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Factors Controlling Riffle-Scale Hyporheic Exchange Flows and Their Seasonal Changes in a Gaining Stream: A Three-

Dimensional Groundwater Flow Model

R.G. Story, K.W.F. Howard, and D.D. Williams2003

Geology 230

Kent E. Parrish, P.G., C.Hg

February 14, 2013

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Lecture Outline

• Introduction• Site• Field Methods• Model Description• Results• Discussion• Conclusions• Limitations

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Introduction

lifeinfreshwater.org.uk

Riffle-pool: Basic unit of exchange area

Hyporheic Flow

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Introduction

Harvey and Bencala, 1993• Used a model to predict that, in mountain streams,

exchange can occur even during aquifer discharge– Discontinuities in stream gradient (>20% slope)

• But they did not describe other properties of streambed or aquifer that are required to allow exchange

• Did not attempt to find relationships between hydrological or geological parameters of the system and vertical or lateral extent of exchange flows

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Introduction

Tracer Studies versus Modeling• Tracers have been main tool to compare storage

sizes and exchange rates• But too many factors to simply describe system• Tracers cannot distinguish between surface and

subsurface storage• Tracers operate from surface water perspective

but hyporheic flow is through porous medium

Color represents a take-home point

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Introduction

Past Modeling Efforts• Hadn’t attempted to evaluate range of controlling factors

for exchange• Had simulated streams as only one cell wide in models• Had been two-dimensional models

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Introduction

This Article’s Objectives1. Identify hydrological and geological conditions that are

required for hyporheic exchange to occur during aquifer discharge

2. Identify key factors that are sufficient to explain seasonal changes in exchange flows

3. Describe differences in vertical versus lateral exchange flows and paths in different parts of streambed

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Site

Speed River, southern Ontario• Gravel bed• Flows across undulating glacial terrain• Low topo relief (2 - 5 m/km)• Dolomite aquifer bedrock 20 m below ground surface• Domomite overlain by low-K till, kame, and outwash

deposits (K = 10-7 m/s (0.028 ft/d) to 10-8 m/s [0.0028 ft/d)

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Site

Speed River, southern Ontario• Stream lies in recent alluvium 1-1.5 m deep and 5-10 m

wide on each side of the stream (K = 2 x 10-4 m/s [57 ft/d])• At Site, stream is 6 m wide; 0.15 – 0.35 cm deep in

summer• Summer baseflow = 0.1 m3/s• Winter baseflow = 2 – 3 times summer baseflow

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Field Methods

Field Studies• Used nested minipiezometers (dia. 1.3 cm)• Each piezo had single 5 mm opening• Nest consisted of piezos 0, 20, 40,60, 80, or 100 cm

below stream bottom• Nests installed about 1 m apart in two transects

– Across stream at upstream end of riffle– Along axis of stream between upstream and downstream end

of riffle

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Field Methods

Field Studies• Measured hydraulic head distributions in 3-D in one 13-

m-long riffle site

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Field Methods

Field Studies• Data collected over four seasons (Aug 1996 to July

1997• Additional measurements over high and low base flow

periods until November 1998

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Field Methods

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Field Methods

Field Studies• Measured temperature variations over 24-hour period in

stream channel and across upper transect• Measured every 3 hours• Criterion for when surface water reached measurement

depth: when piezo temp cycle had amplitude > 10% of stream channel temp cycle

• Criterion based on Silliman et al. (1995)

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Field Methods

Field Studies• Time delay between temp peaks in stream channel and

in each piezo was used to calculate a first-order travel time estimate for surface water down-welling

• NaCl not conservative tracer so only first-order estimate possible

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Model Description

• Model Domain 1,000m x 500 m• E and W boundaries were Speed River catchment

edges• N and S boundaries parallel to groundwater flow• Grid cell sizes

– 8 m x 8 m across domain– Refined to 1 m x 1 m at the riffle

• 12 model layers• Dolomite aquifer bottom, Kx,y = 10-6 m/s (0.28 ft/d); Kz =

10-7 m/s (0.028 ft.d)

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Model Description

• Kx,y of Layer 12 dolomite estimated from specific capacity tests

• Layers 1-3 till and outwash with K = 3 x 10-8 m/s (0.008 ft/d) estimated from slug tests

• Layers 4-11 till and outwash with K = 10-6 m/s (0.28 ft/d) and 10-5 m/s (2.8 ft/d) “calibrated” layers 1-3 to match vertical gradients across piezos and historical stream discharge data (Water Survey of Canada, 1992)

• 3-9 horizontal 0.25 m thick near stream• High K zone (est. via salt tracers) along and 1.5 m

beneath stream K = 2 x 10-4 m/s (57 ft/d)

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Model Description

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Model Description

Note thin,

constant

thickness cells.

Allowed finer-

scale modeling

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Model Description

Stream as

constant heads

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Model Description

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Model Description

Aquifer Recharge• Estimated from stream discharge records• Recharge = summer and winter base flow / catchment area• Applied as constant flux to top model layerModel Application• Steady state runs (36)• Varied parameters to simulate winter/summer conditions

– Stream heads winter and summer (factor of 2)– Groundwater discharge doubled (field-based) [raised heads by 2 m and

doubled aerial recharge. Then doubled groundwater discharge again• K varied over 2 orders of magnitude (field-based)

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Field Results

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Field Results

Temp

expressed as %

of variations in

stream

temperature

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Model Results

Key Model Factors (model most sensitive to these)• Hydraulic conductivity• Boundaries of hyporheic zone

– Head difference between upstream and downstream ends of riffle

– Flux of groundwater entering the alluvial zone from the sides and beneath

– Steeper Summer stream gradient causes increased exchange flux

– Hyporheic flow travel times related to both flow velocity and distance

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Model Results

Vertical versus lateral exchange flows• Vertical exchange in the channel occurred more consistently than

later flows into the stream banks• Downwelling extended to the bottom layer of the alluvial deposits

in majority of simulations

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Model Results

Note flow

direction

change when K

exceeded

threshold

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Model Results

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Model Results

Summer Heads

(2 x Steeper

stream gradient

than Winter)

Hyporheic Flux

vs K

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Model Results

Winter Heads Hyporheic Flux

vs K

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Model Results

Summer Heads

(Steeper

stream

gradient)

Hyporheic Zone

Depth vs K

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Model Results

Winter

Heads

Hyporheic Zone

Depth vs K

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Model Results

Hyporheic

Travel Time vs

K

Summer Heads

(Steeper

stream

gradient)

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Model Results

Winter

Heads

Hyporheic

Travel Time vs

K

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Model Results

Little

exchange

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Model Results

High

exchange

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Model Results

Summer Heads

(Steeper

stream

gradient)

Hyporheic Flux

vs K

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Model Results

Winter

Heads

Hyporheic Flux

vs K

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Model Results

All SeasonsUpstream

Transect

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Conclusions

1. Low-gradient streams, riffle-scale exchange flows are possible only when high-permeability materials (Kx,y =10-

5 m/s [2.8 ft/d)])

2. Moderate- to low-permeability catchment Kx,y = 10-6 m/s (0.28 ft/d) to K = 3 x 10-8 m/s (0.008 ft/d) with alluvial sediments surrounding the stream

3. Amount of exchange flux, lateral and vertical extent of surface water penetration, and travel times through hyporheic zone determined by three parameters:

– K of the alluvium– GW flux to the alluvium– Hydraulic gradient between riffle ends

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Conclusions

4. Exchange flows tend to be stronger but more variable at the sides than at the center of the stream channel

5. Hydraulic conductivity of the streambed can vary by up to 40% with season due to changes in water temperature

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Model Limitations

1. Model not calibrated (fatal flaw)2. Homogenous K in streambed3. No bank storage (not transient model)4. Isotropic conditions in high K zone around stream

(unrealistic)5. Use of constant head cells allows unrestricted flow

into/out of model. Can lead to unrealistic water balance.

6. Drastic changes in model descretization can lead to numerical dispersion (unrealistic and instable results)

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Suggested Improvements

1. Calibrate the model using field data2. Perform more rigorous sensitivity analysis3. Produce table(s) and figure(s) of the calibration and

sensitivity analysis4. Check and present internal water balance of calibrated

model5. Improve model descretization6. Simplify figures

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References

Harvey, J.W. and K.E. Bencala, 1993, The effect of streambed topography on surface-subsurface water exchange in mountain catchments, Water Resources Research, v. 29, p. 89-98.

Silliman, S.E., J. Ramirez, and M.G. Scafe, 1997, The hydrogeology of southern Ontario, Ontario Ministry of Environment and Energy, Toronto, Canada.

Water Survey of Canada, 1992, Historical streamflow summary-Ontario, Inland Waters Directorate, Water Resources Branch, Department of the Environment, Ottawa, Ontario, Canada.