2.0 Overview of Existing Stormwater Drainage System
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El Paso Water Utilities El Paso Stormwater Master Plan The City of El Paso Section 2.0 - Overview of Existing Stormwater Drainage System 2.0 OVERVIEW OF EXISTING STORMWATER DRAINAGE SYSTEM El Paso has an arid, warm climate with hot, low-humidity summers and mild, dry winters. Average daily temperatures range from a high of 55 to a low of 33 degrees Fahrenheit (ºF) in January and a high of 97ºF to a low of 72ºF in July. The mean annual precipitation is approximately eight inches with most of it occurring during July through September. Most of the rain in the El Paso area consists of afternoon thunderstorms caused by the monsoonal flow from the Gulf of California. These thunderstorms are high intensity with short duration. The Franklin Mountains dominate the skyline of El Paso and range approximately 16 miles long and 5 miles wide with a general relief of over 3,000 feet above the surrounding area. The Franklin Mountains are the southernmost extension of a series of north-south ranges across the western United States. They are comprised of pre-Cambrian to Pleistocene-age rocks and are locally overlain by recent sediment. The rocks consist of granite, rhyolite, marble, quartzite, sandstone, limestone, dolomite, conglomerates, colluvium, and alluvium. The Franklin Mountains are characterized by a single range of knife-edge ridges that are uplifted bedding dipping steeply to the east. The central core of the mountains is comprised of vast exposures of bedrock with little sediment accumulation or soil formation. The major canyons, from north to south, that flow toward the eastern face of the range are Hitt, Fusselman, and McKelligon and on the western face of the range, the major canyons are Vinton and Avispa. For the purpose of the SMP, the master plan study area was divided into six regional study areas: • Central; • East Side; • Mission Valley; • Northeast; • Northwest; and • West Central. These study areas were delineated based on the contributing watersheds to unique geographical areas. The regional division of the study area is shown on Figure 2-1 and extends beyond the incorporated limits into the extraterritorial jurisdictional limits. The study areas were further divided into “systems” based on clearly identified needs, historical flooding problems, structural or systematic deficiencies, and potential solutions. The study areas and systems are discussed in detail in this section. Page 2-1 March 2009 El Paso Water Utilities El Paso Stormwater Master Plan The City of El Paso Section 2.0 - Overview of Existing Stormwater Drainage System 2.1 Central Region The Central Region, shown on Figure 2-2, encompasses most of the downtown area and some of the City’s oldest and most historic neighborhoods. This area of the City is fully developed up to the foothills of the Franklin Mountains. Stormwater runoff is conveyed to the Rio Grande through a series of underground conduits. The Central Region is characterized by two different drainage patterns and land types: the Franklin Mountains and the valley. The Central Region consists of three systems: Government Hills System, Cebada System, and Dallas System. The Central Region Drainage System consists of 12 dams, 3 pump stations, 20 stormwater ponds, and 35 miles of identified conveyance systems. Historically, the Central Region suffers from frequent flooding of the downtown area and segments of Interstate Highway 10 (IH-10). There is insufficient detention upstream of the developed areas and inadequate conveyance through the downtown area to prevent flooding. Due to the flat terrain of the valley floor, gravity drainage to the Rio Grande is not always an option. The downtown area is fully developed and space is not readily available for pump station basin enlargements or new pump station basin placements. 2.2 East Side Region The East Side Region, shown on Figure 2-3, is the area just south of the Fort Bliss Military Reservation, north of IH-10 and east of Robert E. Lee Road. The East Side Region is one of the largest areas of town consisting primarily of middle class neighborhoods and is one of the fastest growing. The existing drainage infrastructure for this area primarily consists of a significant number of retention and detention basins. The East Side Region is characterized by three different drainage patterns and land types: the high mesa comprising the majority of the watershed in the upper portion of the region, the escarpment zone between the high mesa and the Rio Grande Valley, and the IH-10 corridor. The high mesa is relatively flat and the primary method for controlling stormwater in this area is through localized ponds. The East Side Region is divided into four systems: Phelps Dodge System, Lomaland System, Americas Basin System, and Americas Ten Basin System. The East Side Region contains one dam and nine open channels. Older development areas utilized large regional basins to address stormwater concerns. However, because the terrain is flat, any flow that has to travel a great distance to a basin or any flow that overtops a basin results in a flooding problem within the local streets. The escarpment zone is where the terrain drops off from the high mesa towards the Rio Grande. The steeper slopes result in higher stormwater runoff velocities, which increase the erosion potential. Page 2-2 March 2009 El Paso Water Utilities El Paso Stormwater Master Plan The City of El Paso Section 2.0 - Overview of Existing Stormwater Drainage System IH-10 is located about midway down the escarpment zone. IH-10 was constructed in the mid 1950s. Since then, adjacent properties have developed into the current commercial and industrial land uses seen today. Surface runoff from these commercial and industrial areas has resulted in large surface flows within the IH-10 corridor. 2.3 Mission Valley Region The Mission Valley Region, shown on Figure 2-4, is considered the oldest settled area of El Paso, dating back to the late 17th century. This region is bounded by the Rio Grande and IH-10 and lies just east, and south of the Central and East Side Regions, respectively. This area of town has extremely flat topography and a variety of land uses, from agricultural to industrial. The existing predominant drainage features are irrigation drains that parallel the Rio Grande. The Mission Valley Region is downstream of IH-10 and consists of the escarpment zone and the lower valley. The lower valley is characterized by relatively flat terrain, is impacted by discharges from the East Side, and experiences some of the same drainage problems as the Central downtown area. The Mission Valley Region has been divided into three systems: Basin A System, Basin G System, and Mesa Drain System. The Mission Valley drainage features consist of 71 ponds, 8 pump stations, 8 dams, and 49 miles of identified drainage conveyance systems. Due to the flat terrain of the valley floor, gravity flow to the Rio Grande is not always an option. The agricultural drains, which are included as part of the stormwater conveyance system, are sometimes elevated in relation to the adjacent area. Agricultural feeder canals are always elevated in relation to the adjacent area. This potentially results in the feeder canals and drains acting as impediments to the flow and causes flooding problems upstream adjacent to the canal/drain. Flat slopes result in low velocities and accumulating sediment thereby reducing system performance. 2.4 Northeast Region The Northeast Region, shown on Figure 2-5, is north of the Central Region and east of the Franklin Mountains and has seen an increase in development in recent years. This region includes all watersheds that contribute flow to Pershing Dam. This area of the City consists of older development transitioning to newer development as it progresses northward. Most of the City dams fall within this region. The Northeast Region is characterized by three different drainage patterns and land types: the Franklin Mountains and undeveloped and developed alluvial fan. The Northeast Region has been divided into three systems: Fort Bliss Sump System, Northeast Ponding System, and Range Dam System. The Northeast drainage features consist of 20 ponds, 14 dams, 1 pump station, and 32 miles of open channel. Runoff from the Franklin Mountains acquires large amounts of sediment and debris as it flows down the mountains and hits the undeveloped alluvial fan. Due to the generally Page 2-3 March 2009 El Paso Water Utilities El Paso Stormwater Master Plan The City of El Paso Section 2.0 - Overview of Existing Stormwater Drainage System steep slopes, the runoff has high velocity with an even greater erosive potential. Lack of sediment debris controls and inadequate capacity slows the water enough to drop much of the sediment within the system. This results in the existing system plugging with debris and sediment, which further limits the conveyance capacity. The existing dams reduce the amount of flow experienced by the lower urban areas. However, they are unable to control all the flow coming from the Franklin Mountains. The runoff that makes it to the urban areas is conveyed down to Pershing Dam through a series of east-west and north-south concrete-lined channels. These channels lie within fully developed regions of the City and have limited space for improvements, such as channel widening. 2.5 Northwest Region The Northwest Region, as shown on Figure 2-6, is west of the Franklin Mountains and includes the Upper Valley. Along with the East side, the Northwest Region is one of the fastest growing areas of the City. This region also experienced most of the flood damage associated with Storm 2006. The existing stormwater features in this region include natural arroyos, man-made channels, ponds, dams, gravity drains, and pump stations.