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The Impact of Growth

Sewer Model Helps Fast-Growing Community of New Braunfels Plan for the Future

A computerized sewer model is helping the fast-growing community of New Braunfels, Texas plan for the future by evaluating its utility infrastructure to ensure that adequate capacity is available now and in the future. “A decade ago, we were adding 50 to 100 lots per year, but now the number varies between 800 and 900,” said Wesley Hamff, Water Systems Manager for New Braunfels Utilities. “Several of our main trunk lines will be reaching capacity in the next five to ten years. It's essential that when we repair or replace lines that they be sized to handle our full development potential. On the other hand, we want to maximize available moneys each year but we don't want to make major investments even one year sooner than necessary. The advantage of the computerized model is that it helps us predict exactly what we need and when so we can meet our customers' requirements at the lowest possible cost.”

The city of New Braunfels, located on the I-35 corridor between Austin and San Antonio, is one of the fastest growing in the country, with a population that increased from 27,300 to 46,000 between 1990 and 1999. The growth of the city places a rapidly increasing burden on wastewater collection and treatment systems which are run by New Braunfels Utilities (NBU). NBU was established in 1942 when the City Commission of New Braunfels purchased the electric transmission and distribution system from the Public Service Company of San Antonio. In 1959 the City of New Braunfels assigned operations of the water and sewer systems to NBU. NBU currently employs 160 people in three lines of business: Electric, Water and Sewer/Wastewater. The NBU Sewer/Wastewater System contains approximately 269 miles of line and presently serves 11,790 customers. The three wastewater treatment plants have a total capacity of 8.4 million gallons per day. In addition, NBU operates its own certified laboratory for testing water and wastewater.

Creating a master plan

To deal with the heavy growth in the system, NBU directed Camp Dresser & McKee (CDM) to develop a wastewater collection system master plan. The purpose of the study was to investigate and report on the present and future needs of the wastewater collection system. Another purpose was to comply with state law for imposing impact fees which are charged to new developments to pay for improvements required by the additional load they place on the system. The technical portion of the relevant statute, Chapter 395 of Texas local government code, requires that a capacity analysis be performed by a professional engineer to distinguish between the portion of improvements that are required to serve existing residents and those that can be charged to new developments.

CDM Project Manager Scott Miles created a computer hydraulic sewer model to identify the deficiencies in the system. The software Miles used for this was HYDRA®, from Pizer Incorporated, Seattle, Washington. "Hydra 6 is a very sophisticated storm and sanitary sewer analysis program that is also quite easy to use,” Miles said. "Its GIS Master Module is designed to work with AutoCAD to form a powerful graphical information system. It outperforms many packages that cost more and are harder to use through its dynamic analysis capabilities. They make it possible to graphically illustrate the variations in flow through the system as a storm passes through at different times in relation to peak sanitary flows."

Modeling the collection system

Miles first created a digital model of the collection system in HYDRA showing the network of pipes, the six pump stations, and the three treatment plants. He also identified drainage basins in the study area to help understand the operations of the sewer system and to establish flow characteristics experienced by the sewer trunks and sub-trunks. Miles digitized in the physical components such as pipes, lift stations, force mains and diversion structures from the AutoCAD GIS system used by the utility company. In initially constructing the model, assumptions were made for certain parameters such as roughness coefficients and diurnal curves.

He determined the utility company’s existing wastewater flows by identifying the service area for each pipe. The contributing area was multiplied by the corresponding per acre flow rate to get the average daily flow. The service areas were then subdivided using the future land use and the flows were recalculated based on projected year 2010 conditions. Land use for the 2010 year modeling condition was determined by obtaining projections from the city's planning staff. The land use plan came from the comprehensive master plan.

Running the analysis

Miles then ran the analysis to evaluate the ability of the existing collection system to handle peak wet weather flows. A dry weather and a wet weather design flow condition were studied. The dry weather flow consists of peak sanitary wastewater plus ground water infiltration. This is the flow that would occur during the peak hours of days with no rainfall. The wet weather design flow consists of dry weather flow plus rainflow dependent inflow and infiltration. This is the flow that would occur during a major rainfall event. The process of setting up the analysis model and providing the required information went very quickly. The software's user interface allowed him to perform most functions through the use of icons and menus.

The capacity of the existing and future sewers was computed using Manning's Formula with a friction factor of 0.013. In addition, the capacity was based on the pipe not overflowing during the peak wet weather design flow. Hydra 6 displayed the results in the form of hydrographs, profile details and color-coded plots that can be displayed in AutoCAD. The next step was to compare the output of the model to the real flow data gathered by flow meters. Adjustments to the diurnal curves and ground water infiltration were made to the model to fine tune it to produce similar results to those observed in the field. After the model was verified to represent actual field conditions, it was used to identify problems in the collection system. The model was verified with six flow monitoring locations.

An example of some results for a wet weather run under existing flow conditions. The pipes highlighted in yellow are surcharged

Problems identified by the model

Here are some examples of the problems identified by the model. The Gruene lift station exceeds its firm pumping capacity for year 2000 wet weather flow condition. Recommendations were made to upgrade the lift station to a firm pumping capacity of 1800 gpm with a total pumping capacity of 3150 gpm. The existing collection system was seen to be sufficient to convey the existing dry weather wastewater flow without surcharges. Two pipe segments however, were flowing above 90% full. When rainfall-dependent inflow and infiltration was added to the existing system, potential overflows occur in several areas such as the main trunk, upstream of the Rio lift station and the line along I-35, also upstream of Rio lift station.

When future growth is taken into account, surcharging occurs in several areas of the system even under dry weather conditions. They include the main branch upstream of previous improvements, upstream of the Gruene wastewater treatment plant heading east and several other areas. When rainflow dependent inflow and infiltration was considered, surcharging occurred in a number of additional areas such as from the northern line just upstream of the Gruene lift station. Based on the model, CDM developed a capital improvement plan. The plan is ranked according to areas that fail to meet 1) existing 2000 wet weather flow conditions 2) predicted 2010 dry weather flow conditions and 3) predicted 2010 wet weather flow conditions.

Preparing for the future

Mickey Fishbeck, Owner of Rimrock Consulting, Austin, Texas, is performing the analysis work that will allow the NBU to fairly determine impact fees. “I am using the master plan results to help create recommendations on the impact fees that should be charged to new developments,” she said. “The master plan will make it possible to define the service area, calculate the needs of existing users and predict expected growth. It will also help to determine the new capacity that is required and how much of that capacity can fairly be charged to the new users. Once my study is completed, it will be presented to an advisory committee appointed by the City Council. This committee, in turn, will make recommendations to the city council which will set the maximum possible fees and the NBU Board will adopt the actual fee at or below the maximum set by the City Council.”

“The information provided by the model will provide a valuable guide to repairing and upgrading the sewer system,” Hamff said. “For example, the model will show that based on current conditions an 18 inch pipe needs to be upgraded to 24 inches but that once the city is fully built out, a 36 inch pipe will be required. We can save a considerable amount of money by putting the 36 inch pipe in now, because the cost of materials is small in comparison to installation costs. The fact that we know exactly which areas of the system are surcharging or are in danger of surcharging in the future helps in other ways. Knowing that several of the main trunks will be reaching capacity soon, we can divert flow away from those trunks as we connect new developments. Postponing capital improvements makes them less expensive because we can accrue impact fees that will pay for a portion of their cost, reducing our borrowing expenses.”

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