Temporary Seasonal Lake Lowering...Houston, and the temporary Seasonal Lake Lowering initiative • Summary of the 2018 and 2019 Seasonal Lake Lowering • Status of downstream mitigation

Temporary Seasonal Lake Lowering

Overview• Pertinent facts and details about Lake Conroe, Lake

Houston, and the temporary Seasonal Lake Lowering initiative

• Summary of the 2018 and 2019 Seasonal Lake Lowering

• Status of downstream mitigation projects 2





Entire Upper Watershed: 2,800 square miles

Lake Conroe Watershed: 450 square miles

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West Fork15.8%

East Fork 13.6%

Luce Bayou 7.8%

Peach Creek 5.7%

Caney Creek7.7%

Lake Creek 10.3%

Cypress Creek 7.8%

Lake Conroe 14.0%

Spring Creek 17.4%

100-YR (ATLAS 14) Volume Percentages Lake Houston Inflow Volume= 2.0 Million ac-ft

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Facts about Lake Conroe• Surface acres: 19,640

• Capacity: Not to exceed 430,260 acre-feet

• Permitted water: 100,000 acre-feet

• Conservation pool elevation: 201 ft msl

• Main spillway: five gates, 40 ft by 30 ft

• SJRA obtained a flowage easement around the reservoir up to elevation 207 ft msl

• All water stored above elevation 201 ft msl must be released

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Facts about Lake Conroe• Constructed as a water supply reservoir –

not intended to be a “constant level lake”

• 1/3rd of permitted water belongs to SJRA

• 2/3rd of permitted water belongs to City of Houston

• City of Houston pays 2/3rd of the operating cost of Lake Conroe

• City of Houston has the ability to call for the release of their permitted water at any time, regardless of the water surface elevation of Lake Conroe 7

Key Points regarding Gate Operations• Tainter gates must be raised as lake rises• Cannot allow over-topping as it compromises the structure.

Downstream side of Gate

Top of Tainter Gate

Top of Water Surface

18” Freeboard

Upstream side of Gate 8

Seasonal Lake Lowering Initiative

• Conservation pool level is 201’ msl

• Seasonal lowering will occur: • Spring: April 1-May 31 to 200’ msl

• Gradual reduction starts April 1st

• Recapturing starts on June 1st

• Fall: August 1-September 30 to 199’ msl• Gradual reduction starts August 1st

• Recapturing starts on October 1st9

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Joint Reservoir Operations Strategy with CoH

• Temporary Seasonal Lowering of Lake Conroe

• City of Houston lowers Lake Houston prior to major rainfall events:• Conservation Pool Elevation is 42.5’ msl

• Lake Level reduced to 41.5’ msl when 3 inches or more of rainfall is forecasted

• Lake Houston was lowered to elevation 41.5’ msl or lower nine times in 2019 11

Why Not Pre-Release from Lake Conroe?

• Weather predictions not accurate enough

• Risk of making downstream conditions worse

• Downstream partners want empty rivers prior to a storm

• Potentially impacts properties immediately downstream of the dam

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49 Miles

13

12 Miles

15 Miles

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Decision Based on Engineering StudiesTwo engineering studies were completed prior to the SJRA Board taking action on the seasonal lake lowering initiative.

1. Analysis of potential impacts of lowering Lake Conroe conservation pool elevation on lake storage and elevation, available diversions from the lake (average and firm), and downstream water rights.

2. Analysis of potential impacts on lake levels and downstream maximum water surface elevation of a two-foot and three-foot reduction in the conservation pool level of Lake Conroe.

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Engineering Study: Water Supply

Scenario Firm Yield(ac-ft/yr)

Change from Baseline(ac-ft/yr)

Conservation Pool 80,200 0

200’ (temp) 80,200 0

199’ (temp) 78,800 -1,400

*Models were developed based on an assumption that reductions of conservation storage for seasonal lake lowering were limited to August and September

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Engineering Study: Maximum Downstream WSEL

201(ft msl)

199(ft msl)

Difference(ft)

100-year Water Surface Elevation at Cross Section, ft mslDS Lake Conroe 153.94 152.47 -1.47

US SH 105 149.57 148.10 -1.47Lake Creek 136.88 136.36 -0.52

IH-45 124.44 123.70 -0.74500-year Water Surface Elevation at Cross Section, ft mslDS Lake Conroe 159.31 157.74 -1.57

US SH 105 154.33 153.00 -1.33Lake Creek 141.02 140.37 -0.65

IH-45 129.69 128.89 -0.80 17


425 ft

WFSJR at I-45

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6,000+ ft

WFSJR at I-45, 100-Year Storm Event Starting Lake Conroe Elevation: 201 ft msl

River estimated to be 8 ft out of bank

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14,000+ ft

WFSJR at I-45, 500-Year Storm Event Starting Lake Conroe Elevation: 201 ft msl

River estimated to be 12 ft out of bank

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Rainfall Averages for Lake Conroe

3.65

2.67

3.27 3.36

5.334.87

3.47

4.114.34 4.48

4.053.55

0.00

1.00

2.00

3.00

4.00

5.00

6.00

January February March April May June July August September October November December

INCH

ES

1973-2019

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Lake Level Averages for Lake Conroe

199.54

199.82199.98

200.32200.44 200.43

200.26

199.92199.75

199.62

199.86200.06

198.50

199.00

199.50

200.00

200.50

201.00

January February March April May June July August September October November December

ELEV

ATIO

N (M

SL)

1973-2019 Conservation Pool: 201 ft msl

23

24

1.351.18

1.04

2.86

0.80

2.49

9/1/2018, 199.04 9/29/2018, 199.04

10/17/2018, 201.29

10/20/2018, 201.67

10/31/2018, 201.07

0

0.5

1

1.5

2

2.5

3

3.5

197.50

198.00

198.50

199.00

199.50

200.00

200.50

201.00

201.50

202.00

8/1/18 8/8/18 8/15/18 8/22/18 8/29/18 9/5/18 9/12/18 9/19/18 9/26/18 10/3/18 10/10/18 10/17/18 10/24/18 10/31/18

INCH

ES

ELEV

ATIO

N (M

SL)

FALL 2018 SEASONAL LAKE RELEASEDAILY RAINFALL AND LAKE LEVEL (AUGUST-OCTOBER 2018)

Daily Rainfall (Inches) Lake Level (MSL)

3.28

Seasonal Releases

Fall 2018 Summary • Lake Conroe conservation pool elevation = 201.00 ft msl

• The lake level was 0.40 feet below conservation pool elevation prior to August 1st due to external losses (i.e. evaporation, customer sales)

• Seasonal releases ended on August 31, 2018

• 18,265 acre-feet of Houston’s water was released

• Reached conservation pool elevation October 17, 201825

0.68

1.04

0.38

2.062.15

0.6

2.11

0.92

0.76

0.56

4/1/2019, 200.96

5/1/2019, 200.08

5/9/2019, 201.43

5/31/2019, 200.06

0

0.5

1

1.5

2

2.5

198.50

199.00

199.50

200.00

200.50

201.00

201.50

202.00

4/1/2019 4/8/2019 4/15/2019 4/22/2019 4/29/2019 5/6/2019 5/13/2019 5/20/2019 5/27/2019

INCH

ES

ELEV

ATIO

N (M

SL)SPRING 2019 SEASONAL LAKE RELEASE

DAILY RAINFALL AND LAKE LEVEL (APRIL-MAY 2019)

Daily Rainfall (Inches) Lake Level (MSL) 26

Seasonal Releases Seasonal Releases

Spring 2019 Summary

• The lake level was at conservation pool elevation prior to April 1st

• Seasonal releases ended on May 24, 2019


• Did not return to conservation pool between end of May and August 1st

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0.05

2.062.15

0.6

2.11

0.12

0.92

0.32

4/29/2019, 200.19

5/7/2019, 200.88

5/9/2019, 201.43

5/13/2019, 201.21

0

0.5

1

1.5

2

2.5

199.00

199.50

200.00

200.50

201.00

201.50

202.00

4/29/2019 5/1/2019 5/3/2019 5/5/2019 5/7/2019 5/9/2019 5/11/2019 5/13/2019

INCH

ES

ELEV

ATIO

N (M

SL)

MAY 2019 EVENT DAILY RAINFALL AND LAKE LEVEL (APRIL 29 - MAY 13, 2019)

Daily Rainfall (Inches) Lake Level (MSL) 28

May 2019 Event

29

30

0.21 0.17

0.91

0.13

0.51

4.0

0.25

0.47

1.23

0.20

0.66

0.43

2.43

0.84

8/1/2019, 200.44

9/18/2019, 198.74

9/21/2019, 199.1610/11/2019, 199.12

10/31/2019, 199.05

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

197.50

198.00

198.50

199.00

199.50

200.00

200.50

201.00

8/1/19 8/8/19 8/15/19 8/22/19 8/29/19 9/5/19 9/12/19 9/19/19 9/26/19 10/3/19 10/10/19 10/17/19 10/24/19 10/31/19

RAIN

FALL

(IN

CHES

)

ELEV

ATIO

N (M

SL)

FALL 2019 SEASONAL LAKE RELEASEDAILY RAINFALL AND LAKE LEVEL (AUGUST-OCTOBER 2019)


Most Likely Wind Driven Anomaly

Seasonal Releases

Fall 2019 • The lake level was 0.56 feet below conservation pool prior to

August 1st due to external losses (i.e. evaporation, customer sales)

• Seasonal releases ended on August 31, 2019


• Lake Conroe has not been at, or above, elevation 201.00 ft msl since May 9, 2019

31

32

0.51

4.00

0.25

0.47

9/17/2019, 198.73

9/18/2019, 198.74

9/19/2019, 199.02

9/20/2019, 199.06

9/21/2019, 199.16

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

198.50

198.60

198.70

198.80

198.90

199.00

199.10

199.20

9/17/2019 9/18/2019 9/19/2019 9/20/2019 9/21/2019

RAIN

FALL

(IN

CHES

)

ELEV

ATIO

N (M

SL)

TROPICAL STORM IMELDADAILY RAINFALL AND LAKE LEVEL (SEPTEMBER 17-21, 2019)


September 2019 – Tropical Storm Imelda

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September 2019 – Tropical Storm Imelda

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Rainfall in late 2019

Month 1973-2019 Rainfall Averages (Inches)

2019 Observed Rainfall(Inches)

Rainfall Difference(Inches)

July 3.47 5.72 2.25August 4.11 1.29 -2.82

September 4.34 1.76 -2.58October 4.48 5.84 1.36

November 4.05 2.30 -1.75December 3.55 1.20 -2.35

* During the last 6-months of 2019 Lake Conroe received 5.89 inches less rainfall than normal. 35

Drought Monitor – January 16, 2020

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San Jacinto River Dredging• Dredging completed on August 30, 2019. Original contract removed

approximately 1.8 million cubic yards (CY). Contract modification removed additional 500,000 CY from mouth bar in Lake Houston area.

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• The CoH issued a Notice to Proceed for additional dredging in Lake Houston near WFSJR the week of January 6, 2020

• The total cost of this phase of the dredging effort is $40 million • City of Houston • Harris County Flood Control District • Texas Water Development Board

• Expected to remove additional 400,000 CY of material over the next 12 months

San Jacinto River Dredging

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Lake Houston Spillway

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Gates on Lake Houston Dam

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Other Mitigation ProjectsThe following mitigation projects were included in the Harris County Bond Election and may have a direct benefit on the Lake Houston Area:

• Multiple Channel Maintenance, Conveyance Improvements, and/or Restoration Projects

• Multiple Detention Basin Construction/Improvements Projects • Multiple Subdivision and General Drainage Improvements Projects • Multiple Right-of-Way Acquisition and Floodplain Preservation Projects • Spring Creek Reservoir Construction • Countywide Storm Repairs

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San Jacinto Regional Watershed Master Drainage Plan

• Develop H&H models that will help predict flood risks in the study areas

• Evaluate flood mitigation measures

• Evaluate sediment management strategies

• Analyze existing watershed conditions

• Identify flood mitigation projects that will potentially reduce the flood risk in the study area

• Study Goal: to identify vulnerabilities to flood hazards and develop a comprehensive flood mitigation plan 44

Schedule• Staff will continue to coordinate with the City of

Houston on their plans related to the release of the city’s water from Lake Conroe

• The SJRA Board will further consider this initiative on February 20th at 6:00pm at the Lone Star Convention Center

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Online Resources• KnowYourWatershed.com• SanJacinto.onerain.com• HarrisCountyfws.org• SanJacStudy.org

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Documents

Temporary Seasonal Lake Lowering...Houston, and the temporary Seasonal Lake Lowering initiative • Summary of the 2018 and 2019 Seasonal Lake Lowering • Status of downstream mitigation