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Civil Engineering

Hydrology Reports

Pre- and post-development drainage analysis using Rational Method and unit hydrograph calculations to demonstrate that your project manages stormwater runoff per local requirements.

What Is a Hydrology Report?

A hydrology report quantifies stormwater runoff from your project site before and after development. The analysis determines peak flow rates and volumes for design storm events (typically the 100-year storm in San Diego County) and demonstrates that the proposed drainage system can convey or detain runoff without causing flooding or erosion downstream.

Moment Engineering prepares hydrology reports using the Rational Method for small sites and modified rational or unit hydrograph methods (HEC-HMS) for larger watersheds. Our reports comply with the City of San Diego Drainage Design Manual, County of San Diego Hydrology Manual, Orange County Hydrology Manual, and LA County Hydrology Manual as applicable to your project location.

100-Year

Design storm event for drainage infrastructure in San Diego County

Q = CiA

Rational Method — the standard approach for small drainage areas

CFS / AC

Peak runoff rates sized per acre to design pipes, channels, and inlets

Need a Hydrology Report? Get a fixed-fee proposal — typically within 24 hours.

Request a Proposal ✆ (619) 374-8677

Typical Investment

$3,000 – $6,000

Varies by watershed complexity, number of drainage areas, and jurisdiction requirements.

Fees vary by site conditions, scope, and jurisdiction. Request a proposal for a fixed-fee quote tailored to your project.

Scope of Work

What's Included

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Rainfall Intensity Analysis

Design rainfall intensities calculated from local IDF curves (NOAA Atlas 14 or county-specific data) for the applicable storm frequency and duration at your project site.

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Runoff Calculations

Pre- and post-development peak flow rates calculated using the Rational Method (Q = CiA) or unit hydrograph methods. Runoff coefficients are selected based on land use and soil type.

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Drainage Area Mapping

Tributary drainage areas delineated on the grading plan for pre- and post-development conditions, showing flow paths, concentration points, and time of concentration calculations.

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Storm Drain Sizing

Pipe sizes, slopes, inlet capacities, and hydraulic grade line calculations for the proposed storm drain system using Manning's equation and energy/momentum methods.

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Detention / Retention Design

Where required, we design detention basins, underground storage, or retention facilities to attenuate post-development peak flows to pre-development levels.

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Drainage Plan Exhibit

A plan-view exhibit showing drainage areas, flow arrows, pipe layout, inlet locations, and discharge points — submitted with the hydrology report for plan check.

When You Need a Hydrology Report

Most development projects in Southern California require a hydrology report to demonstrate proper stormwater management. Common triggers include:

New Development

Residential and commercial construction that increases impervious area (roofs, driveways, parking) changes the site's runoff characteristics and requires hydrologic analysis.

Grading Permits

Hydrology reports are typically required alongside grading plans to demonstrate that proposed drainage facilities are adequately sized.

Storm Drain Connections

Connecting private storm drains to a public system requires demonstrating that the added flow does not exceed the capacity of the downstream system.

Detention Requirements

Many jurisdictions and HOAs require post-development peak flows to not exceed pre-development flows, necessitating detention or retention design.

Floodplain Development

Projects near FEMA-mapped floodplains or floodways may require detailed hydrologic and hydraulic analysis to demonstrate no adverse impact.

Subdivision Maps

Tentative map applications require drainage studies to size backbone storm drain infrastructure and demonstrate adequate capacity for the entire subdivision.

Our Process

From Analysis to Drainage Approval

Data Collection

We gather the topographic survey, grading plan, soils report (for infiltration data), and applicable county/city hydrology criteria for your project location.

Hydrologic Analysis

We delineate pre- and post-development drainage areas, calculate runoff coefficients, and determine peak flow rates for the required design storm events.

Hydraulic Design

Storm drain pipes, inlets, and detention facilities are sized using the calculated flows. Hydraulic grade line analysis confirms the system operates within design constraints.

Report & Plan Check

The hydrology report is submitted with the drainage plan exhibit. We respond to plan check comments from the drainage reviewer until the report is approved.

Want to learn more about what a hydrology reports involves?

Read: What Is a Hydrology Reports? →

Related Services

Often Paired With Hydrology Reports

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Grading Plans

Grading plans and hydrology reports are developed together — the grading design determines the drainage areas and flow paths analyzed in the hydrology report.

Learn More →

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Stormwater / LID / WQMP

Stormwater quality requirements (LID and WQMP) address water quality, while hydrology reports address water quantity — most projects need both.

Learn More →

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Erosion Control Plans

Erosion control plans manage sediment during construction, while hydrology reports size the permanent drainage infrastructure.

Learn More →

Common Questions

Hydrology Reports FAQs

The Rational Method is a widely used formula (Q = CiA) that calculates peak stormwater runoff based on a runoff coefficient (C), rainfall intensity (i), and drainage area (A). It's the standard approach for small drainage areas (under 1 square mile) in San Diego, Orange, and LA counties.

The City and County of San Diego require drainage infrastructure to be designed for the 100-year storm event (1% annual chance). Some jurisdictions in Orange and LA counties use different frequencies depending on the facility type — we apply the criteria specific to your project location.

In many cases, yes — particularly if the project requires a grading permit, connects to a public storm drain, or is in a hillside area. Some jurisdictions allow simplified drainage calculations for minor residential projects in lieu of a full report. We'll advise on what's required for your specific situation.

A hydrology report addresses water quantity — how much stormwater runoff is generated and how the drainage system handles it. A WQMP (Water Quality Management Plan) addresses water quality — how pollutants in stormwater are treated before discharge. Most projects need both, and we prepare them together.

Most residential hydrology reports take 1–2 weeks once we have the grading plan and survey. Larger sites or projects requiring detention design may take 2–3 weeks. The report is typically prepared concurrently with the grading plan to streamline the process.

Need a Hydrology Report?

On a deadline? Tell us your project address, jurisdiction, and permit stage and we'll scope the hydrology study and send a fixed-fee proposal same day.

Request a Proposal ✆ (619) 374-8677