Frequency Curve Method Descriptions

Stream Methods

HEC-HMS Uniform Rain
HEC-HMS Uniform Rain + Reservoir Study
HEC-HMS 2004 USGS Uniform Rain
HEC-HMS NOAA Atlas 14 Uniform Rain
Results from the calibrated HEC-HMS rainfall runoff model using frequency rainfall depths from either NOAA Atlas 14, or the 2004 USGS rainfall atlas for Texas, which have been reduced uniformly across the basin.
HEC-HMS Elliptical Storm
HEC-HMS 2004 USGS Elliptical
HEC-HMS NOAA Atlas 14 Elliptical
Results from the calibrated HEC-HMS rainfall runoff model using either NOAA Atlas 14, or the 2004 USGS rainfall atlas for Texas, frequency rainfall depths which have been applied in an elliptical shaped frequency storm. In the elliptical frequency storm, rainfall depths match the NOAA Atlas 14 point rainfall depths at the storm center and decrease outward from the storm center.
SPF (1949 DPR)
Alternate statistical hydrology analysis using the USACE Fort Worth Floodway Detailed Project Report (DPR) to extend the period of record analysis to include the May 1949 annual peak streamflow event.
2D HEC-RAS Uniform Rain Results
A detailed 2-dimensional hydraulic model was built for the study reach in HEC-RAS, and peak streamflow was simulated using the rain-on-mesh capabilities of HEC-RAS.
Storm Shift Study (June 2000)
A storm shift study is performed by shifting a historical storm event of substantial magnitude to optimize peak streamflow in the target basin.
Historical alternate
Statistical no Historical
Statistical with Historical
Statistical flood frequency analysis of the observed period of record at a stream gage location on the stream using Bulletin 17C guidelines. An alternate flood frequency analysis of the observed period of record at a stream gage using Bulletin 17C guidelines, where a different assumption was made regarding the inclusion of historic information or other information to fill in gaps in the record.
Historical lower
Historical upper
Historical alternate upper
Historical alternate lower
The upper and lower 95% confidence intervals of the corresponding statistical results.
Flood frequency analysis of the extended simulated period of record from a Riverware model using Bulletin 17C methods. The Riverware method simulates the watershed with all current reservoir regulation in place for the entire period of record.
Riverware lower
Riverware upper
The upper and lower 95% confidence intervals of the corresponding Riverware statistical results.
FIS effective
FIS preliminary
Preliminary FEMA FIS (RAMPP Team)
Preliminary FEMA FIS (RAMPP Team) 1%+
FIS Effective - Currently effective discharges from a published FEMA Flood Insurance Study (FIS). FIS Preliminary - Discharges from a published Preliminary FEMA Flood Insurance Study (FIS)
Flood of Record
Horizontal Line denoting the highest recorded peak discharge at this location
Lower Guadalupe Feasibility Study
Discharges from a previous 2013 USACE study of the Lower Gualuape River basin
Reservoir Study
Flood frequency results from a stochastic analysis using RMC-RFA software and USACE's dam safety procedures.
USGS regression
Approximate flood frequency results calculated from USGS regression equations

Reservoir Methods

Easement Pool
The easement is privately owned land on which the Federal government has acquired certain perpetual rights, including the right to flood it in connection with operation of the reservoir. The elevation of this easement is referred to as the easement pool.
FEMA effective
Effective FEMA Pool Elevation Frequency values.
The Risk Management Center - Reservoir Frequency Analyis software was used to simulate rainfall floods using the reservoir's infow-frequency curve, adopted flood seasonality, and selected storm inflow hydrographs as outlined in the main report.
Pool elevation frequency curve was developed by routing the flow period of record data through the dam in HEC-HMS.
Inflow Design Flood and Flood Easement
The inflow design flood is the flood hydrograph entering the reservoir that is used to design and/or modify a specific dam and its appurtenant works; particularly for sizing the spillway and outlet works, and for evaluating maximum storage, height of dam, and freeboard requirements. The flood easement refers to water that lies above the conservation pool in reservoirs with flood storage function. A reservoir's flood pool is used to regulate floodwaters.
Spillway Crest
The elevation of the top, or crest, of the reservoir’s spillway. The spillway is (mostly) a trough-like structure over a dam for water to flow past dam into downstream channel.
Top of Conservation Pool
The top of the conservation pool is determined based on the volume water required to sustain the authorized purposes during the Drought of Record.
Top of Dam
The elevation of the top of the dam.
Top of Flood Control & Gates
Top of Flood Control & Tainter Gates
The elevation of the top of the spillway gates in their closed position.


NOAA Atlas 14

Perica, Sanja; Pavlovic, Sandra; St. Laurent, Michael; Trypaluk, Carl; Unruh, Dale; and Wilhite, Orlan, 2018, Precipitation-frequency atlas of the United States: Texas, National Oceanic and Atmospheric Administration Atlas 14, vol. 11, ver. 2.0, National Weather Service, Silver Spring, Md., 280 p.

Data available online through the NOAA Atlas14 Point Precipitation Frequency Estimates (Texas)

2004 USGS Rainfall Atlas for Texas

Asquith, W.H., and Roussel, M.C., 2004, Atlas of depth-duration frequency of precipitation annual maxima for Texas: U.S. Geological Survey Scientific Investigations Report 2004–5041, 106 p.

Bulletin 17C

England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2019, Guidelines for determining flood flow frequency — Bulletin 17C (ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 p.


Hydrologic Engineering Center, 2021, HEC-HMS user’s manual, ver. 4.10, U.S. Army Corps of Engineers, Davis, Calif.

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