having:
o full capacity;
o slightly less capacity if silt and blockages are included; and/or
o no capacity (e.g. strategic flood risk mapping using direct rainfall).
 the rivers having either a design or observed baseflow; and
 the above ground system having a design or observed level of saturation.
These simplifications have meant that while the understanding of the urban flood
mechanisms has been greatly improved, the manner in which catchment saturation has
been applied has, on occasion, been inconsistent. For instance, most models developed for
SWMPs will apply baseflow to rivers, a level of saturation for runoff models (including 2D
runoff), but not necessarily a corresponding and consistent level of infiltration into the sewers
for design storm events. If infiltration is applied it will often be included as a “runoff”
generation process, which overlooks the fundamental mechanisms of groundwater infiltration
(groundwater availability and sewer condition). Whilst this approach has been a solution to
the problem, its simplification of the mechanisms has led to issues surrounding the
simulation of design storms. Considering that groundwater flooding, as well as surface
water induced flooding due to infiltration, are areas of risk that LLFAs need to manage, this
is a significant inconsistency for catchments that are highly permeable and could be
considered to be infiltration prone.
It is beyond the scope of this brief technical paper to describe in detail how the various
models, or indeed approaches, represent either infiltration into sewers, or baseflows in
rivers. However, it is important to note is that the solution to this inconsistency is not in
reworking our historically siloed approaches (e.g. residual inflow files, adjusted runoff
parameters). The solution lies in recognising that the groundwater system and catchment
saturation needs to be fully integrated and explicitly represented, so that it can become
consistently applied and better understood.
The Catchment
In 2013 Atkins was commissioned by West Sussex County Council (WSCC) and Southern
Water (SW) to prepare a SWMP for targeted areas (18 km²) in a catchment (137 km2) in
West Sussex, UK. It has been long known that the catchment suffers from sewer infiltration
when groundwater levels are high (Grey, 1998) and is at risk of flooding from surface water,
fluvial, and coastal sources.
The study catchment is large and is characterised by a mix of small villages and farmland to
the central and north with urban conurbations spread along the coast to the south. The rural
areas of the catchment are drained via a network of rivers and intermittent/ephemeral
ditches, and the urban areas are largely drained by a separate sewer system that includes a
4
wide use of property soakaways. The main rivers outfall to the sea at Bognor Regis and foul
flows are treated at Lidsey Wastewater Treatment Works, as is shown in Figure 1 below.
Figure 1 Study Area
The geology and soils across the catchment are varied, but it is dominated by a Primary
Chalk aquifer that is situated beneath soil (Brickearth), sands and gravel, and in some areas,
London Clay. The soils are typically UK WRAP classes 1 to 3, the average SPRHOST value
is 20%, and the average BFIHOST value is 70%. These classifications indicate that the
catchment is relatively permeable and that groundwater could have a bearing on flood risk,
which is in line with what has been found to be one of the primary flood risk mechanisms for
the catchment (Grey, 1998).
Our Approach
In recognising that groundwater plays an important role in the flood dynamics for the
catchment, it was decided to develop an explicit and lumped representation of the
groundwater system within InfoWorks ICM, as this would allow interactions to be easily
applied and also understood. Ideally the authors would have wanted to pursue the
5
development of a linked and bespoke groundwater model, but this was beyond what could
have been achieved during the project.
In some respects the approach was simple: Make use of InfoWorks ICM’s Groundwater
Infiltration Model (GIM) to configure a mass balance representation of soil saturation and
groundwater rebound, then use a routing arrangement that solves Darcy’s Law for flow
through a porous media to allow level to vary across the catchment, as the GIM does not
currently allow for this. From this, connections to the drainage and river network could be
made hydraulically and thus represent the dynamic effects of groundwater availability. Such
an approach makes best use of the existing capability of InfoWorks ICM and can be
replicated in other catchments.
For design events, it was decided to implement consistency throughout the modelling of the
respective systems and adopt the ReFH loss model for informing catchment saturation
dynamics.
The development of both aspects is discussed below.
Developing a Mass Balance Groundwater Model in InfoWorks ICM
To convert rainfall and evaporation into
groundwater volume and rebound, InfoWorks ICM’s
GIM was applied to a “contributing groundwater
subcatchment”.
This was defined by the
topographic catchment and the significant open
channels to the west and east, as these will be the
primary aspects that will contribute to the local
groundwater volume via catchment infiltration and
losses due to baseflows being fed to rivers. It is
recognised that groundwater systems can extend
far beyond topographic catchments and the
“contributing groundwater subcatchment” may not
necessarily represent all of the groundwater
processes, but it is nevertheless the area that could
be considered to directly contribute to the local
groundwater rebound in the catchment.
The
delineated catchment area is shown in Figure 2.
Figure 2 Groundwater Catchment Area
The contributing groundwater subcatchment was setup to drain to a set of nodes
representing boreholes east of Barnham town centre known as “Barn Rise”, as this was the
primary area of focus for the study. A GIM landuse was developed whereby “runoff” and
loss were disabled by setting the fixed runoff coefficient and initial losses to zero, so that
once simulated all of the rainfall would be applied to the GIM landuse. The GIM was setup
with an elevated infiltration threshold (5m) to allow the groundwater level to build up within
the GIM and represented using parameters obtained from literature review and calibration.
The landuse and the parameters that were used to represent the GIM are provided in Table
1 to Table 3 in the Appendix.
The GIM was calibrated to the boreholes at Barn Rise using rainfall data collected at the
Arundel gauge and the MORECS real landuse medium potential evaporation data for the
August 2010 to July 2011 period. The rainfall data recorded at the Arundel gauge was
adopted, as rainfall data collected at the Westergate gauge, which is located in the
catchment and could be considered to be more representative, reflected that this period was
wetter than average when compared to Standard Average Annual Rainfall (SAAR) whereas
6
the Westergate gauge did not. This shows how the variability of rainfall can have a
significant impact on models and it is recommended that future modelling studies using the
approach set out in this paper adopt radar-rainfall inputs, as this will better represent any
spatial and temporal patterns.
The calibration of the GIM to the Barn Rise boreholes is shown in Figure 3 below. The
model (red line) reproduces the peaks and troughs in the rise of the groundwater levels
recorded at the “Barn Rise” bores (green line); reaches a peak level of 8.56m AOD, and tails
off at a higher level than that was recorded. The peak level predicted by the model is slightly
lower than the recorded 8.83m AOD level, but does broadly represent the dynamics of the
response. The GIM model is converting approximately 85% of the rainfall volume into
ground store volume, which is in line with SPRHOST for the catchment (20%) and is
therefore considered both reasonably well configured from a dynamic response and
volumetric perspective.
10
0
1
2
3
9
4
8.5
5
6
8
7
Recorded Rainfall
Ground Store Level (m AOD)
9.5
8
7.5
9
7
01/08/2010 00:00
10
09/11/2010 00:00
17/02/2011 00:00
28/05/2011 00:00
Date / Time
Rainfall (Rainfall intensity (mm/hr))
Ground Store Level (m AOD)
Barn Rise - BH3 (m AOD)
Figure 3 Calibration of the GIM to the Barn Rise Boreholes
Developing a Groundwater Routing Model in InfoWorks ICM.
In appreciating that groundwater level will vary across the catchment and that the GIM only
allows for one groundwater store level to be represented, it was decided to develop a routing
model representing the groundwater system draining to the sea. As part of this arrangement
it was decided to convert the setup of the GIM to a fixed runoff surface, so that the model
could be simulated efficiently and the contributing catchment area could be sub-divided into
smaller contributing catchments.
The conversion of the GIM response to a fixed runoff surface for the August 2010 to July
2011 period is provided in Figure 4 below. This shows that the fixed runoff surface
(hereafter referred to as the groundwater runoff surface) replicated the peak ground store
inflows and volume (4% difference) and is then adequately converted to a fixed runoff
surface.
7
Figure 4 Comparison of Ground Store Inflow and Fixed Runoff Surface Flow
The routing model was developed by including nodes at boreholes located at Woodgate and
Shripney (refer to Figure 2) and connecting conduits that solved the "permeable” Darcy’s
Law equations between the respective nodes. The conduits were set to be equivalent to the
catchment width, a nominal height, and draining to the sea.
The contributing groundwater subcatchment was then spliced at the nodes representing
boreholes, so that the routing model would receive representative contributions of inflow.
The August 2010 to July 2011 period was then re-simulated in the groundwater routing
model and calibrated to the Barn Rise, Woodgate, and Shripney boreholes to provide
confidence in model predictions over the full length of the “contributing groundwater
subcatchment”.
The setup of the routing model is shown in Figure 5 below.
Figure 5 Groundwater Routing Model Setup
8
Connecting the Groundwater Model to the Sewer and River Systems
Some parts of a sewer system are likely to be more resilient to groundwater infiltration than
others, therefore applying connections from the groundwater model to all of the modelled
piped system would be an over-simplification. To ensure that groundwater infiltration would
be correctly applied to the model an Inflow and Infiltration (I&I) investigation was undertaken
during the January 2014 floods when the catchment was saturated and groundwater levels
were high.
The I&I investigation identified areas that were used to inform where connections between
the sewer system and groundwater model should be made and showed that groundwater
infiltration into the sewer system was greatest in the north of the catchment and in Barnham.
Connections made between the sewer system and the groundwater model were then
focused in Barnham, but limited to the bounds at which the groundwater model was
configured (between 8.6m AOD and 3.3m AOD). The ground levels of the manholes were
used as a guide for where in the groundwater model the connection should be made on the
assumption that depth to groundwater would be a function of catchment topography. An
example of a connection is shown in Figure 6 below.
Infiltration flap valve
Groundwater conduit
Sewer model
Figure 6 Groundwater Conduit Connections
In lieu of modelling units that could adequately describe the condition of assets and thus the
ability of groundwater to infiltrate into the sewers (functionality that is available in InfoNet),
flap valve units were used to connect the sewer system with the groundwater model. This
modelling unit was chosen as they do not simulate in-conduit storage or allow flow to reenter the groundwater model, even though this would be useful functionality for investigating
exfiltration issues. The invert of the flap valve was defined as the invert level of the sewer
and the opening height configured to re-produce a short term flow survey undertaken
between March – April 2013. The invert controls the magnitude of pressure head which is
applied from the groundwater conduit to the manhole and the diameter of the conduit
controls the upper limit of infiltration flow into the manhole. Such mechanisms would not be
possible using more standard approaches, for example runoff models representing
groundwater infiltration.
9
Dynamic Groundwater Interaction Modelling
The groundwater model setup replicated the groundwater and infiltration processes as
expected. It visually fitted well with recorded rainfall-driven infiltration tails from the short
term flow survey (refer to Figure 7 below) and what would typically be expected to arrive at
the waste water treatment works – 20 to 30% of the total flow being due to infiltration (refer
to Figure 8 below).
Areas of uncertainty did, however, remain and this was assumed to be in response to the
short term flow survey rainfall data not adequately representing the spatial and temporal
patterns of rainfall, as in the case of the longer term calibration, or other processes that were
not accounted for in the lumped representation of the groundwater system. The study
recommended that the calibration to a short term flow survey be revisited with refined rainfall
data (radar-rainfall) and assessed over a timescale more representative of groundwater
processes (multiple years), rather than just the one as considered in this study.
Figure 7 Example of a Short Term Flow Survey Calibration
10
0
120
5
Flow (l/s)
100
10
80
15
60
20
40
25
0
30
09/04 00:00
09/04 07:30
09/04 15:00
09/04 22:30
10/04 06:00
10/04 13:30
10/04 21:00
11/04 04:30
11/04 12:00
11/04 19:30
12/04 03:00
12/04 10:30
12/04 18:00
13/04 01:30
13/04 09:00
13/04 16:30
14/04 00:00
14/04 07:30
14/04 15:00
14/04 22:30
15/04 06:00
15/04 13:30
15/04 21:00
16/04 04:30
16/04 12:00
16/04 19:30
17/04 03:00
17/04 10:30
17/04 18:00
18/04 01:30
18/04 09:00
18/04 16:30
19/04 00:00
19/04 07:30
19/04 15:00
20
Rainfall (mm/hr)
140
Date and Time (dd/mm hh:mm)
FTW Total l/s
Total Infil l/s
Rainfall
Figure 8 Flows and Infiltrated Flows Arriving at the Waste Water Treatment Works
Design Storm and Groundwater Event Modelling
A key area that needed to be addressed in the study was the establishment of antecedent
conditions for the groundwater model, so that design storms could be simulated. Given that
ReFH is the UK industry’s accepted rainfall-runoff model for river modelling it was decided
to use ReFH’s loss model as the basis for establishing consistency between river, sewer,
and groundwater elements. This was achieved by developing relationships between the
Barham Rife flow gauge and borehole data for the Barn Rise boreholes, so that ReFH’s
design antecedent conditions could be used for groundwater and thus sewer elements.
The relationships were developed in three parts. Firstly, baseflow was separated from river
flow data from the Barnham Rife using the Master Depletion Curve technique described in
Engineering Hydrology (Wilson, 1990) in order to separate the rapid response from the
slower infiltration response and aspect of interest. Secondly, a relationship between River
baseflow and baseflow stage was developed. Thirdly, a relationship between river baseflow
stage and groundwater level was developed, so that groundwater level from a design ReFH
baseflow in the river could be inferred. These relationships are provided in Figure 9 below
where on review it can be seen that the relationships have a good level of linear fit, but there
are some outliers in the data sets.
8.8
2.17
2.12
2.07
2.02
y = 0.737x + 1.9369 11
R² = 0.924
1.97
0.1
0.2
0.3
Rife Baseflow (m³/s)
0.4
Ground Water Level at Barn Rise
borehole (mAOD)
Rife Baseflow level (mAOD)
2.22
8.6
8.4
8.2
8
7.8
7.6
y = 7.4333x - 7.097
R² = 0.8168
7.4
7.2
1.9
1.95
2
2.05
2.1
Rife Baseflow level (mAOD)
2.15
Figure 9 – Relationship between Rife Baseflow to Barn Rise Groundwater Level
To ensure the design baseflow predicted by ReFH was reliable, a 2 year return period
baseflow from ReFH was validated against the peak recorded baseflow for two years’ worth
of flow gauging at the Barnham Rife. Ideally, this would have been done for a longer record,
but this was all that was available at the gauge.
In Figure 9 below, the peak event over the two years’ worth of flow record is shown (peak
flow of 1m3/s) where it can be seen that the baseflow level during this peak event is not a
function of one individual event, but rather the succession of three events. The baseflow at
the start of the peak flow event is approximately 0.2m3/s, which is in line with what ReFH
predicts for this event (0.2m3/s). On this basis, it was considered appropriate to adopt the
peak baseflow predicted by ReFH, but it is noted that the timings of events has had a
significant role in determining whether ReFH would be appropriate. The level of baseflow
would be improved with further records of flow at the site, but moreover highlights the
deficiencies with using design storms and it would be better to adopt time series inputs for
assessing the performance of the catchment.
1.2
Flow at Barnham (m³/s)
1
0.8
0.6
0.4
0.2
31/01/11
30/01/11
29/01/11
28/01/11
27/01/11
26/01/11
25/01/11
24/01/11
23/01/11
22/01/11
21/01/11
20/01/11
19/01/11
18/01/11
17/01/11
16/01/11
15/01/11
14/01/11
13/01/11
12/01/11
11/01/11
10/01/11
09/01/11
08/01/11
07/01/11
06/01/11
05/01/11
04/01/11
03/01/11
02/01/11
01/01/11
0
Date
Figure 10 2-year Peak Flow at Flow Gauging for the Barnham Rife at Barnham
The ReFH model was adopted for determining baseflow and thus groundwater level for all
design events up to the 1 in 1000 year event. This showed that the groundwater levels for
all design events in excess of a 1 in 2 year would be at, or just below, the surface at the start
of a design storm and (by extension) the sewers would start in a full state. This:


questioned the suitability of using traditional design storms and antecedent
conditions for assessing river and sewer performance, as this approach highlighted
that this may be significantly underestimated;
questioned whether the sewer system should have been constructed in the manner
they have been because from a design standard perspective a high groundwater
level would stipulate the sewers being constructed:
o above the design groundwater level; or
12
o

using construction techniques that would be highly resilient to deterioration if
constructed below the design groundwater level.
highlighted that strategic surface flood risk mapping, or indeed other SWMPs, should
account for groundwater dynamics within sewers that are in infiltration prone
catchments, as it can have a marked impact on the predicted risk.
The Benefits
The incorporation of the groundwater system into InfoWorks ICM resulted in a number of
benefits for the SWMP. The first being that the partners were able to be co-ordinated on:



planning conditions (e.g. reducing groundwater infiltration through design - fusion
jointed MDPE pipes);
maintenance regimes; and
engagement with the residents and stakeholders of the catchment. This is a key
outcome because obtaining funding to alleviate all risk is unattainable and learning,
as well as gradually adapting, to live with risk is central for delivering improvements
in performance.
InfoWorks ICM allowed this because the software provided a holistic approach to flood risk
in the catchment. The approach adopted to representing groundwater infiltration in the
InfoWorks ICM model showed that:



SWMPs prepared for infiltration prone catchments need to consider the effects of
groundwater infiltration and by not doing so may underestimate local flood risk;
the use of design storms underestimated the value of the sewer system in times of
low groundwater and that a time series approach to assessing sewer performance
was required for infiltration prone catchments such as this; and
because groundwater infiltration modelling was physically based, mitigation decisions
(e.g. sewer sealing, CSO spills) could be meaningfully assessed (flows shown at the
works in Figure 8 are as a result of upstream contributions that can be removed to
determine benefit).
Recommendations
If future users adopt the approach outlined in this paper it is recommended that:




All of the data required to undertake this type of modelling must be available. The
approach is data intensive and the level of risk must match the scale of the problem /
investment required;
Radar-rainfall data is used in preference to point rainfall, as this will better represent
the spatial and temporal patterns;
Site specific infiltration head-discharge curves are created using short term flow
surveys and asset integrity information (e.g. the UKWIR strategic infiltration tool
within InfoNet), so that these can be used in place of the flap valves as used in this
study. Flap valves were adopted for this study, as they do not simulate in-conduit
storage or allow flow to re-enter the groundwater model;
The approach set out in this paper be used for simulating groundwater infiltration
model within InfoWorks ICM (data permitting), but seek to link an actual groundwater
13
model to InfoWorks ICM in preference because of the inherent improvements that a
groundwater model offers.
Where to Next
The Atkins team are currently engaged with another complex urban flood study where
groundwater plays a significant role in the flood dynamics. This is looking to either build on
the approach developed in this study or link a FEFLOW model to InfoWorks ICM. To
support the work undertaken in this study and indeed future work it is suggested that:


A modelling unit that describes asset condition be included in InfoWorks ICM, so that
asset integrity can be used alongside groundwater models for simulating infiltration
into sewers; and
Research be taken into the possibility of groundwater modelling capability being
explicitly included in InfoWorks ICM, as this is the direction the industry should be
taking in understanding groundwater flood risk, infiltration into sewers, and of course
developing mitigation/management measures for these issues.
References
DEFRA. (2004). Making Space for Water - Developing a New Government Strategy for
Flood & Coastal Erosion Risk Management in England.
DEFRA. (2010). Surface Water Management Plan Technical Guidance. London: DEFRA.
F&WM Act. (2010). Flood and Water Management Act.
Grey, I. (1998). Surcharging of Sewers in the Barnham Area. EA.
HR Wallingford & BGS. (2010). BGS. Retrieved 01 06, 2014, from Modelling Groundwater
Systems: http://www.bgs.ac.uk/research/groundwater/modelling/home.html
Lockie, T., & Joseph, T. (2008). Selection of an appropriate hydrological model to simulate
inflow and infiltration. NZWWA Conference.
Ofwat. (2011). Future Impacts on Sewer Systems in England and Wales. Birmingham:
Ofwat.
UKWIR. (2012). Strategic Infiltration. London: UKWIR.
14
Appendix
Table 1 Groundwater Landuse and Runoff Surface
Subcatchment
Landuse
GIM
Barn Rise
Dummy
Groundwater
GIM
Woodgate
Dummy
Groundwater
GIM
Shripney
Dummy
Groundwater
GIM
Landuse
Runoff Surface 1
Dummy Groundwater
Groundwater
Infiltration
Runoff Surface %
100
Runoff Surface
Runoff Model
Loss type
Initial Loss
Routing Model
Fixed PR
Groundwater
Infiltration
Fixed
Abs
0
Wallingford
0
Table 2 Groundwater Infiltration Parameters – Soil Store
Parameter
Definition
Adopted Value
Soil Depth
Depth of soil store
1.5m
Average value determined through a review of
boreholes available on the Magic Website.
Porosity of Soil
A coefficient representing the porosity of the soil (upper storage
reservoir)
45%
As provided in “Rural and Urban Hydrology”
(Mansell, 2003) Table 6.2 for clay soil types.
Percolation Threshold
The percentage saturation level of the soil at which water starts
to percolate downwards
1%
Adopted to essentially disable the soil store,
given that the primary interest is groundwater
level.
Percolation Coefficient
A time coefficient, determined by calibration from existing data. It
15
1 Day
Comments
Adopted through calibration.
Parameter
Percolation % Infiltrating
Definition
is recommended that the value should be between 0.1 and 10
Percentage of percolation flow that infiltrates directly into the
drainage network.
Adopted Value
1%
Comments
Value adopted for simulating the model.
Table 3 Groundwater Infiltration Parameters – Groundwater Store
Parameter
Definition
Adopted Value
Porosity of Ground
A coefficient representing the porosity of the ground (lower
storage reservoir)
20%
As provided in “The Physical Properties of
Major Aquifers in England & Wales” (EA, 1997)
Figure 4.26 for the upper chalk band with dips
of 30º.
Baseflow Threshold
Level and Type
The groundwater level at which secondary infiltration occurs.
-3m
Arbitrary value set to be well below the lowest
topographic value included within the InfoWorks
ICM sewer & river model.
Infiltration Threshold
Level and Type
The level of the groundwater storage reservoir at which
groundwater infiltration occurs. It is recommended that the
Infiltration Threshold Level should be between 0 and 5 m above
the Baseflow Threshold
5m
Value set to ensure that there is no
groundwater flow into the model, so that
groundwater level is allowed to build up.
Baseflow Coefficient
A time coefficient, determined by calibration from existing data. It
is recommended that the value should be between 100 and
10000
1300 Days
Adopted through calibration.
Infiltration Coefficient
A time coefficient, determined by calibration from existing data. It
is recommended that the value should be between 0.1 and 10
1 Day
Adopted through calibration.
16
Comments