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)
  • afflux upstream of bridges and culverts
  • uncertainty in accuracy of input data and output.

CES Development

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 Estimation System

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.

AES Development

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.

CES/AES Integration

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 packages, ISIS 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 or debris
  • 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 with time
  • 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.

Environment Agency

  • 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)
  • Rob Lamb (Afflux technical focus for ongoing research in the area)


Last updated: April 2009