Assignment 4:

The "Dirty" Virgin

The Virgin River near Confluence Park, La Verkin, UT

12 Feb. 2021

pictures dated 24 July 2020

If you ask my grandpa why they call it the Virgin River, he'll tell you "because it's kicks up so much dirt, you can't see it's bottom!" Interestingly, the Virgin River is was actually named by the Spanish for the Virgin Mary, and was originally la Rio de la Virgen. For the Hydraulics Field Observations, I chose to take a closer look at the la Rio de la Virgen through a project I worked on last summer during an internship.

I completed an Ordinary High Water Mark (OHWM) delineation and created cross sections for a study area of the Virgin River near Confluence Park in La Verkin, UT. Conflulence Park runs along the Virgin River and is accessible only by foot. There are a couple of popular swimming holes in the park, and the study area for these observations is home to a swimming hole, a rope swing, and a basalt cliff that is bolted for recreational climbing. Another fun feature is the USGS Stream Gage located on the left side of the river, but only a river engineer might notice that (;

Fig. 1. Vicinity Map including locations of pictures taken in the study reach

Location Map

Figure 1 shows a vicinity map; the large boulder on the north end of the park marks the northernmost extent of the study area. Fig. 2 shows a hand sketch of the north portion of the study area, with hydraulic features marked.

The following video shows the reach from an oblique angle, looking upstream to downstream and at the hydraulic jump.

Virgin River at Confluence Park

Field Map Sketch & Cross Section

The Virgin River runs north past Confluence Park in La Verkin, UT. The study area extends along a dirt road with a steep cliff marking the leftmost boundary of the river, and a sandy beach marking the right side. Several boulders fill the channel, and a USGS Stream Gage has been installed on the left of the channel, shown below in a field map sketch and cross-sectional view of the channel (Figs. 2, 3).

Fig. 2. Cross section at A-A' including relative flow depth and OHWM from previous delineatio

Fig. 3. Hand sketch of study area, with hydraulic features and cross section A-A' marked

Channel Features & Estimated Discharge

The channel is roughly 5-8 ft in width and runs past a basalt cliff on the left and a sandy bank littered with boulders on the right. Total channel discharge was roughly 3-9 cfs, based on a back-of-a-napkin calculation. Flow is slowest in the glide (~3 cfs) and fastest following the hydraulic jump (~9 cfs).

Q= AV

A = 3 ft^2

V = 1 to 3 ft/s

Hydraulic Features

This section features specific hydraulics in the Virgin River, with photo locations marked in Figs. 1 and 2.

Flow magnitude was much higher near the center or thalweg of the channel, and lower near the right side of the channel, as shown by a flow vector field shown in Fig. 4. This is likely due to the fact that the left side of the channel has cut into basalt, and the softer sand banks on the right side of the channel makes it easier for water to pool and eddy.

Convergent flow is evident at the cascade (Fig. 5), where water comes together at a high velocity. Divergent channel flow is caused by protruding boulders, creating a "V" shape down the channel (Fig. 6). These boulders also cause a small wake flow behind the point of flow separation. Uniform flow is evident at the downstream end of the study area, and in the glide (Figs. 7, 8), where flow is steady over time and no turbulence is evident.

Fig. 4. Flow Magnitude, longer arrows indicate higher velocities in the channel.

Convergent Flow

Fig. 5. Convergent Flow: flow comes together as it approaches the cascade and hydraulic jump

Divergent Flow

Fig. 6. Divergent Flow: flow is diverted around boulders protruding from the channel bottom

Uniform Flow

Fig. 7. Uniform Flow: flow is shallow and uniform in the downstream end of the study reach

The deep glide, where water moves relatively slowly before reaching the hydraulic jump is shown in Fig. 8. Further upstream, flow accelerates through the thalweg on the left side of the channel, leaving a slow flow and eddy zone on the right side of the channel. The fast and slow flows are separated by a flow seam, which detaches at the large boulder shown in Fig. 9. The northernmost shear zone is shown in Fig. 10, where flow re-attaches as velocity becomes uniform throughout the channel.

Deep Glide, USGS Stream Gage

Fig. 8. Deep glide, where flow is subcritical and uniform before converging into a cascade (see Hydraulic Jump section). The USGS Stream Gage is located on the basalt cliff on the left side of the channel.

Eddies & Flow Seam

Fig. 9. Flow Seam denotes a separation of flow velocities, with eddies and slow flow on the left of the channel.

Flow Separation &

Re-Attachment

Fig. 10. Shear flow separation, where flow reattaches as entire channel flow becomes uniform.

Hydraulic Jump

The most notable hydraulic jump in the study reach is located at the cascade, just downstream of the deep glide. Water comes together and flow is supercritical as it crests the cascade. The resulting hydraulic jump at the end is evident in white water and turbulent flow. After the jump resolves, flow becomes uniform and subcritical downstream. Fig. 11 and the following video show this transition.

Fig. 11. Hydraulic jump: subcritical flow converges, cascades in supercritical flow, creates a white water jump, and resolves in shallow, uniform, subcritical flow.

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