The following memo was supplied by Metroplan.
Photographs accompanied the memo and are
available by clicking each picture below;
INTER OFFICE MEMORANDUM
DATE: September 9, 2009
TO: Phillip L. McConnell, Assistant Chief Engineer - Design
FROM: Charles Clements, Engineer of Roadway Design Division
Palarm Creek and
HYDRAULICS REFERENCE NUMBER 8-108
The Hydraulics Section has completed a review of the highway drainage structures across Palarm Creek and Lake Conway at Highway 365, Interstate 40, and Highway 89 in Mayflower. It was determined that the Highway 365 and Interstate 40 stream crossings have no effect on flood elevations at Grassy Lake Road. The Highway 89 crossings (1 culvert crossing and 1 bridge crossing) have only a very minor affect on flood elevations on Lake Conway.
The Little Rock District, U.S. Army Corps of Engineers provided stream gage (water surface elevation) information from Lock and Dam #7 (Murray), Lock and Dam #8 (Toad Suck), as well as the HEC2 stream hydraulic models for Palarm Creek that were developed for the Faulkner County Flood Insurance Study. The Corps also provided the most recent plot of flood profiles for various estimated frequency flood events on Murray Lake (Pool #7) between Murray Dam and Toad Suck Dam, and a HEC1 basin hydrologic (rainfall/runoff/peak flow estimation) model for the Palarm Creek/Lake Conway drainage basin. Palarm Creek’s confluence with the Arkansas River is located within Murray Lake, at Nautical Mile 136.9, or about 1/3 of the distance between Murray Dam and Toad Suck Dam, nearest Murray Dam (see attached aerial photo).
The Arkansas River gage data supplied by the Corps was for the period of record from January 1, 1975 until December 31, 2008. This data was reviewed to determine how many occurrences of, and for how many days during each occurrence, flooding at Grassy Lake Road could be directly attributed to backwater from the Arkansas River. During that 34 year period of record, only 3 flood events produced a stage on the Arkansas River that was sufficient to be the primary cause of flooding at Grassy Lake Road. The flood profile plot for Murray Lake indicates that a flood event in excess of approximately a 20% annual chance (5-year) flood on the Arkansas River would have to occur before backwater from the river would cause Grassy Lake Road to be inundated.
Although it is clear that flood stages on the Arkansas River can often influence the depth and duration of flooding that occurs at Grassy Lake Road and the agricultural and private properties in the vicinity of the I-40 crossing over Palarm Creek, it is also evident that backwater from the river is not the primary cause of the flooding during more frequent storm events. It has been observed that the flooding of Grassy Lake Road typically occurs more than 1 time in any given year, and this cannot be explained simply by backwater from the river.
The hydraulic stream model for Palarm Creek was revised to estimate the effects of several different scenarios: 1) a 50% annual chance (2-year) backwater flood stage from the Arkansas River, 2) an average daily flow stage from the Arkansas River; these were made to determine the effects of the Arkansas River flood stage on the flooding at Grassy Lake Road. Then 2 additional scenarios were modeled to estimate the effects of: 3) complete removal of the Highway 365 crossing over Palarm Creek, and 4) complete removal of both the Highway 365 crossing and the Interstate 40 crossing over Palarm Creek. All of these stream models were run with various flow rates on Palarm Creek.
A review of the results of these model runs indicates that backwater from the Arkansas River has little effect on flood depths at Grassy Lake Road during 50% annual chance (2-year), and more frequent (lesser magnitude) floods. During floods of between a 50% annual chance (2-year) and 20% annual chance (5-year) the flood stage on the river will tend to flatten the water surface profile slope on Palarm Creek, leading to slightly higher (a few inches) flood stages and longer times to drain for a given flow rate on Palarm Creek . But during floods of greater than that of a 5-year event on the Arkansas River, backwater from the river will control the depths and duration of flooding in the area. Grassy Lake Road appears to be flooded when the flows on Palarm Creek exceed approximately 1,500 to 2,000 cfs.
The HEC1 (rainfall/runoff/peak flow estimation) basin model for Palarm Creek/Lake Conway was modified to estimate the peak flows along Palarm Creek for various flood events. It was learned that the peak flows on Palarm Creek at Grassy Lake Road are greater than approximately 2,000 cfs after a 2-year storm event.
It was also noted that complete removal of the Highway 365 bridge and roadway embankment, and the Interstate 40 bridge and roadway embankment, would have a negligible effect on the flood depths and duration at Grassy Lake Road and surrounding properties during any of the flood events that were studied.
It appears that the flooding in the vicinity of Grassy Lake Road on Palarm Creek is controlled by the limited capacity of the existing channel downstream from Interstate 40. Some improvement may be realized by channel clearing, widening, or straightening downstream from the interstate, but the magnitude of improvement realized would tend to decrease as flood stages on the Arkansas River climb above that of the 5-year flood.
Highway 89 crosses a small cove on Lake Conway just east of the Interstate 40 interchange at Mayflower. There are currently two 48” corrugated metal pipe culverts under the highway to drain the surface water runoff approaching the cove under the highway and into the larger body of Lake Conway north of the highway. We estimate that surface water runoff from approximately 1.4 square miles approaches the pipe culverts, producing a 50-year peak flood flow rate of 530 cfs.
The existing pipe culverts only have a peak capacity of approximately 200 cfs before headwater over their inlet ends would be sufficient to overtop the highway. However, the temporary storm water storage volume available in the cove has decreased the peak flows approaching the Highway 89 culverts and prevented the water from overtopping the highway during most rainfall events. The water surface elevations on the cove will be slightly higher than those on the main body of Lake Conway for a brief period of time following an intense rainfall event, but should equalize within a matter of hours.
Lake Conway bridge (Highway 89, Section 4, Log Mile 9.94)
The existing Highway 89 Bridge over Lake Conway is 180 ft. long and was constructed in 1968. The Palarm Creek HEC2 hydraulic model provided by the Corps of Engineers indicates that when Palarm Creek/Lake Conway basin experiences a 10-year flood flow, the Highway 89 bridge would cause approximately 4 ½ inches of backwater. This is not normally considered a large amount of backwater, and lengthening the bridge to decrease the amount of backwater it causes would prove very expensive with little effectiveness.
As an example, lengthening the bridge to 360 ft. long would only result in lowering the amount of backwater produced during a 10-year flood to 2 ½ inches. Raising the bridge was also found to be relatively ineffective at reducing the backwater. Our analysis indicated that a 360 ft. long bridge with the low beam elevation raised 1 ft. would decrease the amount of backwater produced during a 10-year flood to 2 inches. These results were very similar to the results produced by an analysis of the Highway 89 crossing site with the entire highway bridge and roadway embankment removed.
Grassy Lake Road is flooded when the peak flows on Palarm Creek (primarily the release rates from Lake Conway Dam) exceed 1,500 to 2,000 cfs, and/or when flood stages on the Arkansas River are greater than that of a 5-year flood. Lake Conway was not designed for flood mitigation and does not have sufficient capacity to retain a significant volume of surface water runoff from a large magnitude rainfall event without its water surface elevation encroaching onto private properties surrounding the lake.
An existing double 48” C.M. pipe culvert under Highway 89 just east of the Interstate 40 interchange appears to be slightly undersized, but the floodwater storage capacity of the cove south of the highway prevents the highway from being overtopped by floodwaters during most storm events. Increasing the number of culverts under the highway would not result in a major decrease in flood elevations on those properties surrounding the cove following a heavy rainfall event. Lake Conway flood management practices (dam releases) have a greater affect on flood depths and durations in this area than do the highway culverts.
The existing Highway 89 Bridge over Lake Conway was constructed in 1968 and causes a relatively small increase in backwater on that portion of the lake north of the bridge after most rainfall events. Lengthening and/or raising the bridge would do little to decrease the depth and duration of flooding experienced on those properties surrounding the lake north of the bridge. Maintaining the lake at a lower elevation and/or increasing the release rates from the Lake Conway dam preceding and during a moderate to intense rainfall event would be less costly and more effective in reducing flooding on those properties surrounding the lake. Unfortunately, increasing the dam release rates would have the undesirable effect of increased flooding of Grassy Lake Road given the roads current centerline profile elevation.
An annotated aerial photograph of the area in question is attached for a location reference.
If there are any questions concerning this review, please contact Brooks Booher in the Hydraulics Section.