ITB 24-42TM Addendum 3 Attachment 4 Coliseum Lake Weir HH Analysis.pdf

HYDROLOGIC &

HYDRAULIC REPORT

for

Coliseum Lake Weir Replacement Hampton, VA

August 25, 2020

Prepared for:

City of Hampton

Department of Engineering Services 22 Lincoln Street, 4th Floor Hampton, VA 23669 Contact: Brian Lewis 757 727 6284

Prepared by:

Kimley-Horn and Associates, Inc.

4525 Main Street, Suite 1000 Virginia Beach, Virginia 23462

KHA Project No. 116227196 © Kimley-Horn and Associates, Inc. Augustp 2020

TABLE OF CONTENTS

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF APPENDICES

1.0 INTRODUCTION

1.1 SUMMARY

1.2 PROJECT BACKGROUND

1.3 SITE DESCRIPTION

2.0 ENGINEERING METHODOLOGY

2.1 H&H MODEL DESCRIPTIONS

2.2 HYDROLOGY PARAMETERS

2.4 PRECIPITATION

2.5 TAILWATER CONDITIONS

3.0 HYDRAULIC STRUCTURE: ALTERNATIVE ANALYSIS

3.1 HYDRAULIC STRUCTURE ALTERNATIVE MODEL RESULTS

3.1.1 Proposed 2-ft Raised Weir

3.1.2 Proposed 4-ft Raised Weir

3.1.3 Proposed Adjustable Weir

3.2 HYDRAULIC STRUCTURE ALTERNATIVE CONCLUSION

4.0 HYDRAULIC ANALYSIS: RECOMMENDED ALTERNATIVE

4.1 CHANGES TO DCR REQUIREMENTS

4.2 PMF COMPARISON

4.3 HYDRAULIC ANALYSIS

4.3.1 Modeling the Existing Weir Structure

4.3.2 Modeling the Proposed Gate Structure

5.0 SUMMARY & CONCLUSION

August 2020

LIST OF TABLES

Table 1. All modeled conditions and the corresponding HGL.

Table 2. Probable Maximum Precipitation (PMP) Comparison Table Table 3: Probable Maximum Flood (PMF) Comparison Table

LIST OF FIGURES

Figure 1. Comparison of Water Surface Elevation (NAVD 88) of the Average Tide vs. Dorian Tide.

Figure 2. Comparison of flow through the Existing Coliseum Weir during a 100-year rainfall event with tailwater conditions of the Average Tide vs. Dorian Tide.

Figure 3. Comparison of flow through the Existing Coliseum Weir during an Average Tide with rainfall events of 10- and 100-year.

Figure 4. Comparison of flow through the Existing Coliseum Weir during a Dorian Tide with rainfall events of 10- and 100-year.

Figure 5. Comparison of all conditions - measuring depth within Coliseum Lake during a 100-year rainfall event with an Average Tide tailwater condition.

Figure 6. 2014 PMF Hydrographs for Coliseum Lake Catchment Figure 7. 2016 PMF Hydrographs for Coliseum Lake Catchment

LIST OF APPENDICES

Appendix A: Project Location Map Appendix B: Impervious Area Map Appendix C: FEMA Flood Insurance Rate Map (FIRM) Appendix D: Drainage Area Map Appendix E: Modeled Pipe Information Map Appendix F: Tidal Gauge Comparison Appendix G: Gauge Location Map Appendix H: 2016 (Current) 6-Hour PMP vs. 2014 12-Hour PMP, Existing Condition Breach Inundation Appendix I: Dam Breach Data and Schematic Appendix J: Gate Initially Closed vs. Gate Initially Open Inundation, Dam Breach at EL. 5.6ft, Current Standards – 100-year Huff

1.0 INTRODUCTION

1.1 SUMMARY

The City of Hampton has retained the services of Kimley-Horn and Associates, Inc. (Kimley- Horn) to conduct a hydrologic and hydraulic analysis for the Coliseum Lake Weir. The subject site is bounded by the Hampton Coliseum, the H2O Housing Development, and I-64 (refer to Appendix A). This report contains two interrelated studies:

1. The analysis completed to determine drainage improvements for Coliseum Lake including the upstream and downstream areas. The analysis yields a recommended alternative, which not only improves drainage but also mitigates tidal influence on Coliseum Lake. In addition, this study contains the preliminary design of the recommended alternative.

2. Hydraulic analysis of the recommended alternative with updated Virginia Department of Conservation and Recreation (DCR) standards, which includes determining the inundation depth and extents for upstream and downstream.

The first part of this report documents the parameters used to set up the Hydraulic & Hydrological (H&H) models. These parameters are the same for both the studies in this report. It considers the hydrologic parameters used for the PCSWMM model, as well as, precipitation, tailwater and tidal information.

The Report is then split into two sections: Hydraulic Structure: Alternative Analysis (Section 3.0) and Hydraulic Structure: Recommended Analysis (Section 4.0). The alternative analysis focuses on determining the optimal drainage improvement for the Coliseum Lake drainage area. The recommended analysis examines the upstream and downstream impacts of the recommended alternative by comparing a previously completed Dam Breach Analysis in 2014 (“Coliseum Lake Dam Break Analysis,” September 2014), and the current DCR standards (approved in March 23, 2016 and revised on March 29, 2018). The comparison will be used to determine if the previous analysis is still valid in light of the changes.

1.2 PROJECT BACKGROUND

The weir at Coliseum Lake has been studied previously because the City requested a condition assessment of the existing weir. In a Technical Memorandum prepared by Kimley-Horn on October 10, 2016 titled, “Coliseum Lake Weir Repair Study” (Repair Study), the weir on Lake Coliseum is reported to have been built between 1968 and 1971. Due to the age and condition of the dam in another inspection in 2013, minor repairs were recommended and subsequently completed on the weir. However, the erosion on the embankment and erosion/degradation near the repaired areas warranted another inspection in 2016. The Repair Study following this inspection recommended three options: place pressure grout to fill the voids in the concrete;

install a sheet pile cut-off wall parallel to the dam and weir; or completely replace the weir structure.

Replacement of the weir presented the most durable option and longest lifespan. The City chose to pursue a replacement option rather than installing more temporary measures to address the condition of the weir.

1.3 SITE DESCRIPTION

Coliseum Lake and surrounding areas have presented the City of Hampton (City) extensive opportunities for growth and development. The Hampton Coliseum sits on Coliseum Lake and attracts tourism and a wide range of events, which have stimulated economic growth in the surrounding areas. There are commercial and residential properties downstream of the lake and weir, as well as, major roads like North Armistead Avenue and LaSalle Avenue. Upstream of the Lake, there are multiple commercial developments including malls, hotels and the Interstate 64 and West Mercury Boulevard interchange.

Since there is relatively dense commercial development around Coliseum Lake, the area draining into the Coliseum Lake contains mostly impervious land, approximately 320 acres, which is 83% of the entire drainage area. Appendix B illustrates the impervious area captured within the drainage area. The average onsite slope is approximately 0.5%.

The site is located on FEMA FIRM Panel 5155270017H revised May 16, 2016 (Appendix C).

The site is located within Zone AE and Zone X, with Zone AE encompassing the area surrounding Coliseum Lake. Zone AE SFHA refers to the 1% annual chance (100-year) floodplain determined by FEMA via approximate methods with Base Flood Elevations (BFEs) determined.

2.0 ENGINEERING METHODOLOGY

2.1 H&H MODEL DESCRIPTIONS

For the first part of the study, three replacement options for the existing weir were considered.

Models were set up to compare the existing weir (existing condition) with three conceptual alternatives:

1. A fixed, raised weir;

2. A manual, adjustable gate; and

3. An automated, adjustable gate.

Using PCSWMM, the existing condition and these three proposed conditions were analyzed to present a recommendation to the City of Hampton. The proposed conditions include a 2-foot raised weir, 4-foot raised weir, and an adjustable gate. Since the manual and automated gate alternatives are the same in terms of function, these alternatives were represented by one single condition.

For the second part of the study, the recommended alternative determined in the first part of the study was used to determine the upstream and downstream impacts of the proposed flood gate construction.

2.2 HYDROLOGY PARAMETERS

Each SWMM model contains 15 subcatchments which make up the entirety of the drainage area (Appendix D). These subcatchments were delineated using existing survey data, contour lines generated in ArcGIS, existing landcover, and the Virginia LiDAR Dataset from the Virginia Geographic Information Network (VGIN). The Hydrologic Soil Groups (HSG) and landcover data were utilized to understand the weighted Curve number (CN) of the entire drainage area.

Onsite drainage was modeled using a two-dimensional (2-D) gridded analysis to evaluate multiple conditions for the site. The 2-D grid boundary extends along the catchment area to allow the model to show stormwater runoff buildup along the Coliseum Lake and further upstream/downstream.

2.3 STORM DRAINAGE MODEL LIMITATIONS

The model is limited to showing only backflow adjacent to the Coliseum Lake due to the lack of pipe information further upstream. Appendix E shows the pipe information inputted into PCSWMM from existing survey data and Kimley-Horn as-builts of the H2O Housing Development. As seen in the appendix, all backflow shown in the models is limited to the encompassed area between Pine Chapel Road, Coliseum Drive, Freeman Drive, and I-64.

2.4 PRECIPITATION

All existing and proposed conditions tested the 10- and 100-year, 24-hour storm events.

Per NOAA Atlas 14 Precipitation Frequency Data Explorer, the 24-hour total rainfall for the 10 and 100-year event are 5.44 inches and 9.33 inches, respectively. The NOAA Type C Distribution is recommended for use in this area of Virginia and was used to estimate the rainfall distribution for the 2-D model area.

2.5 TAILWATER CONDITIONS

Tidal tailwater information was incorporated into all conditions at the outlet of the Coliseum Weir. Water Surface Elevation (WSE) was measured at Coliseum Weir and collected from May 2019 to June 2019 to obtain an understanding of the water surface elevation at high and low tide. To verify whether the tidal information taken in 2019 was reliable, the information was compared with historical tidal data recorded at Coliseum Weir in 2011. Additionally, high and low tides from USGS Tidal Gauge at Newmarket Creek were reviewed and compared with the 2019 tidal data. As seen in Appendix F, the data overall matches the trends of the other tidal gauge data.

Using the tidal information gathered at the weir (identified in Appendix G as Kimley-Horn Tidal Gauge), an average tide event was identified. In addition, information from Hurricane Dorian (occurring Sept. 5 - Sept. 7 of 2019) was gathered to create a high tide event in comparison with average tides. Both tidal events were valuable in understanding when the tide controls the flow through the weir. Graph 1 below shows the Water Surface Elevation at Coliseum Weir for Average Tide and Dorian Tide.

Figure 1: Comparison of Water Surface Elevation (NAVD 88) of the Average Tide vs. Dorian Tide.

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Dorian Tide Average Tide

3.0 HYDRAULIC STRUCTURE: ALTERNATIVE ANALYSIS

To derive a conceptual solution and recommendation to improve existing stormwater infrastructure, a PCSWMM model was created. The existing condition model was set up using the following information:

• GIS and CAD topo information to define overall drainage area for the project

• Virginia LiDAR and Digital Elevation Models (DEM) to define subcatchments

(drainage to the Coliseum Lake)

• DEM to define cross-sections of the weir and parts of Coliseum Lake

• Web Soil Survey from USGS

• Previous Kimley-Horn project GIS information, As-Built plans, and existing hydraulic models to compare existing PCSWMM model information and results.

Each proposed condition model was run to determine whether that solution could balance both stormwater runoff from the catchment area and tidal influence. This was determined by analyzing the Coliseum Lake hydraulic grade line (HGL), the storage in the lake, and the flow through the Coliseum Weir.

3.1 HYDRAULIC STRUCTURE ALTERNATIVE MODEL RESULTS

The results of the hydraulic analysis yielded a balance of stormwater runoff and tidal influence. The first observation was made when running two Existing Condition models (100-year rainfall) with the two having different tailwater conditions (Average vs. Dorian Tide). The results presented that in a 100-year storm event, the stormwater runoff controlled the weir. The existing weir was modeled in PCSWMM by using a transect taken from LiDAR to capture the cross-section of the weir. Figure 2 below displays the results of these models as a hydrograph.

To better understand when the tide does control the flow through the weir, two more models were analyzed to best show the Existing Condition (10-year rainfall) with the two again having different tailwater conditions (Average vs. Dorian Tide). The two graphs below (Figure 3 and Figure 4) show the relationship between the Existing Condition 10-and 100-year storms for the Average Tide and Dorian Tide. The conclusion drawn from the model results was that the tide controls the flow through the weir during a 10-year storm event.

Figure 2: Comparison of flow through the Existing Coliseum Weir during a 100-year rainfall event with tailwater conditions of the Average Tide vs. Dorian Tide.

Figure 3: Comparison of flow through the Existing Coliseum Weir during an Average Tide with rainfall events of 10- and 100-year.

Figure 4: Comparison of flow through the Existing Coliseum Weir during a Dorian Tide with rainfall events of 10- and 100-year.

Once the relationship between stormwater runoff and tidal influence was recognized, the conceptual alternatives were modeled to better understand the best long-term solution for repair of the Coliseum Weir. Both Raise conditions were modeled by taking the existing transect of Coliseum Weir and raising each point by 2 or 4-ft not including the embankments. The Adjustable Weir condition was modeled similarly, although control rules were applied to monitor when the weir would open and close according to water depth within the lake. Figure 5 below represents the water depth within Coliseum Lake during a 100-year rainfall event at each condition; Existing, 2-ft Raise, 4-ft Raise, and Adjustable Weir. All resulting HGLs of Coliseum Lake are listed in Table 1 corresponding to the condition, storm event, and tailwater condition.

Figure 5: Comparison of all conditions – measuring water depth within Coliseum Lake during a

100-year rainfall event with an Average Tide tailwater condition

Table 1: All modeled conditions and the corresponding HGL.

Condition Storm Event

(24hr Duration) Tide Max HGL of

Lake (ft)

Existing Weir 10-yr Average 1.65 Dorian 3.86

100-yr Average 4.33 Dorian 4.33

Proposed 2ft Raise 10-yr Average 2.65 Dorian 3.2

100-yr Average 6.03 Dorian 6.03

Proposed 4ft Raise 10-yr Average Dorian

100-yr Average 8.45 Dorian 8.45

Proposed Adjustable Weir

10-yr Average 1.65 Dorian

100-yr Average 4.76 Dorian 4.76

3.1.1 Proposed 2-ft Raised Weir

As seen in Figure 5, the Proposed 2-ft Raise condition increases the depth within the lake by approximately 1-ft. The results show additional stormwater being held behind the raised weir as well as an average tide remaining in Newmarket Creek.

3.1.2 Proposed 4-ft Raised Weir

The 4-ft Raise condition increases the depth within Coliseum Lake by 4-ft compared to the Existing condition. Within this model, a significant amount of stormwater is being held back by the weir, raising the HGL of the lake to 8.45 ft in a 100-year event.

Although this raise in HGL is substantial, this condition prevents tides with surface water elevations of 4ft from entering Coliseum Lake.

3.1.3 Proposed Adjustable Weir

The Adjustable Weir model results show water depths similar to the Existing condition model. The variations in depth area due to the weir opening to allow more storage for stormwater runoff, which occurs at low tide to prevent high tides from entering Coliseum Lake.

3.2 HYDRAULIC STRUCTURE ALTERNATIVE CONCLUSION

Both the 2-foot and 4-foot raised weir held back significant quantities of water behind the weir structure creating potentially adverse upstream conditions. The adjustable gate was found to be the best alternative for design based on the hydraulic modeling. The conclusion was made due to the similarities in depth between the Adjustable Weir and Existing models and the weir’s ability to best support the lake through varying storm events and tides.

4.0 Hydraulic Analysis: Recommended Alternative

In Section 4.0, the adjustable gate (i.e. recommended alternative) will be further analyzed. First the changes in the DCR rainfall and associated hydrographs will be assessed as compared to the 2014 Dam Break Study. If the changes represent no significant change to the results, the previous Dam Break Study will still be valid.

Secondly, the worst-case scenarios with the proposed gate opened and closed will be modeled to see the effects on the depths and extents of inundation.

4.1 CHANGES TO DCR REQUIREMENTS

The goal of this study was to determine the effect revised PMPs and resulting PMFs had on the inundation extents and depth upstream and downstream of Coliseum Lake.

In the “Coliseum Lake Dam Break Analysis Report” prepared in September 2014 (Dam Break Report), the weir was classified as a “low risk dam” as defined by the DCR. This study will take the most critical PMF (controlling event) in the 2014 analysis and the most critical PMF in the current analysis and compare the resulting inundation.

In the Dam Break Report, the PMP depths were obtained from the Hydrometeorological Report No. 51 (HRM 51) PMP estimates: United States East of the 105th Meridian. The 6-hour, 12-hour and 24-hour PMP depth was applied to an SCS Type II hyetograph for the 6-hour, 12-hour and 24-hour storms, respectively. The PMP depths are shown in Table 2.

The cumulative hyetograph was then input into HEC-HMS to generate the appropriate hydrographs. According to Paragraph C of 4VAC50-20-50, the PMF should be the maximum peak discharge from the hydrographs for the 6-hour, 12-hour and 24-hour duration storm generated from the PMP. The resulting PMFs for the 6-hour, 12-hour and 24-hour storms are shown in Table 3.

According to “Guidance Document on New Probable Maximum Precipitation (PMP) Implementation” (DCR Guidance Document) published by the DCR (approved on March 23, 2016 and revised March 29, 2018), the PMP depths have been updated and the methodology used to determine the Probable Maximum Flood (PMF) depth has also changed.

Using the PMP Study and Evaluation Tool as provided by DCR, the revised PMP depths were determined, which are shown in Table 2. According to guidance from DCR, using the SCS Type II hyetograph for the 6-hour, 12-hour and 24-hour storm is no longer recommended. The Huff Distribution was believed to be a more representative of rainfall in the area than the SCS Distribution, since the SCS Distribution assumes a significant spike in rainfall at hour 12 of a 24-hour storm event. The Huff Distribution has the same volume of rain spread over a slightly more gradual and less intense peak.

These findings coupled with the findings of “Probable Maximum Precipitation Study for Virginia” prepared by Applied Weather Associates, LLC. in November 2015, led this current analysis to apply the PMPs to the Third Quartile Huff Distribution. The maximum peak discharge for the 6-hour, 12-hour and 24-hour duration storm generated from the PMP (i.e. the PMFs) can be found in Table 3.

Table 2: Probable Maximum Precipitation (PMP) Comparison Table

Rainfall Duration/Event 20141 Depth (in) 20162 Depth (in) 6-hour PMP 28.9 33.0 12-hour PMP 34.3 37.6 24-hour PMP 39.5 37.6

100-year, 24-hour 9.33 9.33

1 2014 refers to the PMP depth found in the Kimley-Horn report, “Coliseum Lake Dam Break Analysis,” published in September 2014. 2016 refers to the PMP depths per “Guidance Document on New Probable Maximum Precipitation (PMP) Implementation” published by the DCR (approved on March 23, 2016 and revised March 29, 2018).

2 The results for the 2016 analysis 12- and 24-hour durations are the same in terms of depth but vary in peak discharge. The reason for this is due to the distribution used for the 2016 PMP analysis. The 2016 analysis used the Third Quartile Huff Distribution, which unlike SCS Type II (used in 2014 analysis), spreads the precipitation out over a larger period of time, resulting in lower peak discharge values. While the depth values are the same for the 12- and 24-hour durations in the 2016 analysis, the peak discharge is lower due to the chosen distribution.

Table 3: Probable Maximum Flood (PMF) Comparison Table

Rainfall Duration/Event 20141 Peak Discharge (cubic feet per second)

20162 Peak Discharge (cubic feet per second)

6-hour PMP 3,660 3,159 12-hour PMP 3,683 2,358 24-hour PMP 3,573 1,404

100-year, 24-hour 1,860 1,514

1 2014 refers to the PMP depth found in the Kimley-Horn report, “Coliseum Lake Dam Break Analysis,” published in September 2014. 2016 refers to the PMP depths per “Guidance Document on New Probable Maximum Precipitation (PMP) Implementation” published by the DCR (approved on March 23, 2016 and revised March 29, 2018).

2 The results for the 2016 analysis 12- and 24-hour durations are the same in terms of depth but vary in peak discharge. The reason for this is due to the distribution used for the 2016 PMP analysis. The 2016 analysis used the Third Quartile Huff Distribution, which unlike SCS Type II (used in 2014 analysis), spreads the precipitation out over a larger period of time, resulting in lower peak discharge values. While the depth values are the same for the 12- and 24-hour durations in the 2016 analysis, the peak discharge is lower due to the chosen distribution.

4.2 PMF COMPARISON

The PMP depths were applied to the appropriate hyetographs as discussed in Section

4.1. The cumulative hyetographs were input into HEC-HMS and the resultant hydrographs for the 6-hour, 12-hour and 24-hour storm events were plotted in Figures 6 and 7. Figure 6 shows the hydrograph using the 2014 PMPs applied to the SCS Type II Distribution hyetograph. Figure 7 shows the hydrograph using the current PMPs applied to the Third Quartile Huff Distribution hyetograph. Both hydrographs are for the entire Coliseum Lake drainage area.

Tables 2 and 3 as well as Figures 6 and 7 show that although the PMP depth increased in the current standards as compared to the 2014 analysis, the resultant PMF in the current standards is actually less than in the 2014 analysis. This is largely due to the type of distribution used to apply the PMPs (i.e. Huff vs. SCS).

Figure 6: 2014 PMF Hydrographs for Coliseum Lake Catchment

Figure 7: 2016 PMF Hydrographs for Coliseum Lake Catchment

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4.3 HYDRAULIC ANALYSIS

To gain a better understanding of the differences between 2014 and 2016 PMPs and PMFs respectively, a hydraulic model was set up using HEC-RAS version 5.0.7. HEC- RAS unsteady flow module was used to account for multiple inflow hydrographs upstream and downstream of the weir/gated structure. A downstream stage hydrograph was used to simulate tidal fluctuations. The input hydrographs and tidal information were input at the same cross-sections as the 2014 analysis but with the revised precipitation depths and hydrographs (Third Quartile Huff Distribution instead of SCS Type II).

4.3.1 Modeling the Existing Weir Structure

The first HEC-RAS model plan aimed to compare the inundation in the 2014 and current analyses. The model was set up with 30 cross sections, 3 bridges and 1 inline structure.

This model added three cross-sections on the south side of the subject site where water flows under Interstate 64 to immediately downstream of the weir structure. Detailed survey at the weir structure, Coliseum Lake and west of the Lake were incorporated and merged with the available 1-foot LiDAR contours.

The controlling events (the most critical events) were then compared. In the 2014 analysis, the controlling event – the highest peak discharge when comparing the 6-, 12-and 24-hour PMFs – was the 12-hour PMP. However, according to the revised standards, the controlling event in the current analysis is the 6-hour PMP. Thus, the 2014 12-PMP and 2016 6-hour PMP were compared. In each scenario, the existing weir (inline structure) was modeled with the dimensions in the existing condition.

Appendix H shows the two inundation extents, the 2014 12-hour PMP and 6-hour PMP, overlaid on one another. As seen in this appendix, the current inundation extents cover a slightly larger area but only on the fringes of the inundation boundary. Even though there was deeper inundation associated with the current 6-hour PMP because of the higher intensity (extreme rainfall in a shorter time), the inundation boundary did not change significantly.

4.3.2 Modeling the Proposed Gate Structure

The goal of this second model was to examine the inundation extents and depth with the proposed slide gate installed (i.e. the recommended alternative from Section 3.0). In this section, the HEC-RAS model was set up to see what would happen in the case where the gate was not functioning properly (worst-case scenarios): either the gate was stuck and not opening or if it did not close. In both scenarios, the model assumes a dam breach occurs when the water surface elevation reaches the top of bank at the gate (elevation 5.6 feet).

This model added and extended the cross-sections of the previous model. Additional cross-sections were added both on the east and south side of Coliseum Lake to add more definition and stability to the model. Extended cross-sections were implemented to better model inundation on properties near Coliseum Lake. The LiDAR with merged survey was used for this model as well. The input hydrographs for the Lake Coliseum subcatchment were input at Cross-section 20517 instead of 19199.

The revised model has 38 cross-sections, 3 bridges and 1 inline structure. In this model, the inline structure is modeled as an opening with the proposed slide gate dimensions.

At a specified elevation, the dam structure was set to breach in an immediate and catastrophic manner, consistent with the 2014 Dam Break Report. The method of dam breach was set to overtopping and the breach was triggered at elevation 5.6 feet.

Overtopping was chosen because it was the most conservative method of modeling this dam breach when compared to the piping failure. Elevation 5.6 feet was chosen because it matches the elevation of the banks leading up to the dam. In a worst-case scenario, water would be allowed to pond behind the dam up until the elevation of the existing bank. This would represent the greatest amount of flooded water behind the inline structure and thus the most conservative scenario when all that water is released in a dam breach.

Appendix I shows the dam breach scenario as set up in HEC-RAS. The red line depicts the extents of the dam breach at the inline structure. The bottom elevation of the dam breach assumes that all the concrete also fails at the toe of the dam.

There were two scenarios modeled with overtopping failure and breach set to trigger at elevation 5.6 feet:

1. Gate initially open, 100-year, Third Quartile Huff Distribution

2. Gate initially closed, 100-year, Third Quartile Huff Distribution

According to the DCR Guidance Document, the dam breach analysis only needs to account for the 100-year, 24-hour storm. Thus, the hydrographs input into the HEC-RAS model were derived by using the Third Quartile Huff Distribution and the total rainfall of

9.33 inches over 24 hours, per NOAA Atlas 14.

In the “gate initially open” scenario, there is free flow both in and out of the Coliseum Lake. As precipitation fills the Lake and as the high tide creates a tailwater in the system, the lake water surface elevation starts rising. At Elevation 5.6 feet, the entire dam breaches per the dam breach plan in Appendix I.

The “gate initially closed” scenario is the worst-case scenario. It assumes no flow between the lake and downstream watercourse up until elevation 5.6 feet, at which point the dam breaches per the plan shown in Appendix I. As discusses previously, elevation 5.6 feet represents the highest allowable ponding of water behind the gate and is thus the most conservative scenario when all that water is released downstream.

To better compare these scenarios, Appendix J shows the two inundation extents overlaid on top of one another.

By comparing the scenarios, it can be seen that the depth of inundation in the “gate initially closed” scenario is greater than the depth of inundation in the “gate initially open” scenario. As seen in Appendix J, the differences between these two scenarios are most apparent downstream and on the southern side of Coliseum Lake. The differences in inundation extents downstream are likely due to the significant volume of water released in the “gate initially closed” scenario when the dam breaches. The differences between the two scenarios on the southern side is likely due to the water immediately downstream of the gate backing up into the southern part of the model area (tailwater effect) when the dam breaches.

Overall, the differences between these two scenarios as seen in Appendix J is not significant (i.e. one model does not flood a completely different area or property than the other model). This consistency indicates that even in the worst-case scenario, there are no additional structures being inundated.

5.0 SUMMARY & CONCLUSION

This report built on the weir condition assessments and recommendations in previous reports and by modeling alternatives, made a recommendation, which was tested with the revised DCR requirements. The first part of this Report set the stage for the modeling completed for the entire report. The precipitation, tailwater, land use and tidal conditions were set and modeled in PCSWMM.

Then, three alternatives were investigated in Section 3: a 2-foot weir, a 4-foot weir and an adjustable gate structure. The adjustable gate was found to be the best alternative for design based on the hydraulic modeling. The conclusion was made due to the similarities in depth between the Adjustable Weir and Existing models and the weir’s ability to best support the lake through varying storm events and tides.

Based on changes to the PMP depths and the way PMFs are generated, a separate hydraulic analysis was needed. The analysis was based on the previously completed dam breach analysis in 2014, which found this dam to be a low-hazard dam. The recommended alternative (the adjustable gate) was analyzed with the critical, 6-hour PMP using the Third Quartile Huff Distribution. This was compared to the critical, 12-hour PMP in the 2014 Dam Break Analysis using SCS Type II Distribution. Even though the PMP depth is higher in the current standard, the resultant PMF is lower than was found in the 2014 dam breach analysis. Even with more rainfall falling over half the time, the current day standard inundation was only slightly more extensive as seen in Section

4. This indicated that the previous dam break analysis completed in 2014 is likely conservative even to current-day standards.

To check what would happen in the case where the gate was not functioning properly (worst-case scenarios), a HEC-RAS model was used to determine the extents and depths of the 100-year, 24-hour inundation for two scenarios: “gate initially closed” and “gate initially open.” The results appear to show that there is no inundated infrastructure upstream or downstream.

Collectively these analyses indicate that with the changes to the PMP depths, the peak discharges are less than those used in the 2014 Dam Break Analysis. The resultant inundation due to a proposed gate structure does not appear to affect upstream or downstream structures within the subject area even in the worst scenario. As such, the dam break analysis in 2014 is conservative even to the current-day standards and even with the addition of a gate structure. Thus, no additional dam breach analysis is warranted.

Alison.Wiles Text Box

APRIL 2020

Legend !( Coliseum Lake Weir

Impervious Area Newmarket Creek Subcatchment

± PROJECT NO.: 116227175

APRIL 2020

IMPERVIOUS AREA

MAP

COLISEUM LAKE WEIR

HAMPTON, VA

APPENDIX B

0 0.25 0.50.125 Miles

APPENDIX F

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Alison.Wiles

APPENDIX H:

APPENDIX I

Alison.Wiles

Dam Breach Data and Schematic

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APPENDIX J: