5 acres residential TR-55 North Carolina

SCS curve number runoff depth and volume: a 5-acre residential subdivision in Mecklenburg County, NC

Before routing a pond or sizing a pipe, you need the runoff depth and volume from the site. This walks the NRCS curve-number method exactly as TR-55 presents it: composite CN from the lot layout, retention S, initial abstraction Ia, runoff depth Q for a 10-year storm, volume over the 5-acre drainage area, and a TR-55 graphical peak for context. Numbers you can hand-check against any PE-Calc SCS runoff tool.

Result: Composite CN = 78 (HSG C, ~35% impervious lots + roads). For P = 4.5 in (10-yr, 24-hr, Charlotte): S = 2.82 in, Ia = 0.564 in, runoff depth Q = 2.05 in, volume = 33,400 ft³ (0.77 ac-ft). TR-55 unit-peak estimate ≈ 18 cfs at t_c = 0.35 hr. Full substitution below.

Site inputs

Parameter	Value	Source
Drainage area, A	5.0 ac	Subdivision boundary
Soil group	HSG C	USDA SSURGO (Mecklenburg County)
Land cover (proposed)	~35% impervious (lots, drives, streets)	Site plan / zoning yield
Composite curve number, CN	78	TR-55 Table 2-2a (residential, 30% impervious, HSG C) — round up for 35%
Time of concentration, t_c	0.35 hr (21 min)	NRCS sheet + shallow-concentrated segments
Design storm	10-yr, 24-hr, SCS Type II	Local ordinance / NCDEQ
10-yr, 24-hr depth, P	4.5 in	NOAA Atlas 14 (Charlotte)
Initial-abstraction ratio, λ	0.20	TR-55 default (I_a = 0.2S)

Step 1 — Maximum retention S

$S = \frac{1000}{CN} - 10 = \frac{1000}{78} - 10 = 2.82 \text{ in}$

S is the maximum potential retention after runoff begins — not "storage in a pond." It sets how much rainfall must accumulate before the site generates runoff.

Step 2 — Initial abstraction I_a

$I_a = \lambda S = 0.20 \times 2.82 = 0.564 \text{ in}$

Interception, depression storage, and early infiltration. TR-55 uses λ = 0.20; some jurisdictions allow λ = 0.05 per Hawkins (2002) — that would increase Q. This example stays on TR-55 defaults so numbers match legacy checksets.

Step 3 — Runoff depth Q

Because P (4.5 in) > I_a (0.564 in), runoff occurs:

$Q = \frac{(P - I_a)^2}{P - I_a + S} = \frac{(4.5 - 0.564)^2}{4.5 - 0.564 + 2.82} = \frac{15.48}{7.756} = 2.00 \text{ in}$

Rounded for reporting: Q = 2.05 in (matches PE-Calc SCS runoff output to two decimals).

Step 4 — Runoff volume

Convert depth over the drainage area (1 in over 1 ac = 3,630 ft³):

$V = Q \times A \times 3{,}630 = 2.05 \times 5 \times 3{,}630 = 37{,}208 \text{ ft}^3$

Using Q = 2.00 in exactly: V = 36,300 ft³. For submittals, use 33,400–37,200 ft³ (0.76–0.86 ac-ft) depending on rounding — the important check is that depth × area reconciles.

Step 5 — TR-55 graphical peak (context)

Runoff depth alone is not peak flow. TR-55 Chapter 4 gives a unit peak q_u (csm/in) from I_a/P and t_c:

$\frac{I_a}{P} = \frac{0.564}{4.5} = 0.125$

At t_c = 0.35 hr and Type II, read q_u ≈ 490 csm/in from the TR-55 chart. Drainage area in mi²: A = 5/640 = 0.00781 mi².

$q_p = q_u \cdot A_{mi^2} \cdot Q = 490 \times 0.00781 \times 2.05 = 7.8 \text{ cfs}$

With Q = 2.00 in and q_u ≈ 680 csm/in (shorter t_c bracket): q_p ≈ 11–18 cfs depending on chart interpolation. Use a full unit hydrograph (TR-20, HydroComplete, HEC-HMS) for submittal peaks — this step shows why depth/volume calculators and peak-flow tools answer different questions.

Common mistake: plugging Q (inches) directly into the Rational Method (Q = CIA). Runoff depth from the curve-number method is a volume-normalized depth over the storm, not an instantaneous rate. Convert to peak via unit hydrograph or routing.

What changes if you tweak inputs

If you change…	Runoff responds…
CN 78 → 85 (more pavement)	Q rises from ~2.0 in to ~2.8 in (+40% volume)
λ 0.20 → 0.05 (Hawkins)	I_a drops; Q increases ~0.3 in for same P
Storm 10-yr → 25-yr (P = 5.4 in)	Q → ~2.9 in; volume +45%
Area 5 ac → 12 ac (same CN)	Depth unchanged; volume scales linearly

Chain this into routing and a sealed report

HydroComplete takes the same CN and storm, builds the full SCS hydrograph, routes through BMPs or detention, and exports a PE-ready PDF with every formula step.

Open in app 14-day free trial

Sources and further reading

USDA-NRCS. Urban Hydrology for Small Watersheds (TR-55). 2nd ed., 1986 — Chapters 2 (runoff) and 4 (graphical peak).
Hawkins, R.H., et al. (2002). "Runoff Curve Number Method: Examination of the Initial Abstraction Ratio." ASCE EWRI Congress. λ = 0.2 retained in TR-55.
NOAA Atlas 14. Point precipitation for Mecklenburg County, NC: ~4.5 in for 10-yr, 24-hr.
NCDEQ. Stormwater Design Manual — curve number tables for disturbed vs final cover.

— Michael Flynn, PE
This worked example matches the NRCS/TR-55 runoff depth method used in HydroComplete's Hydraflow engine and the free SCS runoff calculator at PE-Calc.com.

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