|Speciale, 3. modul, 2008, id:250|
|Findes på RUb:||Nej|
The effect of socalled PEMs (Pressure Equalization Modules) on beaches is examined through laboratorie experiments, computer modelling and beach profile analysis. The effect of a REM on drainage ability of a sand is examined in a self designet laboratorie experiment. The results of the experiment shows no significant effect of the PEM on drainage, but it is suspected that internal friction in the setup have affected the results to som degree. Computer modelling of s setup similar to the experimental setup shows a clear increase in drainage of around 10 percent as a result of insertion of a PEM. The increase depends on the hydraulic conductivity of the sand and is highest for the low conductive sand. Increased infiltration of around 40 percent was found for the low conductive sand, but this result results mainly from the unnaturally high pressure gradient in the model setup, and can not be directly translated into natural conditions on a beach. Modelling of the effect of PEM on a beach are carried out in a twodimensional profile model. The effects on wave induced beach groundwater circulation are examined for combinations of PEM and gravel layer. Circulation is found to increase between 5 and 11 percent for different sand types as a result of a PEM. The effect of a gravel layer alone is also an increase in circulation, and the effect is much larger for the low conductive sand than for the high conductive. Combination of a PEM and a gravel layer showed increased infiltrations, and the effects of PEM and gravel layer seemed to be additive. Profile analysis showed a tendency to a slightly larger foreshore beach in the area with a PEM, but the difference is within the uncertainty of the estimation, and the statistical basis is poor. Further more it could not be determined that an increase in slope is caused by the PEMs or other natural conditions like nearshore morphology og wave characteristics. It is concluded that PEM can in principle have som effect on beach groundwater flow, but the effect is considered to be small, and no clear evidence for the impact on the profiles are found.