Erosion to bedrock downstream of a sediment-retention basin, Tijuana, Baja California, Mexico.


"HUNGRY WATER"

Rivers transport two types of suspended sediment: (1) bed-material load, and (2) wash load. Bed-material load is the fraction of sediment load whose particle sizes are significantly represented in the channel bed. Wash load is the fraction of sediment load whose particle sizes are not significantly represented in the channel bed. The bed-material transport rate depends on the hydraulics of the flow, while the wash load concentration is independent, for the most part, of the hydraulics of the flow.

Under steady conditions, there is a sediment rating curve, i.e., a unique relation between water discharge and corresponding sediment (bed material only) discharge. A calculation of sediment transport rate calculates a point--or points--of the rating curve. This sediment discharge is often referred to as the "sediment transport capacity," to denote that it is the amount of sediment that the river will always carry under steady equilibrium conditions.

Under unsteady conditions, several scenarios are possible, and the resulting effects are noted:

  1. An increase in the amount of sediment discharge at the upstream boundary, without a corresponding increase in water discharge, would lead to aggradation.
  2. A decrease in the amount of sediment discharge, without a corresponding decrease in water discharge, would lead to degradation.
  3. An increase in the amount of water discharge, without a corresponding increase in sediment discharge, would lead to degradation.
  4. A decrease in the amount of water discharge, without a corresponding decrease in sediment discharge, would lead to aggradation.

The second and third cases, in particular, merit careful consideration due to the significant practical implications. This situation usually happens downstream of a dam impoundment. The dam ponds the water and retains most of the sediment. The water subsequently released is typically almost devoid of sediment; therefore, it is "hungry water." This water will have the tendency to pick up sediment as it moves downstream.1

The "hungry water" condition is exacerbated in the case of a sediment-retention basin. Holding on to the sediment behind the dam and releasing the water immediately, without the sediment, will produce channel and bank erosion downstream. Depending on the rate of water release, the amount of erosion will be commensurate with the quantity of sediment retained at the basin. Thus, a sediment-retention basin is generally not an effective strategy for sediment control in natural streams.

1 Lane, E. W. (1955). The importance of fluvial morphology in hydraulic engineering. Proceedings, ASCE, Vol. 81, Paper 745, July.