Thursday, May 19, 2016

How big will the I-49 Connector retention pond be? Where will we put it?

Retention/detention pond behind the new Costco store in Lafayette, Louisiana.
photo credit: M. Waldon, no rights reserved

The Lafayette I-49 Connector project envisions a new 5.5 mile highway project going through Lafayette. Where will its storm water drain? The 2002 Environmental Impact Statement simply says it will drain directly into the Vermilion at its southern end, and into local drainage which flows to the Vermilion in its central and northern sections. Even in 2002, it must have been apparent that this could impact flooding. Today, we generally require retention/detention ponds to hold back flows from new construction so that no added flooding results from new projects. These ponds also reduce water pollution by settling out some pollutants How could this not be a major topic of discussion today?

Let's do a "back-of-the-envelope" calculation to get an idea of how large the pond or ponds must be. The project is 5.5 miles (29,040 feet) long, and average width is, I'm guessing, about 600 feet. Multiplying gives 17.424 million square feet of total project area. There are 43,560 square feet in an acre, and division gives 400 acres for the entire project area.

A big 3-day rain event in Lafayette can drop a lot of rain, and we need to plan for the really big storms to avoid flooding. At times we have gotten over 20 inches of rain in one day, so I will assume for planning that we get 30 inches in a 3-day event. Of this rain, some runs off, and some is retained or evaporates. For a typical residential, industrial, or open area, 20% to 60% of the rain might run off. Here, I will assume that 30% currently runs off to the Vermilion. For developed areas with significant impervious surface and sloped, compacted, and drained soil, 80% to 95% might run off. Here, I assume 85% runs off. So, 55% more of the rain is expected to run off after the project is completed. This is 16.5 inches (1.375 feet) of new runoff.

The total volume of added runoff is therefore 400 acres times 1.375 feet, or 550 acre-feet of water. Therefore, the pond needs to provide a storage volume of 550 acre-feet. If our retention pond has an average depth of 3 feet above the dry weather water level, then the pond must be 183.33 acres. Making room for shoreline and fence line (maybe even a jogging track), I assume the pond and related features will take up about 200 acres or 8.712 million square feet.  If square, this requires a property 2952 feet (0.56 miles) on each side.

Where in developed Lafayette can  we place such a feature? The pond must be downhill from the I-49 Connector to avoid the costs and uncertainties of pumping. Therefore, feasible placement of this pond is limited generally to the area between the roadway and the Vermilion channel. Further, any destruction of wetlands or residential areas should be avoided. Agricultural land might be an ideal choice, but we are unlikely to find such within the developed urban area within a feasible distance.

Thus, the question remains - How will we deal with increased flooding from the I-49 Connector? Will this be addressed by DOTD in their public meetings?


  1. OR.... We could have a CUT & COVER interstate designed with water guzzling fauna and other features that capture water and redirect any excess in many other areas around downtown and the urban corridor. Water that can be used by neighbors and the city during dry spells and just to delay the water reaching the bayou. But THE important thing to note is that a cut & cover design IS A WATERSHED. As such it eliminates the need for retention ponds. Actually it will REDUCE the amount of water **** currently shed **** by the Evangeline Thruway. So, after its all said and built this design makes the water retention concerns... A wash😊

  2. Robert,
    You are kidding - I hope.

    Would any downtown business owner, or anyone, really accept having flood water redirected onto their property or business rather than properly managed? What plants can "guzzle" a 20-inch in a day rain storm? You are correct on one point, cut and cover WOULD provide its own retention as the "cut" roadway fills with water that can't drain to an already full drainage infrastructure and Bayou Vermilion.

    The Con design includes additional features like a depressed University Avenue passing under I-49 at the Airport. This will likely flood just as the emergency needs generated by big storms and hurricanes occur. Flooding at University will isolate those east of the Con from hospitals on the west.

    If you really support this cut option then contact DOTD and ask them to do a full hazard assessment (it is called a phase II study) of the rail yard toxic contamination. So far, they refuse to look at the problem, and we think that this is because they, like us, pretty well know the answer. As DEQ already said, cleanup (i.e. excavating and removing the waste) is economically and environmentally infeasible. In-situ remediation is the only feasible option, and that precludes cutting a roadway, digging underpasses, or driving pilings. DOTD simply does not want to risk their federal cash flow by learning the truth too early. They prefer to waste millions in tax dollars on designs for infeasible schemes.

    Face it Robert, even if cutting through the toxic rail yard were not a billion dollar plus cleanup job, it would still be infeasible because of drainage and flooding. The failure of this or any roadway EIS to even consider flooding is unacceptable. A new EIS must be prepared incorporating proper engineering and economic analysis with actual alternatives including east and west bypass. The old FEIS being pushed on us is more of a political than an engineering statement. We need facts not a political fantasy.

    1. Except that a 20' rainfall in 24 hours event is very much the most extreme event you can get; and simply doesn't happen every month.

      Also, the degree of rainfall is actually unaffected by whatever runoff does occur in a particular rainfall event. My guess is that extreme events will occur whether the Connector freeway is built or isn't; the difference is that whatever structure or design is built will be constructed with legitimate provisions for handling runoff, and consideration for how to deal with changes in drainage flow. will not cost $1 billion dollars to remediate the Southern Pacific railyard contamination; it won't even cost $1 billion dollars for constructing the entire project and implementing all the land use changes proposed by the Evangeline Corridor Initiative. The $750 million dollar figure put out by LADOTD refers to the maximum amount of funding they have earmarked out of projected funds for building the Connector freeway; it does not reflect the actual costs of right-of-way acquisition and construction, or the costs of mitigation. Most estimates say that cleanup of the SP railyard to a suitable level would be no greater than $3 million to $5 million dollars; and the last actual estimate of construction of the Selected Alternative came out to nearly $410 million dollars. Updated cost estimates for the Selected Alternative and any modifications proposed or enacted will be done as part of the Supplemental EIS process now ongoing; and I'll bet they don't even come close to $1 billion or even $750 million dollars. (I'll say closer to $500 million would be more accurate.)

  3. No. I'm not joking. And I am familiar with rain gardens used to absorb water. The bottom line is that what we have now sheds more water than a cut & cover interstate. Michael... Tell me please. When it rains now... Where does the water go? If the current flat water shedding area with minimal fauna is covered with water absorbing and retaining landscaping then it would be a net reduction. Period. As far as the cut section retaining water that's very misleading. Could it retain water IF we wanted it to? Sure. But we don't want it to so we will design it so it does not. It's COVERED first and last of all so water entering the area will be minimal. On either end we can install large French drains and water entering through the walls and ceilings (which are designed to eliminate water entering the structure by the way) will also be minimal. Finally, the grade where this cut & cover will go is 20' above the 100 year flood plain. You'd know that IF you had attended the Immaculate Mary meeting.

    The hazard study will come once we have a final or 2 final concepts. This is why it is critical everyone support the cut & cover, including you��

  4. Robert, I am a retired PE environmental engineer, and in my last job I worked for the Federal Government as a senior hydrologist. I'm just saying that to let you know that I am not uninformed on this. I agree that water gardens are wonderful, but they do nearly nothing to mitigate a very large rains storm's runoff. The project will need to be drained. Water is quite heavy, and retention of water on a roof can cause collapse, so soil depth and any water retention will be very limited. I find it incredible, and really unprofessional, that the EIS does not even address flood impacts. Simply telling us the project either drains through local drains or directly to the Bayou is not an analysis.
    Local 100 year flood base flood elevation varies from below 15 feet to over 35, but hopefully drains would be designed to go toward the 15 foot BFE area of the Bayou. Ground height is around 35 feet in much of the cut area, so you and Steve Oubre are correct that there is about there is around 20 feet of elevation to use for drainage. Now, when you cut ten feet away you are left with ten feet. This is assuming a 100-year storm is an appropriate design risk (many would choose less risk for a large project). With only ten feet of head, the roadway will need to gravity drain to an as-yet undetermined retention area which will fill a few (maybe 5 feet) deep as it collects water. That doesn't leave a lot of gradient over the length of the project. The retention area must be high enough to then drain to the bayou under a managed flow. This may be feasible, but the failure to perform any analysis in either the EIS or new proposals leave me to just guess, and my best estimate is that flood mitigation for the project is infeasible.

    Of course, there is a big problem with toxic seepage that will be much larger under the cut design. Unless the government actually is willing to spend a billion dollars or more to excavate, haul away, and securely dispose all of the waste in the toxics contaminated area, we will need to collect all seepage from the side walls of the cut and the underpasses and soil slopes for proper disposal by treatment or deep well injection. That will be a huge continuing cost and risk.

    The hazard study you speak of and that we have been requesting is called a Phase II study. It is insanely wasteful of tax dollars to continue with these discussions, designs, and contracts without this. Relative to what they are spending it will cost very little money; maybe $50,000 to at least get preliminary results. Why won't they do it - simple, they don't want us to know the answer before they spend a maximum amount of OUR federal tax dollars on an infeasible project that then must be abandoned.

    Still - the decision seems like a no-brainer to me. Until the DOTD tests the contamination and gives a definitive plan on what they will do, and then provide practical preliminary plans for these alternatives, I will be skeptical and will support a bypass. The defense of the DOTD design should have been in the FEIS, but it is a totally empty document. Either the east or west bypass plan will cost far far less, not represent a risk to our water supply and our health, not disrupt traffic and evacuation during construction, not displace over a thousand people, not destroy established neighborhoods, not reduce airport safety, not impact our treasured outdoor spaces churches and schools with noise, and will give a reliable evacuation route when we get a lot of rain.

    Finally - yes I did attend the

  5. Ummmm...sorry, Michael, but you are wrong on mostly all counts.

    First off, the issue of drainage will be mitigated by these facts: 1) the depressed alternative will be only 10 feet below ground level, rather than the original 20' proposed in depressed alternatives in the 1990's, placing it well above the 100 year flood level; 2) the existing drainage system can be supplemented to handle any runoff from the covered and sloped sections; and 3) more detention ponds and additional wetlands can be utilized to handle runoff in other areas.

    I'm pretty sure that LaDOTD will provide detailed plans for cleanup of the Southern Pacific former railyard property, especially since it will be utilized as additional development and increased access either underneath or over the Connector freeway ROW. That is not conjecture; that is FEDERAL LAW that says that a full cleanup must be executed and completed before one shovel is turned on this project. And no, Michael, it will not cost "a billion dollars" or even $10 million to clean up the railyard property.

    And once again, you simply don't get it about a bypass costing "much less". The Lafayette Regional eXpressway bypass would be so far west of the Connector ROW to render it useless in drawing any traffic away from the Thruway/US 90 corridor...and it would have it's own concerns with development there. Meanwhile, Teche Ridge would still, even with the best case, remove only 10% of current traffic from the Thruway/US 90 corridor; meaning that you would still have nearly 60-80K of traffic using the Thruway. That would remain a serious noise issue, plus still be a major hazard for pedestrian traffic along the neighborhoods. Even with Teche Ridge AND the LRX fully operational, you will still have the majority of traffic using the Evangeline Thruway because it is the ONLY major route that gets to the major destinations within Lafayette (ULL, Downtown, Lafayette Regional Airport), as well as further south in Broussard and Youngsville. Also, how is an 6-lane freeway reinforced by a 4- to 6-lane access road system less effective at hurricane evacuation than a 4-lane bypass that doesn't even access major areas? (Broussard, for example?)

    And as for the actual costs? Yes, Teche Ridge probably would be a bit less expensive, but since it would attract little if no traffic from the US 90 corridor, and it would still require serious mitigation due to loss of prime farmland and wetlands, it would actually be less cost-effective. At best, Teche Ridge would cost in the order of $700-800 million (using the #601M figure put forth by DOTD in its analysis of the St. Martin Parish "study" of Teche Ridge back in 1994, and adjusting for possible inflation); essentially the same amount as the Connector ($750 million). The LRX, on the other hand, could cost as much as $1.2 billion based on the recommended outer corridors..and that doesn't include the $400M of the northwest quadrant from I-10 west of Scott to I-49 near Carencro. Even with half that project funded by tolls, that's a major hit. While there is no money as of yet earmarked for the Connector, it is very much the top priority for LADOTD for full funding, and as part of the overall I-49 South project, I'm certain it will get major Federal funding down the line too. Teche Ridge or LRX can't say that in their best days.

    I respect your right to oppose the Connector project, Michael...but I do reserve the right as do other principled supporters to correct what I see are distortions and misassumptions. Let the process work itself out, however you may feel, and rely on facts rather than rumors or NIMBY feelings.

  6. It should also be noted in regards to Michael's comments that the Partially Depressed/Covered Mainline alternative has only been introduced in late May, and has only been partially vetted by LADOTD and the ECI. It was not part of any alternatives discussed in the 2003 FEIS/ROD. With this concept now advancing to the Tier II stage of analysis in the Conceptual Design Study/SEIS; it will now get its well deserved full vetting and analysis.

    Regarding the concern of adequate drainage for the capped section: It should be noted that tunneled freeways will tend to collect less runoff than open trenches, and that would mitigate to a great deal the degree of runoff. The existing drainage system that parallels the current Evangeline Thruway, with minor upgrades, should be able to handle any runoff from the sloped covered sections. In addition, there is more than adequate wetlands on the Vermillion River watershed that can handle the drainage, and new detention ponds along the freeway at the Willow Street interchange can handle the rest.

    1. And that adding drainage mitigation measures into the design and all along the length of the urban connector will inevitably be a part of the final design. The cut & cover will significantly IMPROVE the current drainage problems.