Friction, Roughness and Boundary Layer Characteristics of Freshwater Biofilms in Hydraulic Conduits

Hydraulic conduits are susceptible to deterioration in their carrying capacity over time due to biological growths on their internal surfaces. This result is often termed biofouling. It is generally recognised that biofilms cause greater resistance to flow by increasing the effective roughness of the surface in contact with water. However, the detailed mechanisms by which biological growths affect the flow are not well understood. The present study was carried out with the cooperation of Hydro Tasmania, which is responsible for electricity generation in Tasmania through its hydro-electric schemes. Many of their facilities rely on open channels and pipelines for conveying water to power stations. Biofouling in open-channel networks requires increased flow depths to sustain design flows. This reduces the available freeboard, which increases the risk of overtopping and subsequent damage to channel structure and the surrounding environment. Maintaining design free board in the presence of biofouling will reduce channel flow capacity and power output. For pipe networks supplying hydro-electric power stations, biofouling will also reduce power generation capacity due to the increased energy loss lowering the effective head at the turbines. To research the effect biofouling has in reducing the efficiency of hydraulic conduits, a multi-faceted program was undertaken. The core of the work was completed on a purpose built water tunnel. Total drag and boundary layer measurements were undertaken in this tunnel. Analysis of the results from the water tunnel looked at the friction and roughness effects of clean and fouled smooth and rough test plates. Measured roughness was compared to physical roughness derived from photogrammetric methods. Novel photogrammetry methods were used to map the surface of biofilms on the smooth and rough test plates. This data was compared to water tunnel drag results. A field program supported the laboratory work. This consisted of a series of headloss tests of before and after the cleaning of biofouling material. Paint trials were also undertaken to find suitable candidates for the refurbishment of open channel structures. Water quality and temperature monitoring was undertaken to better characterise the water in which the biofilms grow. The final aspect of the field program was the mapping of biofilm surfaces using the novel close range photogrammetry. Plates with fine and coarse surface roughness were deployed in the field and allowed to have biofilms develop. Mapping of the biofilm at intervals was completed to investigate the character of biofilms on rough surfaces.