Model help:AgDegBW: Difference between revisions
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==Model parameters== | ==Model parameters== | ||
= | = Input Files and Directories = | ||
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!Parameter!!Description!!Unit | !Parameter!!Description!!Unit | ||
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|width="20%"| | |width="20%"|Input directory | ||
|width="60%"| | |width="60%"|path to input files | ||
|width="20%"| | |width="20%"| | ||
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|Site prefix | |||
|Site prefix for Input/Output files | |||
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|Case prefix | |||
|Case prefix for Input/Output files | |||
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= | = Run Parameters = | ||
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!Parameter!!Description!!Unit | !Parameter!!Description!!Unit | ||
|-valign="top" | |-valign="top" | ||
|width="20%"| | |width="20%"|Flood discharge | ||
|width="60%"| | |width="60%"| | ||
|width="20%"|< | |width="20%"| m<sup>3</sup> / s | ||
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|Intermittency | |||
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| - | |||
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|Channel Width | |||
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| m | |||
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|Grain size | |||
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| mm | |||
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|Bed Porosity | |||
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| - | |||
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|Roughness height | |||
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| mm | |||
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|Ambient Bed Slope | |||
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|Imposed Annual Sediment Transfer Rate from Upstream | |||
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| tons / annum | |||
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|Imposed water surface elevation | |||
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| m | |||
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|Intervals | |||
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|Length of reach | |||
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| m | |||
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|Number of Time Steps per Printout | |||
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|Number of printout | |||
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|Upwinding coefficient (1 = full upwind, 0.5 = central difference) | |||
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|Coefficient in Manning-Strickler Resistance Relation | |||
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|Coefficient in Sediment Transport Relation | |||
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|Exponent in Sediment Transport Relation | |||
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|Critical Shield stress | |||
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|Fraction of bed shear stress that is skin friction | |||
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|Submerged specific gravity of sediment | |||
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|Time step | |||
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| year | |||
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|} | |} | ||
= | = About = | ||
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!Parameter!!Description!!Unit | |||
|-valign="top" | |||
|width="20%"|Model name | |||
|width="60%"|name of the model | |||
|width="20%"| - | |||
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|Author name | |||
|name of the model author | |||
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<headertabs/> | <headertabs/> | ||
==Uses ports== | ==Uses ports== |
Revision as of 17:04, 4 May 2011
AgDegBW
It is the Calculator for aggradation and degradation of a river reach using a backwater formulation. This program computes 1D bed variation in rivers due to differential sediment transport. The sediment is assumed to be uniform with size D. All sediment transport is assumed to occur in a specified fraction of time during which the river is in flood, specified by an intermittency. A Manning-Strickler relation is used for bed resistance. A generic Meyer-Peter Muller relation is used for sediment transport. The flow is computed using a backwater formulation for gradually varied flow.
Model introduction
The model calculates a) an ambient mobile-bed equilibrium, and b)the response of a river reach to either 1) changed sediment input rate at the upstream end of the reach starting from t = 0 or 2) changed downstream water surface elevation at the downstream end of the reach starting from t = 0, where t is the temporal coordinate. The code is very similar to AgDegNorm. The main difference between the two codes is in the procedure to compute the water depth. In AgDegNorm the flow is assumed normal (i.e. steady and uniform), while in AgDegBW the flow is assumed steady and it is computed solving the backwater equation. The case of Froude-subcritical flow, for which Fr < 1, is considered herein. This implies that integration of the backwater equation must proceed upstream from x = L, with x streamwise coordinate and L length of the modeled reach. Both a Chezy and a Manning-Striclker formulation can be used to compute the flow.
Model parameters
Uses ports
This will be something that the CSDMS facility will add
Provides ports
This will be something that the CSDMS facility will add
Main equations
- Computation of the flow
The backwater equation
[math]\displaystyle{ {\frac{dH}{dx}} = {\frac{S - S_{f}}{1 - F_{r} ^2}} }[/math] (1)
- Friction slope
[math]\displaystyle{ S_{f} = C_{f} F_{r} ^2 }[/math] (2)
- Froude number
[math]\displaystyle{ F_{r} ^2 = {\frac{U^2}{g H}} = {\frac{q_{w} ^2}{g H^3}} }[/math] (3)
- Flow velocity
[math]\displaystyle{ U = {\frac{q_{w}}{H}} }[/math] (4)
- The bed friction coefficient ( assumed to obey a Manninbg-Strickler resistance )
[math]\displaystyle{ C_f ^ \left ( {\frac{-1}{2}} \right ) = C_{z} = \alpha_{r} \left ( {\frac{H}{k_{c}}} \right ) ^{\frac{1}{6}} }[/math] (5)
- roughness roughness
[math]\displaystyle{ k_{s} = n_{k} D }[/math] (6)
- The relation between bed slope S and bed elevation η
[math]\displaystyle{ S = -{\frac{\partial \eta}{\partial x}} }[/math] (7)
- Water surface elevation
[math]\displaystyle{ \epsilon = \eta + H }[/math] (8)
- Shields number
[math]\displaystyle{ \tau^* = {\frac{\tau_{b}}{\rho R g D}} = {\frac{C_{f} U^2}{R g D}} = {\frac{C_{f} {\frac{q_{w} ^2}{H^2}}}{R g D}} }[/math] (9)
- Bed shear stress
[math]\displaystyle{ \tau_{b} = \rho C_{f} U^2 }[/math] (10)
[math]\displaystyle{ R = {\frac{\rho_{s}}{\rho}} - 1 }[/math] (11)
- Computation of the sediment transport
equation of the type of Meyer-Peter and Muller
- τ* > τc *
[math]\displaystyle{ q_{t} ^* = \alpha_{t} \left ( \varphi_{s} \tau ^2 - \tau_{c} ^* \right ) ^ \left ( n_{t} \right ) }[/math] (12)
- τ* <= τc *
[math]\displaystyle{ q_{t} ^* = 0 }[/math] (13)
- Einstein number
[math]\displaystyle{ q_{t} ^* = {\frac {q_{t}}{\sqrt{R g D} D}} }[/math] (14)
- 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] (15)
[math]\displaystyle{ q_{t} = {\frac{G_{tf}}{\rho_{s}Bl_{f} t_{a}}} }[/math] (16)
- Computation of bed variation
- Exner equation
[math]\displaystyle{ \left ( 1 - \lambda_{p} \right ) {\frac{\partial\eta}{\partial t}} = - {\frac{\partial q_{t}}{\partial x}} }[/math] (17)
[math]\displaystyle{ \left ( 1 - \lambda_{p} \right ) {\frac{\partial \eta}{\partial t}}= - {\frac{\partial l_{f} q_{t}}{\partial x}} }[/math] (18)
Symbol | Description | Unit |
---|---|---|
Q | flood discharge | m 3 / s |
x | streamwise coordinate | m |
η | bed elevation | m |
t | time step | year |
B | channel width | m |
D | grain size | mm |
l | bed porosity | - |
R | submerged specific gravity | - |
ξd | downstream water surface elevation | m |
qw | water discharge per unit width | m2 / s |
kc | roughness height | m |
G | imposed annual sediment transfer rate from upstream | tons / annum |
Gtf | upstream sediment feed rate | - |
ξd | downstream water surface elevation | m |
L | length of reach under consideration | m |
qw | water discharge per unit width | m2 / s |
i | number of time steps per printout | - |
p | number of printouts desired | - |
M | number of spatial intervals | - |
u | upwinding coefficient (1 = full upwind, 0.5 = central difference) | - |
r | coefficient in Manning-Strickler Resistance relation | - |
a | coefficient in sediment transport relation | - |
n | exponent in sediment transport relation | - |
P | fraction of bed shear stress that is Skin Friction | - |
R | submerged specific gravity of sediment | - |
Sf | friction slope | - |
Fr | Froude number | - |
U | flow velocity | m / s |
Cf | bed friction coefficient | - |
g | acceleration of gravity | m/ s^2 |
k | roughness height | mm |
αr | coefficient in Manning-Stricker, dimensionless coefficient between 8 and 9 | - |
ks | grain roughness | m |
nk | dimensionless coefficient typically between 2 and 5 | - |
τ* | Shield number | - |
ρ | fluid density | kg / m3 |
ρs | sediment density | kg / m3 |
τc | critical Shields number for the onset of sediment motion | - |
ψs | the fraction of bed shear stress | - |
qt * | Einstein number | - |
qt | volume sediment transport rate per unit width | - |
If | flood intermittency | - |
tf | cumulative time the river has been in flood | s |
Gt | the annual sediment yield | tone/yr |
ta | the number of seconds in a year | - |
λp | the porosity of the bed deposit | - |
Qf | sediment transport rate during flood discharge | |
αt | equals to 8 | - |
nt | exponent in sediment transport relation, equals to 1.5 | - |
τc * | reference Shields number in sediment transport relation, equals to 0.047 | |
Cf | bed friction coefficient, equals to τb / (ρ U2 ) | - |
CZ | dimensionless Chezy resistance coefficient. | |
Sl | slope of the bed surface | - |
x | downstream coordinate | m |
τ | shear stress on bed surface | N / m2 |
qb | bed material load | tons / year |
Δx | spatial step length | m |
Qw | flood discharge | m3 / s |
Δt | time step | year |
Ntoprint | number of time steps to printout | - |
Nprint | number of printouts | - |
aU | upwinding coefficient (1=full upwind, 0.5=central difference) | - |
αs | coefficient in sediment transport relation | - |
Output
Symbol | Description | Unit |
---|---|---|
η | bed surface elevatioon | m |
H | water depth | m |
ξ | water surface elevation | m |
τb | bed shear stress | kg / (s^2 m) |
S | bed slope | - |
qt | total bed material load | m2 / s |
Notes
The model 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. Water surface elevation at the downstream end is prescribed. The model is based on a calculation of total bed material load. The model is 1D, assumes a rectangular channel and neglects wall or bank effects.
By modifying the upstream sediment feed rate Gtf and/or the downstream water surface elevation ξd, the river can be forced to aggrade or degrade to a new equilibrium. The program computes this evolution.
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 downstream water surface elevation must exceed the Hc, critical depth, which is equal to (Qw 2/(Bc 2g))1/3, otherwise the user is alerted, and the program exits.
In the original version of the code embedded, the Manning-Strickler formulation is implemented
The water depth is calculated using a Chézy formulation, and the Manning-Strickler formulation is implemented, when only the roughness height, kc, and the coefficient αr are given in input file. When all the three parameters are present, the program will ask the user which formulation they 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:
- Upload file: http://csdms.colorado.edu/wiki/Special:Upload
- Create link to the file on your page: [[Image:<file name>]].
See also: Help:Images or Help:Movies
Developer(s)
References
- Paola, C., Heller, P. L. & Angevine, C. L. 1992 The large-scale dynamics of grain-size variation in alluvial basins. I: 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.