June 12, 2000
Thomas Kelly, Chairman
Re: Calendar No. 99-1284/1285
Dear Chairman Kelly:
On behalf of the Manayunk Neighborhood Council (MNC) and Friends of the Manayunk Canal (FMC), hydrologic and hydraulic analyses have been performed of flooding along the Schuylkill River in the vicinity of Venice Island. These analyses have been performed in response to the proposed construction of three separate housing developments, comprising a total of 518 residential apartment units. These housing developments, known as the Cotton Street Landing (270 units), Venice Island Apartments (88 units), and 4601 Flat Rock Road (160 units), have been proposed in response to the 1999 re-zoning of Venice Island for residential use by the City of Philadelphia.
The requested hydrologic and hydraulic analyses were conducted to address the MNC and FMC's concerns over the construction of residential units on the highly flood-prone Island. According to a 1993 computer model of the Schuylkill River developed by the U.S. Army Corps of Engineers (USACE) for the Federal Emergency Management Agency (FEMA), Venice Island at the location of the three proposed developments is located entirely within the Schuylkill River's 100-Year Floodway, a high velocity portion of the River's 100-Year floodplain. According to the USACE's
model, peak 100-Year flow velocities may exceed 8 feet per second and peak 100-Year water surfaces may exceed10 feet in depth along portions of the Island at or near the proposed developments. These high depths and velocities, in combination with both the large number of expected new residents and limited vehicular and pedestrian routes from the Island, have led to extreme concerns over the following issues:
In order to address the MNC and FMC's concerns, the Schuylkill River analyses described herein focused on three aspects of the proposed developments and their exposure to flooding:
These aspects were selected to assist the MNC and FMC in their efforts to demonstrate the inadvisability of the proposed developments and to convince the Philadelphia Zoning Board to deny their applications for construction. A summary of each aspect of the analyses is presented below.
As noted above, a hydraulic analysis has previously been performed on behalf of the Venice Island developers in order to demonstrate that the proposed apartment construction will not cause an increase in peak 100-Year flood levels in the Schuylkill River along and upstream of Venice Island. These analyses were performed in order to show compliance with FEMA's requirements for construction within a Floodway. As specified in Section 60.3(d) of Title 44 of the Code of Federal Regulations, these requirements prohibit any construction in the Floodway that causes an increase in the 100-Year flood level. The analysis was based, in part, upon the 1993 USACE computer model of the Schuylkill River noted above, which was then used to develop a new model of the River purported to more accurately reflect existing and proposed hydraulic conditions on the Island.
It is important to note that our review of this analysis was limited to various reports and correspondence prepared by J. Richard Weggel, Ph.D., P.E. all of which included hard copies of the new computer model's output data. Unfortunately, we were not provided copies of the new model's input data. As such, we were unable to verify the accuracy of the input data relative to the 1993 USACE's model or to investigate the accuracy and sensitivity of the model's output. Therefore, despite the recent approval of Dr. Weggel's analysis by FEMA, we have prepared the following list of questions regarding the new computer model that we strongly encourage both FEMA and the Planning Commission to investigate prior to reaching a decision on the developers' applications:
1. Through a combination of geometric data and surface roughness coefficients, the original USACE's model as officially adopted by FEMA purports to accurately depict 1993 hydraulic conditions on Venice Island, including the presence of the Connelly Container Corporation mill building, the Arroyo Grill Restaurant substructure, and the National Milling and Chemical Company building. Therefore, why are these structures added a second time to the new River model of existing conditions prepared by Dr. Weggel? The addition of these structures a second time has the effect of minimizing the effects of the proposed apartment construction and masking their impact on the existing 100-Year flood levels. A more correct assessment of the new construction's impacts on flood levels may perhaps be to add only the new buildings to the model and compare the resultant water surfaces to those in the official Corps' model.
2. In adding the proposed apartments to the new computer model, only the proposed piers on which the buildings will supported have been added to the existing River cross sections. This ignores the likelihood that these piers will collect River-borne debris and, rather that act individually, will form a solid barrier to flow. While such consideration for debris may not be required by FEMA standards, it would allow more accurately determination of the proposed apartment's impacts on existing flood levels.
3. In performing the analysis, only the impacts to 100-Year flood levels along the River have been considered. However, an analysis of the 1993 Corps of Engineers model indicates that portions of the River's floodplains both along and upstream of Venice Island are susceptible to flooding by more frequent (i.e., lower recurrence interval) floods. As such, it is unknown at this time whether the proposed apartments will cause any increase in these flood levels. Once again, while it may not officially required by FEMA, the impact analysis should be expanded to assess the impacts of the proposed apartments on a range of flood frequencies up to and including the 100-Year event to insure that the proposed construction will not adversely impact flood levels for any frequency flood.
Please note that we would be happy to address each of these questions if a copy of the input data for Professor Weggel's new model of the Schuylkill can be obtained.
In order to determine whether the proposed apartment construction complies with Philadelphia's own floodplain regulations, we have reviewed Section 14-406 of the City's Flood Plain Controls. According to Paragraph 5 of these Controls:
"(5) Special Controls. The following special controls are imposed to regulate setbacks in the flood plain, construction, and earthmoving activity along watercourses subject to Department of Environmental Resources:
(a) Within the Floodway:
(1) No encroachment (including fill; new construction; or any development) is permitted except that public utilities are permitted as long as they cause no increase in the One Hundred Year Flood level." (Bold face added)
As noted in the bold face type above, Philadelphia's own Flood Plain Controls unconditionally prohibit fill, new construction, or any development in the Floodway regardless of its impacts to the 100-Year flood level. In our professional opinion, the construction of columns, piers, and pilings constructed within the floodway for the purpose of elevating a structure (residential apartments) above the floodway is included as a prohibited development activity.
According to the Controls, the only activity in the Floodway that is given the opportunity to meet the no 100-Year flood level increase criteria (as allowed by FEMA for all proposed Floodway activity) is the construction of public utilities. As such, the Philadelphia requirement can be seen to be more stringent than the FEMA regulation. In fact, it could also be considered to more prudent and well-informed than the FEMA regulation, since it more comprehensively addresses all of the dangers of Floodway development, not simply the threat of increased upstream flood levels.
In conclusion, due to their location within the Schuylkill River Floodway, the City's own Flood Plain Controls specifically prohibit the proposed apartment developments on Venice Island.
The Board should carefully review the "compliance" letter (Attachment A) issued by FEMA for the Venice Island hydraulic analysis (letter addressed to Barbara J. Kaplan, Executive Director, Philadelphia Planning Commission, as prepared by Mr. Thomas M. Majusiak, P.E., Director, Mitigation Division, FEMA, dated May 15, 2000). The review letter simply states that the analysis is in compliance with Title 44 of the Code of Federal Regulations; it is not an approval letter for the proposed residential development. Specifically, in the letter, Mr. Majusiak states the following:
"This is not an approval of the proposed project. It simply indicates that the proposed project complies with the necessary floodway regulations. All Federal, state, and local laws, including all sections of the local floodplain management ordinance must be adhered to."
The above quoted paragraph specifically states that the Philadelphia development ordinance must be enforced. Therefore, any waiver from Section 14-1606. Flood Plain Contols (5) Special Controls would be in violation of the "compliance" letter.
Mr. Majusiak further states the following:
"The Federal Emergency Management Agency is not in the position to advocate floodway development. Development within the designated floodway is contradictory to our Agency's mission of reducing the loss of life and property associated with natural and man-made disasters." (Bold face added)
In the above quoted paragraph, FEMA clearly states that, in their opinion, development within the floodway is dangerous and is attributable to loss of life and property.
In additional to the technical questions and legal prohibitions regarding the proposed developments on Venice Island, it was also considered important to describe in more direct, less technical terms the flood threat that would be faced by residents occupying the proposed apartments. This description has been developed in two steps. First, a determination of how frequently flooding problems can be expected on Venice Island has been made. Secondly, the exact nature of the flood threat to these residents has been characterized by attempting to recreate the flooding that occurred on the Island during Hurricane Floyd on September 16-17, 1999. This characterization includes an estimation of the rate of rise of the Schuylkill floodwaters on the Island and a review of the accuracy and timeliness of National Weather Service Flood Warnings that were issued during the Hurricane Floyd flood.
Basis of Analysis: The determination of expected flood frequency on Venice Island and the characterization of the flooding that occurred on the Island during Hurricane Floyd were based upon the following:
1. 1993 HEC-2 hydraulic computer model of the Schuylkill River prepared by the U.S. Army Corps of Engineers for FEMA. This is also the same model used by Professor Weggel in his development, on behalf of the developers, of the new hydraulic model of the River described in Section in Section 2 above.
2. Provisional flow data recorded by and obtained from the U.S. Geological Survey (USGS) on the Schuylkill River at Fairmont Dam (USGS Gage 1474500 - Schuylkill River at Philadelphia) and the Wissahickon Creek at its confluence with the Schuylkill (USGS Gage 1474000 - Wissahickon Creek at Mouth).
3. Flood Potential Outlooks and River Flood Warnings issued prior to and during Hurricane Floyd by the Mt. Holly, New Jersey Weather Forecast Office of the National Weather Service (NWS). The Mt. Holly WFO is the NWS office responsible for issuing such flood bulletins for the Philadelphia area, including the Schuylkill River.
Estimated Flood Frequency: Based upon an analysis of cross section and stage-discharge data generated by the Corps of Engineers' HEC-2 computer model at various locations along Venice Island, it is estimated that flooding will occur on the Island when the discharge rate in the Schuylkill River exceeds approximately 55,000 CFS. This flooding is expected to begin in the vicinity of River Cross Sections 73395, which is located immediately upstream of the proposed Cotton Street Landing apartments, and 76046, which is located immediately upstream of the proposed 461 Flat Rock Road apartments. Based upon a discharge-frequency analysis of the Schuylkill River at Venice Island, this threshold discharge rate of 55,000 CFS has an expected average annual recurrence interval of approximately 7-Years and expected annual probability of approximately 15 percent.
This means, in general, that flooding of the Island to varying levels of severity can be expected to occur, on average, approximately 6 times in a 50-year period or approximately 12 times in a 100-Year period. It also means that, during each year they reside on the Island, apartment residents would face an approximately 15 percent chance that a flood will occur on the Island. In light of both Philadelphia's and FEMA's extensive regulatory efforts to date to protect people and property from flooding from even a 100-Year flood (which has on average only a 1 percent chance of occurring each year), construction of residential developments that would face a 15 times greater risk of flooding must be considered contrary to sound floodplain management and public safety principles.
Please note that copies of the stage-discharge and discharge-frequency analyses noted above are presented in Attachment B - Threshold Flooding Analysis.
Characterization of Venice Island Flood Threat: Based upon an analysis of the USGS gage data on the Schuylkill River and Wissahickon Creek noted above, it is estimated that the peak discharge that occurred in the River at Venice Island during Hurricane Floyd was approximately 80,800 CFS. This estimate was developed by both adjusting the times of the Schuylkill River flow data at Fairmont Dam to account for the travel time between the Dam, Wissahickon Creek, and Venice Island and then subtracting the Wissahickon Creek flows to create an estimated Schuylkill River hydrograph at Venice Island. The results of this process are presented in Attachment C - Flood Threat Characterization. Based upon a discharge frequency analysis of the Schuylkill River at Venice Island, this estimated Hurricane Floyd peak discharge has an average annual recurrence interval of approximately 25-Years. This analysis is also presented in Attachment C.
According to the NWS flood bulletins issued by and obtained from the Mt. Holly WFO, the first official NWS Flood Warning was issued at 3:54 PM Thursday, September 16, 1999. At this time, it is estimated from the Hurricane Floyd hydrograph at Venice Island described above that the discharge and water surface levels in the Schuylkill River at Venice Island was approximately at the flooding threshold levels (55,000 CFS and top of channel banks, respectively) described above. As such, it can be seen that the first official flood warning issued by the National Weather Service was not issued until the Schuylkill River had reached approximate flood stage at Venice Island. This means that if the proposed apartments had been constructed at the time of Hurricane Floyd, their residents would have received virtually no advanced warning of flooding on the Island from the NWS.
Please note that copies of the NWS Flood Warnings issued by the Mt. Holly WFO are also presented in Attachment C.
In addition to the above, further analysis of the estimated Hurricane Floyd hydrograph at Venice Island indicates that once flooding began, the water levels on the Island would have initially risen approximately 1 foot per hour. This relatively rapid rate of rise would have seriously hindered any evacuation efforts by the residents of the 518 apartments, particularly in view of the large number of residents and the limited vehicular and pedestrian egress routes.
In addition, it should be noted that once flooding began on Venice Island, it occurred mostly between sunset Thursday and sunrise Friday, with the peak flood level occurring at approximately 1 AM Friday, September 17th. As such, any evacuation or rescue efforts on the Island necessitated by the apartment construction would have had to been conducted in darkness.
Additional Comments: In addition to the above, the following additional comments regarding the impacts of flooding on Venice Island are offered:
1. Following a flood event on the Island, it will be necessary to thoroughly inspect the piers supporting the apartment buildings for damage and undermining. As noted above, these piers will be impacted by river-borne debris (including abandoned residents automobiles), that can then be expected to collect on the piers, thereby increasing both the dynamic and hydrostatic forces on them. In addition, high velocity flows past the piers may scour the ground surface at their base, undermining their foundations. Any damage to the piers must be discovered and repaired prior to readmitting residents to the apartments above. Responsibility for conducting such inspections and authorizing the readmittance of the residents must also be determined.
2 Also following a flood event on the Island, debris clean-up must be conducted and inspections must made of the sediment deposited by the River to insure that no unsanitary, hazardous, or toxic materials are present. These activities must also be conducted before readmitting residents to the apartments.
5. Summary and Conclusions
Hydrologic and hydraulic analyses have been performed of flooding along the Schuylkill River in the vicinity of Venice Island. These analyses have been performed in response to the proposed construction of 518 apartment units at three proposed housing developments on the Island. Based upon these analyses, the following conclusions have been reached:
1. Several questions have been raised regarding the new computer model of the Schuylkill River developed on behalf of the apartment developers. It is recommended that these questions be addressed before final action is taken by the Planning Commission;
2. A review of Philadelphia's official Flood Plain Controls indicates that they prohibit construction of the proposed Venice Island apartments since they will be located within the Floodway of the Schuylkill River;
3. The issued compliance letter prepared by FEMA regarding the hydrologic analysis prepared by Dr. J. Richard Weggel, P.E., clearly states that the all local floodplain ordinances must be adhered to; and further states that development within the floodway is dangerous and is attributable to loss of life and property and;
4. An analysis of the flood threat to the residents of the proposed apartments indicates that they will annually face a 15 percent chance of flooding on the Island. In addition, if the apartments had been constructed and inhabited at the time of Hurricane Floyd, the residents would have received virtually no advanced flood warning from the National Weather Service. Finally, flood waters on the Island are expected to have risen as rapidly as 1 foot per hour and that the worst of the flooding would have occurred in darkness.
If you have any questions regarding this letter report, please contact our office directly. Thank you.
Geoffrey M. Goll, P.E.