Above-Ground Solution For Underground Problem

Atlanta is working urgently to comply with the Decree, which requires the City to evaluate its entire collection system and eliminate all overflows taking all remedial actions necessary.

Wallingford Software's InfoWorks CS has been the primary tool in defining the capacity of the collection systems in tandem with the ongoing sewer evaluation survey. The solution has enabled the city to identify the appropriate remediation projects, and a number of these are already underway, including sewer rehabilitation programs and construction of additional relief sewers.

The City identified outfalls that would have insufficient capacity and it was anticipated that a combination of replacement or relief sewers with a tunnel extension would need to be constructed to achieve the zero spill target. However, this was extremely invasive in this heavily populated downtown area so another solution was needed.

Project background
The Clear Creek combined sewer basin, which is some 5,000 acres in area, serve the basin, into which all of the local tributary areas drain.

The Clear Creek Basin-North Avenue area is undergoing significant revitalisation. Capacity relief was required for the main trunk sewer, and flooding issues in the North Avenue area had to be addressed. There was also a wider need for a project that could promote the ongoing revitalization of the area for more shared public space.

The Highland Trunk sewer, which serves the area, varies from 8ft. to 10ft. in diameter. Due to drainage issues on the surface and capacity issues in the Highland Trunk the Clear Creek Basin was prone to flooding, notably at the vast City Hall East complex and a major shopping center.

The two million square foot City Hall East complex is due to be renovated as part of the area's ongoing revitalisation. The intention is to remodel the massive building into a mix of 1,200 residential units and 182,000 sq.ft. of retail space as well as 154,000 sq. ft. of office space. However, for this project to progress, the flooding problems in the City Hall East loading dock area had to be resolved.

Storm flows followed overland routes, collecting at the low-level loading dock area. Another significant area of flooding downstream needed to be examined to resolve whether this was due to insufficient capacity in the trunk sewer or because the sewer was simply not permitting flows to pass quickly enough through the system.

The area under consideration is a complex mix of steep urban terrain and parkland; therefore the City's modeling group decided to utilize the InfoWorks CS 2D module.

To resolve the flooding issue at the loading dock, it was proposed to infill that low-lying part of the complex. However, to ensure that this did not simply aggravate flooding elsewhere, the solution provided needed to detain these flows.

The area is highly urban so open-cut solutions are difficult to achieve, while trenchless options would be expensive. This made the option of a stormwater detention pond attractive, but to derive accurate dimensions the modelers needed to calculate the true drainage area of City Hall East and evaluate the capacity of the existing trunk main for various design storms.

The area was evaluated using a 100-year, 24-hour design storm. It was anticipated that if the stormwater flows could be prevented from entering the combined trunk sewer, this would have sufficient capacity to serve both downstream and upstream areas. The sewer had to be evaluated across a range of design storms as the Consent Decree stipulates that no spills or overflows of any kind should enter the river. The Consent Decree does not address flooding, but floodwaters play a key role in reducing the capacity of trunk sewer systems.

The City opted to adopt a capacity relief project called the Old Fourth Ward Park, a five-acre park system containing a 2.5 acre stormwater detention pond. During storms, flows would be channeled to the pond and then slowly fed back into the trunk sewer system under gravity. The surrounding park consists of open green space with walking trails, an amphitheater and pavilions. This was considered an ideal solution, effective in retaining storm flows, while providing a functional public amenity.

Capacity modeling
When this option was adopted, the City commissioned an assessment of preliminary data to obtain estimates for the necessary volume of the pond. The available data was limited and based on an old competitor model that was not calibrated for flow meter data-meaning the analysis contained many assumptions.

The first of these assumptions was that under a 100-year design storm, the combined sewer system would be at full capacity. This appeared to be a reasonable assumption, but presupposed adequate inlet capacity although this was not proven. As the model was not calibrated, the original consultants opted to calculate runoff using the SCS method. The results suggested that a storm of this magnitude would create a total volume of 7.9 in. To calculate runoff, given that the sewers were assumed to be at full capacity during the storm, the sewer capacity was subtracted from the total volume of rainfall on the drainage area to obtain a runoff figure.

The consultants also defined the naturally-occurring stormwater detention areas based on local topography. It was assumed that any runoff would either remain at these detention locations until it could access the combined sewer system, or would overtop and the water would continue downstream. Flows were simply routed downstream from the detention areas to the low-lying City Hall East site.

The original assumption was that the volume of runoff contained at City Hall East under the 100-year design storm would be around 975,000 cu.ft or nine million gallons. This was approved as a reasonably conservative estimate.

Ideally, in tandem with the design of the detention pond, the City would also undertake some of the capacity relief projects planned for the area. At that point the City project management team, Clean Water Atlanta, was appointed to look at the proposed detention pond solution to determine whether it could also be used as part of the capacity relief project.

Fortunately, the team had access to significantly more data than was available in the original project, and they were using a current InfoWorks CS model. The City had also installed a number of meters to which the model could be calibrated.

Given the complexity of this highly-developed urban area, the consultants opted to calculate runoff by creating a Land Use that utilized the SCS method for permeable areas and a fixed runoff for impermeable areas. The modelers also created inlet restrictions where known, and then ran the model using the InfoWorks CS 2D module to gauge surface runoff conditions.

The results highlighted a considerable difference between the new and old approaches. The disparity was due to the runoff calculation approach - in the original model, using the SCS methodology for both permeable and impermeable surfaces, when ground saturation was achieved runoff levels across the area increased dramatically. By specifying the SCS method for permeable surfaces and using a fixed runoff level for impermeable areas, the projected 100-year design storm volume reduced from around 145 million gallons per year to 70 million gallons.

Even with the improved data resources, problems remained. The models were calibrated to real rainfall events, approximately one-year design storms, and for storms of increasing rarity such as 1 in 100 year events, interpolation was necessary and the margin of error therefore increased substantially. In addition, while the City was aware of some restrictions on inlets to the network, it did not have information on the condition of all of these structures.

The pond was then relocated some distance upstream of its original position during the course of the design, which created some challenges in enabling water ingress to the new location.

Using the InfoWorks 2D module it was identified that the actual City Hall East direct drainage area was far smaller than anticipated because various obstacles in the catchment retained a portion of the flows. The model accurately identified the correct drainage area and enabled a precise visualization of the exact amount of runoff that would enter the pond, and its volume. It was then possible to identify potential problem areas that would remain after the pond was constructed. The City team was also able to identify projects that would enable better utilization of the pond.

Storm sewers were also added to the network to provide an outlet for the flows for which the re-located pond could no longer access directly. Additional flows from another roadway were incorporated into the model and the extra volume that these would represent was calculated.

It was decided as a result of the modeling exercise that the proposed nine million gallon capacity of the pond was appropriate, as it could be fully utilized by ensuring flows were routed appropriately using the InfoWorks 2D module.

Conclusions
The modeling exercise allowed the City to reject the original proposal for a tunnel extension and sewer upsizing program in favor of the more sustainable and cost-effective option of a detention pond and additional storm sewers. The new park forms an integral part of the area's development, and has been hailed as a solution that the entire neighborhood has embraced.

Proposed future model enhancements include inputting inlet restrictions that are discovered, and adding roughness polygons to utilize the streets for conveyance. It is anticipated that viewing using Google Earth will also provide benefits.

This article is based on a paper presented at the Wallingford Software North American User Conference by Sharon Matthews of the City of Atlanta DWM and Kai Iaukea of Clean Water Atlanta PMT.