WaterMarks Interview with Robert A. Dalrymple

Engineer, educator and author of numerous publications, Dr. Robert A. Dalrymple is the Willard and Lillian Hackerman Professor of Civil Engineering at Johns Hopkins University. On behalf of the American Society of Civil Engineers, Dalrymple participated on the New Orleans levee assessment teams that gathered information on the levees’ performance in the wake of Hurricane Katrina. Dalrymple presented his team’s findings to the U.S. House of Representatives’ Subcommittee on Water Resources and Environment in October 2005. In this WaterMarks interview, Dalrymple talks about the lessons we can learn from Katrina.

WATERMARKS: We’ve heard a lot about the failure of New Orleans’ flood walls and levees after Katrina. From your perspective as an engineer, what went wrong?

Dalrymple: We understand the general principles of the failure but we don’t yet know the specifics. For example, we know that factors such as the composition of the soil beneath the levees and the scouring effect that overtopping waves have on the levees caused problems. We also know that the overall strategy to protect New Orleans was sound.

WATERMARKS: Does that mean that it may not be necessary to increase the protection level of the levees?

Dalrymple: The present levees are designed to withstand a category 3 hurricane. Another category 4 storm hitting the city will overtop the levees. When levees are designed and constructed correctly, you’ll see flooding in the streets only for a matter of hours, not days. The levees would hold and the damage from floodwaters could be minimized.

WATERMARKS: So why all the discussion about the need to increase the level of protection?

Dalrymple: There is the possibility of a storm stronger than Katrina. Although a category 5 hurricane is perhaps a 500-year event, no one knows when it might occur. The Netherlands has built levees to protect itself from a 10,000- year event. But the decision about the level of protection in southern Louisiana isn’t an engineering decision, it’s a political decision: Is the nation willing to invest the necessary funds to protect Louisiana’s coastal wetlands and communities to withstand a category 5 storm?

WATERMARKS: The Corps of Engineers is restoring the levee system to a category 3 level of protection. Will that happen before the next hurricane season starts on June 1?

Dalrymple: The breaches have already been repaired in the city proper, and by June the major part of the city’s protective system will be back to pre-Katrina levels. What won’t be repaired is the damage to the barrier islands and coastal wetlands.

WATERMARKS: Just how important are Louisiana’s barrier islands and coastal wetlands to storm protection?

Dalrymple: There’s no question that these buffers are the first line of defense against a storm. It’s estimated that every mile or two of wetlands reduces storm surge by about a foot. So when we talk about creating protection equal to conditions before Katrina, we must include wetland and barrier island restoration.

WATERMARKS: And the dollars that restoration requires?

Dalrymple: It will take a significant financial investment. But it doesn’t make much sense to put money into rebuilding levees if we haven’t put money into restoring our natural buffers on the coast.

WATERMARKS: In your testimony before the House Subcommittee on Natural Resources and Environment in October of 2005, you said that we must come to “the painful realization that some areas of the coast should not be rebuilt or inhabited again.” Were you referring to New Orleans?

Dalrymple: No. Unlike other parts of southern Louisiana, New Orleans already has a network of levees and floodwalls in place. But the reality is that the combination of sealevel rise and subsiding land makes protecting some areas of southern Louisiana very expensive — the magnitude of damage from storms is going to increase, not decrease. We have to limit the loss of life and property, and that means making politically difficult decisions.

WATERMARKS: What do we need to focus on in our efforts to restore the coast?

Dalrymple: It’s been talked about often, but it’s worth emphasizing. We have to take maximum advantage of the sediment carried in the Mississippi River. Sediment — from the river or from dredging — is a primary resource for coastal restoration. We can’t afford to lose it by allowing it to drop off the continental shelf.

WATERMARKS: Is there anything positive to look at in the aftermath of Katrina?

Dalrymple: Katrina was a terrible disaster, but it brought the crisis in southern Louisiana to the nation’s attention. This country has begun to understand the connection between the destruction and the loss of coastal wetlands and barrier islands. We’ve seen that a hurricane in southern Louisiana can threaten the nation’s oil supply, and we’ve experienced first-hand what that does to our pocketbooks — gas at three dollars a gallon gets our attention. It just may be that because of this crisis, southern Louisiana will get the federal funding it needs to address the terrible loss of wetlands that’s occurred in recent years — and that would be the silver lining in a very dark cloud.