**\[DNFSB LETTERHEAD\]**

March 7, 2003

The Honorable Linton Brooks

Acting Administrator

of the National Nuclear Security Administration

U.S. Department of Energy

1000 Independence Avenue, SW

Washington, DC 20585-0701

Dear Ambassador Brooks:

The staff of the Defense Nuclear Facilities Safety Board (Board) has
reviewed the status of flood mitigation measures following the Cerro
Grande wildfire at Los Alamos National Laboratory (LANL). The wildfire,
which occurred in summer 2000, increased soil hydrophobicity and led to
a much greater potential for flooding at LANL. As discussed in the
enclosure to this letter, recovery of the severely burned areas has been
very slow because of drought conditions at Los Alamos. In addition, it
has not been possible to validate and refine hydrological modeling of
the potential for flooding because of limited rainfall, so the risk of
flooding remains only partially understood. It is the Board's
understanding that Cerro Grande funding for hydrologic modeling expires
in fiscal year 2003.

Defense nuclear facilities at Technical Area 18 (TA- 18) are protected
by a flood retention structure designed and built under emergency
conditions. This structure apparently will be needed longer than
expected because a return to prefire conditions will take more than the
typically observed 3 to 5 years. The structure will probably be needed
until the relocation of the TA- 18 mission is essentially complete. The
Board understands that the Department of Energy has not yet received a
report on material and construction quality assurance for the flood
retention structure. The need for test results on the properties of the
concrete of the flood retention structure was identified earlier in a
Board letter dated November 5, 200l.

It is important that hydrologic modeling be completed to confirm the
adequacy of design assumptions for the TA-18 flood retention structure,
as well as to ensure that other flooding risks are no greater than
currently understood. It is also important that the Department of Energy
confirm the adequacy of the flood retention structure. Therefore,
pursuant to 42 U.S.C. § 2286b(d), the Board requests within 90 days of
receipt of this letter, a report that documents:

-   Plans for continued hydrologic modeling beyond fiscal year 2003
    until the modeling is validated following additional significant
    precipitation.

-   Actions the Department of Energy will take to assure the adequacy of
    the flood retention structure for as long as needed to protect
    defense nuclear facilities.

Sincerely,

***John T. Conway***

***Chairman***

c: The Honorable Jessie Hill Roberson

Mr. Mark B. Whitaker, Jr.

Enclosure

**DEFENSE NUCLEAR FACILITIES SAFETY BOARD**

**Staff Issue Report**

February 27, 2003

**MEMORANDUM FOR:** J. K. Fortenberry, Technical Director

**COPIES:** Board Members

**FROM:** D. Ralston

**SUBJECT:** Flood Mitigation at Los Alamos National Laboratory
Following the Cerro Grande Wildfire

The staff of the Defense Nuclear Facilities Safety Board (Board)
recently reviewed the status of flood mitigation measures at Los Alamos
National Laboratory (LANL). Staff members A. Jordan, A. Hadjian, B.
Jones, and D. Ralston and outside expert P. Rizzo participated in the
review.

The Cerro Grande wildfire in May 2000 severely burned thousands of acres
of Department of Energy (DOE) property and surrounding hill slopes. The
fire decreased vegetation and increased soil hydrophobicity in burned
areas, and greatly increased runoff potential and the risk of flooding.
To protect Technical Area 18 (TA-18) from increased storm flows in
Pajarito Canyon, DOE contracted with the U.S. Army Corps of Engineers to
build a flood retention structure (FRS) upstream of TA-18. The FRS was
constructed of roller-compacted concrete to a height of 72 ft above the
streambed, and was completed in August 2000. The FRS was designed to
retain the volume of a 6-hour, 100-year storm and drain within 96 hours.

After the fire, LANL modified existing hydrologic models of the
laboratory to account for changes in runoff characteristics of the land
surface, but runoff increased much more than the initial estimates. On
June 28, 2000, a small rainfall of 0.69 inches in Pajarito Canyon,
predicted by the models to yield a peak flow of 11 cubic feet per second
(cfs) at TA-18, actually produced a peak flow of 150 ±30 cfs. The model
input parameters were adjusted again so that the results matched this
observed flow, and these changes were incorporated into the design of
the FRS.

In the 2 years since the Cerro Grande fire, Los Alamos has experienced
severe drought conditions. During the drought, there have been no
significant storms or stream flows in Pajarito Canyon to augment the
event of June 28, 2000, used for model calibration. Without additional
data for model evaluation, the input parameters cannot be validated or
refined. The hydrologic models are highly sensitive to the input
parameters describing the condition of the watershed, so without
additional field data, the estimates of storm flows at TA-18 remain
highly uncertain.

Another consequence of 2 years of drought is that recovery of the
severely burned areas has been very slow. Precipitation spurs growth of
vegetation, washes away ash and debris, and breaks down the hydrophobic
layer of soil that limits infiltration. Work crews have raked, seeded,
and watered burned areas on LANL property to aid recovery, but the most
severely burned watersheds are upstream on U.S. Forest Service property
and have seen little or no recovery work. Changes in infiltration
characteristics can be monitored in the field, but thus far the field
investigations of recovery have been very limited.

Because of the drought, runoff from severely burned watersheds is
unlikely to return to prefire conditions within the typically observed 3
to 5 years. To protect TA-18 from a continuing flood risk, the FRS will
likely be needed for longer than initially thought. An extended life
span for the FRS makes an upcoming Corps of Engineers report to DOE on
material and construction quality assurance for the FRS even more
critical. Although the FRS was constructed in the summer of 2000 and
data should have been available in late 2001, this report will not be
completed until March 2003. The report is particularly critical since it
will provide data on the strength of the roller-compacted concrete used
in the FRS. The need for such a report was discussed in a Board letter
dated November 5, 2001. Test results for unconfined compressive strength
at 1 year, direct shear strength at 1 year, and density are particularly
important to understand the structural margin of safety for the FRS.
Data on the potential for alkali reactivity associated with the
aggregate used in the FRS are also of interest, particularly if the life
of the FRS were to be as long as about 20 years.

Because of the slow recovery of the watersheds and because removal of
the FRS will be a major, expensive undertaking, the FRS will be part of
the safety basis for TA-18 for the foreseeable future, perhaps until the
relocation of the TA- 18 mission is complete. Effective implementation
of inspection and maintenance plans will be an essential component of
the TA-18 safety basis.

LANL continues to assess the risk of flooding by updating the hydrologic
models with the changing watershed conditions. The modeling group is
currently funded as part of the Cerro Grande recovery efforts, but those
funds expire in fiscal year (FY) 2003. While the plan is to shift the
responsibility for flood modeling to the hydrology group at LANL, DOE
does not yet have a plan for funding of the work for FY 2004 and beyond.
Substantial uncertainty remains in the models because of the lack of
field data and the slow watershed recovery.