YARDLEY FLOOD ELEVATIONS
Flood Control "Pennsylvania Ave & Vicinity"
FEMA Base Flood Level versus Actual 100-Yr Flood Level
By Larry Hale - May 7, 2007

The previous Flood Article #4 focused on Pennsylvania Ave & Vicinity, described the flooding in detail, listed possible flood mitigation projects, and suggested how the area might be dried-up for floods up to the April 4, 2005 level. This new Flood Article #5 compares estimated flood levels derived by working "top-down" from the FEMA Base Flood Elevation to results obtained by flood measurements on the ground.

Thanks to U. Hundskopf for providing information on flood levels above Yardley and for mentioning that the Schoor DePalma Report is now posted on the web. His house elevation certificate includes the building elevations measured by a licensed engineer, and the flood elevations measured by the owner (Hundskopf):

The 41.1-ft flood elevation in the above table is a good estimate of the true flood elevation in April 2005 immediately above Yardley in the vicinity of the boat access area --Ditto for the 40.6-ft flood elevation at towpath just below Lock 6 and other values shown in the table below. Hundskopf House is on the same latitude as Lock 6 but much closer to the river.

Schoor DePalma Report Dated Oct 13, 2006:
Subject report appears to show "river flood" elevations that are too high both at Belmondo (below Yardley) and "Area 1" (above Yardley). The "High Stage" flood elevations (25.0-ft stage at Trenton) shown in report versus the "April 4, 2005" flood (25.3-ft stage at Trenton) measured by this writer are summarized in the following table. The Schoor DePalma report employs engineering methods undoubtedly sufficient to arrive at the conclusions and recommendations presented in their report and as established by the experienced engineer "on the ground." But having accurate as possible flood levels is of critical importance for flood control and flood mitigation work in the "Pennsylvania Ave & Vicinity " basin.

The focus of this comment is strictly on the "Pennsylvania Ave & Vicinity" basin (PA Basin) where it may be possible to control and perhaps eliminate flooding up to the April 4, 2005 flood crest. The maps in the Schoor DePalma report show the entire area between the canal and the river in vicinity of Letchworth Avenue completely inundated by a Trenton Stage 25.0-ft flood. Field investigations and measurements of the same ground during the crest of April 4, 2005 by Hale indicate that high ground along the river (up to 300-ft wide in some places) was not flooded. It should be noted, that the Trenton flood stage at this time was 25.3-ft!

ANALYSIS
There are several possible ways to account for the 2-4 foot difference in the flood levels at Belmondo:

a. Mapping or Datum Errors. The L.M.T. 2-ft contour map showing contours extending into Yardley Borough in combination with an experienced engineer "on the ground" is believed to be more accurate than method based on FEMA maps.

b. If river surface is higher on one side at bends in the river, FEMA flood elevation straight-line contours (even where drawn slanted away from the perpendicular) may fail to make a distinction between the floodwater in the river, and the floodwater that is trapped and held artificially high above the river.

c. One possible way of accounting for the 2-4 foot difference in the flood levels at Belmondo in the simplest terms, would be if the engineer is working from the "top down" and therefore buying into whatever special considerations and/or inaccuracies are built-in to the FEMA insurance maps.

FEMA Base Flood Elevation (B.F.E.):
Irregardless of whether or not the B.F.E. is set several feet too high, owners of existing houses are not required by law to elevate their houses, but family decisions whether or not to elevate absolutely depend on having accurate flood elevation data. In cases where actual flooding is deep and the decision is a foregone conclusion, the incremental cost of protecting against a flood even as high as FEMA says it is may make sense. But on higher ground like "Pennsylvania Ave & Vicinity" it makes perfect sense to want accurate data on which to base a decision to spend as much as $100,000, or for a community of 100 homes to spend up to $10,000,000. Here the homeowner is going to want to know all the facts and then some. A 2-ft error would be intolerable, and even a 1-ft error could make a difference.

As a general rule elevations derived using indirect methods developed at the agency level as a way of more easily dealing with vast tracts of land should always be subject to modification where justified by detailed engineering findings. FEMA has created a "world class" web based flood mapping system, and is attempting to improve elevation accuracy through use of new technology soon to be adopted. In the meantime there is nothing more valuable than results obtained by on ground investigation by an "experienced engineer." The idea expressed by an official advisor at the August 16, 2005 public hearing on the zoning ordinance that "local implementation of flood plain regulations does not require any engineering at the local level because the engineering has already been done at the agency level" is definitely not correct!

Unlike many parts of the country where flow probability estimates may be based on only decades of data, the Delaware River gauge readings for nearby Trenton are much more solidly based and up to date as confirmed in the following report:

New Jersey Flood Mitigation Task Force Report
Dated August 22, 2006...

The flood peak on the Delaware River at Trenton, New Jersey [April 4, 2005 Flood] was the third highest recorded since 1902. The peak flow of 242,000 cfs on April 4 at 1430 hours exceeded the 100-year recurrence-interval flood. Flood frequency statistics for this site are based on peak flow data from 1913 through 2005 and a historical peak from 1904, as well as historic flood-peak information from earlier floods. The peak stage recorded for this flow was 25.33 feet.

The theoretical flood elevations (based on FEMA) for a "high stage" flood at the lower Yardley Borough line as derived in the Schoor DePalma report are believed to be 2-ft higher than the trapped water inside Belmondo and 4-ft higher than the "high stage" flood in the river in front of Belmondo as measured by Hale. The same "high stage" flood as measured 3-mi distance downriver at Trenton is 25.0-ft and as reported above, the April 4, 2005 flood measuring 25.3-ft is identified as exceeding a 100-yr flood. Therefore the High Stage flood (25.0-ft at Trenton) must indeed be the 100-yr flood or very close to it. On this basis lands along the river, which were anywhere up to 2-ft above the April 4, 2005 flood crest, must clearly be above the 100-yr flood level. And those lands in the "Pennsylvania Ave & Vicinity" basin flooded 2-3 or more feet deep of which 2-ft is artificially trapped above the river would present a tremendous incentive to the owners at the very minimum, to carefully study the situation and keep informed.

The arithmetic is very straightforward. Homeowners in some cases know the level of the April 4, 2005 flood by the line of debris it left. The flood rose very slowly and remained long enough at the crest to leave definite evidence if viewed soon after the flood. Based on the current 100-yr flood probability cited above, you may assume that the said line of debris roughly marks the 100-yr flood, and thereby provides a base elevation from which you may subtract flood reduction estimates, add any desired safety factor, and craft a question for your technical advisor.

LEGISLATION & REGULATION

The FEMA Base Flood Elevation and related regulations have all kinds of various implications depending on your point of view. The original intention was obviously to provide as much protection as possible. But the response of various government bodies responding to the public complaints about flooding has included a torrent of more stringent regulations. FEMA has an obvious financial need to broaden the insurance program by extending coverage to homeowners less likely to be flooded.

Will insurance be mandated for all properties in 500-yr floodplain?

"The High Ground"
Useful News from First American Flood Data Services
Volume 2, Issue 11
November/December 2005

Key Flood Policy Bill Awaiting Further Action

Last month the House Committee on Financial Services approved, with changes, H.R. 4320. The bill would alter several aspects of federal flood policy and significantly affect lenders, insurers and consumers. The full House is expected to consider the bill after the holiday break.

Key provisions of the bill include:

* Mandating a study regarding the impact of including the 500-year floodplain in the Special Flood Hazard Area designation, thereby extending flood insurance requirements to borrowers owning properties in those areas. The 500-year floodplain is any area with a 0.2 percent or greater annual chance of flooding.

H.R. 4320(109th]  Government Mandated Insurance for All Properties in the
500-Yr Floodplain

Here's what appears to have happened...
This is a multi-faceted problem:

• FEMA set the original Base Flood Elevation at a level that turned out to be several feet higher than current 100-yr probability flood and never corrected it
  
• The government set up a program where most of the people applying for insurance are the people most likely to get flooded
  
• Several floods occurred and the program is seriously underfunded
  
• The people through their government promises vast amount of financial aid to Katrina victims but can't deliver
  
• Congress authorized FEMA to investigate the possibility of broadening the insurance program by extending it to the 500-yr flood level thereby including more people who are less likely to get flooded
  
• One way of looking at it is that people who live at an elevation between a 100-year flood and a 500-yr flood are being asked to finance Katrina
  

The new technology not yet employed has greater accuracy and may in some cases actually lower the Base Flood Elevation (BFE) but it remains to be seen if FEMA will lower the elevation. No matter how this turns out, I suspect homeowners in the "Pennsylvania Ave & Vicinity" basin would like to know their true flood levels corresponding to various stages at Trenton as a basis for deciding whether to elevate, and because a difference of 1 or 2 feet can affect insurance premiums. If the BFE is set higher than the 100-yr requirement, then homeowners even though they might have to pay insurance, should nevertheless be exempt from certain unrealistic floodplain ordinance restrictions not directly related to safety or insurance considerations, as long as they are proved to be above the 100-yr flood level.

Flood Matrix

Looking at a large 1"=100' map of the Yardley showing the area between the canal and the river drawn onto grid provided by L.M.T. maps, and by sliding April 4, 2005 flood surface "elevation markers" into their most logical hydrologic positions based on known elevations and other information, it's immediately obvious what must have happened in 1955 in order to account for the high flood level at Pennsylvania Ave. This scenario can be refined later if elevation data including data for lesser floods is ever assembled. The land between the canal and the river from the Yardley Borough line to Afton is long and narrow. Floodwater enters from the floodway at the upper end of Morgan building up slowly. In the main part of the river, fast moving floodwater coming off Scudders Falls builds up against the land where the river bends to the left entering the roads and finally topping the berm along the river. Gradually the increasing debris build-up against the bridge partially blocks the waterway particularly on the New Jersey side. This causes the flood to rise 2-ft or more than it would now that the bridge is no longer there.

At Ferry Street 1/3 of the overland water is diverted back to the river by the ridgeline that begins at N. Bell. Of the remaining 2/3 flowing overland less than half of what remains flows over 90% of the width and makes its way uphill toward College continuing to be squeezed between the canal and Bell, while more than half flows rapidly down the narrow but wide open and flat canal reaching its maximum velocity between Afton and where it hits the bridge at College. Starting at College the width of the land suddenly and greatly increases toward Letchworth causing the high level of floodwater at the canal to be widely dispersed across the land. Whereas on April 4, 2005 the overflow was restricted to the roads cut through the ridgeline at College and Belmondo, and through the railroad opening below Lock 5, in August 1955 there is flow across the entire ridgeline, although only 6-in deep over the high ground along the river at Letchworth. On April 4, 2005 the trapped floodwater heading toward and exiting to the left of Lock 5 has a significant 3-ft elevation advantage, causing an extremely high velocity flow ripping through the towpath, moving fast, and contained inside the canal for a few hundred feet or more past the railroad while the backed up water in the swamp remains about 3-ft lower elevation than the floodwater in the canal.

The DRBC proposed $500,000 model of the Delaware River should include analysis of the very special hydraulics involved in Yardley Borough including 1) the special role played by the canal, 2) the effect of the debris buildup at the Wilburtha Bridge in 1955, and 3) the still existing Railroad Embankment, as well as allow for 4) inputting local flood surface elevation data as a means of avoiding the dichotomy that apparently exists today between estimating a flood using a "top-down" approach based on FEMA and results obtained for the same flood based on results obtained by on ground investigation during and following an actual flood."