The National Geospatial Program

Fragmentary river segments have to be reconnected before addressing various routing and tracking problems. Elevation determines drainage directions, so the partial heights available through LiDAR may provide useful hints on how the segments should be joined. However, it is not trivial how this information can be applied. This paper bridges this gap by proposing the induced structure approach, which first approximates a terrain compatible with those observations, and then derives a river network from that induced terrain. Since the network is derived from an induced terrain that honors the partial observations, we expect that the derived river network will enforce most restrictions imposed by the partial observations. This paper also provides specifics on the implementation. In the first step regarding terrain reconstruction, we find that the optimal scheme depends on the height sample distribution. If the samples are sparsely yet evenly distributed, natural neighbor interpolation with stream burning (NN-SB) is the most cost-effective. If the samples are offered only at the given river locations, the hydrology-aware version of Over determined Palladian Partial Differential Equation (HA-ODETLAP) should be used instead. In the second step concerning river derivation, we find it necessary to favor those given river locations. Otherwise they will be missed out. We set their respective initial water amounts to the critical accumulation level to ensure a river flows across them. In the subsequent branch thinning process, those locations are protected from being trimmed. We foresee applications of our solution framework in a few 2D and 3D network tracing problems with similar observation distribution, like dendrite network reconstruction.     

Surficial sediments of the wave-dominated Orange River Delta and the adjacent continental margin off south-western Africa

The textural and compositional characteristics of the surficial shelf sediments north and south of the Orange River Delta are reviewed and compared. Sediments are fractionated and dispersed both north- and southwards of the Orange River mouth by wave action, longshore drift and subsurface currents. The mean grain size decreases both offshore and southwards in response to decreased wave influence at the seabed and the competence of the weak poleward undercurrent respectively. The increasing dominance of marine biogenic components in sediments south of the prodelta indicates a greater marine influence, modifying previous inferences that the Namaqualand mudbelt is primarily derived from the southward transport of Orange River sediments. A sharp distinction can be drawn between sediments of the Orange Shelf to the south and the Walvis Shelf to the north. Foraminiferarich deposits that dominate the Orange middle shelf and slope indicate that upwelling is an inner-shelf phenomenon. On the Walvis Shelf, foraminiferal sediments are confined to the slope and outer shelf. Fish debris is more common in Walvis Shelf sediments. Although phosphorite and glauconite sands often occur together in the same deposits on the Orange Shelf, the two minerals are concentrated in separate deposits on the Walvis Shelf.