To enter the Siuslaw River from the Pacific, vessels pass along a narrow sandy channel leading to the town of Florence, through what has been described “the northern gateway to the monumental sand dunes stretching between Heceta Head and Cape Arago.” The river flows beneath the Art Deco obelisks of the historic Siuslaw River Bridge, designed by Conde McCullough and opened in 1936.
The channel of the Siuslaw River bar is very narrow and extends past the jetties. Bar conditions are unpredictable, requiring extreme caution by vessels. Keeping the channel accessible requires ongoing maintenance.
Mott MacDonald was retained to evaluate scenarios for modifying the jetties to reduce the need for dredging to maintain the channel. The work was done in support of a Major Maintenance Report prepared for the Portland District of the US Army Corps of Engineers.
We assessed average shoreline trends by gathering and evaluating aerial photographs of the area from 1939 to 2009. In preparation for numerical modeling, we collected data on streamflow, waves, tides, wind, dredging and disposal, construction, bathymetrics, and sediment transport.
Shoreline data was entered in DSAS, a shoreline analysis program, and trends were used to project future conditions. Numerical modeling was used to test the effect of modifications to the jetties and spurs on sediment shoaling and the control of local transport.
Mott MacDonald developed 10 modeling scenarios of jetty configurations, and modeled and analyzed their performance in controlling channel shoaling. Wave, current, and morphology parameters were quantified in order to assess the effectiveness of each scenario in improving maintenance, navigability, and economy of operation.
The results of the Major Maintenance Report provide a sound basis for planning effective and economical improvements to the Siuslaw River jetties.