A near real-time scour monitoring system at Indian River Inlet, Delaware, USA

The Indian River Inlet is the only connection between the Atlantic Ocean and Rehoboth and Indian River Bays. The inlet has been problematic from an engineering standpoint since locals and state agencies first tried to make the opening permanent in the late 1920\´s. Localized scour is of particular concern to state officials due to its proximity to supporting bridge piers for Highway 1 that serves as a vital link between towns north and south of the inlet and as a hurricane evacuation route. While a new bridge is under construction and slated to be completed in 2011, engineers are concerned over the safety of the present bridge due to ongoing scour. In response to the progression of the scour on the seaward side of the bridge, a near real-time scour monitoring system was developed and deployed in November 2008. The system consists of two 250 kHz Marine Electronics 3D profiling sonars and two 470 kHz Nortek Continental Acoustic Doppler Current Profilers (ADCP). The sonar consists of an acoustic transducer in a sealed, oil-filled dome that is controlled by precise step motors. After a complete scan, assuming the maximum range settings have been used, the sonar returns a full hemispherical scan of the inlet bottom within its "viewing" range. One of each sensor type is paired together and housed in a custom-built stainless steel frame and installed below mean lower low water on each of the seaward bridge piers. Each sensor pair is controlled by a single land-based computer using task scheduling software to wake a sensor from sleep mode and initiate a data capture sequence. At the end of a sampling sequence data are automatically transferred through a wireless link to offsite computers for analysis, including merging the individual sonar data sets, and dissemination. Through comparison to recent USACE multibeam surveys, the initial data shows that the scour hole is well-resolved. In addition, preliminary analysis from ADCP data suggests that macro-turbulence during ebbing- tide associated with irregular bottom topography may be a controlling factor in the growth of the scour hole.