A HEC-SSP alternative that runs Bulletin 17C online

What HEC-SSP is and who it suits

HEC-SSP is the Statistical Software Package from the US Army Corps of Engineers’ Hydrologic Engineering Center (HEC). HEC designed it to combine the capabilities of its older HEC-FFA, STATS, REGFRQ and MLRP programs. The current release is version 2.3, with a 2.3.1 beta also on the download page, and it is supported on 64-bit Windows 10 and 11 as a setup or portable package. The Corps developed it for its own use, but anyone outside the Corps may use it without charge; HEC does not support non-Corps users.

HEC lists eleven analysis types: Bulletin 17 flow frequency (both 17B and 17C), general frequency, volume frequency, duration, coincident frequency, curve combination, balanced hydrograph, distribution fitting, mixed population, correlation and record extension. Gauge data and most output live in HEC-DSS, and each run writes a text report in the style of HEC-FFA. It suits USACE districts, US floodplain and dam studies that feed HEC-HMS or HEC-RAS through DSS, and anyone who needs more than a peak-flow frequency curve.

Where HEC-SSP is the better choice

• Analyses Studio does not have. Volume-duration frequency on daily flows, duration curves, coincident frequency (a stage that depends on two variables), balanced hydrographs for reservoir studies, mixed populations, correlation and record extension. Studio’s Flood Frequency Analysis fits distributions to one peak series.
• The full Bulletin 17C data model. Bulletin 17C adopted a representation of flood data that allows interval and censored types, and an improved method for confidence intervals. Studio takes historical peaks and perception-threshold periods (a year known to lie below a level), but not interval data with an upper and lower bound. Its analytic limits for log-Pearson III use the Bulletin 17B non-central t approximation; the alternative is a parametric bootstrap.
• Expected probability. HEC-SSP plots the expected probability curve with the computed curve and confidence limits. Studio does not compute it.
• HEC-DSS. If the study runs through HEC-HMS, HEC-RAS or HEC-ResSim, keeping the statistics in DSS saves a conversion.
• USACE submissions. The Corps’ own program is what Corps reviewers expect to see.

Where Delta Hydro Studio differs

Runs in a browser. Nothing to install, no Windows requirement, and saved analyses open from any machine.

The Bulletin 17C core. Log-Pearson III by method of moments, with the Expected Moments Algorithm (Cohn et al., 1997) when you enter historical peaks and perception-threshold periods. Low outliers are found with the iterative Multiple Grubbs-Beck test, removed from the fit, and accounted for with the conditional-probability adjustment, which also lets log-based distributions fit records containing zero-flow years. Regional skew is weighted with the station skew by mean square error, using the regional value and MSE you enter.

More distributions and estimators. Thirteen distributions fitted side by side, each by method of moments, L-moments, maximum likelihood or probability weighted moments. Goodness of fit is reported with Kolmogorov-Smirnov, Anderson-Darling and chi-square statistics, the first two with parametric-bootstrap p-values for the top-ranked fits, and AIC and BIC. A composite rank points to the best fit; the frequency plot is there to overrule it. Eight plotting positions are available, including Hirsch-Stedinger for censored records.

Confidence intervals that say where they came from. A parametric bootstrap (500 samples by default) works for every distribution and method. Analytic intervals are offered for Pearson III-family fits by method of moments and for any maximum-likelihood fit, and the report states which method produced each interval.

Data in without retyping. Load an annual peak series by station number from USGS, South Africa’s DWS or the Australian Bureau of Meteorology, upload a CSV or text export from any agency and map its columns, or paste values. Flow units can be m³/s, ft³/s or L/s. A USGS peak coded as historic is moved to the historical data panel automatically.

Checks before the fit. The Mann-Kendall trend test with Sen’s slope flags a non-stationary record. For a group of gauges sent from the Stream Gauge Finder, the tool computes the Hosking-Wallis discordancy and heterogeneity measures; the Stream Gauge Finder searches DWS stations in South Africa, USGS gauges in the United States and BoM gauges in Australia.

Reports and the next step. Word and Excel reports, saving to projects, and Send to: a design quantile goes straight into the Culvert Designer or Flood Routing.

HEC-SSP and Studio side by side

How to move a job across

1. Get the peak series out. Copy the annual peaks (year and flow) from HEC-SSP or HEC-DSSVue, or import a USGS station by number. Year and flow are required; gage height and qualification codes are optional.
2. Historical information. A perception threshold with an unbounded upper limit maps to a Studio threshold period: start year, end year and threshold flow. A known flood goes in as a historical peak. Interval data with an upper bound has no equivalent; note it as a limitation.
3. Skew. Turn on weighted skew and enter the same regional skew and MSE you used in HEC-SSP.
4. Fit the same model first. Select log-Pearson III with method of moments so the comparison is like for like; the EMA applies only to that pairing and to Pearson III by moments. Other distributions in the same run are fitted to the systematic record alone, and the tool warns you.
5. Compare. Quantiles should agree closely for the same inputs. Expect the confidence limits to differ, because the interval methods differ.
6. Then widen the view. Fit the other distributions as a sensitivity check and report the spread at your design return period.

Next steps in Studio

Open Flood Frequency Analysis with your peak series. Find gauges near a site with the Stream Gauge Finder (DWS, USGS or BoM). In South Africa, compare the at-site result with a regional estimate from the index flood method, which runs for South African accounts.