Harris County is a diverse metropolitan area that includes a variety of land use types including residential, urban, industrial and undeveloped natural areas. Each type of land use has a unique interaction with stormwater (rain water plus surface water runoff) each resulting stormwater pathway represents different opportunities for stormwater management.
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The City of Houston, Texas
In the flat coastal Gulf Prairies and Marshes ecoregion, spring rains are abundant. The spring of 2015, inundated with heavy rainfall, was no exception. Average monthly rainfall for six rain gauges throughout Harris County show that from March through May, a total of 28 inches of rain was recorded. 40 inches of rainfall was recorded for the remaining nine months of the year. That means 41% of the 2015 total annual rainfall fell in just three months.
Stormwater = rain water plus surface water runoff
Harris County rain gauges
The impact of rainfall on water quality depends on the amount of rainfall, the number of days since the last rainfall, and the moisture content of the ground cover and sediment. As the number of days without rainfall increases, there is more time for pollutants to accumulate on the land in the watershed. The next heavy rainfall can wash these pollutants into creeks, bayous, lakes and rivers - known as the first flush effect.
The watershed connection
A watershed is an area of land that drains into a stream, marsh, bayou, river, lake or ocean. Watersheds can be small or large and each watershed can be part of a larger watershed. Everyone lives in a watershed. The overall function and health of the watershed can be assessed by analyzing the water quality of water bodies within the watershed.
Comprised of twenty-one watersheds, the Houston-Galveston region is a complex network of bayous, rivers and drainage channels. Activities occurring on land in the watershed can affect water quality due to surface runoff.
Of particular importance to this story are the Clear Creek and Armand Bayou watersheds.
The Clear Creek Watershed
More than 328,000 residents work, live and play in the watershed at locations such as the Lyndon B. Johnson Space Center, Kemah Boardwalk, Nassau Bay Peninsula Wildlife park and the Ghirardi Family WaterSmart park. 59% of the watershed is classified as Developed Land Cover while 6% of the watershed is classified as Freshwaterand Saltwater Wetlands.
Scrolling reveals land cover changes between 1996 and 2010.
The Armand Bayou Watershed
Vividly named waterbodies such as Mud Lake, Big Island Slough and Horsepen Bayou are located in the Armand Bayou watershed, which contains 127,300 residents. The Armand Bayou Nature Center provides recreational opportunities and is part of the 4,850 acres of wetlands in the watershed. 67% of the watershed is Developed Land Cover while 8.2% of the watershed is classified as Freshwateror Saltwater Wetlands.
Scrolling reveals land cover changes between 1996 and 2010.
Stormwater drainage pathways
Stormwater drainage pathways range from the natural (bayous, rivers and wetlands) to developed (channelized bayous, storm drains and stormwater Best Management Practices (BMPs)). Some stormwater pathways filter stormwater pollutants while supporting aquatic life, wildlife and recreation.
Creeks, rivers and bayous help drain the land during rainfall events and prevent flooding. These natural pathways also remove pollutants from stormwater because rain water often flows through a riparian corridor (the floodplain next to the stream) or other vegetation that provides filtration.
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Armand Bayou, Trinity River and Wetlands
The pollutants carried via stormwater runoff can include pet and wildlife waste (bacteria), fertilizers and detergents (nutrients), oil and pesticides (organic toxins), trash and litter, sediment, and chemicals from commercial and industrial sites.
To storm drain
Storm drains are installed to prevent flooding by draining stormwater from impervious surfaces such as roads, parking lots and roof tops. Storm drainage infrastructure's primary purpose is to funnel excess water away from homes and business as quickly as possible. Stormwater remains untreated and flows directly into rivers, bayous, or creeks and, ultimately, Galveston Bay.
Concrete infrastructure does not provide the filtration and infiltration that vegetated surfaces do to provide stormwater pollutant reduction.
To wetlands
A number of pathways release stormwater into wetlands. Wetlands receive flow from stormwater outfalls and sewers, bayous and creeks as well as overland runoff. Wetlands perform vital ecosystem services, such as reducing the amount of damage caused by flooding while providing green space to recreate and revitalize. They serve as natural stormwater storage areas and provide filtration for stormwater. However, they have a limited capacity to reduce stormwater pollutants and control flooding.
Coastal wetland
The ability of wetlands to process stormwater is enhanced when the stormwater first flows through natural channels that begin reducing stormwater pollutants by allowing sediments to settle out of the water column, promoting infiltration into groundwater and serving as vegetated riparian corridors to filter nutrients and bacteria. The network of connections makes the system successful.
To natural waterways
Houston is often referred to as the bayou city due to the large network of meandering bayous, several of which flow through the downtown district. Although some of the natural bayous such as Greens Bayou have been channelized and lined with concrete , many creeks and tributaries remain in their natural state. These natural waterways provide drainage, reduce water quality pollutants, and provide habitat for wildlife.
Armand Bayou
The best practice to reduce impacts to water quality from stormwater and reduce pressure from increasing development is to preserve natural waterways that have intact native riparian corridors.
Natural waterways can help to mitigate flooding. Vegetation, soil, and wetlands have large natural storage capacities that can slow rising water levels. In addition, vegetation helps to prevent erosion and reduce water quality pollution. As surrounding land use shifts farther from pre-development conditions and the amount of impervious surface increases in a watershed, more runoff is created and less filtration and infiltration occurs.
Vegetation and living organisms found in soil can filter bacteria, sediment and nutrients from stormwater. Vegetated waterways reduce costs for drinking water plants to purify water for human consumption and provide the added bonus of supporting recreation and wildlife.
To stormwater best management practices
Stormwater Best Management Practices (BMPs) are structural and nonstructural constructed landscape features such as wet and dry bottom detention ponds, bioswales and wetland basins. To manage stormwater The Harris County Flood Control District develops, installs and maintains strategic BMPs that reduce damage caused by flood events and improve stormwater quality. It is this cumulative BMP network that creates a county-wide tool to reduce nonpoint source stormwater pollution and detain excess stormwater resulting from periods of intense rainfall.
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Armand Bayou Stormwater Treatment Wetlands
Harris County maintains structural stormwater BMPs and a number of detention ponds that serve a growing population. Installations range from Riparian Wetland Channels and Open Water Wetland Basins to devices that control floatables (trash and debris). To supplement and support the wet bottom detention ponds, dry bottom detention ponds have been dispersed throughout the County.
Armand Bayou Wetland Basin
2006
The Armand Bayou wetland basin is located in Pasadena, Texas within the Armand Bayou watershed. Construction on the basin began in 2006. The basin was primarily designed to reduce flood risks but also serves to improve water quality by removing and preventing sediment and bacteria from entering Armand Bayou.
2008
The two structural wetland basins (east and west) with open water surfaces began to take shape. The basins are separated by Armand Bayou, which provides inflows during normal and storm conditions. The basins have a direct drainage area of 3,493 acres, of which 43% is impervious land cover.
2010
A permanent pool of water contains stormwater from the previous storm, which is flushed out and refilled during large storm events. The east basin, larger than the west, has 8.6 acres of surface area and holds 35.5 acre feet of water. The west basin's surface area is 4.8 acres and holds 15.6 acre feet of water.
2016
During storm conditions, the wetland basins serve to detain stormwater runoff and improve water quality. Microorganisms, algae growth and wetland vegetation facilitate the removal of dissolved pollutants, bacteria and nutrients.
Select map points for a virtual tour.
Clear Creek Dry Bottom Detention Pond
Located in the Clear Creek watershed at Dixie Farm Road this BMP features a tree lined riparian channel. The basin was designed to slow peak flow and only holds water for a short time after a rain event. In general dry basins are designed to discharge the captured stormwater at a reduced flow rate over a set amount of time, normally 24-48 hours. Fewer dry basins are being constructed in favor of wet detention basins that offer a variety of additional environmental benefits.
Clear Creek Dry Bottom Detention Pond
The primary method of bacteria removal in detention basins is settling and sedimentation. The total pollutant reduction depends on the pond length and detention period of the runoff. The period of detention also correlates with the reduction of bacteria concentrations through natural decay, thermal death, exposure to solar radiation, competition and predation. The longer the period of detention, the more bacteria are removed from stormwater.
In partnership with the United States Geologic Survey (USGS) HARC sampled the inflow/outflow and upstream/downstream of the Clear Creek Detention Basin and the Armand Bayou Wetland Basin from March of 2014 to May of 2015.