For Scenarios 2 and 3, redistribution of radionuclides was estimated using HEC-RAS to calculate river hydraulic parameters and a post-processing routine devel-oped by the RAD staff to estimate radionuclide transport with sediment. Model-ing redistribution of contaminated sediment proved challengModel-ing, as few models are capable of modeling contaminated sediment transport in rivers and deposition on floodplains. For this reason, an original post-processing model was developed. The development of original models is, of course, fraught with uncertainty, and the lack of validation means considerable caution is necessary when interpreting the results of such models. In this case, the RAD staff developed the model for sediment trans-port based on widely accepted theory and used the results of the computations as a guide for insight into the significance of different processes. Although it is likely that this model will prove to be of limited use in rigorous predictive modeling, it is expected to provide sufficiently reliable qualitative results to allow conservative scoping analyses.
3.3.1 Evaluation and selection of models
The hydraulic model used, HEC-RAS, was designed to perform one-dimensional hydraulic calculations for a full network of natural and constructed channels.
Steady-flow water surface profile calculations are currently supported; unsteady-flow simulations and sediment transport/movable boundary computations are cur-rently being added to the code.
The steady-flow component of the code is used for calculating water surface profiles for steady and gradually varying flow. The system can handle a full net-work of channels, a dendritic system, or a single river reach. It is capable of mod-eling subcritical, supercritical, and mixed-flow regime water surface profiles. The basic computational procedure is based on the solution of the one-dimensional en-ergy equation. Enen-ergy losses are evaluated by friction (Manning’s equation) and contraction/expansion (coefficient multiplied by the change in velocity head). The momentum equation is used in situations where the water surface profile varies rapidly. These situations include mixed-flow regime calculations (i.e., hydraulic jumps), hydraulics of bridges, and river confluences (stream junctions). The steady-flow system is designed for application in floodplain management and flood insur-ance studies to evaluate floodway encroachments. Capabilities are also available for assessing the change in water surface profiles due to channel improvements and levees.
Given the lack of existing contaminated sediment transport models, a post-processing routine developed by the RAD staff was used to evaluate contaminated sediment transport based on the sediment transport theory and using the output from
HEC-RAS. This routine was developed after extensive evaluation of the HEC-6 computer code (USACE, 1993), which was designed to evaluate sediment scour and deposition in rivers. Deposition of contaminated sediment in the floodplains was of primary interest in this study, and the use of HEC-6 was abandoned after extensive evaluation due to limitations concerning evaluating flow and deposition conditions in the floodplain regions. HEC-RAS, which relies on the same body of theory and uses essentially the same computational methods, has considerably more data output options. Because HEC-RAS is capable of providing output on hydraulic conditions in the floodplains where deposition was expected to occur, the decision was made to use the output from HEC-RAS as input to the post-processing routine, rather than the channel-floodplain aggregate hydraulic data available from HEC-6.
Although both the Tom and Yenisei Rivers have predominantly sand and gravel bottoms, it was hypothesized that the radionuclides were mainly associated with fine particles (i.e., silt and clay), and only these particle size classes were evalu-ated. This assumption was based on the limited data available indicating the dis-tribution of contamination by particle size and the fact that sorption is generally proportional to surface area, with small particles having relatively larger surface area per unit mass. A mass of fine particles will thus have a greater capacity for sorbing contamination than an equivalent mass of coarse particles. The underlying theory of fine sediment transport is quite limited in terms of predictive capability;
it is governed by empirical relations based on deposition velocities as a function of particle size and critical shear stresses for deposition and scour. The theory used in HEC-6 for fine particle scour and deposition was used in the post-processing rou-tine summarized below. Although limited, this theory is expected to be satisfactory for the scoping-level analyses provided here.
3.3.2 Cohesive sediment and radionuclide transport model
The river was divided into reaches, denoted by the indexi, and channel and over-bank sections, denoted by the index k (Figure 3.1). HEC-RAS was run with a symmetric channel composed of three lateral sections: the right overbank, the left overbank, and the main channel. Water elevations were calibrated to available dis-charge curves along the river reach of interest by adjusting the Manning’s numbers.
The outputs of HEC-RAS were the surface areas, flow rates, and computed bed shear stresses for the channel and overbank sections.
The sediment scour rate in the post-processing routine was calculated as
SEDM
_
i;k=
ERi;k,SCRCRITSA; (3.1)whereM
_
is the erosion rate in kilograms per hour, ER is the slope of the erosion rate curve, is the bed shear stress in the segment,SCRCRIT is the critical shearS U S PC
Figure 3.1. Conceptual model for sediment transport.
stress for scour, and SA is the surface area of the section. The critical shear stresses for silt and clay were assumed to be 0.7 and 2.1 kg/m2, respectively (A. Brenkert, Oak Ridge National Laboratory, and R. Waters, IIASA, personal communication, 31 July 1997). Due to the lack of site-specific data, the critical shear stresses for scour and deposition were set equal to each other. Therefore, scour was assumed to occur above these critical stresses and deposition was assumed to occur below them.
The conceptual model for activity transport (Figure 3.1) shows the transport of adsorbed contaminants into and out of individual sections in the suspended phase.
Complete mixing is assumed within each box and conservation of mass in terms of water and radioactively contaminated sediment is preserved. The activity transport model was developed for equilibrium, steady-state calculations.
The concentration of the activity suspended in the water column is based on an equilibrium homogeneous compartment model given by
C
=
S_
Q
; (3.2)
where C is the concentration,S is the activity-input rate, and Qis the discharge in the compartment. Activity effluent rates from each section were determined as follows:
EFFS
_
i;k=
Ci;kQi;k: (3.3)The influent input rate to a section in a reach is the activity input due to ad-vection from the previous reach. This rate is the sum of two sources: influent
suspended sediment from the same section in the upstream reach and a cross term indicating input from the opposite section of the upstream reach. Therefore, in-put into a channel section of a particular reach may be composed of inin-puts from the channel in the upstream reach and the overbank (floodplain) section in the upstream reach:
The fractionfi;k indicates the fraction of flow and activity into a given section from the same section in the previous reach; the difference in flows between the two reaches in a section is equalized by the flows in the opposite section of the previous reach. For example, if overbank water discharge in reach iis 1,000 m3/sec and is 4,000 m3/sec in reach i,
1
, thenf= 0
:25
, indicating that 25% of the overbank activity of reach i,1
flows into the overbank of reachi; the remaining 75% of flow and activity is diverted to the channel of reachi.The input rate of activity into a section by scouring,SCRSi;k, is obtained by multiplying the mass input rate due to scour in a reach, SEDMi;k, by the activ-ity concentration in the soil or sediment of that reach, SEDCi;k. We assume that the radionuclides are homogeneously distributed within a reach and the activity concentration is constant within a section:
SCRS
_
i;k=
SED Ci;kSEDM_
i;k: (3.6)Releases from the site to the river (Scenario 3) were assumed to be discharged into the overbank or into the channel when no overbank is present. The mechanism for the release is not specified. Possible mechanisms for release include (1) flood-waters washing into the pond and suspending the contaminated pond sediments;
(2) a pond failure causing overland flow of water and sediments to the river; or (3) heavy rains causing overtopping of the pond banks and subsequent overland flow.
The amount of activity released,PNDS, can be adjusted by a release fractionf to account for less-than-total release of activity from the pond:
PNDS
_ =
f PNDCPNDQ=
fPNDCVReleaset
Release
; (3.7)
wherePNDCis the concentration of radionuclides in the release, andVReleaseand
t
Releaseare the volume and duration of the release, respectively.
The suspended concentration in a reach section is determined as follows:
The product of the deposition velocity,, and the concentration of suspended activity determines the areal deposition rate. This rate is adjusted by a factor P to account for the dependence of deposition on the bed shear stress. This factor is sometimes interpreted as the probability that a settling particle will remain on the bed:
whereDEPCRIT is the critical shear stress for deposition.
The areal deposition rate is multiplied by the time step to obtain the total areal concentration, which is then converted to an activity density by dividing by a mix-ing depthdand a bulk soil density:
In each reach, a check was performed to indicate the proportion of the activity deposited in a time step, and total activity in the reach was reduced accordingly to conserve total activity. At low flows (and consequently low bed shear stresses), 100% deposition in a section was possible, indicating that the activity was sub-sequently unavailable for further transport and deposition downstream. However, at higher flows, 100% deposition was generally not expected and the radioactivity was available for downstream transport. The deposition pattern is dependent on the assumed critical shear stress. Plots of the channel and floodplain shear stresses are provided in Chapters 4 and 5. The qualitative pattern of deposition at low flows and scour at high flows was expected because the potential range of critical stresses was within the range of predicted shear stresses at both sites.