Tag Archive for: Freese and Nichols

Operational Statistics from Lake Conroe Dam During Harvey Raise Troubling Questions

Two affidavits in a lawsuit filed against the SJRA for flooding downstream residents during Harvey contain statistics that raise several troubling questions about the operation of gates during the storm.

  • Did the SJRA wait too long to begin releasing water in significant volumes?
  • As a consequence, did it create an unnecessarily high peak discharge?
  • Did it maintain high discharge rates longer than it needed?
  • As lake levels declined, why did the SJRA continue releasing 2X to 10X more water than it was taking on when it had up to 3 feet of storage capacity in the Lake Conroe?
  • Why did it never let the level of Lake Conroe reach its flowage easement max?
  • Could different procedures have reduced downstream flooding?
Part of one page of seven pages of gate operation statistics in affidavits.

Affidavits of Gilman and Olmos Contain Insights

The first affidavit comes from Chuck Gilman, the SJRA’s Director of Flood Management. It contains a gold mine of statistics. Tables at the end of the affidavit show the date, time, average lake level, total inflow, and total discharge (cubic feet per second), and the exact time of gate changes. The statistics start August 26, 2017 at 10 p.m. They end three days later at the same time. The seven pages of statistics capture a snapshot of the storm and the SJRA’s response hour by hour during Harvey. At the peak, the SJRA recorded changes every 15 minutes.

The second affidavit comes from Hector Olmos, a Principal and Vice President of consulting firm Freese and Nichols, Inc. Olmos helped develop the gate operations policy for at Lake Conroe for the SJRA. The Olmos affidavit contains the same statistical information in Gilman’s. However, it also contains more details of the Gate Operations Policy in place at the time of Harvey. And the two affidavits assert different facts.

Download Gilman Affidavit

Download Olmos Affidavit

Inflow Vs. Outflow and Flowage Easement Max

In the summary that follows, outflow vs. inflow rates are significant. Gilman swore in his affidavit that the gate operation “…policy is programmed so that even in the most extreme situations, peak outflow will never exceed 70% of inflow.” “Peak” is the key word there. Olmos swore in his affidavit that 80% was the limit. However, statistics show that it never significantly exceeded 60%.

As you review the following, keep in mind another key point. SJRA had the ability and authority to increase the lake level to 207, but stopped short at 206.23 for some reason that the affidavits don’t explain.

Summary of Key Statistics and Actions

Key statistics show that:

  • Lake Conroe started to rise at 11:30 p.m. on August 26, 2017 in response to 1,722 cfs flowing into the lake.
  • After the lake level reached 201.04 feet at 12:15 a.m. on August 27, SJRA first opened its gates at 12:25 a.m. and started releasing 529 cfs.
  • After that, inflow generally increased for the next 24.5 hours, though the increases were not a straight line. Inflow fluctuated up and down, likely in response to feeder bands passing over the watershed or variations in readings due to wave and wind action.
  • More than 24 hours after the start of the storm, at 1 a.m. on August 28, inflow peaked at 129,065 cfs. By then, the lake level had reached 205.65 feet and the SJRA was releasing 62,082 cfs, less than half of the inflow.
  • From that point on, the inflow generally declined, but not in a straight line.
  • The water level in Lake Conroe peaked six hours later at 7 a.m., August 28, at 206.23 feet. That’s roughly three-quarters of a foot BELOW the SJRA’s flowage easement.
  • After that, water continued to go down for the duration of the storm, but the SJRA continued increasing its release rate for five more hours, until 12 noon on the 28th. The water level was 206.17 feet, almost a foot below its flowage easement. Inflow was 63,986 cfs (less than half the peak), yet discharge peaked at 79,141. So the lake level and inflow were going down, but the discharge rate kept increasing when the lake had room to spare.
  • SJRA kept the release rate above 70,000 cfs until 4:15 a.m. the morning of the 29th, more than 16 hours. By then, the lake level had gone down to 204.58. And the discharge rate was still three times higher than the inflow (71,538 cfs discharge vs 20,287 inflow).
  • For the rest of the storm, lake level, inflow rates and discharge rates all continued to decline. The table ends at 10 p.m., August 29th. Lake level equaled 203.44, discharge 22,033, and inflow 6,579.

Turning Points in the Storm

During the entire day of August 27th, outflow fluctuated roughly from 16% to 50% of inflow as the inflow kept building relentlessly.

Outflow exceeded inflow by 8:30 a.m. on the 28th and stayed that way for the duration of the storm even though the lake had up to 3 available feet of storage capacity.

By the morning of the 29th, downstream areas were flooding badly. The SJRA had roughly three feet of extra storage capacity in Lake Conroe within its flowage easement. Yet it kept releasing, on average, 2 – 10X more water than it was taking in. At one point the ratio exceeded 100:1.

Could the SJRA have used more of Lake Conroe’s available storage capacity as lake levels declined to help reduce downstream flooding?

Neither Mr. Gilman’s, nor Mr. Olmos’ affidavits shed light on these issues.

Please Note

Chuck Gilman inherited this problem. The SJRA did not hire him until well after Harvey.

Also note that conditions during an emergency can be chaotic. Keyboard quarterbacking after the fact is much easier.

If the SJRA wishes to respond to this post, I will print its position verbatim.

Posted by Bob Rehak on 6/11/2020

1017 Days after Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.

Freese and Nichols study finds more gates on Lake Houston dam could have lowered flood during Harvey

A study by consulting firm Freese and Nichols looked at the value of adding flood gates to the Lake Houston Dam. It found that during Harvey, new gates would have lowered the level of flooding around Lake Houston by up to 1.9 feet depending on the number of gates added. Obviously, the gates by themselves won’t protect us from another Harvey, but they are an important part of a comprehensive solution that includes ongoing river dredging, ditch maintenance, debris removal, additional upstream retention, and better warning systems and more.

Modeling the Effect of More Gates on a Harvey-type Flood

Freese and Nichols conducted the gate study, which modeled flood levels only for the Hurricane-Harvey case. It did not address the impact of adding gates on smaller floods, such as those that occurred in 1994, 1998, 2001, 2015 and 2016.

Extent of modeling in Freese and Nichols study on the effects of adding floodgates to Lake Houston

Would Pre-release Help with a Storm as Big as Harvey?

Additional gates were originally proposed as a concept that could enable pre-release of water from Lake Houston as a flood mitigation strategy. The idea behind a pre-release strategy is to lower the level of a lake BEFORE a storm. The lake then has more capacity to absorb heavy rains before overflowing its banks and flooding residents, much as the City of Houston did before a small flood at the end of March.

Pre-release is currently difficult for Lake Houston because the dam consists mainly of a spillway with a fixed height – 42.5 feet above mean sea level. The Lake Houston dam does have two small gates, but they have less than one tenth the capacity of the flood gates on Lake Conroe. This makes it difficult to coordinate discharges between the two lakes.

The SJRA repeatedly cited fear of overloading the Lake Houston watershed as a reason for delaying its release from the Lake Conroe dam during Harvey. Additional gates might have reduced those concerns, encouraging the SJRA to release water earlier.

Theoretically, that could have reduced the volume of water coming down the west fork at the peak of the storm. At the peak, Lake Conroe’s release constituted one third of all the water coming down the West Fork where most of the damage occurred. It’s therefore easy to see how reducing the peak flow down the west fork could have spared hundreds of homes and businesses.

However, Freese and Nichols found that the volume of water coming into the lake during Harvey from multiple sources was too great to realize much benefit from pre-release. The amount pre-released would have quickly filled back up again  – within a few hours.

Primary Benefit Comes from Additional Discharge Capacity

This does not mean that Freese and Nichols recommended against adding gates. They found that gates would have benefitted the community, but in a different way than originally anticipated. Surprisingly, they found that the largest reduction in flood levels came simply from the additional discharge capacity that the gates provided during the peak of the flood.

Freese and Nichols states in its conclusion, “Adding additional gates to the spillway at Lake Houston would be a feasible alternative to allow for additional discharge capacity to reduce the impact of very large flood events.  … Though additional gates would provide the ability to lower the lake quickly in advance of an anticipated major flood event, the additional capacity of the gates would have far more impact on the flood level than any preliminary lowering of the lake.”

Cost/Benefit Ratio

Like many engineering studies, Freese and Nichols says that any decision to build the gates depends on whether elected officials find benefits worth the costs. However, the scope of the study did not include cost savings to home and business owners. So let’s look at that.

FEMA, the agency that would likely pay for most of the gates evaluates projects primarily on the number of people helped. They want to provide the most “benefit-per-buck” possible.

Looking at the world from FEMA’s Point of View

The City of Houston is currently in the process of developing the FEMA grant application. Mayor Sylvester Turner stated at a community meeting in Kingwood in March that he supported 10 additional gates, which he estimated would cost $47 million.

FEMA estimated in November of 2017 that 16,000 homes and 3,300 businesses in the Lake Houston area were damaged. Therefore, to reduce the impact of flooding, this project would require an expenditure of less than $2,500 per structure. Repairs to flooded structures in virtually all cases cost 10 to 100 times more than that. I know of at least one case where repairs cost $600,000. It doesn’t take many of those to equal the cost of the additional gates that the mayor proposed – $47 million.

The gates would completely eliminate flooding at the edge of the flood, and would reduce the depth of flooding inside of that.

It’s not clear at this point how many homes sit within that band where flooding could have been  eliminated. Nor is there a precise estimate of damage to those homes.

Calculating the Value of Flood Reduction

Looking at homes that would seen reduced flooding, it’s important to note that the cost of repairs correlates highly with the level of flooding. According to Bill Fowler, a real estate tax expert and Co-Chair of the Lake Houston Area Grass Roots Flood Prevention Initiative, the Harris County Appraisal District is lowering valuations on homes by the amount of flooding they experienced.

Lowering flood levels usually lowers repair costs. Lower flood levels can also lower flood insurance costs, losses to insurance companies, damage to contents, and damage to vehicles. Value can be measured many ways.

It’s also important to note when calculating value in floods smaller than Harvey, pre-release might actually become a viable strategy and greatly reduce or eliminate flooding altogether. Freese and Nichols did not evaluate additional gates from that perspective; they considered only Harvey-level flooding.

Adding floodgates to Lake Houston will be a valuable flood mitigation tool. It must be viewed as an essential PART of the solution, not THE solution. Consider its value within the context of other mitigation efforts, such as dredging, ditch maintenance, and additional upstream storage capacity.

Posted June 10 by Bob Rehak

285 Days since Hurricane Harvey