Practical Channel Hydraulics - Roughness, Conveyance and Afflux
is available on the Routledge website
The Conveyance and Afflux Estimation System (CES/AES) is a software
tool for the improved estimation of flood and drainage water
levels in rivers, watercourses and drainage channels. The
software development followed recommendations by practitioners
and academics in the UK Network on Conveyance in River Flood
Plain Systems, following the Autumn 2000 floods, that operating
authorities should make better use of recent improved knowledge
on conveyance and related flood (or drainage) level estimation.
This led to a Targeted Programme of Research aimed at improving
conveyance estimation and integration with
other research on afflux at bridges and structures at high
flows. The CES/AES software tool aims to improve and assist
with the estimation of:
- hydraulic roughness
- water levels (and corresponding
channel and structure conveyance)
- flow (given slope)
- section-average and spatial velocities
- backwater profiles
upstream of a known flow-head control e.g. weir (steady)
upstream of bridges and culverts
- uncertainty in accuracy
of input data and output.
In 2001, a Scoping Study by
the UK Network on Conveyance in River Flood Plain Systems
confirmed the potential for reducing the uncertainty associated
with estimating flood levels. The main drivers
were the advances in the understanding of flow phenomena
in complex river and coastal flood systems; the availability
of over twenty years of data from the EPSRC Flood Channel
Facility at HR Wallingford;
the advent of computing power that enables more sophisticated
solution techniques; and concern that this new knowledge
was not well transferred into practice within the UK
flood management community. This led to a Targeted
Programme of research to develop a new Conveyance
The final product is a software tool for estimating
conveyance (water levels, rating curves, etc), spatial
velocities and boundary shear stresses at river sections
as well as undertaking simple reach-based backwater calculations. It
also provides a comprehensive database of river roughness,
integrating diverse information from over 700 references,
including photographs (linked to the River Habitat Survey)
and advice on vegetation cutting and regrowth.
In 2003, a Scoping study into the 'Hydraulic
performance of bridges and other structures, including
effects of blockage, at high flows' confirmed the potential
for drawing together much improved guidance for practitioners
on the estimation of afflux at in-channel structures.
In 2004, a second
complementary project was commissioned to develop the
Afflux Estimation System (to operate with the CES) and
the Afflux Advisor (an Excel spreadsheet application).
The aim of the project was to improve
the understanding of the effects of in-channel structures
on flood water levels at high flows, relating particularly
to the representation of afflux (the increase in upstream
water levels attributable to the structure). The project
researched a range of methods of calculating afflux,
to determine their applicability to the various types
of bridges and culverts found in UK channels and incorporated
the most appropriate methods and algorithms, following
detailed testing, into the AES software.
The CES and the AES have been integrated
into a single software module so that they can operate together
where needed to estimate water levels along an open channel
reach containing bridge or culvert crossings. The CES/AES
software has also been integrated into the 1D river-modelling
Flow and InfoWorks
RS. The source code for the CES and AES calculation may be made available under licence to other software
houses and bona fide researchers. The CES and AES are
subject to on-going review and development to incorporate
beneficial advances in knowledge and use.
The combined CES/AES software can be downloaded as a stand-alone executable package.
What does the CES/AES include?
The AES/CES software includes the:
- Roughness Advisor, which is a database of
roughness information including descriptions, photographs
and unit roughness values for a range of natural and
man-made roughness types. These are sourced from over
700 references (including the River Habitat Survey)
and include aquatic vegetation, crops, grasses, hedges,
trees, substrates, bank protection and irregularities.
The Roughness Advisor also includes information on
seasonal variations in vegetation roughness, cutting
and suggested regrowth patterns following cutting.
- Conveyance Generator,
which estimates the channel conveyance based
on the roughness information and cross-section
description. The calculation is based on
a lateral distribution method, where the unit flow
rate is estimated at 100 points across the channel
and the overall flow is then determined by integrating
across the river section. The available outputs
with depth include water level, flow, rating curves,
velocity, area, perimeter, Froude Number, Reynolds
Number etc. It is also possible to obtain
spatial distributions (across a section) of velocity,
boundary shear and shear velocity.
- Uncertainty Estimator, which provides
an upper and lower credible scenario, providing
some measure of the uncertainty associated with
each predicted water level. These values
are derived from the upper and lower roughness
estimates from the Roughness Advisor.
- Backwater Module, which includes a simple
calculation of the backwater profile upstream of a
control i.e. a point of known stage and flow. This
is based on a simple energy balance, working from downstream
to upstream, and includes the option to incorporate
the velocity head term (kinetic energy term). Available
within CES/AES Standalone.
- Afflux Estimator, which is a code for use with gradually varied flows. AE
models arch and beam bridges with up to 20 openings, and
pipe, box and arch culverts with up to 10 identical barrels. This
code is embedded within the bridge and culvert units in
the CES/AES stand-alone desktop application, and produces
a longitudinal water surface profile.
What additional tools are there?
- Afflux Advisor, which is a separate spreadsheet application to provide a relatively quick calculation of the afflux for simple bridge and culvert structures in a uniform channel. The calculation is based on the same field and laboratory data as the Afflux Estimator but assumes a uniform flow and does not provide a longitudinal water surface profile. It requires data for a single channel cross section and returns an afflux rating curve as an output.
What can the CES/AES do for me?
The CES/AES software tools can
assist the practitioner with:
- Calculating water levels, flows and velocities for
rivers, watercourses and drains
- Providing upper and
lower uncertainty scenarios
- Assessing flood or extreme
water levels, and the sensitivity of these to channel
adaptation or management options (particularly dredging
and plant management)
- Assessing the impact of timing
and nature of vegetation cutting
- Assessing the impact of blockage due to vegetation
- Understanding the influence of in-stream structures
on water levels
- Finding holistic solutions which address
both environmental (e.g. Water Framework Directive)
and Flood Risk Management or Land Drainage objectives
- Implementing guidance and procedures e.g. supporting
Work Instructions for channel maintenance and performance
specification (in the UK).
We also expect the CES/AES to be a valuable teaching and research tool in its different forms.
What will the CES/AES not do?
The CES/AES does not consider:
- Unsteady flows, i.e. where the flow changes significantly
- Super-critical flow conditions, except within a bridge or culvert structure
- Complex looped river networks
- A more comprehensive range of structures e.g. sluice
gates, trash screens (to come), weirs, etc.
For more complex river modelling, incorporating the above, users are advised to consider full 1D hydrodynamic modelling e.g. ISIS, InfoWorks
RS, HEC-RAS etc.
Where can I find out more?
The CES/AES was developed under two
separate projects. For further technical and project
details see the CES Development and AES Development pages,
and follow-up related links and documentation as necessary.
Who is involved in the ongoing
CES support and development?
The ongoing CES support and development is being undertaken by a multi-disciplinary team comprising representatives from the Environment Agency and other funding organisations, the original CES/AES development teams and the ongoing software support team.
- Eleanor Heron, Senior Scientist, Flood Risk Science
- Peter Robinson, Technical Advisor, Asset Management - Strategy and Data Systems
- Geoff Baxter, Principal Scientist and Sustainable Asset Management Theme Manager, Flood Risk Science
Other UK organisations
- Stephen Dawson, Northern Ireland Rivers Agency
- Stan Irvine, Flooding Policy Team, Scottish Government
HR Wallingford / JBA Project Team
- Rob Millington
(Project Manager for CES Roll-Out
and ongoing software support)
- Caroline Hazlewood (Conveyance
technical focus for ongoing research in the area)
Lamb (Afflux technical
focus for ongoing research in the area)
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Last updated: April 2009