Abstract
Two approaches were used to quantify the spatial distribution of hydrofacies in braided stream deposits. One approach involved mapping a 50 by 60 by 3.3 meter section of a proximal braided stream deposit. In a second study, we generated a 400 by 400 by 2.6 meter section of a medial braided stream deposit using a computer model. In both cases we produced three-dimensional images showing connected hydrofacies with high permeabilities that form preferential flow paths. This information was input to a groundwater flow model and flow paths were analyzed by following the transport of imaginary particles. In both systems, particles that were uniformly distributed at the up-gradient end of the model clustered along preferential flow paths during transport, showing that connection among high permeability facies is a critical factor in hydrogeological investigations involving assessment of contaminant movement and remediation.
Table 1
Description of lithofacies at the Stoughton site.
| Lithofacies | Description | Interpretation |
| Gmc | Coarse, massive gravel with cobbles and boulders; sandy matrix, trace silt; weakly imbricated | Deposited during periods of very high meltwater discharge |
| Gm | Massive to poorly stratified medium to coarse gravel | Deposited as flow decreased from a higher discharge associated with Gmc |
| Gs | Stratified fine to medium sandy gravel | Deposited during sustained, relatively low flow conditions on the flanks of bars or in chutes cut between bars |
| Gp | Planar-bedded fine to coarse gravel | Formed as bars migrated into deeper water |
| Gow | Moderately well sorted, open-work gravel with silt-filled upper surfaces | Deposited during conditions of fluctuating discharge; gravel aggraded during periods of high discharge and then became filled with suspended material when discharge decreased. |
| Gt | Trough-bedded fine to medium gravel | Fills concave-up scours cut into coarser gravel lithofacies |
| St | Fine to medium grained trough-bedded sand | Formed as migrating dunes in the low flow regime |
| Sh | Horizontal to low angle bedded fine to medium sand | Formed as high velocity flow diverged over bar tops or in the low flow regime as aggradation on a plane or low angle bed in channels on bar flanks |
| Sr | Ripple-laminated very fine to fine sand | Formed as flow waned near the limit of sand bedload transport, either on the tops of bars or in emerging chutes along bar flanks |
| Fm | Massive very fine- to fine-grained sand and silt | Deposited in stagnant flow conditions where suspended material settled out rapidly in cut-off channels or bar-top pools; occurs as a thick drape along or filling channel scours |
| Fl | Laminated very fine- to fine-grained sand and silt | Similar to Fm, except that periodic fluxes of sediment were followed by stagnant periods |
Table 2
Hydrofacies in the Stoughton deposit .
| Hydrofacies | Percent of deposit | Average Median Grain size (mm) | Sorting | Geometric mean hydraulic conductivity (cm/sec) |
| Gow | 4 | 9.13 | poor | 1.2 |
| Gmc | 34 | 12.40 | very poor | 0.17 |
| Gm | 25 | 2.99 | very poor | 0.088 |
| Gs/Gp | 26 | 2.67 | very poor | 0.090 |
| Gt | 2 | 0.55 | poor | 0.053 |
| St/Sh | 5 | 0.57 | medium well | 0.030 |
| Fm/Fl | 4 | 0.93 | medium well | 0.00068 |
Table 3
Description of lithofacies in the synthetic deposit.
| Lithofacies | Description | Interpretation |
| G | Imbricated clast-supported gravels | Form channel lag deposits |
| Gm | Matrix-supported gravel with massive to crudely planar bedding | Deposited from bedload |
| Sh | Planar-bedded fine to coarse sand | Deposited in the upper flow regime |
| St | Medium to very coarse trough cross-bedded sand | Deposited in the lower flow regime as dunes |
| Sp | Medium to very coarse sand planar-tabular cross beds | Forms the foresets of advancing sand sheets |
| Sr | Ripple-bedded fine to coarse sand | Deposited in the lower flow regime |
| Sw | Wavy-bedded fine sand with no structures | Deposited in the lower flow regime |
| Ss | Massive medium to coarse sand with no structures | Deposited from suspension |
| O | Organically dominated fines with minor sand and no structures | Deposited as overbank or drape deposits |
| Fm | Massive or finely laminated fine material with no structure | Deposited as overbank or drape deposits |
Table 4
Hydrofacies for the synthetic deposit arranged in order of decreasing
hydraulic conductivity.
| Hydrofacies | Percent of deposit | Mean grain size (mm) | Froude number | Mean hydraulic conductivity (cm/sec) |
| G/Gm | 4 | -- | 99.0 | 1.0 |
| Sh | 5 | 0.18 | 0.900 | 0.5 |
| St | 20 | 0.35 | 0.550 | 0.1 |
| Sp | 24 | 0.62 | 0.310 | 0.05 |
| Sr/Sw | 34 | 0.14/ 0.13 | 0.210 | 0.01 |
| Ss/Fm/O | 13 | 0.20/ 0.09/ 0.12 | 0.080 | 0.005 |
Figures
Figure 1. Generalized profile of the Stoughton deposit. Descriptions of the lithofacies are given
in Table 1.
Figure 2. Distribution of values of ln K for sediment samples from various hydrofacies in the
Stoughton deposit (adapted from Riemersma, 1997).
Figure 3. Three-dimensional image of the hydrofacies in the Stoughton Scott-type proximal
deposit. The dimensions of the deposit are 60 m in the direction of flow, 50 m wide and 3.3 m thick. Colors are interpreted as follows: red is Gow; yellow is Gmc; blue is Gm; purple is Gs/Gp; black is Gt and Fm/Fl; dark pink is St/Sh. The inset represents the downgradient or exit face through which particles left the flow system during particle tracking experiments. The colors in the inset represent particle concentrations where red, yellow and blue are high concentrations. High particle concentration clusters are evident in the area labeled C and in the lower right hand corner of the inset, just below the area labeled D. The particles within the area labeled D traveled through the tabular sheet-like body of Gmc shown near D1.
Figure 4. Photograph of lithofacies Gs in a channel scour in the Stoughton deposit. Inclusions
of Gow within Gmc are also shown.
Figure 5. Distribution of hydraulic conductivity in hydrofacies in the synthetic deposit.
Figure 6. Three-dimensional image of the hydrofacies in the synthetic Donjek-type medial
deposit (Webb, 1994). Colors are interpreted as follows: dark blue is G/Gm; light blue is Sh; green is St; yellow is Sp; red is Sr/Sw; orange is Ss/Fm/O.