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HomeMy WebLinkAboutFlood Plain Information Mineral Creek Valdez, AK 1976e8pug yaoiD [ereul ey} JO Weasysdn szYSZIey] POOjy enn *pooyy Ieah-QQ] 94} JO JU9}xe oy} SuMmoYs qiodo oy} wos deur vare papoory & osye pue sore pepooy jo drysuorejor yeoryioA pue ye}UOZTIOY dy} SuNeIsN]E soyojoys ore Iapjoy oy} oprsuy “M990 pnoys ‘poor Ie94-9OS dy} “poor osrey AOA B JE poyoeel oq pynom yeyy JYSIOY POOL pojewnyse oy} st payeorpur ospy “1eoA Aue ut In990 plnoo ‘19AeMOY POOL SIL *oseroAR ay] uo sivak QO] Ul sou popssoxs 10 poyenbo ACTION is needed Development adjacent to Mineral Creek has increased markedly in the past few years; however, there are still large tracts of flood-prone land that are undeveloped. With the rapid growth that can be expected in the future, especially in housing development, damage caused by floods will continue to increase unless action is taken. Effective regulatory measures such as zoning ordinances and building codes can be designed to help prevent increased flood damages. Flood proofing can help reduce potential damages to properties already subject to flooding and additional works to modify flooding and erosion damage can also be a part of the long range solution. The Valdez area is not the only area with flooding problems. Flood plain information reports have already been provided for many of several thousand flood-plagued communities throughout the nation. Over 1300 of these communities having FPI Reports by mid 1975 have adopted or strengthened regulations, while 1200 others have them under study. An additional 2000 communities have used the FPI Reports to establish interim land use controls. This folder has been prepared for the City of Valdez by the U. S. Army Corps of Engineers from data in the report “Flood Plain Information, Mineral Creek, Valdez, Alaska. Copies of the report and this folder are available upon request from the City of Valdez, P. O. Box 506, Valdez, Alaska 99686 BuIssoro a8pliq ay} Jo wearysdn ysnf{ yaerD [eLOUTI, . punossyorq ayy Ul ZapleA og YMA yaar [eeu JO YJNouI oy} pwMo} BuLyooT Gravel extraction operation near the mouth of Mineral Creek Port Valdez; Mineral Creek Islands are in the foreground and the old townsite of Valdez in the background. aq [ITM ‘poopy 1edA-QO] oY} ‘Pools o8se] B IO} UMOYS WSIOY Pooly oy, "YD esouT JOAo BUTSSOIO a8plq A[UO oY} ye s}YBIOY Pooyy [eUsjod sinjny Surmoys ydessojoyd ke st Japfoy sty} UL pepnpouy “proday UOTRULIOJU] UTe[g POOL OY} UT papnyour st sdnuns OARM IBOA-QOS PUB -OO] SY} JO UOT}eoUT[Op B pue stureuns} pue sdn-uni oara [e}svoo JO UOISsNdsIC “ZUIPOOTJ [eJSBOd 0} yOOLqns OsTe st A}IO BY} “ZOP[eA og Jo pray oy} ye ‘ZapfeA JO UOT}eOO] 0} ong “spooyj ainynj JO AyLIaAes pure ju9}xo ofqissod oy} 97e1\SNI]I 0} popnpour useq aaky suOTIOeS ssolo pue ‘softjord ‘sdew “wodey Idj 2U} Ut spooy omyny 0} UdATs st stseydurg ‘oInjNJ 9Y} UL INDO UL SPOOTf JOBIe] UdAD ye} a]RoIpur sarpnys “jsed oy} UT spooyy Woy odeurep posayyns sey Yoolg [eIOUTP| Suoye kore oy} YsnoyIy “poyedisnyfi st Jey uey] SuIpooy Jo spas, JoysTy YIM pue snopiezey sou Ay[etjuRysqns spoojy 9]e019 uv sBUIssOIO oyenbapeul JO UIvel}s oY} uo SuTYyOROIOUS JUSUIdOTOASP MOU “IOAOMOY ‘SPOOTS IedA-QOS OY} pur Ied4-QO] SY} YO OJ MOIICU ST ureld poopy oy], ‘SeseaIOUT OsTe soseUTeP WIO}s pue Ppooyj 1OJ erjUajod oy} JUOIJI0}eM [e}SvOD oY} pur yaoi [e1oul, Suoje yUoUdopeAop SUTseOIOUT YIM ‘uoTjezIIN ure[d poojy SuLUIIDUOD sUOTSIOOP juowroseueul Joy pue Suruuejd osn purl Io; siseq pUNOs v oprAOId [JIM YOTYA spiezey Poolj pue [eud}0d pooyj uO sjoRj ay} yUasod 0} st Yoder oY} jo asodind oy] ..eYxseTy ‘ZopleA “YoodD [eIOUlL ‘yaoday (Id) woreurrojuy urejq POOTY,, OU} 0} juourojddns pure jo juowoeounouur ue St Jopfoj STL W33YNO TVYSNIAN uo | ll | on MINERAL CREEK VALDEZ, ALASKA BUILDING os this in the LY f, ENCROACHMENT FLOOD PLAIN t - can change can make q FLOODS Small Flood WIDER be la natural stream wit deealcomient into a and bank-full flooded encroachments MAJOR DEEPER FLOOD TOOLS of FLOOD PLAIN MANAGEMENT for the reduction of Flood Damage and Human Suffering MEASURES TO MODIFY FLOODS OTHER are often required to alleviate existing problems and ME ASURES sometimes fo forestall future problems - - - aid the Flood Plain occupant in coping with floods - - - Tax Adjustments Flood g MEASURES TO REDUCE vutnerasuiry nsuranc TO FLOODS provide for a future with more freedom from flood damage, often at minor cost and with little Wa rning & adverse effect on the environment: : °°: ° REGULATIONS — ZONING, BUILDING Emergency CODES, URBAN RENEWAL, SUBDIVISION Plans FLOOD PROOFING - RELOCATIONS MINERAL CREEK FLOODED AREAS VALDEZ, ALASKA APPROXIMATE SCALE IN FEET ALASKA POWER AUTHORITY LIBRARY COPY PLEASE, DO NOT REMOVE FROM OFFICE!! ORIENTATION FOLDER INSIDE FLAP CONTENTS Page PREFACE .. 1... . ee ee i BACKGROUND INFORMATION ....... 0.00000 2 eee ee eee 1 Settlement . 2... ee 1 The Stream and Its Valley... 2... 2 ee ee 5 Developments in the Flood Plain .............. 00... 00 0005 6 FLOOD SITUATION ... 2... 20.00.0000. ee eee 8 Source of Dataand Records... 2... ee 8 Flood Season and Flood Characteristics... 2... ee es 8 Factors Affecting Flooding and theirImpact ...............0-6. 8 Obstructions to Floodflows .... 2.2.2... 02.0.2 000 000004 8 Flood Damage Reduction Measures ..........-.0+--0+.00 05 11 Other Factors and their Impacts .... 2... 2... eee ee eee 11 Flood Warning and Forecasting ...............2--. 11 Flood Fighting and Emergency Evacuation Plans ......... 11 Material Storage on the Flood Plain .............04- 11 PAST FLOODS .. 1... 0... cee tt es 13 Summary of Historical Floods... 2... 2. ee 13 FUTURE FLOODS ............ 0.0.00... pee eee 14 100 Year Flood .. 1... ee ee ee ee 14 500 Year Flood .. 1... ee 14 Frequency .. 1... 15 Hazards of Large Floods... 2... 1. ee 15 Flooded Areas and Flood Damages ..............00005 15 Obstructions .. 1... ee 17 Velocities of Flow .. 2... 2... 17 Rates of Rise and Duration of Flooding ................. 17 Photographs, Future Flood Heights ................0.0. 17 GLOSSARY 2... ee 18 Table CONTENTS (continued) TABLES Page Drainage Areas 2... ee 5 PLATES General Map . 2.1... ee Opposite Page i Index Map .. 1... eee At end of report Flooded Areas... 1... . ee At end of report Profiles 2... ee ee At end of report Selected Cross Sections ... 2... ...-2. 000022 eee At end of report FIGURES Page Looking from the small boat harbor toward the old townsite of Valdez in the background. Mineral Creek Islands are in the foreground. ........... 3 Small boat harbor at the new townsite, near the mouth of Mineral Creek. ... 3 View of the lower portion of Mineral Creek, near its mouth. Port Valdez is in the background. 2.2... ee 4 Typical of the material through which the channel of Mineral Creek flows. In this area, near the mouth, the material is being commercially excavated. ... . 4 View of lower portion of Mineral Creek. Alyeska housing is in the right foreground. Note the undeveloped areas on the west (left) side of Mineral Creek. 7 View of Mineral Creek near the upper study boundary. Stream is confined by the steep mountains. Note the heavy underbrush in the floodplain. ...... 7 Looking upstream at Mineral Creek bridge... 2... 2. ee eee ee ee ee 9 Mineral Creek downstream from the bridge. .... 2... - 2. ee eee eee 9 Future flood heights just upstream of Mineral Creek bridge. .......... 10 VICINITY MAP SQ PRINCE WILLIAM Tidal Flat “*s/ Mineral Creek Islands” SCALE IN MILES DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA FLOOD PLAIN INFORMATION VALDEZ ALASKA GENERAL MAP JULY 1976 PLATE | PREFACE The portion of the community of Valdez covered by this report is subject to both riverine flooding from Mineral Creek and to coastal flooding. The developed properties along Mineral Creek are primarily residential and have been moderately damaged by floods in past years. The open spaces in the flood plains which are now under pressure for future development are extensive. Although large floods have occurred in the past, studies indicate that even larger floods are possible. This report has been prepared because a knowledge of flood potential and flood hazards is important in land use planning and for management decisions concerning flood plain utilization. It includes a history of flooding in Valdez and identifies those areas that are subject to possible future floods and coastal wave run ups. Special emphasis is given to these floods through maps, photographs, profiles, and cross sections. The report does not provide solutions to flood problems; however, it does furnish a suitable basis for the adoption of land use controls to guide flood plain development and thereby prevent intensification of loss and damage. It will also aid in the identification of other flood damage reduction techniques such as works to modify flooding, and adjustments, including flood proofing, which might be embodied in an overall Flood Plain Management (FPM) program. Other FPM program studies — those of environmental attributes and the current future land use role of the flood plain as part of its surrounding — would also profit from this information. At the request of the City of Valdez and the endorsement of the State of Alaska, Department of Natural Resources, this report was prepared by the Alaska District, Corps of Engineers under continuing authority provided in Section 206 of the 1960 Flood Control Act, as amended. Assistance and cooperation of the National Weather service, U.S. Geological Survey, Alaska Disaster Office, and private citizens in supplying useful data for the preparation of this report are appreciated. Additional copies of this report can be obtained from the City of Valdez. The Alaska District, Corps of Engineers, upon request, will provide technical assistance to planning agencies in the interpretation and use of the data presented, as well as planning guidance and further assistance, including the development of additional technical information. BACKGROUND INFORMATION Settlement The community of Valdez, located 115 miles east of Anchorage, was established in 1890 as a debarkation point for men seeking a route to Interior Alaska and the Klondike gold region. A post office was established in the community in 1899. Valdez soon became a supply center for gold and copper mining in the immediate area. The city is in a setting of natural beauty situated in mountainous terrain at the head of Port Valdez. It is the farthest north ice-free sea port in Alaska and serves as southern terminus of both the Trans-Alaska Pipeline and the Richardson Highway. Unlike the other port communities of Seward and Whittier, Valdez is not served by the Alaska Railroad and is only linked to the interior by road and air travel. Port Valdez is the northeastern most extension of Prince William Sound and is surrounded by steep mountain walls of the Chugach Mountains. It is a 3 mile wide steep walled glaciated fiord that extends east-west about 14 miles. At its western end the fiord bends to the southwest and necks down to a mile in width at Valdez Narrows before it opens out into the Valdez Arm of Prince William Sound. The steep mountain slopes extend beneath the water producing a steep-sided, flat bottomed trough 400 to 800 feet deep. The shore of Port Valdez is rocky everywhere except where deltas and moraines have been built into the fiord by streams and glaciers. Sediment, transported by the Lowe River and the streams draining from Valdez Glacier, has formed a large delta which makes up the entire eastern end of Port Valdez. It was on the edge of this delta that the initial townsite of Valdez was located. The constantly shifting streams from Valdez Glacier subjected the site to intermittent flooding problems. As a result, the community was eventually protected and contained within a V-shaped dike. The area had, and still has, a potential for large floods because of the glacially dammed lakes along Valdez Glacier which could ‘‘dump” at any time. Called the “Switzerland of Alaska”, Valdez favored a developing tourist trade when disaster struck in March 1964. The Alaska Earthquake brought overwhelming destruction to the community. The entire waterfront area was totally destroyed and Seismic Sea waves penetrated deep into the heart of town. In fact, the town was virtually destroyed. A large portion of the damage resulted from tsunamis that were caused by submarine landslides. Compounding the problem was the fact that the whole town subsided because of the compaction of the deltaic material during the tremors. The subsidence, which was as much as 9 feet, was the greatest at the waterfront and gradually decreased away from shore. In addition to the fact that the subsidence contributed to wave damage, approximately 40 percent of the homes and most of the larger commercial buildings were seriously damaged by the heaving ground. Many buildings had large fissures extending under or near them. Because of the unstable sediments upon which the town was built, it was condemned as a hazard and was vacated in favor of a new townsite at Mineral Creek, which is underlain by stable rock. After initial disaster recovery efforts, planning and construction efforts for the new townsite began almost immediately. The new site was developed on a chain of bedrock hills about 4 miles to the northwest, near Mineral Creek. The new city has been growing rapidly since it was relocated, especially so with the tremendous influence of the Trans-Alaska Pipeline activity. Under this influence, the economy of Valdez is booming. Considering the fact that it will be the terminus of the pipeline, the economic future looks bright. In addition, government will continue to play a large part in the economy. Fishing and canning industries will also contribute to a limited extent. Although served by air, road, and sea, Valdez is relatively isolated from other population centers in southcentral Alaska. As previously mentioned, Anchorage is located 115 miles to the west, but physical barrers considerably increase the distance by highway to 306 miles. FIGURE 1 - Looking from the small boat harbor toward the old townsite of Valdez in the background. Mineral Creek Islands are in the foreground. FIGURE 2 - Small boat harbor at the new townsite, near the mouth of Mineral Creek. 3 FIGURE 3. - View of the lower portion of Mineral Creek, near its mouth. Port Valdez is in the background. FIGURE 4 - Typical of the material through which the channel of Mineral Creek flows. In this area, near the mouth, the material is being commercially excavated. 4 The Stream and Its Valley Mineral Creek, a tributary of Port Valdez, drains an area of 45.9 square miles and is located west and north of the new townsite of Valdez. The stream gradient is steep, and the topography is extremely mountainous, with approximately 30 percent of the basin covered by glaciers. Elevations in the drainage basin range from sea level to over 5,000 feet. The flood plain through which the channel migrates is composed chiefly of silt, sand, gravel, and boulders. The overbank areas of the stream are generally very heavy with timber and brush. Drainage areas for the stream, which contribute to runoff at various locations in the study area, are shown in Table I. TABLE I DRAINAGE AREAS Location River Mile Drainage Area Square Miles Mineral Creek at Mouth 0 45.9 Mineral Creek at Bridge 1.6 43.3 Mineral Creek at Upper Study Boundary 3.8 41.3 Valdez’s climate is directly related to its location. Port Valdez, a large natural deep water bay, off Prince William Sound, provides both an excellent ice-free harbor and a moderate, maritime climate for the city. The climate is typified by cool summers, mild winters, and large amounts of snow and rainfall. Temperatures are relatively mild, and daily variations are minimal. The moderating influence of the Japanese current gives the area warmer winter temperatures than are normal for these latitudes. Seasonal variations are also minimal with normal temperatures ranging from 19.3°F in January to 53.2°F in July. Annual precipitation averages 60.7 inches with most occurring in late summer and fall. The average annual snowfall is 254 inches. 5 Development in the Flood Plain The portion of Mineral Creek included in this study commences at its mouth at Port Valdez and extends upstream approximately four miles. The coastal area that is included extends eastward approximately three miles from the mouth of Mineral Creek. The development that presently exists in this area is minor, and in the case of lands west of Mineral Creek, is non-existent. However, with the influence of the Trans Alaska Pipeline, new development can be expected. At the present time, only one stream crossing, a bridge, exists in the study area. If, as is anticipated, land is sought for development on the west side of the stream, near the mouth, there will be a need for additional stream crossing in this area. FIGURE 5 - View of lower portion of Mineral Creek. Alyeska housing is in the right foreground. Note the undeveloped areas on the west (left) side of Mineral Creek. FIGURE 6 - View of Mineral Creek near the upper study boundary. Stream is confined by the steep mountains. Note the heavy underbrush in the floodplain. 7 FLOOD SITUATION Source of Data and Records There are no stream gaging records for Mineral Creek. Records of temperature, precipitation, and wind are available from 1931 to the present. To supplement these records, newspaper files, historical documents, and records were searched for information concerning past floods. From these investigations and from the studies of possible future floods and storms, the local flood situation, both past and present, has been developed. Maps used for this report were prepared utilizing photogrammetric methods from aerial photographs taken in 1964 immediately after the March 27 earthquake. Stream cross sections and data on the bridge were obtained by field surveys in 1975. Flood Season and Flood Characteristics Floods on Mineral Creek occur most frequently in late summer and fall. Although they can result from a combination of factors, including rapid snow melt and precipitation, the primary cause is rapid runoff during heavy rains. The flooding potential is also affected by the sequence of events. For example, spring floods may occur as a result of above-normal snowfall during the winter, followed by an unusually cold spring and, finally, a rapid snow melt. Typically, although streams rise from normal to extreme flood peaks in a relatively short period of time, the floods are of short duration. Factors Affecting Flooding and Their Impact Obstructions to Floodflows — Natural obstructions to floodflows include trees, brush, and other vegetation growing along the stream banks in flood plain areas. Man-made encroachments on or over the stream, such as roads and the bridge, can also create more extensive flooding than would otherwise occur. During floods, trees, brush, and other vegetation growing in the flood plain impede floodflows, thus creating backwater and increased flood heights. Trees and other debris may be washed away and carried downstream to collect at the bridge or other obstructions. As floodflows increase, masses of debris could break loose and surge downstream until another obstruction is encountered. In general, obstructions restrict floodflows and result in overbank flows and FIGURE 7 1 Looking upstream at Mineral Creek bridge. FIGURE 8 - Mineral Creek downstream from the bridge. 9 5 i 00 YEAR oy rl NIZAS| SVN FIGURE 9 - Future flood heights just upstream of Mineral Creek bridge. 10 unpredictable areas of flooding, possible destruction of any bridges or culverts, and an increased velocity of flow immediately downstream. It is impossible to predict the degree or location of the accumulation of debris; therefore, for the purposes of this report, it was necessary to assume in the development of the flood profiles that there would be no accumulation of debris along the streams or at the bridges. Flood Damage Reduction Measures — There are no existing flood control structures, such as levees, dikes, etc., on Mineral Creek. Neither is there an existing city zoning ordinance, building code, or other regulating measure specifically for the reduction of flood damage. Other Factors and Their Impact — Because of its steep slope, Mineral Creek has high velocity flows. With these high velocities, erosion can become a major problem, and should be considered in both the siting and design of any type of structure in the flood hazard area. Flood Warning and Forecasting — The National Weather Service, Alaska River Forecast Center (RFC) located in Anchorage, is responsible for flash flood warnings for the Valdez area, including Mineral Creek. These warnings are based on current and forecast precipitation and are disseminated to the public by commercial radio and television stations and by radio station KEC-43 on a frequency of 162.55 mhz. Weather forecasts are also issued by the National Weather Service using the same communication facilities. Flood Fighting and Emergency Evacuation Plans — Although there are no formal flood fighting plans for the Valdez area, provisions for alerting area residents in time of emergency are accomplished by the Alaska Disaster Office through the City of Valdez. This office maintains communication with the National Weather Service at its control center, establishing a “flood watch” during the early stages of flood threat. Residents along the stream are warned by radio and telephone of approaching flood conditions. Subsequent flood fighting, evacuation, and rescue activities are coordinated on a city-wide basis with city officials. An evacuation plan has been established by the Alaska Disaster Office for use during tsunami warnings. This plan could be utilized for coastal storm warnings or modified for use during flooding conditions. Material Storage on the Flood Plain — Due to the size and nature of the development along Mineral Creek, there are no significant quantities of floatable 11 PAST FLOODS Summary of Historical Floods Very little information is available concerning historical floods on Mineral Creek, since records of past floods on Mineral Creek are meager and, in some cases, non-existent. Information on historical floods was obtained primarily from interviews with residents of the area. Field investigations and office computation supplemented what data were available and were used to develop the flood profiles of this report. 13 FUTURE FLOODS Floods of a magnitude equal to or larger than those that have occurred in the past will occur in the future. Larger floods have been experienced in the past on streams with similar geographical and physiographical characteristics as those found in the study area. Rainfall and runoff combinations like those which caused these floods could also occur in the Mineral Creek area. Therefore, because of the lack of hydrologic data in the study area, it was necessary, in order to determine the flooding potential of Mineral Creek, to consider storms and floods that have occurred in regions of like topography, watershed cover, and physical characteristics. Discussion of future floods in this report is limited to those that have been designated as the 100-year flood and the 500-year flood. The 500-year flood represents a reasonable upper limit of expected flood flows in the study area. The 100-year flood may reasonably be expected to occur more frequently although it will not be as severe as the infrequent 500-year flood. 100-Year Flood The 100-year flood (IRF) is defined as one that will occur once in 100 years on the average, although it could occur in any year. The peak flow of this flood was developed from a statistical analysis of stream flow and precipitation records and a study of runoff characteristics; however, limitations in Mineral Creek Basin data required analysis on a regional rather than a watershed basis. In determining the 100-year flood for Mineral Creek, statistical studies were made using flood data from U. S. Geological Survey gaging stations and precipitation data from National Weather Service climatological stations in the vicinity of Valdez, Alaska, and relating the basin characteristics. The peak flow developed for the 100-year flood in the study is 11,800 cubic feet per second (cfs). 500-Year Flood The 500-year flood is defined as a major flood that can be expected to occur once in 500-years on the average or one that has a 0.2 percent chance of occurrance in any given year. The Corps of Engineers, in cooperation with the NOAA Weather Service, has made comprehensive studies and investigations based on the past records of experienced storms and floods and has developed generalized procedures for estimating 14 the flood potential of streams. The peak discharge for the 500-year flood on Mineral Creek is 16,000 cfs. Frequency A frequency curve of peak flows was constructed on the basis of available information and computed flows of floods up to the magnitude of the 500-year flood. The frequency curve thus derived, which is available upon request, reflects the judgement of the engineers who have studied the area and are familiar with the region; however, it must be regarded as approximate and should be used with caution in connection with any planning of flood plain use. Floods larger than the 500-year flood are possible, but the combination of factors necessary to produce such a large flood would be extremely rare. Hazards of Large Floods The extent of damage caused by any flood depends on the topography of the area flooded, depth and duration of flooding, velocity of flow, rate of rise, development in the flood plain, and the effectiveness of flood fighting efforts. Floodwaters flowing at high velocity and carrying floating debris would create conditions hazardous to persons and vehicles attempting to cross flooded areas. In general, floodwater three feet deep or more and flowing at a velocity of three feet per second or more, could easily sweep an adult person off his feet, thus creating a definite danger of injury or drowning. Water lines could be ruptured by deposits of debris and the force of floodwaters, and wells could be flooded, thus creating the possibility of contaminated domestic water supplies. Isolation of areas by floodwater could create hazards in terms of medical, fire, or law enforcement emergencies. Flooded Areas and Flood Damage — Because of Valdez’s location at the mouth of Mineral Creek and at the head of Port Valdez, the city is subject to both riverine and coastal flooding. Coastal flooding can result from either storm driven waves or tsunamis. Like other communities on the Pacific Ring, Valdez is in a zone with a relatively high probability of strong earthquakes, a major cause of tsunamis. The city experienced major tsunamis during and immediately following the 1964 earthquake, which caused extensive damage to the water front. The frequency of these tsunamis, however, exceeded the 500-year event, as determined by extensive research by the 15 Corps of Engineers Waterways Experiment Station in Vicksburg, Mississippi. Coastal storms are a much more frequent event than tsunamis and, therefore, were used as the basis in determining the coastal high hazard zone. The streams in the Valdez area emerge from the mountains and flow seaward over alluvial fans to empty into Port Valdez. Normally the flow in these streams is small, but peak flows from glacial melt occur after seasonal periods of warm temperatures, warm rains in the uplands, and extended summer weather. Mineral Creek is a heavy contributor to flood problems in the Valdez area. The constant shifting of the glacially fed river has resulted in an alluvial plain composed of silt, sand, gravel and boulders, lightly consolidated and easily eroded. Flood flows in the river carry down debris and gravel and create erosion problems. The areas in Valdez that would be inundated by the 100-year and the 500-year coastal storm and floods on Mineral Creek are shown on Plates 3 through 8. The actual limits of overflow may vary somewhat from those shown on the maps because the five foot contour interval and the scale of the maps do not permit precise plotting of the flooded area boundaries. In addition, it should be emphasized that the flood hazard areas were delineated on a topographic map which was based on 1964 aerial photography. Changes have occurred since this time, especially with the development of the coastal waterfront area. These changes could affect the actual flood hazard area and should be evaluated when using the information presented in this report. Since development exists along the east side of Mineral Creek below the bridge and along the coast, flood damages will result when overbank flooding or coastal storms occur. Adequate Topographic mapping was not available for the area west of Mineral Creek, below the bridge. Therefore the extent of the 100-year or 500-year floods could not be delineated here. If any portion of this area is to be safely developed, it should be filled to a height that exceeds the water surface elevation of the 100-year flood at the corresponding location or the first finished floor of any structure should be elevated to this level. These elevations and the depths of flow can be determined from Plates 9 through 11, which show the high water profiles for Mineral Creek. Because of the steep gradient above the bridge, high velocity flow will occur during periods of flood flows. Virtually all flow would remain within channel banks, 16 and any hazard from inundation would be minor. The high velocities, however, could cause erosion damage and subsequent shifts in the channel. Obstructions — During floods, debris collecting at the bridge could decrease the capacity for flow and cause greater water depths (backwater effect) upstream of this structure. Since the occurrence and amount of debris are indeterminate factors, only the physical characteristics of the bridge were considered. Both the 100-year and 500-year floods produce water surface elevations that exceed the low chord of the bridge. Depending on the flood’s duration, and flood fighting efforts, the bridge could be washed out and destroyed. Velocities of Flow — Stream velocities during floods depend largely on the size and shape of the stream and the bed slope, all of which vary on different streams and at different locations on the same stream. During the 100-year flood, velocities of main channel flow would range up to 16.7 feet per second, but average much less. Velocities of this magnitude are sufficient to cause erosion to stream banks and roadway fills. Velocities of overbank flow would vary widely, depending upon location, but generally would be less than three feet per second. Water which is flowing at two feet per second or less would deposit debris and silt. Velocities during the 500-year flood would be slightly higher than those during the 100-year event. Rates or Rise and Duration of Flooding — Intense rainfalls that accompany severe storm fronts usually produce the floods occuring in the Valdez area. Characteristically, there is a short time lag between the time rainfall begins, and the time when flooding actually occurs. Floods generally rise rapidly to their peak discharge and then recede almost as fast. Photographs, Future Flood Heights — The levels that the 100-year and 500-year floods are expected to reach at the Mineral Creek Bridge are indicated on the following photograph. SEE PHOTOGRAPH ON PAGE 10 17 GLOSSARY Backwater. The resulting high water surface in a given stream due to a downstream obstruction or high stages in an intersecting stream. Flood. An overflow of lands not normally covered by water that are used or usable by man. Floods have two essential characteristics: The inundation of land is temporary; and the land is adjacent to and inundated by overflow from a river, stream, ocean, lake, or other body of standing water. Normally a “flood” is considered as any temporary rise in streamflow or stage, but not the ponding of surface water, that results in significant adverse effects in the vicinity. Adverse effects may include damages from overflow of land areas, temporary backwater effects in sewers and local drainage channels, creation of unsanitary conditions or other unfavorable situations by deposition of materials in stream channels during flood recessions, rise of ground water coincident with increased streamflow, and other problems. Flood Crest. The maximum stage or elevation reached by the waters of a flood at a given location. Flood Peak. The maximum instantaneous discharge of a flood at a given location. It usually occurs at or near the time of the flood crest. Flood Plain. The areas adjoining a river, stream, watercourse, ocean, lake, or other body of standing water that have been or may be covered by floodwater. Flood Profile. A graph showing the relationship of water surface elevation to location, the latter generally expressed as distance above mouth for a stream of water flowing in an open channel. It is generally drawn to show surface elevation for the crest of a specific flood, but may be prepared for conditions at a given time or stage. Flood Stage. The stage or elevation at which overflow of the natural banks of a stream or body of water begins in the reach or area in which the elevation is measured. Hydrograph. A graph showing flow values against time at a given point, usually measured in cubic feet per second. The area under the curve indicates total volume of flow. 100-Year Flood. A flood having an average frequency of occurrence in the order of once in 100 years although the flood may occur in any year. It is based on statistical analyses of streamflow records available for the watershed and analyses of rainfall and 18 runnoff characteristics in the general region of the watershed. Left Bank. The bank on the left side of a river, stream or watercourse, looking downstream. Right Bank. The bank on the right side of a river, stream, or watercourse, looking downstream. 500-Year Flood. The flood that may be expected from a severe combination of meteorological and hydrological conditions that is considered reasonably characteristic of the geographical area in which the drainage basin is located, excluding extremely rare combinations. This flood has a 0.2 percent chance of occurrance in any given year. 19 VALDEZ LEGEND 6) PLATE NUMBER 2000' 4000° APPROXIMATE SCALE IN FEET INDEX MAP FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ, ALASKA PREPARED BY THE DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA JULY 1976 LEGEND OVERFLOW LIMITS EXTENT OF 100 YEAR COASTAL FLOODING EXTENT OF 500 YEAR COASTAL FLOODING ~ — GROUND ELEVATION IN FEET SEA LEVEL DATUM, (POST QUAKE). NOTES . MAPPING BASED ON AERIAL PHOTO— GRAPHS TAKEN IN JULY 1964. . LIMITS OF OVERFLOW SHOWN MAY VARY FROM ACTUAL LOCATION ON GROUND AS EXPLAINED IN THE REPORT. . AREAS OUTSIDE THE FLOOD PLAIN MAY BE SUBJECT TO FLOODING FROM LOCAL RUNOFF. 4. CONTOUR INTERVAL IS 2 FEET. o' 400' SCALE IN FEET FLOODED AREA MAP FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ, ALASKA PREPARED BY THE DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA JULY 1976 LEGEND OVERFLOW LIMITS __ ___ EXTENT OF 100 YEAR COASTAL FLOODING EXTENT OF 500 YEAR COASTAL FLOODING ——-35~— GROUND ELEVATION IN FEET SEA LEVEL DATUM, (POST QUAKE). NOTES . MAPPING BASED ON AERIAL PHOTO— GRAPHS TAKEN IN JULY 1964. . LIMITS OF OVERFLOW SHOWN MAY VARY FROM ACTUAL LOCATION ON GROUND AS EXPLAINED IN THE REPORT. . AREAS OUTSIDE THE FLOOD PLAIN MAY BE SUBJECT TO FLOODING FROM LOCAL RUNOFF. 4. CONTOUR INTERVAL IS 2 FEET. QO 400' (Beep edhe tie SCALE IN FEET FLOODED AREA MAP FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ, ALASKA PREPARED BY THE DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA JULY 1976 LEGEND OVERFLOW LIMITS EXTENT OF 100 YEAR COASTAL FLOODING EXTENT OF 500 YEAR COASTAL FLOODING 35~ GROUND ELEVATION IN FEET SEA LEVEL DATUM, (POST QUAKE). NOTES . MAPPING BASED ON AERIAL PHOTO— GRAPHS TAKEN IN JULY 1964. . LIMITS OF OVERFLOW SHOWN MAY VARY FROM ACTUAL LOCATION ON GROUND AS EXPLAINED IN THE REPORT. . AREAS OUTSIDE THE FLOOD PLAIN MAY BE SUBJECT TO FLOODING FROM LOCAL RUNOFF. 4. CONTOUR INTERVAL IS 2 FEET. o' 400' a eae SCALE IN FEET FLOODED AREA MAP FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ, ALASKA PREPARED BY THE DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA VULY 1976 LEGEND OVERFLOW _ LIMITS — raat} oovear moon | __ __ EXTENT OF 100 YEAR COASTAL FLOODING EXTENT OF 500 YEAR COASTAL FLOODING MILES ABOVE MOUTH CROSS SECTION GROUND ELEVATION IN FEET SEA LEVEL DATUM,(POST QUAKE). NOTES . MAPPING BASED ON AERIAL PHOTO— GRAPHS TAKEN IN JULY 1964. . LIMITS OF OVERFLOW SHOWN MAY VARY FROM ACTUAL LOCATION ON GROUND AS EXPLAINED IN THE REPORT. . AREAS OUTSIDE THE FLOOD PLAIN MAY BE SUBJECT TO FLOODING FROM LOCAL RUNOFF. 4. CONTOUR INTERVAL IS 2 FEET. eo SCALE IN FEET FLOODED AREA MAP FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ, ALASKA PREPARED BY THE DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA JULY 1976 LEGEND OVERFLOW LIMITS STRRAN, 7 100 YEAR FLOOD ee agar M+l MILES ABOVE MOUTH [M=5 ] cross SECTION 7—35-—— GROUND ELEVATION IN FEET SEA LEVEL DATUM, (POST QUAKE). NOTES MAPPING BASED ON AERIAL PHOTO— GRAPHS TAKEN IN JULY 1964. LIMITS OF OVERFLOW SHOWN MAY VARY FROM ACTUAL LOCATION ON GROUND AS EXPLAINED IN THE REPORT. AREAS OUTSIDE THE FLOOD PLAIN MAY BE SUBJECT TO FLOODING FROM LOCAL RUNOFF. CONTOUR INTERVAL IS 2 FEET. SCALE IN FEET FLOODED AREA MAP FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ, ALASKA PREPARED BY THE DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA JULY 1976 500 YEAR (POST QUAKE). 100 YEAR FLOOD INFORMATION LEGEND PREPARED BY THE JULY 1976 DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS MILES ABOVE MOUTH CROSS SECTION SEA LEVEL DATUM SCALE IN FEET MINERAL CREEK VALDEZ, ALASKA ANCHORAGE, ALASKA OVERFLOW LIMITS FLOODED AREA MAP BE SUBJECT TO FLOODING FROM LOCAL FLOOD PLAIN FROM ACTUAL LOCATION ON GROUND AS RUNOFF. MAPPING BASED ON AERIAL PHOTO— EXPLAINED IN THE REPORT. GRAPHS TAKEN IN JULY 1964. 2. LIMITS OF OVERFLOW SHOWN MAY VARY 3. AREAS OUTSIDE THE FLOOD PLAIN MAY Kb Ww WwW we = 2 3S - < > Ww a Ww a 2 2 3 iid °o ! LINN) _AGNLS 4. CONTOUR INTERVAL IS 2 FEET. = e a 8 iy Ls < ui o z < wi = KE WW Wi re Zz ELEVATION a 2000 3000 4000 DISTANCE IN FEET ABOVE MOUTH OF STREAM LEGEND 500 YEAR FLOOD 100 YEAR FLOOD STREAM BED CROSS SECTION 50 YEAR FLOOD 10 YEAR FLOOD HIGH WATER PROFILES FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ , ALASKA PREPARED BY THE DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA JULY 1976 LEGEND SOO YEAR FLOOD —=<= =< 100 YEAR FLOOD QR STREAM BED [u=5 | CROSS SECTION 50 YEAR FLOOD —-—--- 10 YEAR FLOOD = - <q a a ww > iff a <q W o z < Wl = Ee WW Wi ww = z 9° < > ww J Ww HIGH WATER PROFILES FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ ALASKA PREPARED BY THE DEPARTMENT OF THE ARMY - ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA 9000 10,000 11,000 , DISTANCE IN FEET ABOVE MOUTH OF STREAM JULY 1976 Bi ATF IN = ec a pu WwW > WW 4 < uJ on z < Ww = EL uJ WW Ww = ELEVATIONS 15,000 DISTANCE 16,000 17,000 IN FEET ABOVE MOUTH OF STREAM LEGEND —_—— 500 YEAR FLOOD ——— 100 YEAR FLOOD STREAM BED CROSS SECTION 50 YEAR FLOOD 10 YEAR FLOOD ‘HIGH WATER PROFILES FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ ALASKA PREPARED BY THE DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA JULY 1976 LEGEND 500 YEAR FLOOD —--— 100 YEAR FLOOD —-—— 50 YEAR FLOOD —-—-—-— 10 YEAR FLOOD NOTE 300 400 SECTIONS TAKEN LOOKING DOWN- DISTANCE IN FEET STREAM. ADDITIONAL SECTIONS CROSS SECTION 5 NOT SHOWN BUT AVAILABLE AT DISTRICT OFFICE. = 2 - <z a I Ww > Ww 4 <a w ao 2 < WwW = EE Ww Ww we z 2 © Ee <z > w a Ww SELECTED CROSS SECTIONS FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ, ALASKA PREPARED 8Y THE DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS 150 200 ANCHORAGE, ALASKA DISTANCE IN FEET CROSS SECTION 9 JULY 1976 PLATE 12 = 5 = < a a wW > Ww a a WwW no Zz <4 WwW = eK w wW uw z z 2 S > i — Ww 100 DISTANCE IN FEET CROSS SECTION |2 600 DISTANCE IN FEET CROSS SECTION 18 800 LEGEND 500 YEAR FLOOD —--— 100 YEAR FLOOD —-—— 50 YEAR FLOOD —-—-—-— 10 YEAR FLOOD NOTE SECTIONS TAKEN LOOKING DOWN- STREAM. ADDITIONAL SECTIONS NOT SHOWN BUT AVAILABLE AT DISTRICT OFFICE. SELECTED CROSS SECTIONS FLOOD PLAIN INFORMATION MINERAL CREEK VALDEZ, ALASKA PREPARED BY THE DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS ANCHORAGE, ALASKA JULY 1976 PLATE 13