Sarnwick's Sewer Overflows: Green Streets or a Tunnel

Resolution: Sarnwick should meet its overflow reduction requirement with distributed green infrastructure rather than with a deep storage tunnel.

Background. Sarnwick is a fictional city of about 140,000 people, and every figure in this kit is invented for the debate. Its older half was built with a combined sewer, a single pipe that carries both wastewater and street runoff to the treatment plant. In dry weather the plant treats everything. When rain fills the pipe past its capacity, a relief structure discharges the mixture into Hollin Creek instead of backing it up into basements. That discharge is a combined sewer overflow.

Sarnwick's overflows now happen about 38 times a year and release roughly 410 million gallons of diluted sewage into the creek. A state consent order requires the city to cut that volume by 85 percent within fifteen years. Two paths meet the requirement on paper.

The green infrastructure path builds thousands of small features across the older neighborhoods: bioretention cells in curb extensions, permeable alleys, street trees in structural soil, and downspout disconnections that route roof water into planted beds. Each feature holds a small volume and lets it soak into the ground or return to the air, so less water reaches the combined pipe.

The tunnel path bores a single storage tunnel beneath the old city, sized to hold the overflow from a design storm. Water fills the tunnel during a storm and is pumped back to the treatment plant afterward, when capacity is available.

Roles. Each team of four assigns a speaker for the opening case, one for the hydrology and soils argument, one for the water quality and creek health argument, and one for rebuttal. The judging panel scores with the rubric below and must name the evidence that decided each criterion. A moderator keeps time: four minutes per opening, three minutes for each of two exchanges, and two minutes per closing. Each speaker drafts a written case or rebuttal brief before the round and turns it in with the debate, so the argument can be scored on paper as well as aloud.

Every figure below is invented for the fictional Sarnwick scenario, and fictional sources are labeled as such.

Case for distributed green infrastructure

1. Most overflows come from small storms. The fictional Sarnwick Sewer Study reports that 31 of the 38 overflow events last year followed storms of less than three quarters of an inch. Features that capture the first half inch of rain would prevent most events, even though they would not handle the largest storms.
2. It treats the first flush where it lands. The same study measured the highest bacteria and metals concentrations in the first twenty minutes of runoff from streets. Bioretention soil captures that water at the curb, and plants and soil microbes break down much of the load rather than sending it downstream.
3. It restores summer flow. Water that soaks into the ground moves slowly toward the creek and supports baseflow. Hollin Creek fell to 2 cubic feet per second last August, and the fictional watershed council attributes part of that low flow to the fact that most rain in the old city leaves as runoff within an hour.
4. It reduces erosion from ordinary storms. A tunnel does nothing for the frequent small storms that scour the creek bed in the newer half of the city, because those flows reach the creek through separated storm drains that never enter the tunnel. The same curb features can be extended along those streets as the program grows, while the tunnel serves only the combined sewer.
5. It can be built in stages. Each block that is finished starts working immediately, and the city can adjust the design as it learns which features perform. A tunnel delivers nothing until the whole project is finished.
6. It brings added benefits. Street trees and planted beds lower summer pavement temperatures and improve the look of a block, which the fictional Sarnwick Sewer Study values at $3 million a year in avoided cooling costs and property effects.

Case for the deep storage tunnel

1. Half of the old city has soil that will not accept water. Borings reported by the fictional Sarnwick Sewer Study found clay with infiltration rates below 0.1 inches per hour across the riverside neighborhoods, and a water table within four feet of the surface near the creek. Infiltration features there would stand full for days and fail.
2. A tunnel captures a known volume. Engineers can size the tunnel to the design storm and state with confidence how much it will hold. Green infrastructure performance depends on thousands of separate installations, their soils, and their upkeep.
3. Maintenance is concentrated rather than scattered. One tunnel and one pump station need one crew. The green path would add about 6,000 separate assets that must be cleaned of leaves and sediment on a schedule, and the fictional public works department reports that neglected bioretention cells lose most of their capacity within four years.
4. The largest storms cause the largest releases. Seven storms last year produced 240 million gallons of the total overflow volume. Features sized for a half inch of rain would overflow in those storms, so the biggest pollution events would continue.
5. Captured water gets full treatment. Water stored in the tunnel is pumped to the plant and passes through the entire process, including disinfection. Water infiltrated at the curb receives only what the soil column removes.
6. The tunnel offers schedule certainty. The consent order sets a deadline. The fictional city engineer estimates the tunnel at $520 million with a twelve-year schedule, while the green path would require acquiring easements on thousands of properties, with no way to compel a private owner to accept a feature.

Shared background both sides may use. The two approaches are not strictly exclusive; several cities have built a smaller tunnel alongside a green program. Any hybrid proposal must still state what volume each part captures.