Understanding Hydrographs in Stormwater Design
Hello stormwater designers! Welcome back to Clear Creek Solutions’ Hydrology Education Series. In our previous lessons, we explored the SCS runoff method, the Rational Method, and the water cycle. Now, let’s take a deeper dive into hydrographs—what they are, how they're used in hydrology, and why they matter in stormwater analysis.
What is a Hydrograph?
A hydrograph is a graph that shows how a watershed responds to a rainfall event over time. Specifically, it plots discharge (Q)—measured in cubic feet per second (CFS) for English units—on the y-axis, and time—measured in hours or minutes—on the x-axis.
This graphical representation reveals key characteristics of the watershed, helping engineers and designers analyze and model how water flows through a system after a precipitation event.
Why Use Hydrographs?
While methods like the Rational Method or the SCS Runoff Method are excellent for estimating runoff and precipitation relationships, hydrographs provide visual and analytical insights into the performance of a watershed. This makes them particularly useful in planning, stormwater system design, and impact assessments.
Hydrographs help identify:
How quickly a watershed responds to rain
The volume of runoff generated
The peak discharge
How long runoff continues after a storm
Components of a Hydrograph
A typical storm hydrograph includes several key features:
Base Flow: The normal, sustained flow of water in a stream, typically from groundwater seepage.
Rising Limb: The part of the graph where discharge increases due to rainfall.
Peak Discharge: The maximum flow rate observed after rainfall.
Falling Limb (Recession Limb): The portion of the graph showing a decrease in discharge after the peak.
Lag Time: The time between the peak rainfall and the peak discharge.
Understanding these components allows us to interpret how a watershed manages stormwater.
What is a Unit Hydrograph?
A unit hydrograph represents the direct runoff hydrograph resulting from one unit (usually one inch) of rainfall excess occurring uniformly over a watershed in a specified time period.
Key Equation:
Q=12×Peak Flow×DurationQ = \frac{1}{2} \times \text{Peak Flow} \times \text{Duration}Q=21×Peak Flow×Duration
Unit hydrographs can vary in form:
Standard Unit Hydrographs
Synthetic Unit Hydrographs
SCS (Soil Conservation Service) Unit Hydrographs
Each has its own shape factors. For example, a common peaking factor is 484, though this can vary depending on watershed characteristics and geographical location.
Determining Peak Flow
To calculate peak flow using a unit hydrograph, use the equation:
Peak Flow (CFS)=K×A×VTp\text{Peak Flow (CFS)} = \frac{K \times A \times V}{T_p}Peak Flow (CFS)=TpK×A×V
Where:
KKK = Peaking constant (commonly 484)
AAA = Drainage area (in square miles)
VVV = Volume of direct runoff (in inches, usually 1 for unit hydrograph)
TpT_pTp = Time to peak (in hours)
For example, if:
Drainage area = 0.25 square miles
Volume of runoff = 1 inch
Time to peak = 1.1 hours
Then:
Peak Flow=484×0.25×11.1≈110CFS\text{Peak Flow} = \frac{484 \times 0.25 \times 1}{1.1} \approx 110 CFSPeak Flow=1.1484×0.25×1≈110CFS
Practical Application: Plotting a Hydrograph
To create a hydrograph:
Prepare a table with time (x-axis) and discharge (y-axis).
Plot the data to form the curve.
Identify components like base flow, peak discharge, and lag time.
This visual representation helps us understand how the watershed reacts over time and supports better stormwater design decisions.
Summary
Hydrographs are powerful tools in hydrology. They:
Visualize how watersheds respond to rainfall
Help identify key flow characteristics
Support design and analysis in stormwater projects
For a more detailed breakdown of hydrological methods—including continuous simulation, single-event models, and graphical techniques—check out our Ultimate Hydrology Guide linked in the video description.
Thank you for watching, and we’ll see you next time!
