Buffalo Bayou HURRICANE HIKE
Stop 3 // SAND // Geologic Archive
Footbridge NW of Johnny Steele Dog Park
Figure 3.1 View looking north ~100 days after Harvey. The peak water level was ~6 m (~20 ft) above the top of the bridge across the bayou. The top of the early to middle adjustment phase dog park sandbar is represented by the short dashed green line. The base of the late adjustment phase channel cut is represented by the long dashed green line.
The view to the north (Fig. 3.1) shows the remnants of the dog park sandbar that formed during the early and middle adjustment phases, the dog park channel that cut the bar during the late adjustment phase, and the bulldozed sand from the recovery phase. This image shows the area where the sand peels were collected. It dates from ~100 days after Harvey. Today (2019), due to removal efforts and reworking, there is almost no evidence remaining of Hurricane Harvey in this area. The 11 sand peel sculptures taken in this area are the remaining geologic archive of the Harvey flood at the dog park.
Figure 3.2. Google Earth image of the dog park area in 1944. Note the natural channel (yellow line) flows from north to south through where the sand peels are taken.
Figure 3.3. Google Earth image of the dog park area ~60 days after Harvey. The active channel (magenta line) cut the natural channel meander (yellow line) in the 1970s as part of flood abatement efforts. Note the dog park sandbar (light areas) and the dog park pond drainage that cuts through the bar where the sand peels were collected (Fig. 3.4 shows details).
Natural Channel
Figure 3.4. Google Earth image of the dog park channel cut with locations of sand peels.
The oldest geologic archive of Buffalo Bayou in this area is the remnants of the pre-human-intervention meander to the north of Buffalo Bayou. A comparison of an image taken before straightening of the bayou in the 1970s (Fig. 3.2) and the image taken ~60 days post-Harvey (Fig. 3.3) shows how the dog park pond and the dog park channel cut inherited their locations from the pre-1970s bayou channel. The location of the sand peels is seen in Figure 3.4. A view from inside the dog park channel to the north (Fig. 3.5) reveals the old topography of the natural Buffalo Bayou channel.
Figure 3.5. View looking north out of the dog park drainage channel at Buffalo Bayou. The sand peel 1 site (Plate 1) is on the left and that of peel 2 (Plate 12) is on the right. On the left are primarily sand bar deposits. The red lines mark the basal erosional surface of the flood phase. On the right are remnants of the sand bar (between the red and green line). This is overlain by the dog park channel cut deposits (between green and blue lines). Above this are sands reworked by bulldozers. Across Buffalo Bayou a bridge spans a small tributary that lies in the old (1944) channel. To the right of the bridge at water level are outcrops of Pleistocene red mudstone coastal plain deposits of the Beaumont Formation.
Harvey Deposits
In the first ~100 days after Harvey, sand peels were collected from the dog park sandbar and the dog park channel cut (Fig. 3.6). These sand peels are the geologic archive of Hurricane Harvey at this location. A special feature of these sand peel sculptures is their ability to pinpoint a geologic event in human time and geologic time. It is rare in the geologic community that a rock or a sediment deposit can be pinpointed to a specific and exact time of creation on any human time scale. Using the Shepherd bridge stream gauge as a context provides the opportunity to tie individual layers in the sand peel sculptures to precise time frames. Meyer et al. (2019) interpreted 3 phases of Harvey archived in the sand peels.
Figure 3.6. Panoramic view along the axis of the dog park channel cut ~100 days after Harvey, showing the flood, adjustment, and recovery deposits and the sand peel locations. Note most of the volume of the dog park sandbar from ~ 40 days after Harvey when peels A, B, C, D and E were collected has been removed or reworked.
Flood Phase
Figure 3.7. Early-middle adjustment phase, interpreted sand peel A (Plate 2). The base of the peel is pre-Harvey deposits. The red line marks the scour at the peak of the flood phase. The early adjustment phase between the red and dashed light green lines is characterized by planar cross-stratification and high energy bedforms. The middle adjustment phase is above the dashed green line and is characterized by lower energy bedforms (low angle trough cross-stratification and current ripples). The late adjustment phase is evidenced by the small faults (solid white lines and arrows) that formed as the dog park sandbar was collapsing into the growing dog park channel cut. Flow was to the east (toward the viewer).
The flood phase of Harvey is archived by a gap preserved in the sand peels and marked by a red line in Plates 1-12. The interpreted image of sand peel A (Fig. 3.7, Plate 2) shows the irregular basal scour formed by the peak water level and peak flow energy.
Adjustment
The early adjustment phase saw the biggest drop in energy and consequent deposition of sediments. The layers between the red and dashed green line in sand peel A (Fig. 3.7 and Plate 2) are characteristic of these high energy archives. The flow was to the east (toward the viewer). The early adjustment phase is also seen in sand peel 1 (Plate 1).
The middle adjustment phase saw lower energy recorded in the sand deposits as the water level continued to drop. The section above the green dashed line in sand peel A (Fig. 3.7 and Plate 2) is characteristic of this phase. Flow indications are still to the east. Ground penetrating radar (GPR) recorded in the dog park channel (Sammy, 2019) agrees with a flow to the east (Figs. 3.8 and 3.9). The middle adjustment phase is recorded in Plates 3-7.
The late adjustment phase is archived by the dog park channel cut deposits seen in sand peel 3, (Fig. 3.10 and Plate 8). The cut formed when the water level dropped back into the active channel after ~30-40 days. The solid green line in Figure 3.10 and Plates 8-12 marks the scour surface at the base of the late adjustment phase. Flow was to the north.
Figure 3.8. Interpreted 2d ground penetrating radar (GPR) image from Sammy (2019). This GPR was collected less than 0.5 m (1.6 ft) into the sandbar where sand peel D was collected. The red and green lines are interpreted layer geometries (Sammy, 2019).
Figure 3.9. Sand peel D (Plate 5) overlain on coincident GPR image in Figure 3.8. The GPR appears to be resolving the 3-5 cm (1.2-2 in) bed sets of the trough cross-stratified middle adjustment phase of flow archived in the upper 1/3 of sand peel D (modified from Sammy, 2019). A 3d GPR volume collected ~ .5 m (18 in) east of the sand peels (Sammy, 2019) indicates flow was to the east, in agreement with the flow direction indicated in the adjustment phase archived in the sand peels.
Figure 3.10. Late adjustment phase, interpreted sand peel 3 (Plate 8). The base of the sand peel is pre-Harvey deposits. The red line marks the scour at the peak of the flood phase. The late adjustment phase is archived by the trough cross-stratification above the solid dark green line. Flow was to the north (left for the viewer). The mud layer marked by the dashed dark green line is the archive of a storm event ~63 days after Harvey that flooded the dog park area. The middle adjustment phase was completely eroded by the dog park channel cut. The early adjustment phase between the red and solid dark green line is characterized by deposits from suspension. Flow was to the east (away from the viewer) during this phase.
Figure 3.11. Recovery phase, interpreted sand peel 2 (Plate 12). The recovery phase is archived in the unstructured sands, above the blue line, that were pushed west into the dog park channel cut by bulldozers grading the area during the sand removal process. Over 272 million kg (600 million lbs) of sand were removed during the recovery efforts (BBP, 2018).
Figure 3.12. Hurricane Harvey Buffalo Bayou hydrologic archive with human impact and geologic archive overlays. This is Figure 2.2 with added boxes represent the geologic depositional packages. The red box marks the flood phase when the rising waters created the scour seen at the base of Plates 1, 2, 7, 8, 9, 10, 11, and 12. The light green box with dashed lines marks the early adjustment phase when water level rapidly fell and the high energy deposits seen in Plates 1 and 2 formed. The dark green box with dashed lines represents the middle adjustment phase as the sandbar continued to grow with lower energy bedforms (Plates 3-7). The dark green box with solid outline represents the main late adjustment phase when the bayou returned to its channel and redistributed the earlier deposits. The pond formed in the dog park started to drain to the north, cutting a channel through the dog park sandbar. Plates 9-11 show the archive of this phase. The blue box represents the recovery phase when bulldozers were reworking the deposits. The top 1/3 of Plate 12 records this phase.
Recovery
The recovery phase involved bulldozers reworking and hauling sand from the dog park site. Sand peel 2 (Fig. 3.11, Plate 12) shows the massive sands above the blue line archiving this reworking. The bulldozers were pushing the sands to the west.
A plot of the geologic phases overlain on the representations of hydrologic and human impact archives illustrates how all three Hurricane Harvey flow archives record the events in their own ways (Fig. 3.12).
Flow evolution
The dog park channel cut is unique in that it archives flow in 4 different directions over time: To the south before the bayou was diverted, then east during the Hurricane Harvey flood, then north in the channel cut during the late adjustment phase, and finally to the west by bulldozers (Figure 3.13).
Flow to the south: Until Buffalo Bayou was straightened in the 1970s, the Buffalo Bayou channel formed a sharp meander through what is now the dog park drainage channel (yellow).
Flow to the east: Flood, early and middle adjustment phases: During the waning flow, Harvey floodwater dropped from 12.8 m (42 ft) to ~3 m (~10 ft), flowing east (red) and depositing the thick sandbar and filling the dog park channel.
Flow to the north: Late adjustment phase: When the bayou dropped below ~3 m (~10 ft) approximately 30 days after Harvey, the pond that formed in the dog park started draining to the north (dark green), cutting through the sandbar. This channel continued to cut and fill for approximately 60 days.
Flow to the west: During the recovery phase the bulldozers reworked and removed the sands, transporting them to the west.
Figure 3.13. Detail of dog park area showing water bodies and flow directions at different phases. This is a detail of the Buffalo Bayou Park Map (Fig. 0.2).
GeoGulf 2019 Field Guide Collaborator: Sarah Meyer
As an undergraduate student at University of Houston, Sarah completed interpretations of all 12 sand peels and published a poster at the GeoGulf Convention 2019.
Sarah Meyer
GeoGulf Convention 2019