Hydrology VS Hydraulics. What’s the difference?

Welcome back, stormwater designers! At Clear Creek Solutions, we often get asked about the difference between hydrology and hydraulics—two essential components of stormwater engineering. In this article, we’ll break it down in a simple, straightforward way, touching on what each field entails, how they relate to one another, and why both are vital for sustainable water management.

And hey—if you’re just getting started or want a quick reference, don’t forget to grab our free Hydrology Terms Guide (link in the description). It’s a handy breakdown of key concepts and definitions every stormwater professional should know.

What Is Hydrology?

Hydrology is the study of the hydrologic cycle—also known as the water cycle. You probably first learned about it in grade school: water evaporates, forms clouds, precipitates, and returns to oceans, lakes, or the ground. While that’s the basic version, real-world hydrology dives much deeper into the intricacies of how water moves through the environment.

In stormwater design, hydrology focuses on how rainfall becomes runoff, how it infiltrates into the soil, or how it contributes to groundwater. Understanding the quantity and movement of this water is crucial for designing systems that manage flooding, erosion, and water quality.

Key Hydrologic Concepts:

• Precipitation

• Evaporation and transpiration

• Surface runoff

• Infiltration and percolation

• Groundwater flow

What Is Hydraulics?

Hydraulics, on the other hand, is the study of how water flows through engineered systems. While hydrology looks at how water behaves in nature, hydraulics focuses on how we control and convey that water—often through systems like pipes, channels, and treatment facilities.

In short: hydrology tells us how much water is coming, and hydraulics tells us where it goes and how it gets there.

Examples of Hydraulic Systems:

• Storm drains and sewer systems

• Water distribution networks

• Channels, culverts, and canals

• Dams and levees

• Treatment plants

How They Work Together

Hydrology and hydraulics are closely intertwined. To design an effective hydraulic system, you need accurate hydrologic data. For example, if a development generates a certain amount of runoff during a storm, hydraulic systems must be designed to handle and route that volume efficiently.

A real-world example:

• A detention pond (hydrologic facility) captures runoff from a developed area.

• That water is then funneled into a storm drain system (hydraulic facility) that conveys it to a treatment plant or a discharge point.

Common Hydrology Methods

Here are a few key methods used to calculate runoff in stormwater design:

1. Continuous Simulation Hydrology

• Tracks water through the entire cycle

• Measures soil saturation, runoff, and flow over time

• Ideal for modeling complex storm events

2. Rational Method

• A simpler, widely used equation:
  Q = CiA
  (Runoff = Coefficient × Rainfall Intensity × Area)

• Great for quick peak runoff estimates on small sites

3. SCS Runoff Method (Curve Number Method)

• Developed by the USDA Soil Conservation Service

• Uses land use, soil type, and rainfall to estimate runoff

Common Hydraulics Methods

Hydraulic calculations focus on flow rates, pipe sizing, and system behavior. Here are some go-to methods:

1. Manning’s Equation

• Used for open channel flow

• Factors in channel roughness, area, slope, and radius

2. Hazen-Williams Equation

• Calculates flow in pressurized pipes

• Considers pipe material and friction loss

3. Bernoulli’s Equation

• Based on energy conservation

• Helps determine pressure and velocity changes in a system

Wrapping It All Up

In summary, hydrology is about understanding water in its natural state—its source, movement, and distribution. Hydraulics is about engineering solutions to manage that water effectively. Both fields are essential for designing systems that are safe, efficient, and environmentally responsible.

Whether you’re building a detention pond or sizing storm drains, you’ll rely on both hydrologic and hydraulic methods. From tools like the Western Washington Hydrology Model to EPA SWMM, these processes help ensure that stormwater systems meet both public and environmental needs.

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Thanks for reading, and we’ll see you next time!