The first meeting of the Hydrologic Frequency Analysis Work Group was convened in Room 4145, Main Interior Building, Washington, DC on January 20, 2000. The attendees were: Steve Chase, Federal Highway Administration; David Conrad, National Wildlife Federation; Mike Grimm, Federal Emergency Management Agency; Zhida-song James, Dewberry & Davis; Ken Bullard, Bureau of Reclamation; Bill Kirby, U.S. Geological Survey; Will Thomas, Michael Baker, Jr.; Lesley Julian, National Weather Service; Don Woodward, Natural Resources Conservation Service; and Martin Becker. Don Woodward and Martin Becker, acting co-chairman, conducted the meeting.

The meeting began by discussing the three priorities of the work group as assigned to us by the Subcommittee on Hydrology. These priorities are given in a December 21, 1999 letter from Don and Martin that announced the first meeting of the work group. These priorities are:

1. Develop a set of frequently asked questions and answers on the use of Bulletin 17B.

2. Prepare a draft position paper that reflects the most appropriate computational methodology for a determination of the flood flow frequency for an ungaged watershed.

3. Prepare a draft methodology for frequency analysis of gaged streams whose upstream flows are impeded.

Each work group member was asked to comment on the applicability and utility of the above priorities.

Steve Chase - Federal Highway Administration (FHWA)

Steve described FHWA's practice of estimating scour for the 500-year event at bridges and emphasized the importance of estimating the 500-year discharge. He agrees that estimating flood discharges for ungaged sites is an important issue since most bridges and culverts are on ungaged streams. He believes that some type of watershed modeling or routing is needed to compute flood flows for regulated watersheds

Lesley Julian -National Weather Service (NWS)

Lesley briefly described how theNWS uses L-moments for estimating the magnitude and frequency of precipitation data. She thought the priorities of the work group were reasonable but indicated she was not knowledgeable enough to offer any comments on the three priorities.

Mike Grimm - Federal Emergency Management Agency (FEMA)

Mike noted that priority #2 is important to FEMA and briefly described procedures that FEMA is using to judge the reasonableness of flood discharges for ungaged watershed. He indicated that FEMA defers to procedures used by the U.S. Army Corps of Engineers (USACE) for flood frequency analyses for regulated watersheds.

Bill Kirby - U.S. Geological Survey (USGS)

Bill commented that it may be difficult to provide meaningful guidance on priority #2 since this a difficult issue to address. He believes that regulated flood frequency analysis is an important issue and is related to the issue of flood frequency analysis for ungaged watersheds. Bill commented that partial-duration series data are needed for defining flood discharges less than the median peak flow. Bulletin 17B guidelines utilize annual peak data and really only appropriate for estimating flood discharges above the median.

Zhida-Song James - Dewberry & Davis

Zhida asked who the audience is for the Frequently Asked Questions - the lay public or the technical community. The implication is that the audience will determine the type of questions and responses. She indicated that mixed population frequency analysis (such as floods from rainfall and snowmelt) is an issue that the work group might want to address in the future. Relative to priority #2, she commented that the USGS regression equations need updating in some states and that the hydrologic regions as defined by USGS may be too large in some states. She commented that regulated flood frequency analysis should be acomplished through a modeling approach and that this approach is time consuming and expensive.

Ken Bullard - Bureau of Reclamation

Ken indicated that the Bureau is interested in estimating the magnitude and frequency of low discharges such as the bankfull discharge (about 1.5 year event) for sediment transport studies. He indicated that the Expected Moments Algorithm (EMA), a new approach for estimating the moments of flood data, should be made available to the public.

David Conrad - National Wildlife Federation

David indicated that he is interested in procedures for extrapolating frequency curves to extreme events and flood mapping issues.

Will Thomas- Michael Baker, Jr.

Will indicated that the Frequently Asked Questions should be used to extend and clarify issues in Bulletin 17B. An example is to develop questions and answers that will clarify the applicability of the regional skew map in Plate I of Bulletin 17B. He commented that priority #2 should be to develop a methodology for judging the reasonableness of flood discharges for ungaged watersheds, not to determine which methodology is best. He handed out an example of how FEMA determines the reasonableness of flood discharges estimated from watershed models by comparison to confidence limits about regression equations and gaging station data. This approach could be a starting point for satisfying priority #2. Will believes that the procedures used by (USACE) for regulated frequency analysis is also a good starting point for defining priority #3.

Action items

1. By February 20, 2000, each work group member is to draft at least one frequently asked question and response and email it to all work group members. The questions should be related to Bulletin 17B and how to make the guidelines work better.

2. Mike Grimm will email out the hydrology review guidelines developed by FEMA to all work group members for comment and review. Each member is to provide comments to Mike and Will Thomas by March 20, 2000. Mike will provide an introductory paragraph explaining why FEMA developed the guidelines and the intended audience.

3. By April 20, 2000, each member of the work group is to define what they mean by regulated flow. The definition should include information about how much upstream storage is required for the flow to be regulated.

Next meeting

The next meeting will be the morning of July 13, 2000 in Washington, DC. At the next meeting, a chairman and vice chairman will be chosen.

Will Thomas
January 31, 2000