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HomeMy WebLinkAboutAPA3183FL001 INSUR MATANU KA - U ITS!A BOROUGH, A LA AKA 4ATANU I A- U ITNA DIVISION €'VEMBER 1, 1984 Federal Emergency Management Agency COMMUNITY NUMBER - 020021 TABLE OF CONTENTS Page 1.0 INTRODUCTION.................................................... 1 1.1 Purpose of Study ........................................... ] 1.2 Authority and Acknowledgments ............................. 1 1.3 Coordination .............................. ................. 1 2.0 ARFA STUDIED ..................................................... 2 2.1 Scope of Study ........................— ----------------- 2 2-2 Community Description ...................................... 2.3 Principal Flood Problems......... 4......................... S 2.4 Flood Frotection Measures .................................. 7 33.0 ENGINEERING METHODS .............................................. 7 3.1 Hydrologic Analyses .................. ...4**--............ 7 3.3 Hydreulin- Analyses ....................... ...«............. 8 4.0 PLOOD PLAIN MANAGES NT ........ 11 4.1 Flood HOUR(Series........................................... 11 4.2 Flaadways ................................................. 12 5.0 INSURANCE APPLICATION ............................................ 27 5.1 Reach Dekerninations....................................... 27 5.2 Flood Hazard Factors ....................................... 2B 5.3 Flood Insurance Zones ... .... ............................. 28 5.4 Flood lnsuranoe Rate Map Description ....................... 32 6.0 OTHER STUDIES ... ....................................... ........ 32 7.0 LOCATION OF DATA ................................................. 33 9.0 BIBLIOGRAPHY AND REFERENCES.,------- ............................. 33 TABLE OF CONTENTS ( Con t' d } Page FIGURES Figure 1 -- Vicinity map....... ..................................... 3 Figure 2 - Floodway Scb erratic .......................................... 27 TABLES 'Fable 1 - Historical Flooding ............ ............................ 6 Table 2 - Summary of Discharges ........................................ 9 Table 3 - Floodway Data ................................................ 13 Table 4 Flood Insurance Zone Data .................................... 29 EXHIBITS Exhibit 1 - Flood Profiles Deception Creek Deception Creek Tributary l Deception Creek Tributary Deception Creek Tributary 3 Willow Creek Willow Creek Tributavy Little Susitna River Little Susitna River Split Flaw 1 Little Susitna River Split Flow 2 Little Susitna River Split Flow 3 Exhibit 2 - Flood Boundary and Floodway Map index Flood Boundary and Floodway map PUBLISHED SEPARATELY: Flood Insurance bate Map Index Flood Tnsueance mite Map Panels DIP-07P Panel 08P Panel a9P Panel 1OP pknels IIP-17P Panels 13P-21P Panels 22P-37P Panel 38P Panels 39P-40P Panel 41P a FLOOD INSURANCE STUDY 1,0 INTRODUCTION 1.1 Purpose of Study This Flood Insurance Study investigates the existence and severity of flood hazards in the unincorporated areas of Matanuska-Susitina Borough, Alaska, and within the incorporated Cities of Palmer, Houston, and wasilla, and aids in the administration of the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973. This study will be used to convert Matanuska-Susitna Borough to the regular program of flood insurance by the Federal EOMergency Management Agency. Local and regional planners will use this study in their efforts to promote sound flood plain manage- ment. In some states or communities, flood plain management criteria or regulations may exist that are more restrictive or comprehensive than hose on which these federally supported studies are based_ These criteria take precedence over the minimum Federal criteria for purposes of regulating development in the flood plain, as set forth in the Code of Federal Regulations at 44 CFR, 60.3. Ta such cases, however, it shall be understood that the State (or other Jurisdictional agency) shall be able to explain these requirements and criteria. 1.2 Authority and Acknowledgments The source of authority for this Flood Insurance Study is the National Flood Insurance Act of 1968, as amended. The hydrologic and hydraulic analyses for this study were performed by the U.S. Army Corps of Engineers, Al aeka District, for the Pederal Emergency Management Agency, under Inter -Agency ,Agreement No. IAA-H-1878, Project Order No. 15. This work, which was completed in April 1982, covered all signiflpar4t flooding sources affectirxg Matar►uska-Susitna Borough. 1.3 Coordination A meeting was field on July 20, 1977, and attended by representatives of the study Contractor, the Federal Emergency Management Agency, and matanuska-Susitna Borough_ The purpose of khe meeting wan to determine which streams would require detailed study and which would require approximate study. A priority list of the streams to be studied was determined; however, tape to the lack of funds, the highest priority streams (Matanuska, Susitna, and Knfk Rivers) were eliminated from this study. Instead, three atreams which have already been partially studied by the U.S. Army Corps -of Eag ineers were funded for this study. The other streams will be studied at a later date when funds are available. Coordination was made with the U.S. Geological Survey ar�d the U.S. Soil 2.0 Conse€vation Service. A draft of this study was reviewed by Matanuska- Susitna Borough. Three final coordination meetings were held during the week of September 29, 1983. These meetings were attended by representatives of the Federal Emergency Management Agency, the study contractor, and the community. All problems and questions that were raised at the meeting have heen resolved AAEA STUDIED 2.1 Socpe of Study This Flood insurance Study covers the incorporated Cities of wasilla, Eouston, and Pa3.mer, and the unincorporated areas of Matanuska- Susitna Borough, Alaska. The area of study is shown on the Vicinity Map (Figure 1) . Deception Creek Deception Creek Tributaries 1, 2, and 3; willow Creek; Willow Creek Tributary; Little Susitna River; and Little Susitna River Split Flows 1, 2, and 3 were studied by detailed methods. Each stream was studied from a downstream location, below which little development is expected by the borough due to wetland oonditions, to an upst[aam location where the 100-year flood plain is less than 200 feet wide_ Flooding in the Bodenburg Butte area from the Matanuska River was stiAted by approximate methods. Those area$ studied by detailed methods were Chosen with Consider- ation given to all proposed construction and forecasted development through 1967. Approximate analyses were used to study those areas having a low development potential or minimal flood hazards. The scope and methods of study were proposed to and agreed upon by the Federal Emergency Management Agency and the borough_ 2.2 Community Description Matanuska-Susitna Borough, located in the Third Judicial District in south --central Alaska, governs an artea of over 23,000 square rtiles extending from the Municipality of Anchorage in the south to Mt. McKinley National Park in the north. it is surrounded by unorganized area to the north, east, and west, and by Greater Anchorage Area and Kenai Peninsula Bovoughs to thr south. The population of Matanuska-Susitna Borough was 6,509 in 1970 and increased 174 percent to 17,816 by 1980 (Reference 1). It is estimated that over 2.000 people live near the streams studied by detailed methods. t axv F Vaex L"�tr I Aaduson C . 'eRek JW b- H✓'r5T x iiln T}fO'S�iCn4' A{i leki7 L tl 7 ro n± • ]$Ji,s o dbllr MIn urrMnd7Ts, +I' 'F La ;} "6 + Heal ,y.-114bnm �y4.�j i 1YL� rein Srrr +. I "3 } U Jnl•uK55 f \ j'+r-.�n i'. 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' ' 4+sY I Y - {^ 2J Fi if r V,. - z Q r�Iil{lip: '' 'DEL f:;4 QnU9 �ft W ) r p/1 r y� rt r7 µJJ55YTM �4VV 'fTa Cr �45.a.r ..� .. x "'�.�� .. appy VaFleyFr i �A { W Cf} d 7 !rlYinFA3s p4 I �I� rrl�` 3 J 2 ae . ` c- 5 & Ila I W lLIrrCWEMAll $4Y AT[ P �� y ❑ Se wi ew r ErJ�1Fal U 'rGahs d I . w sr,�Tf cn 55 �' � Er I FIGURE 1 Most of the land along Deception Creek, Willow Creek, and the Little Susitna Raver is developing as a low --residential area. The water- shed basins for these streams are located in south-central Alaska, approximately 30 air miles and 70 miles by highway north of Anchorage. The area has been a focal point of increasing use for recreational activities such as boating, hiking, snowmvbiling, fishing, and hunting. This increased recreational usage can be attributed to the area's esthetic qualities and closeness to Anchorage, the largest city in the State. Tremendoo s subdivision activities stressing recreational lots have been occurring in recent years_ The streams studied by detailed methods originate in the Talkeetna Mountains and flow west to the Susitna Rioter or Cook inlet. Physiographfc characteristics are quite varied having developed from glacial activities and volcanic action. The study area is underlain primarily by bedrock consisting of weakly consolidated{ coal -bearing rocks of Tertiary Age. It has been glaciated several times, so them are thick deposits of glacial drift and alluvial sediments made up of sandy and g'raVelly material. The relative proportions of the materials in the glacial drift vary quite a bit, as does the compactness of the soil. Thus, Permeability and internal drainage are highly variable, even over short distances. poorly drained soil$ often occur on the slopes of moraines in close association with well -drained soils. Most of the area is also covered with a mantle of silty loess probably derived from the Susitna River flood plains to the west. The loess ranges from a few inches to several feet in thickness_ Poorly drained peat is common in scattered depressions, shallow basins between moraine hills, and other low-lying areas, The Willow area contains fifteen varying vegetative habitat types composed of mature stands of mixed coniferous and deciduous forests with an understory of a variety of (orbs and woody plants, muskeg -black spruce bays, and grassland areas_ Ferns, horsetails, and clubmosses are present throughout the entire borough. Generally, the vegetative ground cover is dense and provides substantial protection from erosion activity, particularly in the higher elevations where better drained soil conditions are found. Willow and hirchfaapen stands ender 10 feet high are generally lacking, with low woody shzLjbs such as vaccinum being very common throughoiut the borough. Elevations range from 1O,Q4ff feet in the r4o=tains to less than 100 feet in the southern valleys. The region is in a transitional climatic zone between maritime and rontinF*ntal conditions. Fronounc@A temperature variatiott5 and In cloudy weather ace cormnon during a large portion of the year. Mountain ranges to the south act as a barrier to the influx of warm air from the Gulf of Alaska, resulting in an average annual precipitation which is only 10 to 15 percent of that at stations looted on the Gulf of Alaska. Annual precipitation in the study area averages 25 inches with annual snowfall of 80 inches. Rainfall is generally heaviest in August and September with monthly precipitation amounts approxi- mately equal for the rest of the year. The Alaska Mountain range lies in a long arc, approximately 70 miles north of the detailed study area and serves as an effective barrier to the flaw of extreme cold winter weather from the north. The streams remain frozen during the winter with ice jars flooding occurring occasionally in the spring. Annual temperatures range from -20OF to 80°F. 2.3 Principal Flood Problems Floods in Matanuukd-Susitna Borough can occur as a result of a, combination of factors, including heavy snow pack, temperature, sunshine, and precipitation, The sequence of events affects the flooding potential. Spring floods on streams may occur as a result of an above -normal snowfall during the winter followed by an unusually cold spring and a rapid snowmelt. Summer and fall floods usually result from intense precipitation. In addition, are ice jam could occur during the winter or during spring breakup causing overbank flooding. Ice jams have caused the highest flooding on these streams, but no frequency hks been applied to this type of flood. Typical of most of Alaska, there is little information available concerning historical floods in Matanuska-Stxsit" Borough_ Public agencies and longtime residents, however, substantiate that floods have occurred. Information of historical floods was obtained primarily from interviews with residents in the area. A tahulation of floods in recent years and an analysis of conditions resulting from these floods are shown in Table 1. The principal flood problems are natural obstructions such as trees and vegetation along the banks, manmade obstructions such as bridges and boatdecks, ice jams, the accumulation of brush and debris along and within the streambed which can be carried downstream by high water and block bridge openings or other constrictions, and inadequately -sized culverts. willow Creek crosses the Parks highway at mile 72. It originates in the Talkeetna Mountains and generally flows west to join the Susitna River. It has a total length of approximately 35 miles of which only the lower 18 is developable. The two major tributaries to axis stream are Peters Creek and Deception Creek. The lower reaches of the strum, especially above the Parks Highway, are under intense pressure for subdivision and development in spite of the fact that there are obvious flood hazards within the area. 5 Fable 1. Historical Flooding Year Flooding SourCt and Resulting Damage 1938 Willow Creek; water overtopped the railroad, caused by ice jaaa. 1943 Little Susitna Divert pier in railroad bridge washed out. 1949 Little Susitna Rives; rain on rapid snowmelt caused roads to wash out, damaged culvert. 1955 Willow Creek; heavy rainfall damaged railroad. 1959 Little Susitna River; massive road washouts at Houston and Little Susitna inn, track and culverts washed out. 1963 Little Susitna River; roads washed out, damaged culverts. 1�64 Little Susitna River; ice jam flooding. 1564 Willow Creek; ice jam flooding. 1971 Willow Creek; log jars caused flooding near Willow, damage to highways and residences. 1971 Little Susitna hives; railroad urkdermined at Houston caused derailment of 13 oars. Man-made dam broke during rainfall. lower Hatcher Pass Road bridge over the Little Susitna River washed out. 1R71 Matanuska River; flooding resulted when a landslide -formed dam on Granite Creek (a tributary to the Matanuska River) broke during a period of Cainfall and snowmelt. Water overtopped Old Palmer Highway in the Bodenhurg i1Utte area, and residential and commercial buildings were flooded. Discharge was estimated at a0,000 cubic feet per second (cfs). Estimated 100-year discharge for the Matanuska River at Palmer is 40,000 cfs. 1975 Willow Creek; ice and lag jams caused flooding. Approximately five homes were flooded off, Hatcher Pass Road, 2 to 5 miles east of the Parks Highway. Deception Creek .also originates in the Talkeekna fountains and gene€ally flows north and west for approximately 20 miles to Join Willow Creek just upstream of the Parks Highway. At the present time, the entire length of Deception Creek is sparsely developed with, very few crossings. The Little Susitna Rivet drains the southern slopes of the Talkeetna mountains and has its headwaters in the mountains. The land form is such that the river intercepts numerous minor tributaries directly from the mountain slopes to the north. It is an extreme meandering stream and has a total length of approximately 75 miles. 2.4 Flood Protection Measures There are no existing flood -control structures on any of the streams in this study. Matanuska-Susitna Borough recently passed a zoning ordinance to restrict development iFX areas noted for flood hazard. These areas have been determined by previous U.S. Army Corps of Engineers or U.S. Geological Survey studies. 3.0 ENGINEERING METHODS Por the flooding sources studied in detail in the borough standard hydro- logic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equalled or exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having special significance for flood plain management and for flood insurance premium rates. 'these events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equalled or exceeded during any year. Although the recurrence interval represents the long term average period between floods of a speoific magnitude, raze floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (1 percent chance of annual occurrence) in any 50-year period is approxi- mately 40 percent {4 in 10), and, for any 90-year period, the risk in- creases to approximately 60 percent (6 in 10). The analyses reported here reflect floodiN potentials based on conditions existing in the borough at the time of completion of this study. Maps and flood eleva- tionr�, will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak dis-- charge-frequency relationships for floods of the selected recurrence 11 intervals for each flooding source studied in detail affecting the borough. Peak discharges for selected recurrence intervals on Deception Creek; Deception Creep Tributaries 1, 2, and 3; Willow Creek; and Willow Creek Tributary were determined utilizing Clarks time -aeea unit nydrograph analysis sub --routine in the computer p{ogram HEC-1 developed by the U.S. Army Carps of Engineers (Reference 2}. Precipitation was determined from the U.S. Weather Bureau "technical Paper No. 53 (Reference 3) and used in the HEC-1 program. These frequencies were confirmed through a regional -frequency anaiys;is developed for other gaged basins in the sarde geographic area. Peak discharges for selected recurrence intervals on the Little Susitna River were determined utilizing a regional analysis of drainage area -peak discharge relationships for other stream -gaging stations within the geographic area of the Little Susitna River. Peak discharge -drainage area relationships for streams studied by detailed methods are shown in Table 2. The hydrologic analysis included a review of all existing flood frequency data for the area and the utili2ativn of analytical techniques best suited to the specific stream data_ Statistical analyses were conducted in accordance with approved procedures recommended by tha U.S. Water Resources Council guidelines for determining flood flow frequency (Reference 4). 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of the flooding sources studied in the borough were carried out to provide estimates of the elevations of floods of the selected recurrence intervals along each of these flooding sources, Water -surface profiles were Computed for Willow Creek, Deception Creak, and the Little Susitna River utilizing s computerised HEC-2 step -backwater program developed by the U.S. Army Corps of Engineers (reference 5). Cross sections were developed from survey notes, field reconnain- sance, photographs, previous studies, and the use of topographic reaps at a scale of 1:4,800 (References 5 and 7). Intermediate cross sections were developed utilizing cross -sectional area of the stream in conjunction with the topographic maps. The cross sections were located at close intervals in the vicinity of struc- tures to determine the backwater effects of these structures. In addition, numerous intermediate cross sections have been added when the slope of Lhe stream or the total loss was excessive between any two cross sections. Sxc�pt where noted in the computations, B W -- ru o 0 0 0 in o o CD o to sv D CD ti r ti k \ ti ti f } ai Vp di � q � 0) ti Xi Fi w 4-1 SQ O O fl p O b Q O w N 'b-I IDS -W m co Ow iT N to Ln W ED sD I q ri ra kO V} Ln 413 T:3 r� V k� 4D in Q Q Q KO n Q} 0f Q {V Li} rn 4 O O c n %0 Ln C7 19 ti-p 0) ;T QN ti VI 4 tl 0 74 0 .X M O O O Q 4 C3 O C? 4F ifh •--4 LM < a g O Cl C; * 4 t i r 3 jib co C+ Ui V 0 +I L=-a i.l u �+4 � �+L fa IN � H SL z z " .--E r- z M rd En -,d m L4 V1 {'x2 w 14 E ?y O � � w Aj >1 ON U � 1.4 ru to m p�p i$ -3-3 -P 7 V w � _r_ .� -k w VI 0 u LA s4 rl :5 > P-4 rt N E-1 F4 a �t M � v T 0 9 1*4 o °- w m w w -W, S.,v1-= sr e U3 ,: �c al w N A N 0 V G V as i! C �+ w ki W k+ 71 1.1 ka sj o ;IC) � o o c � in ji .0 .i UAC 0� u r-q C24 C3 c �t 4 9 road and bridge failure was not considered in this study. In effect, the backwaters were computed so as to show the maximum flooding effect regardless of the structure being present. Locations of selected crass sections used in the b draulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway is computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and Flc way Map ( Exhibit 2). ChaurMel roughness factors (Mannipg"s In") for these oomputations were assigned on the basis of field inspection of the flood plain areas and standard }published factors for utilization of n values through pipes ar culverts. The Manning"a 'n" values used are as follows: Stream Deception Creek Deception Creek Tributary 1 Deception Creek Tributary 2 Deception Creek Tributary 3 Willow Creek Billow Creek Tributary Little Susitna River Little Susitna River Split k lov 1 Little Susitna liver Split Flow 2 Little Susitna River Split Flow 3 Roughness Factors rhannpl rJuArhnnk 0.430-0_035 0.035 0_035 0.035 0.035 0.035 0.035-0.075 0.035-0.075 0.080-0.120 0.080 0.120 0.120 Starting water -surface elevations were based an utiiization of the slope -area method. During the early stages of examination of both Willow Creek and the Little Susitna River, it was determined there were several possibilities of divided flow, in essence, split flow throughout the strum reach. It was, therefore, necessary to split these areas, computing one side as a tributary and balancing the flows between the two. In some cases, as will be noted is the Computa- tions and on the work raps, floodways were necessary for bath portionA rf the 5 split flow condition. The Little Susitna River has extensive flow divisions, and three extremely braided areas required split flow analysis. The secondary channels (braids) were modeled as tributaries with water Surfaces balancing at the upstream division point. 10 Approximate 100-year flooding from the Matanuska River along Old Palmer Highway in the Bodenburg Butte area was studied using high- water marks (torn a flood which ocru{Led in August 1971. High-- water marks were determined from field surveys and interviews with local residents. No recurrence interval was assigned to this event. `load profiles were drawn showing computed water --surface elevations to an accuracy of U.5 foot for floods of the selected recurrence intervals (Exhibit 1). All elevations are referenced to the National Geodetic vertical Datum of 1929 (NGVD) . Elevation Reference marks used in the study are shown on the maps. 4.0 FLOOD PLAIN MANAGEKENT APPLICATIONS The National Flood Insurance Program encourages State and local governments to adopt sound flood plain management programs. Therefore, each Flood Insurance Study includes a flood boundary map designed to assist communi- ties in developing sound flood plain management measures. 4.1 Flood Boundaries In order to provide a national standard without regional discrimina- tion, the 100-pear flood has'be en adopted by the Federal Emergency Management Agency as the base flood for purposes of flood plain management measures. The 500-}year flood is employed to indicate additional areas of flood risk iri the c:0n unity. For each stream studied in detail, the boundaries of the 100- and 500-year floods have been delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were inter- polated using topographic maps at a scale of 1:4,800, with a contour interval of S feet (References 6 and 7) . Approximate flood boundaries for flooding from the Matanuska River along oia Fakmer Highway in the Bodenburg Butte area were determined using high-water marks and delineated using topographic maps at a scale of 1:2,400, with a contour interval of 2 feet (Reference 8). Approximate flood boundaries on Susitna River, Kroto Creek, and Froto Slough (in the vicinity of their confluences) were delineated based on information supplied by the community. Approximate flood boundaries in some portions of the study area were taken from the Flood Hazard Boundary Map and from Flood Hazard Studies (References 9 through lb) . Flood boundaries for the 100- and 500-year floods are shown on the Flood Boundary and Floodway Map (Exhibit 2). In cases where the 100- and 500-bear flood boundaries are olose together, only the 100-pear flood boundary has been shown. Sma11 areas within the flood boundaries may lie above the flood elevations and, there- fore, not be subject to flooding; owing to limitations of the map scale, such areas are not shown. 4.2 Floodwrsys Encroachment on flood plains, such as artificial fill, reduces the flood -carrying capacity, increases the flood heights of streams, and increases flood hazards in areas beyond the encroachment itself. Qne aspect of flood plain management involves belarxcing the economic gain from flood plain development against the resulting increase in fled hazard. For purposes of the National Flood Insurance Program, the concept of a floodwE�y is used as a tool to assist local communities in this aspect of fled plain management. Under this concept, the area of the 100 -year flood is divided into a floodway and a floodway fringe. The floodway is the channel of a stream plus any adjacent flood plain areas that must bs kept free of encroachment in order that the 10-year flood may be carried without substantial increases in flood heights. Minimum standards of the Federal Emergency Management Agency Limit such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. The floodways in this report are presented to local agencies as minimum standards that can be adopted or that can b'e used as a basis for additional studies. These floodways were Computed on the basis of equal conveyance reduction from each side of the flood plain. The results of these computations were tabulated at selected doss sections for each stream segment for which a floodway was computed (Table 3), Weir flow can occur only to the north of khe Alaska Fa,i,lroad bridge{ and a floodway must be left clear along the railroad embankment between Willow Creek and Willow Creek Tributary to permit flow from the main channel to the weir area. On the Little Susitna River in two of the split flow areas, a split floodway was designed, and its some other areas the floodway is to fallow the natural 100-year boundary. The latter was necessary because of excessive velocities and the large number of high ground areas. The extreme meandering nature of this stream required that the floodways go from meander to meander rather than attempt to follow the stream. As shown on the Flood $oundary and Floodway Map (Exhibit 2), the fl.oadway widths were determined at crass sections; between cross sections, the boundaries were interpolated. in cases where the boundaries of the floodway and the 100-}year flood are either close together or ccllinuar, only the floodway boundary has been shown. The area between the floodway and the boundary of the 100-year flood is termed the floodway fringe. The floodway fringe thus encompasses the portion of the flood plain that could be completely obstructed without increasing the water -surface elevation of the 100--year flood more than 1.0 foot at any point. Typical relation- ships between the floodway and the floodway fringe and their signif- icance to flood plain develroprment are shown in Pigure B. l-, i BASE FLOOD FLOODING SOURCE F'LC}ODWAY WAFER SURFACE ELEVATION SECTIO] MrAIN WITHOUT WITH CROSS SEC"iIC}N DIST#NCE WIDTH AREA VELOcil Y REGUI`ATCRY' F'LOODWAY Fi�00D4�1P.Y INCREASE (FEET`) (SQUARE (FEET PER FEET) SECOND} .(FEET NGVD ) Deception Creek A 600 500 1,706 3.6 174.7 174.7 175.7 1.0 0 750 500 1,903 3.2 176.2 176.2 176.2 0.0 C ©a0 500 2,153 2.8 176.4 176.4 176.5 0.1 D 2,140 350 1,249 4,9 178.2 178.2 178.8 0.5 E .3,420 290 864 7.1 183.8 183.8 183.9 0.1 F 4,540 350 1,410 4.3 188.3 188.3 188.4 0.1 G 6,360 243 850 7.1 194.1 194.1 1.94.2 0.1 H 7,1380 210 970 6.2 1.98.9 198.9 199.5 0.6 I 91655 300 1,231 4.9 201.9- 201.9 202.7 0.8 10,415 450 1,161 5.2 207.8 207.8 208.7 0.9 K 11,635 450 1,558 3.9 211.9 211.9 212.9 1.0 L 13,735 450 1,253 4.9 218.2 21B.1 218.7 015 m 14,505 632 1,269 4.8 222.0 222.0 222.E 0.6 N 15,305 550 1,252 4.8 227.3 227.3 227.4 0.1 0 17,255 650 1,823 3.3 232.4 232.4 232.9 0.5 P 18,330 800 1,483 4.0 236.1 236.1 236.4 0.3 Q 18,620 800 11580 2.6 237.6 237.6 238.0 0.4 R 19,705 400 686 4.3 240.9 240.9 240.9 0.0 5 20,985 400 1,557 3.2 244.1 244.1 244.4 0.3 T 22,285 500 11085 3.7 246.6 246.6 246.9 0.3 u 23,400 285 666 s.3 251.2 251.2 251.2 0.0 v 24,765 250 1, 018 2.7 253.1. 253'.1 253.8 0.7 w 26,130 52 155 8.a 257.9 257.9 258.2 0.3 x 27,18G 439 721 3.9 253.3 263.3 263.7 0.4 Y 28,23d 132 343 9.6 269.2 269.2 269.2 0.0 z 29,730 500 1,019 4.7 276.7 276.7 277.3 0. 1Finet Above Confluence With Willow Creek FEDERALEMERGENCYMANAGE MENTAGENCY FLVODWA! DATA ca r- MATANU KA• U ITNA BOROUGH, AS WATANUSICA-SUSITNA DIVISION) DECEPTION CREr-K w r rn Li FL(K)DING SOURCE FLOODWAY EASE FLOOD WATER SURFACE FLEVATION SECTION MEAN WITHOUT WITH CRASS SECTION DISTANCE WIDTH AREA VELOCITY REGULATORY FLOODWAY FLDODWAY INCREASZ (FEET) (SQUARE (FEET PER FEET) SECOND) (FEET NGVB) Deception Creek (Cont'd} AA 31,230 925 1,296 3.0 282,=. 282.5 2B2.8 0.3 AC 32,950 375 726 4.8 288.3 288.3 286.6 0.3 AD 33,400 162 303 7.4 289.1 289.1 289.5 0.4 AE 34,930 544 772 2.9 296.4 295.4 296.4 O.O AF 36,460 503 633 3.5 301.9 301.9 302.0 0.1 AG 37,195 579 1,272 3.2 306.0 306.0 306.8 0.8 AH 37,705 300 664 5.6 31D.3 310.3 310.3 0.0 AI 3B,340 500 1,271 2.9 312.3 312.3 312.6 D.3 AK 41,350 295 596 5.0 330.7 33iJ.7 330.9 0.2 AL, 43,160 331 574 5.2 346.1 346.1 346.1 0.0 ASS 44,255 45.0 677 6.3 357.4 357.4 357.4 0.0 AN 45,235 324 658 6.4 369.9 369.9 370.4 0.5 AC 46,190 98 402 10.5 378.0 378.0 378.2 0.2 AP 47,400 400 753 5.6 391.3 391.3 391.,a 0.5 AQ 48,525 184 473 9.0 405,8 405.8 406.9 0.0 AR 49,700 154 468 9.1 41B.0 41B.0 418.7 0.7 AS 50,150 144 414 10.3 431.2 431.2 431.2 0.0 AT 51,705 90 365 1.1.6 444.1 444.1 444.1 0.0 AU 52,470 259 656 6.5 452.2 45.2 452.3 0.1 AV 53,020 188 523 7.7 459.1 459,1 459.1 0.0 AW 53,410 1-88 521 7.7 464.5 464.5 464.5 0.0 AX 53,800 300 652 6.1 471.1 471.1 471.2 0.1 AY 54,620 300 660 6.1 479.E 479.6 479.9 0.3 Az 55,530 3Q0 6811 5.8 485.7 485.7 485.7 0.0 1Feet Above Confltene with Willow Creels FEDERAL EMERGENCY MANAGEMENT AGENCY MATANUSKA- USITNA BOROUGH, AK IMATAi USKA-5U51TNA 01VISION) FLOODWAY DATA DECEPTION CREEK r- r" w FLOODING SOURCE FL DWAY SE WATERER BLIE�` SURF FACE ELEVATIQN SECTION MEAN WITHOWITH CROSS SEC�'ION DISTANCEII+ICl.ABE WIDTH AREA VELOCITY REGUT-A'�ORy( 1,00DWAY FLOODWAY (FEET) (SQUARE �FEE'T PEP FEE'() $ECOND) (FEET N477M) Deception Creek (font"d) BA 55,7001 134 434 9.2 06.8 486.8 48.6.9 0.1 BE 56,675 490 864 4.6 501.9 501.9 502.6 0.7 BC 58,1201 536 915 4.9 518.0 518.0 519.0 1.0 SD 55,410 142 445 8.9 533.9 533.9 533.9 0.0 DE 59,650 1 15 41.9 8.6 536.3 5315.3 536.3 0.0 BF 59,9201 97 350 10.5 539.E 539.6 539.7 0.1 BC 60,610 175 475 7.7 550.5 550.5 550.5 0.0 BH 51,385, 267 623 5.9 556.7 556.7 551.1 0.4 131 61.7851 136 410 9.2 561,.3 561.3 561.3 0.0 BJ 62,410 135 405 8.7 567.7 567.7 5-67.7 0.0 peception Creek Tributary 1 A 3802 400 945 1.9 237.6 237.6 238.0 0.4 B 1,03D2 400 978 3.4 240.4 240.4 240.5 0.1 C 1,710 40CI 396 2.5 245.5 245.5 245.5 0.0 D 2,2402 400 859 1.8 241.2 247.2 247.B 0.6 F 2,7102 40G 641 3.3 250.4 250.9 251-1) o.1 F 3,1202 500 1,392 2.0 252.4 252.4 252.9- 0.5 G 4,370 600 1,191 3.5 258.1 258.1 258.3 0.2 H 5,1702 600 1,140 2.3 262.0 262.0 262.2 0.2 1 6,520 330 604 3.6 268.9 268.9 269.4 0.5 a 7,6602 NO 196 2.7 276.6 276.8 277.3 0.5 K 8,5602 500 586 2.4 282.8 292.8 283.1 0.3 1Fee Above Confluence with Willow Creek 2Fee.t Above Confluence With Deception Creek FWERAL EMERGENCY MANAGEMENT AGENCY f LOOD A 1 DATA NEATANUSKA-SU ITNA BOROUGH, AK IMATANUSKA-SUSiTNA DIVISION) DECEPTION CREEK -DECEPTION CREEK TH[BUT ARY 1 FLOODING SOURCE FLOpI]WAY BASE FLOOD WATER SURFACE ELEVATION 1 SECTIONAREA MEAN W T.THOUT' WITH CROSS SFCI'ION DISTANCE WIDTH (SQUARE VELOCITY (FEET PER REGCTLA"IOFtY FLDO!?WAY FI.00}[aW�1Y INCREASU (FEET) FEET) SECOND) (FEET NWD) Willow Creek A Q 1,600 5,438 3.1 107.5 107.5 108.5 1.0 B Rio 949 2,335 7.2 109.6 109.6 109.9 0.3 c 3,200 1,505 5,509 3.1 115.4 115.4 116.0 0.6 �i 41710 1,6110 5,357 3,2 116.9 116.9 117.6 0.7 6,870 650 2,732 is.2 119.8 119.8 120.7 0.9 F 8,670 1,500 6,993 2.4 123.4 123.4 124.4 1.0 G 12,230 1,704 5,514 3.1 130.3 130.3 130.7 0.4 H 17,780 1,600 5,831 2.9 139.5 139.5 140.5 1.0 1 20,000 11600 4,315 3.9 144.5 144.5 1.45.4 0.9 22,655 11320 4,831 3.5 151.t, 151.5 152.0 0.5 �{ 22,785 2,0fla 11,745 1.4 155.4 155.4 155.4 0.0 L 23,100 2,100 11,077 1.5 155.5 155.5 155.5 0.0 m 24,590 -675 2,163 4.6 156.7 156.7 156.8 0.1 N 27,215 85 738 13.6 163.3 163.3 164.2 u.9 0 27,320 1,649 5,355 2.7 167.5 167.5 168.0 0.5 P 33,170 400 1,950 5.9 182.6 182.6 183.3 0.7 Q 33,870 500 2,762 4.2 184.9 184.9 185.5 0.6 R 34,680 Soo 2,176 4.6 1.86.3 1845.3 187.0 D.7 s 36,630 550 1,766 .3.7 197.1 197.1 197.2 0.1 T 39, M 793 2,493 i1.0 207.5 2Q7.5 208.2 0.7 U 41,9840 373 1,226 8.2 218.4 218.4 218.7 0.3 V 43,700 1,001 2,854 3.5 227.3 227.3 227.5 0.2 W 48,000 450 1,583 6.3 248.6 248.6 249.3 0.7 x 48,115 400 1,694 5.9 250.4 250.4 250.4 0.0 Y 50,220 8Q0 1,820 5.5 261.2 26.1,2 261.2 0.0 z 53,370 811 1,786 5.6 274.8 274.8 275.5 0.7 1Feet Above Downztxeam Limz.k; of Detailed Study FEDE RAL EMERGENCY MANAGEMENT AGENC`f MATANUSKA-SUSITNA BOROUGH, ABC (MATANUSKA USITNA DIVISION) w FLDODAY DATA WILLOW CREEK r R7 w FLOODING SOURCE FIX)ODWAY BASE PLOOD WATEP SURFACE ELEVATION SECTION MEAN WITHOUT WITH CROSS SECTION DISTANCE WIDTH AREA VELOCITY REGULATORY FLOODWAY FLOODWAY INCREASE I {'EET) (SQUARE (FEET PER FEET) SECOND) (FEET NGVD) Willow Creek (Cont"d) AA 56,250 180 818 12.2 301.5 301.5 301.5 0.0 AB s8,550 177 698 14.3 325.4 325.4 325.4 0.0 AC 59,070 159 1,073 14.'8 331.5 331.5 331.5 0.0 AD 62,470 203 1,162 11.7 367.8 367.8 367.8 0.0 AE 66,110 163 1,085 14.7 407.8 407.8 407.6 0.0 AF 67,500 190 11158 13.6 427.2 427.2 427.6 0.4 AG 68,430 132 1,004S 15.8 439.1 439,1 439.1 0.0 Ali 68,470 133 1,009 15.8 452.2 452.2 452.2 0.0 �Feet Above Downst.Team Limit of Detailed Study FEDERAL EMERGENCY MANAGEMENT AGENCY MATANUSKA-SU ITNA BOROUGH, ABC ( MATANUSKA USITNA ENVISION) FLOODWAY DATA WILLOW CREEK rm w IF FLOODING SOURCE FLOODWAY BASE FLOOD WATER SURFACE ELEVATION SECTION I MEAN WITHOUT WITH CROSS SL•'CTION DISTANCE ITY PLOODWAY FLICODWAY INCREASE (FE) (SQUARE (�'�'E�T PERREGULATORY FELT) SECOND) (FEET NGVD) willow Creek Tributary A 1,185 11450 3,961 1.7 155.7 155.7 156.4 0.7 B 2,665 1,250 5,535 1.2 160.0 150.0 150.1 0.1 c 2,725 931 1,70+4 4.0 164.0 164.0 164.0 0.0 D 9,700 517 833 3.0 181.2 181.2 161.4 0.2 E 10,920 500 1,313 1.9 185.3 185.3 165.4 0.1 F 12,355 450 1,102 4.7 192.7 192.7 192.7 0.0 G 14,205 500 2,219 2.3 198.0 196.0 198.2 0.2 H 15,650 600 1,255 4.1 204.4 204.4 244,5 0.1 I 17,940 450 1,439 3.6 212.8 212.8 213.3 0.5 ,] 21,400 650 1►751 3.0 224.5 224.5 225.4 0.9 K 23,50Q 970 1,780 2.9 231.7 231.7 231.8 0.1 L, 26,740 1,197 2,010 2.6 246.5 246.5 246.8 0.3 M 28,545 800 1,548 3.3 254.4 254.4 254.8 0.4 N 29,13D 650 1,244 4.2 259.E 259.6 259.8 0.2 0 30,2BO 500 1,061 4.9 269.5 269.5 269.5 D.0 P 31,110 250 933 5.6 273.7 273.7 273.7 0.0 Q 31,730 500 1,202 4.3 281.0 281.0 281.1 0.1 R 35,08D 600 1,+412 3.7 310.4 310.4 310.9 0.5 s 36,56Q 177 429 12.1 325.5 325.5 325.7 0.2 IFeet Abcve Confluence with willow Creek FEDERAL EME RG ENCY MANAGEMENT AGENCY MATANU KA•SUSITNA BOROUGH, AK IMATANU KA,SUSITNA DIVISION) LLOODWAY DATA WILLOW CREEK TRIBUTARY FLOODING SOURCE VLOODWAY BASE FLOOD WATER SURFACE ELEVATION 1 SECTION N WITH CROSS SECTION DISTANCE WIDTH WEFT) (SQUARE VELOCITY {FEET PER REGULATORY FLOODA INCREASE T`LOOD4dAY �`LOO??WAY FEW SECOND) (FEET NGVD) Tittle Susitna River A 0 1,650 '1,?38 2.0 230.5 230.5 231.0 0.5 B 1,520 2,400 7,729 2.0 232.4 232.4 232.8 0.4 C 6,070 2,140 7,995 1.9 236.7 236.7 237.4 0.7 D 10,760 300 3,012 5.1 241.6 241.6 242.1 0.5 T 10,880 290 4,R12 3.1. 245.3 245.3 245.5 0.2 r 15,06D 1,360 6,506 1.8 247.0 247.0 247.3 0.3 G 18,350 350 3,991 3.8 248.5 248.5 246.9 0.4 H 18,720 500 4,165 3.7 249.2 249.2 249.5 0.3 1 21,915 1,060 6,067 2.5 251.3 251.3 251.6 0.3 ,7 26,400 1,235 8,62 1.8 253.3 253.3 254.3 1.0 K 27,730 1,305 7,118 2.1 254.5 254.5 255.3 0.8 L 2$,630 1,350 7,030 2.2 256.5 256.5 257.2 0.7 M 32,580 1,050 6,006 2.5 258.7 258.7 259.3 0.6 N 35,790 1,500 9,803 1.6 260.5 260.5 261.5 1,0 0 38,000 11200 7,791 2.0 262.1 262.1 263.0 0.9 p 41,140 1,180 7,225 2.1 264.8 264.8 265.4 0.6 Q 45,360 1,20D 6,629 2.3 267.6 267.6 269.4 0.8 R 47,685 1,000 4,513 3.4 271.0 271.0 271.5 0.5 g 49,995 1,200 6,725 2.3 274.5 274.5 274.9 0.4 T 53,400 1,100 6,484 2.4 277.5 277.5 278.0 0.5 U 57,025 1,220 6,980 2.2 260.6 280.6 281.3 0.7 V 59,180 1,350 61876 2.2 2B2.5 282.5 283.4 0.9 w 61,950 1,450 8,137 1.9 264.8 284.8 285.7 0.9 x 64,000 1,395 5,890 2.6 237.8 287.8 238.5 0.7 Y 67,090 1,100 4,836 3.2 293.5 293.5 294.0 0.5 Z 69,060 11059 5,657 2.7 297.4 297.4 297.8 0.4 1Feet Above Dowrns-yeam Limit of Detailed Study FEDERAL WERGENCY MANAGEMENT AGENCY MATANUSKA USITNA BOR0UGH, AFC IMATANUSKA-SUSITNA DIVISION) w FLOODWAY DATA LITTLE SU ITNA RIVER PLGODING SOURCE FLOODWAY SASE FLOOD WATER SURFACE ELEVATION 1 SECTION MF-AN WITHOUT WITH CROSS SEOTION DISTANCE WIDTH (FEET) AREA (SQUARE VELOCITY (FEET PER U�GULATORY FLOODWAY FLOODWAY INCREASE FEET) SECOND) (F'F.ET NGVD) Little Susitna River (Conn' ) AA 70,7BO 1,06D 6,3�9 2.2 299.8 299.8 300.1 0.3 AB 731690 1,060 4,983 2.8 303.9 303.9 304.2 0.3 Ac 75,510 11250 7,734 1.8 305.9 305.9 306.6 0.7 AD 78,600 1,10 Q 5,045 2.6 308.4 308.4 309.2 0.8 AE 80,050 1,200 6,740 2.0 310.4 310.4 311.0 0.6 AF 80,160 1,140 7,520 1.8 310.6 310.6 311.1 0.5 AG 81,183 875 2,864 4.6 312.0 312.0 312,.3 0.3 AH 83,355 768 2,671 5.0 318.7 316.7 319.0 0.3 Al 86,615 1,337 6,510 2.0 324.0 324.0 324.8 0.8 Ah7 88,840 1,200 4,666 2.9 326.7 326.7 327.5 0.8 AK 91,090 1,08fi 4,048 3.3 331.1 331.1 331.6 0.5 AL 94,910 1,150 5,574 2.2 336.4 336.4 336.8 0.4 AM 97,145 1,550 4,052 3.0 341.5 341.5 342.1 0.6 AN 99,555 1,445 7,106 1.7 345.6 345.6 346.2 0.6 AO 101,590 1,765 1,305 9.4 349.0 349.0 349.0 0.0 AP 103,145 2,454 1,837 6.7 355.3 355.3 355.3 0.0 AQ 103,340 3,309 9,264 1.3 357.4 357.4 357.4 0.0 Ali 103,870 3,003 6,637 1.9 357.8 357.8 357.6 M AS 104,610 2,598 4,329 2.8 -3G0.9 350.9 360.9 0.0 AT 105,130 2,474 8,195 1.5 363.0 363.0 363.0 0.0 AU 105,635 2,482 7,813 1.6 363.7 363.7 363.7 0.0 AV 1n6,o50 11840 4,863 2.5 363.1 363.7 363.7 U.0 AW 107,440 1,060 2,728 4.5 368.0 368.0 368.0 0.0 AX 109,D60 1,010 3,549 3.5 372.9 372.9 373.1 0.2 AY 110,055 1,020 2,11.3 5.8 376.4 376.4 376.6 0.4 AZ 111,985 630 2,518 4.9 360.0 380.0 380.9 0.9 Feet Above Downskream Limit of Detailed Study FEDERAL EMERGENCY MANAGEMENT AGENCY MATANUSKAE-SUSITNA BOROUGH, AK (MATANUSKA*SMS1TNA DIVISION) M FLOOD MAT DATA LITTLE SUSITNA DIVER FE�OODI�i{a 9DL�RCE I'IAO'�4�TA,Y RASE FLOOD WATER SURFACE ELEVATION SECTION] MEAN WITHOUI.1 WITH CROSS SE 10N DISTANCE wYr1T��l (FEET) AREA (SQUARE VELOCITY (FEET PER REGULATORY FLOODWAY PI�OO]]WAY INCREASE FEET) SECOND) (FEET NC;VD) Little SuSitna River (Cant'd) SA 113,200 745 2,243 5.5 382.8 362.8 382.9 0.1 Be 114,385 444 1,536 8.0 386.2 386.2 386.2 0.0 $C 116,365 990 4,405 2.8 391.7 391.7 392.2 0.5 $D 119,335 676 1,625 6.7 396.4 395.4 396.4 0.0 BE 120,085 1,949 4,043 2.S 401.2 401.2 401.4 0.2 BF 121,860 165 770 12.5 405.9 405.9 405.9 0.0 BO 123,249 66D 1,984 4.8 41.2.9 412.9 412.9 0.0 SH 123,470 222 11550 6.2 413.4 413.4 413.4 0.0 BI 124,055 109 772 12.4 413.7 413.7 413.7 D-0 BJ 1.24,520 1,732 3,967 2.8 418.2 418.2 419.2 0.0 BK 125,095 200 979 3.7 419.2 419.2 419.2 0.0 SL 125,620 91 697 8.0 420.2 420.2 420.2 0.0 Biel 126,510 96 444 12.6 423.6 423.6 423.E 0.0 BN 127,190 100 535 10.4 429.0 429.0 420.0 0.0 BO 128,135 195 716 7.9 434.0 434.0 434.0 0,0 BP 129,035 160 1,021 5.5 437.1 431.1 437.2 0.1 BQ 130,495 120 550 10.1 440.1 440.1 440.2 0.1 ER 131,835 758 1,559 3.6 446.4 446.4 446.4 0.0 8S 132,060 840 2,424 2.3 447.2 447.2 447.2 0.0 BT 132,280 750 1,685 3.0 447.4 447.4 447.4 0.0 BU 133,260 449 1,603 3.5 449.0 449.0 449.0 0.0 2V 134,115 970 2,349 4.8 451.5 4S1,5 451.5 0.0 SW 135,645 720 21315 4.5 457.7 457.7 458.0 0.3 EX 136,925 750 3,168 3.3 462.8 462.8 463.7 0.9 BY 138,315 1,025 5,337 3.1. 468.2 468.2 469.2 1.0 13Z 139,685 1,135 3,378 3.1 475.0 475.0 475.5 0.5 Ireet Above D wnstream Limit of Detailed Study FEDERAL EMERGENCY MANAGEMENT AGENCY MATANUSKA-SU ITNA BOROUGH, AK (MATANUSKA-SUS[TNA 01VFSION; w FLOODWAY DATA LITTLE SUSiTNA RIVER FLOODING SGURC'E FLO)OWAY 13ASE FLOOD WATER SURFACE ELEVATION 1 SECTION MEAN WITH �i�TI'TIOD CROSS SECTION DISTANCE WI1}TH AREA VELOCITY REGULATORY F'LOODTh'DWA �4Y FLt?OQIJWp,Y INCREASE (FEET) (SQUARE (FEET PER FEET) SECOND) (FEET NGVD) Little Susitna River (Cont'd) CA 140,770 1,260 3,421 3.1 481.0 481,11 482.0 1.0 C5 141,815 980 3,164 3.3 486.7 486.7 487.6 0.9 cc 142,640 1,02o 2,500 4.2 492.2 452.2 492,4 0.2 CD 143,505 820 2,424 4.3 496.2 496.2 496.6 0.6 CE 144,835 620 1,637 5.7 502.5 502.5 502.7 0.2 CF 146,170 365 1,478 7.1 5D9.8 509.E 510.0 0.2 CC 147,455 1,100 2,691 3.9 516.3 516.3 516.6 0.3 CH 148,590 495 2,241 4.7 519.7 519.7 520.6 0.9 CI 149,985 1,000 2,390 4.4 526.8 526.8 527.1 0.3 C.7 151,365 74D 2,975 2.9 532.7 532.7 533.5 0.3 CR 151,815 171 451 9.5 535.1 535.7 535.7 0.0 CL 152,320 80b 1,901 2.3 537.9 537.9 538.5 0.6 CM 152,450 691 1,694 2.6 541.3 541.3 541.7 0.4 CN 154,110 700 1,943 2.3 547.3 547.3 548.2 D.9 co 155,405 350 768 5.6 554.3 554.3 554.5 0.2 cp 15.6,755 239 615 7.0 560.9 56U.5 560.9 0.0 CQ 157,620 200 416 10.3 563.2 563.2 563.2 0.0 CR 158,070 250 475 9.1 570.6 'S70.8 570.8 0.0 CS 158,955 115 452 9.5 576.6 576.6 576.6 0.0 CT 159,665 200 391 11.R 583.3 585.3 583.3 D.0 cu 160,330 2,006 1,595 3.1 597.4 587.4 587.4 0.0 CV 161,430 2,000 1,898 2.3 595.5 595.5 595.5 0.0 cw 162,125 2,280 4,516 1.9 597.7 597.7 597.7 0.0 cx 162,980 800 2,16 D 4,0 602.6 602.6 602.6 D.0 Cy 163,945 1,500 1,846 4.7 609.3 609.3 609.3 0.0 ca 164,715 2,400 4,455 1.9 617.1 617.1 617.1 0.0 1FQet Above Downstream Limit of Detailed Study FEDERAL EMERGENCY MANAGEMENT AGENCY MATANUSKA- USITNA BOROUGH, AEA WATA{5 USKA-SUSITNA DIVISION) W FLODDWAY DATA LITTLF SUSITNA DIVER BASE FLOOD FLOODING 90URCE FLOODWAY WATER SURFACE ELEVATION SECTION MEAN WITHOUT WITH CROSS SECTION DISTANCE WI DTI- AREA VELOCr'I'Y REGULATORY F'LCK)DWAY FLOODWAY INCREASE (FEET) ( SQUARE I FEET PER rEET) SECOND) (FEET NGVD) Little Snsitna River (Cont'd) DA 165,510 1,000 3,151 2.7 622.0 622.0 622.0 U.0 DS 166,405 1,040 3,265 2.6 626.9 526.9 626.9 0.0 Dc 167,230 2,640 4, 32 2.0 612.0 632.0 632.0 0.0 DD 168,070 1,520 4,215 2.0 636.1 636.1 636.1 0.0 DE 168,905 1,600 2,176 3.5 644.8 644.8 644.6 0.0 DF 169,565 1,800 5,267 1.5 647.8 647.8 647.8 0.0 DG 170,245 800 2,607 3.0 651.9 651.9 651.9 0.0 DH 171,1.15 450 1,557 4.9 657.7 657.7 657.7 0.{l UI 171,980 620 ,250 3.4 666.0 666.0 666.0 0.0 03 172,300 500 1,638 4.7 670.5 670.5 670.5 0.0 DK 172,60B 550 1,475 5.2 674.1 674.1 674.1 0.0 UL, 173,225 1,250 2,159 3.6 681.0 681.0 681.0 0.0 DM 1.73,870 1,600 2.,401 3.2 687.2 687.2 6B,.2 0.0 nN 174,210 1,880 3,147 2.4 691.7 691.7 691.7 0.0 DO 174,565 1,250 2,774 2.8 595.9 695.9 695.9 0.0 DP 1.74,950 1,300 2,628 2.7 699.6 699.E 699.6 0..0 DQ 1.75,250 1,000 2,318 3.3 703.9 703.9 703.9 0.0 DR 11.5,815 850 2,703 2.8 710.1 71G.1 710.1 0.0 aS 176,400 609 11556 718.5 716.5 718.5 0.0 DT 176,970 700 1,720 4.5 726.3 726.3 726.3 0.0 DU 177,770 906 2,143 3.6 734.3 734.3 734.3 0.0 UV 178,015 70B 1,947 4.0 736.5 736.5 73.6.5 0.0 DW 178,265 294 720 10.7 746.7 746.7 746.7 0.0 DX 178,695 294 720 10.7 746.7 7+46.7 746.7 0.0 DY 179,125 329 837 9.2 752.9 752.9 752.9 0.0 UZ 179,470 1B7 694 11.J 758.7 758.7 758.7 0.0 yFeet Above Downstream Limit of Detailed Study FEDERAL EMERGENCY MANAGEMENT AGENCY FLOODWAY DATA r- MATANUSKA-SUSITNA BOROUGH, A rM WATANUSKA•SUSSTNA DWISIGN) LITTLE SUSITNA RIVER r rri C4 FLOODING SOURCE FLOOC}4JAY SASE FesD WATER SURFACE ELEVATION 1 SECTION MEAN WIT 4VI'TFjWA CROSS SECTION DISTANCE WIDTH AREA VELOCITY I GULF4TORY FLOODINCRIE�ASE F`T�04DWAY FLDQD4$AY (FEET (SQUARE (FEET .���{ FEET) SECOND) (FEET N[PVD} Little SuSitna River split Flow 1 A 200 3,309 9,264 1.3 357.44 357.4 357.4 0.0 $ 600 3,003 6,61 1,19 357.8 357.8 357-B 0.0 C 1,150 2,596 4,329 2.8 360.9 360.9 360.9 0.0 D 11550 2,474 8,195 L.5 363.0 363.0 363.0 0.0 E 1,800 2,482 7,813 1..6 363.7 363.7 363.7 0.0 Little Susitnm River Split Flow 2 A 120 1,949 4,446 2.8 401.2 401.2 401.4 0.2 B 2,220 1,164 1,459 1.2 407.2 407.2 407.2 0.0 c 3,424 515 1,D26 1.7 414.0 414.0 414.0 0.0 D 3,890 773 1,321 1.3 417.4 411.4 417.4 0.0 E 4,070 816 3,955 2.9 418.1 418.1 418.1 0.0 F 4,530 1,731 3,955 2.9 418.1 418.1 418.1 0.0 G 5,720 450 2,151 2.7 420.2 420.2 420.3 0.1 H 7,025 300 1,462 3.9 423.4 423.4 424.4 1.0 I 81650 620 2,190 2.6 427.8 427.9 428.E 1.Q 1 9,780 600 1,795 3.2 432.1 432.1 432.5 0.4 x 10,560 500 1.,119 5.1 436.4 436.4 436.6 0.2 L 11,110 600 2,174 2.6 438.4 438.4 438.9 0.5 m 11,995 800 1,995 2.9 442.7 442.7 442.7 0.0 N 12,490 1,125 5,232 1.1 444.5 444.5 442.7 0.0 0 12,620 1,194 2,405 2.4 447.Of 447.0 447.0 0.0 P 13,475 820 2,837 2.a 448.5 449.5 44A.5 0.0 1Pect Above ti--pnfluence With Lattle Qu.sit_na RivEf FEDERAL EMERGENCY MANAGEMENT AGENCY MATANUSKA- USIT A BOROUGH, AK WATANUSKA-SUSITNA RIVIS10N) FLOODWAY DATA LITTLE SUSITNA RIVER --SPLIT FLOW 1 LITTLE SUSITNA RIVER -SPLIT FLOW 2 � � � rr'l w FLOODING SOURCE r 5 Y SE FLOOD TER SURrACE ELEVATION EAERWIC AR 0 WITH. CROSS SECTIONDISTANCE l WIDTH VELOCITY REGULATORY Rio Y E ¥I RZE (FEET) (SQUARE (FEET PER FEET)SECOND) (PEET§T.WD) Little Sitna Rives Split Flo 3 A 450 620 1,048 4.1 535.6 535.6 536.1 0.7 B 1.265 330 1.131 3.a 540.5 540.5 540.7 0.2 C 1,380 340 811 5.3 542.4 542.4 542.5 0.1 D 2,940 480 1.548 2.6 548.4 548.4 549.0 O.§ £ 4.240 425 1.177 3.7 555.7 555.7 555.7 0.0 E 5.775 630 1,593 2.9 566.5 566.5 567.3 0.8 G 6,515 816 1.747 2.5 570.6 570.6 570.E 0.0 E 7.285 1,020 11680 2.6 576.5 576.5 576.5 0.0 I 7,960 680 926 4,6 580.7 560.7 580.7 D.0 J 6,610 740 1,184 3.6 586.9 586.9 586.9 B.0 R 9.240 491 1.294 3.3 589.6 5B9.6 Sb9.6 0.0 5 � 9.880 950 1.34E 3.2 593.9 593.9 593.9 0.0 eet Above Confluence With LiEtle Su.Sitna kip y FEDERAL EMERGENCY MANAGEMENT AGENCY ¥ATANUSKA-U&TNABOROUGH, AK |RATAN Ksl NA DION| [LOODW AY DATA LITTLE SUSITNA RIVERSFLITFLOW 3 i0a-YEAR FLOOD PLAIN F LOODWA Y FRINGE PI~DOD ELEVATION WH9N CONFINF;DWITHIN FLOODWAY ENCROACHMENT AREA OF FLOOD PLAIN TH k7 COULD eE USED FOR DE VI5LOP PAENT BY FIAiVNG GROUNp PLOODWAY 41 F11-001D14+AY FRINGE STREAM CHANNEL k)RCHARGE' E NCFIp A C HM ENT FLOOD ELEVATION BEFORE ENCR4ACHM€N7 ON FLOOD PLAfN LrNE AD IS THE FLOOD ELEVATION BEFORE r!NI~RGACHIVIENT- LINE CO IS THU FLOOD ELEVATION AFTER ENCSOACF#MENT. 'SURCHARGE IS NOT To EXCEED 1,0 FOOT �FEk1,q REQUIREMEIM OR LESUR AMOUNT IF SPECIFIED ST STATE. Figure 2. Floodway Schematic 5.0 INSURANCE APPLICATION In order to establish actuarial insurance rates, the Federal Emergency Management Agency has developed a process to transform the data from the engineering study into flood insurance criteria. This process includes thoe determination of readies, Flood Hazard Factors, and ±load insurance zone designations for each flooding source studied in detail affecting Matanuska-Susitna Borough. 5_1 Reach Determinations Reaches are defined as lengths of watercouzses having Telatively the same flood hazard, based on. the average weighted difference in water -surface elevations between the 10- and 100-year floods. This difference does not have a variation greater than that indicated in the following table for more than 20 percent of the reach: 27 Average Difference Between 10- and 100-Year Floods Variation Less than 2 feet 0.5 foot 2 to 7 feet 1.0 foot 7.1 to 12 feet 2.0 feet More than 12 feet 3.0 feet The locations of the reaches determined for the flooding sources of Matanuska-Susitna Borough are shown on the Flood Profiles (Exhibit 1) and summarized in Table 4. 5.2 Flood Hazard Factors The 'load Hazard Factor (FEiF) is the Federal Emergency Management Agency device used to correlate flood information with insurance rate tables. Correlations between property damage from floors and their PW are used to set actuarial insurance premium rate tables based on FHFs from 005 to 200. The FHF for a reach is the average weighted difference between the 10- aad 100-year flood water-surfaca elevations expressed to the nearest one-half foot, and shown as a three -digit code. POE example, if the difference between water -surface elevations of the 10- and 100-year floods is 0.7 foot, the FHF is 005; if the difference is 1.4 feet, the FHF is 015; if the difference is 5.0 feet, the FHF is 050. When the difference between the 10- and 100-year water -surface elevations is greater Lhan 10.0 feet, accuracy for the FHF is to the nearest foot. 5.3 Flood Insurance fines After the determination of reaches and their respective FHrs, the entire unincorporated area of Matanuska-Susitna Borough was divided into Zones, each having a specific flood potential or hazard. Each 20ne was assigned one of the following flood insurance Zone designations; Zone A: Special Fled hazard Areas inundated by the 100-yeas flood, determined by approximate methods; no base flood elevations shown or FHFs determined. Zones Al through A5 Special Flood Hazard Areas inundated by and A7: the 100-year flood, determined by detailed methods; base flood elevations shown, and 20nes subdivided according to PHFs. Zone 8: Areas between the Special. Flood Hazard Areas and the ZiAdts of the 500-gear flood, including areas of the 500 -year 2.8 r� rri ,_ _';.: SOURCE J I.A.,I �' ELEVATION DIFFERENCE BETWEEN 1% (100-YEAR) FLOOID AND FLOOD HAZARD FACTOR ZONE BASE FLGQD ELEVATION � {FEET 17GVD) { 2i 0 {IU-}SEAR) (50-YEAR) (SOO-YEAR) Deception Greek Reach 1 7945 -1.0 -0.2 0.9 010 A2 Varies - See map Reach 2 7945,7965 -0.7 -0.2 0.5 005 Al Varies - See Map Roach 3 7965,8780 -1.1 -0.3 0.7 010 A2 Varies See Map Fzeach 4 8760,8785 -0.5 -0.2 0.4 005 Al Varies - See Map Reach 8 8790,8785 -1.7 -0.4 1.0 015 A3 Varies - See Map Reach 6 8785 -0.9 -0.2 0.6 010 A2 Varies - See Map Reach 7 8785 -1.5 -0.3 0.9 015 A3 Varies - See Map Deception Creek Tribu- tary 1 Reach 1 7965 -0.5 -0.1 0.3 005 Al Varies - See Map Reach 2 7965,3780 -1.1 -0.3 0.7 010 A2 Varies - See leap Deception Creek Tribu- tary 2 Reach l 8780 -1.' -0.3 0.7 010 A2 Varies - See Map Reach 2 E8780 C. = -0.1 0.3 00v Al Varies - See Mafia Deception Creek Tribu- tary 3 Reach 1 8780,8785 -0.4 -0.1 0.3 005 Al Varies - See Map Willow creek Reach 1 7940,7945 -1.07 -0. 9 G.78 010 A2 Varies - See Map Reach 2 7945 -1.71 -0.73 1.00 015 A3 Varies - See Map Reach 3 7965 -1.10 -0.16 0.62 010 A2 Varies - See Map Reach 4 7965,7970 -2.15 -0.47 1.29 020 A4 Varies See Map 1Flood insurance Rate Map Panel 2Weighted Average FEDERAL EMERGENCY MANAGEMENT AGENCY 3Rounded to Nearest Foot FLOOD INSURANCE ZONE DATA MATANUSKA- USITNA BOROUGH. A (MATANUSKA•SUSITNA DJ VISION) DECEPTION CRE:EKrDE EPTION CREEK TRIBUTARY I - DECEPTION CREEK TRIBUTARY2-DECEPTION CREEK TRIBUTARY 3-WILLOW CREEK rn r2 FLOODING SOURCE 1 PANEL ELEVATION DIFFFRENCE 13ETWEE N 1(100-YEAR) FLOOD ANU FLOOD HAZARD FACTOR ZONE BASE FLOOD ELEVATIO24 , (FEET :ACVD) , 10% 2% 0.24 {14-YEAR} (50-YE ) {500--YEAR} Little Susitna River Reach 1 8795,9610 -1.45 -0.47 0.92 015 A3 Varies See Map 9630 Reach 2 8795,8815 035 ,A7 Varies - See Map 9610,9630 Reach 3 8815,8920 -1.84 -0.51 1.23 020 A4 S.aries See Map 8840,8845 p 8865,963G Reach 4 8865 -1. 1 -0.37 0.58 j10 A2 varies See Map Reach 5 6865 -2.21 -0.35 0.91 020 A4 varies - See Map Reach 6 8865,6870 -1.50 -0.35 0.72 015 X. Varies - See Map Reach 7 8870 -2.65 -0.56 0.95 025 A5 Varies - See Map Reach E3 8880,5890 -1.14 -0.37 0.76 010 A2 Varies May 8850, B870 Little Susitna river - Split Flow 1 Beach 1 9845 -1.99 --0..31 0.90 020 A4 Varies - See Map Little-Susitna River - Split Flow 2 !teach 1 8845,8865 -2.1-2 --0.60 1.10 020 A4 Varies - See may Little Susitna River - Split Flaw 3 Reach 1 8865,8870 -1.50 -0.36 0.72 015 A3 Varies - See Map Reach 2 BS70 -2. 65 --0.56 0.95 025 A5 varies - See Map Reach 3 B670 --O. B5 -0. 32 0.91 010 A2 Varies - See Map IFlood insurance Rate flap Panel 2Weighted Average FEDERAL EMERGENCY MANAGEMENT AGENCY 3Rounded to Nearest Foot FLOOD INSURANCE ZONE DATA MATANU KA U ITNA BOROUGH, AK (MATAN US KA-SUS I TN A DIVISION LITTLE: SUSITNA RIVER -LITTLE SUSITNA RtVER -SPLIT FLOW 1- LITTLE SUSITNA RIVER -SPLIT FLOW 2•LITTLE SLI5i7NA RIVER -SPLIT FLOW 3 flood plain that are protected from the 100--year flood by dike, levee, or other water -control structure; also areas subject to certain types of 100-year shallow flooding where depths are less than 1.0 foot; and areas subject to 100-year flooding from sources with drainage areas less than 1 square mile. Zone 5 is not stibdi.vided. Zone C: Areas Of minimal flooding. Zone D: Areas of undetermined, but passible flood hazard. The flood elevation differences, it's, flood insurance zones, and base flood elevations for each flooding source studied in cat tail in the community are summarized in Table 4. 5.4 Flood insurance Rate Map Description The Flood Insurance Rate !slap for Matanuska--usitna Borough is, for insurance purposes, the principal result of the FL.00d Insurance Study. This map (published separately) contains the official delineation of flood insurance zones and base flood elevation lines. Base flood elevation lines show the locations of the expected whole -- foot water -surface elevations of the base (100-year) flood. This Wrap is developed in accordance with the latest flood insurance map preparation guidelines published by the Federal Emergency Management Agency. 6.0 OTHER STUDIES The U.S. Soil Conservation Service has prepared three Floor} Hazard Studies, two Vlaod Plain Management Studies and a Flood Plain inventory Report for various streams in Matanuska-Susitna borough (References 11 through 16). These reports were the sources of some of the approximate flood boundazies presented in this study. The Expanded Flood Plain Information Study for Willow, Alaska (Refer- ence 17) utilized the sane hydrologic and hydraulic prooed= es; however, through the use of spatial analysis, it reported the effects of flooding and development on the environment and considered the effects of evacua- tion, flQadproofing, and zoning on existirkg conditions in the year 2AQ4. This study is in agreement with the Flood Plain Information Study. A Flood Insurance Study has been prepared for the Municipality of Anchorage (Reference 18). This study is in agreement with the Anchorage Florid Insurance Study. Flood Fla zatrd Boundary Maps have been prepared for the unincorporated areas of Matanuska-Susitna Borough and the City of Palmer (References 9 and 10) . Doe to the more detailed nature of this study, it supersedes the Flood Hazard Boundary maps. 3: This study is authoritative for the Purposes of the National Flood insur- ance Program; data presented herein either supersede or are compatible with all previous determinations_ 7. LOCATION OF DATA Information concerning the pertinent data used in preparation of this study can be obtained by contacting the Natural and Technological Hazards Da'vlsion, Federal EWaergency Management Agency, 130 228th Street SW., Bothell, Washington 98011_ 8.0 BIBLIOGRAPHY AND REFERENCES 1. U.S. Department of Commerce, Bureau of the Census, 1980 Census of Population, November 1981 2. U.S. Department of the Army, Corps of ,Engineers, Hydrologic Engineer- ing Center► HEC-1 Flood Eydroqraph Package, Davis, California, 1973 3. U.S. Department of Commerce, Weather Bureau, "Teohnic,al Paper No. 53," 1973 4. U-S. Water Resources Council, "A Uniform Method for Determining Flood FloW Frequency," Bulletin 17, March 1976 5. U-S, Department of the Army, Corps of Engineers, Hydrologic Engi- neering Center, Computer Program HEC-2 water -Surface Profiles, Davis, California, 1976 6. North Pacific Aerial Survey$, Inc., MR22gra2hic Maps, Willow and Deception Creeks, Scale 1:4,800, Contour Interval 5 feet. 1977 7. North Pacific, Aerial Surveys, Inc., 'lMgraphic Maps, Little Susxtna River, Scale 1:4,800, Contour interval 5 feet, 197a S, Borth Pacific Aerial Surveys, Inc,, Topographic Maps, Bodenburg B�j tte Area -Palmer, Scale 112,400, Contour Interval 2 feet. 1980 9. U.S. Department of Housing and €7rhaa Development, Federal Insurance Administratinn, Flood Eazard Boundary map, Matanuska-Susxtna Borough Alaska, Sca3.e 1;24,044, 1979 10. U.S. Department of Housing and Urban Developmerit, Federal Insurance Administration, )Flood Hazard Boundary Map, City of Palmer, Alaska, Scale 1:6,000, 1979 N 11. U.S. Department of Agriculture, Soil Conservatiop Service, Flood Eagard Study, 196 Mile, Caswell, Sheep, Goose, Montana, Answer, and Birch Creeks and Tributaries, Matanuska-Susitna Borough,_Alras%a. Scale 1:25,944, April 1981 --- 12. U.S. Department of Agriculture, Soil Conservation Service, Flood tiasard Study, Troublesome, Byers, and Eonol.ulu Creeks, East and _Middle Forks of hulitna River, Matanuska-Susitna Borough, Alaska, Scale 1:25,263, September 1981 13. U_S_ Department of Agriculture, Soil Conservation Service, Flood Hazard Stud Xroto, Rabideux, Trapper, and Peters Creeks, Matanuska- Susitna Borough, Alaska, Scale 1:25,263, January 1982 14. U.S_ Department of Agriculture, Soil Conservation Service, Flood Plain Inventory Report, Lower Little Susitna river and Tittle Willow_ Creek, Matanuska-Susitna Borough, Alaska, Scale 1:63,360, February 1980 _.,. 15. U.S. Department of Agriculture, Soil Conservation Service, Flood Plain Management Sttidy, Kashwitna River, Wasilla, Cottonwood, and Lucile Creeks, Matanuska-Susitna Borough, Alaska, Scales 1:6,000 and 1:25,O00, May 198 — - - 16. U.S. Department of Agriculture, Soil Conservation Service, Pload Plain Management Stud , Belu a Sub sin Streamm , Katanaska-Susitna and Kenai Peninsula Boroughs, Alaska, Scale 1:63,360, September 198 1.7. U.S. department of the Army, Corps of Engineers, Expanded Flood Plain Information Study for Willow, Alaska, June 1980 18. U.S. Department of Housing and Urban Develnpmerxt, Federal Insurance Administration, ]Flood Insurance Study, Municipality of Areoharage, Alaska, March 1979 U.S. Department of the Army, Corps of Engineers, North pacific Division, SSARR (Stream Flow Synthesis and Reservoir Regulation), Portland, Oregon, September 1972 U.S. Department of the Array, Corps of Engineers, Water Resources Development Oy the U.S. Artily Corps of Engineers in Alaska, 1977 L.S. Department of Commerce, Weather Bureau, '"Technical Paper No. 47" Waterways Experiment Station, Technical Report H743, May 1974, Vicksburg, Mississippi 34