Bay_Pines_Model_Memo_FINAL.pdf

Memo

To: Taylor Brewer, Beaufort County

From: Crystal Muller, Woolpert

Date: April 21, 2025

Subject: Bay Pines Flood Assessment

Beaufort County has received repeated reports of flooding during rain events from residents along Bay Pines Drive on Hilton Head Island. The County-owned road is parallel to and just west of Highway 278, just south of the bridge over Broad Creek. Stormwater runoff from the homes along Bay Pines Drive is collected in roadside swales and conveyed through pipes to a ditch that runs behind the homes along the north side of the road. This ditch runs to a pipe system and down to a tidal creek that is conveyed under Helmsman Way and then discharges to Broad Creek. There is a second system that collects runoff from the Brighton Bay Villas and conveys the runoff through two ponds and an outlet control structure to the tidal creek that goes under Helmsman Way. This second system was determined to not influence the issues along Bay Pines Drive.

Woolpert previously studied this general area as part of a larger Point Comfort watershed study on behalf of the Town of Hilton Head Island in 2012 and 2020, and the Town agreed to allow the County to use this original model as a baseline for a more detailed evaluation of the Bay Pines Drive area. Woolpert performed field investigations to verify the model inputs and then used the updated model to evaluate the Bay Pines area and determine the extents of flooding attributed to a range of design storms, identify causes and choke points, and evaluate potential solutions to lessen the flooding. This memo summarizes this effort and the findings.

Data Collection Field crews collected data to confirm existing inventory information at critical locations such as along Bay Pines Drive, in the ditch north of Bay Pines Drive, Paddle Boat Lane, and Helmsman Way. In addition to confirming the quantitative data used in the model, crews also made notes about the condition of pipes and channels and observations of water in the downstream system.

There is one 12-inch pipe that conveys runoff from the cul-de-sac to the ditch and runs between 33 and 35 Bay Pines Drive, which will be referred to as Pipe 1 in this memo. A second 12-inch pipe collects runoff further south along Bay Pines Drive and runs between 19 and 21 Bay Pines Drive, which will be referred to as Pipe 2. This pipe runs in close proximity to a privately owned swimming pool. A concrete pad around a pool at 21 Bay Pines Drive was constructed over this pipe, which discharges about one foot below the bottom elevation of the receiving ditch (Figure 1). There also appears to be some sort of blockage in the downstream pipe that is causing standing water inside the pipe in the area behind 37 Bay Pines Drive, which was observed using the haloptic pole camera (Figure 2).

Figure 3 shows the point where both pipe systems (Bay Pines Drive and Brighton Bay Villas) discharge into the tidal creek.

Both culverts under Helmsman Way are submerged (Figure 4), with one pipe appearing completely clogged with sediment.

Figure 1: Upstream and Downstream End of Pipe 2

Figure 2: Standing Water in Downstream Pipe

Figure 3: Discharge into Tidal Creek

Figure 4: Submerged Crossing at Helmsman Way

Model Development The field data was used to refine the prior watershed model, which was developed as a 1D XP-SWMM model in 2012 and then converted to a 1D/2D XP-SWMM model in 2020 using 2019 Lidar data. No updates were necessary for the previous hydrology and there have been no changes in the land use that would require additional model updates. Woolpert evaluated the storms included in the prior assessment, including the 2-year, 10-year, 25-year, 50-year, and 100-year, 24-hour design storms. The 2-year and 10-year storm events were used for this evaluation.

Existing Conditions The model showed that, under existing conditions, the northern portion of Bay Pines Drive in the cul-de-sac experiences roadway overtopping during the 2-year storm event. Properties at 32 and 34 Bay Pines Drive show significant ponding in their front yards under this event, with conditions worsening during larger storm events resulting in potential structural inundation. Similarly, the southern portion of Bay Pines Drive shows flooding during the 2-year storm event, with both the roadway and adjacent properties experiencing overtopping and inundation during all modeled storm events. The existing conditions model revealed that the flooding was attributed to undersized drainage systems. The following section provides a detailed evaluation of potential alternatives to address these drainage deficiencies.

Figures 5 and 6 below show the areas of inundation in the 2-year and 10-year storm events under existing conditions, with stormwater pipes shown as black arrows. The blue shading represents the depth of the flooding of the modeled area, with the darker blue showing areas with deeper depths of water. Figures 7 and 8 show the same existing conditions, but represented as the limits of inundation that will be used for comparisons in the alternatives analysis. These images show where the roadway inundation is centralized at the upstream ends of the pipes, suggesting the system may be undersized. Homes are impacted during both modeled storm events.

Figure 5: Existing Flood Depths, 2-year Storm Event Figure 6: Existing Flood Depths, 10-year Storm Event

Figure 7: Existing Flood Extents, 2-year Storm Event Figure 8: Existing Flood Extents, 10-year Storm Event

Alternatives Analysis As noted above, the initial modeling effort revealed that the cause of flooding was undersized pipes between the roadway and the receiving ditch. Three alternatives were modeled to investigate opportunities to increase the capacity of these pipes to minimize the inundation seen along Bay Pines Drive. Each alternative and the perceived improvements are described below.

Alternative 1- Lowered Channel and Upsized Pipes Alternative 1 increased the capacity of both pipes and lowered the channel to meet the downstream invert of Pipe 2.

Pipe 1 was increased from a single 12-inch pipe to dual 12-inch pipes and Pipe 2 was increased from a 12-inch pipe to a single 18-inch pipe. Pipe 2 was also realigned to provide more space between the pipe and adjacent swimming pool. The results can be seen in Figures 8 and 9 below. The model results indicate that the street still overtops in the cul-de-sac during the 2-year storm event as seen in Figure 8, but the inundation at Pipe 2 appears to be limited to the roadside ditches. Homes at 16, 18, and 20 Bay Pines Drive appear to still have impacts. There is still road inundation and flooding of homes during the 10-year storm event, but the impacts are less at 17 and 19 Bay Pines Drive. Note that the existing limits of inundation are shown in dark blue, and the modeled limits of inundation are shown in light blue.

Figure 8: Alternative 1, 2-year Storm Event Figure 9: Alternative 1, 10-year Storm Event

Alternative 2- Lowered Channel and Larger Pipes Alternative 2 further increased the capacity of Pipes 1 and 2, and maintained the lowered ditch invert from Alternative 1.

Pipe 1 is increased from a 12-inch pipe to an 18-inch pipe under the road and then transitions to two parallel 18-inch pipes down to the ditch. Pipe 2 is upsized from a 12-inch pipe to an 18-inch pipe under the road and expanded from 12-inch pipe to a 24-inch pipe down to the ditch. The pipe is also realigned to move away from the swimming pool. As shown in Figures 10 and 11, this results in substantial improvements to both roadway and home inundation during both the 2-year and 10-year storm events. In the 2-year storm event, the street is not overtopped and no yard flooding is observed.

This indicates that these improvements can effectively handle 2-year storm event. The 10-year storm event, Figure 11, shows limited street overtopping at each pipe and some property impacts that appear limited to front yards. Note that the existing limits of inundation are shown in dark blue, and the modeled limits of inundation are shown in yellow.

Figure 10: Alternative 2, 2-year Storm Event Figure 11: Alternative 2, 10-year Storm Event

Alternative 3 – Lowered Channel and Pipe 3 The third alternative is built upon the previous two alternatives and includes lowering the channel and upsizing multiple pipes. Pipe 1 was upsized from a 12-inch pipe to an 18-inch pipe, like in Alternative 2. An existing 12-inch pipe runs parallel to Bay Pines Drive on the east side from lots 18 to 24; this pipe was increased to dual 12-inch pipes. A pipe running parallel to the west side of Bay Pines Drive along a portion of lot 19 was also upsized from a 12-inch pipe to a 15-inch pipe. Pipe 2 was upsized from a 12-inch pipe to a 24-inch pipe from the east side of the road to the ditch. Model results show that during the 2-year storm event, there is no overtopping of the street and no yard flooding. In the 10-year storm event, the street still overtops near the cul-de-sac, but still flooding is reduced compared to the existing conditions and Alternative 1. Note that the existing limits of inundation are shown in dark blue, and the modeled limits of inundation are shown in red.

Figure 12: Alternative 3, 2-year Storm Event Figure 13: Alternative 3, 10-year Storm Event

Results The results of the alternate scenarios and the effect on the system in the cul-de-sac of Bay Pines Road are shown in Table 1 below. The numbers shown in red indicate flood staging elevations for the 2 and 10- year storm events that would overtop the road elevation at this location of 6.67 feet. Alternative 2 shows the most flood reduction.

Table 1: Bay Pines Road Cul-de-Sac Existing and Alternatives Summary

Catch Basin 1

Rim = 6.37 Catch Basin 2

Rim = 6.32

Inv 2-Yr Stage 10-Yr Stage Inv 2-Yr Stage 10-Yr Stage

Existing 5.12 7.48 7.90 4.82 7.47 7.90

Alt1 5.12 6.94 7.65 4.82 6.89 7.64

Alt2 4.46 5.50 6.73 4.30 5.43 6.63

Alt3 4.48 6.59 7.43 4.27 6.54 7.39

Figure 14 shows existing conditions and all three alternative scenarios layered during a 2-year storm event, using the color schemes used above (light blue for Alternative 1, yellow for Alternative 2, and red for Alternative 3). The improvements modeled in Alternatives 2 and 3 diminished flooding so that the streets remains passable and yards are not flooded.

Figure 14: Alternatives Analysis, 2-year Storm Event

During the 10-year event (Figure 15) flooding is reduced slightly to the light blue areas with Alternative 1, though yards are flooded, and the street remains overtopped. Alternative 2 is shown in yellow and is the most improved scenario. The street is only overtopped at the low point and there is less yard flooding compared to the other alternatives. The red shading shows the flooding when Alternative 3 was modeled, which has less yard flooding than Alternative 1 but does not eliminate flooding in the street.

Figure 15: Alternatives Analysis, 10-year Storm Event

CATCH

BASIN 1

CATCH

BASIN 2

CATCH

BASIN 2

BASIN 1

The results of the alternate scenarios and the effect on the system near 19 Bay Pines Road are shown in Table 2 below.

The numbers shown in red indicate flood staging elevations for the 2 and 10- year storm events that would overtop the road elevation at this location of 7.67 feet. Alternative 3 shows the most flood reduction in this area.

Table 2: Area of 19 Bay Pines Road Existing and Alternatives Summary

Catch Basin 3

Rim = 6.75 Manhole

Rim = 7.93 Pipe (HD240123)

Inv 2-Yr 10-Yr Inv 2-Yr 10-Yr Inv 2-Yr 10-Yr Existing 5.15 7.66 7.87 4.82 7.62 7.85 5.91 7.67 7.87

Alt1 5.15 7.18 7.71 4.82 7.08 7.62 5.91 7.24 7.71 Alt2 5.15 6.61 7.39 4.30 6.46 7.10 5.91 6.75 7.58 Alt3 4.90 6.47 7.40 4.27 6.32 7.10 5.91 6.52 7.49

Flooding associated with the 2-year storm event under all three alternative scenarios can be seen in Figure 16. The flooding is contained to the roadside ditches and front yards under all alternatives.

Figure 16: Alternatives Analysis, 2-year Storm Event

Figure 17 shows flooding during existing conditions and all three alternative scenarios during the 10-year storm event.

Yards remain flooded and there is still significant street overtopping under Alternative 1, represented by the light blue shading. Alternative 2, shown in yellow, overtops the street in a smaller area, but minor yard flooding is projected.

Alternative 3 resulted in no street overtopping and minor yard flooding.

CATCH

BASIN 3

MANHOLE

PIPE

Figure 17: Alternatives Analysis, 2-year Storm Event

Recommendation The recommended solution would be to implement Alternative 2 for the cul-de-sac, upsizing the existing 12-inch pipe to an 18-inch pipe under the roadway and then to dual 18-inch pipes down to the ditch. For the area closer to 19 Bay Pines Dr, Alternative 3 should be implemented, upsizing the roadside pipes to 15- and dual 12-inch pipes and both realigning and increasing Pipe 2 to a 24-inch pipe from the road to the ditch. This combination would provide the most benefit for this area, nearly eliminating road overtopping in storms up to the 10-year storm event and minimizing yard flooding. The model was used to verify that there were no adverse downstream impacts, and the water remaining within the banks of the ditch with only a slight increase in water surface elevations.

Note that no survey data was collected for this effort at this time and that adequate cover could not be verified. The next steps of this effort would be to obtain easements along the proposed pipe paths, collect detailed survey data, and design the upsized pipes within the site constraints.

BASIN 3

MANHOLE

PIPE