Amite River Flooding After Three New Bridges


It seems obvious that during flood events after the bridges are completed, levels will be lower above the bridges and higher below the bridges. My question is, have any studies been made as to how much higher an '83 flood will be at Manchac point and Port Vincent once the three new bridges are completed?


Over the past two years there have been two meetings (sponsored by EBR's Office of Homeland Security & Emergency Preparedness) of various local, state and federal agencies to discuss local flooding and to start a dialog to address this very issue.

Let me give you MY opinion based on my chats with the U.S. Geological Survey (USGS) and the NWS River Forecast Center (NWS/LMRFC) experts, the agencies in charge of Amite River monitoring (USGS) and flood forecasting (NWS).

Given the changes in the Amite drainage channel resulting from the construction and modifications for the new bridges, it is almost a certainty that water levels (for the same amount and distribution of rain) will be lower at the bridges (Florida Blvd and I-12 bridges. (I admit that I am less clear about the Magnolia Bridge work).

In other words, the same storm SHOULD mean slightly lower water levels near Florida Blvd -- because the channel will be wider. If this is true, then this is good news for western Denham Springs (especially along River Road). And again SLIGHTLY lower, not several feet lower.

I have two concerns:
#1 -- Many residents regularly affected by high water on the Amite have learned over time to set their own "warning/benchmark" number for a certain stage at the Amite/Denham Springs gage.  For example, I've met people that tell me something like "my property is fine until Denham Springs gets to 31 feet."

The problem for these people, I fear, is that the new channel will mean a change in those "benchmark" numbers. Not a major change, necessarily, but maybe something on the order of 6" to 12" or so.  If this is true, in the future, 31 ft at Denham Springs may "look like" 31.5ft or even 32.0ft (in the past) for some residences.

Now admittedly, this change is far from huge, but for some people, 6" means the difference between water and no-water in the house or garage.

#2 -- The answer to the impact farther downstream is MUCH, much less clear. Since the downstream channel(s) will remain essentially unchanged, the real question is whether upstream changes at the bridges will mean a change in "HOW FAST" the water arrives at places like Bayou
Manchac Point, Port Vincent, etc.

If the water gets downstream faster, then the flood threat may be increased.  But my guess is that by the time you get to (or below) Bayou Manchac, the impact of the new bridges will be minimal.

[As to your question,] the simple answer is NO.

Until all three bridges are completed, there is only guesswork. Even after all of the construction is completed, computer modeling will not give us the kind of detailed answers that we really need to assess exactly how the changes will affect impacted neighborhoods and communities downriver.  Sadly -- but realistically -- we will need one or two bigger events (but hopefully nothing like Dec'82 or Apr'83) to better assess how the construction (and associated channel changes) will affect
the flows.

Probably NOT the answers you were looking for ... but in my opinion, we will be more uncertain about Amite flood threats in the next couple of years than we have since before I joined WAFB.

Jay Grymes
WAFB Storm Team