Runoff Analysis
Compute peak discharge and runoff volume using the Rational Method and NRCS methods.
Section progress
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Water Resources · Watershed hydrology, stormwater, channels, culverts, networks and hydraulic structures.
Deliverable: Runoff analysis calculation package with peak discharge for each design storm.
Minimum tables, figures and equations for Runoff Analysis
Tables — at least 7
- Table — trial sections or sizes considered, with the capacity of each and the selection decision
- Table — final selected geometry for every element: dimensions, thickness, grade, spacing, elevation
- Table — inlet, pipe, culvert and outlet sizing with slope, capacity and headwater depth
- Table — ultimate limit state check summary: demand, capacity, ratio, pass or fail, governing clause
- Table — serviceability check summary: deflection, crack width, settlement, freeboard or velocity against its limit
- Table — factors of safety achieved against the factor required, per failure mode
- Table — pre- versus post-development peak flows by design storm
Figures — at least 5
- Figure — free body diagram of each isolated element, fully labelled with loads, reactions, dimensions and axes
- Figure — shear and moment (or pressure and velocity) diagrams for each force-carrying element
- Figure — dimensioned section or plan of each designed element
- Figure — capacity versus demand plot, interaction diagram, or rating curve as applicable
- Figure — hydraulic grade line and energy grade line profile
Equations — at least 8
- Equation — equilibrium equations written out for each free body (sum of forces and sum of moments, or continuity and energy)
- Equation — the internal force relations V(x) and M(x), or the momentum/thrust relation, used to compute each element's demand
- Equation — the resulting demand at the critical section of each element, with numeric substitution
- Equation — the capacity expression for each element type, shown with full numeric substitution and units
- Equation — the sizing criterion that sets the final dimension (for example required area, depth or diameter)
- Equation — each limit state check written as demand over capacity with numbers substituted
- Equation — the factor of safety calculation for each failure mode checked
- Equation — outlet scour velocity and riprap sizing check
Number every table and figure (Table 4.x, Figure 4.x), caption it, and refer to it by number in your text. Number displayed equations and show the substitution with units. These counts are minimums — add whatever else your design needs.
Engineering documentation standard — required in every Chapter 4 subsection
These rules are graded on every subsection. Work that misses them is capped on technical accuracy, exhibits, codes and communication, whatever the quality of the prose.
Code and standard references
- Every requirement, factor, coefficient, limit and allowable you apply cites the governing document AND the exact section, article or sub-article number — e.g. ACI 318-19 §22.5.5.1, AISC 360-22 Chapter J, Section J3.6, AASHTO LRFD 10th Ed. Article 3.6.1.2.2, ASCE 7-22 §12.8.1, ASTM D2487, state DOT manual section, local stormwater manual chapter.
- Give the edition or year of every document the first time it appears, then use a consistent short form.
- Where a code equation is used, quote the equation number (e.g. Eq. 22.5.5.1) next to your displayed equation.
- Where you depart from a code provision, state the clause you are departing from and the engineering justification.
- List every code, standard and manual actually used in a Codes and Standards table at the start of the subsection.
Citations for statements
- Every statement of fact, value taken from elsewhere, material property, soil parameter, rainfall depth, unit cost or published method carries an in-text citation (APA) to its source.
- Field and lab data cite the report, boring log, gauge, survey file or test number and its date.
- Manufacturer data cites the product literature and revision date; software results cite the program, version and model file name.
- Uncited assertions are treated as assumptions and must appear in the assumptions table with a justification.
- Every in-text citation resolves to a full entry in the reference list.
Step-by-step calculations
- Structure every calculation the same way: (1) objective, (2) governing code clause, (3) equation in symbolic form with the equation number, (4) definition of each symbol, (5) numerical substitution, (6) result with units, (7) comparison against the limit and the pass/fail statement.
- Show the substitution line — never jump from the formula to the answer.
- Number displayed equations sequentially (Eq. 4.1, 4.2, …) and refer to them by number in the text.
- State the load or flow combination governing each calculation by name.
- Carry consistent significant figures and round only at the reported result; state the rounding convention once.
- Present repetitive element checks in a calculation table with one row per element and the same column order throughout.
Free body diagrams and figures
- Draw a separate free body diagram for each isolated element — no combined sketches standing in for several members.
- Dimension every FBD: span, depth, thickness, cover, eccentricity, embedment, slope, pipe diameter, wall height — with the dimension lines and values shown.
- Label every force, pressure, reaction and moment with its symbol, magnitude and units, and show the sign convention and coordinate axes.
- Show supports and boundary conditions explicitly (pin, roller, fixed, elastic, buoyant, hydrostatic).
- Accompany each FBD with its shear, moment, thrust, pressure or hydraulic grade diagram at the same scale reference.
- Number and caption every figure (Figure 4.x) and refer to it by number in the narrative; add a scale or north arrow to plans.
Units and notation
- Every number in text, tables, figures and equations carries its unit — no bare numbers.
- Use one unit system throughout (US customary or SI); if both appear, give the converted value in parentheses consistently.
- Check dimensional homogeneity of each equation and say so — the units of both sides must match.
- Provide a nomenclature table defining every symbol with its unit.
Checking and verification
- Every calculation is checked by an independent route — hand check against software, alternative method, order-of-magnitude estimate, or a published worked example — and the check is shown, not just claimed.
- Report demand-to-capacity ratios and factors of safety against the required values, with the source clause for each required value.
- Include a verification/checking table: item, method of check, expected, obtained, difference, accept or revise.
- Sanity-check every result (magnitude, direction, plausibility) and state the conclusion.
- Record who checked the work and on what date; flag anything still unverified as an open item.
- State limitations and the range over which the result is valid.
How to complete this section
Do this next: Read the Runoff Analysis lecture and the worked example so you know what "Runoff analysis calculation package with peak discharge for each design storm." has to contain.
Not sure how to start or how much depth is expected? Read the fully written model example for this deliverable first — it shows the structure, tables and level of justification your advisor grades against.
Hydraulic Design Office — what this workspace teaches
Watershed hydrology, stormwater, channels, culverts, networks and hydraulic structures.
- Watershed delineation, land use, curve number and imperviousness
- Rainfall: IDF curves, design storms, return periods
- Rational Method and NRCS/SCS runoff methods with validity limits
- Hydrograph development and flood routing (HEC-HMS concepts)
- Open channel flow: Manning's equation, normal and critical depth, froude number
- Culvert hydraulics: inlet vs. outlet control, headwater, performance curves
- Storm sewer network design and hydraulic grade line
- Detention basin sizing, stage-storage-discharge and outlet structures
- Weirs, orifices and energy dissipation structures
- Stormwater BMPs, water quality volume and EPA guidance
End-of-term milestones
- Tuesday, November 17, 2026 — Poster printed and ready. 36 in × 48 in poster finalized and printed one week before the November 24 showcase.
- Wednesday, November 18, 2026 — Final document package uploaded for scoring. Chapters 4–5, calculation package, drawings and appendices uploaded in the app for advisor scoring.
- Wednesday, November 18, 2026 — Poster presentation to faculty and industry. Wednesday poster session — printed 36 in × 48 in poster presented in person; industry reviewers score communication and impact.
- Wednesday, November 25, 2026 — Oral presentation and defense (scored). Scored oral presentation and defense held on Wednesday, November 25.
Runoff Analysis
Compute peak discharge and runoff volume using the Rational Method and NRCS methods.
Section B
Engineering story
A real project situation that frames this module
The team opens week 4 believing runoff analysis is a formality, because the proposal treated it in a single sentence. Compute peak discharge and runoff volume using the Rational Method and NRCS methods. The first review question is not about arithmetic — it is where the basis for rational Method applicability limits (drainage area typically < 200 acres) came from.
Applying the Rational Method to a drainage area well beyond its size limitation. Because runoff coefficient (C) selection by land cover and soil type, the error does not stay local: it is carried into the design of record that drawings, quantities and cost are generated from, and every downstream product inherits it before anyone notices.
Downstream property owners, emergency responders and the utility carry the consequence. On this module specifically, the exposure runs through nRCS curve number method for runoff depth on larger or complex basins, and the cost of correction rises every week the hydrologic and hydraulic model with the sized conveyance moves closer to issue.
Decisions the engineer must make
- What record establishes rational Method applicability limits (drainage area typically < 200 acres), and is that record in the project data inventory?
- Does NRCS TR-55 (Current), Ch. 2, govern here — and is that the edition adopted by the jurisdiction?
- What is the acceptance criterion for runoff coefficient (C) selection by land cover and soil type, and was it written before the result was known?
- Is Q = C·i·A valid over the parameter range this project actually occupies?
- If the check fails, does the team revise the hydrologic and hydraulic model with the sized conveyance or raise a change request against the locked baseline?
Photo 1. Inlet capture during rainfall: gutter spread and inlet capacity decide whether the lane floods.
Capstone Studio instructional photograph
Section C
Why this matters
Professional
A licensed engineer defending runoff analysis cites NRCS TR-55 (Current), Ch. 2, and shows the record behind each input. Your runoff analysis calculation package with peak discharge for each design storm. is reviewed the same way — traceability is assessed before arithmetic.
Technical
Rational Method applicability limits (drainage area typically < 200 acres) is what makes Q = C·i·A usable on this project rather than a formula copied from a reference. Get it wrong and every quantity derived from it is wrong by the same factor.
Safety
The failure mode this module guards against is a conveyance or storage element overtopped by the design event. It reaches people through peak discharge (qp) via NRCS graphical peak discharge method, which is why the safety check is recorded explicitly here rather than inferred from a passing strength or performance check.
Economic
The design of record that drawings, quantities and cost are generated from is priced from this work. Quantities, unit costs and schedule float all trace to rational Method applicability limits (drainage area typically < 200 acres); a late correction here is paid for as a change order, not a redline.
Environmental
Environmentally, this module fixes receiving-water quality, channel erosion and altered baseflow. Choosing conservatively without justification is not free — the excess shows up as material, energy and land that the project consumes for no measurable gain.
Community
Runoff analysis using outdated rainfall frequency data understates flood risk as urbanization and climate patterns change. Households and businesses in the floodplain the design is meant to protect live with that outcome long after the semester ends.
Section D
Learning objectives
By the end of this module you will be able to:
- 1.Justify rational Method applicability limits (drainage area typically < 200 acres), using this project's own conditions rather than a textbook case.
- 2.Evaluate runoff coefficient (C) selection by land cover and soil type, using this project's own conditions rather than a textbook case.
- 3.Apply nRCS curve number method for runoff depth on larger or complex basins, using this project's own conditions rather than a textbook case.
- 4.Analyze peak discharge (qp) via NRCS graphical peak discharge method, using this project's own conditions rather than a textbook case.
- 5.Compute the governing quantity from Q = C·i·A and Q = (P − 0.2S)²/(P + 0.8S), with a unit audit on every term.
- 6.Apply NRCS TR-55 (Current), Ch. 2, and cite the section that governs your acceptance decision.
- 7.Reproduce the worked example for a 15-acre commercial site has C = 0.75 and design intensity i = 4.2 in/hr and defend the interpretation of the result.
- 8.Produce runoff analysis calculation package with peak discharge for each design storm. at a standard the floodplain administrator would accept without a second revision cycle.
Section E
Instructional content
Full lecture notes with figures and governing equations
Runoff Analysis — what the work actually is
Compute peak discharge and runoff volume using the Rational Method and NRCS methods. That single sentence hides the substance of the module: rational Method applicability limits (drainage area typically < 200 acres), and runoff coefficient (C) selection by land cover and soil type. Both must be established from project evidence before anything downstream is credible.
In water resources engineering, this work is the input to the hydrologic and hydraulic model with the sized conveyance. NRCS curve number method for runoff depth on larger or complex basins — which is why this page asks you to record the source of every quantity, not just its value. The design of record that drawings, quantities and cost are generated from depends on it.
- Rational Method applicability limits (drainage area typically < 200 acres)
- Runoff coefficient (C) selection by land cover and soil type
- NRCS curve number method for runoff depth on larger or complex basins
- Peak discharge (qp) via NRCS graphical peak discharge method
Photo 1. Runoff Analysis — what the work actually is in practice — Subsurface investigation: SPT samples and the boring log that every geotechnical number traces back to.
Capstone Studio instructional photograph
Governing relationships and how they are applied here
The relationships below govern runoff analysis. Q = C·i·A; Q = (P − 0.2S)²/(P + 0.8S) — each is valid only inside the parameter range this project occupies, so state that range before substituting.
Runoff coefficient (C) selection by land cover and soil type sets the values you place into these expressions. Any code-prescribed factor must match NRCS TR-55 (Current); a factor lifted from a different edition silently changes the answer.
Q = C·i·A
- Q = peak discharge (cfs)
- C = runoff coefficient
- i = rainfall intensity (in/hr)
- A = drainage area (acres)
Q = (P − 0.2S)²/(P + 0.8S)
- Q = runoff depth (in)
- P = rainfall depth (in)
- S = potential maximum retention = (1000/CN) − 10 (in)
Photo 2. Governing relationships and how they are applied here in practice — Inlet capture during rainfall: gutter spread and inlet capacity decide whether the lane floods.
Capstone Studio instructional photograph
Constraints, adopted standards and the safety case for runoff analysis
NRCS TR-55 (Current), Ch. 2, governs this module: Runoff curve number method
The safety case is explicit here. The failure mode is a conveyance or storage element overtopped by the design event; the people exposed are downstream property owners, emergency responders and the utility; the control that prevents it is peak discharge (qp) via NRCS graphical peak discharge method together with an independent check by someone who did not perform the work.
- Controlling criterion for this module: rational Method applicability limits (drainage area typically < 200 acres).
- Adopted reference: NRCS TR-55 (Current) — cite Ch. 2 by number.
- Failure mode guarded: a conveyance or storage element overtopped by the design event.
- Evidence produced: Runoff analysis calculation package with peak discharge for each design storm..
Photo 3. Constraints, adopted standards and the safety case for runoff analysis in practice — Inlet capture during rainfall: gutter spread and inlet capacity decide whether the lane floods.
Capstone Studio instructional photograph
Where this method stops being valid
The worked example — a 15-acre commercial site has C = 0.75 and design intensity i = 4.2 in/hr — holds only while its assumptions hold. This peak discharge sizes the storm sewer and inlet capacity serving this drainage area for the selected design storm frequency. Outside that envelope the arithmetic still returns a number, and the number is wrong in a way no unit check will catch.
For this project, the boundary you are most likely to push is peak discharge (qp) via NRCS graphical peak discharge method. If you cross it, say so in writing, bound the error, and carry the limitation into your results chapter. A disclosed limitation is professional practice; a silent extrapolation is not.
Photo 4. Where this method stops being valid in practice — Inlet capture during rainfall: gutter spread and inlet capacity decide whether the lane floods.
Capstone Studio instructional photograph
Section F
Engineering workflow
Steps
- 1. Assemble the inputs this module needs — rational Method applicability limits (drainage area typically < 200 acres); runoff coefficient (C) selection by land cover and soil type — each with a unit and a source record.
- 2. Confirm NRCS TR-55 (Current) is the adopted edition and locate Ch. 2.
- 3. State the assumptions and the acceptance criterion for rational Method applicability limits (drainage area typically < 200 acres).
- 4. Evaluate Q = C·i·A and Q = (P − 0.2S)²/(P + 0.8S) term by term, carrying one extra significant figure.
- 5. Test the result against nRCS curve number method for runoff depth on larger or complex basins.
- 6. Audit units and run an order-of-magnitude check by hand before the number leaves your desk.
- 7. Obtain an independent check from a teammate who did not perform the work, and record their name and date.
- 8. Assemble runoff analysis calculation package with peak discharge for each design storm. and submit it to the floodplain administrator for review.
Decision points
- Is every input behind rational Method applicability limits (drainage area typically < 200 acres) traceable? If not — stop and collect the record.
- Does the result satisfy runoff coefficient (C) selection by land cover and soil type? If not — revise the work, never the criterion.
- Would the correction change the design of record that drawings, quantities and cost are generated from? If yes — raise a change-control request before proceeding.
- Have you ruled out the most common error on this module — applying the Rational Method to a drainage area well beyond its size limitation?
Quality checklist
- Documented: rational Method applicability limits (drainage area typically < 200 acres)
- Documented: runoff coefficient (C) selection by land cover and soil type
- Documented: nRCS curve number method for runoff depth on larger or complex basins
- NRCS TR-55 Ch. 2 cited by section number
- Units audited on every expression
- Acceptance criterion recorded before the result
- Independent check signed and dated
- Runoff analysis calculation package with peak discharge for each design storm. attached and named per the course convention
Section H
Interactive visualization
Runoff Analysis — step-through
Advance one frame at a time. Each frame adds one engineering decision to the previous state.
Step 1 of 6
Confirm method applicability by basin size
Section I
Applicable codes and standards
NRCS TR-55
Current · Ch. 2
Adopted design/analysis reference governing this module.
Relevance: Runoff curve number method
Reference the section number and edition in your calculation package. Do not reproduce code text.
Section J
Worked examples
Full engineering solution format
Section K
Common mistakes and how to avoid them
- Applying the Rational Method to a drainage area well beyond its size limitation
- Using a runoff coefficient that ignores post-development imperviousness
- Mixing curve number method outputs (runoff depth) directly into a peak-discharge equation meant for a different method
- Treating rational Method applicability limits (drainage area typically < 200 acres) as a given instead of establishing it from a project record.
- Producing runoff analysis calculation package with peak discharge for each design storm. without showing how runoff coefficient (C) selection by land cover and soil type was satisfied.
- Substituting into Q = C·i·A outside the range where it is valid, and reporting the number anyway.
- Missing peak discharge (qp) via NRCS graphical peak discharge method, which is exactly the path to a conveyance or storage element overtopped by the design event.
- Designing to the average condition when the governing condition is the controlling one.
- Freezing a design before the constructability and access review that would have changed it.
- Using inputs that no field record, laboratory report, or published source supports.
- Stopping at output and skipping verification — an unverified number is not an engineering result.
- Confusing results (what the analysis produced) with conclusions (what the engineer decided).
Section L
Industry case study
Houston Addicks and Barker Reservoirs — Hurricane Harvey
Houston, TX, 2017
Official findings
- USACE post-storm review found rainfall totals and resulting runoff volumes vastly exceeded the reservoirs' original design assumptions, causing controlled and uncontrolled releases that flooded upstream and downstream neighborhoods.
Field observations
- The controlling assumption was documented nowhere in the design record.
- No independent check existed at the stage where the error entered the work.
Engineering interpretation
- Interpretation below is student analysis for instructional purposes, not an official finding.
- Map the failure to a step in your own workflow and state where your process would have caught it.
Lessons learned
- Runoff analysis using outdated rainfall frequency data understates flood risk as urbanization and climate patterns change.
Source: Summarize the published investigation; cite it in your reference list. Do not reproduce copyrighted report text.
Section M
FE Civil exam connection
Handbook FE Reference Handbook — water resources engineering section (record the section number from your handbook edition).
Exam topics
Handbook formulas
- Q = CiA
- Q=(P-0.2S)²/(P+0.8S)
Weak results here feed your FE Civil Academy weak-area queue for targeted practice.
Question 1 of 2
Score: 0/2In runoff analysis, which item must be established BEFORE the analysis is run?
Section N
Apply it to your project — Runoff Analysis
Complete this using your own capstone project data. Every field is saved to your project record and routed to your advisor with this module's submission.
Inputs and sources
Every value needs a traceable source.
| Quantity | Value | Unit | Source / record |
|---|
Assumptions and consequences
| Assumption | Basis | Consequence if wrong |
|---|
Self-check before submission
Section O
Design challenge
Consulting challenge — Runoff Analysis
Your firm has been retained to deliver the runoff analysis scope for a municipal client on a compressed schedule. Produce the technical position your firm would defend at a public meeting.
Client request: The client wants a defensible recommendation, the basis of design, and an honest statement of what remains unresolved.
Constraints
- Adopted local code edition governs; no exceptions without written variance.
- Budget and schedule are fixed; scope changes require change control.
- Public safety and accessibility requirements are non-negotiable.
Deliverables
- One-page basis of design
- Supporting calculation extract
- Risk and limitation statement
Evaluation
- Technical correctness
- Standard compliance
- Clarity of engineering judgment
- Honest treatment of uncertainty
Section P
Documentation workspace
Write the report section for this module in the academic editor
Section Q
File uploads
Accepted: PDF, DOCX, XLSX, CSV, PNG, JPG, ZIP, HEC-HMS
No files uploaded yet.
Section R
Deliverable and advisor review
Runoff analysis calculation package with peak discharge for each design storm.
Submissions route to your assigned faculty advisor and are scored independently by faculty and administrator rubrics.
Reflection
What was the hardest engineering judgment in this module, and how did you resolve it?
Section S
ABET outcome mapping
Runoff analysis calculation package with peak discharge for each design storm. with advisor review and dual scoring.
Assessment: Faculty rubric score and administrator rubric score on this module's submission.
Rubric: Engineering design · Target: 70% of students at or above 'meets expectations'.
Runoff analysis calculation package with peak discharge for each design storm. with advisor review and dual scoring.
Assessment: Faculty rubric score and administrator rubric score on this module's submission.
Rubric: Engineering design · Target: 70% of students at or above 'meets expectations'.
Section T
References and further study
NRCS TR-55 (Current)
Adopted reference — cite section numbers, do not reproduce text.
Runoff Analysis — instructor design procedure
Course template for the calculation package format expected in the final report appendix.
NCEES FE Reference Handbook
Locate the equations used here and note the handbook section for exam recall.
Advisor meeting agenda item
Bring the unresolved decision from this module to your next weekly advisor meeting.