Water and Wastewater Systems

In one line: Development changes where rain goes. Water engineering estimates the runoff, sizes the pipes and basins to carry it, protects wetlands and supplies, and treats wastewater before it returns to the environment.

Core idea

Hydrology studies how water occurs, moves, is stored and is distributed across the earth. Hydraulics studies how it flows through channels and pipes. Together they answer the questions behind water supply, flooding, drought, drainage and water quality.

The key concept is the watershed: all the land that drains to a single outlet, bounded by a divide that you can trace on a contour map. Runoff flows downhill, perpendicular to the contours.

Developing land changes the watershed’s behavior. Roofs and pavement replace soil and trees, with these results:

• More runoff, sooner. Peak flows rise and the time of concentration drops.
• Less infiltration, so groundwater and the base flow of streams decline.
• Lower water quality: sediment, nutrients that cause algal blooms, and warmer streams once the shade is cleared.
• Downstream, more flooding, erosion, channel widening and habitat loss.

How it works

Estimating peak runoff: the rational method. For small watersheds, typically under about 200 acres, peak flow is estimated as:

$Q=CiA$

• $Q$ is the peak flow in cubic feet per second.
• $C$ is a runoff coefficient for the surface: about 0.1 to 0.3 for woods and lawns, and 0.7 to 0.95 for roofs and pavement.
• $i$ is the rainfall intensity in inches per hour, for a storm lasting as long as the watershed’s time of concentration.
• $A$ is the area in acres.

Example: a 10-acre wooded site with $C=0.2$ and $i=4$ in/h produces about 8 cfs at its peak. Paved over, with $C=0.7$, the same storm produces about 28 cfs, three and a half times as much. In reality it would be even more, because runoff arrives faster and so the design intensity is higher. The method is simple and conservative. Larger watersheds and the sizing of detention basins need hydrograph methods, such as the federal TR-55 procedure.

Sizing pipes: the open-channel flow equation. For gravity flow in U.S. units, engineers use a standard empirical formula from the 19th century:

$Q=\frac{1.49}{n}A{R}^{2/3}{S}^{1/2}$

• $n$ is the pipe’s roughness, about 0.012 for smooth concrete or plastic.
• $A$ is the flow area.
• $R$ is the hydraulic radius, which is $D/4$ for a full circular pipe.
• $S$ is the slope.

A 12-inch pipe at a 1% slope carries about 3.9 cfs when flowing full.

Detention. Regulations usually require that peak flow after development not exceed the peak before it. Basins and underground chambers store the difference and release it slowly.

Protecting wetlands. Wetlands filter water, absorb floods, buffer storms and support wildlife, so they are protected by law. In Massachusetts, for example, work in or within 100 feet of a wetland is reviewed by a local conservation commission:

• An applicant who is unsure can ask whether the law applies.
• If it does, they file a notice of intent and attend a public hearing.
• The commission then issues an order of conditions, or denies the project.

Wastewater. There are two main approaches:

• On-site septic systems: a tank where solids settle, a distribution box, and a leach field where the soil completes the treatment.
• Municipal treatment plants: physical, biological and chemical stages.

Many older cities have combined sewers, which carry stormwater and sewage in the same pipe. In large storms they overflow untreated into rivers. Separating them is very expensive, which makes funding the separation as much a policy problem as an engineering one.

In practice

For a leader, the lesson is that water problems are systems problems (see Thinking in Systems). A parking lot built upstream causes flooding downstream, and the costs land on people who weren’t at the table. Environmental review is often where a project’s real constraints appear, so it pays to engage it early rather than treat it as a final permit.

Questions to ask the experts

• How do peak flow and runoff volume change between pre- and post-development, and how are they mitigated?
• Are there wetlands, floodplains or protected buffers on or near the site?
• Where does the site drain, and who is affected downstream?

Connections

• The discipline: Civil and Environmental Engineering. Where it shows up in plans: Land Development.
• The ground that absorbs it: Soils and Foundations. A water-system failure through corrosion: Process Safety.
• Systems view: Thinking in Systems.
• Capabilities exercised: Connecting Across Disciplines, Inquiry, Ethical Actions and Integrity.

Sources

• Course lectures on hydrology, stormwater, water and wastewater; no public source.