PondPack at PebbleCreek
B&R Engineering of Sun Lakes, AZ (United States) successfully applied PondPack in the design of a complex drainage system that includes an intricate network of over 30 ponds. The engineering firm performs the land development design work for Robson Communities, a developer of large Planned Area Developments (2,000 acres or more) in Arizona and Texas. In addition to PondPack, they rely on Haestad Methods’ full suite of products to perform all of the hydraulic and hydrologic design work for the developments.
Mark A. Maloney is B&R’s project manager for Robson’s PebbleCreek project, a long-term masterplanned adult community in Goodyear, AZ, near Phoenix. He uses Haestad Methods’ PondPack software to perform the required detention and retention design. B&R previously used the U.S. Army Corps’ HEC-1 software along with another commercial hydrology package to perform this work, but desired a change because of the tedious iterative process involved. PondPack attracted Mr. Maloney because of its technically sound interconnected pond modeling algorithms. As Mr. Maloney put it when he first saw PondPack, “This is exactly what we do. No other software on the market can do this as seamlessly.” PondPack’s ease of use, customization capabilities, and advanced hydraulics sold his management on the product.
Project Description
Construction of the PebbleCreek project is occurring in three phases (see aerial photograph), with full build-out anticipated in 12 to 15 years. The complete development will include 6,200 homes and a 54-hole golf course.
Roadway gutters collect surface runoff from the lots, which is then intercepted by curb openings or scuppers and directed to the golf course through improved channels. The golf course includes dry detention ponds and wet lakes connected together by a system of storm drains used for both conveyance and retention of the runoff. Backwater effects and reverse flow were extremely important design considerations. “We have found that PondPack works extremely well for modeling this type of situation” said Maloney.
To maintain water quality (i.e. minimize stagnant water), a pump station recirculates water from the most downstream retention basin to the most upstream lake, which is also fed with fresh water. To overcome water restrictions, treated wastewater is also injected into the system at various points.
Design Requirements
The City of Goodyear requires the development to retain the 6-hour, 100- year storm onsite. Minor streets must convey the 5-year storm discharge without exceeding the curb height, and the 50-year storm must be conveyed within the right-of way. Collectors and arterials must convey the 10-year and 100-year storms, respectively, without overtopping the curb. Another design consideration is to match the PondPack results with the HEC-1 results from the original 1997 Master Drainage Study. The comparison of the two studies yielded “amazingly similar results,” said Maloney.
Modeling Approach
The runoff hydrographs for the project were generated using the SCS method. The project consists of three types of drainage areas: residential (CN = 89), golf course (CN = 67), and roads/rights-of-way (CN = 94). Individual drainage areas are relatively small, with a minimum time of concentration of 10 minutes.
All ponds in the extremely flat network use PondPack’s ICPM option (interconnected pond modeling) to account for the backwater effects and reverse flows. Outlets for dry detention/retention basins are modeled with weir overflows and grate structures and lakes use overflow standpipes. Emergency overflow structures are provided at critical points and at the most downstream retention pond.
At peak flows, which are the primary design flows, discharges from the ponds are typically limited by the connecting storm drains, not the initial outlet structures. Thus, it is important that the storm drains be represented as part of the interconnected system. This is effectively done by modeling sewer pipes as culvert-type outlet structures in PondPack. Manholes, located where the storm drain changes direction or slope, are modeled as ponds using the area-elevation method, and do not provide storage. PondPack calculates the HGL at these critical points to determine if flooding is occurring. Ponds are also used to model street flow interception in sag locations (a split flow situation), since no bypass flow exists and storage does occur. The top of the pond correlates with the top of the 6-inch curb, and the outlet structure is the curb opening.
Percolation in dry retention basins is handled using PondPack’s Average Infiltration Rate method, whereby an average infiltration rate is applied over the water surface area at each time step. Modeling percolation in the lakes is a bit trickier, since the lake is lined below the normal water surface. Percolation only occurs on the side slopes above this elevation. To model this situation, a large, artificial pond is created within the model to accept the infiltration flow. The infiltration rate is defined by setting up an outlet structure from the lake with a user-defined rating table corresponding to infiltration flows.
Mr. Maloney is a PondPack power user exercising the product to its fullest capabilities, and stretching the limits by developing creative approaches to handle unusual situations. He even leveraged PondPack’s .NET framework to develop customized reports using Visual Basic.