What's the difference between hydrology and hydraulics?

Hydrology calculates how much water (runoff volumes and flow rates). Hydraulics designs the system to convey that water (pipe sizes, grades, velocities). You need both for a complete drainage design.

What's a 100-year storm?

A storm event that has a 1% chance of occurring in any given year. It's a design standard, not a prediction. Drainage systems are typically designed for 10-year, 25-year, or 100-year storms depending on the risk level and jurisdiction requirements.

Can I discharge stormwater to the street?

Usually, yes — that's how most residential drainage works. But the discharge has to be at approved locations (typically the gutter), and in some cases you may need to limit the rate of discharge through detention.

What if my neighbor's property drains onto mine?

The hydrology report accounts for offsite drainage entering your property. You can't block it, but you can design your system to intercept and convey it safely through your site.

Do I need a hydrology report for a simple residential addition?

It depends on the jurisdiction and how much impervious area you're adding. Many cities require it if you're adding more than 500-1,000 SF of roof or hardscape. Check with your local building department.

Home›Resources›What Is a Hydrology Report?

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What Is a Hydrology Report?

The analysis that calculates stormwater runoff and sizes the drainage infrastructure to manage it.

When It Rains, That Water Has to Go Somewhere

Every project generates stormwater runoff. Roofs, driveways, patios — all that impervious surface sheds water during storms, and if it's not properly managed, it floods yards, overwhelms storm drains, causes erosion, or worse: gets your project red-tagged for non-compliance with drainage codes.

A hydrology report is the civil engineering analysis that calculates how much runoff your project will generate, determines the peak flow rates during design storms, and sizes the drainage infrastructure needed to handle it safely. This is what your civil engineer uses to design catch basins, storm drains, detention basins, and surface drainage systems.

Building departments require hydrology reports because they need to verify that your project won't make downstream flooding worse, won't overload the public storm drain system, and won't cause erosion or property damage. Your drainage system has to be designed, not guessed.

What's in a Hydrology Report

A complete hydrology report includes the calculations and analysis needed to design drainage systems:

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Rainfall Data

Uses historical rainfall intensity data for your location, typically based on the 10-year, 25-year, or 100-year storm event.

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Drainage Basin Delineation

Identifies which parts of your site (and sometimes offsite areas) drain to each collection point.

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Runoff Calculations

Calculates runoff volumes and peak flow rates using the Rational Method or other approved hydrologic models.

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Inlet & Pipe Sizing

Sizes catch basins, area drains, and storm drain pipes to convey the calculated flows without surcharging or flooding.

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Detention/Retention Analysis

If required, calculates detention basin volumes needed to limit post-development runoff to pre-development rates.

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Hydraulic Grade Line

For complex systems, shows that the drainage system has adequate capacity at all points without backing up.

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Why Drainage Can't Be an Afterthought

Undersized drainage infrastructure causes real problems. We've seen projects where the developer skipped the hydrology study and "winged it" — the result was a 12-inch pipe that should have been 18 inches, a system that backs up during every moderate storm, and a grading contractor who gets sued for flooding the neighbor's yard.

Building departments require hydrology reports because stormwater management is a public health and safety issue. Inadequate drainage causes flooding, property damage, erosion, and water quality violations. They need to see calculations proving your system works before they'll approve it.

And here's what most people don't realize: many jurisdictions now require that post-development runoff doesn't exceed pre-development rates. That means if you're adding a lot of hardscape, you may need detention basins or underground storage to "slow down" the runoff. The hydrology report is what sizes those facilities.

When You Need a Hydrology Report?

Common project types and triggers:

New Construction

Required for any project that modifies site drainage or adds significant impervious area.

Grading Permits

If you're regrading and changing drainage patterns, the jurisdiction needs hydrology to verify the new system works.

Subdivisions

Always required. Multi-lot developments must show that the drainage system can handle the cumulative runoff from all lots.

Detention Basin Design

If your project requires stormwater detention, the hydrology report sizes the basin and outflow control structures.

Downstream Flooding Concerns

If neighbors or the city have concerns about increased runoff, a hydrology report documents that your project won't make things worse.

Common Questions

What clients typically ask about a hydrology report?:

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We handle a hydrology report? for projects across Southern California.

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