Given the brief description on the two most common ways to construct a pipeline across a river or stream (open trench or horizontal directional drilling); compare and contrast the potential environmental impacts of each construction method.
1. Open Trench: Open trench technology causes physical disruption of the waterway and nearby riparian areas. While these disturbances can be managed to reduce impacts, open trench technology has the potential to disrupt water flow and cause sediment to be dislodged and carried downstream. Additionally, the trees and shrubs in the riparian areas on both side of the waterway need to be cut and removed during open trench construction which can increase the amount of sunlight hitting the waterway, increasing water temperatures. This can be especially important in small cold water streams.
2. Horizontal Directional Drilling (HDD): HDD is often the construction method that causes the least environmental impact to the waterway and riparian areas on both sides of the waterway. However the process often requires a larger footprint on each side of the waterway for the HDD related equipment. Also, the HDD process utilizes a water-clay mixture to lubricate the drill string and return cuttings. This introduces the risk of producing an ‘inadvertent mud return’ or leaking the drilling mud through shallow rock fissures into the waterway. There are also some site specific limitations to using HDD technology such as the approach and departure angles leading to and from the waterway and the distance of the horizontal bore.