Model help:AgDegNormal

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AgDegNormal

This module calculates a) the equilibrium sediment transport rate and b) the morphodynamic evolution of a reach due to a change in sediment input rate.

Model introduction

The module computes variation in river bed level η(x, t), where x denotes a streamwise coordinate and t denotes time, in a river with constant width B. The bed sediment is characterized in terms of a single grain size D and submerged specific gravity R. The reach under consideration has length L. Bed elevation at the downstream end is assumed to be fixed. The model is based on total bed material load. The model is 1D, assumes a rectangular channel and neglects wall effects.

By modifying the sediment feed rate (Gtf) at the upstream end, the river can be forced to aggrade or degrade to a new equilibrium. The module computes this evolution.

Model parameters

Parameter Description Unit
Input directory Determine if you want to use the "GUI" interface to provide input parameter values or use a text file with the input parameters by providing the location of the file on the server. [-]
Site prefix Part of the input and output file name e.g. the name of the geographic location, or project [-]
Case prefix Part of the input and output file name that provides you the opportunity to do different scenario simulations for e.g. the same location, or project [-]
Parameter Description Unit
Flood discharge [m2/s]
Intermittency [-]
Channel width [m]
Grain size [mm]
Bed porosity [-]
Roughness height [mm]
Ambient Bed Slope [-]
Imposed Annual sediment transport rate from upstream [tons/year]
Length of reach [m]
Time step [days]
Number of time steps per printout [-]
Number of printouts [-]
intervals [-]
Upwinding coefficient (1 = full upwind, 0.5 = central difference) [-]
Coefficient in Manning-Strickler resistance relation [-]
Coefficient in sediment transport relation [-]
Exponent in sediment tranpsort relation [-]
Critical Shields stress [-]
Fraction of bed shear stress that is a skin friction Fraction of bed shear stress that is a skin friction [-]
Submerged specific gravity of sediment [-]
Parameter Description Unit
Model name The name of the model [-]
Author name The developer of the model [-]

Uses ports

This component has no uses ports.

Provides ports

  • Model: Provides IRF functionality.

Main equations

  • Flow in the channel (using Manning-Strickler formulation)
[math]\displaystyle{ C_{z}={\frac{U}{u_{*}}}=\alpha _{r}\left ( \frac{H}{K_{c}} \right )^{\frac{1}{6}} }[/math] (1)
  • grain roughness (Used as roughness height when bedforms are absent)
[math]\displaystyle{ k_{s} = n_{k} D }[/math] (2)
  • water conservation for a quasi-steady flow
[math]\displaystyle{ Q = q_{w} B = U B H }[/math] (3)
  • Boundary shear stress
[math]\displaystyle{ \tau _{b} = \rho u_{*} ^2 = \rho g H S }[/math] (4)
  • Shields number (Shields stress)
[math]\displaystyle{ \tau ^* = {\frac{\tau _{b}}{\rho R g D}} = {\frac{H S}{R D}} }[/math] (5)
  • Submerged specific gravity
[math]\displaystyle{ R = {\frac{\rho _{s}}{\rho}} - 1 }[/math] (6)
  • Water depth
[math]\displaystyle{ H = [{\frac{\left (k_{c} \right ) ^{\frac{1}{3}} Q_{w}^2}{\alpha _{r} g B^2 S}}]^{\frac{3}{10}} }[/math] (7)
  • Computation of the sediment transport (Meyer-Peter and Muller equation )
[math]\displaystyle{ q_{t} ^* =\left\{\begin{matrix} \alpha_{t} \left ( \varphi_{s} \tau ^2 - \tau_{c} ^* \right ) ^ \left ( n_{t} \right ) & \tau ^* \gt \tau_{c} ^* \\ 0 & \tau ^* \lt = \tau_{c} ^*\end{matrix}\right. }[/math] (8)
  • Einstein number
[math]\displaystyle{ q_{t} ^* = {\frac {q_{t}}{\sqrt{R g D} D}} }[/math] (9)
  • Cumulative time of the river has been in flood
[math]\displaystyle{ t_{f} = I_{f} t }[/math] (10)
  • Equilibrium (graded) states
  • Annual sediment yield with a graded state at this slope
[math]\displaystyle{ G_{t} = \rho_{s} q_{t} Bl_{f} t_{a} }[/math] (11)
  • Volume sediment transport rate per unit width obtained at the graded state
[math]\displaystyle{ q_{t} = {\frac{G_{tf}}{\rho_{s}Bl_{f} t_{a}}} }[/math] (12)
  • Computation of bed variation
  • Exner equation of sediment continuity (assume that qt is zero for most of the time)
[math]\displaystyle{ \left ( 1 - \lambda_{p} \right ) {\frac{\partial\eta}{\partial t}} = - {\frac{\partial q_{t}}{\partial x}} }[/math] (13)
  • Exner equation of sediment continuity (average over many floods)
[math]\displaystyle{ \left ( 1 - \lambda_{p} \right ) {\frac{\partial \eta}{\partial t}}= - {\frac{\partial l_{f} q_{t}}{\partial x}} }[/math] (14)
  • Spatial derivative of the total bed material load per unit width
[math]\displaystyle{ \frac{\Delta q_{t,i}}{\Delta X}=a_{u}\frac{q_{t,i}-q_{t,i-1}}{\Delta X}+\left ( 1-a_{u} \right )\frac{q_{t,i+1}-q_{t,i}}{\Delta X} }[/math] (15)
  • Bed slope computed in each node
[math]\displaystyle{ S_{i}=\left\{\begin{matrix} \frac{\eta _{1}-\eta _{2}} {\Delta x} & i=1\\ \frac{\eta _{i-1}- \eta _{i+1}} {2\Delta X} & i=2...M \\ \frac{\eta _{M} - \eta _{M+1}}{\Delta X} & i=M+1 \end{matrix}\right. }[/math] (16)
  • Initial bed elevation
[math]\displaystyle{ \eta_{i}=S \left ( L - x_{i} \right ) }[/math] (17)
  • non-dimensional total shear stress
[math]\displaystyle{ \tau _{b} ^* = {\frac{\tau _{b}}{\left ( \rho _{s} - \rho \right ) g D}} }[/math] (18)

Notes

Actual rivers tend to be morphologically active only during floods. That is, most of the time they are not doing much to modify their morphology. The simplest way to take this into account is to assume an intermittency If such that the river is in flood a fraction I of the time, during which Q = Qf and qt is the sediment transport rate at this discharge (Paola et al., 1992). For the other (1 – If) fraction of time the river is assumed not to be moving sediment.

Output is controlled by the parameters Ntoprint and Nprint. The code will implement Ntoprint time steps. In addition to output pertaining to the initial state, the code implements Nprint outputs, to that the total number of time steps executed is equal to Nprint x Ntoprint.

  • Note on model running

The maximum number of computational nodes, M, is 99 (this is the case for all of the AgDeg functions).

The model calculates the water depth with a Chezy formulation, if only the Chézy coefficient is specified in the input file. The code uses a Manning-Strickler formulation, when only the roughness height, kc, and the coefficient αr are given in the input text file. If all these parameters are in the text file, the program will ask the user which formulation he would like to use.

Examples

An example run with input parameters, BLD files, as well as a figure / movie of the output

Follow the next steps to include images / movies of simulations:

See also: Help:Images or Help:Movies

Developer(s)

Gary Parker

References

  • Paola, C., Heller, P.L., and Angevine, C.L., 1992. The large-scale dynamics of grain-size variation in alluvial basins. 1: Theory. Basin Research, 4, 73-90.
  • Meyer-Peter, E., and Müller, R., 1948. Formulas for bed-load transport Proceedings, 2nd Congress International Association for Hydraulic Research, Rotterdam, the Netherlands, 39-64.

Links