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.
Sonya Webb P.E., M.ASCE
Jensen Design & Survey, Inc.
Sent: 10-09-2019 15:15
From: Michael Bloom
Subject: Standard of Care for Drainage and Floodplain Planning and Design?
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.
Michael Bloom P.E., BCEE, ENV SP, M.ASCE
Sustainability Practice Manager
R.G. Miller Engineers, Inc.