The Diversion Ditch project would help address several potential problems. Expanding it would remove 62 structures from inundation areas and another 586 structures would benefit from improved local drainage. In addition, the project:
Can divert floodwater from Ben’s Branch, which will be a much more complicated project, taking more time.
Has a 300-foot right-of-way, of which only half is being used
Has bridges that already span the entire 300 feet.
Will help carry floodwaters from rapidly growing south Montgomery County.
History of Diversion Ditch
In the early days of Kingwood, Friendswood Development Company built the Diversion Ditch to reduce water flowing into Ben’s Branch. But since then, upstream development and larger rains have stressed the capacity of both Ben’s Branch AND the Diversion Ditch. Engineers estimate that peak flows have doubled since 1985.
Most of Ben’s Branch is Natural Channel
Ben’s Branch cuts diagonally through the heart of Kingwood. See red lines below. More than half its length – between Woodland Hills and Rocky Woods Drive is natural channel. Widening it will be complicated and take much time.
Red Line indicates approximate path of Ben’s Branch through Kingwood.
Ben’s Branch Now at 2-Year Level of Service
However, areas on both sides of Ben’s Branch are threatened by flooding as you can see in the image below from FEMA’s Flood Hazard Viewer.
Ben’s Branch once had a 100-year level of service, meaning it had enough carrying capacity to prevent homes from flooding in everything but a 100-year rain. Models based on new Atlas-14 rainfall probability frequencies indicate that the channel’s capacity is now down to a 2-year level of service. That means it will flood in minor rains, exactly as St. Martha School did last year.
Worse yet, Ben’s Branch has decreased to a 2-year level of service throughout its length.
HCFCD Kingwood Area Drainage Analysis
Source: Fema’s Flood Hazard Layer Viewer. Cross-hatched equals floodway, aqua = 100 year floodplain, brown = 500-year floodplain.Floodplains shown above are based on pre-Atlas-14 rainfall probability statistics. An Atlas-14 hundred-year rain is about 30-40% higher than the old hundred-year rain.
When flood maps are updated based on Atlas-14 statistics, those floodplains will likely expand…unless we do something to handle more floodwater before then.
However, Ben’s Branch will not move to preliminary engineering right away.
How to Protect Against Bigger Rains and More Upstream Development
The Kingwood Area Drainage Analysis sought to understand what we need to do to restore a 100-year level of service to all ditches and streams based on Atlas 14. Of the 19 ditches and streams studied, nine need improvement. The level of service for some, including Ben’s Branch, has been reduced to 2 years.
Expanding the Diversion Ditch is the fastest way to take pressure off of Ben’s Branch.
The Diversion Ditch intersects Ben’s Branch at the new St. Martha Church. It then flows south to Deer Ridge Park and then winds through River Grove Park. See the white line below.
Kingwood Diversion ditch (white line) intersects Ben’s Branch near the new St. Martha Church.
Expansion Capacity Already Built into Diversion Ditch
Engineers foresaw the day when Kingwood would need more drainage capacity due to upstream development in Montgomery County. They built the Kingwood Diversion Ditch to handle the extra stormwater. They also made the bridges over the diversion ditch wider than they needed at the time. Finally, they dedicated a flood easement on both sides of the ditch that was wider than they needed, so they could expand the ditch later without encroaching on neighboring properties. Here’s how it looks from the air.
Looking north across Northpark Drive toward Bens Branch, which cuts diagonally from left to right through the middle of the frame. Note the ample clearance under the bridge and the wide shoulders of the ditch. St. Martha Church is in the upper left.
Looking south toward Kingwood Drive at the Diversion Ditch. King’s Mill is on right in foreground.
Both Kings Mill and Kings Manor now drain into the Diversion Ditch. But they came long AFTER Diversion Ditch construction. Other new upstream developments that drain into the Diversion Ditch and Ben’s Branch include Brooklyn Trails and Woodridge Forest, both in Montgomery County.
As a result, the Diversion Ditch itself has decreased to a 2- to a 25-year level of service in places. However, it still offers a 100-year level of service in others.
Looking NE toward Deer Ridge Park from over Hamblen Road. A corner of Deer Ridge Estates is on the left. The diversion ditch cuts in front of the park (upper left to lower right) and goes into an area largely undeveloped on its way to the river (out of frame to the right).
Impact on River Grove Park
Once the Diversion Ditch passes through the area shown in the photo above, it enters wetlands and winds through River Grove Park. Two questions arise. How do we protect, from additional flow:
The park?
People downstream on the West Fork?
The first question is simple: split the flow in two. Take part through the undeveloped area west of the park. See the green below.
Green Line represents one possible route for diversion of the diversion ditch.
The second question is more complicated. We need a retention basin to hold the extra stormwater until the peak of any flood passes on the West Fork. But where? The closer you get to the river, the lower the elevation. Because of that, the basin could fill with floodwater from the river before it fills with floodwater from upstream. Fortunately, some large tracts of land exist on higher ground that could be purchased. HCFCD estimates the need at 1248 acre-feet. Preliminary engineering should start soon to address these issues.
Upstream Development Not Addressed by Analysis
Unfortunately the Kingwood Area Drainage Analysis did not address upstream development issues in Montgomery County. That was beyond the scope of work. Regardless, such issues must be addressed somehow, someday soon. Otherwise, even the improvements we invest in today could soon be overwhelmed by additional floodwaters.
In that sense, these channel improvements represent a stopgap measure. The real solution lies in making everyone in the region realize that we are all in this together.
The ten remaining channels/streams already offer a 100-year level of service. Therefore, no improvements are needed. HCFCD felt Taylor Gully should be the next priority after the Diversion Ditch. But the possible purchase of Woodridge Village may require re-thinking project requirements. Specifically, if Woodridge turns into a giant detention basin, the channel may not need as much deepening or widening.
Posted by Bob Rehak on 10/20/2020
1148 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/10/RJR_4304.jpg?fit=1500%2C1000&ssl=110001500adminadmin2020-10-20 21:09:102020-10-21 09:32:19HCFCD Recommends Expanding Diversion Ditch as First Priority in Kingwood
The Harris County Flood Control District has released a Report Summary of results from the 600-page Kingwood Area Drainage Analysis. HCFCD will hold an online community meeting Tuesday night to discuss the results. It may help to review this summary or HCFCD’s before the meeting.
Objective: Protect Homes/Businesses from Bigger Rains
The objective of the Kingwood Area Drainage Analysis: to provide the knowledge needed to protect homes and businesses from flooding in a 100-year (1% chance) rain as defined by the new, higher Atlas-14 Rainfall Probability Statistics. Said another way, the engineers want to make sure that if you bought a home outside the 100-year flood plain, that you STILL won’t be flooded in a 100-year rain. Engineers call that “the 100-year Level of Service (LOS).”
Steams, ditches and channels evaluated as part of the Kingwood Area Drainage Analysis. Please note ditch numbers for any comments you make as the flood control district identifies them by that number.It will help communication.
What Study Included
To accomplish their objectives, engineers:
Evaluated historical floods and mapped flood damage
Created hydrologic and hydraulic models to quantify flooding risks along streams, channels and open ditches
Identified drainage issues associated with storm sewers and streets
Performed an overland flow analysis
Used LiDAR, record drawings, previous surveys and data collected in the field
Incorporated the impact of potential storm sewer improvements on channel capacity
Quantified detention capacity needed to prevent increasing flows into Lake Houston
Determined how many structures would benefit from each improvement (or planned future improvement)
Researched rights-of-way and flowage easements
Recommended channel-capacity improvements
Recommended bridge and culvert improvements
Developed preliminary cost estimates
Recommended construction sequencing
In places where ditches provide less than a 100-year level of service, storm sewers cannot empty into them during a 100-year rain. When that happens, water backs up into streets and can flood homes. So engineers are looking at the performance of the drainage system from end to end in conjunction with the City of Houston.
Limitations: What Study Does NOT Include
The study does NOT:
Include new topographic surveys
Evaluate improvements for greater than 100-year events
Examine Montgomery County issues except for North Park Drive
Identify sites for detention basins
Duplicate the San Jacinto River Basin Master Drainage Plan or associated sedimentation and sand trap studies
No construction is included as part of this project. This project will only lay the groundwork for subsequent construction projects.
Study Identified Nine Channels That Need Improvement
Not surprisingly, the Kingwood Area Drainage Analysis found that many channels do not need improvements. Said another way, they already provide the required level of service. However, engineers identified nine channels that DO need improvement to protect homes and businesses.
Six of those nine have rights-of-way controlled by HCFCD. They include:
For summary sheets of each recommended Kingwood Area Drainage Analysis project, click on the associated links. The Taylor Gully project will need to be re-analyzed if the Woodridge Village purchase goes through; that area could turn into a regional detention facility.
This table contains a summary of streams, channel types, ownership, current level of service, improvements, rights of way needed, cost estimates and detention estimates for all the projects considered in the Kingwood area.
Two Projects Recommended Immediately
Based on the results of the Kingwood Area Drainage Analysis, HCFCD recommends two projects: G103-38-00 (Kingwood Diversion Ditch) and G103-80-03.1B (Taylor Gully) move to the next phase: engineering design. Additionally, HCFCD recommends the Taylor Gully project be reanalyzed to determine how the use of Woodridge Village for detention could modify the recommended plan.
These two projects were chosen because:
They help the largest number of people.
HCFCD owns land to expand and deepen the Diversion Ditch.
Diversion Ditch enhancement will immediately take pressure off Ben’s Branch, and help flooding there.
Thus, these two projects address the three biggest needs. The remainder will have to wait for Capital Improvement funds.
First Step of Many
The Kingwood Area Drainage Analysis is a feasibility study that helped identify the problem areas. As you can see from the lifecycle diagram below, it represents the first step of several. The Flood Control District included $10 million in the bond fund (See F14) for Kingwood projects. That can be local-match seed money for attracting state and federal grants.
HCFCD will hold an online meeting Tuesday night. Engineers will describe the project and recommendations in more detail. You will also have a chance to interact with team members, make suggestions, and ask questions.
Or by phone at 855-925-2801 with Meeting Code: 9541
Please login a few minutes early. The Flood Control District has a brief survey on the login page that will help them track your concerns.
What the Flood Control District Needs from You
The Flood Control District needs to know about local issues that they may NOT have identified near you.
For example, culverts under Kingwood Drive for a ditch near me are almost totally blocked by sediment. That could have affected their analysis. And the stream may have correctable issues that could easily prevent future flooding.
Another example. The recommendations include taking out the low water crossing near Bear Branch Elementary School. Many kids use that to walk or bicycle to school. Removal without replacement would create a major inconvenience. It might also negatively affect downstream bridges damaged in previous storms.
A final example: the study did not recommend any changes to the bridge over Taylor Gully at Rustling Elms. However, many Elm Grove residents identified that as a major issue in two floods last year.
The Rustling Elms Bridge is a road over twin culverts, not a true open bridge like the one farther downstream in the background.
Please join the meeting Tuesday night and be prepared to discuss such issues. Volunteer your local knowledge. Speak up now or live with the results.
Credits
The Kingwood Area Drainage analysis cost $700,000. Funds from the 2018 Harris County Flood Bond and TIRZ 10 paid for this study.
Posted by Bob Rehak on 10/19/2020
1147 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/10/hvr7ksjet8llq3oonj3l.jpeg?fit=1599%2C1188&ssl=111881599adminadmin2020-10-19 16:50:002020-10-20 20:26:12Sneak Peak at Results of Kingwood Area Drainage Analysis; Review Before Meeting Tuesday Night
On Friday, October 16, 2020, stakeholders in the Lake Houston Spillway Improvement Project met at the dam to review gate alternatives and progress on the project. The main item of interest: a review of options still under consideration to increase the outflow during major storms, such as Harvey.
Phase One includes preliminary engineering and environmental permitting. It should take 18 months and is on schedule at this point. A key deliverable for phase one is verification of the benefit/cost ratio. But, of course, to determine that, you need to know the cost.
Alternatives Still Under Consideration
FEMA allotted 18 months for Phase 1. We’re six months into that. Work to date has focused on determining the optimal alternative. Five remain:
Expanding the existing spillway by adding new tainter gates
Adding a new gated spillway within the east embankment
Creating a new uncontrolled spillway within the east embankment
Building crest gates within the east embankment
Developing crest gates within the existing spillway
Tainter gates lift up from a radial arm. The tainter gates on Lake Conroe have 15X more release capacity than the lift gates on Lake Houston.
Above: the current conditions at the Lake Houston Dam.Looking slightly upstream. East is on the right.
The east embankment is a solid earthen area 2800 feet long east of the spillway and existing gates. Water cannot get over it in a storm. By adding various structures in this area, engineers could widen the current spillway capacity, allowing release of more stormwater.
One main benefit: additional gates would reduce uncertainty associated with pre-releases. Operators could wait longer until they were certain an approaching storm would not veer away at the last minute. That would avoid wasting water.
Reverse angle.Note the difference in height between the east embankment (left) and the spillway (right).Looking downstream toward Galveston Bay.Looking west toward Beltway 8. This shows the major segments of the dam.
Benefit/Cost Ratio Must Be > 1.0
The lake-level reduction benefits of these gate alternatives during major floods range up to roughly 8x. The costs also vary by roughly 4x. Those are order-of-magnitude, back-of-the-envelope estimates and far from final. Much hard work remains to develop tighter costs and tighter estimates of flood-level reductions. The latter will determine flood-prevention savings in a storm. And the benefits divided by the costs will determine the benefit/cost ratio.
In FEMA’s eyes, the benefit/cost ratio must exceed 1.0 to justify the project. Said another way, it must produce more benefits than it costs.
FEMA allotted 18 months for phase 1. We’re six months into that phase with a year left. Project partners expect results of the alternatives analysis before the end of the year.
Benefit/Cost Calculation
Given the ballpark costs of some of these gate alternatives, we will need very tight estimates of the benefits.
Potential Benefits include:
Upstream flood risk reduction
Reduced maintenance (debris management) for CWA
Improved Water Quality (post storm)
Potential Impacts include:
Increased scour and erosion potential to wetlands downstream
Increased water surface elevations to structures downstream
Calculating Benefits
Benefit Cost Ratio = (Net Present Value of Benefits)/(Project Costs)
Project Costs = (Capital costs) + (Net Present Value of Operations and Maintenance Costs)
Major Tasks Remaining in Phase 1
Barring surprises, the preliminary engineering report is due in February 2021 and environmental permitting should be complete by the Fall of 2021. Other tasks that must be completed by then include:
Hydrologic modeling of flows into and out of Lake Houston using the latest Atlas 14 data
Hydraulic modeling of Lake Houston, its Dam, and the San Jacinto River downstream of the dam to Galveston Bay
Calibration of models to historic storms
Examination of upstream benefits to residents/businesses removed from flood impacts
Examination of downstream impacts associated with additional flow release scenarios
It’s important to understand that not everyone who flooded in the Lake Houston Area did so because of the lake level. Some on the periphery of the flood flooded because water backed up in streams leading to the lake. If you got two feet of water in your living room, it doesn’t automatically mean a lake level reduction of two feet would eliminate your flooding by itself.
Congressman Dan Crenshaw (left) reviews the project with team members at the Lake Houston Dam and listens to their needs.
Phase 2 Still Not Certain
Assuming all goes well in the planning, accounting, and conceptual validation, FEMA will make a go/no go decision on construction at the end of next year or the beginning of the following year. Construction should take another 18 months.
Credits
Funding for this project comes from FEMA, Texas Division of Emergency Management, City of Houston and Harris County Flood Control. Other stakeholders include the Coastal Water Authority, Harris County, Fort Bend County, Baytown, Deer Park, and other communities adjacent to Lake Houston.
Posted by Bob Rehak on 10/17/2020
1145 Days since Hurricane Harvey and 394 since Imelda
HCFCD Recommends Expanding Diversion Ditch as First Priority in Kingwood
At the Harris County Flood Control District’s (HCFCD) Kingwood Area Drainage Analysis meeting tonight, HCFCD recommended that expansion of the Kingwood Diversion Ditch should be the community’s highest priority.
The Diversion Ditch project would help address several potential problems. Expanding it would remove 62 structures from inundation areas and another 586 structures would benefit from improved local drainage. In addition, the project:
History of Diversion Ditch
In the early days of Kingwood, Friendswood Development Company built the Diversion Ditch to reduce water flowing into Ben’s Branch. But since then, upstream development and larger rains have stressed the capacity of both Ben’s Branch AND the Diversion Ditch. Engineers estimate that peak flows have doubled since 1985.
Most of Ben’s Branch is Natural Channel
Ben’s Branch cuts diagonally through the heart of Kingwood. See red lines below. More than half its length – between Woodland Hills and Rocky Woods Drive is natural channel. Widening it will be complicated and take much time.
Ben’s Branch Now at 2-Year Level of Service
However, areas on both sides of Ben’s Branch are threatened by flooding as you can see in the image below from FEMA’s Flood Hazard Viewer.
Ben’s Branch once had a 100-year level of service, meaning it had enough carrying capacity to prevent homes from flooding in everything but a 100-year rain. Models based on new Atlas-14 rainfall probability frequencies indicate that the channel’s capacity is now down to a 2-year level of service. That means it will flood in minor rains, exactly as St. Martha School did last year.
However, Ben’s Branch will not move to preliminary engineering right away.
How to Protect Against Bigger Rains and More Upstream Development
The Kingwood Area Drainage Analysis sought to understand what we need to do to restore a 100-year level of service to all ditches and streams based on Atlas 14. Of the 19 ditches and streams studied, nine need improvement. The level of service for some, including Ben’s Branch, has been reduced to 2 years.
The Diversion Ditch intersects Ben’s Branch at the new St. Martha Church. It then flows south to Deer Ridge Park and then winds through River Grove Park. See the white line below.
Expansion Capacity Already Built into Diversion Ditch
Engineers foresaw the day when Kingwood would need more drainage capacity due to upstream development in Montgomery County. They built the Kingwood Diversion Ditch to handle the extra stormwater. They also made the bridges over the diversion ditch wider than they needed at the time. Finally, they dedicated a flood easement on both sides of the ditch that was wider than they needed, so they could expand the ditch later without encroaching on neighboring properties. Here’s how it looks from the air.
Both Kings Mill and Kings Manor now drain into the Diversion Ditch. But they came long AFTER Diversion Ditch construction. Other new upstream developments that drain into the Diversion Ditch and Ben’s Branch include Brooklyn Trails and Woodridge Forest, both in Montgomery County.
As a result, the Diversion Ditch itself has decreased to a 2- to a 25-year level of service in places. However, it still offers a 100-year level of service in others.
Impact on River Grove Park
Once the Diversion Ditch passes through the area shown in the photo above, it enters wetlands and winds through River Grove Park. Two questions arise. How do we protect, from additional flow:
The first question is simple: split the flow in two. Take part through the undeveloped area west of the park. See the green below.
The second question is more complicated. We need a retention basin to hold the extra stormwater until the peak of any flood passes on the West Fork. But where? The closer you get to the river, the lower the elevation. Because of that, the basin could fill with floodwater from the river before it fills with floodwater from upstream. Fortunately, some large tracts of land exist on higher ground that could be purchased. HCFCD estimates the need at 1248 acre-feet. Preliminary engineering should start soon to address these issues.
Upstream Development Not Addressed by Analysis
Unfortunately the Kingwood Area Drainage Analysis did not address upstream development issues in Montgomery County. That was beyond the scope of work. Regardless, such issues must be addressed somehow, someday soon. Otherwise, even the improvements we invest in today could soon be overwhelmed by additional floodwaters.
In that sense, these channel improvements represent a stopgap measure. The real solution lies in making everyone in the region realize that we are all in this together.
Additional Resources
If you missed the presentation, you can view it on YouTube.
Here is a PDF that contains the District’s summary of the Kingwood Study. It includes a spreadsheet comparing the improvements plus data sheets on the nine recommended projects.
The ten remaining channels/streams already offer a 100-year level of service. Therefore, no improvements are needed. HCFCD felt Taylor Gully should be the next priority after the Diversion Ditch. But the possible purchase of Woodridge Village may require re-thinking project requirements. Specifically, if Woodridge turns into a giant detention basin, the channel may not need as much deepening or widening.
Posted by Bob Rehak on 10/20/2020
1148 Days since Hurricane Harvey
Sneak Peak at Results of Kingwood Area Drainage Analysis; Review Before Meeting Tuesday Night
The Harris County Flood Control District has released a Report Summary of results from the 600-page Kingwood Area Drainage Analysis. HCFCD will hold an online community meeting Tuesday night to discuss the results. It may help to review this summary or HCFCD’s before the meeting.
Objective: Protect Homes/Businesses from Bigger Rains
The objective of the Kingwood Area Drainage Analysis: to provide the knowledge needed to protect homes and businesses from flooding in a 100-year (1% chance) rain as defined by the new, higher Atlas-14 Rainfall Probability Statistics. Said another way, the engineers want to make sure that if you bought a home outside the 100-year flood plain, that you STILL won’t be flooded in a 100-year rain. Engineers call that “the 100-year Level of Service (LOS).”
What Study Included
To accomplish their objectives, engineers:
In places where ditches provide less than a 100-year level of service, storm sewers cannot empty into them during a 100-year rain. When that happens, water backs up into streets and can flood homes. So engineers are looking at the performance of the drainage system from end to end in conjunction with the City of Houston.
Limitations: What Study Does NOT Include
The study does NOT:
Study Identified Nine Channels That Need Improvement
Not surprisingly, the Kingwood Area Drainage Analysis found that many channels do not need improvements. Said another way, they already provide the required level of service. However, engineers identified nine channels that DO need improvement to protect homes and businesses.
Six of those nine have rights-of-way controlled by HCFCD. They include:
Other entities control the rights-of-way for the three remaining ditches/streams that need improvements:
For summary sheets of each recommended Kingwood Area Drainage Analysis project, click on the associated links. The Taylor Gully project will need to be re-analyzed if the Woodridge Village purchase goes through; that area could turn into a regional detention facility.
This table contains a summary of streams, channel types, ownership, current level of service, improvements, rights of way needed, cost estimates and detention estimates for all the projects considered in the Kingwood area.
Two Projects Recommended Immediately
Based on the results of the Kingwood Area Drainage Analysis, HCFCD recommends two projects: G103-38-00 (Kingwood Diversion Ditch) and G103-80-03.1B (Taylor Gully) move to the next phase: engineering design. Additionally, HCFCD recommends the Taylor Gully project be reanalyzed to determine how the use of Woodridge Village for detention could modify the recommended plan.
These two projects were chosen because:
Thus, these two projects address the three biggest needs. The remainder will have to wait for Capital Improvement funds.
First Step of Many
The Kingwood Area Drainage Analysis is a feasibility study that helped identify the problem areas. As you can see from the lifecycle diagram below, it represents the first step of several. The Flood Control District included $10 million in the bond fund (See F14) for Kingwood projects. That can be local-match seed money for attracting state and federal grants.
Online Meeting Details Tuesday Night
HCFCD will hold an online meeting Tuesday night. Engineers will describe the project and recommendations in more detail. You will also have a chance to interact with team members, make suggestions, and ask questions.
Tuesday, October 20, 2020
6:30 p.m. to 7:30 p.m.
Join online at PublicInput.com/Kingwood
Or by phone at 855-925-2801 with Meeting Code: 9541
Please login a few minutes early. The Flood Control District has a brief survey on the login page that will help them track your concerns.
What the Flood Control District Needs from You
The Flood Control District needs to know about local issues that they may NOT have identified near you.
For example, culverts under Kingwood Drive for a ditch near me are almost totally blocked by sediment. That could have affected their analysis. And the stream may have correctable issues that could easily prevent future flooding.
Another example. The recommendations include taking out the low water crossing near Bear Branch Elementary School. Many kids use that to walk or bicycle to school. Removal without replacement would create a major inconvenience. It might also negatively affect downstream bridges damaged in previous storms.
A final example: the study did not recommend any changes to the bridge over Taylor Gully at Rustling Elms. However, many Elm Grove residents identified that as a major issue in two floods last year.
Please join the meeting Tuesday night and be prepared to discuss such issues. Volunteer your local knowledge. Speak up now or live with the results.
Credits
The Kingwood Area Drainage analysis cost $700,000. Funds from the 2018 Harris County Flood Bond and TIRZ 10 paid for this study.
Posted by Bob Rehak on 10/19/2020
1147 Days since Hurricane Harvey
October 2020 Gate Update
On Friday, October 16, 2020, stakeholders in the Lake Houston Spillway Improvement Project met at the dam to review gate alternatives and progress on the project. The main item of interest: a review of options still under consideration to increase the outflow during major storms, such as Harvey.
History of Project
Shortly after Harvey, representatives from the Lake Houston Area identified “increasing outflow” as one of the main strategies to reduce flooding. Harris County Flood Control District even conducted a pilot study. Flood Control included $20 million for the project in the 2018 bond fund (CI-028). With matching funds identified, grants were then written.
FEMA approved the project. And in April of this year, the clock started ticking on the first of two phases.
Phase One Includes…
Phase One includes preliminary engineering and environmental permitting. It should take 18 months and is on schedule at this point. A key deliverable for phase one is verification of the benefit/cost ratio. But, of course, to determine that, you need to know the cost.
Alternatives Still Under Consideration
FEMA allotted 18 months for Phase 1. We’re six months into that. Work to date has focused on determining the optimal alternative. Five remain:
Tainter gates lift up from a radial arm. The tainter gates on Lake Conroe have 15X more release capacity than the lift gates on Lake Houston.
Crest gates flop down from a bottom hinge.
The east embankment is a solid earthen area 2800 feet long east of the spillway and existing gates. Water cannot get over it in a storm. By adding various structures in this area, engineers could widen the current spillway capacity, allowing release of more stormwater.
Benefit/Cost Ratio Must Be > 1.0
The lake-level reduction benefits of these gate alternatives during major floods range up to roughly 8x. The costs also vary by roughly 4x. Those are order-of-magnitude, back-of-the-envelope estimates and far from final. Much hard work remains to develop tighter costs and tighter estimates of flood-level reductions. The latter will determine flood-prevention savings in a storm. And the benefits divided by the costs will determine the benefit/cost ratio.
In FEMA’s eyes, the benefit/cost ratio must exceed 1.0 to justify the project. Said another way, it must produce more benefits than it costs.
FEMA allotted 18 months for phase 1. We’re six months into that phase with a year left. Project partners expect results of the alternatives analysis before the end of the year.
Benefit/Cost Calculation
Given the ballpark costs of some of these gate alternatives, we will need very tight estimates of the benefits.
Potential Benefits include:
Potential Impacts include:
Calculating Benefits
Benefit Cost Ratio = (Net Present Value of Benefits)/(Project Costs)
Project Costs = (Capital costs) + (Net Present Value of Operations and Maintenance Costs)
Major Tasks Remaining in Phase 1
Barring surprises, the preliminary engineering report is due in February 2021 and environmental permitting should be complete by the Fall of 2021. Other tasks that must be completed by then include:
Phase 2 Still Not Certain
Assuming all goes well in the planning, accounting, and conceptual validation, FEMA will make a go/no go decision on construction at the end of next year or the beginning of the following year. Construction should take another 18 months.
Credits
Funding for this project comes from FEMA, Texas Division of Emergency Management, City of Houston and Harris County Flood Control. Other stakeholders include the Coastal Water Authority, Harris County, Fort Bend County, Baytown, Deer Park, and other communities adjacent to Lake Houston.
Posted by Bob Rehak on 10/17/2020
1145 Days since Hurricane Harvey and 394 since Imelda