tidal and rainfall flooding evaluation...sep 07, 2010 · tidal and rainfall flooding evaluation...

Submitted to: City of Virginia Beach, Department of Public Works

Tidal and Rainfall Flooding Evaluation

OCEAN PARK

9/7/2010

Tidal and Rainfall Flooding Evaluation for Shore Drive and Ocean Park

September 7, 2010 Page i

TABLE OF CONTENTS

Executive Summary ............................................................................................................. 1

Introduction .......................................................................................................................... 4

November 11th through 14th Northeaster .............................................................................. 5

Recent Tidal Floods ............................................................................................................. 8

Previous Tidal Floods .......................................................................................................... 9

Tropical Depression Ernesto ................................................................................................ 9

Tidal Flood Frequencies ..................................................................................................... 11

The Limits of Flood Protection in Ocean Park .................................................................. 12

Ocean Park Stormwater Evaluation ................................................................................... 12

Tidal Flood Sources in Ocean Park.................................................................................... 15

Potential Flooding Improvement Analysis ........................................................................ 17

Conclusions ........................................................................................................................ 26

APPENDICES

Appendix A – Estimate of Probable Construction Cost Appendix B – A Note About Datums, Elevations, and Tide Levels

EXHIBITS

Exhibit 1 – Elevation Map Exhibit 2 – Tidal Overflow Drainage Boundaries Exhibit 3 – Stormwater Pump Stations Drainage Area Isolation Plans


September 7, 2010 Page 1

EXECUTIVE SUMMARY

Shore Drive is one of three major east west traffic corridors connecting the ocean front beaches to the business and population centers in Virginia Beach and other Hampton Roads communities, and served as a military corridor during World War II. Residential and commercial areas along Shore Drive west of the Lynnhaven Inlet to Pleasure House Creek were originally platted prior to 1920 well before the incorporation of this area into the City of Virginia Beach. These developments predate all zoning and flood plain ordinances adopted by the City of Virginia Beach. Recent tidal events in November and December of 2009 caused flooding of low lying areas of Virginia Beach. The Ocean Park area west of the Lynnhaven Inlet experienced significant extended flooding due to the tidal surge, rainfall and the combination of both concurrently. The flooding was the result of strong sustained northeast winds in combination with heavy rains. The resultant floods approached the level of March 1962 northeaster and the August 1933 hurricane. Gauged tide records from the National Oceanic and Atmospheric Administration (NOAA) Chesapeake Bay Bridge Tunnel tide gage are presented in this report to display the magnitude and timing of the recent tidal flooding events. Elevations shown on the graphs should not be used to estimate the peak tidal flooding elevations or times in the subdivisions. Actual peak elevations were established from photographs and high water marks in the subdivisions. These peak elevations vary from one subdivision to another and are substantially different from the levels recorded at the NOAA gage. Beginning November 11, 2009, the strong northeast winds continued for most of the four day period causing the tide level to remain three or more feet above normal over six tide cycles. This extended tidal flood prevented rainfall runoff from draining to the Lynnhaven River. According to the gage at Oceana Naval Air Station, Virginia Beach experienced approximately 9.7 inches of rain for the two day period of 11 to 12 November 2009. The highest surface water elevation in Ocean Park area which was influenced by tide and rain was about 5.2 feet North Atlantic Vertical Datum (NAVD) 88. On December 19, 2009, about one month after the November northeaster, another northeaster produced tide levels that caused flooding in the low lying areas along the Shore Drive corridor. The peak tide elevation on December 19th was about one foot lower than the November northeaster. Approximately one inch of rain fell on that morning. Most residents remember Tropical Depression Ernesto of September 1, 2006 as the largest storm along the Shore Drive corridor in recent memory. Like the November ‘09 northeaster, Ernesto produced strong northeast winds, elevated tides, and over 8 inches of rain in 24 hours with periods of high intensities. The old electric railroad right-of-way, which is now Shore Drive, bisects Ocean Park. The Shore Drive corridor and the adjacent area are the lowest areas in Ocean Park. The dune line adjacent to the Chesapeake Bay to the north and a ridge line along Marlin Bay and Chesterfield Avenue are much higher than the Shore Drive corridor. Much of Shore Drive east of Pleasure House Creek is below the



ten (10) year tidal flood level. Because of the low elevations, the effect of tides, and the distance to outfalls, it is difficult to provide adequate drainage to the Shore Drive corridor for all tidal and rainfall conditions. There is no current Capital Improvement Project that address the tidal and rainfall flooding improvements needed for this area. This report addresses potential improvements to minimize flooding and provides recommendations to assist City staff to forcast future funding needs. Stormwater Pump Stations Two stormwater pump stations have been installed to provide relief from heavy rains for the lower areas on and adjacent to Shore Drive and portions of Ocean Park. Most of the area below elevation 4.0 NAVD88 is served by one of the stormwater pump stations. Protection for these areas from tidal flooding is limited for tides running three or more feet above normal. Several stormwater systems drain directly to Pleasure House Creek, Crab Creek, and the Lynnhaven River by gravity. During periods of normal tides the stormwater pump stations and the gravity outfall systems are capable of discharging the ten year rainfall event. However, during periods of tide levels greater than 3.5 feet NAVD88, the tidal waters cross the drainage boundaries of the gravity fed systems, and enter the drainage system of the pump stations. The pump stations were not designed and do not have the capacity to handle the tidal waters entering the pump station drainage area by overflowing the low points of the drainage divides The Powhatan Stormwater Pump Station (SWPS) located north of Shore Drive, constructed in 1984, serves an area of about 55 acres. The pump station and force main have the capacity to discharge the ten year rainfall event. However, the drainage system feeding the pump station can only deliver flow comparable to a two year rain event. As a result, ponding of low lying areas for brief periods can be expected during the ten year rain event. The Winston Place SWPS, located south of Shore Drive, was constructed in 1994, and serves an area of about 35 acres. This station has the capacity to handle ten year rainfall flows from an area larger than it currently serves. However, during periods of high tides, tidal waters can enter the pump station drainage system from the gravity systems outfalls at Crab Creek and Marlin Bay East, and by overland flow along Shore Drive from Pleasure House Creek. Unlike the area east of the Lynnhaven Inlet, adequately sized stormwater pumping facilities capable of discharging the design rainfall event are in place. Improvements outlined in this study are intended to isolate the drainage areas of these stormwater pump stations in order to prevent tidal flooding from entering the drainage systems. Due to the low elevations, however, tidal flooding can only be prevented up to the 10 year tidal event. The improvements include reconfiguring the gravity fed drainage systems and creating elevation boundaries to prevent tidal waters from entering the SWPS service areas. The implementation of these measures would allow the storm water pump stations to provide proper drainage for the ten year rainfall event even during tidal events equal to the ten year tidal flood, and should be done concurrently.



Shore Drive Corridor The section of Shore Drive from Pleasure House Creek east to the eastern intersection with DuPont Circle is below the 10 year tidal flood level (Elevation 4.9 NAVD88). In some areas the road elevation is more than one foot below the ten year tidal flood level. Stormwater Management Pond Serving the Winston Place SWPS Tide flap gates have been installed at the downstream end of the twin 48 inch culverts running from the stormwater management pond that serves as the discharge point for the Winston Place SWPS. The stormwater management pond is located south of the Winston Place SWPS and south of the Loch Haven Crescent an unimproved street. The tide flap gates are not functioning properly and should be removed to allow for the proper functioning of the discharge from the stormwater management pond. Low Lying Areas Adjacent to Waterways Areas adjacent to Crab Creek and Pleasure House Creek are not separated from tidal flooding by higher ground and are subject to direct tidal flooding. Methods of protecting those areas affected by direct overland flow of tidal waters from Crab Creek or Pleasure House Creek were not considered in this report.

Tidal Flood Warnings The normal high tide in the Lynnhaven River is about one foot NAVD88. Higher than normal high tide levels are wind influenced tides. Levels above 3.0 feet NAVD88 will cause flooding in the lower areas of Ocean Park. Residents in these areas can expect flooding at the time of high tide when the weather stations are predicting tides two or more feet above normal.



Introduction Shore Drive is one of three major east west traffic corridors connecting the ocean front beaches to the business and population centers in Virginia Beach and other Hampton Roads communities, and served as a military corridor during World War II. Residential and commercial areas along Shore Drive west of the Lynnhaven Inlet to Pleasure House Creek were originally platted prior to 1920 well before the incorporation of this area into the City of Virginia Beach. This development predates all zoning and development ordinances adopted by the City of Virginia Beach. The first flood plain ordinance was adopted after the Flood Plain Information Report prepared by the Norfolk District Corps of Engineers in 1969. The map below is a section of the USGS map titled “Cape Henry.” Roads and structures shown in red or black were in place at the time the original map was produced in 1964. Roads and structures shown purple were built between 1964 and 1986 which is when the map was revised. Note that portions of the Ocean Park area predate the map and therefore where in place before the City of Virginia Beach incorporated the region.

Figure 1. Shore Drive Corridor



Major tidal flooding has occurred along the Chesapeake Bay beaches and in the Lynnhaven River during the August 1933 hurricane, the March 1962 northeaster, and several other tropical storms to a lesser degree. Prior to 2009 the most severe recent tidal flood since 1962 was associated with Hurricane Isabel. Recent tidal/rainfall events in November and December of 2009 caused flooding of low lying areas of Virginia Beach. On the west side of Lesner Bridge, the impacted areas include Shore Drive and all of the Ocean Park subdivisions. The flooding was exacerbated by heavy rains during the tidal flooding events. As a result, Virginia Beach directed Parsons Brinckerhoff to evaluate the level of flooding during the northeasters to identify the primary factors causing the level and duration of the flooding. In addition to the evaluation of the recent storm the study included the quantification of risks of tidal flooding in combination with rainfall for the 10 year, 5 year, and 2 year flood event and lesser events. Cautionary Note: Tidal levels from the graphs in Figures 2, 3, 5 and 6 below show the relative magnitude and timing of the tidal event but cannot be directly converted to an elevation in Ocean Park. The vertical datum of the tide gages shown in the figures is different from the vertical datum currently adopted for use in the City of Virginia Beach.

November 11th through 14th Northeaster Strong northeasterly winds began in the early morning hours of November 11th and continued though November 14th. By 3 PM of the 11th, the tides were running three feet above normal. The tidal flooding peaked on the evening of November 12th at an elevation of about 6 feet above mean sea level or about elevation 5.2 feet NAVD88. (See Appendix B for an explanation of datum and tidal elevations). Above normal tides continued through November 14th. Figure 2 below shows the normal tide level and the gauged tide level for the November 11th through 14th period. Heavy rains accompanied the strong winds on November 11th and 12th. Virginia Beach (Oceana Naval Air Station) experienced approximately 9.7 inches of rain for the two day period and an intensity of over 3 inches/hour for a 10 minute period on November 12. The combination of wind driven tides and heavy rains produced significant flooding in Ocean Park. The strong northeast wind continued for most of the four day period causing the tide level to remain three or more feet above normal over six tide cycles. This extended tidal flood prevented rainfall runoff from draining to the Lynnhaven River through the existing storm drain pipe system. Thus, this rainfall runoff compounded the volume and depth of ponding of tidal water.



Figure 2. Chesapeake Bay Bridge Tunnel NOAA Tide Gauge November 11 through 14, 2009. (Note: This chart is not intended to illustrate the tides that were actually experienced in the Lynnhaven River or Pleasure House

Creek, but to compare a normal tide to the storm tide during the November 2009 Northeaster.) Figure 3 below shows the recorded rainfall at the Norfolk Airport and the Oceana Naval Air Station weather stations compared to the gauged flood level at the Chesapeake Bay Bridge Tunnel. One can see that the heaviest rain occurred when the gauged tide level was four feet above normal.

Figure 3.

Rainfall Distribution and Tide Levels - Nov. 12th. (Note: Tides are relative to Mean Sea Level (MSL) and cannot be converted to tidal flood elevations within the study area. For a detailed description of MSL and

how it relates to ground elevation, see Appendix B)

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The peak flood levels in areas along the Shore Drive Corridor were estimated at various locations based on photographs and videos. The highest level in Ocean Park was about 5.2 feet NAVD88. Based on the peak elevations, the November 12th flood stage was somewhat greater that a 10 year event but less than a 25 year event.

Figure 4. 3753 Stratford Rd. Approximate Flood Elevation 5.2 NAVD88.

Photograph taken around 5PM November 12, 2009. Actual peak flood elevation was slightly higher than shown in photograph.



Recent Tidal Floods In December 2009, about one month after the November northeaster, another northeaster produced tide levels that caused flooding in the low lying areas along the Shore Drive corridor. The peak tide elevation on December 19th was about one foot lower than the November 2009 northeaster. Approximately one inch of rain fell on that morning which was significant because of the ground saturation at the time. Figure 5 charts the tide levels at the Chesapeake Bay Bridge Tunnel gage on December 19, 2009.

Figure 5.

Chesapeake Bay Bridge Tunnel NOAA Tide Gauge December 19, 2009. (Note: This chart is not intended to illustrate the tides that were actually experienced in the Lynnhaven River or Pleasure House Creek, but to

compare a normal tide to the storm tide during the December 2009 Northeaster.)



Previous Tidal Floods The following is a list of the highest tidal floods recorded at the NOAA Sewells Point Tide Gage. The tidal floods are listed in order of magnitude at Sewells Point. Actual levels in the Lynnhaven River may differ from levels recorded at the gage.

• August 23, 1933, Hurricane • September 18, 2003, Hurricane Isabel • November 12, 2009, Northeaster • March 7, 1962, Ash Wednesday Northeaster • September 18, 1936, Hurricane • November 22, 2006, Thanksgiving Northeaster • February 5, 1998, Twin Northeaster • October 7, 2006, Columbus Day Northeaster • April 11, 1956, Northeaster • January 28, 1998, Twin Northeaster • September 16, 1999, Hurricane Floyd • September 26, 1956, Hurricane Flossy • September 12, 1960, Hurricane Donna

Based on the information listed above, no significant tidal flooding occurred between the Ash Wednesday Northeaster of 1962 and the Twin Northeaster of 1998, a period of almost 35 years. However, since 1998, there have been several. Thus, though there can be several years of no flooding, tidal flooding in the Ocean Park area is not unusual, and should be expected in such a low lying coastal area.

Tropical Depression Ernesto Not listed above is Tropical Depression Ernesto, which also produced some localized flooding, though not as severe. Because of the excessive rainfall coupled with tide levels of near 2.5 feet above normal, it is considered a severe storm. Tropical Depression Ernesto made landfall on September1, 2006. Like the November northeaster Ernesto produced strong northeast winds, elevated tides, and over 8 inches of rain in 24 hours with periods of high intensities. Figure 6 charts the tide levels associated with Tropical Depression Ernesto. Though the November northeaster was also greater than a 100 year design storm, it is important to note some differences between Ernesto and the November northeaster:

• Most of the Ernesto rain fell prior to noon on September 1st, which was during the rise of the high tide but before the peak of the high tide.

• The winds associated with Ernesto were easterly, while the winds associated with the northeaster

were north to northeast. Northeasterly winds tend to “push” the tide into the Lynnhaven and Broad Bays which not only creates “higher” high tides, but prevents the tides from receding.



• Tropical Depression Ernesto occurred on September 1, 2006 following generally dry antecedent

conditions. Thus, evaporation and dry soil conditions allowed the excess runoff to infiltrate into the soil, and evaporate more readily than during the November 2009 northeaster.

• The peak tide experienced during Ernesto was almost that of a 5 year tidal event, whereas the northeaster was greater to a 10 year tidal event.

Figure 6. Chesapeake Bay Bridge Tunnel NOAA Tide Gauge September 1, 2006.

(Note: This chart is not intended to illustrate the tides that were actually experienced in the Lynnhaven River or Pleasure House Creek, but to compare a normal tide to the storm tide during the Ernesto.)



Tidal Flood Frequencies The FEMA Flood Insurance Study for the City of Virginia Beach was revised effective May 4, 2009. Part of the revision included listing the tidal flood events in the NAVD 88 datum which is the vertical datum currently used by the City of Virginia Beach. The tidal flood frequency curve shown in Figure 7 below shows the flood stage at return periods from 1 year to 500 years. The plotted points were taken from the FEMA Flood Insurance Study revision. The flood levels in the Lynnhaven River are less than those in the Chesapeake Bay. Based on Figure 7 below, the five and 10 year tidal flood levels are 4.3 and 4.9 ft. respectively.

Figure 7. Tidal Flood Frequency Curve based on FEMA Flood Insurance Study City of Virginia Beach

(Revised May 4, 2009)

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The Limits of Flood Protection in Ocean Park As mentioned previously, the Ocean Park area is a coastal community with ground elevations in some areas only 2 feet above normal tide levels. Tidal flooding is to be expected. While some measures can be taken to provide protection for rainfall and tidal flooding in the majority of the Ocean Park area, there is no feasible way to provide protection for tides that exceed 5.1 feet in any area. At this point, the tidal waters overflow over all drainage barriers. Moreover, due to the extremely low elevations, properties that are closer to the water’s edge in the gravity fed basins are particularly susceptible to flooding at even lower tide levels. Exhibit 1 at the rear of this report is a color coded map showing the elevations in Ocean Park.

Ocean Park Stormwater Evaluation Ocean Park, located east of Pleasure House Creek and west of the Lesner Bridge, is a densely populated area consisting of approximately 50% single family homes, 25% multifamily residential condominiums and apartments, and 25% commercial. Shore Drive, a major artery, cuts through the entire area from the beaches on the north end and the populated areas east of the Lynnhaven Inlet. Ocean Park is a peninsula bounded on the north by the Chesapeake Bay, on the south and west by Pleasure House Creek, and on the east by Crab Creek and the Lynnhaven Inlet. Figure 8 below is an aerial photograph of the Ocean Park development.

Figure 8. Shore Drive and Ocean Park



The old electric railroad right-of-way, which is now Shore Drive, bisects Ocean Park. The Shore Drive corridor and the adjacent area are the lowest areas in Ocean Park. The dune line adjacent to the Chesapeake Bay to the north and a ridge line along Marlin Bay and Chesterfield Avenue are much higher than the Shore Drive corridor. Much of Shore Drive east of Pleasure House Creek is below the ten (10) year tidal flood level. Because of the low elevations, the effect of tides, and the distance to outfalls, it is difficult to provide adequate drainage to the Shore Drive corridor for all tidal and storm conditions. Two stormwater pump stations have been installed to provide relief from heavy rains for the lower areas on and adjacent to Shore Drive and portions of Ocean Park. This protection from tidal flooding is limited for tides running three or more feet above normal. Since several stormwater systems drain directly to Pleasure House Creek, Crab Creek, and the Lynnhaven River by gravity, tidal waters can backflow through the storm system as well flow overland. Shore Drive During a ten year tidal event all of Shore Drive from Pleasure House Creek east to Rockbridge Road would be inundated. Portions of Shore Drive are more than one foot below the ten year tidal elevation. Tidal flooding at this or higher levels can render Shore Drive impassable. Ocean Park For study purposes, the Ocean Park area is broken into six drainage basins as shown in figure 9:

• Crab Creek Basin • Winston Place Stormwater Pump Station (SWPS) Basin • Marlin Bay East Basin • Marlin Bay-Ocean Tides Basin • Powhatan Stormwater Pump Station (SWPS) Basin • Western Shore Drive Drainage area

The Winston Place and Powhatan basins are served by stormwater pump stations; the other four basins drain directly to Crab Creek, Pleasure House Creek, or the Lynnhaven Bay.

• Crab Creek Basin – The Crab Creek basin is located south of Shore Drive and east of and including Dinwiddie Road. The outfalls to Crab Creek area include a 36 inch concrete pipe on Piedmont Circle and a 42 inch concrete pipe from the unnamed right-of-way east of Tazewell Road. These outfalls flow directly into Crab Creek. Because of the extremely low elevations in this basin, tidal waters from Crab Creek can backflow into the surrounding area. The Crab Creek basin also includes some small areas north of Shore Drive at East Stratford Road. The drainage area of the Crab Creek Basin is about 20 acres. The drainage system in the Crab Creek basin was designed to handle the 10 year rainfall event.

• Winston Place Stormwater Pump Station (SWPS) Basin – Most of the Winston Place SWPS

basin is south of Shore Drive between Mystic Cove Drive and East DuPont Circle, but also includes the area north of Shore Drive between DuPont Circle and E. Stratford Rd. The drainage area served by the Winston Place SWPS is about 35 acres. The SWPS can discharge the 10 year



rainfall from a larger drainage area than the area currently served. Large areas of this basin are below elevation 4.0 with some areas below elevation 3.0 NAVD88.

• Marlin Bay East Basin – The Marlin Bay East Basin includes Mystic Cove Drive south of

Pendleton Avenue and Rockbridge Road south of E. Stratford Road, and discharges into Pleasure House Creek. The basin overlaps the Winston Place SWPS drainage area. As a result, during high tides, tidal waters can backflow into the drainage area and subsequently flow into the Winston Place SWPS drainage area.

• Marlin Bay - Ocean Tides Basin – The Marlin Bay –Ocean Tides Basin is approximately 8

acres along Marlin Bay drive, and includes Ocean Tides Drive, Whispering Sands Lane, and a portion of Clipper Bay Drive.

• Powhatan Stormwater Pump Station (SWPS) Basin - The Powhatan SWPS is located north of Shore Drive on Powhatan Avenue between Stratford Road and Surry Road. The drainage area is approximately 55 acres, and includes most of the area north of Shore Drive and west of DuPont Circle as well as some smaller areas south of Shore Drive. The outfall for the pump station is a 36 inch concrete pipe discharging into Pleasure House Creek on the south side of Shore Drive. The SWPS is capable of discharging the runoff from a 10 year rain event. However, the pipe drainage system flowing to the pump station is not capable of delivering more than about two year rain event. Thus minor ponding for brief periods would be expected during a 10 year rain event.

• Western End of Shore Drive Drainage Area – The Western End Drainage Area covers the low

lying areas of the western end of Shore Drive that is not covered by the other five drainage basins. This 2.5 acre area includes areas west of Marlin Bay Dr including the swales in the center of Shore Drive. The outfall is on Pleasure House Creek.

Exhibit II shows the locations where the tidal water overflows from one drainage system to another.



Figure 9. Ocean Park Drainage Areas

Tidal Flood Sources in Ocean Park Above normal tides, heavy rainfall, or a combination of both causes flooding in the Ocean Park area. The extremely low lying, flat terrain coupled with interconnected drainage basins allows for the flooding to cross drainage divides from one drainage system to the others. Once the tide level reaches elevation 3.5 feet NAVD88, the tidal waters backup through the direct drainage systems, and overflows the drainage inlets even without any rain. From there, the water flows overland to drainage structures in adjacent drainage areas in the Shore Drive/Ocean Park area. There may also be flooding during heavy rains when the tide is normal. In the Ocean Park area, normal tides range between elevations 1 ft. and -1 foot. Some road elevations area as low as elevation 2.1 feet. Thus there is only two feet of vertical separation above normal tides before flooding occurs in the roadway, and subsequent spreading from one drainage system to another. When heavy rains are accompanied by higher than normal tides, the flooding is much more pronounced due to the combination of events.



In the Shore Drive Ocean Park area the basins that are gravity fed are Crab Creek, Western End, Marlin Bay East, and Marlin Bay Ocean Tides. Crab Creek Basin

• Flows exceed pipe capacities for the 2 year rain storm.

• Water surface elevations exceed the rim elevations (elevation of the top of drainage structure) on Shore Drive near DuPont Cir. for the 2 year tidal event.

• Water surface elevations exceed the rim elevations on Shore Drive and Dinwiddie for the 2 year rain storm with no tidal event.

Figure 10. (left) 36” outfall on Piedmont Circle (right) 42” outfall from Tazewell Road

When the stormwater drainage system in the Crab Creek Basin is above capacity, the water then ponds on Shore Drive, and ultimately flows down into the Winston Place SWPS drainage basin, where it is then pumped back into Crab Creek. When tidal waters reach elevation 3.5 feet, the tidal waters back up the drainage system, and overflow into the Winston Place SWPS service area. Western End Shore Drive

• Flows exceed pipe capacities for the 2 year rain storm.

• Water surface elevations exceed the rim elevations on Shore Drive for the 2 year tide and rain event.

When the drainage system along the Western end of Shore Drive is backed up by tidal and/or rainwater, the overflow drains eastward along the surface adjacent to Shore drive into the Powhatan Avenue SWPS drainage system where it is pumped back into Pleasure House Creek. The outfall of the SWPS is near



the out fall of the Western End Shore Drive drainage system which is gravity fed. As a result, the Powhatan SWPS is overwhelmed by tidal water during the 5 year tidal event. Marlin Bay East Basin

• Water surface elevations exceed the rim elevations on Rockbridge Rd. during a 5 year tidal event.

When the stormwater drainage system along Rockbridge Rd. is backed up by tidal water, it flows northward onto E. Stratford Road and into the Winston Place SWPS drainage basin, where it is pumped out into Crab Creek. Marlin Bay- Ocean Tides Basin

• Water surface elevations should not exceed the rim elevations unless tides are above the 50 year level. Thus, though this drainage basin is not isolated from the other drainage basins, it does not serve as a conduit for tidal waters to enter the other low lying areas during lesser tidal events.

Potential Flooding Improvement Analysis Four of the six drainage systems in Ocean Park described above discharge directly to tidal waters. Once tide levels reach elevation 4.0 NAVD88, three of the direct discharge drainage systems back up to the point that flooding from tidal waters is apparent on sections of Shore Drive and in lower sections of Ocean Park. The three drainage areas that overflow tidal waters to SWPS drainage areas are the Crab Creek Basin, Marlin Bay East Basin and the Western End Shore Drive Basin. The following are methods investigated to provide flood relief in the Ocean Park area and the Shore Drive corridor west of the Lynnhaven Inlet. Any improvements to the Ocean Park area are primarily intended to elevate the drainage boundaries of the two stormwater pump stations such that the drainages area of the stormwater pump stations are not affected by tidal flooding up to the level of a ten(10) year tidal event or about elevation 5.0 NAVD88. More than one alternative may exist to prevent a drainage basins listed above from overflowing in a SWPS drainage area. Although the individual drainage basin projects are listed separately, the SWPS drainage areas will not be fully protected from tidal water below the ten year tidal event unless the improvements to all three basins are installed. More than one method or option may exist to prevent the overflow of tidal waters to a SWPS drainage area. The improvements described below are listed in order of the elevation of the drainage boundary breech starting at the lower elevation. Crab Creek Basin Alternative 1 Provide tide gates, self regulating tide gates, or tide flex “J” valves at the 42 inch and 36 inch outfalls to Crab Creek. See figure 11. The tidal waters back up these pipe systems and begin ponding of tidal waters in the Shore Drive Right of Way at elevation 3.0 NAVD88 or the one year tidal flood level.



Ponded water within the Shore Drive Right of Way would then be picked up by inlets draining to the Winston Place SWPS. The benefit of the tide gate method would be to prevent tidal waters from backing up into Shore Drive and over flowing to the Winston Place SWPS drainage system.

This alternative is not recommended because of the pressure required to open the valves. The pressure required to open the tide valves may cause flooding from runoff to a greater level than would be produced during the high tide event. The fact that the outfalls are partially full of silt and have crustacean growth would prevent the tidegate/valve from opening and /or closing properly. See Figure 10 above for photographs of the outfalls. Estimated Cost: $70,000

Figure 11. Crab Creek Basin- Alternative 1

Install tide gates Crab Creek Basin Alternative 2 - Implementation 1 The alternative to providing tide gates as described above involves two separate and necessary implementations. The first is the creation of a higher drainage divide on Shore Drive between DuPont Circle and East Stratford Road by increasing the elevation across the road and shoulder to elevation 5.0 feet NAVD88. The purpose of the divide is to prevent tidal waters from entering the Winston Place SWPS drainage area. The existing 24 inch pipe that crosses Shore Drive east DuPont Circle would be plugged. As a result, flow from the north side of Shore Drive at the DuPont Circle intersection would be



redirected east along the north side of Shore Drive through 270 feet of 24 inch concrete pipe (proposed) along the north to East Stratford Road. This 24 inch drain would then cross Shore Drive and tie into the existing 36 inch concrete pipe on the south side of Shore Drive. The existing 36 inch pipe on the south side of Shore Drive would be plugged upstream of the tie in with the proposed 24 inch pipe. The existing drainage structure located on the south side of Shore Drive east of DuPont Circle would be connected to the Winston Place SWPS drainage system by installing an 18 inch pipe from the drainage structure to tie into the existing drainage structure on the south side of Shore Drive approximately 300 feet to the west. This action would prevent the tidal waters from flooding Shore Drive and entering the Winston Place SWPS system when the tidal flood reaches 5.0 NAVD88. See figure 12. Estimated cost: $100,000

Figure 12. Crab Creek Basin Alternative 2A

Shore Drive Drainage Divide

Alternative 2 -Implementation 2 The second implementation involves redirecting flows along Dinwiddie from Crab Creek at Tazewell to the Winston Place SWPS. This requires plugging the 34” X 53” elliptical concrete pipe on E. Stratford Rd. at its highest point, and installing 375 feet of 36” pipe along E. Stratford Rd between Dinwiddie and Chesterfield where it would be connected to the existing 72 inch concrete pipe.



It is important to note that this action should be accomplished along with elevating the drainage divide as described above in order to prevent tidal flows from Crab Creek from entering the Winston Place SWPS below the 10 year tide level. See figure 13. Estimated Cost: $90,000 The total cost for alternative two, which is recommended, is $190,000.

Figure 13. Crab Creek Basin - Alternative 2B

Redirect Flow along Stratford to Winston Place SWPS

Western End Shore Drive Basin

Create a drainage divide on Shore Drive between Clipper Bay and Marlin Bay Drive by increasing the elevation across the road and shoulder to elevation 5.0 feet NAVD88. The intersection of Marlin Bay Drive and Shore Drive would continue to drain to Pleasure House Creek to the west through existing storm drainage. Shore Drive would be elevated beginning at the Marlin Bay intersection and rise to a high point near the intersection with Clipper Bay then fall back to existing grade at a point approximately 100 feet east of Clipper Bay Drive. This drainage divide would provide a separation between the western Shore Drive drainage area and the Powhatan SWPS drainage area up to the ten (10) year tidal flood level. See figure 14.



As currently envisioned, no driveways should be impacted by constructing the drainage divide. The intersection at Clipper Bay and Shore Drive will have to be regraded. The roadway surface elevation changes will not adversely affect traffic flow after construction is completed. Estimated Cost: $210,000

Figure 14. Western End Shore Drive Basin

Create Drainage Divide

Marlin Bay East Basin

Redirect stormwater flow in Rockbridge Road south of Marlin Bay Drive by plugging the existing 36 inch concrete pipe extending north along Rockbridge Road upstream of the intersection with Mystic Cove Drive. Connect the existing drainage structures on Rockbridge Road between Mystic Cove Drive and East Stratford Road to the drainage system in East Stratford Road. This will prevent tidal waters from entering the Winston Place SWPS basin. See figure 15. Estimated cost: $10,000



Figure 15.

Marlin Bay East Basin Redirect Flow to the North – Rockbridge Road

Powhatan SWPS Basin Below are listed two alternatives to relieve flooding along Shore Drive in the area between E. Stratford Road and Clipper Bay Drive which experiences poor drainage and ponding even during relatively small storms. Both involve redirecting flows from the Powhatan Ave. pump station to the Winston Place pump station. Alternative 2 redirects a larger runoff area than Alternative 1. Both are acceptable. Alternative 1

Reroute flows through the 12 inch and 18 inch concrete pipes under Shore Drive at the intersection with Roanoke Avenue/Stratford Road from the Powhatan SWPS to the Winston Place SWPS. This would include installing approximately 200 feet of 24” pipe along Stratford Road south to Pendleton Avenue. The flows can drain to either the Winston Place SWPS or the Powhatan SWPS. Remove and replace the existing damaged manhole on the corner of Pendleton and E. Stratford Rd. for a larger one. See figure 16.

Estimated cost: $40,000



Figure 16. Redirect flow to Winston Place SWPS

Alternative 1 Powhatan SWPS Basin Alternative 2 Alternative 2 involves redirecting the area on the south side of Shore Drive at the intersection with Mystic Cove Drive, Ocean Tides Drive, and Stratford Road from the Powhatan SWPS to the Winston Place SWPS drainage system. Flows from the south side of Shore drive would be redirected by installing 18 inch and 24 inch storm pipe along Shore Drive from Mystic Cove Drive to Stratford Road and installing 30 inch and 36 inch pipes from Shore Drive south along Stratford Road to Rockbridge Road to connect to the existing 72 inch storm drain. Storm drain pipes crossing Shore Drive at the three intersections would remain open to allow flows to either Winston SWPS, Powhatan SWPS, or both. See figure 17. Estimated Cost: $470,000



Figure 17. Reroute flows from Powhatan Pump Station to Winston Place SWPS

Alternative 2

Winston Place SWPS Tide Gates Removal There are two heavy duty tide flap gates located at the downstream end to the twin 48 inch culverts serving the stormwater management pond that is the discharge point for the SWPS. It appears that sediment is preventing one of the gates from closing during high tide events. The other gate appears to be fixed in the closed position preventiing outflow from the stormwater management pond. Even if these gates were functioning properly, the pressure needed to open these flap gates to allow for adequate discharges of water from the pond is so high that it would interfere with the proper functioning of the pond. The tide flap gates should be removed to allow for free flow of the twin 48 inch culverts. The berm elevation of the stormwater management pond is above the ten year tidal flood so that flooding of the pond would have no negative impact to the Winston Place SWPS drainage area. See figure 18 and 19. Estimated cost: $20,000



Figure 18. Tide Flap Gates at Stormwater Management Pond Outfall

Figure 19. Remove Existing Flap Gates



Conclusions Ocean Park is a low-lying, highly urbanized coastal community bisected by Shore Drive, a major traffic corridor. Recent tidal and storm events have caused severe flooding in parts of Ocean Park and along Shore Drive. Flooding protection can be provided for most of the Ocean Park area and along Shore Drive, up to a tide level of approximately 5 feet (NAVD88). In order to provide this protection there are four drainage modifications that must be made:

• Crab Creek: Create drainage divide and redirect flows (Implementation 1 and 2) - $190,000 • Western End Shore Dive – Create drainage divide - $210,000 • Marlin Bay East - Redirect flows - $10,000

Because these improvements are interdependent, at minimum the upgrades to the three basins as listed above must be implemented concurrently to provide protection for most of Ocean Park for a tidal elevation of 5 feet NAVD88. These improvements address tidal flooding only. Shore Drive will continue to experience ponding and localized flooding even after these implementations are completed. To address some of the localized ponding and slow drainage along Shore Drive, improvements to Shore Drive drainage and rerouting flows to the Winston Place SWPS as described for the Powhatan SWPS Basin should be implemented. Table 1 below describes the options available for addressing the tidal flooding and drainage improvements in Ocean Park. Exhibit III shows the improvements described in this report. There is a need to create a Capital Improvement Project to systematically fund these improvements. This report provides a basis to develop a program for needed drainage improvements. Currently there is no Capital Improvement Project that is addressing these needs.



Table 1. Summary of Options Available to provide flooding protection and drainage improvements for Ocean Park.

OPTION 1 Tidal Flood Protection Only

Basin Alternative Approximate Cost

Crab Creek 2 $190,000

Western End 1 $210,000

Marlin Bay East 1 $10,000

Total Estimate $410,000

Tidal Flood Protection and Drainage Improvements

OPTION 2 OPTION 3 OPTION 3

Basin Alternative Approx. Cost Alternative Approx. Cost

Crab Creek 2 $190,000 2 $190,000

Western End 1 $210,000 1 $210,000

Marlin Bay East 1 $10,000 1 $10,000

Powhatan SWPS 1 $40,000 2 $470,000

Winston Place 1 $20,000 1 $20,000

Total Estimate $470,000 $900,000



Table 2. Summary of Actions for each Drainage Basin

Action Description Benefits Disadvantages Approximate Cost Remarks

Crab Creek Basin Alternative 1

(pg. 18) Install tide gates, tide flex, or “J” valves at outfalls to Crab Creek.

Will prevent major flooding caused by tides in the Ocean Park area.

Does not allow for drainage of stormwater in the event of a major storm during a tidal event, and requires regular maintenance

$70,000 Not recommended

Alternative 2 Implementation 1

(pg. 19) Create drainage divide across Shore Drive and redirect flow to Crab Creek Basin

Will provide flooding protection in a 10 year tidal event.

Provides minimal protection for tidal events above the 10 year level.

$100,000 Recommended

Implementation 2 (pg. 20)

Install 350 feet of 36” pipe to redirect storm drainage to the Winston Place SWPS.

Will prevent major flooding caused by high tides in the Ocean Park area.

None $90,000 Recommended

Western End Shore Drive Basin

(pg. 21) Create a drainage divide across Shore Dr. between Marlin Bay and Mystic Cove

Will provide flooding protection in a 10 year a tidal event.

Provides minimal protection for tidal events above the 10 year level.


Marlin Bay East Basin

(pg. 22) Redirect flows on Rockbridge Rd. Prevent tidal waters from entering

the Winston Place SWPS basin. None $10,000 Recommended

Powhatan SWPS Basin Alternative 1

(pg. 23) Reroute flows from Powhatan Basin that run under Shore Drive to the Winston Place SWPS.

Will improve drainage on Shore Drive.

Lower construction costs. May not prevent surcharging and temporary flooding during large storm events.

$40,000 Recommended

Alternative 2 (pg 24) Reroute flows from Powhatan

Basin that run under Shore Drive to the Winston Place SWPS.

Will improve drainage on Shore Drive and prevent surcharging during high flows.

Higher construction costs




Action Description Benefits Disadvantages Approximate Cost Remarks

Winston Place SWPS Tide Gates

(pg. 25) Remove tide flap gates at downstream end of culverts serving stormwater management pond.

Prevent flooding due to failure of gates to open during a major storm event.

None $20,000 Recommended

PARSONS BRINCKERHOFF

Appendix A

Appendix A

PROJECT: Ocean Park BASIS FOR ESTIMATELOCATION: Virginia BeachCLIENT: City of Virginia Beach [ X] PRELIMINARY DESIGN

DISCIPLINE/ TOTALSUMMARY NO. UNIT PER TOTAL PER TOTAL COST

UNITS MEAS. UNIT UNIT

TideFlex Valves 2 EA $15,000 $30,000 $7,000 $14,000 $44,000

Subtotal $30,000 $14,000 $44,000

Tax & OH&P(5%& 15%) $2,100 $2,100Subtotal $30,000 $16,100 $46,100

Engineering & Oversight (20%) $6,000 $3,220 $9,220Contingency (35%) $10,500 $5,635 $16,135

Total $46,500 $24,955 $71,455

TOTAL ESTIMATE $47,000 $25,000 $70,000

ESTIMATE OF PROBABLE CONSTRUCTION COST

CRAB CREEK ALTERNATIVE 1QTY MATERIAL LABOR

A1

Appendix A

IMPLEMENTATION 1DISCIPLINE/SUMMARY TOTAL

NO UNIT PER TOTAL PER TOTAL COSTUNITS MEAS. UNIT UNIT

Milling 200 SY $0 $0 $10 $2,000 $2,000Filling Compact 66 CY $25 $1,650 $20 $1,320 $2,970Asphalt 135 TONS $65 $8,775 $30 $4,050 $12,825Pavement Marking 160 LF $1 $160 $0 $0 $16024" RCP 270 LF $28 $7,560 $21 $5,670 $13,23018" RCP 75 LF $24 $1,800 $21 $1,575 $3,375Connect to Existing 3 Each $250 $750 $1,500 $4,500 $5,250C&G Removal 200 LF $0 $0 $4 $800 $800C&G Replacement 200 LF $13 $2,630 $3 $550 $3,180Pavement Removal & Replace 200 SY $30 $6,000 $35 $7,000 $13,000E&S 1 LS $1,000 $1,000 $1,500 $1,500 $2,500

Subtotal $30,325 $28,965 $59,290Tax & OH&P(5%& 15%) $4,345 $4,345

Subtotal $30,325 $33,310 $63,635Engineering & Oversight (20%) $6,065 $6,662 $12,727Contingency (35%) $10,614 $11,658 $22,272

Total $47,004 $51,630 $98,634

TOTAL ESTIMATE $47,000 $52,000 $100,000

CRAB CREEK BASIN ALTERNATIVE 2QTY MATERIAL LABOR

A2

Appendix A

IMPLEMENTATION 2DISCIPLINE/ TOTALSUMMARY NO. UNIT PER TOTAL PER TOTAL COST

UNITS MEAS UNIT UNIT

36" RCP 375 LF $65 $24,375 $40 $15,000 $39,375Drainage Structures 1 Each $1,300 $1,300 $800 $800 $2,100Connect to Existing 1 Each $0 $0 $1,000 $1,000 $1,000Pavement Removal & Replace 160 SY $30 $4,800 $35 $5,600 $10,400E&S 1 LS $500 $500 $1,000 $1,000 $1,500

Subtotal $30,975 $23,400 $54,375Tax & OH&P(5%& 15%) $3,510 $3,510


Total Estimate $48,011 $41,711 $89,722

TOTAL ESTIMATE $48,000 $42,000 $90,000

CRAB CREEK ALTERNATIVE 2QTY MATERIAL LABOR

A3

Appendix A

DISCIPLINE/ TOTALSUMMARY NO UNIT PER TOTAL PER TOTAL COST

UNITS MEAS. UNIT UNITMilling 100 SY $0 $0 $10 $1,000 $1,000Asphalt 800 Tons $65 $52,000 $30 $24,000 $76,000Pavement Marking 160 LF $0 $1 $0 $48 $49C&G Removal 140 LF $0 $0 $3 $466 $466C&G Replace 140 LF $13 $1,841 $3 $385 $2,226Cement sidewalk 60 SY $45 $2,700 $5 $300 $3,000Standard C & G 140 LF $13 $1,820 $3 $420 $2,240Fill 400 CY $15 $6,000 $5 $2,000 $8,000Drainage Modifications 3 EA $7,500 $25,000 $2,700 $8,100 $33,100Radial C & G 50 LF $25 $1,250 $7 $350 $1,600Landscaping 1 LS $750 $750 $100 $100 $850Subtotal $91,362 $37,169 $128,531Tax & OH&P(5%& 15%) $0 $5,575 $5,575


Total $141,610 $66,254 $207,864

TOTAL ESTIMATE $142,000 $66,000 $210,000

QTY MATERIAL LABORWESTERN END SHORE DRIVE BASIN

A4

Appendix A



36" Endwall with Cut-Off Wall 1 Each $100 $100 $1,200 $1,200 $1,300Concrete Pipe Fill to Reverse Flow250 LF $7 $1,750 $1 $125 $1,875Bulkhead cut thru 1 Each $250 $250 $1,500 $1,500 $1,750Pavement Removal & Replace 2 SY $30 $60 $35 $70 $130E&S 1 LS $500 $500 $200 $200 $700

Subtotal $2,660 $3,095 $5,755Tax & OH&P(5%& 15%) $464 $464

Subtotal $2,660 $3,559 $6,219Engineering & Oversight (20%) $532 $712 $1,244Contingency (35%) $931 $1,246 $2,177

Total $4,123 $5,517 $9,640

TOTAL ESTIMATE $4,000 $6,000 $10,000

MARLIN BAY EAST BASINQTY MATERIAL LABOR

A5

Appendix A

DISCIPLINE/ TOTALSUMMARY NO. UNIT PER TOTAL PER TOTAL COST


24" RCP 200 LF $28 $5,600 $21 $4,200 $9,800Remove Manhole 1 Each $0 $0 $700 $700 $700Install Manhole 1 Each $925 $925 $680 $680 $1,605Connect to Existing 1 Each $0 $0 $500 $500 $500C&G Removal 200 LF $0 $0 $4 $800 $800C&G Replacement 200 LF $13 $2,630 $3 $550 $3,180Pavement Removal & Replace 90 SY $40 $3,600 $35 $3,150 $6,750E&S 1 LS $500 $500 $820 $820 $1,320

Subtotal $13,255 $11,400 $24,655Tax & OH&P(5%& 15%) $1,710 $1,710


Total $20,545 $20,321 $40,866

TOTAL ESTIMATE $21,000 $20,000 $40,000

POWHATAN SWPS BASIN ALTERNATIVE 1QTY MATERIAL LABOR

A6

Appendix A



36" RCP EQUIV 340 LF $100 $34,000 $53 $18,020 $52,02030" RCP 280 LF $50 $14,000 $31 $8,680 $22,68024" RCP 480 LF $30 $14,400 $21 $10,080 $24,48018" RCP 290 LF $25 $7,250 $21 $6,090 $13,340Reg Drainage Structures (4'Dia) 6 LF $800 $4,800 $455 $2,730 $7,530Lg Drainage Structures (6' Dia) 5 LF $2,700 $13,500 $1,200 $6,000 $19,500Connect to Existing 2 Each $250 $500 $1,500 $3,000 $3,500C&G Removal 290 LF $0 $0 $4 $1,160 $1,160C&G Replacement 290 LF $13 $3,814 $3 $798 $4,611Pavement Removal & Replace 1300 SY $30 $39,000 $35 $45,500 $84,500Utility Adjustments 1 LS $10,000 $10,000 $15,000 $15,000 $25,000Traffic control 15 Days $1,500 $22,500 $22,500E&S 1 LS $500 $500 $1,000 $1,000 $1,500

Subtotal $141,764 $140,558 $282,321Tax & OH&P(5%& 15%) $21,084 $21,084


Total $219,733 $250,544 $470,277

TOTAL ESTIMATE $220,000 $250,000 $470,000

QTY MATERIAL LABORPOWHATAN SWPS ALTERNATIVE 2

A7

Appendix A

DATE 07/12/2010DISCIPLINE/SUMMARY TOTAL

NO UNIT PER TOTAL PER TOTAL COSTUNITS MEAS. UNIT UNIT

Remove Flap Gates 1 LS $0 $10,000 $10,000 $10,000

Subtotal $0 $10,000 $10,000Tax & OH&P(5%& 15%) $1,500 $1,500

Subtotal $0 $11,500 $11,500Engineering & Oversight (20%) $0 $2,300 $2,300Contingency (35%) $0 $4,025 $4,025

Total $0 $17,825 $17,825

TOTAL ESTIMATE $0 $18,000 $20,000

WINSTON PLACE TIDE GATES

QTY MATERIAL LABOR

A8

PARSONS BRINCKERHOFF

Appendix B

Appendix B A Note About Datums, Elevations, and Tide Levels

B1 Tidal and Rainfall Flooding Evaluation| Parsons Brinckerhoff

Ground Elevations

There is a great deal of confusion concerning elevations and tide levels, and it is important for thestakeholder to determine not only the units of elevations such as whether elevations are feet or meters, but towhat vertical datum it is referencing.

A vertical datum is a starting point from which elevations can be compared and referenced. Until 2004,the official vertical datum for the City of Virginia Beach was the National Geodetic Vertical Datum of 1929(NGVD29). Most homes built in the area would reference this datum in the original design drawings. In 1988,the United States Coast and Geodetic Survey (USCGS) established a new datum called the North AmericanVertical Datum of 1988 (NAVD88). Details explaining the difference between the two datums and thescientific basis of measurement are beyond the scope of this document, but generally the difference betweenNGVD29 and NAVD88 is between .8 to 1.1 feet in this area.

The transition from using the NGVD29 datum to the NAVD88 datum has been gradual as it wasadopted by various agencies, so there was not one particular year in which the change was universally accepted.To mitigate any confusion and facilitate change, both datums were used in the Hampton Roads documentsthroughout the 1990’s. The December 1996 FEMA Flood Insurance Study for the City of Virginia Beach usesthe NGVD-29 datum, but the revised flood insurance study dated May 4, 2009 uses the NAVD88 datum.

As a result, homes that received flood elevation certificates prior to May 2009 have elevations thatreference the NGVD29 datum, which in some cases can be up to 1.3 feet above the current NAVD88 datum.To illustrate, if a home in Cape Henry was built in 1984 had a final floor elevation of 5 ft., when converted tothe new datum, its elevation is considered to be about 4.2 feet in the new datum (NAVD88).

All elevations in this document reference the NAVD88 datum, so it is important for the homeowner todetermine which datum their flood insurance documents reference.

Tides

Being able to determine the tide level with regard to a ground elevation can also be confusing. There areseveral ways to reference a tide level: mean sea level (MSL), mean low water (MLW), and mean low low water(MLLW). Both the mean low water and mean low low water have nothing to do with ground elevation. Meansea level (MSL) is actually a number that is based on local conditions, so a direct conversion from MSL toNGVD29 or NAVD88 is not actually possible, but in the Cape Henry area, the difference is in the order of 1foot. Thus in Virginia Beach, a tide that is 6 feet MSL can be considered close to 5 feet in the NAVD88 datum.

Determining the actual tide level in the Long Creek area is also challenging. Since there are no tidegauges in Long Creek, the only tidal information available in the area is the Sewells Point tide gauge and theChesapeake Bay Bridge Tunnel (CBBT) tide gauge. Readings for the tide gauges at these points which arereported in the media do not reflect the actual tide levels in Long Creek.

Generally, because Long Creek is further inland, the range and height of tides are less than those ofSewells Point and the CBBT. The range of spring tide levels in Long Creek is approximately 2 feet, whereasthe range of tide at the CBBT is 3 feet, and 3 feet at Sewells point. Moreover, the high tide occursapproximately 1.5 hours later than that of the Chesapeake Bay Bridge Tunnel and Sewells Point. To illustrate,the high tide on May 11 is predicted to be 2.3 feet at 7:00 am, whereas in Long Creek, the high tide is predictedto be 1.6 feet at 8:15am.

tidal and rainfall flooding evaluation...sep 07, 2010 · tidal and rainfall flooding evaluation...

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