As Tropical Storm Beta bears down on the Houston Area, many people in the Elm Grove/North Kingwood Forest area worry that they might flood again. How likely is that, given the current predictions of 6-10 inches? After all, on May 7 last year, Elm Grove flooded on what was officially a six inch rain according to the nearest gage at West Lake Houston Parkway and the West Fork.
…But Detention Based on Pre-Atlas 14 Rainfall Rates
Even though that’s far more than Woodridge Village had during the May or September floods, the detention calculations by LJA Engineering were based on pre-Atlas 14 rainfall rates. A 100-year rainfall then was about 40% less than the official 100-year rainfall now.
So, the questions is, “How much rain would Beta have to dump on Woodridge Village before it overwhelmed the detention ponds that exist today?”
Figures Used by LJA
The chart below shows the rainfalls that the ponds were designed to hold without flooding. The bench mark its the 24-hour, hundred year rain.
These figures come from the hydrology report submitted by LJA to Montgomery County. A table buried on page 32 of the PDF shows that they based their analysis on a pre-Atlas 14, 100-year storm that dropped 12.17 inches of rain in 24 hours.
From Page 2.1 of LJA Hydrology Report Addendum, 8/28/2018 (page 32 of pdf.)
The ponds should also hold any of the shorter-duration rainfalls in the last column above.
Assumptions Underlying the Answer
To answer the question – How much would it take to flood Elm Grove again? – we need to make several assumptions:
LJA correctly calculated the volume needed.
Contractors actually excavated the volume promised.
With those caveats in mind, it would take 12+ inches of rain in 24 hours to exceed the capacity of the detention ponds currently on Woodridge Village. After that, water would start to overflow.
Short, High-Intensity Downpours Can Cause Different Type of Flooding
However, consider the other durations in the chart above. Seven inches in three hours or nine inches in six hours could also exceed the capacity.
Actually, as you get into these short-duration, high-intensity rainfalls, you introduce the risk of flooding from a second source: overwhelming the capacity of storm drains.
Storm Drains Designed for 2″ Per Hour
The storm drains in Kingwood are designed to convey about two inches of rain per hour. When you exceed that, water begins to back up in the streets. Exceed it enough, and water could actually enter homes – without sheet flow from Woodridge Village.
NHC Rainfall Prediction Spans 5 Days
The six-to-ten inch prediction issued by the National Hurricane Center for Beta spans five days. That’s good news. If ten inches were evenly spread out over five days, the streets, drains and ditches could easily handle two inches per day.
But those short, high intensity rainfalls – when you get two inches in five or ten minutes – represent a real danger. There’s just nowhere for the water to when it comes down that quickly.
Perhaps the Biggest Danger
Even if we got the predicted 6-10 inches all in one day, that’s still, at most, about 80% of the old 100-year rain which the detention was designed for.
I suspect the biggest danger from Beta may be those short, high-intensity cloud bursts or training feeder bands that dump a couple inches in five or ten minutes.
So keep your eye on the rain gage. Sign up for alerts at the Harris County Flood Warning System. Also, keep your eye on the forecasts; uncertainty still exists with Beta, its track and rainfall potential.
Posted by Bob Rehak on 9/19/2020
1117 Days after Hurricane Harvey and 1 year after Imelda
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/09/Rainfall-Table-Page-32-LJA-report.png?fit=730%2C350&ssl=1350730adminadmin2020-09-19 20:11:192020-09-19 20:21:44How Much Rain Would It Take To Flood Elm Grove Again?
Beta has turned toward the Texas coast. Adverse conditions will likely begin Sunday along coast and spread inland. The main forecast change since this morning? An increase in rainfall amounts, which could be significant. See rainfall section below.
A Flash Flood Watch will likely be issued on Sunday for the area.
Jeff Lindner, Harris County Meteorologist
Discussion
There has been little change in the overall organization of Beta today. Forecasters expect slow intensification as Beta approaches the TX coast.
Track:
There have been no significant changes in the track reasoning today and the official forecast remains similar from earlier this morning. With that said, there is some uncertainty on when Beta will turn toward the N and NE near the mid Tx coast early next week, but generally impacts will be the same for much of SE TX.
Intensity:
Given the lack of organization of the system and the lurking dry air to the west, it is now unlikely that Beta will attain hurricane intensity. The National Hurricane Center now keeps the system as a strong tropical storm as it approaches the TX coast. It should slowly as it moves NE or ENE up the coast.
Impacts
Storm Surge:
Water levels will rise along the upper TX coast beginning Saturday and worsen Sunday into Monday due to the expansion of the tropical storm force wind field. Expect swells reaching 15-25 feet over the NW Gulf. Four- to eight-foot waves could break on beaches. These large swells will lead to wave run up on top of the elevated tides.
Results:
Significant beach erosion
Damage to fragile dune systems.
Damage to coastal infrastructure
Inundation of low lying roads near the coast.
Once the water begins to rise on Saturday evening, some low lying areas may remain flooded into the middle of next week.
Overwash is likely on Hwy 87.
Ferry operations at both the Galveston-Bolivar and the Lynchburg sites may be impacted.
Rainfall:
While there is still some uncertainty, the storm could produce very heavy rainfall rates and amounts.
Jeff Lindner, Harris County Meteorologist
The rainfall forecasts have been adjusted upward, even though some models suggest only meager amounts of rainfall with Beta. Jeff Lindner, Harris County Meteorologist, currently predicts:
Coast: 12-18 inches
Coastal counties: 10-15 inches
South of I-10: 10-15 inches
South of Hwy 105: 6-10 inches
North of Hwy 105: 4-6 inches
Note that the rainfall amounts above are for a five-day total.
Given the forecast rainfall amounts…flooding of creeks and bayous across Harris County may flood depending on how the rain falls and how much falls at one time.
By Tuesday, when the ground is saturated, NOAA gives the Lake Houston Area a slight chance of flash flooding.
Six inches could fill smaller creeks; 7-8 inches could fill larger streams and bayous such as Buffalo.
Addicks and Barker Reservoirs are currently empty.
Winds:
Tropical storm force winds will be possible starting Sunday evening around Matagorda Bay with sustained 40mph winds likely spreading northward along the upper TX coast on Monday including Galveston Bay. Sustained winds of 55-65mph will spread up the TX coast Tuesday into Wednesday. Across Harris County wind will increase into the 25-35mph range on Monday and possibly 40-45mph range on Tuesday into Wednesday with higher gusts, especially near Galveston Bay.
Note: Tropical storm force winds will begin to reach the coast on Monday…well ahead of any landfall of the center…due to the expanding wind field. Tropical storm force winds now extend 175 miles outward from the center of the storm.
Prepare now for a long period of coastal flooding and periods of heavy rainfall.
Voluntary evacuations have been recommended for:
Western end of Galveston Island, west of the seawall
Bolivar Peninsula
Low-lying areas of Chamber, Brazoria and Matagorda Counties (outside levee protection)
Low-lying areas of Seabrook.
AlertHouston Recommends Taking These Steps Now
Ensure your family has 5-7 days of food, water, and necessary supplies.
This includes enough prescription medication for at least this duration, in case pharmacies and doctor’s offices are closed.
Consider the unique needs of small children, seniors, family members with access and functional needs, and pets.
Decide what you and your family will do if the storm impacts your area. Most City of Houston residents are not vulnerable to storm surge and do not need to evacuate before a hurricane or tropical storm. Vulnerable residents who require electricity may also consider evacuation in advance of a major storm. For a map of hurricane evacuation zones and mandatory evacuation areas, visit: houstonoem.org/hurricanes
Develop a family communication plan, so that you know who to check in with after a storm. Visit readyhoustontx.gov for more information on developing a plan.
Know how to turn off your utilities. This includes electricity, water, and gas. Only turn off gas if instructed by local officials or by CenterPoint Energy.
Posted by Bob Rehak on 9/19/2020 with data from the NHC, HCFCD, and City of Houston
1117 Days since Hurricane Harvey and 1 Year since Imelda
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/09/210245WPCERO_sm.gif?fit=1090%2C692&ssl=16921090adminadmin2020-09-19 18:06:232020-09-19 18:07:07Beta Rainfall Estimates Increasing; 6-10 Inches Now Predicted for Lake Houston Area
As of 4am Saturday, 9/19/20 (the 1-year anniversary of Imelda), the National Hurricane Center issued:
Hurricane Watches for Texas coast from Port Aransas to High Island
Tropical Storm Watches east of High Island into Louisiana and from Port Aransas to the Rio Grande
Storm Surge Watches from Port Mansfield, TX to Cameron, LA including all inland bays.
Discussion
There is an increasing risk of heavy rainfall and flooding along the northwest Gulf Coast Sunday through at least the middle of next week. Beta is forecast to move slowly toward and along or offshore of the coast through that time and create life-threatening storm surge.
Lake Friday, a USAF mission found a stronger tropical storm with winds of 60mph and a fairly large area of 40-50mph winds. Ships and oil platforms in the north-central Gulf of Mexico have reported winds above 40mph. The wind field is starting to expand toward the north.
Track:
Beta is moving NNE at 12mph. Faster than expected. Beta will turn west around mid-day Saturday and decrease forward speed into early next week. This will bring Beta toward the middle Texas coast.
As Beta approaches the mid TX coast, weakening high pressure to the north will turn the storm toward the north and northeast. However, forecasters have lower-than-normal confidence as to when all this will happen.
The current National Hurricane Center forecast track is near the center of consensus for major global models, but shifts in this track will remain possible both toward the north or south.
Intensity:
Beta intensified despite wind shear. As Beta moves over very warm western Gulf of Mexico waters, intensification will likely continue for the 24-48 hours. NHC now forecasts Beta to become an 80mph hurricane. As Beta begins to turn northeast and skirt the middle and upper TX coast, the storm may weaken. Some uncertainty remains in the intensity forecast.
Tropical storm force winds will likely begin to arrive along the middle TX coast Sunday evening and spread northward into Matagorda Bay and along the upper TX coast by Monday. Hurricane conditions could reach the Matagorda Bay region on Tuesday.
Impacts
Storm Surge:
Water level rises along the upper TX coast will begin on Saturday and worsen Sunday into Monday due to the expansion of the tropical-storm-force wind field. This large wind field will also develop large swells over the NW Gulf of Mexico by Sunday into Monday with swells reaching 15-25 feet. On Sunday into Monday, expect 4-8 feet waves breaking on beaches. These large swells will lead to wave run up on top of elevated tides.
Large waves will accompany the higher tides. The prolonged nature of the event will result in significant beach erosion and damage to fragile dunes.
Expect inundation of low lying roads near the coast. Once the water begins to rise on Saturday evening, some low lying areas may remain flooded into the middle of next week.
Jeff Lindner, Harris County Meteorologist
Ferry operations at both the Galveston-Bolivar and the Lynchburg sites may be impacted.
For SE Harris County:
From Saturday night into next week, coastal flooding will be possible in El Lago, Nassau Bay, Shoreacres, Seabrook and portions of Baytown near Goose Creek and Cedar Bayou.
This includes all areas around:
Clear Lake
Along Taylor’s Bayou in Shoreacres and Taylor Lake Village
Around the San Jacinto Monument and Lynchburg Ferry landing
In Seabrook areas along and east of Toddville Rd and in the waterfront area east of HWY 146.
The lower San Jacinto River at Rio Villa and other low lying areas south of US HWY 90.
Low lying areas in Galena Park and Jacinto City near the Houston Ship Channel
Lower portions of Greens and Hunting Bayous south of I-10.
Most flooding will happen in streets, low marsh lands, roads near Galveston Bay and around Clear Lake and some subdivisions streets.
Wave action from ENE/E winds across Galveston Bay of 2-4 feet will be possible on top of these tides along the west side of Galveston Bay and 1-3 feet in Clear Lake which may damage docks, bulkheads, and moored vessels.
Rainfall
Rainfall will strongly depend on the track of Beta. Small changes could result in significant differences in rainfall amounts. Heaviest rains will likely be near the coast and offshore.
Any deviation of the track northward would bring the heavy rainfall further inland, likewise any south or east shift in the track would keep the heaviest rains off the coast.
Coast: 8-12 inches
Coastal counties: 6-8 inches
South of I-10: 4-6 inches
South of Hwy 105: 3-5 inches
North of Hwy 105: 1-3 inches
HCFCD models suggest that basin average rainfall totals of at least 6 inches would fill smaller creeks and bayous in Harris County (Willow Waterhole, Keegans Bayou, Willow Creek, Upper Little Cypress Creek).
Basin average amounts of 7-8 inches would fill Clear Creek, Hunting Bayou, Armand Bayou, Cypress Creek, Halls Bayou, Greens Bayou, White Oak Bayou, Buffalo Bayou, Cedar Bayou, and various smaller tributaries.
Addicks and Barker Reservoirs are currently empty.
Winds:
Tropical storm force winds may start Sunday evening around Matagorda Bay. Sustained 40mph winds likely spread northward along the upper TX coast on Monday including Galveston Bay.
Hurricane conditions will be possible near Matagorda Bay on Tuesday and across portions of coastal Matagorda, Brazoria, and Galveston Counties into Wednesday.
Across Harris County wind will increase into the 25-35mph range on Monday and possibly 40-50mph range on Tuesday into Wednesday with higher gusts, especially near Galveston Bay.
Posted by Bob Rehak on 9/19/2020at 4ambased on data from the National Hurricane Center and Harris County Meteorologist Jeff Lindner.
1117 Days since Hurricane Harvey and 1 Year since Tropical Storm Imelda
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/09/image003.gif?fit=800%2C561&ssl=1561800adminadmin2020-09-19 04:20:102020-09-19 04:23:40Hurricane Watch for Port Aransas to High Island
How Much Rain Would It Take To Flood Elm Grove Again?
As Tropical Storm Beta bears down on the Houston Area, many people in the Elm Grove/North Kingwood Forest area worry that they might flood again. How likely is that, given the current predictions of 6-10 inches? After all, on May 7 last year, Elm Grove flooded on what was officially a six inch rain according to the nearest gage at West Lake Houston Parkway and the West Fork.
Additional Detention in Woodridge Village Now…
First of all, understand that upstream conditions have changed. On May 7th, only about 11% of the planned detention pond capacity had been constructed. And only 23% was constructed by Imelda. Today, 100% is in place.
…But Detention Based on Pre-Atlas 14 Rainfall Rates
Even though that’s far more than Woodridge Village had during the May or September floods, the detention calculations by LJA Engineering were based on pre-Atlas 14 rainfall rates. A 100-year rainfall then was about 40% less than the official 100-year rainfall now.
So, the questions is, “How much rain would Beta have to dump on Woodridge Village before it overwhelmed the detention ponds that exist today?”
Figures Used by LJA
The chart below shows the rainfalls that the ponds were designed to hold without flooding. The bench mark its the 24-hour, hundred year rain.
These figures come from the hydrology report submitted by LJA to Montgomery County. A table buried on page 32 of the PDF shows that they based their analysis on a pre-Atlas 14, 100-year storm that dropped 12.17 inches of rain in 24 hours.
The ponds should also hold any of the shorter-duration rainfalls in the last column above.
Assumptions Underlying the Answer
To answer the question – How much would it take to flood Elm Grove again? – we need to make several assumptions:
With those caveats in mind, it would take 12+ inches of rain in 24 hours to exceed the capacity of the detention ponds currently on Woodridge Village. After that, water would start to overflow.
Short, High-Intensity Downpours Can Cause Different Type of Flooding
However, consider the other durations in the chart above. Seven inches in three hours or nine inches in six hours could also exceed the capacity.
Actually, as you get into these short-duration, high-intensity rainfalls, you introduce the risk of flooding from a second source: overwhelming the capacity of storm drains.
Storm Drains Designed for 2″ Per Hour
The storm drains in Kingwood are designed to convey about two inches of rain per hour. When you exceed that, water begins to back up in the streets. Exceed it enough, and water could actually enter homes – without sheet flow from Woodridge Village.
NHC Rainfall Prediction Spans 5 Days
The six-to-ten inch prediction issued by the National Hurricane Center for Beta spans five days. That’s good news. If ten inches were evenly spread out over five days, the streets, drains and ditches could easily handle two inches per day.
But those short, high intensity rainfalls – when you get two inches in five or ten minutes – represent a real danger. There’s just nowhere for the water to when it comes down that quickly.
Perhaps the Biggest Danger
Even if we got the predicted 6-10 inches all in one day, that’s still, at most, about 80% of the old 100-year rain which the detention was designed for.
I suspect the biggest danger from Beta may be those short, high-intensity cloud bursts or training feeder bands that dump a couple inches in five or ten minutes.
So keep your eye on the rain gage. Sign up for alerts at the Harris County Flood Warning System. Also, keep your eye on the forecasts; uncertainty still exists with Beta, its track and rainfall potential.
Posted by Bob Rehak on 9/19/2020
1117 Days after Hurricane Harvey and 1 year after Imelda
Beta Rainfall Estimates Increasing; 6-10 Inches Now Predicted for Lake Houston Area
Beta has turned toward the Texas coast. Adverse conditions will likely begin Sunday along coast and spread inland. The main forecast change since this morning? An increase in rainfall amounts, which could be significant. See rainfall section below.
Discussion
There has been little change in the overall organization of Beta today. Forecasters expect slow intensification as Beta approaches the TX coast.
Track:
There have been no significant changes in the track reasoning today and the official forecast remains similar from earlier this morning. With that said, there is some uncertainty on when Beta will turn toward the N and NE near the mid Tx coast early next week, but generally impacts will be the same for much of SE TX.
Intensity:
Given the lack of organization of the system and the lurking dry air to the west, it is now unlikely that Beta will attain hurricane intensity. The National Hurricane Center now keeps the system as a strong tropical storm as it approaches the TX coast. It should slowly as it moves NE or ENE up the coast.
Impacts
Storm Surge:
Water levels will rise along the upper TX coast beginning Saturday and worsen Sunday into Monday due to the expansion of the tropical storm force wind field. Expect swells reaching 15-25 feet over the NW Gulf. Four- to eight-foot waves could break on beaches. These large swells will lead to wave run up on top of the elevated tides.
Results:
Rainfall:
The rainfall forecasts have been adjusted upward, even though some models suggest only meager amounts of rainfall with Beta. Jeff Lindner, Harris County Meteorologist, currently predicts:
Given the forecast rainfall amounts…flooding of creeks and bayous across Harris County may flood depending on how the rain falls and how much falls at one time.
Six inches could fill smaller creeks; 7-8 inches could fill larger streams and bayous such as Buffalo.
Addicks and Barker Reservoirs are currently empty.
Winds:
Tropical storm force winds will be possible starting Sunday evening around Matagorda Bay with sustained 40mph winds likely spreading northward along the upper TX coast on Monday including Galveston Bay. Sustained winds of 55-65mph will spread up the TX coast Tuesday into Wednesday. Across Harris County wind will increase into the 25-35mph range on Monday and possibly 40-45mph range on Tuesday into Wednesday with higher gusts, especially near Galveston Bay.
Note: Tropical storm force winds will begin to reach the coast on Monday…well ahead of any landfall of the center…due to the expanding wind field. Tropical storm force winds now extend 175 miles outward from the center of the storm.
Summary of Watches/Warnings In Effect
As of 4PM Saturday, 9/19/20:
A STORM SURGE WARNING IS IN EFFECT FOR
A STORM SURGE WATCH IS IN EFFECT FOR:
A HURRICANE WATCH IS IN EFFECT FOR:
* PORT ARANSAS TEXAS TO HIGH ISLAND TEXAS
A TROPICAL STORM WARNING IS IN EFFECT FOR:
A TROPICAL STORM WATCH IS IN EFFECT FOR:
Protective Actions
AlertHouston Recommends Taking These Steps Now
Posted by Bob Rehak on 9/19/2020 with data from the NHC, HCFCD, and City of Houston
1117 Days since Hurricane Harvey and 1 Year since Imelda
Hurricane Watch for Port Aransas to High Island
As of 4am Saturday, 9/19/20 (the 1-year anniversary of Imelda), the National Hurricane Center issued:
Discussion
There is an increasing risk of heavy rainfall and flooding along the northwest Gulf Coast Sunday through at least the middle of next week. Beta is forecast to move slowly toward and along or offshore of the coast through that time and create life-threatening storm surge.
Lake Friday, a USAF mission found a stronger tropical storm with winds of 60mph and a fairly large area of 40-50mph winds. Ships and oil platforms in the north-central Gulf of Mexico have reported winds above 40mph. The wind field is starting to expand toward the north.
Track:
Beta is moving NNE at 12mph. Faster than expected. Beta will turn west around mid-day Saturday and decrease forward speed into early next week. This will bring Beta toward the middle Texas coast.
As Beta approaches the mid TX coast, weakening high pressure to the north will turn the storm toward the north and northeast. However, forecasters have lower-than-normal confidence as to when all this will happen.
The current National Hurricane Center forecast track is near the center of consensus for major global models, but shifts in this track will remain possible both toward the north or south.
Intensity:
Beta intensified despite wind shear. As Beta moves over very warm western Gulf of Mexico waters, intensification will likely continue for the 24-48 hours. NHC now forecasts Beta to become an 80mph hurricane. As Beta begins to turn northeast and skirt the middle and upper TX coast, the storm may weaken. Some uncertainty remains in the intensity forecast.
Tropical storm force winds will likely begin to arrive along the middle TX coast Sunday evening and spread northward into Matagorda Bay and along the upper TX coast by Monday. Hurricane conditions could reach the Matagorda Bay region on Tuesday.
Impacts
Storm Surge:
Water level rises along the upper TX coast will begin on Saturday and worsen Sunday into Monday due to the expansion of the tropical-storm-force wind field. This large wind field will also develop large swells over the NW Gulf of Mexico by Sunday into Monday with swells reaching 15-25 feet. On Sunday into Monday, expect 4-8 feet waves breaking on beaches. These large swells will lead to wave run up on top of elevated tides.
Large waves will accompany the higher tides. The prolonged nature of the event will result in significant beach erosion and damage to fragile dunes.
Ferry operations at both the Galveston-Bolivar and the Lynchburg sites may be impacted.
For SE Harris County:
From Saturday night into next week, coastal flooding will be possible in El Lago, Nassau Bay, Shoreacres, Seabrook and portions of Baytown near Goose Creek and Cedar Bayou.
This includes all areas around:
Most flooding will happen in streets, low marsh lands, roads near Galveston Bay and around Clear Lake and some subdivisions streets.
Wave action from ENE/E winds across Galveston Bay of 2-4 feet will be possible on top of these tides along the west side of Galveston Bay and 1-3 feet in Clear Lake which may damage docks, bulkheads, and moored vessels.
Rainfall
Rainfall will strongly depend on the track of Beta. Small changes could result in significant differences in rainfall amounts. Heaviest rains will likely be near the coast and offshore.
HCFCD models suggest that basin average rainfall totals of at least 6 inches would fill smaller creeks and bayous in Harris County (Willow Waterhole, Keegans Bayou, Willow Creek, Upper Little Cypress Creek).
Basin average amounts of 7-8 inches would fill Clear Creek, Hunting Bayou, Armand Bayou, Cypress Creek, Halls Bayou, Greens Bayou, White Oak Bayou, Buffalo Bayou, Cedar Bayou, and various smaller tributaries.
Addicks and Barker Reservoirs are currently empty.
Winds:
Tropical storm force winds may start Sunday evening around Matagorda Bay. Sustained 40mph winds likely spread northward along the upper TX coast on Monday including Galveston Bay.
Hurricane conditions will be possible near Matagorda Bay on Tuesday and across portions of coastal Matagorda, Brazoria, and Galveston Counties into Wednesday.
Across Harris County wind will increase into the 25-35mph range on Monday and possibly 40-50mph range on Tuesday into Wednesday with higher gusts, especially near Galveston Bay.
Posted by Bob Rehak on 9/19/2020 at 4am based on data from the National Hurricane Center and Harris County Meteorologist Jeff Lindner.
1117 Days since Hurricane Harvey and 1 Year since Tropical Storm Imelda