Atlas 14: Analysis Method and Preliminary Results

The Developers Council of the Greater Houston Builders Association asked me to present an overview of the Atlas 14 analysis method and the preliminary results for Harris County.

Here’s a link to the presentation I made on December 6, 2017:

Additional information about Atlas 14 can be obtained as follows:

Harris County Close-Up of Preliminary Atlas 14 Data

On November 20, 2017 I posted about the release of the preliminary Volume 11 of Atlas 14, the wholesale update of statistically based rainfall intensity, duration, and frequency estimates for the state of Texas.  In that post I shared a screen shot of the Houston region from the statewide map published by the National Weather Service showing the new projected depths of the 1% annual chance, 24-hour duration rain event.  In this post I will discuss the changes depicted for Harris County.

First, let’s look at the official depths in use before Atlas 14.  The map below shows the three regions and the associated rainfall depths used by Harris County, Harris County Flood Control District, and all civil engineers working the area.

Source: Harris County Flood Control District, hydrology and hydraulics guidance manual. color and labeling by m. bloom.

This map shows three regions with areas closer to the Gulf of Mexico generally getting more rain than those more inland.  The rainfall depths range from 12.4 inches to 13.5 inches. These values were based on an analysis of observed rainfall events between the early 1900’s (the exact date varies by rain gauge) and 2001 (just after Tropical Storm Allison). The map above has been shaded to match the color legend used in the new Atlas 14 map, so that the changes can be more easily seen.

The map below is the preliminary Atlas 14 map zoomed and cropped to show just Harris County.

Source: National Weather Service, preliminary atlas 14 data released for peer review. zoomed, cropped, and labeled by m. bloom.

This map shows five regions with rainfall depths ranging from 14.01 inches to 18.80 inches. The table below presents the magnitude of the proposed, preliminary changes in each region.  The percentage is calculated as follows:

Percent Change = (After – Before) / Before

So, in some parts of the county the depth of the so-called “100-year” event is proposed to be increased by only 13%, while in some parts of the county it is proposed to increase by 39% percent.

Draft / Provisional Atlas 14 Data Out for Peer Review

Tonight I received an email from Sanja Perica, Ph.D., Chief, Hydrometeorological  Design Studies Center, Office of Water Prediction, National Weather Service, National Oceanic and Atmospheric Agency inviting me (and many others) to peer review a preliminary version of Volume 11, Version 1 of Atlas 14.

This preliminary volume is a wholesale update of statistically based rainfall intensity, duration, and frequency estimates for the state of Texas.

I’ve written about Atlas 14 four times before on this blog.  Please check out these earlier posts if you need a primer.

In one of my earlier posts I estimated what the new depth of the 1% annual chance, 24-hour duration rain event might be. I did this by looking at a location in Louisiana that was a similar distance from the coast as Houston.  The Louisiana Atlas 14 volume which had been completed earlier, so it was a good reference. In this older post I guessed that the updated 100-year event in Houston might be around 14 to 16 inches in 24-hours.

Well, drum roll please, now that the preliminary Atlas 14 numbers have been released for peer review we can see if I my guess was close.  Take a look a the exhibit below.

My estimate was actually low. The 1% annual chance, 24-duration event in downtown Houston is now estimated to be between 16 and 17 inches.

A few notes about this figure:

  1. This figure is based on historic rainfall up to and including Harvey;
  2. These values should not be used for design purposes;
  3. Official floodplain maps have not yet been updated to reflect these new estimates;
  4. These values don’t reflect a pending NOAA analysis of potential climate change effects;
  5. The values in this figure might change as a result of peer review; and,
  6. Final, official numbers will not be released until May 2018.

If you’d like more information about the Atlas 14 project, come to the December 12, 2017 lunchtime panel discussion being hosted by the Houston Chapter of the Environmental & Water Resources Institute of the American Society of Civil Engineers.

If you’d like to comment on the preliminary data download it here. Send your comments to Comments are due January 19, 2018.  Feel free to share this with others who may be interested.

Rule Changes, Projects, and Other Post-Harvey Discussions

If you are having trouble keeping up with all of the discussions about possible post-Harvey rule changes, projects, or other activities here’s a handy summary of what is going on (that I know about):

  1. West Houston Association: The association has proposed construction of a set of projects to address flood risk including buyouts; widespread bayou and channel enlargements to convey runoff from 12″ of rain in 1 day; a third reservoir in the Katy/Cypress area; fixing Addicks and Barker reservoirs; and enlargement of Buffalo Bayou from Highway 6 to downtown so that it can convey 15,000 cubic feet per second (the full release rate during Harvey). The Association’s plans were covered by Channel 2 news.
  2. Judge Ed Emmett: Harris County Judge Ed Emmett presented a 15 point plan which is detailed in another post on this blog.
  3. Governor Greg Abbott: Governor Abbott compiled a list of 300 projects from all County Judges and Mayors in declared disaster areas.  I published highlights here.
  4. County Engineer John Blount, P.E.: Mr. Blount has proposed changing floodplain rules to require the finished floor of occupied structures to be higher than previously required. This was based on the County’s analysis of documented flood damages from Harvey and the rules in place at the time that the developments with damaged homes were designed and built. A comparison sheet with existing and proposed rules can be reviewed here.
  5. City of Houston Chief Resilience Officer: Stephen Costello, P.E. has moved forward with a task force that was planned long before Harvey. The task force is charged with evaluating three things: (a) Detention rules for redevelopment and development; (b) Rules regarding placement of fill dirt above or outside the regulatory floodplain; and (c) Protection and preservation of the city’s rights of way, especially roadside ditches.
  6. Low Impact Development (Harris County): The county has a task force working on an update to their 2011 rules regarding low impact development / green infrastructure. One reason for the update is to more clearly address how LID can meet the “retention” requirement in “Supplemental Guidelines” approved by Harris County in March 2016 addressing drainage and development in some areas the Cypress Creek / Addicks / Barker system.
  7. Low Impact Development (City of Houston): The Chief Resilience Officer of the City of Houston is applying for a grant from the Houston Endowment to develop a program to encourage private development to use green infrastructure in redevelopment and development projects in the city.  This effort might include experts from the Natural Resources Defense Council and the Urban Land Institute.
  8. Greater Houston Flood Mitigation Consortium: The Kinder Foundation, the Cynthia and George Mitchell Foundation, the Houston Endowment, and the Walton Family Foundation have jointly funded the Greater Houston Flood Mitigation Consortium. The group will support leading researchers from various institutions to come together to “compile, analyze and share a rich array of scientifically-informed data about flooding risk and mitigation opportunities.”  The program manager is Christof Spieler of Huitt-Zollars (formerly Morris Architects), Rice University, and the Metro Board.
  9. Bayou City Initiative: Jim Blackburn, and others, have created a new non-profit organization called the Bayou City Initiative.  As I understand it, the organization’s mission will be to develop a “vision document” for what a more resilient Houston would look like; create a network of 100,000 citizens by February 2018; advocate for sufficient funding for buyouts; advocate for project funding; advocate for advanced flood warning systems; advocate for better public information about flooding and flood risks; and prepare and distribute a how to guide for “surviving Houston flooding” for residents of the region.
  10. Lawsuits: A large number of residents in the western fringe of the Addicks and Barker flood pools, whose homes were inundated by the rising water inside the reservoirs, and a large number of residents along Buffalo Bayou whose homes flooded as a result of the United States Army Corps of Engineers (USACE) releasing a higher than normal amount of water from both reservoirs, have sued the USACE. The lawsuits are seeking financial compensation for the government’s use of private land to hold or convey flood waters, a legal concept known as “inverse condemnation.” This is similar to condemnation, but in a different order. Normally the government pays a private land owner for the right to use or own the land first and then the government uses the land.  In an inverse condemnation case, the government uses the land first and then compensates the owner afterwards.  I am not an expert in inverse condemnation law and I’m not a lawyer, but from what I’ve read, I don’t believe the flooded home owners are likely to prevail.

If you know about any other post-Harvey activities, plans, or projects, please leave a comment about them.

Incentives for Natural Drainage in the Houston Region?

[This is an article I wrote on Linkedin back in 2016.  I’m reposting it again here now because, first, the City of Houston has a task force working on rule updates related to detention required in redevelopment projects, fill placement outside of the regulatory floodplain, and protection of the drainage right of way; and, second, Harris County has a task force working on updating their green infrastructure design requirements.]

The Houston-Galveston Area Council (H-GAC) hosted a May 20, 2016 workshop on the use of natural drainage systems in land development projects. The event was part of H-GAC’s rollout of it’s new Designing for Impact: A Regional Guide to Low Impact Development.

Low impact development (LID) – also called “green infrastructure” or “natural drainage systems” — is an approach to stormwater management and drainage design that uses natural systems to reduce the cost of drainage infrastructure, lower detention requirements, and create natural amenities in the properties it serves. The approach is called “low impact” development, because it reduces the volume and rate of stormwater runoff from most rain events, and thus reduces the rain’s impact on downstream properties.

Natural drainage subdivision concept rendering prepared by R. G. Miller Engineers, Inc. and Asakura Robinson.

I was asked to participate in the H-GAC workshop and to present a summary of incentives used by county and city land development permitting officials to promote the use of LID techniques in the Houston region. I initially agreed to present without giving the requested topic much thought, but as the event date approached, I started considering the content of my presentation.

I was shocked by a realization: There are no local incentives.

Sure, we have design criteria and guidelines for how LID must be implemented. We define how LID must be integrated into site designs in ways that also achieve our region’s serious and strict drainage and flood damage reduction rules for new development and redevelopment. We even have post-installation performance and acceptance testing for certain LID techniques. But our local authorities do not provide any credit, relief, or break from the current land development and permitting rules for the use of LID techniques.

Want proof? Check out these facts:

  • Houston area land owners don’t get a discount on their stormwater drainage utility fee for using LID, as do land owners in Washington DC. Folks in our nation’s capital can earn a discount of up to 55% on their stormwater fee when they install LID features.
  • Houston area land owners or operators are not eligible for grant funding to retrofit LID facilities on their properties, as are land owners in Philadelphia, PA. Folks in the City of Brotherly Love can obtain cash to retrofit LID facilities on their properties.
  • Houston area land owners are not offered the discounted permitting fees or accelerated permitting timelines in exchange for the use of green building techniques that are available to developers in Sarasota County, FL.

So why should a Houston area land developer consider LID if there are no incentives?

  • Reduced Detention Volume, Lower Cost, and More Developable Land: LID reduces the volume and speed of stormwater runoff from the development area. When this is demonstrated to Harris County and Harris County Flood Control District to using a drainage analysis, they will approve a lower detention rate, as long as the post-development peak flow is less than or equal to the pre-development peak flow. This is not available for projects that must be approved by the City of Houston. Less detention means more developable land, more lots, and a lower cost per lot for drainage infrastructure. We demonstrated a 45% drainage system cost reduction in a side by side comparison of traditional vs. LID drainage on 38-acre tract in northwest Harris County. Lot yield increased from 88 to 95, and every lot was next to a trail along a natural creek system.
  • LID Costs are Reimbursable: LID drainage systems can be constructed using private developer financing. Once constructed, and after sufficient property tax revenues are realized, the water utility district served by the drainage facilities can sell tax-free municipal bonds and use the bond sale proceeds to reimburse the developer for the cost of the drainage system. This process is now fully authorized by the Texas Commission on Environmental Quality for those portions of the LID facilities that serve a drainage purpose.
  • Reduced Imperviousness Equals Reduced Detention: The City of Houston determines detention requirements solely on the basis of impervious cover changes (pre-project vs. post-project). LID techniques such as permeable pavers, permeable asphalt, permeable concrete, or similar approaches can be used to reduce the magnitude of the impervious cover change. The graph below illustrates how the detention rate is reduced if the amount of new impervious area is restricted.

So there you have it. While it is true that Houston area land development authorities don’t currently offer any LID incentives, the current development rules, the cost structure of traditional drainage infrastructure, the space required for detention, lot yields, and changes in imperviousness frequently provide a compelling economic rationale for using these approaches.

PS: The locations that do offer incentives are under enforcement orders to mitigate overflows from combined sewer systems that carry wastewater and stormwater in the same pipes. Those communities have found that its much more cost-effective to reduce overflows by detaining, retaining, capturing, and otherwise managing rainfall using LID approaches in combination with grey infrastructure solutions than with grey solutions alone.

What Does the 2017 Climate Science Special Report Say About Precipitation?

As required by Section 106 of the U. S. Global Change Research Act of 1990, the U. S. Global Change Research Program has published the 2017 Climate Science Special Report, which is part of the Fourth National Climate Assessment.

The report is a full assessment of the science of climate change, with a focus on the United States. The report includes an executive summary written for a lay person audience along with 15 chapters of more technical information relating to global changes; physical drivers of climate change; detection and attribution of climate change; climate models; circulation and variability; temperature changes; precipitation changes; droughts, flood, and wildfires; extreme storms; land cover changes; arctic changes; sea level rise; ocean acidification; mitigation; and potential surprises. The report also includes large appendices with information on data; model weighting; detection and attribution methods; acronyms; and a glossary.

This post provides a summary of the technical information relating to precipitation, extreme storms, floods, and hurricanes.  The post will use the vocabulary from the report used to describe the confidence we have around the predictions and trends discussed as well as the likelihood of predictions and trends occurring in the future the way we believe. The vocabulary is defined in the image below:


The report looks at precipitation in two ways. First, the report describes historical changes observed, based on gauge measurements. Second, the report describes possible future trends based on modeling results.  Key findings are copied directly from the report below:

  1. Annual precipitation has decreased in much of the West, Southwest, and Southeast and increased in most of the Northern and Southern Plains, Midwest, and Northeast. A national average increase of 4% in annual precipitation since 1901 is mostly a result of large increases in the fall season. (Medium
  2. Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (high confidence). There are important regional differences in trends, with the largest increases occurring in the northeastern United States (high confidence). In particular, mesoscale convective systems (organized clusters of thunderstorms)—the main mechanism for warm season precipitation in the central part of the United States—have increased in occurrence and precipitation amounts since 1979 (medium confidence).
  3. The frequency and intensity of heavy precipitation events are projected to continue to increase over the 21st century (high confidence). Mesoscale convective systems in the central United States are expected to continue to increase in number and intensity in the future (medium confidence). There are, however, important regional and seasonal differences in projected changes in total precipitation: the northern United States, including Alaska, is projected to receive more precipitation in the winter and spring, and parts of the southwestern United States are projected to receive less precipitation in the winter and spring (medium confidence).

Historical changes vary by region and by season. The graphic below illustrates the observed changes in precipitation by calculating the difference between the average depth of rain during the period from 1986 to 2015, minus the average depth of rain during the period from 1901 to 1960, divided by the average for the 1901-1960 period.

The current average annual rainfall in the Houston area is reported as about 50 inches per year, but this is based on averaging the entire period of record.  The maps above illustrate how the average changed from the 1901-1960 period to the 1986-2015 period. The scale of the map makes it a bit difficult to see the effect in the Houston region, but here’s what I see for the Houston area:

  • Annual Precipitation: Historic increase of 5 to 10%, with perhaps an area to the east with historic increases of between 10 and 15%.
  • Winter Precipitation: Historic increase of -5 to +5%.
  • Spring Precipitation:  Historic increase of 0 to 10%.
  • Summer Precipitation: Historic increase of >15%.
  • Fall Precipitation: Historic increase of >15%.

Just a quick reminder, these annual averages are not used in the design of our drainage systems or our floodplain management infrastructure. We use the so-called 1% annual chance 24-hour duration event, which is about 13 inches of rain in one day.


The frequency of extreme storms observed in the United States have either increased or decreased depending upon the region; in Texas, they have increased.

Observed Trends in 20-Year, 1-Day Events

The authors present information about the change in depth of 5% annual chance, 24-hour duration rain event (about 6.2 inches in Houston). The figure below shows historic changes in the 20-year return value (5% annual chance event) of the seasonal daily precipitation totals for the contiguous United States over the period 1948 to 2015 using data from the Global Historical Climatology Network.

The figure illustrates that the depth of rain associated with the 5% annual chance (20-year recurrence interval), 24-hour duration event in Texas has historically increased by 0.19 to 0.48 inches from 1948 to 2015 (depending upon the season). The current 5% annual chance event in Houston is 6.2 inches of rain in 24 hours, so these depth increases represent historic increases of about 3.0% to 7.7%.

Observed Trends in 20-Year, 2-Day Duration Events

What about the 5% annual chance (20-year recurrence interval), 2-day duration event? The figure below, which includes rain event observations from the entire contiguous United States – not just Texas, requires some explanation. It was created from a series of steps.

First, the authors looked up the number of 2-day duration rain events observed at rain gauges around the contiguous United States that exceeded the 5% annual chance, 2-day event depth at each of the associated gauges. Then they compared that number to the mean number of events larger than the 5% annual chance, 2-day event depth measured at each gauge during the period from 1901 to 1960. This is called the “Relative Number of Extreme Events” in the graph. Gauges experiencing more frequent 5% annual chance, 2-day rains during the period from 1960 on, would have a positive number and those experiencing less would have a negative number.

To see how these historic increases or decreases in the frequency of these storms vary with time, the authors grouped the data into five year periods called “pentads.”  Each pentad is identified in the graph by a year that matches the end of each of the five year periods used in the calculation. These years are shown along the horizontal axis.

The authors also grouped the rain depth measurements at the various gauges spread across the United States into area grids and then into a single area for the entire United States. So this graphs does not tell us anything about historic regional differences.

Its pretty clear from the graph, that, on the whole, the United States experienced a higher number of 5% annual chance, 2-day duration, rain events in the later part of the 1900’s and early 2000’s than in the earlier part of the 1900’s.


Many factors influence projected precipitation amounts and patterns. The authors indicate that “projecting regional changes is much more difficult [than global changes] because of uncertainty in projecting changes in the large-scale circulation that plays an important role in the formation of clouds and precipitation.”  For the contiguous United States, precipitation amounts will be a mix of increases, decreases, or little change, depending on location and season.

Globally, high-latitude regions are generally projected to become wetter while the subtropical zone is projected to become drier. Because the United States is located between these two regions, the authors state that “there is significant uncertainty about the sign and magnitude of future anthropogenic changes to seasonal precipitation in much of the region, particularly in the middle latitudes of the Nation.” They add that “confidence is high that precipitation extremes will increase in frequency and intensity in the future throughout the contiguous United States.”

The figure below illustrates the projected change (%) in seasonal precipitation as compared to the average precipitation measured during the period from 1976 to 2005. Stippling (dots) indicates that projected changes will be large compared to
natural variations. Hatching (diagonal lines) indicates that projected changes will be small compared to natural variations.

This graphic shows that Texas (and the Houston region) are expected to experience small changes in season precipitation as compared to natural variations (except perhaps for some portions of the Houston area that might experience 10 to 20% less rainfall in the spring of each year). All variations for the Houston area appear to range between -10 to +10%, except for the spring season.


The report presents the following key finding:

Detectable changes in some classes of flood frequency have occurred in parts of the United States and are a mix of increases and decreases. Extreme precipitation, one of the controlling factors in flood statistics, is observed to have generally increased and is projected to continue to do so across the United States in a warming atmosphere. However, formal attribution approaches have not established a significant connection of increased riverine flooding to human-induced climate change, and the timing of any emergence of a future detectable anthropogenic change in flooding is unclear. (Medium confidence).

Trends in measured peak stream flowrates vary across the contiguous United States. Data from 200 stream gauges indicates areas of both increasing and decreasing frequency of flooding but these data do not suggest that these trends are attributable to human influences. Significant increases in flooding frequency have been detected in about one-third locations in the central United States. Less significant increases in flooding magnitude were observed in the same gauge locations.

The authors explain the discrepancy between observed extreme precipitation increases and flooding trends by highlighting the seasonality of the two phenomena. “Extreme precipitation events in the eastern half of the United States are larger in the summer and fall when soil moisture and seasonal streamflow levels are low and less favorable for flooding. By contrast, high streamflow events are often larger in the spring and winter when soil moisture is high and snowmelt and frozen ground can enhance runoff.”

The authors add that “floods may be poorly explained by daily precipitation characteristics alone; the relevant mechanisms are more complex, involving processes that are seasonally and geographically variable, including the seasonal cycles of soil moisture content and snowfall/snowmelt.”


Regarding hurricanes (called “tropical cyclones” in the report), the authors state:

“It is likely that global mean tropical cyclone maximum wind speeds and precipitation rates will increase; and it is more likely than not that the global frequency of  occurrence of tropical cyclones will either decrease or remain essentially the same.

Confidence in projected global increases of intensity and tropical cyclone precipitation
rates is medium and high, respectively, as there is better model consensus.  Confidence is further heightened, particularly for projected increases in precipitation rates, by a robust physical understanding of the processes that lead to these increases.

Confidence in projected increases in the frequency of very intense tropical cyclones is generally lower (medium in the eastern North Pacific and low in the western North Pacific and Atlantic) due to comparatively fewer studies available and due to the competing influences of projected reductions in overall storm frequency and increased mean intensity on the frequency of the most intense storms. 

Both the magnitude and sign of projected changes in individual ocean basins appears to depend on the large-scale pattern of changes to atmospheric circulation and ocean surface temperature. Projections of these regional patterns of change—apparently critical for tropical cyclone projections—are uncertain, leading to uncertainty in regional tropical cyclone projections.”

New U.S. Government Accountability Office Report on Green Infrastructure

In December 2015 United States Senator Sheldon Whitehouse (D – Rhode Island) asked the United States Government Accountability Office (GAO) to research the use of green infrastructure by municipalities to meet Environmental Protection Agency (EPA) stormwater quality permitting requirements. On October 30, 2017 the GAO publicly released a report of their findings. The report:

  • Describes the extent to which selected municipalities are incorporating, and funding, green infrastructure in stormwater management efforts.
  • Describes what challenges, if any, municipalities reported facing in incorporating green infrastructure into stormwater management efforts.
  • Examines efforts EPA is taking to help municipalities use green infrastructure.

GAO obtained information about the first two items using a survey of 20 randomly selected municipalities with stormwater pollutant discharge permits (Table 1) and 11 randomly selected municipalities that have entered into consent decrees with EPA to mitigate combined sewer overflows (CSOs) (Table 2).  Both groups were randomized to include a wide range of sizes and geographic locations.

Table 1 – Surveyed Municipalities with Stormwater Discharge Permits

Table 2 – Surveyed Municipalities with CSO Mitigation Consent Decrees

GAO obtained information about EPA assistance efforts by conducting interviews of EPA officials and reviewing documents, web pages, and other references.

Here’s a list of the key findings from the report:

Use of Green Infrastructure: Of the 31 municipalities surveyed, 30 reported using at least one type of green infrastructure. While many reported using green infrastructure somewhere, the area within these municipalities that are served by green infrastructure is generally small. Twenty-seven municipalities provided information about how much of the areas subject to CSO mitigation of stormwater permitting drains to gray or green infrastructure.  Of these 27 municipalities, 15 reported that less than 5% of the relevant areas drained to green infrastructure facilities, 6 reported that between 5 and 20% of their area drained to green infrastructure facilities, and 6 reported that more than 20% of the relevant area drained to green infrastructure facilities.

Funding Sources: About three-quarters of the municipalities said that they fund green infrastructure through general revenues and stormwater fees.

Use of Green Infrastructure Deemed Challenging in Certain Ways: Municipalities were asked about how challenging it was to obtain land, estimate capital costs, estimate operations and maintenance costs, design installations, install facilities, afford implementation, be confident in the effectiveness of the installation, and secure political and public support when implementing either gray or green infrastructure solutions.  Municipalities most often listed the following aspects as more challenging for green infrastructure projects than for gray infrastructure projects:

  • Estimate capital costs (22 of 30 municipalities)
  • Estimate operations and maintenance costs (16 of 30 municipalities)
  • Design installations (16 of 30 municipalities)

Greater Awareness, But Not Yet Institutionalized: EPA interviewees report that while many municipalities are aware of green infrastructure, many are not yet using it extensively to manage stormwater. EPA officials have concluded that green infrastructure approaches are not yet institutionalized in a similar fashion to gray infrastructure.

Long Term Planning Efforts Launched: EPA officials reported that they initiated a pilot project with 5 municipalities to develop long-term stormwater plans that incorporate green infrastructure over a 20 or 30 year implementation period, instead of the typical 5 year stormwater permit cycle. GAO recommended that EPA enter into a formal agreement with the five pilot cities to strengthen the collaborations, increase accountability, improve outcomes, and clarify roles and responsibilities, among other things.