Someone recently asked me if I could estimate the financial impacts to the Houston area from the release of Atlas 14, Volume 11, Version 2.0. The short answer is no, but I can write a blog post with a rough qualitative and directional assessment of impacts!

Flood damage reduction infrastructure will need to be bigger to achieve the 1% annual chance risk level we have generally settled upon. This will mean that pipes or structures that carry the runoff that is generated from the 1% annual chance, 24-hour rainfall will be larger. Thankfully pipes, culverts, and earthen ditches don’t increase in cost linearly with their carrying capacity, but they do tend to get more pricey as they get bigger.

As you drive around the Houston/Harris County region you might sometimes notice concrete “notches” in the banks of some of the bayous or “flumes” that connect the bayou to a detention basin.  You might also see the same type of structure in smaller detention basins in and around neighborhoods.  Those structures allow the runoff from larger storm events to flow to or from the bayou and the channel with less chance of erosional damage.  Because they often are designed to carry the runoff from larger storms, they will be larger if they are designed to carry the flow arising from the updated rainfall stats. Here’s a screenshot of one of these structures from Google Maps:

Pipes that carry smaller storms may not increase in size that much or at all, because the Atlas 14 information did not change the smaller, more frequent rain event depths that much. For example, the 50% annual chance, 24-hour event reported for the center of Harris County near Houston in “Technical Paper No. 40: Rainfall Frequency Atlas of the United States, For Durations From 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years” (affectionately known as “TP-40”) (1961) is somewhere in the 5.1 to 5.2 range on the national scale map (screenshot below) and the same event in Atlas 14 is reported as 5.11 inches; so they are pretty close.  This means that the smaller neighborhood storm sewer pipes that are designed to carry runoff from these frequent, smaller events, might be able to stay the same size and, therefore, cost about the same.

One of my co-workers evaluated a hypothetical 160-acre green-field development site to see if detention ponds would need to be larger as a result of the initial draft release of Atlas 14, using a storm event depth of 17.7 inches in 24 hours. The evaluation showed that both the pre-development and the post-development flows increase, so the difference was not large enough to exceed the applicable regulatory minimum detention rates of 0.50 acre-feet or 0.65 acre-feet (Houston and Harris County). The results of this evaluation are shown below.

This suggests that most detention basins will still be sized to provide between 0.5 and 0.65 acre-feet per acre of development because most local governments won’t let folks install smaller ones.  This will have to be evaluated for more sites to be sure, but that’s what we saw for the one site we evaluated.

Regulatory floodplains will be remapped over the next three to four years using the more accurate rainfall depths. Harris County Flood Control District is already moving forward with this work.  This will enlarge them and more existing structures will then be located in the regulatory floodplain. This will increase the cost of insurance premiums to obtain coverage for many buildings and facilities. Approximately 37% of the county is inside one of the regulatory floodplains defined by the National Flood Insurance Program (NFIP). These include areas that have greater than a 1% annual chance riverine flooding, greater than 0.2% annual chance of riverine flooding, or greater than a 1% annual chance of coastal flooding.  The updated rainfall data will increase the area of the county that is in a regulatory floodplain by some amount.

Based on the elements above, it appears that the financial impact of the publication of the rainfall statistics update will be less significant than the financial impact of the floodplain map updates coming in a few years.

If you have other thoughts about cost or financial impacts, please leave a comment.