A Brief Analysis of River Flooding Along Major Basins in the Albany and Buffalo, NY WFO County Warning Areas

                                                                            

   

Jessica  Najuch

Department of Earth and Atmospheric Sciences

University at Albany, State University of New York
and National Weather Service, Buffalo, NY

   

 

Abstract

 

A comprehensive and statistical analysis was done for major river basins in the Albany and Buffalo, NY weather forecast office county warning areas to determine the distribution and magnitude of river flooding.  This was done on a monthly and seasonal basis along with an examination of the causes of each of the flooding events.  Results from this research are an important advancement towards increased operational awareness of distinct flooding patterns in both regions.  This study both identifies and ranks the months, seasons, locations, and causes of floods in the Albany and Buffalo county warning area basins.

 

  

1. Introduction

 

River flooding is a major forecast problem for many National Weather Service Offices (NWS).  There are several large basins within the Albany and Buffalo NWS county warning areas (CWA), which are prone to flooding.  Forecasters in these offices have river forecast models and other tools to aid them in making river forecasts and warnings of river flooding.  However, climatology of river flooding specifically created for their CWA could prove to be another valuable forecasting tool.  This research was provided for the central NY region (Binghamton office) by Matthew Noyes (a student out of Cornell), and is viewed by the forecasters as a very informative reference for river flooding.  Thus, further research on this subject was encouraged in other regions of NY.  This study and the previous are comprehensive and statistical analyses, aimed to identify the months and locations of most frequent flooding along major NY state river basins.  A preliminary analysis of the causes of these floods is also provided.

 

This study presents the results of a statistical analysis of river crest data from dating back to the beginning of the 20th century.  These data used was obtained from for 17 gauges along the Albany CWA river basins and 17 gauges along the Buffalo CWA river basins (Fig.1), (Table, 1).    Frequency and magnitude of flooding at several of these gauges may have been affected by flood control efforts taken during the 20th century.  Accounting for these flood control efforts during this study was not possible (Noyes, 2000).

 

 

 

Table 1. Basins and Gauge Locations

CWA 

Basin

Gauge Locations

 

 

Albany

Mohawk River

Utica, Little Falls, Tribes Hill, Schenectady, Burtonsville, Cohoes 

Hudson River

Albany, Mechanicville, Hadley, Troy, Ft Edward, Schuylerville

Housatonic River

Gt Barrington, Falls Village, Coltsville, Roxbury, Stevenson Dam

 

 

Buffalo

Buffalo Area

Batavia, Rapids, Williamsville, Lancaster, Gardenville, Ebenezer, Gowanda

Genesee River

Wellsville, Portageville, Avon, Garbutt, Churchville

Allegany River

Olean, Salamanca, Falconer

Black River

Booneville, Watertown

 

 

 

 

 

 

 

Figure 1: New York State Rivers and Gauge Locations

 

 

2. Data Analysis and Results

 

In order to distinguish between large-scale and small-scale flooding events, it is important to consider the areal extent of the flooding.  Though a measure of square kilometers affected by these events would be the most effective way to analyze this, limitations on river gauge data prevents such an in depth analysis (Noyes, 2000).  Therefore, the initial step to this research was to acquire the top ten above flood stage (AFS) observations for each of the gauge locations (not all gauges had a full period of 10 AFS floods, but were included based on their location).  Thus, only the floods of the highest magnitudes were considered.  These data were taken from the HYDROVIEW program from both the Albany and Buffalo NWSs’ AWIPS.  These data were then sorted by month and a monthly flood frequency was plotted for each CWA, (Figs. 2a-b) along with a seasonal flood frequency (Figs. 3a-b).   The seasons were based on standard meteorological seasonal definitions, for Winter (December-February), Spring (March-May), Summer (June-August) and Fall (September-November).  These figures show that for the Albany CWA, approximately 55% of all river flooding observations for the 17 gauges occurred during the spring months, which is identical to the percentage of springtime observations for the central NY region (Noyes, 2000).  However in Buffalo, the springtime percentage of observations was slightly lower at 48% due to a higher number of winter observations, 29%.

 

 

 

 

 

 

The monthly flood frequency, not magnitude (examined later), analysis shows that for both county warning areas the month of March had the highest number of all flooding observations (Figs. 2a-b).  This is again paralleling the research done in central NY (Noyes, 2000).  It is in the month of January and the early summertime months, that there are obvious differences.  Buffalo has a larger frequency in January than Albany, which is likely a result of the larger amount of ice jam observations in the Buffalo region.  Buffalo’s rivers and creeks are much narrower than those of the Albany CWA basins, in particular the Hudson and Mohawk Rivers.  This also causes the larger percentage of winter observations in Buffalo.  The Buffalo CWA also has a much larger monthly flood frequency in June than the Albany area basins.  This may be a result of flash floods occurring in smaller creeks of the Buffalo area basin from storms developing along the lake breeze convergence zones.   

 

In order to consider the climatological magnitude of flooding in these areas, the height above flood stage (HAFS) was calculated.   For this calculation, the actual crest and flood stage for each observation was taken from the gauge information and subtracted.  The monthly median HAFS charts (Figs. 4a-b) reveal that there are many differences between the Albany and Buffalo gauges with respect to magnitude.  The data for HAFS is not normalized thus it is highly skewed for the months of August and November.  High HAFS observations for those months can be disregarded based on no or few AFS observations in both basins for November and August.  The results from Buffalo for HAFS are rather level from month to month compared to those of Albany.  However for the fall months, the median HAFS is fairly high for both regions considering the smaller percent of AFS observations for September to November.  This is primarily due to the increased amount of tropical systems, which impact the Northeast.  For 6 of the 17 Buffalo CWA gauges the highest flood recorded was due to a tropical system, mainly Tropical Storm Agnes and Tropical Storm Frederick.  Due to these two storms, almost all of the gauges had an AFS observation within their top ten floods.  This is also true for Albany however it was for such storms as Hurricane Diane.

 

 

 

 

 

 

To further examine the reasons for these differences in monthly distributions, it was necessary to examine the possible cause of each flood (Figs. 5a-b).  This was gone about in several ways.  One of these processes was to actually examine the antecedent weather conditions for those observation dates using archived weather maps (stored in the Buffalo NWS) and NCEP/NCAR Eta reanalysis datasets.  However, over half of the gauge observations included comments, which described the specific reason for the flood.  Thus, all of the flooding events were organized into 6 categories: heavy rain (flood caused by a prolonged period of rain with no snowmelt associated), snowmelt (flood due to increased surface temperatures and no rainfall), ice jam (blockage of rivers due to an extreme thaw and no precipitation), heavy rain/snowfall (flood with a thaw and associated precipitation), flash flood (mainly an early to mid-summertime event), and heavy precipitation associated with tropical systems.  For lack of time and large amounts of data (over 300 flooding events), this part of the research is not complete.  In Albany’s comments the term “FRESHET” was used to indicate some sort of thaw but does not indicate whether there was heavy precipitation in addition to the thaw.  Therefore, Albany has only five categories for flood causes.

 

 

  

 

 

Buffalo’s main reason for severe flooding is prolonged periods of rain creating saturated soil conditions.  The second largest reason for severe flooding is periods of heavy rain with increased surface temperatures (heavy rain/snowmelt), (Fig. 5b).  This is in conjunction with the monthly median HAFS and the monthly AFS frequencies (Figs. 2b and 4b).   Since March has the highest AFS frequency, it would be expected that there must be abundant events with heavy rain and early season thaws to cause the bulk of flooding events.  Tropical events are the second minimum for both Albany and Buffalo but again in comparison to HAFS, they are the biggest reasons for large-scale severe flooding with respect to magnitude.

 

Since favorable months for floods AFS and months with the largest HAFS were evaluated, along with possible causes of these floods, the last step would be to examine where flooding is typically occurring.   Again since the top ten floods for each gauge were used, the areas susceptible to the greatest HAFS will be the best indicators of these flood prone areas. For the Buffalo gauges, highest values of HAFS were predominant in the Genesee river basin (Fig. 6b) but the largest HAFS occurred at the Olean gauge along the Allegany River.  This may be a result of snowfall maximum across the Allegany Plateau in the upper Genesee River basin, giving way to large snowmelts with associated heavy rain caused floods.  The rather large HAFS in the Buffalo area basin are likely due to abundant ice jams along those smaller creeks and streams.  Albany gauges indicate large magnitude floods along the Mohawk River at the Utica and Schenectady gauges (Fig. 6a).  Again, this may be an indication of large amounts of snow in the higher elevations of the Adirondacks causing runoff into the Mohawk River.  Large flooding is also evident in the Housatonic River Basin at the Stevenson Dam gauge.  This is primarily a result of tropical storms, which caused 3 of the top 10 floods for that gauge.  Both the Genesee and Mohawk Rivers had near record flooding from tropical events, skewing the HAFS, although the more frequent occurrence of severe flooding, and most of the top 10 HAFS, were the result of rapid snowmelt.  It must be noted that results from gauge to gauge are highly dependent upon the shape and depth of the river at that site and also the topography of the land along the vicinity of the river.  

 

 

 

  

 

3. Interpretation of Results

 

On average, March is the month with climatologically warming daily temperatures causing heavy snowmelt.  Thus, river basins in both the Albany and Buffalo region can expect a flood threat maximum.  Heavy rainfall in conjunction with these warmer temperatures can enhance the magnitude of flooding.  Also aiding in the early spring floods is runoff from higher elevations into streams, creeks and rivers, especially if the ground is still relatively frozen.  Therefore, the 55% and 48% spring seasonal distribution of AFS observations (Figs. 3a-b) can be directly related to heavy rainfall, increased temperatures, and ice jams (Figs. 5a-b) for both Albany basins and Buffalo basins. 

 

During the summer months, the increased HAFS can be attributed to heavy rainfall associated with convective systems or systems with training echoes.  Localized flash flooding (from convective storms along the Lake Erie/Lake Ontario lake breeze boundaries) occurring in the smaller creeks and streams of the Buffalo area basin are most likely the reason for a 10% higher summertime percentage than that of Albany basins.  However, even though both regions show signs of increased fall flooding due to tropical systems, the Albany region is 8% more vulnerable to excessive tropical rainfall (being closer to the coast).  These tropical systems have caused several of the most intense floods magnitude wise for both regions.

 

It should be noted that flooding at certain gauge locations might be highly skewed since certain efforts of flood control in recent years.  For example, the Avon gauge in the Genesee River basin was prone to many problems and as a result, flood control was installed at Mt. Morris.  On the other hand, the Stevenson Dam gauge off the Housatonic River, mirrors the Albany region’s late summer, early fall increase in flooding due to tropical systems.      

 

4. Summary and Future Work

 

Again, the month of March has the highest threat of flooding along with the highest HAFS for both regions (both in frequency (AFS) and magnitude (HAFS)).  Though AFS observations may be lower (in frequency of occurrence) in the summertime and fall months in both regions, the HAFS plots indicate that when these floods occur, they tend to be large in magnitude.   The majority of these results are in perfect agreement with the previous work done in Central NY.

 

This brief analysis of river flooding along major basins of the Albany and Buffalo county warning areas has allowed for a climatological monthly evaluation of flood threat.   The climatology considers not only AFS observations by month but also median HAFS observations both monthly and by location.  Basic causes of the top ten flooding events for each gauge are also used to explain the monthly differences in the climatology for each region.  Thus, areas susceptible to minor or moderate flooding can be distinguished based on time of year. 

 

In order to continue this analysis, there needs to be more in depth examinations of the synoptic and mesoscale conditions preceding and during flooding events.  This mainly needs to be done for the Albany region events in which the term “FRESHET” was used

to indicate the possible cause of flooding.  However, it would be of great interest to study the ground conditions prior to the event (soil moisture and how much snow is on the ground) along with the exact time in which the flood occurred.  This entire analysis would prove to be useful to many other areas outside of NY and even outside of the Northeast.  Though combining these results and those of the Central NY region into one easily accessible dataset would give forecasters a basic reference to past flooding events and also allow them to compare flooding events by region.

 

5. Acknowledgements

 

Both the Albany NWS and Buffalo NWS provided much of the motivation for this research.  Special thanks to Tom Niziol (SOO), Steve McLaughlin (hydrology focal point), and Bill Hibbert of the Buffalo NWS for their assistance and training on usage of the Hydroview program.  Also thanks to Tom Wasula of the Albany NWS.

 

6. Reference                                  

Noyes, Matthew C., A Preliminary Climatological Analysis of River Flooding Along Major Central New York River Basins, 2000.