HCFCD watersheds:

ADDICS ACSON

ARMAND LITTLE CYRESS

BARER LUCE

BRAYS SAN ACINTO RIER

SAN ACINTO RIER BUFFALO GALESTON BAY

CARENTERS SIMS

CEDAR SRING CREE

SRING CREE CLEAR CREE GOOSE CREE

CYRESS CREE INCE

Davis Bayourinity River GREENS WHITE OA

aney reekLake reek Beason reekBraos River Briefing Document #2 East Fork San acinto River HUNTING WILLOW CREE Peach reekaney reek Frontal Lake Houston

rystal reekWest arkington BayouLuce Bayou How do Addicks &Fork SanBarker acinto River Reservoirs work? Pine Island Bayou New Year reekBraosAddicks Dam/Reservoir and Barker Dam/Reservoir are two federal flood control projects located in western Harris County that are designed to River store water during large rainfalls to reduce downstreamWalnut flooding reekSpring reek along Buffalo Bayou within the City of Houston.

lear reekBraos River

Whites Bayouurtle Bayou Frontal Lake Houston Watershed

Little ypress reek ypress reek Watershed ld Riverrinity River reens Bayou Watershed

reens Bayou Adlong Ditchedar Bayou Watershed

White ak Bayou ill reekBraos River Addicks Reservoir Watershed Halls Bayou

White ak BayouBuffalo Buffalo BayouSan ADDIS Bayou Watershed Hunting Bayou acinto River Watershed RESERVIR

Buffalo Bayou Barker Reservoir Watershed BARER East Fork Doule Bayou Bessies reekBraos River RESERVIR

Brays Bayou Vince Bayou Brays Bayou Watershed Upper yster reek Watershed Sims Bayou Sims Bayou Watershed

UpperFigure San 1 Watersheds impacting or impacted by Addicks and Barker (shaded in GREY) Bernard River lear reekFrontal alveston Bay Watershed History of Addicks and Barker Benefits of Addicks and Barker Resevoirs Big reekBraos River Watershed Addicks and Barker Dams were constructed between 1942 and 1948 Addicks and Barker Reservoirs and Dams hold water during large as part of a larger U.S. Army Corps of Engineers 1940 flood protection rainfall events, reducing the flow of water into Buffalo Bayou and plan for the City of Houston. At the time, around $4 million were spent on mitigating the risk of flooding along the bayou. ThisDickinson protects Bayou areas HCFCD ublicly announced projects construction of these two dams, including land acquisition for the dams of significant economic impact in Houston, including Downtown, scale 1:600,000 and for some of the land area behind the dams within the design flood pool Memorial, River Oaks, Washington Avenue, Montrose, Fourth of the reservoirs. The reservoirs were intended as part of a larger system Ward,ustang and Bayou the East End. Without the reservoirs, Harvey would 0 1mi 4mi 6mi 10mi 18mi iddle San Bernard River that was never implemented: likely have caused significantly more flooding in these areas.

1 A levee to prevent flood water from Cypress Creek iddlewatershed yster reek from Austin Bayou Halls Bayou overflowing into the Addicks Reservoir. 2 A reservoir on upper White Oak Bayou. 3 Bypass channels to direct high volumes of water being released from the dams out of Buffalo Bayou and around Houston. The two dams were redesigned and reconstructed in the 1980s to meet updated safety criteria for federal dams. Initially, these two dams and reservoirs were far from developed areas of Houston; however, today, development has occurred in and around both, Figure 2 Shows Congress and Milam intersection during a flood (1929) including thousands of single-family homes built within the reservoirs’ design flood pool for each dam, and within the pathway of floodwater releases from these dams along and outside the banks of Buffalo Bayou. The reservoirs are designed to hold more rainfall than a 1% event, so land upstream of the reservoir is considered “safe” because it is protected from flooding due to rainfall in a 100-year event. Therefore, FEMA does not map these areas within the 100-year floodplain, but in an event larger than the 100-year, they will flood. Similarly, during events larger than the 1%, dam releases can occur, flooding areas downstream, along Buffalo Bayou, that are not mapped in the FEMA 100-year floodplain. This scenario occurred during Hurricane Harvey. Figure 3 Shows Congress and Milam intersection during Harvey (2017) How do the dams/reservoirs work? Addicks and Barker capture and store excess rainwater that runs off of the land upstream from the dams, creating a flood control reservoir, or pool, of water for an extended period of time until the stored water can be released through outlet pipes or conduits located at the bottom of each dam. Where Buffalo Bayou starts, both dams have controlled outlet gates that allow outflow to be released at a desired rate. The flood control reservoirs hold floodwater and then slowly release through gated outlet pipes. The gates at the ends of the pipes can be incrementally opened to regulate the amount of water released from these dams so as to mitigate flooding downstream. Emergency spillways are located at the far ends of both dams. Emergency spillways are built to allow water to flow out of reservoirs during extreme conditions in order to prevent dam failures, similar to an emergency overflow drain on a bathtub or sink. Thus, as the reservoirs start filling up, at a certain level some of the water stored behind the dams can begin to flow over the spillways. The spillways all have neighborhoods and commercial developments downstream of them. The reservoir levels have reached high enough to spill over, and it is not known what flooding might result from an overflow.

ADDIS outlet gates Normal Flow RESERVIR outlet flow ULE TO OF DAM: 121’ controlled

ADDIS HAREY HIGH OINT : 109.1’ RESERVIR WAERSHED BOTTOM OF DAM: 70’ BUFFAL ADDIS CONTROLLED RELEASE OUTLET STURCTURE BAYU RESERVIR BARER BUFFAL/ RESERVIR WHIE A WAERSHED WAERSHED

BARER RESERVIR BARER outlet gates RESERVIR outlet flow BRAYS ULE TO OF DAM: 115’ WAERSHED controlled gate controlled spillway HAREY HIGH OINT : 101.5’

BOTTOM OF DAM: 70’ BUFFAL CONTROLLED RELEASE OUTLET STURCTURE BAYU ADDIS Harvey did not exceed emergency spillway levels RESERVIR At its highest point near the outlet structure, Addicks DamSPILLWAY is 121 feet above mean sea level, while Barker Dam is about 115 feet. The top of the dams slope downward to a ground level approximately 70uncontrolled feet above mean sea level, making the dams about 45-50 feet tall. For Addicks Dam, the spillways are at elevation 112 feet to 115 feet; for Barker, the spillways are at elevation 106 feet. Large areas behind each of the dams and within the reservoirs’ footprints are inundated when the dams are filling up. CONTROLLED RELEASE OUTLET STURCTURE ADDIS Harvey did not exceed emergency spillway levels Spillway Flow TO OF DAM @ OUTLET: 121’ RESERVIR top of dam SPILLWAY top of uncontrolled TO OF DAM @ SILLWAY: 112’-115’ spillway

ADDIS HAREY HIGH OINT : 109.1’ RESERVIR BUFFAL/ WAERSHED WHIE A BAYU BOTTOM OF DAM: 70’ WAERSHED

ADDIS RESERVIR BARER BUFFAL/ RESERVIR WHIE A WAERSHED WAERSHED

BARER RESERVIR BARER top of dam TO OF DAM @ OUTLET: 115’ RESERVIR top of BRAYS SPILLWAY spillway WAERSHED controlled gate uncontrolled TO OF DAM @ SILLWAY: 106’ spillway HAREY HIGH OINT : 101.5’ BRAYS BAYU WAERSHED BOTTOM OF DAM: 70’ Addicks and Barker During Harvey

Figure 4 North Addicks Before Harvey Figure 5 North Addicks After Harvey Upstream Flooding within the Reservoir Pool During Harvey, unprecedented amounts of rainfall caused water levels to reach 109.1 feet above mean sea level inside Addicks Reservoir and 101.5 feet inside Barker Reservoir, both of which were record pool levels. In Addicks, the water level was high enough to flow out of the reservoir, going around the northern spillway that meets up with the natural ground at 108 feet elevation. More than 10,000 homes located inside of these two reservoirs are estimated to have experienced some level of flooding during Hurricane Harvey. As the water level in these reservoirs began to rise significantly, water was released through the gated flood control structures at the outlets of each dam.

MA OSSIBLE OOL LEEL HAREY OOL FEDERAL GOERNMENT ROERTY TO OF DAM: 121’ TO OF SILLWAY: 112-115’ HAREY HIGH OINT : 109.1’

BOTTOM OF DAM: 70’ land outside federal govt property, but inside resevoir not to scale

federal government property

floodway

%1annual chance floodplain (100-year)

%0.2 annual chance Shown in Figure 4 and 5 floodplain (500-year)

extent of 109.1’ elevation pool levels from Harvey

Figure 6 Diagram of 109.1’ level pool in Addicks as a result of Harvey rainfall Hurricane Harvey Post Event Imagery - Texas Esri Disaster Response Program    Hurricane Harvey Post Event Imagery - Texas Esri Disaster Response Program   

Compare before and after photos of Hurricane Harvey Compare before and after photos of Hurricane Harvey

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

S Dairy Ashford Rd @ Buffalo Bayou S Dairy Ashford Rd @ Buffalo Bayou + + Imagery courtesy of NOAA Remote Sensing Imagery courtesy of NOAA Remote Sensing Division Division − −

Figure 7 South Dairy Ashford Road @ Buffalo Bayou - before controlled release Figure 8 South Dairy Ashford Road @ Buffalo Bayou - during controlled release

top of spillway Downstream Flooding due to Dam Releases In order to minimize upstream flooding, risks of spillway overflows from the reservoirs, and the risk of further rainfall exceeding the addic reservoir capacity, the Army Corps of Engineers released water at a s ele vation controlled rate through the outlet gates during and after Harvey. The combined amount of water released through the gates of both dams exceeded 15,000 cubic feet per second (cfs), well above the capacity of Buffalo Bayou, resulting in flooding in neighborhoods downstream of the dams. Such reservoir releases have never happened before. ddics Reservoir levation Feet Typically, combined reservoir releases have not exceeded 2,000 cfs. ug ug ug Sep. Sep. Sep. Sep. Neighborhoods between State Highway 6 and Beltway 8 were especially affected by the releases and an estimated 4,000 homes were flooded. top of spillway What is the status of the reservoir projects? New outlet gate structures will replace the existing ones at both Addicks and Barker Dams, expected completion in 2019. Like all dams, Addicks arer elevatio n and Barker are at some risk of failing, and failure could be catastrophic. More information about this can be found in later fact sheets. Risk of Future Dam Releases

arer Reservoir levation Feet Addicks and Barker Dams could again release large amounts of water downstream or flood homes upstream during extreme rain events like ug ug ug Sep. Sep. Sep. Sep. Harvey, or as a result of a number of smaller rains occurring over a short period of time. HARVEY RAINFALL Some solutions that have been suggested to avoid future releases rmy Corps release of water to clear reservoir include excavating and/or clearing vegetation in the reservoirs’ storage areas to increase their volumes, expanding Buffalo Bayou to increase top of an s its capacity and allow more flow, adding more flood control structures tre am e upstream, and additional regulations for upstream development to lev at ion reduce the amount of water entering these reservoirs.

uffalo ayou levation Feet KEY POLICY QUESTIONS ug ug ug Sep. Sep. Sep. Sep. Could a new regional flood management strategy help use our Figure 6 Stream gages 08/12/17-09/30 current structures to their full ability and help target future funds For more information visit for improvements? HCFCD on Flooding Impacts with the Reservoirs: Could regional policy restricting development inside the reservoir https://www.hcfcd.org/hurricane-harvey/flooding-impacts-in-connection-with-the- reservoirs/ pool or overflow path help prevent damages in future events? USGS Stream Gages: Could a policy requiring better water storage for developments https://nwis.waterdata.usgs.gov/usa/nwis/uv/ lessen the demand for new storage structures in the future, or Greater Houston Flood Mitigation Consortium: even prevent flooding in areas not near the reservoirs? http://houstonconsortium.com