**3.1. Understanding the problem**

Sediment transport is concerned with entrained soil materials carried in water by erosion on the catchment and within channels. Sediment particles are categorised as follows (i) the saltation load (not discussed here); (ii) bedload (not discussed here) and (iii) suspended load including clay (< 62*μm* in particle diameter), silt and sand. Suspended load (both as "finegrained sediment" and "wash load") is directly a result of the turbulence in water and forms a large proportion of the transported load, where the turbulence is a measure of the energy in the water to carry the load.

*Sediment discharge* is a measure of the mass rate of sediment transport at any point in space and time and determines whether the load is being transported or deposited. The whole process comprises soil erosion, sediment transport and sediment yield, where the deposited load delivered to a point in the catchment is referred to as sediment yield and is expressed as tons per unit area of the basin per year, measured at a point. Estimation of sediment yield (and soil erosion) is essential for management but these and mathematical models are used to gain an insight into the underlying processes. Sediment yield is estimated by (i) direct measurement, (ii) using local time series models to predict future states; (iii) using mathematical models to study jointly both erosion and sediment processes.

Suspended sediment forms most of the transported load and can be affected by many parameters including rainfall, land use pattern, slope, soil characteristics, e.g. soil moisture content but their considerations lead to distributed models, which are complex. Recorded suspended sediment derives distributed models by serving them as boundary conditions or input sources but their inherent information is not tapped on. There is a case for local models to study the information contained in recorded sediment loads alone in terms of flow and sediment hydrographs. This chapter is concerned with the study of the suspended load of a river, as discussed below.

Inter-Comparison of an Evolutionary Programming

Model of Suspended Sediment Time-Series with Other Local Models 261

**Figure 2.** Variation of Daily Suspended Sediment and Flow Data in the Mississippi River Basin

**Figure 1.** Mississippi River Station at Tarbert + RR Landings (http://pubs.usgs.gov/circ/circ1133/geosetting.html).

## **3.2. Study area**

The flow–sediment time series data of a Mississippi river Station is used in the study, the location of which is shown in Figure 1. The gauge is situated at Tarbert + RR Landings, LA (USGS Station no. 07373291, latitude 30°57′40″, longitude 91°39′52″) and it is operated by the US Geological Survey (USGS) – the location map is shown in Figure 1. The Mississippi River discharges an average of about 200 million metric tons of suspended sediment per year to the Gulf of Mexico and to the ocean.
