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