# Sediment transport

The current sediment transport module only works with the MMF soil erosion model. The sediment transport module incorporates a transport capacity equation, which is forced by two model parameters, i.e. TCβ and TCγ. Prosser & Rustomji (2000) suggest to adopt a value of 1.4 for both model parameters. However, these two parameters are frequently used for model calibration, for instance, when measured sediment concentration or reservoir sediment yield data are available.

The sediment transport equation includes a roughness factor, that accounts for the effect of vegetation on sediment transport. The roughness factor is based on the vegetation characteristics, as provided in the land use specific mmf\_table. This approach works well for headwater areas, where overland flow can be assumed on the hillslopes. However, for larger study areas, where the water accumulates into river channels, the roughness factor has an important influence on how much sediment is transported downstream. When a river flows through a densely vegetated land use class (e.g. a forest), much of the sediment will be deposited. This might be unrealistic, because the sediment transport will be unaffected by the forest. Instead, the flow is mostly affected by the composition of the river bed, which mostly consists of sand and/or gravel, with a lower Manning’s roughness coefficient than the surrounding land use. To overcome this, the model may account for a much channel bed specific Manning’s roughness value. To use this feature, the user needs to set manningChannelFLAG to 1. Next, the upstream area need to be provided (upstream\_km2), from which river channels are assumed. And finally, the channel Manning’s roughness coefficient should be provided, which will override the Manning’s values determined by the vegetation characteristics. Typical channel Manning’s value can be obtained from [Chow (1959)](https://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Mannings_n_Tables.htm).

When the reservoir module is used, the sediment transport module accounts for the trapping efficiency of the reservoirs. The trapping efficiency should be provided by the user in the TrapEffTab look-up table:

Table 29: TrapEffTab

| Reservoir ID | Trapping efficiency (TE) |
| ------------ | ------------------------ |
| -99          | 1                        |
| 1            | 0.95                     |
| 2            | 0.89                     |
| 3            | 0.64                     |
| …            | …                        |

In this table, the reservoir IDs should correspond to the ones used in the reservoir module. The trapping efficiency can be based on the equation by Brown (1943), which uses the reservoir capacity and drainage basin as input. This advanced sediment transport algorithm uses a 2-phase approach. In the first phase the transport capacity is applied to the sediment taken into transport, as determined by the soil erosion model. In the second phase the trapping efficiency determines the fraction of the sediment that is trapped by the reservoir and the fraction that is routed in downstream direction of the reservoir. When several reservoirs are located in the same river (i.e. in sequence), this 2-phase approach is repeated as many times as reservoirs are present in the same river. The user needs to provide a table that indicates the order of the reservoirs, starting with the most upstream located reservoir (with a value of 0) down to the downstream located reservoirs. Figure 1 shows an example of the Segura River catchment, where the most upstream located reservoirs all have an order of 1 (but get a value of 0 I the ResOrder table). The order increases in downstream direction, where the last reservoir (which is actually the catchment outlet), has the highest order of 6 (5 in the ResOrder table).

Table 30: ResOrder

| Reservoir ID | Trapping efficiency (TE) |
| ------------ | ------------------------ |
| 1            | 0                        |
| 2            | 0                        |
| 3            | 0                        |
| 4            | 1                        |
| 5            | 1                        |
| 6            | 2                        |
| …            | …                        |

<figure><img src="https://2432255304-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FbpkOf6MyIYzEi0TyV0In%2Fuploads%2FoeK4UFrEYfK8L73FRa6p%2Fafbeelding.png?alt=media&#x26;token=7511f00d-f119-4e48-926b-523ae70e8832" alt=""><figcaption><p>Figure 43: Example of the subcatchment order in the sediment transport routine.</p></figcaption></figure>

The sediment transport model generates several model output variables. This include sediment deposition (SedDep), which shows where and how much sediment is deposited in the study area, this output variable is usually stored as a yearly map. The sediment flux (SedFlux) shows how much sediment is transported through each grid cell. The sediment flux can be stored as a yearly map or can be obtained as a time series at the station cells. The latter can be used for model calibration when time series of sediment concentrations are available. In that case, the sediment transport should be divided by the discharge to get sediment concentration. When the reservoir module is used, the model can obtain sediment yield data at each of the reservoirs using the SedYld variable. Depending on the application, a time series or map output can be used.

Table 31: Model parameters

| Model parameter                   | Model variable | Unit    | Range/default |
| --------------------------------- | -------------- | ------- | ------------- |
| Model parameter TCβ               | β              | -       | 1-1.8         |
| Model parameter TCγ               | γ              | -       | 0.9-1.8       |
| Manning channel flag              |                | -       | 0 or 1        |
| Upstream area for channel Manning |                | km2     | > cell area   |
| Channel Manning’s roughness value | n              | m s-1/3 | 0.025-0.15    |
| Trapping efficiency               | TE             | -       | 0-1           |
