Based on the new information available (#1 below) and the information becoming available over the next few years (#2 below) we feel it is premature to build either of the Rodanthe bridges now, and it is best to wait until the key uncertainties about nourishment are resolved. NC 12 will be protected by emergency nourishment in the meantime.
1. We now have much better data about the success of sand nourishment from the Nags Head project ($36 million, 10 miles, 4.6 million cy of sand). There have been two major hurricanes and several major, long-lasting nor'easters. The sand loss is as expected, about 300,000 cy per year, although high in variability (e.g., very low for Irene, very high for Sandy).
2. We will have even better data in a few years because USACE/NCDOT emergency nourishment for northern Rodanthe should begin this coming fall (2013) and Dare County is funding initial studies on a longer term nourishment project.
3. The Bonner Bridge merger team panel of coastal science experts recommended against nourishment because of "issues" with (a) a suitable sand source, (b) the amount of sand and dredging required, (c) dredging limitations (time), and (d) the cost of regular nourishment (see Section 3.3.2 of the the Final Report of the Peer Exchange Group available from NCDOT).
4. Regarding 3a, the Final Report acknowledges that sand sourcing is an unknown but assumed that sources offshore of Rodanthe and the outer bar at Oregon Inlet were most likely (Appendix B in the Peer Exchange Final Report). We will know much more about sand sources in the next year or two.
5. Regarding 3b, the information gained from 1 and 2, above, will greatly reduced this uncertainty. The panel relied on the Overton & Fisher (2005) report that assumed 2 million cy every 4 years for a 1 mile stretch of beach. Given 1, above, this seems to be a serious overestimate (i.e., compared to 300,000 cy per year for a 10 mile stretch in Nags Head), even given that the erosion rate is much higher in northern Rodanthe compared to Nags Head (say 2x or even 3x).
6. Regarding 3c, viable time periods for dredging due to weather and wildlife concerns remains an issue, but the currently planned emergency nourishment will establish a precedent about how these concerns can be successfully resolved.7. Regarding 3d, the Final Report acknowledges that cost estimates are very dependent on 3abc, so to the extent that these uncertainties are resolved, more accurate cost estimates will be possible. A simple benchmark is that an annual cost of $5 million (e.g., $20 million every four years) has a 50 year present value of $69 million (@ the 7% discount rate used by Overton & Fisher, $91 million @ 5%). The currently estimated cost of either bridge option is $100 million to $150 million (and those costs seem to be just to build the bridge and do not include 50 years of maintenance).