Drainage infrastructure and hazards inventory

Elise Moore, P.E., CFM, Flood Control/Traffic Section Chief, Pinal County Department of Public Works, Florence, Arizona; Hernan Aristizabal, P.E., CFM, Director of Water Resources, and Chris Christensen, MAS-GIS, Senior GIS Analyst, Entellus, Inc., Phoenix, Arizona

Over the last several decades, Arizona has experienced tremendous growth, recording the second-highest statewide population growth rate in the nation, second only to Nevada in 2006. The majority of this growth has been concentrated between Prescott in the northwest through the Phoenix Metro area to Tucson in the southeast, an area referred to as the Sun Corridor. Centrally located within the Sun Corridor lies the (until recently) relatively undeveloped Pinal County.

Pinal County is a rural county of approximately 5,000 square miles, and in the year 2000, home to just under 180,000 people. By 2007, the County had surpassed 300,000 residents, an approximate seventy percent increase. This made Pinal County the fastest-growing county in the nation as determined by the U.S. Census Bureau. Much of the population increase can be attributed to development pressure from the Phoenix Metro area in the northwest and Tucson area in the southeast. In the midst of the housing boom, thousands of acres of desert rangeland and agricultural fields were transformed into urban development. This sudden increase in development was difficult for the County to handle, and it became apparent that as the washes and ditches of the desert and agricultural fields gave way to drainage channels and culverts of the new urbanization, it was increasingly critical to inventory the drainage infrastructure to get a better handle on the drainage system before it became unmanageable.

The County needed to know the extent of the existing drainage infrastructure and how this infrastructure affects the drainage patterns of the County. The County was only aware of the structures that they maintain, such as some culverts, bridges, and wet crossings along County roads. However, these County-maintained and owned structures constitute a small fraction of the total existing drainage infrastructure, the larger portion of which are either privately owned, or owned and maintained by other public agencies.

Schnepf Bridge over Queen Creek Wash, one of the many drainage structures inventoried as part of the Pinal County ADMP, experienced over ten feet of scour during a single storm event.

In order to accomplish this task, the County hired Entellus, Inc. to develop a comprehensive drainage facilities inventory (of particular interest were regionally significant structures) and a countywide Area Drainage Master Plan (ADMP). The goals of the inventory and ADMP were to gather information on both private and public agency structures, acquire a better understanding of current drainage conditions, and identify critical areas where improvement was needed. This would also help the County with enforcing the maintenance of drainage structures.

Given the rural nature and history of Pinal County, the majority of the pre-housing boom drainage structures were constructed in support of the ranching and farming operations. Ranchers constructed stockponds, small- to medium-sized impoundment areas utilized to capture surface runoff to water cattle, and farmers built intricate systems of canals and dikes to both irrigate and protect their crops. The latter has the most significant impact to the drainage system by potentially rerouting historic ephemeral flows. The railroads, mining companies, developers, ADOT and even the County themselves play a role in the history of Pinal drainage. Unfortunately, there is limited information concerning the majority of these drainage structures, and the accuracy of the information available is somewhat questionable.

Since the County was interested in the drainage system and not just the perceived drainage infrastructure, the question arose: What is a drainage structure? Though the answer seems obvious—dams, roadway crossings, drainage channels, etc.—other man-made structures that may not have been intended for drainage purposes often, by their very nature, become drainage structures by intercepting runoff or otherwise changing the course of the natural drainage pattern. Roadway and railroad embankments, irrigation canals, mining pits, fences and others all fall within this category. The ADMP identified all major man-made features that impact the natural drainage system regardless of ownership or original intent of the structure.

Due to the lack of information and questionable accuracy available for the drainage infrastructure, it was decided early on in the project that the drainage facilities inventory would primarily encompass field data collection. Data collection was not limited to type, size and shape of the structure, but incorporated conditions both structurally and of the surrounding area that would help the County prioritize maintenance of the inventoried structures. The facilities inventory consisted of three basic steps:

  1. Office identification of potential facilities
  2. Field collection of facilities
  3. Office check of the data collection

Field personnel utilize a tablet PC with GPS and a built-in camera to collect the facility inventory data. The operator records his observations on the GIS forms and takes photographs of the structures.

First, the approximate location of potential structures was identified in the office utilizing ArcMap. Aerial photos, hydrologic data generated utilizing ArcHydro, as-builts, design reports and other information assisted in the identification of these structures. Next, the potential locations were loaded onto the field tablet PC which is equipped with a GPS unit and field camera directly linked to ArcPad. Field personnel then locate the drainage structures, creating a feature in ArcPad and populate preset fields with information about the structure. Field photos were taken and linked directly to each structure. Field personnel are trained to find and identify drainage facilities that may not have been identified in the office. The final step in the facility inventory consists of an office check where an independent operator checked the field data against other available information (i.e., aerial photography, as-built drawings, design reports, etc.). The data is then either accepted or marked for additional field confirmation.

During the first two phases of this four-phase project, several thousand structures were identified, located and entered into the County's GIS system where the information is available to County personnel. Additionally, the data was converted into KLM format which can be viewed in Google Earth and is available to any user with Internet access. The facility inventory provides the County with a tool to schedule maintenance for their own facilities and to enforce maintenance requirements from other entities. However, the inventory of these facilities by itself does not adequately describe the County's drainage system. Additional information is required in order to understand the complete drainage system of the County. The ADMP also identified specific flood hazards and special consideration areas associated with both man-made structures as well as natural drainage conditions. Items of particular interest included ponding areas, areas downstream from embankments, split flow and uncertain flow path areas, sheet flow areas, alluvial fans, earth fissure locations and agricultural areas.

The ponding areas were developed based on physical evidence of ponding, while the areas downstream of embankments were identified to describe areas that would most severely be affected by an embankment structure failure. Sheet flow, split flow, alluvial fans, and fissures identify natural flow patterns and hazards that have unique characteristics and require special attention. Agricultural areas were also identified due to the significant effect that the irrigation infrastructure has on the natural drainage system, the amount of storage that can potentially be provided by these agricultural areas as well as the likelihood of increasing historical flows once these areas are developed and the fields are no longer available for stormwater storage.

Associated with these hazard areas, the ADMP developed a set of "rules of development" to assist the County and municipalities in guiding and regulating new development. These rules of development provide guidelines for analysis that will help protect the new development from the identified drainage hazards and minimize negative impacts to surrounding properties. The County now has a complete package consisting of a facilities inventory, identified drainage hazards and rules of development. These tools will assist the County in minimizing drainage problems, providing and enforcing maintenance on drainage infrastructure, and guiding the integration of future development into the County's drainage system.

Elise Moore can be reached at (520) 866-6638 or elise.moore@co.pinal.az.us; Hernan Aristizabal can be reached at (602) 244-2566 or ahernan@entellus.com; and Chris Christensen can be reached at (602) 244-2566 or cchristensen@entellus.com.

Entellus employs 60 professional engineers, designers, drafters, technical assistants and support staff. The firm is located in Phoenix, Arizona.