|CONCEPTS - Overview|
Incision of alluvial, channelized streams is widespread in the Mid-south and Midwestern United States, with the concomitant loss of land and stream habitat. The highly erodible loess soils are unable to halt the incision and widening of many of these stream systems, leading to increased sediment production and yields as material is eroded from beds and banks. Post- disturbance, incised channels follow an evolutionary sequence of degradation, continuing degradation with channel widening, followed by aggradation and widening, and final attainment of quasi-equilibrium. The aggradational phase can also be accompanied by incipient channel meandering. This sequence can last well over 100 years. To evaluate remediation strategies, it is very important to obtain a proper understanding and description of these processes. Further, there is a need for comprehensive computer models that simulate the long-term morphology of incised channels.
The National Sedimentation Laboratory has developed the CONservational Channel Evolution and Pollutant Transport System (CONCEPTS) computer model to simulate the evolution of incised streams and to evaluate the long-term impact of rehabilitation measures to stabilize stream systems and reduce sediment yield. CONCEPTS simulates unsteady, one-dimensional flow, graded sediment transport, and bank-erosion processes in stream corridors. It can predict the dynamic response of flow and sediment transport to instream hydraulic structures. It computes channel evolution by tracking bed elevation changes and channel widening. The bank erosion module accounts for basal scour and mass wasting of unstable cohesive banks. CONCEPTS simulates transport of cohesive and cohesionless sediments, both in suspension and on the bed, and selectively by size classes. CONCEPTS also includes channel boundary roughness varying along a cross section, for example due to varying vegetation patterns.
CONCEPTS assumes the flow in stream systems to be one-dimensional along the centerline of the channel. It computes the flow as a function of time simultaneously at a series of cross sections along the stream using the Saint Venant equations. The governing equations are discretized using the generalized Preissmann scheme, and the resulting set of algebraic equations are solved by a variation of the Gaussian elimination method. CONCEPTS can handle flashy runoff events, common to the size and locale of disturbed streams in the Mid-south and Midwestern US.There are four types of hydraulic structures included in CONCEPTS: (1) box and pipe culverts, (2) bridge crossings, (3) grade control (drop) structures, and (4) any structure for which a rating curve is available. The mathematical representation of the flow at hydraulic structures is equivalent to that of open- channel flow, resulting in an efficient implementation of
Sediment transport and bed adjustment
Sediment-transport rates are a function of flow hydraulics, bed composition, and upstream sediment supply. The composition of the channel bed may change as particles are eroded from or deposited on the bed, thereby changing flow hydraulics and fractional transport rates. CONCEPTS calculates total-load sediment-transport rates by size fraction from a mass conservation law, and taking into account the differing processes governing entrainment and deposition of cohesive an cohesionless bed material.
For graded bed material, the sediment-transport rates depend on the bed material composition, which itself depends on historical erosion and deposition rates. CONCEPTS divides the bed into a surface or active layer and a substrate or subsurface layer. These layers constitute the so-called 'mixing layer'. The subsurface layer may be composed of several layers reflecting historical deposition patterns. Sediment particles are continuously exchanged between the flow and the surface layer, whereas particles are only exchanged between the surface layer and substrate when the bed scours and fills. The volumetric fraction content by size class in the surface layer is determined by a mass conservation equation. CONCEPTS uses 14 predefined size classes to represent graded sediment: < 0.01, 0.01-0.025, 0.025-0.065, 0.065-0.250, 0.25-0.841, 0.841-2.0, 2.0-3.36, 3.36-5.66, 5.66-9.57, 9.57-16.0, 16.0-26.9, 26.9-38.1, 38.1-64.0, and 64.0-128.0 mm.
For cohesive bed material, erosion rate is calculated following an excess shear-stress approach. The deposition rate is based on local shear stress and particle fall velocity
Channel-width adjustment occurs in a wide variety of geomorphic contexts and is usually accompanied by changes in other morphological parameters such as channel depth, roughness, bed material composition, riparian vegetation, energy slope, and channel planform. The processes responsible for width adjustment are diverse, and the adjustment process itself displays a wide variety of spatial and temporal patterns.
It is unlikely that equilibrium approaches such as regime theory, extremal hypotheses, or tractive force methods can accurately predict width adjustment over time. CONCEPTS simulates channel- width adjustment by incorporating the fundamental physical processes responsible for bank retreat: (1) fluvial erosion or entrainment of bank-material particles by flow, and (2) bank mass failure due to gravity. Bank material may be cohesive or non- cohesive and may comprise numerous soil layers.
The detachment of cohesive soils is calculated following an excess shear-stress approach. An average shear-stress on each soil layer is computed, which increases with distance below the water surface. If the critical shear stress of the material is exceeded, entrainment occurs. CONCEPTS is therefore able to simulate the generation of overhanging banks.
Streambank failure occurs when gravitational forces that tend to move soil downslope exceed the forces of friction and cohesion that resist movement. The risk of failure is usually expressed by a factor of safety, defined as the ratio of resisting to driving forces or moments. CONCEPTS performs stability analyses of planar slip failures and cantilever failures of overhanging banks. The bank's geometry, soil shear-strength (cohesion and angle of internal friction), pore-water pressure, confining pressure exerted by the water in the stream, and riparian vegetation determine the stability of the bank.