Check descriptions - structures

Bypass Spills

Extreme flood events can often overtop bridges and culverts. If a spill is not included then water can artificially back up causing too much flooding upstream and too little downstream. This check will report if a weir, sluice or orifice does not have a bypass spill. The program checks this by finding the first upstream and downstream junctions, ignoring those with only an inflow boundary attached, and looking for a spill connected to both junctions. This approach is not always adequate however due to the complexity of some models. An example of this may be a mill with multiple orifices representing the mill wheels with a mill bypass further upstream. If any structures are found without a bypass spill being identified an error is raised and a list of the structures node names which have failed are displayed.

P1 / P2 Inverts

Inappropriate P values (depth of crest above upstream or downstream bed level) can force Flood Modeller into using incorrect modes, therefore affecting discharge relationships and flow paths. Additionally, in some circumstances wrong P values can create numerical noise that affects model stability. The check will look at the upstream and downstream units attached to every crump, broad crested and sharp crested weir, sluice and orifice and compare the minimum invert levels with the p1 and p2 values. If discrepancies are found then an error is reported and the node name of the structure is reported, whether it is the p1 or p2 value at fault, the invert level of the weir / sluice / orifice, the p1 / p2 value, the minimum elevation of the upstream / downstream unit and the difference between the inverts. In more complex models the upstream / downstream minimum elevation may be difficult to find and then a 'Handcheck’ warning will be given listed beside the node name and whether the p1 or p2 value needs to be checked. When the weir / sluice / orifice is attached to a reservoir unit then the check will not be carried out. If wished the user may set a tolerance by which the difference between inverts and the p1 / p2 value may differ. This can be input in the 'Max P Err (m)’ box.

Weir Crests Above Channel Inverts

Gated, notional and flat V weir units are not included in this check. Weir equations are only valid if the weir has a drop over the structure (see BS3680, Part 4E, Sec 8.3; Part 4F, Sec 8.3.4). Inappropriate positioning of weirs and crest levels can force Flood Modeller into using incorrect modes therefore affecting discharge relationships and flow paths. Additionally, in some circumstances wrong crest values can create numerical noise that affects model stability. This check therefore ensures the difference between the weir invert and the upstream and downstream minimum elevations are always greater than 0.15 m. If discrepancies are found then an error is reported and the node name of the structure is reported, whether upstream or downstream is incorrect, the invert level of the weir, the minimum elevation of the upstream / downstream unit and the difference. In more complex models the upstream / downstream minimum elevation may be difficult to find and then a 'Handcheck’ warning will be given listed beside the node name and whether upstream or downstream needs to be checked.

Weir Modular Limits In Range

The modular limit controls when a weir unit is calculated with free or drowned flow. The appropriate British Standards should be consulted for valid ranges for different structure types. This check looks at the modular limit of every crump, broad crested and sharp crested weir unit and will report an error if any are outside the set limits. The program will list the node name, the modular limit value and whether the modular limit is too high or low. By default the maximum and minimum limits are set to 0.95 and 0.8 respectively, but these can be altered using the same box in which the spill modular limits are set.

Bernoulli Loss Elevation to Invert

The data in Bernoulli loss units should cover the range of water levels expected to occur. Otherwise inaccuracies occur in the calculated losses and this can also be a cause of instability in the model. Therefore it is best practice that the lowest elevation matches the minimum elevation of the upstream and downstream river sections. In this check the program will ensure that the lowest elevation listed in the Bernoulli loss unit matches the minimum elevation of the upstream and downstream units. If a discrepancy is found the node name is listed, whether upstream or downstream is incorrect, the Bernoulli loss unit’s invert, the upstream / downstream section invert and the difference. In some circumstances the upstream / downstream invert cannot be identified and then a 'Handcheck’ error is listed along with the node name and whether upstream or downstream is a problem.

Bernoulli Loss / General Loss Spill Bypass

Loss units are sometimes used to represent the losses which occur at a bridge or small culvert. During extreme flood events bridges and culverts can often overtop. If a spill is not included then water can artificially back up causing too much flooding upstream and too little downstream. This check will report if a Bernoulli / general loss does not have a bypass spill. The program checks this by finding the first upstream and downstream junctions, ignoring those with only an inflow boundary attached, and looking for a spill connected to both junctions. This approach is not always adequate however due to the complexity of some models. If any Bernoulli / general loss units are found without a bypass spill being identified, an error is raised and a list of the node names which have failed are displayed.

Culvert Spill Bypass

Extreme flood events can often overtop culverts. If a spill is not included then water can artificially back up causing too much flooding upstream and too little downstream. This check will report if a culvert does not have a bypass spill. The check will attempt to find the start and finish of a culvert, taking into account culvert bends, manhole units and junctions. Once the extent of the culvert is found a check is made by finding the first upstream and downstream junctions, ignoring those with only an inflow boundary attached, and looking for a spill connected to both junctions. If any culverts are found without a bypass spill being identified an error is raised and a list of the start and end node names which have failed are displayed.

Culverts Without Culvert Inlets and Outlets / Culverts With No Entry or Exit Structures

There can be significant energy losses at the entrance and exit of a culvert, it is therefore important that this is represented in the model when appropriate. The culvert inlet and outlet units are the most appropriate units to use, however many older models contained culverts where entry and exit losses were modelled as energy losses. Culverts may also have sluices or orifices at the downstream end.

For these checks the program first identifies the start and end of all the culverts in the model, taking into account culvert bends, manhole units and junctions. The first check, Culverts Without Culvert Inlets and Outlets, will then look for a culvert inlet and outlet unit attached to the start and end of the culvert. If not found the program will return an error listing the start and or end node names which have failed. It should be noted that this check will not identify if the inlet and outlet structures are the correct way round. The second check, Culverts With No Entry or Exit Structures, will look for a structure at the start and end of the culvert. If none are found then the program will return an error listing the start and or end node names which have failed.

Culvert Inlet and Outlet Orientation

Occasionally models are found with the inlet and outlet units attached to the downstream and upstream ends of a culvert respectively. This can lead to numerical instabilities, and also does not correctly represent the losses at the entrance and exit. This check enables the inlet and outlet units are attached to a culvert in the correct order. It will test every culvert inlet / outlet unit to ensure it is not attached to a culvert unit upstream / downstream respectively. If there is a fault an error is recorded and the node name is listed.

Bridge Bypass Spills

Extreme flood events can often overtop bridges. If a spill is not included then water can artificially back up causing too much flooding upstream and too little downstream. This check will report if a bridge unit does not have a bypass spill around it. The program checks this by finding the first upstream and downstream junctions, ignoring those with only an inflow boundary attached, and looking for a spill connected to both junctions. This approach is not always adequate however due to the complexity of some models, for example a spill around multiple structures. If any bridges are found without a bypass spill being identified an error is raised and a list of the bridge node names which have failed are displayed.

Bridge Spring Points

This check ensures that bridge spring points, for both arch and USBPR1978 bridges, exist in the cross-section data and that the bridge spring points are above or equal to the section data. If this is not true it suggests an error in the data. The program will report an error if the spring points are not found or if the elevation of the spring points is lower than the cross-section elevation at that point. The bridge node name will be listed along with the left / right spring point and Left or Right is displayed as appropriate.

Bridge Manning’s n Values

Often when Manning’s values are altered along a stretch of river, the user can forget to alter the Manning’s values in the bridges. This check therefore looks for bridge Manning’s n values which are not the same as the cross-sections upstream and downstream. If an error is found the program lists the bridge node name, the Manning’s value upstream, the Manning’s value downstream and the bridge Manning’s value.