Urban Hydrology Lab
Reviewed October 2026 · Civil Systems LLCFive short experiments that connect stormwater equations to what water actually does on a site. Each one asks for a prediction first, then lets you change one input and explain what you see.
- 01 Predict
- 02 Change one input
- 03 Explain the graph
Do peak flows add?
Separate the time a sub-area takes to form its peak (Tc) from the time a pipe takes to carry it (Tt), then see how a full storm changes the answer.
Sub-area A’s peak of 10 cfs forms at its outlet 10 minutes into the storm (its Tc), then takes 10 more minutes to travel the pipe to the junction. B’s peak of 20 cfs forms at 5 minutes and travels 5 minutes. What is the peak at the outfall?
Try it: read the timeline under the plan, then drag Travel time, A to junction until both peaks reach the junction together.
The peaks reach the junction 10 minutes apart: B’s at 5 + 5 = 10 min and A’s at 10 + 10 = 20 min. Each passes the junction alone, so the outfall peak is B’s 20 cfs. Set A’s travel time to 0 min, or load Upstream channelized, and both arrive at minute 10: then the outfall sees 30 cfs.
Both times count. Tc is how long a sub-area takes to deliver its peak to its own outlet; Tt is how long the pipe takes to carry that peak on. But a real storm’s peak is not a single instant, so step 2 runs a full storm.
Same network and travel times, but now a 6-inch, 24-hour NRCS Type II storm falls on both sub-areas. Each produces a full hydrograph that peaks at about 10 and 20 cfs. Is the outfall peak closer to 20 cfs (step 1’s answer) or to 30 cfs (the sum)?
Try it: press Play and watch both hydrographs pass the junction. Then raise A’s travel time to 20 min.
About 27 cfs. A real storm keeps each sub-area near its peak for several minutes, so when A’s peak arrives, B is still flowing well above half of its peak. The same 10-minute offset that fully separated two single peaks barely lowers the combined peak here.
Timing matters most when the offset is long compared with how long each hydrograph stays high. With a 20-minute travel time for A, the outfall peak drops to about 21.5 cfs.
- What is the difference between a sub-area’s Tc and the travel time from its outlet to the junction?
- Why did step 1’s answer (20 cfs) underestimate the full-storm peak?
- Runoff volume did not change when A’s travel time changed. What did?
- Where on a real project could travel time shorten without anyone intending it?
Show the calculation
Teaching model Step 1 treats each sub-area’s peak as a single value that forms at its outlet at Tc and reaches the junction Tt later. Pipes delay flow by the travel time without attenuating it, in both steps.
Published method Step 2 builds each sub-area’s hydrograph with the NRCS curve number equations, the NRCS Type II 24-hour distribution, and the NRCS triangular unit hydrograph, as in experiment 03 and the NRCS Hydrograph Calculator.
Which area sets the peak?
Check whether the whole drainage area, or a fast part of it, produces the larger Rational Method peak.
On a 6-acre site, 3 acres of woods (C = 0.20) drain through 3 acres of parking (C = 0.85). A composite C with the full 20-minute Tc gives 12.6 cfs. Could a shorter, more intense storm falling on only part of the site produce a larger peak?
Try it: drag Storm duration under the graph from 20 down to 5 minutes. Watch the contributing area shrink on the plan.
At 5 minutes only the parking lot reaches the outlet, but intensity rises from 4.0 to 6.8 in/hr: Q = 0.85 × 6.8 × 3 = 17.3 cfs, about 38% more than the composite answer.
The composite calculation is not wrong. It answers the full-area question. Good practice checks the whole area at its Tc and any fast, highly impervious part at its own shorter Tc, then designs for the larger. Load Similar sub-areas to see the full area govern.
- Why can a 5-minute storm on half the site beat a 20-minute storm on all of it?
- When would you expect the full-area peak to govern?
- What would change if the woods were downstream of the parking lot?
Show the calculation
Published method Rational Method, Q = C·i·A, with Q in cfs, i in in/hr, and A in acres (the 1.008 unit factor is taken as 1).
Teaching model Intensity is interpolated log-log between IDF table points and extended past the last point on the last segment’s slope. The partial-area check assumes contributing area grows linearly with travel time within each sub-area.
How much of the rain runs off?
See how the curve number splits rainfall into initial abstraction, infiltration, and runoff, and why small storms change most with development.
Development raises this 20-acre site’s curve number from 70 to 92. In a 6.0-inch storm, post-development runoff is 1.8 times pre-development runoff. In a 1.2-inch storm, is that ratio larger, about the same, or smaller?
Try it: drag Rainfall depth from 6.0 down to 1.2 in. Watch the runoff ratio and the rain partition bars.
The ratio jumps to about 22 times. At CN 70 the site holds back the first 0.86 in as initial abstraction, so a 1.2-inch storm barely runs off (0.03 in). After development the initial abstraction is only 0.17 in, and the same storm produces 0.56 in of runoff.
Development changes small, frequent storms the most. That is one reason stormwater rules regulate runoff volume and water quality for small storms, not only peak flow in large design storms.
- In each condition, where does the rain that does not run off go?
- Why does the developed peak arrive earlier, not only higher?
- For this site’s downstream channel, which matters more: the 1.2-inch storm or the 6-inch storm?
Show the calculation
Published method Runoff depth from the NRCS curve number equations in TR-55 (chapter 2), with Ia = 0.2 S.
Teaching model The hydrograph applies the NRCS Type II 24-hour rainfall distribution and convolves each minute of runoff with the NRCS triangular unit hydrograph, a simplified version of the TR-20 approach. Peaks differ somewhat from TR-55 graphical-method results; use TR-55 or TR-20 software for design.
Where does the stored water go?
Watch a pond trade peak flow for time, and separate temporary storage from runoff-volume reduction.
This pond cuts the site’s peak from 138 cfs to about 52 cfs. By the end of the event, how much water has left the pond compared with the runoff that entered it?
Try it: press Play. Watch the pond fill until outflow crosses the falling inflow, then compare volume in with volume out.
Every cubic foot that enters eventually leaves: volume out matches volume in. Storage peaks at the moment outflow equals inflow on the falling limb. After that the pond releases more than it receives, for hours.
Dry detention lowers peaks; it does not reduce runoff volume. Reducing volume takes infiltration, reuse, or evapotranspiration, the runoff-reduction practices.
- What happened to the water after the peak?
- Why does maximum storage occur exactly when outflow crosses inflow?
- Shrink the orifice. What happens to the peak, the stage, and the time the pond takes to drain?
Show the calculation
Published method Orifice and weir equations with typical coefficients (Cd = 0.6, Cw = 3.1). First-cut storage from the TR-55 Figure 6-1 equation for Type II storms, valid for qo/qi from 0.1 to 0.8.
Teaching model Level-pool routing of the continuity equation with vertical pond sides (storage = floor area × depth) and the outlet at the pond floor. Inflow is the post-development hydrograph from experiment 03, so inflow volume equals runoff volume. No emergency spillway is modeled; the lab flags a stage above the embankment instead.
What controls Tc?
Build a time of concentration from sheet, shallow concentrated, and channel flow, and find the segment that controls it.
This 1,400-ft flow path has 100 ft of sheet flow over dense grass, 800 ft of shallow concentrated flow, and 500 ft of channel. Which segment takes the longest?
Try it: change the sheet-flow surface from dense grass to smooth pavement, then back.
The first 100 ft takes about 13 minutes, over two-thirds of the 19-minute Tc, because water moves as a thin sheet at roughly 0.1 ft/s. Paving that strip (n from 0.24 to 0.011) cuts sheet flow to about one minute and Tc by more than half.
Small changes at the top of the flow path can shorten Tc, which raises the Rational Method intensity in experiment 02 and the NRCS peak in experiment 03.
- Why does TR-55 limit sheet flow to 100 ft?
- Which segment would a grading plan most likely change?
- Tc feeds experiments 02 and 03. What happens to the peak when Tc drops?
Show the calculation
Published method TR-55 chapter 3: sheet flow by the kinematic solution with Manning’s n from Table 3-1, shallow concentrated flow from the paved and unpaved velocity equations, and channel flow by Manning’s equation. Enter the site’s 2-year, 24-hour rainfall from NOAA Atlas 14; the default is an example value.
Using this lab
In lecture
Turn on Presentation view (or press P) for larger type and fewer controls. Ask for predictions before changing anything, then use Reset example to return to the lecture values.
Baselines
Each experiment starts with the lecture example as its dashed baseline. Set as baseline saves the current case, so readouts report the change from it.
Model limits
Every calculation panel names what follows a published method and what is a simplified teaching model. The lab is for building intuition, not for design.