Monday, October 18, 2010

The Manhole Mystery at Fort Sam Houston

by Paula Robertson, Member

Ed.: A different version of this article was previously published (pp. 14-15) in the September/October 2010 issue of Public Works Digest.

Have you ever wondered what you would find under the lid of a manhole? While hardly noticeable, embedded in the streets that we drive over, sanitary sewer manholes provide more than an inconspicuous clue to the utility system concealed beneath. They are the only accessible means to begin exploration of a complex system that is vital to the health of troops and civilians who embody the missions at our nation’s military installations.

The Base Realignment and Closure Act (BRAC) of 2005 recommended the expansion of Fort Sam Houston in San Antonio, Texas, to become the premier medical training complex for the U.S. Department of Defense. The consolidation and relocation of multiple service branches’ medical training activities to Fort Sam Houston would mean a significant increase in on-site population, greatly expanded missions, and improved medical care to the military community. As part of an initiative to analyze the current infrastructure and its capacity to support the expansion, in 2007 the Fort Sam Houston Directorate of Public Works (FSH DPW) contracted the U.S. Army Corps of Engineers, Fort Worth District, to provide a comprehensive evaluation of the water utilities at Fort Sam Houston.

The Comprehensive Infrastructure Studies included storm water, water distribution, and wastewater collection system assessments. As technical editor of the reports that documented the findings and recommendations for two of these studies, the most interesting to me was the Fort Sam Houston Wastewater Collection System Study, which delved into the mysterious underworld of the sanitary sewer system.

For the Wastewater Collection System Study, Fort Worth District engineers conducted a Sanitary Sewer Evaluation Survey (SSES). The typical SSES includes the following component tasks:
  • Assessment of the lift stations, which pump wastewater uphill to the collection system of pipes, or mainlines
  • Manhole inventory and assessment
  • Wastewater flow monitoring
  • Mainline inventory and assessment
  • GPS surveying, to update or create an accurate GIS map of the collection system components
  • Development and analysis of a Hydraulic Model of the collection system, based on all the data collected in previous tasks
Wastewater flow monitoring is one of the early activities of the SSES, typically done while collecting the manhole inventory data. Thus we begin to uncover the mystery, peering straight into a manhole and even crawling down inside, thanks to the images captured by our third-party contractors.

Go with the Flow Monitoring

The Fort Sam Houston Wastewater Collection System consists of roughly 232,000 linear feet of gravity sewer mainline and 1,060 manholes. Much of the system was built in the 1930s and has far exceeded its 50-year design life. As stated in the Fort Sam Houston Sanitary Sewer Evaluation Survey Temporary Flow Monitoring Report (February 2009), “Extraneous water from infiltration/inflow (I/I) sources reduces the capacity of the collection system to transport wastewater and may result in sanitary sewer overflows. Groundwater may enter [infiltrate] the collection system through defects, such as open pipe joints, cracks, broken pipe, dropped joints. Inflow, or rainfall [derived] infiltration/inflow (RDII), normally occurs when rainfall enters the sewer system through drains, missing cleanout caps, roof leaders, manhole covers and frame seals, storm sewer cross connections.”

To quantify and determine where groundwater infiltration and RDII could enter the collection system, the study team installed flow monitoring equipment in 18 manholes, chosen at key locations in the FSH system as representative of the system hydraulics. Flow monitoring measures the hydraulic variation of sanitary sewer flows for extended periods under dry- and wet-weather conditions. To measure and record the actual flow, technicians installed a submerged flowmeter inside each of the 18 manholes. Six continuously recording rainfall gauges were also operational, to obtain rainfall intensity and duration for the same monitoring period.

The flowmeter, which is a small probe, is mounted to a steel band installed inside the outgoing pipe at the manhole base. The probe uses sonar to detect fluid velocity; a pressure detector determines the depth of fluid in the outgoing pipe. The probe is connected to a data recorder, which is secured to the side of the manhole chimney. The data recorder captures and stores the flowmeter readings.

Upon activation, each of the 18 meters measured and recorded flow data at 15-minute intervals. Over a 24-hour period, each recorder accumulated 96 data sets, which included velocity (cubic feet per second) and fluid depth (inches). For this study, the 18 flow monitors were active for 39 consecutive days. About once a week, a technician pulled the cover of each meter manhole and connected the recorder to a laptop to upload the data.

Data “Flow”

Using the velocity, fluid depth, and pipe diameter data for each monitored mainline, additional flow data was derived, including Minimum and Maximum Flow, Average Daily Flow, Peak Flow, Ratio of Fluid Depth to Pipe Diameter (d/D), and Percent Full. The wastewater flow monitoring provided adequate hydraulic data to determine the following key information for the system area represented by each meter manhole:
  • Dry-Weather Average Daily Flow. The flow data for a typical dry-weather week (not impacted by rainfall) provided an average flow rate per day.
  • Dry-Weather Peak Flow. Peak flows recorded during dry weather were compared to the full pipe capacity, to determine the total system capacity being used during dry weather.
  • Wet-Weather Average Daily Flow. Wet-weather flows for each recorded rainfall event were analyzed to determine the percentage of rainfall that enters the collection system. Comparing the rainfall event flows with the dry-weather flows established the rainfall-derived infiltration/inflow (RDII).
  • Wet-Weather Peak Flow. Peak flow rates during wet weather are critical to the analysis of the total system capacity. Peaking ratios (Peak Flow Rate to Dry-Weather Average Flow) were compared for dry and wet weather.

Results Revealed, Mystery Solved

In addition to data about hydraulic performance under dry- and wet-weather conditions, flow monitoring provides important information about flow patterns. The study team used all of this information to develop a hydraulic model of the FSH Wastewater Collection System, which integrated the flow data for the single mainlines to establish baseline flow patterns. The model uses those patterns to assess existing and future carrying capacity and to predict the performance of the complete collection system under a variety of simulated operational conditions.

As noted by Dave Bowersock, Fort Worth District Senior Engineer, “With the completion of the FSH wastewater system analysis and the hydraulic model results, FSH DPW will have reliable data as well as access to the measurements and assumptions that the Project Delivery Team used to construct the hydraulic model of the system. Planners, utility design engineers, and maintenance personnel will be able to use modeling results to clearly identify problems and create proper system designs for future development at Fort Sam Houston.”

Further, the flow data was used to quantify the RDII, assess its impact on the Fort Sam Houston wastewater collection system, and prioritize areas with excessive I/I for rehabilitation to meet the BRAC requirements. As a result, the study made recommendations of a $2.7M investment in the FSH Wastewater Collection System alone.

Amazing what can be “uncovered” from the wonderful world inside a sanitary sewer manhole!

Paula Robertson is a technical writer/editor, U.S. Army Corps of Engineers, Fort Worth District

Photo credits: Pipeline Analysis, LLC