Check descriptions - spills

Spill / Floodplain Length vs Distance to Next

Differences between spill length and distance to next section can cause too much or too little water to enter or leave a cross-section in a non-physical manner. The program will check every river cross section and interpolate unit for attached spills and / or floodplain units. It will then compare the "distance to next" of the river section against the length of the spill / floodplain. If they differ by more than the set error percentage an error will be returned and the river section / interpolate node name, distance to next, spill length and the difference will be listed. The percentage error allowed between river distance to next and spill length is set by default to 10%; however it can be adjusted by the user in the 'Spill Length Error %’ box. If a spill / floodplain unit cannot be found corresponding to the name listed in the river section / interpolate unit then a 'Spill Not Found - Handcheck’ error is reported alongside the section node name and the name of the spill / floodplain which could not be found.

Spill / Floodplain Elevations Matches Section Bank Elevations

It is important that river cross-sections match the spills which are attached as spills / floodplains should only allow water to flow at the appropriate level. For instance, if surveyed levels have been used for cross-sections and LiDAR used for spills / floodplains, this can cause flows to occur at the wrong elevations. This check ensures that the spill / floodplain units are consistent with the river cross-section, looking for a point in the spill / floodplain that matches the maximum left or right elevations of the cross section. If a match cannot be found then the program will return an error and list the node name of the river cross-section, the maximum left and right elevations of the river section, the spill / floodplain name, the minimum elevation in this unit and the minimum difference between these.

For example, a cross section, XX234, with maximum left and right elevations of 32.46 and 33.50 mAOD respectively has an attached spill, XX234SpU, which does not have a point corresponding to either of these elevations. The minimum elevation of the spill is 34.00 mAOD. The program will return an error of the form:

If a spill / floodplain unit cannot be found corresponding to the name listed in the cross section unit then a 'Spill Not Found - Handcheck’ error is reported alongside the river section node name and the name of the spill / floodplain which could not be found.

Locate Side Spills / Floodplains with Default Coefficients

The default spill coefficient for a spill unit is 1.7, which is representative of a round-nosed broad-crested weir, and not necessarily of a lateral spill. This check looks for spills which are attached to only one river section / interpolate and reports an error if the spill coefficient is set to 1.7 (the default setting).

Locate Reservoir To Reservoir Spills / Floodplains with Default Coefficients

Similar to the previous check, this check looks for spills which are attached to two reservoirs and reports an error if the spill coefficient is set to 1.7 (the default setting).

Locate Other Default Coefficients

Searches for all other spills and floodplains not checked by the other spill / floodplain coefficient checks. This check can indicate where spills or floodplain units have been unintentionally left with default coefficient values.

Spill / Floodplain High Coefficients

Spills / Floodplains should be set to a value which is reasonable and representative of the material over which water is flowing. The program will check every spill and floodplain unit to ensure the coefficient is equal to or below 2.3. If any units have a coefficient above 2.3 a warning is output, listing the node name and the coefficient value.

Spill / Floodplain Low Coefficients

Spills / Floodplains should be set to a value which is reasonable and representative of the material over which water is flowing. The program will check every spill and floodplain unit to ensure the coefficient is equal to or above 0.5. If any units have a coefficient below 0.5 a warning is output, listing the node name and the coefficient value.

Spill / Floodplain Modular Limits

The modular limit controls when a spill / floodplain 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 spill and floodplain unit and will report an error if any are outside the set limits. The program will list the node name, whether the modular limit is too high or low and the modular limit value. By default the maximum and minimum limits are set to 0.95 and 0.8 respectively, but these can be altered in the relevant boxes in the menu.

Spill Invert above Channel Invert

When spills are situated in-line they should ideally be at least 0.1 m above the invert of the channel. For this check every spill which is attached upstream and / or downstream to a river section, conduit or bridge is tested to ensure its minimum elevation is at least the Spill Level above the upstream /downstream unit minimum elevation. The Spill Level is set to 0.150 m by default but can be altered by the user if required. If a spill is found to be too low an error is reported and the node name of the spill is listed, along with the minimum elevation of the spill, minimum elevation of the neighbouring unit and the difference.

Reservoir Initial Elevations

Appropriate initial water levels in a reservoir unit can affect the validity and calibration of a model. There are two ways to check reservoirs initial water levels. The first checks the initial water level of the reservoir against the minimum elevation of all the spills / floodplains attached to the reservoir. This is appropriate when reservoir elevation area curves are too low and the floodplain needs filling to ensure that excess non-physical storage volume is not created. If the reservoir initial water level is below the lowest minimum spill level an error will be generated and the reservoir node name listed alongside the minimum spill elevation, the initial water level, the difference of these values and the volume of additional storage falsely created is output. If the difference is extremely large (positively or negatively) this would suggest the initial water level has not been set or there is an error with the spill/ floodplain data and a 'Handcheck’ warning will appear alongside the usual output. If no spills are found then no errors or warnings are generated.

The second check should be used when the reservoir units are expected to be empty at the start of the run. This will compare the initial water level in the reservoir to the lowest point given in the geometry data. A tolerance can be set to allow the two values to vary within the set range. This is particularly important as most reservoir units have a small notch at the bottom to allow a smooth transition when they begin to fill. By default the 'Invert Tolerance’ is set to 0.5 m but can also be altered by the user. If the difference between the initial water level and the minimum elevation of the reservoir is outside the set tolerance then an error is recorded and the reservoir name, the minimum elevation, the initial water level, the difference and the volume of the stored water is listed. An error is also recorded if the initial level of the reservoir is below the minimum elevation of the reservoir, as this can cause numerical instability within the model. The reservoir name, the minimum elevation, the initial water level, the difference and 'Initial Level Below Invert' error is reported.