Mifflin County Floodplain Remapping
A Zone Updates
Final Report
Dennis L. Johnson
Executive Summary
This document presents the final report for Phase II of the Mifflin County Floodplain remapping project. Mifflin County is partnered with Juniata College in the completion of
this project, of which, this is Phase II. Phase I was completed in the Spring of 2001.
Phase II was begun in September 2001 and was slated to be completed in 1 year. An
extension was granted as considerably more QA/QC and hand entering of data was
encountered than had previously been expected. For example, there are over 400 flow
location or flow change points in the hydraulic analysis.
Overview
As with many areas of the United States, the delineated flood zones or base flood
elevations (BFE's) of Mifflin County are becoming outdated. Since their original
development, considerable changes have taken place, both within the county and the
contributing watersheds. These changes include alterations to waterways, new industrial
complexes, floodplain development, and watershed alterations. In addition to these
physical changes, a number of discrepancies between the existing BFE's and current
structures and data have been identified.
The recent advent of GIS capabilities and analytical analysis has made the
updating of flood mapping more efficient and accurate than ever before. Mifflin County,
identified as a FEMA - Cooperating Technical Community (CTC), is a regional leader in
Information Technology (IT) and data base development. The Mifflin County
Mapping/MIS Department has developed and/or acquired a considerable volume of
advanced and baseline data that will be invaluable in the development of the updated
BFE study. A partial list of the types and extent of data coverages that the Mifflin
County Mapping/MIS Department has compiled is illustrated in Table 1, below.
Table 1 - Partial listing of Mifflin County GIS layers.
Parcel
Right-of-way (partial data set)
State Forest Cabins
Ten-foot Contours
Digital Elevation Model (DEM)
Aerial Photography (1" = 200' scale and 1" = 50' for Lewistown)
Road Centerline with 9-1-1 address data and Penn DOT data
Hydrography
Flood Plain (100 year uncorrected with 100 foot buffer)
Land Use (residential, commercial, etc.)
Clean and Green Land Use (tillable, pasture, and woodland)
Landmark (schools, churches, etc.)
Wetlands
Soils
This document details the work completed in Phase II as well as an overview of the work
completed in Phase I and recommendations for future phases (i.e Phase III).
Introduction
The proposed updating of BFE's for Mifflin County is divided into 3 distinct
phases of activity. Phase 1 involved initial data collection and the correction of current
BFE's to existing aerial photos and updated digital terrain models (contours). This was
accomplished by overlaying the current flood zones (A and AE zones) onto the
aforementioned, updated, and current GIS data layers. According to Stewart Bruce (than
MIS Director for Mifflin County), the aerial photos and digital terrain models (DTM's)
were obtained in April 1995 from L.R. Kimball & Associates and were produced at a
mapping accuracy of 1:200 scale, capable of producing a vertical accuracy of 5 feet. The
corrections primarily consisted of realigning the flood plains to the contours and to
common and identifiable physical features, such as roads, streams, and railroads, while
maintaining the features of the floodplain such as width and length.
Phase I was competed and delivered to Mifflin County and FEMA in the Spring
of 2001 on schedule. Juniata College received an Outstanding Community Award as a
Cooperating Technical Partner from FEMA Region III and was invited to present the
work and organizational structure at the June 2001 Association of State Floodplain
Managers Conference in Ashville, NC.
Phase II is described in the remainder of this document. Phase III was initially
intended to consist of analyzing those areas that have been identified as requiring detailed
analysis from previous phases; however, recent discussion with FEMA – Region II have
indicated the desire to limit the potential Phase III analyses to AE zone areas only. This
is further discussed later in this report (Future Recommendations). The three phases of
this project, as an entire entity, are dependent on subsequent years of funding; however,
each individual phase was structured such that a deliverable product can and was
produced independent of the next year's funding. Data was produced at the end of each
phase. With this information FEMA will be able to produce updated DFIRM's for those
areas where no expected changes in the BFE are to take place in the subsequent years.
Phase II Process
The basic process for this phase was to perform hydraulic analyses on the A zone
areas of Mifflin County. The U.S. Army Corps of Engineers (USACE) River Analysis
System (HEC-RAS) was used for this analysis. The output of the HEC-RAS analysis
was then used as the updated A zones. These updated A zones now have approximate
elevations associated with them because the geometric configurations for the HEC-RAS
models were obtained from GIS data such as the digital elevations and updated
hydrography for Mifflin County. Details of the overall processes are provided below.
HEC-RAS Preprocessing
Arcview Extension developed by the USACE and ESRI was used in order to
develop the HEC-RAS models in an efficient and precise manner. The extension is
known as HEC GeoRas and is also available from the USACE Hydrologic Engineering
Center website. The processing was completed for the entire county, which was divided
into 4 sections for convenience. Figure 1 illustrates Mifflin County, the four sections,
and hydrograph. The basic preprocessing steps are then outlined below.
Figure 1 – Illustration of Mifflin County, hydrography, section breakdown.
Stream Delineation – In this step the existing hydrography was used and
corrected (from and to nodes altered to properly indicate flow direction). These
streams were then truncated or clipped to include only the A zones and some
distance upstream.
Banks and Flow Paths – Next, digital aerial photographs were used to estimate
the stream banks and existing A zone floodplains were used to estimate the
approximate flow paths for the 100-year flood.
Cross-Sections – Cross-sections were then “cut” at appropriate locations. These
included starting sections, sections above and below all confluences and road
crossings and areas that would require additional sections such as sudden changes
in width , direction, and slope.
Export to RAS GIS Format – After completing these steps, the GeoRas extension
was used to export the geometric data to a format that is ingestible by HEC-RAS.
Figure 2 illustrates a sample stream section with the stream, banks, flow paths and crosssections also present. The bank stations (red) and flowpaths (light blue) may not be
evident because of the scale.
Figure 2 – Illustration of stream, banks, flow paths and cross-sections for a sample
stream section. Roads are also indicated.
100-year Flow Determination
Before continuing on with hydraulic analysis, it was necessary to develop the
100-year flows. This was done in conjunction with the a representative from Dewberry
& Davis (the technical contacts) and a hydrologist with the PA USGS. It was decided
that 100-year flows at gages within and near the basins of interest would be obtained
form the USGS and that an areal reduction of the form indicated in Equation 1 below
would be used:
Q1  A1 
 
Q2  A2 
0.73
Equation 1
Gages Used – Table 2 illustrates the gages that were selected for use in the study.
Table 2 – Gages available for use in the study.
Location
Kish Creek at Reedsville
Augwhwick Creek near
Three Springs
Juniata at Huntingdon
Raystown Branch Juniata
near Huntingdon
Little Lost Creek at Oakland
Mills
Tuscarora Creek near Port
Royal
Cocolamus Creek near
Millerstown
Drainage
Area (mi2)
164
205
816
957
6.52
214
57.2
Flow Values - Three values of the 100-year flow were obtained for each gage. A
regression equation flow, as obtained from the proper PA USGS regression
equation, a gaging station flow, obtained a statistical breakdown of the data at the
individual gage, and a Log Pearson flow, obtained by applying a LPIII
distribution to the data. The gaging station flow and regression equation flows
should obviously be very similar, as the gage data were used in the development
of the regression equations. The 100-year flow ultimately used for areal reduction
and subsequently in the model runs was the regression equation as it was the most
conservative value (generally slightly larger than the others).
Flow Locations – There were over 400 required flow changes or flow definitions
in the final model runs. A flow change or definition is necessary at locations
where tributaries add enough flow to warrant such a change or where the drainage
area reduced enough to warrant a change in flow. A 20% reduction in drainage
area was arbitrarily set on the main channels sections as warranting a new
computation and change in flow for the HEC-RAS model. Figure 3 illustrates the
flow locations throughout the study area.
Figure 3 – Flow locations within the study area.
Hydraulic Analysis
The next step involved importing the geometric files (previously exported from the HECRAS preprocessing). The importing is done through the HEC-RAS geometric data
editor.
Manning’s n – Once the geometry data is imported. Manning’s n values can be
assigned to the cross-sections. Because there were literally thousands of crosssections, general Manning’s n values were assumed. A value of 0.035 was used
for the channel sections and 0.065 was assigned to the bank stations. Detailed site
investigations would be required to adjust these at individual cross-sections. In
general, it was felt that these were somewhat high and thus conservative given the
general nature of the topography.
Expansion & Contraction Coefficients – The default expansion and contraction
coefficients of 0.1 and 0.3 were used for all cross-sections initially. The
expansion and contraction coefficients at raod crossings was set to 0.3 and 0.5, as
and sometimes 0.5 and 0.7, as bridges and culverts were not explicitly modeled
(this would have been far beyond the budgetary allowance of this project). To
facilitate the changing of the expansion and contraction coefficients, a road
crossings shapefile (ArcView native file format) was generated by finding all
points that coincided with the intersection of the roads and streams in each
section. A georegistered image of all road crossings was produced and used as a
background image in the HEC-RAS geometry editor. This allowed for reasonably
fast editing of the expansion and contraction coefficients at road crossings.
Figure 4 illustrates an example of the use of the georegistered background images
in the geometry editor.
Figure 4 – Illustration of HEC-RAS Geometry editor with georegistered
background image.
Starting Water Surface Conditions – The starting water surface was obtained
from one of two methods. First, when available, if the A zone butted up against
an AE Zone, then the elevation of the AE zone at the juncture was used as a
“known starting water surface elevation. If there was no AE zone present, then a
normal depth computation or slope-area method was used. The input for this
method is the slope of the energy grade line and the slope of the ground surface
was used (as is customary for normal depth computations). In order to obtain the
slope, ArcView Spatial Analyst was used to compute a gridded or raster slope
data set for the entire county. The slope of the grid cell at the farthest
downstream cross-section was used as the input to the slope-area method. Table 3
illustrates the starting water surface methods and values for the 24 sections of
stream that were modeled.
Table 3 – Starting water surface methods and values.
Section 1
SubSection
101
102
103
104
105
106
107
108
109
110
111
112
Slope for SlopeArea Method slope
(ft/ft/)
0.023
AE Zone
Starting WSEL
(ft above MSL)
490
0.096
482
590
530
527
565
537
533
526
540
Section 2
201
202
203
204
205
206
207
0.13
0.029
642
514
0.001
0.001
610
Section 3
301
302
670
0.0536
Section 4
401
402
403
540
522
528
Export RAS Analyses – Each stream section was then analyzed in HEC-RAS.
Figure 5 illustrates a three-dimensional output of a model run. Each completed
analysis or model run was then exported to GIS format using the HEC-RAS
interface.
Figure 5 – Typical 3-dimensional HEC-RAS output.
HEC-RAS Post Processing
Post processing consists of importing the “exported” HEC-RAS results and
producing the flood plains. This is again done with the HEc-GeoRas extension.
Floodplain Creation – HEC GeoRas ingests the exported model run data and
creates a water surface TIN (Triangular Irregular Network). This Tin is then
converted to a grid and the ground surface TIN is also converted to a grid. The
ground surface grid (DEM) is then subtracted from the water surface grid to
produce a grid that is essentially a grid of floodplain depths. This is then
converted to a polygon to produce the floodplain. Figure 6 illustrates an “initial”
floodplain produced by this process.
Figure 6 – Illustration of “initial” floodplains produced.
Note that there are numerous “discontinuous” polygons. In some cases there were
also large “hole” produced. This is because of the terrain data that was used in
the initial preprocessing, as well as the nature of the topography that is being
modeled. The study area has floodplains that are rather wide compared to the
meandering streams in these valleys and the floodplains are not all that deep. Any
errors irregularities in the DEM will produce negative floodplain depths and the
resulting discontinuous polygons or holes in the floodplains. It is possible to
eliminate some of these by going back and re-entering or digitizing cross-sections
at key locations (this was done in some circumstances); however, the same
situation still occurs.
Final Smoothing – In order to create the “final” A zones, the floodplains were
individually smoothed or altered to remove irregularities. The Hydrologic
Engineering Center of the USACE was consulted and also recommended this
smoothing as did the technical contacts for this project (engineers at Dewberry &
Davis). Final smoothing or correcting attempted to account for the topography
(i.e. the contours). A final A Zone floodplain is illustrated in Figure 7.
Figure 7 – Example of final “smoothed” floodplain. Contours, roads, banks, stream, and
flowpaths are also indicated.
Future Recommendations
This was a very large study and obviously, there would be any number of areas
that could be further investigated. Overall, the contractors feel that the results are very
acceptable. As part of future recommendations, it is suggested that a site near the
confluence of Kish Creek and the Juniata River be considered for future investigation.
This area is illustrated in Figures 8 and 9 below. Note from Figure 9 that considerable
change has taken place between the original AE Zones form the Q3 product and the AE
zones that were altered as a result of Phase I realignment to the new topography. This
area is the Mifflin County Industrial Park and thus, potentially serious consequences.
As appropriate, a hydrologic investigation will be conducted to determine the validity of
the 100-year (and other) return flows. If available from recent studies (due to road
construction, etc…), cross-sections will be collected backwater analysis (HEC-RAS) will
be completed. The output of these models will be incorporated into the current AE zone
estimates. Field verification of the data will also take place for this site.
Area of interest – see
Figure 9 below.
Figure 8 – Mifflin County and major road system.
Original AE from Q3 Product
Updated AE Zone from Phase I
based on new topography.
Location of Industrial Park
Figure 9 – Proposed area of study for Phase III.