ML20236A878

Forwards Comments on Grand Junction Phase II Preliminary Design.Diversion Ditches & Energy Dissipation Areas Not Adequatelu Designed as Indicated in T Jonson Surface Water Comments

ML20236A878
Person / Time
Issue date:	09/25/1987
From:	Starmer R NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
To:	Fliegel M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
References
REF-WM-54 NUDOCS 8710230206
Download: ML20236A878 (10)

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MEMORANDUM FOR: Myron Fliegel, Section Leader p

Uranium Recovery Section Operations Branch Division of Low-Level Waste Management and Decommissioning FROM:

R. John Starmer, Section Leader Siting Section Technical Branch Division of Low-Level Waste Management and Decommissioning 1

SUBJECT:

GRAND JUNCTION PHASE II PRELIMINARY DESIGN Enclosed are the comments on the Grand Junction Phase II Preliminary Design, some major concerns remain outstanding. The diversion ditches and energy dissipation areas have not been adequately designed as indicated in Ted Johnson's surface water comments.

It appears that gully erosion on the pediment surfaces has still not been considered as indicated in Joel Grimm's geology comments.

DOE's calculations for dewatering rates of the tailings appear to be inadequate and contaminant production concentrations from tailings dewatering appear to be non-conservative as indicated in Mike Young's ground water comments.

Please contact Kristin Westbrook at X74543 if you have any questions or comments about this review.

Original Signed By R. John Starmer, Section Leader Siting Section Technical Branch Division of Low. Level Waste Management and Decommissioning, NMSS

Enclosure:

Grand Junction Phase II Preliminary Design comments.

DISTRIBUTION:

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COMMENTS ON GRAND JUNCTION PHASE II PRELIMINARY DESIGN SURFACE WATER HYOROLOGY 1.

Geomorphic Processes at the Cheney Reservoir Site 1

In DOE's. draft Remedial Action Plan for Cheney Reservoir, headward and lateral erosion of existing gullies was identified as the greatest geomorphic hazard to the site (p. E-49). As a result, 00E calculated and proposed set-backs from existing gullies to assure long-term stability of the disposal area. The pile's proposed location and the second phase design are based upon an assumption that all surface-water impacts upon the disposal will result only from sheetwash and uniform areal erosion.

00E, therefore, based its erosion protection criteria on the Uniform Soil Loss Equation (USLE) (Israelson, 1980) in the Phase II Design.

NRC staff's independent geomorphic analysis indicates that pediments at Cheney Reservoir may exist in a state of incipient landscape instability, and a potential exists for new gully formation unrelated to existing gullies. Based on this potential, staff considers set-backs from existing gullies and allowances for uniform erosion may not provide adequate erosion protection for the site.

If new gully formation occurs, designs based on USLE criteria are unlikely to be adequate to ensure long-term site stability.

Potential gully formation is related to channel gradient and drainage-basin area.

In general, as area increases downstream or as gradient becones oversteepened a channel becomes unstable and is likely to develop a gully.

For its analysis, staff compared natural conditions at Cheney Reservoir with other semiarid drainage basins in Colorado, Nebraska, and New Mexico.

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Pertinent data at Cheney Reservoir are derived from the dRAP and USGS topographic maps:

drainage area above proposed site 1.25 km2 (310 acres) channel gradients: above site 0.038 through site 0.021 Morphometric conditions at Cheney Reservoir were plotted with data derived fromfivewesternwatersheds(WellsandGardner,1985)andarepresented graphically in Figure 1.

The discriminant functions which separate gullied from ungullied valleys represent a statistical determination of threshold conditions.

In each case, however, a few gullied channels plot t

below the threshold, and a few ungullied channels plot above. Functions "a" through "d" represent channels underlain by fine-grained deposits, similar to those at Cheney Reservoir.

Function "e" represents coarse-grained alluvium.

The comparison shows that the Cheney Reservoir site possesses conditions which occur on or beyond a. threshold condition for channel instability.

The analysis suggests (1) the Cheney Reservoir area is unstable, (2) gullies may be expected to form along any of the area's channels, and (3) protection from the effects of future erosion should be based on criteria more conservative than uniform soil erosion and set-backs from distant gullies.

The potential impacts of future gully formation on various aspects of the Cheney Reservoir design are addressed in the following surface-water hydrology comments.

2.

Design of Energy Dissipation Areas at Ditch Outlets Staff's review of energy dissipation areas (EDA) designed for diversion ditch outlets indicates they may not be adequate to prevent long-term erosion.

The designs appear to include several shortcomings:

A.

Depth of Placement of Rock Depth of rock placement at the toe of EDA's has been based on computations using the USLE.

Depth of erosion was computed assuming removal of a certain soil volume across l

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4 the EDA. Maximum predicted erosion depth is 26 inches and is accommodated by placing rock to a depth of three feet.

' Staff do not agree that use of the USLE is appropriate for computing erosion depths for the Cheney Reservoir site. The USLE assumes all erosion results from sheet flow, and has little or no applicability to gully erosion.

In all likelihood soil slopes downstream of EDA's will be eroded, because the slope of the natural ground is 2.5% while the slope of the EDA is about 0.4%.

Allowing a transition from a 0.4% rock slope to an unprotected 2.5% earth slope is questionable, because concentrated flow will eventually occur on the earth slope, and gully erosion should be expected.

To adequately design the rock toe, staff suggests consideration of j

all of the following computational methods and assumptions to arrive l

at a reasonable and conservative estimate of gully depth.

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1.

Depths of gullies neighboring the site should detennined.

For example, southwest of the site, existing gully depths exceed three feet. Correlations should be developed between gully depth and drainage area near the site using readily available maps and field data so that, for the drainage area of the EDA in question, future gully depth can be estimated.

2.

If DOE assumes the total amount of soil eroded will be removed from a single v-shaped gully, it could possibly use the USLE for order-of-magnitude estimates of gully depths. Then, for an assumed EDA width of 300 feet and an assumed 2-foot depth of erosion, a v-shaped gully section with a cross-sectional area 2

of 600 ft could be used for design purposes. To determine a design basis depth of erosion, the gully side-slope should be set to the angle of repose of the soil.

3.

For the EDA east of the pile, DOE must consider potential lowering of base level and bank erosion in the adjacent arroyo.

It appears a rock toe at least as deep as the adjacent stream 3

{g should be provided in order to prevent undercutting (see also l

Question 2.B below).

B.

Protection of EDA Against Erosion in Adjacent Stream Staff's review of Dwg. No. GRJ-DS-10-0220 indicates that the east EDA will be located relatively close to an existing natural arroyo.

It also appears that the proposed 4-inch rock in the EDA may not be capable of withstanding the PMF velocity produced in this stream, especially if bank erosion occurs. Riprap in the EDA should be. designed to resist PMF velocities in the stream, or analyses should be presented to demonstrate that the proposed rock is large enough. Consideration should be given to potential erosion at the outside of channel bends, potential for hydraulic jumps 'or energy dissipation to occur, potential lowering of stream levels due to renewed gullying, and potential lateral erosion or undercutting of the EDA by erosion in the stream (seealsoQuestion2.A.3,above).

3.

Design of Diversion Ditches Design documents show that ditches proposed to divert flood flows upstream of the pile will be constructed on slopes ranging from 0.4% to a maximum of 1.3%.

However, proposed recontouring of the area immediately upstream will produce a ground surface with a slope of up to 3.3%.

It appears the ditches could become clogged with sediment and debris on a routine basis because of the limited ability of a relatively flat ditch to tiansport sediment delivered from steep slopes. Since flows could be blocked at critical areas in ditches, resulting in erosion along the embankment, the clogged ditches would require frequent and regular maintenance. Because of the requirement for such maintenance, EPA long-term stability criteria (40 CFR 192) may not be met.

Based on an examination of the site and information provided from geomorphic analyses (comment 1), it appears additional gullying could occur and, therefore, a potential exists for concentration of runoff into 4

Fi diversion ditches at one or more points. This would occur where the new gullies discharge into the ditch. The design should provide erosion protection.in the ditches to resist forces associated with energy dissipation of concentrated flows entering the ditch perpendicular, or near perpendicular, to the ditch alignment. Accordingly, the ditch design should:be revised to account for the above phenomena or justification should be provided to document that sedimentation and flow concentrations will not. result in failure. The basis for all assumptions and calculations should be provided.

4.

Design of Rock Apron Staff note the rock apron surrounding the pile has been designed to resist headcutting and erosion to a depth of three feet. However, the basis and rationale for this. design have not been provided DOE should provide the rationale for design of the rock apron's vertical extent. Justification should include analyses and discussions of the manner in which the rock apron fulfills design criteria suggested in Comment 2 above.

5.

Rock Durability Criteria Staff notes that DOE intends to use rock acquired from onsite excavation as riprap erosion protection for the pile and diversion ditches.

Furthermore, the rock is of marginal quality, and some rock will require oversizing to meet rock durability standards.

In general, NRC staff concludes that good-quality rock should be used at UMTRAP sites. Lower quality rock might be acceptable only in cases where.

DOE can justify its use on the basis of unreasonableness or impracticality of obtaining acceptable rock.

DOE should document the search for sources of good quality rock and should justify the use of rock source (s) selected. Documentation should include analyses and discussions regarding the location, durability, and costs associated with the most practical source of good quality rock and/or the difficulties and costs associated with its acquisition.

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Additionally,'our, review of durability data indicates that several tested samples show " borderline"'results. DOE should conduct at least several I

additional tests to better enhance.the scoring methodology. We suggest li that tests such as the Schmidt Impact Hammer Tensile Strength, and Slake-Abrasion tests be performed in order to better define the rock quality and to better determine the rock-quality scores. Alternately, DOE should propose additional tests that could be used to better define the quality of this specific rock.

REFERENCES CITED Israelson, 1980, Erosion control during highway construction: U.S Federal Highway Administration, NCHRP Report Number 221.

-Wells, S.G. and Gardner, T.W., 1985, Geomorphic criteria for selecting stable uranium tailings disposal sites in New Mexico: New Mexico Energy Research and Development Institute, NMERDI 2-69-1112, v. 1, 353 p.

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GROUND-WATER COMMENTS ON GRAND JUNCTION PHASE II DESIGN Document: Grand Junction Preliminary Design Phase II Comment: -GW1 Calculation No. 05-654-01-01 Hydrogeology - Dewatering DOE calculated the rate of discharge needed to maintain unsaturated conditions in the zone of excavation during removal of the tailings.

They divided the source of water into four discrete portions (1) water flowing through the slurry' wall, (2) water flowing under the slurry wall, (3) water entering the excavation zone from. sides not bounded by the slurry wall and (4) drainage of water from the saturated tailings.

The fourth water source, denoted Q4, was calculated using assumptions that appear nonconservative. 00E assumed that the specific yield of the tailings (slime and sand / slime mixture) was 0.03.

However, available literature indicates that the specific yield is much higher, possibly ranging from from 0.20 to 0.33 (Mercer, 1982). Using Mercer'.s estimates of specific yield in DOE's calculations, almost an order of magnitude more drainage from the tailings can be expected.

This would greatly increase the total discharge needed to dewater the tailings. NRC staff suggests that DOE recalculate the drainagevolumes(Q4)usingmoreconservativeestimatesofspecificyield,or demonstrate that values used in the Preliminary Design document are appropriate.

Reference:

Mercer, J.W., et al.,1982, Parameters and Variables Appearing in Repository Siting Models, prepared for the U.S. Nuclear Regulatory Commission,NUREG/CR-3066.

Document: Grand Junction Preliminary Design Phase II Coment: GW2 Calculation No. 05-654-01-01 Hydrogeology - Dewatering DOE calculated the effect of ground water discharge from the saturated zone in the excavation area on the water quality of the Colorado River.

DOE concluded that no impact to surface water quality is expected. NRC staff reviewed these calculations and considers the results nonconservative.

00E determinea degradation of surface water quality by diluting the maximum constituent concentration of the groundwater, times the maximum discharge rate, with the low flow discharge rate from the river. However, DOE's maximum concentrations for both NH concentrationslistedint$e(393mg/1)andTDS(6000mg/1)arelowerthan Final Environmental Impact Statement, pages F86, F97, F167. F182, F187 and F188. 00E should consider using these higher concentrations in groundwater effluent calculations.

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I WM-54/GRJ/PH2 DESIGN /MHY/87/09 !

Document ~: Grand Junction Preliminary Design Phase II comment:

GW3 i

Dewatering-DOE performed two calculations on dewatering in the Preliminary Phase II l

design document. These are calculation numbers 05-622-01-00 and 05-654-01-01. -D0E's method of determining the hydraulic conductivity may have been inappropriate. DOE states that the Theim method was used to calculate hydraulic conductivity from five test pit pump tests. However, the Theim l

method assumes steady state conditions.

DOE did not. indicate if steady state existed prior to taking the final measurements. Therefore the values taken i

during the test may not accurately reflect equilibrium conditions. Also, the test pits were constructed in alluvial material along the perimeter of the tailing pile and may not represent conditions in the tailings.

j DOE should indicate.by providing measurements that equilibrium was reached in 1

the testpits used in calculation No. 05-622-01-000. Also, DOE should justify

.use of test pits on the tailing perimeter alluvium as being representative of the main portions of the actual tailings pile, i

j Document: Grand Junction Preliminary Design Phase II Comment: GW4 l

Calculation No. 05-670-01-00 Radon Barrier Design - Thickness DOE considered only radon attenuation when they calculated the radon barrier thickness for the embankment at the Cheney Reservoir disposal site.

No consideration was given to whether the thickness adequately limits infiltration into the embankment, thus protecting ground-water resources.

This aspect of the remedial action must be considered so that both radon exhalatiun and ground-water protection are adequate and acceptable.

The two foot thickness proposed in the Preliminary Design may be adequate to inhibit infiltration rates and protect ground water, but no calculations were provided.

DOE should consider infiltration rates when designing the cover thickness, and include the calculations in future design documents.

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