Modeling Streamflow for Channel and Bank Stability*Larry D. Stetler, John P. Knight, and Micah L. Keller

Department of Geology and Geological EngineeringSouth Dakota School of Mines and Technology

*lstetler@taz.sdsmt.edu Rapid City, SD

ABSTRACTSpearfish Creek flows in a narrow, incised canyon in the northern Black Hills resulting in high velocity flows that place increased stress on channel banks and bottom. Potential results include damage to existing roadways and bridges as well as channel widening and damaged fish habitat. Thus, it would be desirable to predict locations where increased bank and/or channel stress would be expected under varying flow conditions. An approximately 1.6 km reach was surveyed using a laser-digital transit. Channel cross-sections were measured every 30 m producing a detailed topographic map of the channel, banks, and limited floodplains. Flow modeling was performed using discharges that were based on current stream-flow and known historic flood flows. The stream was modeled as segments, using SMS, where inflow to each segment occurred at the upstream boundary. The segment containing Squaw Creek tributary had two inflow boundaries. Results indicted high stress at numerous points along the entire stream channel as a function of river channel morphology. Areas of concern included sections that contained bends or narrowing of the channel. Where the highway was closest to the potential channel scour zones, damage might be expected under severe flow conditions causing immediate damage to the fishery. These results will allow highway engineers and fisheries personnel to maximize efforts to sustain existing roadways and to provide and maintain quality fisheries within Spearfish Canyon.

PROJECT GOALSThis project focused on potential effects that physical and mechanical channel properties (channel substrate, flow conditions, and banks and other structures) had on fish habitat. Three primary project goals were:

model potential effects of increased discharge on fish habitat determine behavior of substrate (scour and deposition of sediment) identify potential locations of increased bank stress

GEOLOGICAL SETTINGThe study area was located approximately 10 miles south of the city of Spearfish on Spearfish Creek between Homestake Mining Co. (now Barric Mining Co.) Hydroelectric Power Plant Number Two (Hydro No. 2) and Maurice intake, a diversion tunnel supplying Hydro No. 1 in Spearfish (Fig. 1). Two streams flow through the study area. Spearfish Creek (the larger of the two) originates from springs emanating from the Mississippian Madison Limestone and flows northward within the confines of Spearfish Canyon. Squaw Creek (the smaller tributary stream) enters Spearfish Creek about 1/3 mile below Hydro No. 2 and sources from springs and seeps within the Cambrian Deadwood formation aquifer east of Spearfish Canyon and flows westward. The stream flows north out of the Black Hills crossing saturated and unsaturated Paleozoic and Mesozoic strata prior to emptying into the Belle Fourche River. Numerous Tertiary intrusions and other structural features, such as faults, also contact the stream channel. As a result, net groundwater contributions and losses are realized from springs and seeps and high permeability zones between Hydro No. 2 and Maurice Intake (Table 1).Spearfish Creek is a low sinuosity single channel within the study reach although it does meander through the bedrock of the canyon through which it flows. State Highway 14A, located immediately to the west of the main channel, is stabilized by using gabion structures (wire mesh baskets filled with cobbles and boulders) to armor the road side channel bank which effectively constrains the creek from natural meandering and bank adjustments during high flows. Thus, increased shear stress generated during high flows would primarily be directed toward the gabions and the channel bottom promoting scour. Potential loss to fish habitat under these conditions would be high. In addition, under extreme flow events, gabions could easily be washed out causing bank and possible road failure, both of which would exacerbate loss of fish habitat.

Figure 1. Study reach on Spearfish Creek. Digital elevation model created using survey data and SMS (Boss, Int.) software.

Locations of discharge measurements shown in Table 1.A B C

SUBSTRATE CHARACTERISTICSWater in Spearfish Creek is super-saturated with respect to calcium and magnesium owing to its source from the Madison Limestone. Water temperature plays a key role in the precipitation process since calcium has a decreased solubility in cold water. Thus, colder water temperatures favor precipitation of the cement rind and warmer waters favor weakening and dissolution of the cement rind. Rind thickness varies throughout the study reach from less than 1/8” to more than 2” (Fig. 2).Consequences of the rind are two-fold:

Cementing of the bottom sediment and subsequent reduction in available fish spawning habitat Immobility of the sediment at shear stresses which would remove it were the cement not present

Thus, determining required shear stress and effect sediment mobility had on fish habitat were explored.

Figure 2. Calcitic rind forms a natural cement on bottom sediments in Spearfish Creek. A shows a thin (~1/8”) rind in various stages of shedding from boulders on the channel bottom. B show a piece of the rind collected from along the stream bank. Thick plates of the rind are common in the channel covering large areas and rendering the substrate unmovable for fish habitat and requires an increased shear stress to mobilize during runoff events.C illustrates heavy and thick rind that is common along channel banks.

A

B C

Sediment transport sites shown in Figure 3 and 4.90° Bend300 MarkerHydro #2

Table 1. Measured discharge in Spearfish Creek at 3 locations. See Figure 1 for measurement locations.

Location Q, cfs A 22.2 B 72.7 C 60.1

Figure 4. Stream locations for sediment transport studies. Flow is from right to left.

Study Methods and resultsA 3-D numerical flow model, Surfacewater Modeling System, was used to evaluate potential effects of various flow conditions on the banks and substrate of Spearfish Creek. The model was run on a topographic base map (Fig. 1) that was prepared for this study from detailed surveys of the channel, banks, floodplains, and surrounding areas (Fig. 5). Head loss along the main channel of Spearfish Creek was 80ft (4463 ft at Hydro No. 2 to 4383 ft at Maurice intake). Model limitations for maximum head loss of <25 ft forced the study reach to be segmented for flow analysis. For the study reach, 3 material types were designated: 1) channel, 2) vegetated bank, and 3) engineered bank. Each of these materials were assigned an erodibility factor (Manning roughness coefficient) with in-channel erosion also controlled through an eddy viscosity value (transverse flow). Adjustment of parameter values were based on actual physical conditions at the stream and from published data. Flow modeling was performed using the mean average discharge (~60 cfs), or the ~50% probability flow (Fig. 6), and a medium flood event discharge (250 cfs), or a 0.5% probability flow. Results from the 250 cfs event for the confluence is shown in Figure 7. The arrows represent flow magnitude vectors, the larger the arrow, the higher the velocity. At the upstream edge, flow is spread across the entire segment and is flowing out of the main channel. At this location, erosion on the vegetated and engineered banks is low due to the velocity slowing due to bank roughness and vegetation. In the main channel above the confluence, sediment is securely cemented forming a mostly impenetrable bottom. flow impinging on the outer, west-side bank, increases in velocity, and thus, shear stress as well. A 1994 flood event had an estimated discharge of +1200 cfs (based on water elevations and cross-sections) and undercut and eroded gabion structures along the entire study reach (Fig. 8). Significant channel erosion occurred as well as the cement was broken by the high shear stresses generated by this magnitude of flow. In 2000, damaged gabions were removed and portions of the channel were re-engineered.

Figure 5. Survey of Spearfish Creek was made using a laser total station instrument. Base map form survey is shown in Figure 1.

SummaryThese results form a preliminary assessment of channel and bank erosion for Spearfish Creek indicate areas of concern exist from both a fisheries and a highway standpoint. During increased flow, increased bank stress (and the potential for bank failures) exists at several points along the highway side of the channel. Many of these banks are protected by gabion structures which in effect, transfers the stress to the channel bottom which can reduce or eliminate fish spawning habitat in these areas. In extreme events, complete failure of the highway side bank is possible by under-washing of the gabions.

Figure 6. Flow duration curves for Spearfish and Squaw Creeks based on the limited available discharge record from 1988 to 1997. Modeled flows were the ~50% probable flow and the ~0.5% probable flow. From these data, the 0.5 P flow is well belong long-term flows in the canyon.

~50% P

~0.5% P

Figure 8. High magnitude flood event (2000 cfs) shows high stress points at bends with velocities ranging from 4.5 to ~8 ft/s. Due to model limitations, only high flow events are able to show entire 1 mile study reach. High flow events are used to predict potential scour and bank erosion sites.

A

B

Figure 7. Velocity vector maps for the medium flood event flow of 250 cfs for Spearfish Creek and 80 cfs for the tributary stream, Squaw Creek. A shows the entire confluence region and B is zoomed in on the confluence. Flow on the right bank downstream of the confluence shows the highest velocity and is consistent with field measurements and observations. A large hole is also present on the right bank (blue vector color). On the left bank opposite the confluence, velocity is low and the presence of fine gravel and sand bar provides usable spawning habitat that is vigorously utilized. This sediment bar is cement free.

EXPERIMENTAL MODELThe in-stream flow characterization was focused on quantifying required flow conditions to mobilize sediment on the stream bed. This was accomplished using natural stream sediment consisting of pebbles and cobbles. These materials were oven-dried and painted with bright yellow highway paint. Three locations were selected based on the potential increased effects of mechanical energy during high flow events and are shown in Figure 1. The sediment was distributed by size as shown in Figure 3 and placed in piles on the stream channel (Fig. 4) where the materials were.

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22 32 45 64 90Size (mm)

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Fre

quen

cyHydro 2 300 Marker 90° Bend

Figure 3. Sediment size distribution for locations