HR Wallingford/JBA - CES/AES Home page

Any software that is made available to download from this server ("Software") is the copyrighted work of the Environment Agency, Scottish Government, Rivers Agency Northern Ireland and Wallingford Software Ltd. and/or their suppliers. Use of the Software is governed by the terms of the end user licence agreement, which is included with the Software ("End User Licence Agreement “ or “EULA"). An end user will be unable to install any Software unless he or she first agrees to accept the terms of the EULA.

A: The unit roughness is the roughness due an identifiable segment of boundary roughness. It does not take account of turbulence generated from the channel plan form or cross-section shape. The values are ideally derived from measurements in wide straight channels, where there is limited influence from the channel banks. The values for the substrate materials were largely derived in this manner. The back-calculated (i.e. from the CES derived flow based on input unit roughness values) Manning n values were then compared to measured Manning n values through depth, and the unit roughness values were altered where appropriate. For channels with vegetation, it is more complex to separate out the different sources of resistance. Here, the measured values have been adopted as 'pure' vegetation values, based on the assumption that the vegetation influence dominates the other sources of resistance. In the absence of measurements, expert judgement has been used. As the knowledge and measurement techniques improve, these unit roughness values may be refined. For further details, see Roughness Review in the CES Development documents.

A: Conveyance is a quantitative measure of the discharge capacity of a watercourse. It relates total discharge to a measure of the gradient or slope of the channel. It is derived from the channel properties, including channel roughness, channel shape (section and plan form) and cross-sectional area which provide insight into the likely flow mechanisms, for example, turbulence generated due to boundary roughness or channel shape. For further details and equations, see Conveyance User Manual .(pdf)

A: A unit roughness may be smaller, larger or equal to a given Manning n value for the same substance e.g. gravel, cobbles, sand. There is no consistent or direct relationship as these values are used in different equations to describe different flow processes. The Manning n is a 'resistance' parameter used in the Manning equation and CES unit roughness is a 'roughness' parameter used in a Lateral Distribution Method. In addition, the Roughness Advisor unit roughness value is referenced to a flow depth of 1m, as this is a typical operating depth for UK rivers. As Manning n also varies with flow depth, for a direct comparison (despite the aforementioned resistance/roughness differences), the Manning n value at a 1m flow depth would need to be used.

A: As there is no direct relationship between a unit roughness and a Manning n value (see above question), there is no conversion method. The Manning’s n value can be back-calculated within the CES as described above so a “comparison” can be done between those values and the unit roughness although it must be recognised that the unit roughness is for a depth of 1m. The CES Development team considered carefully this departure from a Manning n value and concluded that is was necessary in order to simulate the physics of the flow in a more scientifically sound manner. The unit roughness enables one to separate out resistance due to boundary generated turbulence from the other sources of turbulence e.g. secondary circulations, and thus each flow process is independently quantified based on its true contribution to the overall resistance.

A. No, it would be expected for every zone that there would be the minimum of bed material but there may be no vegetation or irregularity. If there is vegetation and/or an irregularity it is essential that a bed material is also incorporated as without this the unit roughness value would not be calculated correctly.

A: Yes. The spatial description of the roughness is used locally at that point in the cross-section to derive the conveyance capacity. Thus a change in, for example, which bank is vegetated and the extent of the vegetation coverage will influence the water level estimates. (The exception is that for a perfectly symmetrical channel, with either one or the other bank being vegetated, the only change would be the velocity distribution across the channel – not the water level.)

A: No. To account for form losses due to consecutive expanding/contracting cross-sections, you would need to move to full 1D hydrodynamic modelling e.g. ISIS, InfoWorks RS. However, transition losses (due to contraction on entry to a structure and expansion on exit) are accounted for by the automatically computed transition lengths and coefficients in the AES. For open channel cross sections, a possible ‘fudge’ would be to increase/decrease the overall cross-section roughness by a percentage e.g. increase for contracting and decrease for expanding.

A. This will affect the water levels due to the higher (or lesser) friction effects offered by these plants. The effects of plants growing locally can be estimated by looking at a single cross-section locally within CES and applying the appropriate roughness values. From the CES a stage-discharge relationship can be generated for the ‘with’ and ‘without’ vegetation situation and the differences between levels for the same flood flows determined. If the impact further upstream were to be assessed, a backwater, with further cross-sections should be included. The backwater impact could then be assessed comparing the ‘with’ and ‘without’ vegetation situations.

For extreme blockage cases, it is possible to set a very large unit roughness value or prescribe a non-conveyance portion of the channel. For further details on non-conveyance zones, see Conveyance User Manual.(pdf)

A. There was not enough reliable information or evidence from the research to provide consistent results for unit roughness values for emergent vegetation as it grows or if it is cut. For emergent vegetation, the impact and relationship on roughness is quite different to floating and submerged vegetation. The relationships are more dependent on drag coefficient and we were not able to represent the relationships in a way which was consistent at this time within the Roughness Advisor.

A:The CES/AES water level calculation assumes normal depth conditions i.e. similar to use of the Manning equation. The assessment of individual cross-sections does not take tidal influences into account. In tidally influenced reaches, the downstream boundary condition is the tidal cycle, which is typically described by a head-time relationship. As the backwater is limited to steady state, this would need to be fixed as the head associated with a given time e.g. highest point in the tidal cycle. The reach upstream of this tidal boundary would be treated as a fluvial channel, subject to normal depth conditions, with no influence other than the downstream head which will provide some backwater influence.

A: The CES/AES assumes clear water flow. The outputs include spatial distributions of boundary shear stress and shear velocity, and thus future CES/AES upgrades may consider more complex methods for mobile beds and sediment transport. #

A. In the addition to the user specifying the channel vegetation, the Roughness Advisor can access information on ecotypes derived from the River Habitat Survey for the site location concerned. If the user does not know the type of vegetation in the channel under investigation and cannot get access to photographs or visit the site, the RHS included in the CES can help. By typing in the grid reference of the site, the CES checks the nearest RHS sites available in database information stored in the CES. The vegetation ecotypes for these nearest sites are given and can be used as a guide to the likely, type and coverage of vegetation to be expected in rivers in that area. Roughness values for most of these ecotypes are provided. This establishes an important link to the natural habitat which might be expected in the location concerned and is of particular relevance to the Water Framework Directive. The data should of course be used with caution and it is recommended that the user visits or refers to photographic evidence of the site to determine the vegetation ecotypes present.

A: This is purely a cosmetic bug in the software (being addressed) and the underlying calculations are being carried out with the correct section separations but the markers are then drawn incorrectly. To view the results in the interim, output the backwater results to a CSV file where the numbers are all correctly reported.

A: This comes about as a result of the backwater calculation encountering an error, but the appropriate error handling not always being called. This bug will be addressed in the next update. In the interim, to ensure you are viewing the correct results, restart the CES if inconsistent results are observed and always use the results from the first backwater run.

A: This only takes place when the ‘omit kinetic energy’ option is selected on the backwater, and a backwater calculation is run. The code writes the calculated slope (as used for the backwater method) back into the slope field on the property sheet. As a result, the user can see different slopes on the section property sheet and in the main summary grid for the sections. If the Conveyance Generator (.GEN) file is then saved, the new slope values are written into the .GEN file and then become visible on the summary grid when the .GEN file is reloaded. The changed slope values do not affect the backwater calculation, as the slopes are recalculated when the backwater is run. However, they will affect the cross-section CES calculation if the user displays the conveyance data for a specific cross-section. This bug is being addressed. In the interim, do not save the .GEN file after running an ‘omit kinetic energy’ option, rather export and save the results.

A. Afflux is an increase in water level that can occur upstream of a structure at high flows. More formally, afflux can be defined as the instantaneous maximum difference in water level, at a location upstream of a structure, if the structure were to be removed. It is caused by energy losses at high flows through bridges and culverts, and it is made worse by the presence of blockage. Afflux can be physically significant in terms of flood levels and leads to a potential for increased losses due to blockage. It therefore needs to be represented in river modelling tools and understood by river engineers and other professionals.

For a more detailed discussion of afflux, see the technical documentation listed on this web site. A simple illustration is given below.

A. Currently there is not explicit 'blockage module' within AES. However, it is possible to represent blockage by adjusting the geometry of a structure so as to reduce the opening dimensions. It is hoped to update the AES in 2008 with a "trash screen simulator" whereby the effect of water level of blockage to differing degrees of a screen upstream of the culvert can be simulated.

A. There must be a section at the upstream face of the structure. This section will either be a culvert or a bridge, depending on the structure being modelled. On the Backwater tab, the distance to the next section must be greater than or equal to the length of the structure on the structure property sheet. Note: in the current version of the software, you must have exactly three roughness zones in the structure AND in the sections upstream and downstream of the structure.

A. Individuals or organisations who simply download the CES/AES Standalone free of charge are not entitled to any support in using the software, other than obtaining further knowledge and understanding though the FAQs and documentation identified in these web pages. We recommend that new users should attend a training course. Users are welcome to submit suggestions, comments and questions to the following email address ces@wallingfordsoftware.com and we will respond where appropriate.

Organisations with a large number of users (over 25) are welcome to contact Wallingford Software through the same email address to discuss individual support arrangements.

The CES/AES Standalone and Afflux Advisor are provided “as is” and are not formally supported. However, users are welcome to submit general feedback, issues and questions to: ces@wallingfordsoftware.com and we will respond where possible.

For further information on the standard 1 day training course as well as dedicated courses tailored to specific applications (e.g. channel maintenance, scheme design, flood risk), please contact Caroline Mc Gahey.