@SanJacintoRiverAuthority @SJRA_1937 Temporary Seasonal Lake - PowerPoint PPT Presentation

@SanJacintoRiverAuthority @SJRA_1937

Temporary Seasonal Lake Lowering

Overview of the January Presentation • Upper Watershed & Volumes • Lake Conroe Facts/Gate Operations • Current Initiative/Watershed Map • Pre-release • Lake Conroe/Lake Houston Distance • Seasonal Lowering Initiative Engineering Studies • Lake Conroe Data – Lake Level and Rainfall • Fall 2018/Spring and Fall 2019 Summary • Downstream Projects 3

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August 31st Historical Lake Conroe Lake Levels 202 201.02 200.75 201 200.53 200.5 200.4 200.36 200.36 200.23 200.09 200.08 199.87 199.82 200 199.71 199 198.42 LAKE LEVEL (FEET MSL) 198 197.68 197.64 197.09 197 196.3 196 195 194 193 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 YEAR 7

Important Considerations SJRA has coordinated with staff from the City of Houston • and the Texas Division of Emergency Management in developing this recommendation. Recent legislation related to flood funding (SB 7 by • Creighton) and flood planning (SB 8 by Perry) emphasize and prioritize regional cooperation. Coordination and cooperative effort will be absolutely critical for future flood mitigation efforts. 8

Important Considerations All water released from Lake Conroe as part of this • initiative is being accounted for from the City of Houston’s 2/3 share and reported to TCEQ by the City of Houston. The action taken by the SJRA Board tonight will be a • recommendation to the City of Houston. It is their water in Lake Conroe that is being diverted under this initiative; therefore, it is ultimately their decision. 9

Primary Objective Create a modified seasonal lake lowering initiative that still benefits downstream communities while achieving the following objectives: 1. Provide a more strategic location for flood mitigation storage 2. Acknowledge the mitigation projects accomplished to date 3. Consider additional data from actual rainfall events that occurred in the Fall seasonal lowering months 4. Reduce the negative impact to water supplies 10

1 - More Strategic Mitigation Storage Seasonal storage in Lake Houston provides multiple benefits to reduce flood risks: • Lake Houston is currently at 41.5’ msl (one foot below conservation pool) to allow for repairs on the dam (creating approximately 11,000 acre-ft of storage capacity). • Provides a risk reduction to the Lake Houston area for rainfall events that do not occur directly over the Lake Conroe Watershed. • Additional capacity in Lake Houston creates space for nearby storm water infrastructure to drain some initial runoff. 11

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Tropical Storm Imelda 2019 13

Tax Day Event 2016 15” 14 14

1 - More Strategic Mitigation Storage Seasonal storage in Lake Houston provides multiple benefits to reduce flood risks: • Lake Houston is currently at 41.5’ msl (one foot below conservation pool) to allow for repairs on the dam (creating approximately 11,000 acre-ft of storage capacity). • Provides a risk reduction to the Lake Houston area for rainfall events that do not occur directly over the Lake Conroe Watershed. • Additional capacity in Lake Houston creates space for nearby storm water infrastructure to drain some initial runoff. 15

2 - Acknowledge Completed Mitigation Projects Completed projects have restored some downstream conveyance capacity in the San Jacinto River. • 2.3 million cubic yards of sand has been removed from the mouth of the San Jacinto River. Enough sand to fill the Astrodome 1.5 times. • Additional dredging is in progress and will continue throughout 2020. 16

3 - Data From Actual Rainfall Events A review of historic rainfall data and corresponding lake rise suggests less than 2-feet of storage is adequate to catch most storm events at Lake Conroe that occur in the fall. • Only two rainfall events that occurred in the months of August and September since 1999 (excluding 2017) resulted in more than one-foot of rise in Lake Conroe. • More than 90% of these events resulted in less than 3- inches of rise in Lake Conroe. • Five named tropical storms in this same period resulted in less than 12-inches of rise. 17

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4 - Reduce Negative Impact to Water Supplies Minimizing the amount of stored water that is released from Lake Conroe will benefit regional water supplies. • Best practice is to store water supplies as high as possible in the basin. • With a drainage basin that covers approximately 3,000 square miles, it is easy for Lake Houston to recover after seasonal lowering or pre-releasing with even a small rainfall event. • The Upper San Jacinto River Basin is currently in Moderate Drought (https://droughtmonitor.unl.edu/). 19

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Recommended Regional Mitigation Strategy (Spring) Lake Houston: • Lake Houston is currently at 41.5’ msl (one foot below conservation pool) creating • approximately 11,000 acre-ft of storage capacity SJRA coordinate with Houston on potential for additional release of water from Lake • Houston when a significant storm is in the forecast Increase in mitigation benefit as compared to 2019 Spring strategy • Lake Conroe: • Seasonally lower to 200’ msl (approx. 20,000 acre-ft of storage) • Begin seasonal lowering on April 1 st • Begin recapturing flows on June 1 st • No change from the 2019 Spring strategy • 21

Recommended Regional Mitigation Strategy (Fall) Lake Houston: • Lake Houston is currently at 41.5’ msl (one foot below conservation pool) creating • approximately 11,000 acre-ft of storage capacity SJRA coordinate with Houston on potential for additional release of water from • Lake Houston when a significant storm is in the forecast Increase in mitigation benefit as compared to 2019 Fall strategy • Lake Conroe: • Seasonally lower to 200’ msl (approx. 20,000 acre-ft of storage) • Begin seasonal lowering on August 1 st • Begin recapturing flows October 1 st • One-foot reduction from 2019 Fall strategy • 22

City of Houston Recommendation Spring: • Seasonally lower Lake Conroe to 200’ msl • Begin seasonal lowering on April 1 st • Begin Recapturing flows on June 1 st • No change from the 2019 Spring strategy • Fall: • Seasonally lower Lake Conroe to 199.5’ msl • Begin seasonal lowering on August 1 st • Begin recapturing flows on October 1 st • Additional lowering to 199’ msl if named storm within five-day forecast • Continue until completion of Lake Houston gate expansion project • 24

@SanJacintoRiverAuthority @SJRA_1937

Board Action Requested Direct staff regarding recommendation to City of Houston Acknowledge that water released for seasonal lowering • belongs to City, and diversions are ultimately their decision Acknowledge that City can call for diversions, including • prior to a named storm (SJRA does not recommend releasing water prior to a storm) Require City to notify SJRA in writing when to make • diversions of City water and the desired quantity 26

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Total Payouts for Lake Conroe Area & Lake Houston/Kingwood Area (Without 2017) $29,000,000.00 $28,000,000.00 $27,000,000.00 $26,000,000.00 $25,000,000.00 $24,000,000.00 $23,000,000.00 $22,000,000.00 $21,000,000.00 $20,000,000.00 $19,000,000.00 $18,000,000.00 $17,000,000.00 $16,000,000.00 $15,000,000.00 $14,000,000.00 $13,000,000.00 $12,000,000.00 $11,000,000.00 $10,000,000.00 $9,000,000.00 $8,000,000.00 $7,000,000.00 $6,000,000.00 $5,000,000.00 $4,000,000.00 $3,000,000.00 $2,000,000.00 $1,000,000.00 $- 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2018 2019 LC Total LH Total

Entire Upper Watershed: 2,800 square miles Lake Conroe Watershed: 450 square miles 2 9

100-YR (ATLAS 14) Volume Percentages Lake Houston Inflow Volume= 2.0 Million ac-ft West Fork Spring Creek 15.8% 17.4% East Fork Lake Conroe 13.6% 14.0% Luce Bayou 7.8% Caney Cypress Creek Lake Creek Creek 7.8% 10.3% Peach Creek 7.7% 5.7% 3 0

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 3 1

Facts about Lake Conroe • Constructed as a water supply reservoir – not intended to be a “constant level lake” • 1/3 rd of permitted water belongs to SJRA • 2/3 rd of permitted water belongs to City of Houston • City of Houston pays 2/3 rd 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 3 2

Key Points regarding Gate Operations • Tainter gates must be raised as lake rises • Cannot allow over-topping as it compromises the structure. Top of Tainter Gate Top of Water Surface 18” Freeboard 3 Downstream side of Gate Upstream side of Gate 3