History of the Pittman Wash
The Pittman Wash is the main path for runoff from storm events in the south and west area of the Las Vegas Valley to be conveyed to the Las Vegas Wash. The contributing watershed extends from the McCullough Range to where the Pittman Wash crosses Boulder Highway at Russell Road, is located within portions of the City and unincorporated Clark County, and is a major tributary to the Pittman, Duck Creek, and Lower Las Vegas Washes. The total drainage area of Pittman Watershed is approximately 121 square miles and flood control facilities within the watershed consist primarily of detention basins connected by conveyance facilities. The City is responsible for the construction, inspection, operation, and maintenance of local and regional flood control facilities within it’s the jurisdictional boundaries. The reach of the Pittman Wash from Pecos Road to the Arroyo Grande Sports Complex is the subject of the current discussion between the public and City. The approximately 2-mile reach is formally identified by the Clark County Regional Flood Control District (CCRFCD) Las Vegas Valley 2018 Flood Control Master Plan Update (2018 MPU) with assigned ID miles PTWA 0275 through 0091. The design flow of the reach is 6700 cubic feet per second, or over 3 million gallons a minute, that is based on the 100-year flood (1 percent chance of annual occurrence) that FEMA has adopted as the base flood for floodplain management purposes. With these significant flows the conveyance capacity and efficiency of the channel may not be compromised.
In pre-development conditions the Pittman Wash was a sparsely vegetated ephemeral arroyo with shallow pockets of surfacing groundwater. Historical aerial photography and drainage studies for development along the reach, shows that the vegetative cover was semiarid rangelands consisting of desert shrub in poor condition with a plant density of less than 30%. The existing reach consists of engineered gabion lined channel from Pecos Road to Green Valley Parkway, natural channel from Green Valley Parkway to the Union Pacific Railroad Bridge, and reinforced concrete arch culvert from the UPRR Bridge to Arroyo Grande Boulevard. Vegetative cover has increased in density, height, type, and size to significantly exceed the pre-development conditions. This is due over-irrigation resulting in a constant flow of water in the wash, the spread of non-native landscape and invasive species with no natural biological controls, and the unnatural collection and seeding of plants. The City owns and operates a large-diameter sanitary sewer main line and numerous lateral pipes that conveys wastewater from a majority of the residents and businesses located between the I-215 Beltway and Warm Springs Road to the lowest elevation of the service area, which is the Pittman Wash. The sanitary sewer main was installed in the late 1980’s as part of the residential development in the Green Valley area and included an access road to maintain system, with the lateral pipe installed as part of the residential and commercial development. Since the 1990’s the Pittman Wash, sanitary sewer system, and corresponding access road has been adversely impacted by increased recurring flood events due to the urbanized watershed, the introduction of invasive plant species that has resulted in increased density and height of vegetation, increased water level elevations due to the overgrowth in vegetation, and the recent surfacing of groundwater as a result of over-irrigation. Below is a brief history of events and corresponding actions:
- The manholes along the sanitary sewer main were compromised during a major storm event in the late 1990’s allowing in rock and sediment to block the pipe resulting in a sanitary sewer overflow (SSO) that discharged raw sewage into the wash.The City expended considerable resources to clean the debris from the sanitary sewer main.
- A reach of sewer main between Valle Verde Drive and the UPRR Bridge was unearthed in a large storm event in the early 2000’s and a portion of the pipe washed away resulting in a large SSO discharging raw sewage into the wash and the downstream pipe again being filled with sand and rocks. The City expended considerable resources to control the SSO during the event, complete emergency repairs to the sanitary sewer system and armor the wash in the impacted reach, was ultimately fined by the Nevada Division of Environmental Protection (NDEP) for the discharge, and was required by NDEP to conduct a system-wide evaluation of the entire sanitary sewer system owned and operated by the City.
- The Sewer Vulnerability Report resulted in the identification of sanitary sewer pipe throughout the City vulnerable to impacts from floods and other emergencies, provided a ranking scale ranging from most to least vulnerable, and a schedule for mitigating the vulnerabilities.The report found that the reach of Pittman Wash between Green Valley Parkway and Arroyo Grande Boulevard is subject to significant erosional forces that have resulted in the bed elevation being decreased in excess of 10-feet in some locations resulting in several projects to protect the sanitary sewer system within the Pittman Wash:
- Reaches of the sanitary sewer main and the lateral pipes between Green Valley Parkway and UPRR Bridge were armored with concrete caps, and a concrete access road and stems walls.
- The sanitary sewer main was lined to prevent groundwater infiltration into the system and to provide structural stability of the pipe.
- Sheet piles were constructed in the reach between Valle Verde Drive and the UPRR Bridge to protect the sanitary sewer main from erosional forces.
- A new manhole was constructed and the sanitary sewer main realigned immediately upstream of the Arroyo Grande bridge to pipe around a large blockage of boulders in the pipe that could not be removed.
- A reinforced concrete arch culvert was constructed in the reach between the UPRR Bridge and Arroyo Grande Boulevard.
- Between 2011 and 2015, the City did not revisit the sanitary sewer main to investigate any of the structural conditions nor conducted any maintenance.Unfortunately, this proved to be too long of a period and in January 2015 there was an SSO west of Green Valley Pkwy that discharged into the Pittman Wash.The City is re-evaluating the maintenance program for the sanitary sewer system in the Pittman Wash to mitigate future environmental concerns.
- In December 2015, the City found a broken sewer line in Ocotillo Point Terrace which connects into the sanitary sewer main.Further investigation found debris had settled in the sanitary sewer main that needed to be cleaned to prevent an overflow.The City re-established the access road to repair the sewer and clean debris from inside the main pipe.
- Between 2015 and 2017 the City repaired significant erosion behind the abutments and under the vehicle travel lanes on the Green Valley Parkway Bridge, repaired significant erosion along the south bank of the wash between Wigwam Parkway and Pecos Road, and repaired significant bank erosion and installed riprap lining between Green Valley Parkway and Valle Verde adjacent to La Mancha Estates.The City re-established the access road as necessary to repair the project areas of the wash.
- In 2019 the City conducted vegetation control activities in the Pittman Wash between the confluence with the Pecos-Legacy Channel west of Green Valley Parkway and Valle Verde Drive in response to complaints from concerned citizens regarding rats and fire danger related to the overgrowth of vegetation.During the activities a stand of mesquite trees were removed to unclog the storm drain pipe discharging at Ocotillo Pointe Terrace, and all trees rooted in the gabion baskets west of Green Valley parkway were removed to prevent further damage to the channel lining and reduce the water surface elevation at the bridge to prevent further scouring behind the abutments.
- The Southern Nevada Health District has been conducting mosquito monitoring and surveillance in the Las Vegas Valley since 2004.Mosquito monitoring reports available on the SNHD website show that mosquitoes in the 89014 and 89074 zip codes that encompass the Pittman Wash have tested positive for West Nile Virus and St. Louis Encephalitis in 2016 and 2017.The Mosquito Surveillance Program conducts ongoing surveillance for the Aedes albopictus or Aedes aegypti mosquitoes known to carry Zika virus as well as chikungunya, and dengue.The SNHD detected Aedes aegypti mosquitoes in Southern Nevada in 2017.
- The Southern Nevada Health District has been conducting rat monitoring and surveillance in the Las Vegas Valley since 2003.During this time the SNHD has set live capture traps at homes from 35 zip codes and trapped nearly 200 Rattus rattus (roof rats) and several desert wood rats. These animals were combed for fleas, and blood samples were collected for plague and Hantavirus testing; all results were negative.