PondMAC Modified Rational Detention Calculator How to Use

1Design Rainfall (IDF)

Search a place or coordinates, or click a point in Texas to pull live NOAA Atlas 14 data — or switch to manual entry.

Basemap
Search or click a point in Texas to fetch its NOAA Atlas 14 (Vol. 11) point IDF.
Intensity must decrease as duration increases; values 0–40 in/hr.

2Existing Conditions

The maximum allowable release Q(allow)

Existing Basin Runoff CoefficientC =
Existing Basin Time of ConcentrationTc =min
Existing Basin AreaA =acres
Maximum Allowable RunoffQ(allow) =cfs

3Proposed Conditions

Proposed drainage area to the pond.

Proposed Basin Runoff CoefficientC =
Proposed Basin Time of ConcentrationTc =min
Proposed Basin AreaA =acres
Method A · Triangular hydrograph
acre-ft
 
Critical duration
Method B · Trapezoidal hydrograph
acre-ft
 
Critical duration

Triangular · Modified Rational

Storage Critical inflow Linear outflow Q(allow)

Trapezoid · Modified Rational

Storage Inflow Outflow Q(allow)
Full routing tables · 1–180 min

4Recommended Preliminary Detention

Averaged triangular & trapezoidal storage for the 100‑yr and 2‑yr storms, then summed. Requires a map point so both return periods come from the same NOAA Atlas 14 cell.

Select a point on the map to populate this summary.