New Tool: Runoff Hydrograph Generator (SCS Convolution)
I am incredibly excited to announce the final piece of our core hydrology suite: the Runoff Hydrograph Generator! 🌊
Over the past few weeks, we built tools to analyze IDF curves from raw gauge data and distribute those storms over time using Design Hyetographs. But ultimately, rain is just the input. As civil engineers, what we really need to know is what happens when that water hits the ground.
This new web-based worksheet uses the industry-standard NRCS/SCS Unit Hydrograph Convolution method to translate a rainfall event into a continuous, time-series flood wave (a runoff hydrograph). It calculates abstractions, handles routing, and even includes a built-in TR-55 Time of Concentration (Tc) calculator!
Why Peak Flow (Q = CiA) Isn't Enough
For small pipes and catch basins, calculating the single maximum "Peak Flow" using the Rational Method (Q = CiA) is perfectly fine. However, modern stormwater management requires much more than just sizing a pipe.
If you are designing a detention basin, a dam spillway, or evaluating downstream channel routing, you need to know the entire volume of water and the shape of the flood wave. A 10-minute burst of heavy rain might produce a high peak flow, but a 24-hour sustained storm will produce a massive volume that will quickly overflow a poorly sized detention pond.
A Hydrograph solves this. It gives you the entire timeline of the storm runoff from start to finish, mapping Discharge (Q) against Time (t).
Built-In Engineering Tools
Instead of just being a dumb calculator, I built this worksheet to act like a fully integrated hydrologic workstation. It handles all the tedious intermediate steps for you:
1. The TR-55 Time of Concentration (Tc) Calculator
No more switching to a different spreadsheet to figure out your Tc. Click the little calculator icon next to the Tc input, and a hidden panel slides out. It allows you to break your flow path into the three standard TR-55 regimes:
- Sheet Flow: Calculates travel time using the Kinematic Wave Equation based on length, slope, Manning's n, and 2-yr rainfall.
- Shallow Concentrated Flow: Automatically calculates velocity based on surface type (paved vs. unpaved).
- Open Channel Flow: Define your channel length and velocity to compute the final leg.
The total is automatically summed and pushed back into your hydrograph calculation!
2. Interactive SCS Curve Number (CN) Matrix
Need to look up a Curve Number? Click the search icon. A searchable matrix pops up containing standard NRCS land use descriptions (Urban, Residential, Cultivated, Pasture, Woods) mapped against Hydrologic Soil Groups (A, B, C, D). Click the value you want, and it instantly updates your model.
3. Dual-Input Rainfall Support
You can generate the runoff using two different methods:
- Synthetic SCS Curves: Simply select Type I, IA, II, or III, enter a 24-hour total depth, and the tool builds the storm for you.
- Manual Hyetograph: (My favorite feature!) You can paste a custom incremental depth series directly into the tool. This means you can use our Design Storm Hyetograph tool to generate a hyper-specific Alternating Block storm, copy the data, and paste it directly in here to route the runoff!
Pro Tips for Modelers
The main visualizer shows two things at once. The solid blue curve is the actual runoff leaving your site (read on the left axis). The inverted bars coming from the top represent the rain falling from the sky (read on the right axis). The gray bars are Total Rain, while the dark purple bars are Excess Rain (rain that didn't soak into the ground). The gap between the peak of the rain and the peak of the flow is your watershed's Lag Time!
Just like in our previous tools, your simulation time step (Δt) is critical. If your watershed has a Time of Concentration of 15 minutes, a 30-minute time step is far too coarse and will mathematically "chop off" the top of your peak flow. As a rule of thumb, ensure your Δt is less than or equal to 20% of your Tc.
Export and Go!
Once you've dialed in your watershed parameters and selected your design storm, click the Export Results button. You'll get a clean CSV file containing the full time-series routing data (Time vs. Runoff Flow). You can easily import this hydrograph directly into HEC-RAS or other hydraulic modeling software to route the flow through channels or bridges.
Head over to the tool page and try it out. If you spot any bugs or have requests for new features (like Kinematic Wave routing or Green-Ampt infiltration), leave a comment below!
Happy Modeling!
- CivilSheets
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