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Composite NRCS Curve Number — Mixed Watershed Worked Example (TR-55)

A subdivision drainage report needs runoff volume, not just peak flow. For watersheds in the 10–2,000 acre range, NRCS TR-55 (1986, USDA-NRCS) is the standard first pass: composite curve number from land-cover sub-areas, then the SCS rainfall–runoff equation for depth and volume. We work a 45-acre suburban catchment with four cover types, compute area-weighted CN (not runoff-weighted), derive potential maximum retention S and initial abstraction I_a, and solve runoff depth for a 2.5-inch design storm. Sources: NRCS TR-55, NRCS NEH Part 630, and the curve number reference.

The watershed

Preliminary plat for a 180-lot subdivision in the Piedmont (suburban Charlotte MSA). NRCS soil survey maps HSG B and C; no dual-group splits required on this tract. Land-cover delineation from plan sheets and NWI screening:

Total drainage area	45.0 ac
Lawn (good condition, HSG B)	18.0 ac (yards, common turf)
Woods (fair, HSG C)	12.0 ac (retained buffer, moderate understory)
Pavement / streets (HSG B)	10.0 ac (roads, sidewalks — directly connected)
Townhome roofs → lawn	5.0 ac impervious footprint draining to pervious lawn
Design rainfall depth (P)	2.5 in (local water-quality / detention basis storm)
Antecedent moisture	AMC II (TR-55 default unless jurisdiction specifies otherwise)

Step 1 · Tabular curve numbers by sub-area

TR-55 Table 2-2 lookups

Curve numbers are looked up by cover type, condition, and hydrologic soil group. Values below are from TR-55 Table 2-2 (residential / urban / agricultural covers):

Lawn, good condition, HSG B → CN = 61 · 18.0 ac
Woods, fair, HSG C → CN = 73 · 12.0 ac
Pavement / roads, HSG B → CN = 98 · 10.0 ac
Townhome cluster (roofs to lawn):
Impervious fraction ≈ 35% of 5.0 ac parcel → weighted sub-area CN:
CN_townhome = 0.35 · 98 + 0.65 · 61 = 34.3 + 39.7 = 74 · 5.0 ac

Source: TR-55 Table 2-2; townhome CN from area fraction of impervious (98) and lawn (61) on the parcel.

Cross-check tabular values on the curve number cheat sheet.

Step 2 · Composite CN — area weighting

CN_c = Σ(CN_i · A_i) / A_total

TR-55 Section 2-3: the composite curve number is a simple area-weighted average. You do not weight by prior runoff depth, peak flow, or imperviousness percentage of the whole watershed.

CN_c = (61 · 18 + 73 · 12 + 98 · 10 + 74 · 5) / 45.0
CN_c = (1098 + 876 + 980 + 370) / 45.0
CN_c = 3324 / 45.0 = 73.9 ≈ 74 (report as CN = 74)

Wrong method: weight by runoff volume from each sub-area after separate Q calcs → biased high
Wrong shortcut: treat entire site as “residential 1/4-acre, B” CN = 77 → +3 on CN, +15% on Q depth

Area-weight, not runoff-weight. Computing runoff from each sub-watershed separately and then blending depths is not the TR-55 composite method. One composite CN, one S, one I_a for the whole 45-acre watershed.

Step 3 · Potential maximum retention S and I_a

TR-55 Equations 2-1 and 2-2

From TR-55 (potential maximum retention after runoff begins, in inches):

S = (1000 / CN) − 10
S = (1000 / 74) − 10 = 13.51 − 10 = 3.51 in

Initial abstraction (TR-55 empirical relation):
I_a = 0.2 · S = 0.2 · 3.51 = 0.70 in

Design rainfall P = 2.5 in > I_a = 0.70 in → runoff occurs

Some state manuals use I_a = 0.05S (NRCS revised formulation); confirm which equation your jurisdiction adopted before permit submittal.

Step 4 · Runoff depth Q (inches)

SCS rainfall–runoff equation (TR-55 Eq. 2-3)

Q = (P − I_a)² / (P − I_a + S)

P − I_a = 2.5 − 0.70 = 1.80 in
Denominator = 1.80 + 3.51 = 5.31 in

Q = (1.80)² / 5.31 = 3.24 / 5.31 = 0.61 in runoff depth

Runoff volume:
V = Q · A / 12 = 0.61 in · 45 ac / 12 = 2.29 ac-ft (74,600 cf)

Run the same inputs in the NRCS curve number calculator.

CN assumed	S (in)	Q depth (in)	Volume (ac-ft)
74 (composite, correct)	3.51	0.61	2.29
77 (single residential shortcut)	2.99	0.72	2.70
70 (ignoring pavement CN 98)	4.29	0.50	1.88

Step 5 · Gotchas that change the answer

AMC II, dual HSG, and connected impervious

AMC II assumption. Tabular CNs in TR-55 Table 2-2 are for Antecedent Moisture Condition II (average). AMC I (dry) lowers CN ~5–10 points; AMC III (wet) raises it similarly. Wet-season designs in the Southeast sometimes require AMC III for detention sizing — here, AMC II Q = 0.61 in would increase to roughly 0.75–0.80 in at AMC III (CN ≈ 82–84).

Dual HSG A/D. NRCS soil maps often show “A/D” for disturbed urban cuts: treat as D (CN one class higher) when impervious cover exceeds ~20%, or as A for full pervious restoration. This site is mapped B and C only, but the 10 ac pavement on B soils would be reclassified to D (CN 98) if the cut fill pad were dual-group A/D — composite CN would jump to ~78.

Roofs to lawn — connected vs. disconnected. TR-55 allows treating disconnected impervious as pervious only if flow travels ≥ 100 ft over pervious area with F_c ≥ 0.05. Townhome downspouts discharging within 20 ft of lawn are connected; we used the 35% impervious parcel CN (74), not lawn CN (61) alone.

Step 6 · What we did NOT do, and when you would

From runoff depth to peak discharge

No unit hydrograph. Runoff depth Q = 0.61 in is volume, not peak. TR-55 Chapter 4 tabular hydrograph or WinTR-55 routes this to peak q_p using T_c and watershed shape.
No detention routing. A 2.29 ac-ft water-quality volume at 0.61 in depth does not fit in a bioretention cell without routing through stage-storage.
No Rational cross-check. At 45 ac, Rational Method is not appropriate for peak; NRCS is correct here. For comparison only: CN = 74 implies C_equiv ≈ 0.50–0.55 for large storms.
No continuous simulation. SCM sizing for MS4 permits may require SWMM or HEC-HMS with long-term rainfall.

Full subdivision hydrology with TR-55 routing and sealed reports: HydroComplete.

Tools used in this example

Reproduce each step in PE-Calc: NRCS curve number / SCS runoff · Rational Method (small-area check only) · time of concentration (for hydrograph step). Reference values from the curve number cheat sheet.

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