DEMO 1: Migrating trench in a 1D channel

Description

In this demo, water flows across a steep-sided trench cut in the sand bed of a flume. The water reaches the upstream edge of the trench with an equilibrium suspended sediment concentration profile. As the flow decelerates over the deeper trench some sediment is deposited. Sediment is then picked back up by the accelerating flow at the downstream edge of the trench. Due to the spatial difference between the areas of deposition and erosion the trench appears to migrate downstream. Initially the model has a constant depth with a trench with vertical walls in the middle of the model area. The goal is to verify whether the trench development due to morphological impact is in agreement with measurements carried out by Van Rijn (1984). The computed results of Delft3D-FLOW as well as the measurements are presented in the Muppet plot.

Model characteristics

• Dimension of model: length of 30 m and a width = 0.5 m
• dx = 0.3 m and dy = 0.1 m
• 10 non-equidistant layers
• Time step of 0.24 seconds
• Simulation period of 10 minutes; morphological changes start after T = 5 min
• Morphological scale factor of 180, which means that at the end of the simulation the position of the trench after 15 hours is computed (5 min. of morphological changes multiplied by 180)
• Algebraic turbulence model
• Horizontal viscosity coefficient of 5e-4 m2/s
• White-Colebrook bottom friction coefficient of 0.025 m

DEMO 2: Simple channel flow

Description

Flow in a simple channel with sloping bathymetry is investigated. A steady solution is reached, in which the vertical viscosity term balances the barotropic pressure gradient. For this steady situation an analytical solution is available from the 2D shallow water equations. Results from Delft3D-FLOW simulations are compared to the analytical solution. For the 3D simulations vertical profiles for velocity are investigated. Solutions are obtained with the k epsilon turbulence model. It is validated whether the models produces logarithmic velocity profiles. A comparison has been made between sigma-layer and with Z-layer model results. The formulas that have been used to obtain the analytical solutions are presented in a document in the measurements folder (Genseberger et al., 2002). Both the full document (testcasesZ-modelD3Dflow.pdf) as well as the relevent part for this demo (Simple channel flow analytical solutions.pdf) can be found. 

Model characteristics

• Length L  = 10000 m
• Constant slope ib = 0.0001 -
• Discharge q = 5 m2/s
• Chézy coefficient C2D = 65 m1/2/s
• Grid size dx=dy = 500 m
• Time step dt = 60 s

DEMO 3: Tidal flume

Description

The model is used to simulate salt intrusion in a river using both sigma and Z - layers. A salt tidal signal enters the model from the west. the model tries to reproduce measurements that have been carried out at Deltares. The scale model of the tidal flume has a basin with a surface area of 120 m2, representing a sea, and a flume with a width of 1 m and a length of 130 m, representing a river. In the numerical simulation, we use a schematisation of the river (130 x 1 grid cells of 1 x 1 m2 each). At the sea side a water level boundary is applied.

• 2DV model (20 uniformly distributed layers for both the - and Z-model)
• Inflow of 12.5 ppt at the sea boundary and a fresh water inflow at the downstream river boundary
• k - epsilon turbulence model
• The set up of the experiment with the tidal flume was chosen such that a partly stratified tidal flow occurs
• minimal salt intrusion of order 20 m
• maximal salt intrusion less than 75 m
• vertical stratification characterised by a gradual transition from salt water to fresh water
• 2-dimensional Chézy coefficient between 65 and 70 m1/2/s
• water depth in the river of 0.2 m
• density difference of 10 kg/m3
• tidal period of 600 seconds such that there is a reasonable displacement due to the tide

DEMO 4: Wave driven longshore current

Description

In this demo, both the FLOW and WAVE module of Delft3D are showcased. This model reproduces a longshore current that is the effect of constant obliquely incident waves. The waves enter the model domain under an angle and propagate towards the shore. When the waves start to break, a longshore current is generated as the alongshore current is a function of the dissipation of the wave energy. This demo does not reproduce any analytical or measured values. 

Model characteristics

• 2DH depth averaged model
• Length model longshore = 600.0 m  
• Length model cross shore = 300.0 m
• Grid size x min = 3.33 m
• Grid size x max = 10.0 m
• Grid size y min = 10.0 m
• Grid size y max = 10.0 m
• Simulation time = 2 hr
• Simulation timestep = 0.1 min
• Wave height = 5.0 m
• Wave period = 10 s
• Wave direction = 225 degrees

DEMO 5: Wind driven channel flow

Description

For the wind induced flow in a straight channel analytical solutions are available [Kocyigit and Falconer, 2004] The paper can be found in the measurements folder. A comparison is made between the analytical solutions in these publications and results obtained with Delft3D-FLOW. This is done for the water levels and vertical velocity profiles. The analytical solution presented by Kocyigit and Falconer reads:

in which zeta is the water level, tau is the wind shear stress, rho is the water density, g is acceleration due to gravity, h is the water depth, Vv is the vertical eddy viscosity, k1 is a linearised bottom friction coefficient and u is the horizontal velocity. Kocyigit and Falconer specified the following conditions and parameters:

In Delft3D-FLOW one can not specify the wind shear stress, but only the friction coefficient. The latter is determined using:

where rho is the air density, cdw is the friction coefficient and u is wind velocity. Wind speeds of 5 and 10 m/s and tau values of 0.1 and 0.235 N/m2 respectively were specified by Kocyigit and Falconer. Using the above-described, friction coefficients cdw should be 0.004 and 0.00325, respectively, in order to arrive at the wind stresses of Kocyigit and Falconer. In this way, the simulations of Kocyigit and Falconer are reproduced with Delft3D-FLOW.