Intake systems may be static structures such as concrete-lined forebays, intake pipes, trash racks and floating barriers. Some structures may be active, such as gates, valves and trash rack cleaning equipment. Eurasian mussels can foul such structures. Irrigation systems that only periodically withdraw water from surface water bodies are particularly vulnerable.
The amount of mussel fouling in an intake may in part depend on the orientation of the mouth of the intake. Intakes which point up seem to develop more fouling than intakes which open in parallel with the bottom. Speed of flow and intake depth are also important variables. Flows over 4.5ft/s discourage initial attachment by Dreissenids. Intakes which draw from depths greater than approximately 30 meters (100 ft) will generally experience the least amount of fouling. This is due primarily to the following factors. In most cases, depths greater than 30 m (100 t) ft) contain water which is permanently cooler than shallower depths. Low ambient temperatures translate into slower growth of mussels. The lack of oxygen in the deeper layers of some water bodies may impact the growth of mussels. In some reservoirs in the Southern US where water is withdrawn from the bottom of the reservoir, dissolved oxygen may plummet at certain times of the year, precluding survival of dreissenids in these intakes.
Water intakes that are of relatively small diameter (60 to 180 cm; 2 to 6 ft), situated in the upper 30 m (100 ft) of water and which are very long (several hundred meters) are particularly vulnerable to fouling. The lake end of the intake usually terminates in a fixed grate designed to exclude large debris. The intake grate is likely to be the most visible point of fouling. Attached mussels can easily close gaps in the grate and decrease the water intake flow rate.
During the intake design, the material roughness is incorporated into required diameter calculations, but when the roughness increases due to mussel settlement, flow rate drops and facilities pumping water near their design capacity will have trouble meeting demand.