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Standard of Care for Drainage and Floodplain Planning and Design?

  • 1.  Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-09-2019 03:20 PM
    My impression is that most engineers in North America (and their insurance companies) believe that if they design projects that don't expose the occupied portion of new structures to more than a 1% annual chance of inundation (and don't increase inundation likelihoods for existing structures), they have met their "standard of care" - to protect public health and safety.

    If you do the math, this approach exposes the lowest structures with no freeboard to a 26% likelihood of inundation over a 30-year mortgage period.  This approach also assumes that the period of record used to estimate the 1% annual exceedance probability (AEP) storm is long enough and that hydrology and hydraulics modeling are adequate.  It also omits consideration of the confidence band around the estimate of the 1% AEP depth.

    Do most folks agree with this approach? A related question is what level of inundation likelihood must be exceeded for a sealed set of plans to not conform with the standard of care?

    Thanks in advance for any thoughts on this.






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    Michael Bloom P.E., BCEE, ENV SP, M.ASCE
    Sustainability Practice Manager
    R.G. Miller Engineers, Inc.
    Houston TX
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  • 2.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-10-2019 08:38 AM
    Here in the Mid-Atlantic region, standard design is to keep the entire proposed structure and grading above of the base flood elevation determined by FEMA, Regulatory Agency or registered engineer.  If this is not feasible, a hydraulic analysis is required to show no harm upstream and downstream of the proposed project.  Due diligence should dictate that climate change is accounted for in the analysis.

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    John Wood P.E., M.ASCE
    Project Engineer
    Pittsburgh PA
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  • 3.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-10-2019 09:52 AM
    It may not be as risky as you assume. As an example, I placed my home's basement level two feet above the 1% flood elevation however due to the ground elevation's rapid change upon coming out of the floodwaters the home basement level was three vertical feet about the flood elevation.

    The insurance people need to use some standard maybe that is why they allow the 1% however it would be "good practice" to augment any decisions with a site visit to set easily met logical safety factors. I would never accept the blind 1% elevation if it really mattered.

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    Charles Taylor P.E., M.ASCE
    Environmental and Project Management
    Greenville SC
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  • 4.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-11-2019 08:05 AM
    Mr. Wood, How is "no harm" demonstrated?  What storm is used?
    Thanks,
    Michael

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    Michael Bloom P.E., M.ASCE
    Sustainability Practice Manager
    R.G. Miller Engineers, Inc.
    Houston TX
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  • 5.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-11-2019 10:48 AM
    'No harm" is demonstrated by showing that an increase in the base flood elevation does not adversely affect structures, roadways , etc. (public safety) and velocities will not increase erosion of streambanks or damage properties.

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    John Wood P.E., M.ASCE
    Project Engineer
    Pittsburgh PA
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  • 6.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-10-2019 11:13 AM
    You raise a good question.  And what about urban areas which may have large undeveloped upstream areas for which plats have not been  filed?  The Urban areas in Texas are developing very quickly.  Unless the policy has changed, the flood zone maps don't reflect runoff for the fully developed condition.  Are Urban governments going to require storm water detention on those plats?  What happens if they don't?

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    James Anderson, MASCE
    Principal
    Anderson Consulting
    North Richland Hills, TX
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  • 7.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-10-2019 12:57 PM
    I believe that the 100-year or 1% chance was selected because it is a reasonable accommodation of cost of construction verses risk of damage.  Insurance companies, with the recommendation of engineers and risk analysis experts set that level of risk/damage.  It would help if the 1% chance was truly a 1% chance to be able to evaluate that risk.

    Most communities that I work in, as do you Mike have decided that the 1% event is not a reasonable value, and so they have added freeboard to the calculations.  I agree with them.  If our structure is designed for a 30 year life expectancy, as are most homes in the Houston area, then perhaps a 100-year event may be too much risk for that time period.  If our structure is designed for a 100 or 200 year life expectancy, such as our hospitals, churches, museums, and downtown buildings, maybe we should be thinking about reducing the risk for those structures so that they only have a 1% chance over their life-cycle, or making them constructed in such a way that there is only limited risk if the waters do rise.  Most of my clients would need a lot of convincing to even consider designing something to protect themselves even 1 inch higher than regulatorily required if it cost them a dollar.  I do think it is worth our effort to try to convince them though.

    I think some examples that Houston area structures are using for that is to move mission critical items such as power stations, transformers, air handling systems well above the first floor of buildings designed to be above the 100-year flood elevation for mission critical or longer term buildings.  The option that is regularly used is to use materials (concrete or stone being one product) that does not mind being inundated for a short period of time.  Then protect the electrical systems so we don't have to continually replace them.  Then put things like parking instead of underground shopping in those areas.  We still have to clean and disinfect, but generally that is better than remove and replace.

    Finally we need to be able to disconnect underground systems between buildings in the flooding risk area from buildings less at risk, such as the Houston Downtown tunnel system.   Floodproofing works to do that.

    As Harvey has taught us, we can select a reasonable design standard, but nature can always find a way to make us look like we have no clue.


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    Dwayne Culp, Ph.D., Ph.D., P.E., M.ASCE
    Culp Engineering, LLC
    Rosenberg TX
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  • 8.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-10-2019 12:59 PM
    Not accounting for risk to life/health, our standard of care for minimizing future structural damage risk to our client, including impacts to neighbors structures, is based on the cost of design vs reduction in actuarial cost of total economic risk, with some reasonable FOS.  Can use cost of insurance as a surrogate for actuarial cost if insurance really reflects total economic risk (all damages, lost income, etc.).  Obviously we shouldn't design something that has an unreasonable future cost to insure, or unreasonably impacts neighbors' cost of insurance.  Think fire and wind risk--in those cases the market-based insurance industry has effectively/efficiently evolved pricing of the risk as well as appropriate design standards for us!

    Community NFIP-driven "standards" may be convenient, but MAJOR limitations and errors are becoming very well known:  a "false-binary" approach to risk (100-yr risk IN vs OUT), outdated info on this simplistic binary, big errors, no allowance for uncertainty, and no allowance for future change--e.g., climate change. Furthermore, NFIP insurance costs often don't reasonably reflect actuarial risk.  In my opinion the NFIP is on the way out as a basis for Due Diligence in financial transactions, flood risk design, or any rigorous decision related to managing flood risk.

    Engineer beware.  Discuss with client!   Just like with wind and fire--actuarially sound analysis is becoming the standard of care for more expensive/risky commercial and industrial investments!

    Email me [email protected] and I will send you:  "Real Flood Risk: The Grassroots Revolution", "Modernizing Post-NFIP, Climate-Change Flood Risk Management"

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    Robert Jacobsen P.E., M.ASCE
    President
    Baton Rouge LA
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  • 9.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-14-2019 05:31 PM
    Good morning!  I tried to email requesting the document, but bounced saying Address Not Found.

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    Maria Price P.E., M.ASCE
    Engineering Manager
    City of Chattanooga
    Chattanooga TN
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  • 10.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-14-2019 11:15 PM
    Hi Maria,

    I also had this issue. There was a typo in the email address, and the correct one should be: [email protected].


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    Vinicius Taguchi S.M.ASCE
    PhD Candidate
    Minneapolis MN
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  • 11.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-10-2019 02:00 PM
    James Anderson: Regarding local runoff, here we are required to detain and filter stormwaters onsite for new developments.

    Regarding hydraulics, we are seeing Cities and our local County 2D modeling watersheds and dynamics between them to attempt to identify risks, especially due to aging and undersized conveyances. It can be a moving target what the 1% AEP is even for the same site, as attempt to predict risk changes what the downstream flows are with the latest models. For communities to maintain points with the Community Rating System, reducing flood insurance rates, they require a minimum of 1-2' of freeboard (added safety factor). Non occupied structures in the inundation areas are required to meet floodproofing requirements. Our local agencies don't allow active floodproofing. It must be passive, not requiring manpower to activate. You are right though, people may modifying structures and damage or injury can be incurred. The issues are that as population and property values increase people find ways to develop in floodprone, typically less expensive properties. As you may already know, structures that are "grandfathered" are only required to upgrade to floodproofed or elevated when considerable improvements are made.

    In this area we don't have hurricanes, but along the Coast, we do have new FIRMs demonstrating what the latest calculated base flood elevations are. Unfortunately, most of the development along the coast is aged, and built even before FEMA developed the concept of BFE or even building to an AEP. Hindsight is 20/20. As we experience more catastrophes, more regulations are written. This is a situation of designers meeting requirements at that time.

    Moving forward, I wonder what others think about building behind certified levees. This is where design can allow for a lower building pad, but if the levee fails, there is the inundation issue (even with habitable structures).

    In general, regarding hydraulic modeling, at the recent Floodplain Modelers Association conference in San Diego last month, there were several that participated in a "Challenge Models" which compared various flooding scenarios, 2-Dimentional  independent modelers, softwares and computer systems. Thomas Plummer mentioned that compared with the 2012 Challenge Models was not only were modelers faster now, but results from all the different programs were more consistent with each other. This indicates that our modeling programs seem to be more accurate, and possibly more trustworthy. Due to safety factors in hydrology selected, more accurate models, and freeboard we feel pretty confident designing using the technology we have, especially with 2D models.

    Regarding more thorough data inputs, at the FMA conference, Chris Huxley, TUFLOW, stated that in Australia, they require an enormous amount of work creating hydrology and combinations of hydrographs and inputting them in the hydraulic model using a complicated combination (Monte Carlo Method) and multiple scenarios to obtain their final hydraulic model. He said in a court of law they are not allowed to just use 1 AEP hydrograph. This results in a more conservative BFE.

    We always have more to learn.​

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    Sonya Webb P.E., M.ASCE
    Civil Engineer
    Jensen Design & Survey, Inc.
    Ventura CA
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  • 12.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-11-2019 12:37 PM
    Thank you James Anderson for bringing up the issue of accounting for stormwater drainage in an urbanizing landscape. As Sonya Webb mentioned, most new land developments are required to manage stormwater on-site, but the requirements vary in different regions/municipalities. I see two main issues that are still not adequately addressed:
    1. Stormwater management requirements usually target the volume of a small rain event (1-2" or so) and the peak flow rate of the 25-year storm or something on that order. That means that larger storms, such as the 1% storm we've been talking about, are not fully addressed in the vast majority of stormwater infrastructure. So even if a developing watershed does not impact downstream flooding for the most common storm events, I imagine that a newly developed watershed might still see some increases in the peak flow rates associated with the larger 1% storm. This also means that runoff could be moving down the watershed at faster rates and lead to worsening flooding downstream.
    2. Older areas of the world that have already fully developed are facing changes in precipitation patterns (sometimes larger storms, sometimes more frequent storms). In these areas, even if stormwater runoff is adequately managed by the current infrastructure, changing conditions mean that the infrastructure may become less effective.

    It's really great to see these conversations happening. I'm looking forward to hearing everyone else's thoughts on the points I raised.


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    Vinicius Taguchi S.M.ASCE
    PhD Candidate
    Minneapolis MN
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  • 13.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-10-2019 02:01 PM
    In Oklahoma, most medium and large municipalities have a freeboard requirement (ordinance) that requires the finished floor of buildings to be 1 foot or 2 foot above the published or calculated BFE.  This should be the minimum for standard of care regardless if the area is governed by a municipality or not.  However, knowing that flood plain management is a dynamic target, would this freeboard be sufficient in a future situation, when more development has occurred, or if the flood is produced from a 0.25% storm?  Each site should have it's own criteria based on the topography, development situation and importance of the structure.

    Some 40% of flood insurance claims are for structures not lying within the mapped FEMA floodplains, so that should tell us something about the unpredictable nature of floodplain modeling.  In my opinion, floodplains and fringe areas should be left for agriculture and nature, but many areas are well past that point.

    In these situations, a standard of care is often dictated to the engineer, via government laws and ordinances.  Since there are many stakeholders (citizens, officials, politicians, owners, engineers) if the engineering suggests a higher threshold than dictated by ordinance, it should be well communicated and documented, but the engineer may not always win the argument.

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    Lyndell Bishop A.M.ASCE
    Vice-President
    Shawnee OK
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  • 14.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-14-2019 12:15 PM
    I hadn't heard the "Some 40% of flood insurance claims are for structures not lying within the mapped FEMA floodplains, so that should tell us something about the unpredictable nature of floodplain modeling. " statistic, but I can believe it.  The FEMA maps for my area use data from the 80's & are in great need of revision.  We've partnered with Army Corps to do the heavy lifting on the modelling side which is yielding many much needed map revisions (LOMRs).

    We do require developments in the 100 yr floodplain to either detain the 100 year storm or check for 25 year backwater conditions.  This can be tricky as we're on the TN River with TVA's extensive dam system.  We have creeks that reverse direction on a regular basis.  (That sawing motion has been brutal for streambank erosion!)

    We are currently in the process of researching some Floodplain & Steep Slope regulations (staff suggestions go to City council soon) as we currently have nothing preventing development in the Floodplain.


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    Maria Price P.E., M.ASCE
    Engineering Manager
    City of Chattanooga
    Chattanooga TN
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  • 15.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-11-2019 01:30 AM
    It would be nice to see the math


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    Harlan Kelly P.E., M.ASCE
    President
    KNE2 Ltd
    Vancouver BC
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  • 16.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-11-2019 08:05 AM
    Mr. Kelly:

    The math is as follows.

    If the annual chance of flooding is 1%, then the annual chance of NOT flooding is 99%.  The total chance of NOT flooding during any period of years is given by:

           Total Chance of NOT Flooding During Any Duration of N Years =  (Annual Chance of Not Flooding)^N

    The total chance of flooding during the same period can be determined by subtraction, as follows:

           Total Chance of Flooding During N Years = 1 - [Total Chance of NOT Flooding During N Years]

    So for my example in the original post:

            Total Chance of NOT Flooding During 30 Years = (0.99)^30 = 0.7397  or 73.97%
            Total Chance of Flooding During 30 Years = 1 - 0.7397 = 0.26 = 26%


    ------------------------------
    Michael Bloom P.E., M.ASCE
    Sustainability Practice Manager
    R.G. Miller Engineers, Inc.
    Houston TX
    ------------------------------



  • 17.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-11-2019 08:37 AM
    Mr. Kelly:

    Also, if you want to see a graphical depiction of the math, for multiple AEP's I created one!
    See below.

    Graph of cumulative likelihood of various AEPs for different time durations.


    ------------------------------
    Michael Bloom P.E., M.ASCE
    Sustainability Practice Manager
    R.G. Miller Engineers, Inc.
    Houston TX
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  • 18.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-11-2019 01:10 PM
    ​Excellent plot.

    And the math is?


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    Harlan Kelly P.E., M.ASCE
    President
    KNE2 Ltd
    Vancouver BC
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  • 19.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-11-2019 01:35 PM
    Please see my previous reply to your question.
    Post 13 in this thread.

    ------------------------------
    Michael Bloom P.E., M.ASCE
    Sustainability Practice Manager
    R.G. Miller Engineers, Inc.
    Houston TX
    ------------------------------



  • 20.  RE: Standard of Care for Drainage and Floodplain Planning and Design?

    Posted 10-14-2019 05:01 AM
    I like your chart.

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    Dwayne Culp, Ph.D., Ph.D., P.E., M.ASCE
    Culp Engineering, LLC
    Rosenberg TX
    [Phone]
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