MIKE 21 BW - Porosity - Scientific Background

Calibration Parameters
143
For uniform particle sizes, Engelund (1953) suggests the following range of
values for the laminar and turbulent resistance coefficients:
Porosity - scientific background
For Boussinesq wave simulations, the equations in MIKE 21 BW have been
modified to include porosity and the effects of non-Darcy flow through porous
media. In this way, it is possible for MIKE 21 to model partial reflection,
absorption and transmission of wave energy at porous structures such as
rubble mound breakwaters (see e.g. Madsen and Warren (1984) and Madsen
(1983)).
The main effects of porosity are introduced by additional laminar and turbu-
lent friction terms for describing losses due to flow through a porous struc-
ture. In most practical cases the pore sizes are relatively large (typically 0.1 m
to 1.0 m), and the turbulent losses will dominate. The laminar loss term has
also been included to allow the simulation of small scale physical model tests.
The flow resistance inside the porous structure is described by the non-linear
term
(5.6)
where and  account for the laminar and turbulent friction loss, respectively,
and U is the velocity.  and  are determined by the empirical expressions
recommended by Engelund (1953):
(5.7)
where:
Laminar Turbulent
Spherical particles 780 1.8
Rounded particles 1000 2.8
Angular particles up to 1500 up to 3.6
n = porosity (pore volume/unit volume)
 = kinematic viscosity of water
d = a characteristic diameter of the structure units (the grain size)

0
= a laminar particle form resistance coefficient

0
= a turbulent particle form resistance coefficient
+ UU

0
1 n–
3
n
2
-------------------

d
2
-----

0
1 n–
n
3
-----------------
1
d
---
=
=