Modeling a proposed residential development
Noble’s Pond is a residential community being developed by Regal Builders in Kent County, Del. It is a medium-density planned unit development consisting of both single-family and duplex dwellings. The project includes a clubhouse and pool surrounded by 850 lots on approximately 293 acres. McCrone Inc. was retained to provide infrastructure design and guidance through the Kent County permitting process for the multi-phase project. One of the key aspects of the permitting process was stormwater management. The site drains to a high-quality watercourse and the downstream effects of stormwater runoff were a major concern to the community.
The site is primarily low-relief, poorly drained farmland including some wooded wetlands and several ponds. The parcel is divided by streams and contains many sumps. The stormwater management study point was identified at the intersection of Fork Branch (the parcel’s boundary to the south) and one of its unnamed tributaries, which runs through the parcel. Although McCrone was not required to study Fork Branch itself, they were required to assess and manage water quality and quantity management at the study point. After meeting with Kent Conservation District (KCD), a pre-development drainage area was modeled in HydroCAD and the results were submitted. Using HydroCAD, McCrone modeled the farm fields, wooded areas, ditches, and sumps. HydroCAD’s modeling flexibility allowed McCrone to effectively and efficiently manage and model numerous subcatchments, as well as the complex, low-relief tailwater conditions present on the site.
Once the pre-development model was approved, a post-development model was created, utilizing large stormwater facilities to provide quantity management. During the design process, the Regional Planning Commission requested utilization of green technology Best Management Practices (BMPs). McCrone identified locations where BMPs could be implemented while minimizing impact to the existing layout. McCrone’s engineering team then modeled the BMPs and micro-practices with HydroCAD to achieve qualitative stormwater management goals. McCrone engineers were able to quickly model the BMPs – such as bioretention areas, bio-swales, and micro-bioretention areas – utilizing the different node types available in HydroCAD. After developing working models for each BMP, the models were easily integrated into the previous quantitative model and, after minor adjustments, McCrone had a working model that demonstrated both quantitative and qualitative compliance.
Information provided by McCrone Inc. and HydroCAD Software Solutions LLC
Evaluating storm impacts in real time
Based in Huntsville, Ala., 4Site Inc. recently adopted Autodesk Storm and Sanitary Analysis software, a fully dynamic hydrology and hydraulic modeling tool that is closely integrated with AutoCAD Civil 3D, the firm’s existing design software. This software combination helps 4Site quickly test multiple design options and evaluate their impacts in real time.
One of 4Site’s first projects with the software was an 85,000-square-foot commercial development project in Madison, Ala. The owner hoped to achieve LEED for Core and Shell certification on the project, which included two office buildings and associated site improvements. 4Site engineers designed the project using Civil 3D and then exported data from the Civil 3D model into Autodesk Storm and Sanitary Analysis software to analyze the stormwater network. During the design process, 4Site’s engineers were able to model the entire project as a whole instead of in parts, adjust stormwater pipe sizes on the fly, see the impact that various storm events would have on the proposed system, and then make adjustments in real time. The analysis software handled flow calculations and hydraulic grade lines, and enabled the firm’s engineers to update pipe sizes in a single step, without lengthy manual calculations.
Using Autodesk Storm and Sanitary Analysis software advanced modeling tools, the firm was able to accurately model the many rain gardens throughout the site by specifying their individual infiltration rates instead of relying on traditional outflow structures. This enabled 4Site to better manage runoff and infiltration throughout the site and limit the size and cost of the associated stormwater infrastructure. The project achieved LEED Gold Certification utilizing site credits for both stormwater quality and quantity, with more than 50-percent reduction in potable water use for irrigation.
By Jackie Whitaker, P.E., LEED AP, 4Site; and Teresa Elliott, Autodesk
Water main criticality analysis
As part of an ongoing capital improvements program (CIP), a main transmission artery known as the Cross Town Tunnel in Washington, D.C., had to be taken out of service for rehabilitation. Hatch Mott MacDonald performed hydraulic analyses in Bentley’s WaterGEMS to identify alternative supply sources and forecast potential outages during the 13,200-foot, 84-inch-diameter main shutdown. It was determined that the tunnel would need to be in service during the peak demands of summer and it became a contract stipulation.
Next, DC Water wanted to know what could potentially go wrong while the main was out of service that could jeopardize supplying water to customers. Hatch Mott MacDonald used WaterGEMS’ criticality tool, which automatically removes one pipe at a time from the model, and then determines the impact of removing the pipe on providing customer demand at adequate pressures.
Hatch Mott MacDonald ran scenarios for 475 transmission main outages and 72 CIPs ongoing during the tunnel closure, including large valve replacement projects and water main renewal projects. The analysis identified 26 transmission main segments that resulted in more than 1 percent loss in demand and that could result in unacceptable service levels.
DC Water is now aware of the areas where transmission mains segments would result in system vulnerability if they failed during the Cross Town Tunnel rehabilitation and is developing emergency response and contingency plans in the event of these failures. The criticality analysis also identified two CIP projects that would need to be performed prior to or after the main rehabilitation to avoid adverse impact to customer service.
Information provided by Bentley Systems Inc.
Sanitary sewer evaluation
BG Consultants Inc., Manhattan, Kan., was tasked with evaluating the sanitary sewer system in Horton, Kan., a town with 2,000 residents about 80 miles northwest of Kansas City, Kan. The sewer system contained sections that, to the best of anyone’s knowledge, dated from as early as 1915. BG Consultants needed to evaluate the existing system, prioritize necessary improvements, and develop the project documents to rehabilitate the system.
Initially, a closed-circuit television inspection of the entire collection system was performed by Mayer Specialty Services Inc. (according to Pipeline Assessment & Certification Program standards) to classify the size, material, and condition of the sewer main pipe. BG Consultants then surveyed the manhole locations using GPS, conducted manhole inspections, and then evaluated the inspection data utilizing IT Pipes software. Rehabilitation methods were then prioritized for approximately 375 manholes and 96,000 linear feet of sewer pipes ranging from 8 inches to 18 inches in diameter.
The Carlson Hydrology module was used by BG Consultants to efficiently build the entire sanitary sewer collection system. In Phase 1 of the project, approximately 32,000 linear feet of sewer main is scheduled for complete replacement through open trench construction. Tools such as "generate plan and profile sheets" were used to significantly expedite the creation of more than 55 plan and profile drawings, which will accurately define the parameters for this construction activity.
BG Consultants worked closely with programmers and support staff at Carlson to adapt Carlson Hydrology to the needs of this sanitary sewer rehabilitation project. While Carlson is able to perform accurate stormwater flow modeling, it is not fully developed to model average daily wastewater flow rates generated at service tap connections. However, BG Consultants was able to utilize custom sewer network report generation tools within the hydrology module to assist in the hydraulic capacity evaluation of the system.
Information provided by Carlson Software
Wastewater utility improves system optimization
The Racine, Wis., wastewater utility provides wastewater collection and treatment to 10 communities in the greater Racine area. The existing service area is approximately 34 square miles and serves approximately 140,000 residents. It is anticipated that the population associated with full build-out conditions could increase by 60 percent, to just over 225,000 residents by 2035.
The utility has a conventional gravity flow interceptor system with sanitary sewer diameters ranging from 8 inches to 84 inches. The utility is responsible for operating and maintaining 14 lift stations and two off-line storage facilities. In general, sanitary sewer flows in the system are very responsive to rainfall. Peak flow rates at the wastewater treatment plant (WWTP) have produced peaking factors (peak flow divided by average daily flow) in excess of 10 during large wet weather events.
On Aug. 18-20, 2007, the city of Racine and surrounding areas received as much as 6 inches of rain on top of an already wetter-than-average month. This event produced safety site bypassing, basement backups, and excess flows at the WWTP. As a result of the system-wide basement backups and bypassing, the utility developed a system optimization plan to mitigate these issues.
System optimization was achieved by modeling the utility’s interceptor system using the collection system model MIKE URBAN. Peak wet weather flow rates – dry weather flow plus inflow and infiltration (I/I) – were calibrated to match flows and levels at more than 30 locations from the Aug. 18-20, 2007, calibration event. The model uses the Rainfall Dependent Inflow and Infiltration (RDII) module to simulate fast (inflow), medium (rapid infiltration), and slow (interflow) I/I components. These components, when added together, make up the overall wet weather flow response for each sub-basin. Sub-basin time series flows are then routed through the sewer network to simulate hydraulic grade lines (HGLs) throughout the system.
Various scenarios and alternatives were evaluated using the model’s RDII and Real Time Control options. Eliminating basement backups and system bypassing were defined as the evaluation/optimization criteria, as well as reducing peak wet weather flow at the WWTP sufficient to meet a wet weather flow threshold. The optimized solution consisted of a combination of storage and conveyance measures which best met the evaluation criteria of the lowest unit cost for total bypass elimination (dollars/gallon removed); the lowest unit cost for system-wide surcharge reduction (expressed as a system-wide average surcharge); and the lowest total cost to meet a flow threshold at the WWTP.
Information provided by DHI Group
Integrated 1D/2D basin model
A complete study of Redwood basin in Josephine County, Ore., was undertaken to assess the existing infrastructure, starting with an existing XPSWMM model, and updating the hydrology and the hydraulics as necessary. Multiple major elements of the basin’s hydraulic elements were missing in the existing model. This triggered a full assessment of the existing 1D model and the basin infrastructure to determine all elements requiring updating and model inclusion.
The hydrology included 92 sub-basins employing the Green-Ampt infiltration methodology. The model was calibrated to three gages maintained by the project team during winter of 2010/2011. Many elements were added to the hydraulic model including all major irrigation canals, natural creeks, pipe conveyance, and updates to some of the existing model infrastructure. The updated 1D model is robust, complete, and appropriately represents the infrastructure currently in place.
However, a 1D schematization cannot accurately model flood water once it surcharges out of the 1D elements. When surcharging occurs, the model simulated this volume of water as lost at the point of surcharge and the overall model was inaccurate as it did not convey these flows overland. Numerous 1D locations surcharged within the model, therefore a 2D module was added to maintain model continuity and increase the model description. The 2D model revealed overland flow paths where the 1D system surcharged and connected downstream 1D elements with overland flows. The 2D model consists of 979,627 grid cells, each 10 feet by 10 feet, with 5.8 miles of large irrigation canals, 4.22 miles of creeks, and 5.44 miles of additional open channels all linked to the 2D module. This linkage of 1D and 2D allows flows in and out of the 1D system.
For example, flows leaving an irrigation canal can flow overland to a downstream irrigation canal or stream. The 2D portion of the model was built and integrated to the 1D model, therefore, a grid cell size of 100 square feet was deemed appropriate while balancing run time and number of cells. The resulting fully integrated 1D/2D model revealed significant drainage paths previously unidentified with 1D modelling. Furthermore, the addition of the 2D module allowed a complete picture of the flooding and shortcomings of the current infrastructure. Without the addition of the 2D module, significant aspects of the system would have been lost and the development of a CIP may have been misguided.
Information provided by XP Solutions (formerly XP Software)