TO: STEVE STORY, P.E., CFM DOUG BRUGGER, EI FROM: MARK MCBROOM, P.E. ANDREW PARK-FRIEND, P.E. WILL THOMAS SUBJECT: THREE FORKS ICE JAM ANALYSIS DATE: JANUARY 22, 2020 CC: RUSS ANDERSON, P.E., CFM Introduction The Montana Department of Natural Resources and Conservation (DNRC) has tasked Michael Baker International (Baker) with performing hydraulic analyses in the vicinity of Three Forks, Montana (Baker 2018). This analysis is meant to supersede the currently effective analysis, which is based on a study performed by Van Mullem Engineering (2004). Initial analysis of historical records indicates that ice plays a significant role in flood hazards in this area (Baker 2018). This memo details historical ice-affected flooding, our technical approach, analysis results, and recommendations. Historical Ice-Affected Flooding The Madison River has historically experienced ice-affected flooding events, which commonly occur during extreme cold periods from December to March and are largely composed of frazil and anchor ice. The first clear description of ice-affected flooding on the Madison River was provided by J.C. Stevens in 1922 where he provided the following: “The Madison River…flows through two agricultural valleys locally known as the Upper and Lower Madison Valleys. In these valleys the river banks are low, and near the lower end of each valley the river divides and subdivides into a network of many brush-lined channels. “In these many channeled parts of each valley, during the cold winter months, ice gorges of varying characteristics are formed. These gorges frequently cause the river to leave its channel entirely and flow across the valley floor, occasionally driving the residents from their homes and leaving the valley covered with solidified frazil ice many feet in thickness. “The winter of 1916-1917 was one of exceptionally sustained, moderately low temperatures, during which an unusual quantity of frazil and anchor ice was formed. This resulted in ice gorges and extensive overflow of agricultural lands in both valleys.” 165 S. Union Blvd., Suite 1000 | Lakewood, CO 80228 Office: 720.514.1100 | Fax: 720.479.3157 “The Madison is probably the largest river in the state in which river overflow conditions [caused by ice gorges] are so pronounced. The reasons are not hard to find. Madison River has a fairly steep gradient throughout its course. In the two valleys the banks are low, the river is shallow and wide, and the bed is strewn with boulders, cobble stones and gravel.” Stevens’ description is not unlike local reports of Madison River ice gorging today that regularly occurs near Ennis in the Upper Valley and Three Forks in the Lower Valley. The term ice gorging continues to be used to describe the Madison River winter ice-affected flooding. Frazil ice, or slush ice, is generated in large quantities during protracted cold weather when the river water becomes supercooled, dropping just a few hundredths of a degree below 32.0° F. Frazil ice most rapidly forms in turbulent water, where conductive cooling of water is accelerated. Frazil ice begins to form as small crystals, either on the surface or within the water column. The crystals agglomerate into larger floating masses having the general appearance of snow in the water. This is consistent with images and video of ice gorging near Three Forks and Ennis available on the web. Stevens points out that the entire water area of the river may become impregnated with frazil ice, forming a mixture more viscous than water. Frazil ice can also generate along the fringe of anchor ice in slower moving channels. Anchor ice forms on large objects projecting into the water column, such as cobble, stones, weeds, and brush. Anchor ice is formed in shallow, low velocity streams and is the results of rapid radiation of heat from the object on clear cold nights. With a slight rise in temperature on clear sunny days the anchor ice may release, float to the surface, and provide a platform for frazil ice formation which rapidly grows as flowing water adheres to the anchor ice. Independent of atmospheric conditions (clear, cloudy or stormy), if air temperatures are lower than freezing the river is continually manufacturing ice, the rate of formation accelerated with even lower temperatures. Stevens goes on to say: “With the accumulation of frazil and floating anchor ice, these channels become completely choked and the river is virtually damned with ice. Overflow is inevitable… There is practically no limit to the extent of overflow or ice accumulation that may occur as along as the critical degree of cold continues. “Often times banks of frazil 7 or 8 ft. high are left along the river’s edge” Stevens’ interviews of old settlers in the valleys suggested ice gorges and overflows were just about an annual occurrence, with prominent flooding occurring in 1867, 1875, 1883, 1898, 1910, and 1917. During the period of 1890-1900 in the Lower Madison Valley, there were reports of ice overtopping fence posts and extending more than 10 miles, matching if not exceeding the flooding observed in 1917. The construction of Hebgen Reservoir and Madison Reservoir has had a notable impact on the magnitude of ice-affected flooding in the Upper and Lower Madison Valley by regulating flows and providing a barrier to transport of upstream ice. Hebgen Reservoir is located above the Upper Valley and 2 Madison Reservoir is located between the two valleys. Winter base flows increase with regulated release. The additional flow 1) reduces anchor ice formation by increasing flow depth, 2) limits frazil ice formation to the surface by reducing turbulence through shallow riffles, 3) increases transport power of the river to carry ice load downstream without gorging or damming, and 4) increases the sensitivity of ice clearing to smaller increases in temperature. Prior to reservoir construction (1913), approximately 150 miles of river could contribute large quantities of ice to the Upper and Lower Valley. The Madison Reservoir now provides a barrier, prohibiting approximately 125 miles of river ice from reaching the Lower Valley. Despite the increased benefits of reservoirs a significant ice gorging event occurred in the winter of 1916-17, after both reservoirs were operational. Stevens found that extremely cold and prolonged winter temperatures were the leading ice-forming factor, quantified as the freezing degree days relative to total number of winter days between November and March. This ice-forming factor does not alone dictate the flooding potential but merely the net quantity of ice forming influence from temperature, which is the dominant parameter for ice formation. Associated flooding results and daily temperature curves provide a better understanding of the gorge produced and associated flooding. The 1916-1917 winter had the second highest ice-forming factor between 1868 to 1921; it was the second coldest winter consistently below zero over the period of record. Two types of ice gorges were identified by Stevens to occur on the Madsion River; “bridging gorge” in which little or no overflow occurs (Figure 1), and an “overflow gorge”. The bridging gorge is caused by sudden and sustained extreme low temperatures (-15° to -30° F). This causes the maximum quantity of frazil ice to form choking the river with a sluggish mixture of ice and water throughout its entire length. The bridging gorge is unique and does not fit the typical description of an ice jam; a bridging gorge does not form solely from downstream transport and accumulation of ice, but rather from continual formation of ice in the local water column. This effectively yields a continual supply of ice until temperatures warm or flow ceases. 3 Figure 1 – Ice Gorging of Madison River from the Ennis Bridge (ERA Landmark Arrow Real Estate 2011) The overflow gorge is caused by sustained moderate temperature (15° to 25° F), where the main channel is left open throughout most of its upper reach but frazil and anchor ice continue to form unabated, moving downstream before gathering in the lower part of the valley. The overflow gorge is descriptive of what is currently identified as a freezeup ice jam, where frazil ice is transported downstream before reaching an obstacle or constriction, where it stops and forms a single layer ice jam. This is known as juxtaposition. This subsequently transitions to undercover deposition(Figure 2), where additional incoming frazil becomes unstable at the upstream ice cover, underturns, and is transported beneath the ice cover and either releases downstream or deposits beneath the ice, thickening the overall cover and becoming a hanging dam (Figure 3) Because these jams are largely comprised of frazil agglomerates and not ice sheets, they behave differently than breakup ice jams (Figure 4). Between the two distinct types of gorges is a myriad of intermediate types. For the purpose of this report a bridging gorge will be referred to as a gorge and an overflow gorge will be referred to as a freezeup ice jam. In all cases we will define these events as ice jams. 4 Figure 2 – Undercover Deposition of Frazil (USACE 1999) Figure 3 – Hanging Dam in the LaGrande River (Ashton 1986) 5 Figure 4 – Breakup Ice Jam (USACE 1999) The release or degradation of a gorge is dependent on warming weather when the cohesive strength of frazil weakens and flow begins to cut through the gorge. During protracted cold temperatures the density and cohesive strength of the gorge prohibits shear failure of the gorge. When overbank flows move into the floodplain, frazil and anchor ice continues to form, further increasing the extent and quantity of ice and its effect on stage. Freezeup jams, which form in warmer temperatures and are comprised of transported agglomerates, have a cohesive strength greater than breakup ice jams but are still susceptible to the shear stresses and mechanical failures descriptive of breakup ice jams.