Jetty Spur Functional Design at Coastal Inlets: Effects on Nearshore Circulation and Potential Sediment Movement

Abstract

PURPOSE: This Coastal and Hydraulics Engineering Technical Note (CHETN) discusses the effects of connected and detached spurs on sediment transport and circulation in the vicinity of a coastal inlet jetty. The influence of spur orientation on the magnitude and pathways of these processes is detailed based on observations in the field and comprehensive laboratory measurements. A companion technical note, CHETN-IV-61 (Seabergh and Krock 2003), discusses existing jetty spurs in the United States and presents initial Coastal Inlet Research Program (CIRP) physical model jetty spur results. BACKGROUND: A jetty spur is a relatively short structure extending at an angle from the main jetty axis that protects a navigation channel. The spur diverts sediment that may shoal in the channel back towards the beach, where it can nourish the beach. Spurs are usually constructed of rock rubble similar to the connecting jetty. If longshore sediment transport is primarily unidirectional, sediment will deposit on the upcoast side of the inlet, and the downdrift beach will erode. For this case, spur jetties can provide a protected area within which land- or water-based equipment can dredge and bypass the accumulated sediment. Spurs are typically nearly perpendicular to the jetty, but may also be constructed at some angle with respect to the jetty up to about 45 deg. These types of spurs have the potential to reduce maintenance dredging volumes and to divert sediment back to the beach. At Siuslaw River Inlet, OR (Figure 1), spurs reduced channel maintenance by 133,800 cu m per year (Bottin 1997). The spur acts as a deflector for the nearshore current that is generated by waves breaking at an angle to the main axis of the jetty, by the alongshore component of the wind, and by the tidal current. This current may entrain sand from the beach. The spur’s basic function is to alter the path of the sediment-laden current, redirecting it shoreward and updrift. Ideally, sediment will be deposited on the beaches adjacent to the jetty. Typically, longshore and tidal currents are turned seaward as they approach a coastal inlet jetty. Flow is usually seaward along the outer side of a jetty and, in the absence of a spur, it is typically drawn into the navigation channel during flood flow. The region on the sea side of the updrift jetty may have a fillet as sand accumulates against the jetty. This sediment may be entrained and carried around the jetty tip and eventually encroach on the navigation channel. During ebb tide, the longshore flow diverted seaward by the jetty may be parallel to the ebbing channel current, and sediment from the shore side of the jetty can be entrained into the ebb current and delivered to the ebb shoal, where it potentially can bypass naturally to the downdrift beach. However, this sediment may be transported back into the navigation channel by the flood current and result in shoaling or continued transport to the ebb shoal or flood shoal.