In-stream sediment transport & deposition

Chin H. Wu, Kenneth W. Potter, John A. Hoopes, Justin S. Roger, and Evan A. Murdock

Goals & Objectives

Methods

Summary of Significant Findings

Publications & Presentations

 

Goals & Objectives

The overall goal of this study has twofold: (i) to elucidate the hydrodynamic and hydraulic processes, including relationships and parameters, regulating the mobility of sediments and bio-available phosphorus (BAP) in streams; (ii) to quantify the hydrologic and water quality functions of wetlands in controlling stream sediments and BAP. The specific objectives were to:

-Determine the size of runoff events required to mobilize streambed sediments, and quantify the amounts of sediment in streams subject to short term and long-term storage during occasional high-energy hydrologic events;

-Examine the effects of stream morphology on the dispersion and storage of dissolved and fine particulate materials in the stream and estimate the residence time for these fine-grained sediments related to BAP; and

-Elucidate the role of a wetland in controlling hydrologic processes and stream sediment dynamics under different intensity of storm events and quantify sediment budget of a wetland.

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Methods

The Dorn Creek watershed is located northwest of Madison, Wisconsin, in a primarily agricultural region. Based on USGS mapping (USGS1983), five major sub-watersheds (A through E) have been identified (Fig. 1). The watershed drains approximately 32.9 square kilometers, of which 16 percent are wetlands and 78 percent are agricultural land (WDNR, 1996). Dorn creek contributes ~ 7% of the annual surface flow into Lake Mendota. The main stream channel is approximately 13 kilometers long, of which roughly 7 kilometers are perennially wet. Over its length the stream drops approximately 57 meters (excluding a very steep portion at the very top of the watershed) giving an average slope of 0.44 %. The lower portion of the watershed is significantly less steep: 14 meters of drop over the 7 km of perennially flowing stream gives an average slope of 0.19 %. Base flow is provided by a number of small springs, and is on the order of 0.03 cubic meters per second at the lower end of the study area. The creek flows through two wetlands having areas of approximately 0.5 and 1.0 square kilometers. The larger is the Dorn Creek Marsh, located at the bottom of the watershed (downstream of the research sites). There is perennial flow in the channel beginning just upstream of the smaller (upstream) wetland. The area receives an average annual precipitation of 840 mm (30 year average 1971 – 2000, from Midwest Regional Climate Center).

Figure 1: Project location with sub-watersheds(A, B, C, D and E)

During the study period from 2004 to 2006, detailed measurements including water level, flow velocity, sediment concentration, streambed sediment profile, sediment particle size and composition, and sediment transient storage were conducted. A total of nine study sites that cover the lower Dorn Creek watershed in the northwest of Madison, Wisconsin were selected. Sites 1 and 2, the boundary of Reach A, are located in the intermittent flow portion of the stream. Sites 3 and 4 establish the boundaries of Reach B, located downstream of a wetland area in a very flat portion of the stream. Sites 5 and 7 serve as the boundaries for Reach C, located in a steep, boulder-filled section of the creek. Site 6, located between sites 5 and 7, continues to serve as the main study site of riffle-pool sequences. Sites 8 & 9, located at the head of the Dorn Creek downstream marsh, form the boundaries of Reach D and have characteristics of heavy sediment deposits. The slopes of Reaches A, B, C, and D are 0.0052, 0.0015, 0.013, and 0.0011, respectively, as shown in Figure 2.

Figure 2: Study Sites and reaches of the Dorn Creek Watershed

Sediment bed Characteristics and Sediment Storage

Regular (approximately every two weeks) monitoring of cross sectional channel profiles was carried out at the studied sites during the study period to examine temporal changes in sediment storage in the stream channel, as well as an estimate of the magnitude of these changes. Bed sediments were sampled on a monthly basis at sites 1, 3, 6 and 9. Sediment samples were taken using Shelby tube cores, which were separated into 4 depths. Particle size and available phosphorus (AP) analysis was carried out by the University of Wisconsin Soil Testing Laboratory.

Transient storage and solute (phosphorus) processes

Transient storage describes the temporary hydrologic retention of stream water apart from the main advection flow in the stream channel. This hydraulic storage increases the contact time of main-channel water with biogeochemically reactive sediments, and thus, increased transient storage is often presumed to increase nutrient retention in stream ecosystems. Transient storage can be a combination of hyporheic flow (within streambed sediments) and turbulent dead zones within the surface water, although most research to date has focused on hyporheic storage. Any flow obstruction in the stream (submerged vegetation, rocks, leaf packs, debris jams, etc.) contributes to channel roughness and subsequent flow resistance, thereby slowing the downstream passage of stream water.

Three reaches, B, C, and D with varying gradients were selected and dye concentration breakthrough curves were recorded at the upstream and downstream ends of each. A one-dimensional hydrologic transport model was used to estimate transient storage area and exchange in streams. A total of ten dye experiments were conducted during the study period. Results were analyzed for mean travel time.

Wetland Investigation

Wetlands are reputed to reduce peak flows and improve water quality by trapping sediment and phosphorus. However, there are relatively few studies that quantify these wetland services. This section reports on a study of a 45-hectare wetland in southern Wisconsin. The wetland is traversed by a stream channel that drains a predominantly agricultural 17.4 km2 watershed. During the spring and summer of 2006, we collected data and water samples at stations upstream and downstream of the wetland, with the former accounting for 82% of the contributing area. Continuous measurements of water stage were used to construct a stream flow record. During storm events water samples were taken automatically during at 2-hour intervals for the first day, and 8-hour intervals for the next 4 days. Samples were analyzed for total suspended solids, total phosphorus, and dissolved reactive phosphorus. Ten events were observed during the observation period; the two largest events were 1 to 2-year storms. One-dimensional unsteady flow routing was used to estimate the maximum extent of wetland inundation for each event.

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Summary of Significant Findings

Sediment Transport and Wetland Investigation

- The critical shear stress in the top 5 cm of a typical sediment core sample is low (i.e., 0.05 Pa) (Fig. 3), suggesting that surface sediment resuspention occurs frequently within a year.

Figure 3: Critical shear stress profile of the sediment core at site 3.

- A loose silty sediment layer was frequently observed at the top 15 cm for almost all sediment cores collected at all study site locations, while the shear stress generally increased with depth indicating cohesive material at greater depths.

- Resuspended sediment is trapped within the wetland during small runoff events and flushed through the wetland for larger events (i.e. one-year recurrence storm event).

- Streambed sediments were susceptible to resuspention when subjected to a 10 ~ 20 cm/s flow velocity.

- The bed shear stress at site 4 is well correlated with sediment transport rate (Fig. 4), further supporting the importance of bed shear stress induced by storm events to resuspend streambed sediments.

Figure 4: Bed shear stress (blue) and seidment transport rate (brown) at site 4

- Overall, these results clearly show that an increase in the shear stress above the measured critical shear value produces an increase in the sediment transport due to resuspention of channel sediments.

Sediment Bed Characteristics and Storage

- Median available phosphorus values increase near the bed surface, suggesting that much of the stored phosphorus is the more active surficial sediment.

- Available phosphorus also increased with silt content, which is likely responsible for the majority of the suspended load carried in the stream.

- An example of sediment bed changes at site 6 during the 2005 season (Fig 5). Overall the magnitude of variations (~ 5 cm), was in good agreement with critical shear stress measurements.

Figure 5: Time series of average sediment bed elecation at site during the 2005 season.

- Soft (unconsolidated) sediment deposits was primarily located in reaches with mild slope, and that steep areas store little to no sediment.

Transient Storage and Solute (Phosphorus) Processes

- Solute transport processes were studied using a fluorometric tracer dye, Rhodamine WT, as shown in Figure 6. Not only stream flow (advection) and mixing (dispersion) are important, streambed morphology (i.e., riffiles and pools, varying stream beds, and vegetation) can play a controlling role in the residence of phosphorus transport.

FIgure 6: Dye released experiments at site 4 for transient storage estimate.

- The most heavily vegetation reach (Reach D), has the highest transient storage, but lower values for the exchange between main channels and stream banks, suggesting that vegetation may provide little mass exchange between the flow and storage area.

- Reaches B and C had smaller transient storage, but larger exchange capability. This may be caused by irregularities in the channel morphology, such as embayments, or in separated flow regions behind obstructions, such as the boulder clusters in Reach B.

- While transient storage zones are small, a large amount of turbulent mixing takes place between water in the flow region and water in the storage area.

- These parameters were used to model solute transport in a 2.4 km reach of Dorn Creek with and without transient storage. The model results with transient storage can have 30% longer residence time in the study reach than those with no transient storage.

Wetland Investigation

- When normalized for flow volume, all peak flows were attenuated by the wetland, with the maximum attenuation corresponding to the intermediate events.

- In the case of sediment, the amount leaving the wetland in the two largest storms, which accounted for 96% of the exported sediment during the period of bservation, was twice the amount entering the wetland.

 

Figure 8. Sediment loading into and out of the upstream wetland in Dorn Creek by event for nine events and the total during the 2006 season.

- The failure of the wetland to trap sediment is apparently due to the role of drainage ditches; which trap sediment during the wetland-filling phase and release it during drainage.

- In the case of total phosphorus, the inflow to the wetland about equaled the outflow, although the wetland sequestered 40% of the incoming dissolved reactive phosphorus (Fig. 9). The discrepancy is almost certainly due to net export of sediment.

Figure 9. Total phosporus loading into and out of the upstream wetland in Dorn Creek for nine events during the 2006 field season.

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Publications & Presentations

Publications

Rogers, J.S., Potter, K.W., Hoopes, J.A., C.H. Wu, Hoffman, A., Armstrong, D., Hydrologic and water quality function of a small wetland in Southern Wiscosnin, To be submitted to Water Resources Research, 2007.

Wu, C.H., Hoopes, J.A., Potter, K.W., and Murdock, E, Transport and storage of sediments and solutes in a small agricultural stream, in preparation, to be submitted to J. of Hydrologic Engineering - ASCE, 2007.

Rogers, J.S., Potter, K.W., Hoopes, J.A., and C.H. Wu, Hydrology, Hydrodynamics and Sediment Transport Investigation in a Small Wetland: Upper Dorn Creek Wetland, Wisconsin, ASCE Environmental Water Resources conference. May 2007.

Rogers, Justin, Hydrologic and water quality function of a small wetland-upper Dorn creek wetland, Wisconsin, Master Thesis, Civil and Environmental Engineering Department, UW-Madison, Fall 2006.

Murdock, Evan, Transport and storage of sediments and solutes in a small agricultural Stream, Dane County, Wisconsin, Master Thesis, Civil and Environmental Engineering Dept, UW-Madison, Spring 2006.

Chan, Yuen Man. Direct Discharge Measurement using current meter method, Independent study Report, Civil and Environmental Engineering Dept. UW-Madison, May 2004.

Ostermann, Susan. Instrumentation selection for stream measurements, Independent study Report, Civil and Environmental Engineering Dept., May 2003.

Presentations

Rogers, J.S. Potter, K.W. Hoopes, J.A. and Wu, C.H., Hydrology, Hydrodynamics and Sediment Transport Investigation in a Small Wetland: Upper Dorn Creek Wetland, Wisconsin, ASCE Environmental Water Resources conference. May 2007.

Potter, K.W., Rogers, J.S., Hoffman, A. Wu, C. H., Hoopes, J.A., and Armstrong, D.M., Quantifying Wetland Services: A Case Study, AGU Spring meeting, 2007.