Loading (50 kb)...'
(continued)
a. There is high hazard to loss of life from large flows downstream of the dam.
b. Dam failure resulting from overtopping would significantly increase the hazard to loss of life downstream from the dam from that which would exist just before overtopping failure.
c. The spillway is not capable of passing one-half of the probable maximum flood without overtopping the dam and causing failure.
3. The above criteria are generally adequate for evaluating most non-Federal dams. However, in a few cases the increased hazard potential from overtopping and failure is so great as to result in catastrophic consequences. In such cases, the evaluation of condition 2c should utilize a flood more closely approximating the full probable maximum flood rather than one-half the flood. An example of such a situation would be a large dam immediately above a highly populated flood plain, with little likelihood of time for evacuation in the event of an emergency.
4. Conditions 2a and 2b require an approximation of housing location in relation to flooded areas. Resources available in Phase I inspections do not permit detailed surveys or time-consuming studies to develop such relationships. Therefore, rough estimates will generally be made from data obtained during the inspection and from readily available maps and drawings. Brief computer routings such as the HEC–1 dam break analysis, using available data, are recommended in marginal cases. The HEC–1, dam break version, is available on the Boeing Computer Services or may be obtained from the Hydrologic Engineering Center, Davis, California. Available resources do not permit detailed studies or investigations to establish the amount of overtopping that would cause a dam to fail, as designated in condition 2c. Professional judgment and available information will have to be used in these determinations. When detailed investigations and studies are required to make a reasonable judgment of the conditions which designate an unsafe dam, the inspection report should recommend that such studies be the responsibility of the dam owner.
5. During the inspection of a dam, consideration should be given to impacts on other dams located downstream from the project being inspected. When failure of a dam would be likely to cause failure of another dam(s) downstream, its designation as an unsafe dam could result in multiple impacts. Therefore, the information should be explicitly described in the inspection report. Such information may be vital to the priorities established by State Governors for dam improvements. Similarly, when the failure of an upstream dam (classified as unsafe) could cause failure of the dam being inspected, this information should be prominently displayed in the inspection report.
6. The criteria established in paragraph 2 for designating unsafe dams because of seriously inadequate spillways are considered reasonable and prudent. They provide a consistent bases for declaring unsafe dams and also serve as an effective compromise between the Recommended Guidelines and unduly low standards suggested by special interests and individuals unfamiliar with flood hazard potential.
7. The Hydrometeorological Branch (HMB) of the National Weather Service has reviewed some 500 experienced large storms in the United States. The purpose of the review was to ascertain the relative magnitude of experienced large storms to probable maximum precipitation (PMP) and their distribution throughout the country. Their review reveals that about 25 percent of the major storms have exceeded 50 percent of the probable maximum precipitation for one or more combinations of area and duration. In fact some storms have very closely approximated the PMP values. Exhibits C–1 thru C–5 indicate locations where experienced storms have exceeded 50 percent of the PMP.
8. There are several options to consider when selecting mitigation measures to avoid severe consequences of a dam failure from overtopping. The following measures may be required by a Governor when sufficient legal authority is available under State laws and a dam presents a serious threat to loss of life.
a. Remove the dam.
b. Increase the height of dam and/or spillway size to pass the probable maximum flood without overtopping the dam.
c. Purchase downstream land that would be adversely impacted by dam failure and restrict human occupancy.
d. Enhance the stability of the dam to permit overtopping by the probable maximum flood without failure.
e. Provide a highly reliable flood warning system (generally does not prevent damage but avoids loss of life).
Table 1_Storms With Rainfall >=150% of PMP, U.S. East of the 105th Meridian (for 10 mi\2\, 6 Hours; 200 mi\2\,
24 Hours and/or 1,000 mi\2\, 48 Hours)
----------------------------------------------------------------------------------------------------------------
Corps assignment Storm center
Storm date Index No. (if ------------------------------------ Latitude Longitude
No. available) Town State
----------------------------------------------------------------------------------------------------------------
July 26, 1819................ 1 ................ Catskill........ NY.............. 42°1 73°53
2[min] [min]
Aug. 5, 1843................. 2 ................ Concordville.... PA.............. 39°5 75°32
3[min] [min]
Sept. 10-13, 1878............ 3 OR 9-19......... Jefferson....... OH.............. 41°4 80°46
5[min] [min]
Sept. 20-24, 1882............ 4 NA 1-3.......... Paterson........ NJ.............. 40°5 74°10
5[min] [min]
June 13-17, 1886............. 5 LMV 4-27........ Alexandria...... LA.............. 31°1 92°33
9[min] [min]
June 27-July 11, 1899........ 6 GM 3-4.......... Turnersville.... TX.............. 30°5 96°32
2[min] [min]
Aug. 24-28, 1903............. 7 MR 1-10......... Woodburn........ IA.............. 40°5 93°35
7[min] [min]
Oct. 7-11, 1903.............. 8 GL 4-9.......... Paterson........ NJ.............. 40°5 74°10
5[min] [min]
July 18-23, 1909............. 9 UMV 1-11B....... Ironwood........ MI.............. 46°2 90°11
7[min] [min]
July 18-23, 1909............. 10 UMV 1-11A....... Beaulieu........ MN.............. 47°2 95°48
1[min] [min]
July 22-23, 1911............. 11 ................ Swede Home...... NB.............. 40°2 96°54
2[min] [min]
July 19-24, 1912............. 12 GL 2-29......... Merrill......... WI.............. 45°1 89°41
1[min] [min]
July 13-17, 1916............. 13 SA 2-9.......... Altapass........ NC.............. 35°3 82°01
3[min] [min]
Sept. 8-10, 1921............. 14 GM 4-12......... Taylor.......... TX.............. 30°3 97°18
5[min] [min]
Oct. 4-11, 1924.............. 15 SA 4-20......... New Smyrna...... FL.............. 29°0 80°55
7[min] [min]
Sept. 17-19, 1926............ 16 MR 4-24......... Boyden.......... IA.............. 43°1 96°00
2[min] [min]
Mar. 11-16, 1929............. 17 UMV 2-20........ Elba............ AL.............. 31°2 86°04
5[min] [min]
June 30-July 2, 1932......... 18 GM 5-1.......... State Fish TX.............. 30°0 99°07
Hatchery. 1[min] [min]
Sept. 16-17, 1932............ 19 ................ Ripogenus Dam... ME.............. 45°5 69°09
3[min] [min]
July 22-27, 193.............. 20 LMV 2-26........ Logansport...... LA.............. 31°5 94°00
8[min] [min]
Apr. 3-4 1934................ 21 SW 2-11......... Cheyenne........ OK.............. 35°3 99°40
7[min] [min]
May 30-31, 1935.............. 22 MR 3-28A........ Cherry Creek.... CO.............. 39°1 104°3
3[min] 2[min]
May 31, 1935................. 23 GM 5-20......... Woodward........ TX.............. 29°2 99°28
0[min] [min]
July 6-10, 1935.............. 24 NA 1-27......... Hector.......... NY.............. 42°3 76°53
0[min] [min]
Sept. 2-6, 1935.............. 25 SA 1-26......... Easton.......... MD.............. 38°4 76°01
6[min] [min]
Sept. 14-18, 1936............ 26 GM 5-7.......... Broome.......... TX.............. 31°4 100°5
7[min] 0[min]
June 19-20, 1939............. 27 ................ Snyder.......... TX.............. 32°4 100°5
4[min] 5[min]
July 4-5, 1939............... 28 ................ Simpson......... KY.............. 38°1 83°22
3[min] [min]
Aug. 19, 1939................ 29 NA 2-3.......... Manahawkin...... NJ.............. 39°4 74°16
2[min] [min]
June 3-4, 1940............... 30 MR 4-5.......... Grant Township.. NB.............. 42°0 96°53
1[min] [min]
Aug. 6-9, 1940............... 31 LMV 4-24........ Miller Isl...... LA.............. 29°4 92°10
5[min] [min]
Aug. 10-17, 1940............. 32 SA 5-19A........ Keysville....... VA.............. 37°0 78°30
3[min] [min]
Sept. 1, 1940................ 33 NA 2-4.......... Ewan............ NJ.............. 39°4 75°12
2[min] [min]
Sept. 2-6, 1940.............. 34 SW 2-18......... Hallet.......... OK.............. 36°1 96°36
5[min] [min]
Aug. 28-31, 1941............. 35 UMV 1-22........ Haywood......... WI.............. 46°0 91°28
0[min] [min]
Oct. 17-22, 1941............. 36 SA 5-6.......... Trenton......... FL.............. 29°4 82°57
8[min] [min]
July 17-18, 1942............. 37 OR 9-23......... Smethport....... PA.............. 41°5 78°25
0[min] [min]
Oct. 11-17, 1942............. 38 SA 1-28A........ Big Meadows..... VA.............. 38°3 78°26
1[min] [min]
May 6-12, 1943............... 39 SW 2-20......... Warner.......... OK.............. 35°2 95°18
9[min] [min]
May 12-20, 1943.............. 40 SW 2-21......... Nr. Mounds...... OK.............. 35°5 96°04
2[min] [min]
July 27-29, 1943............. 41 GM 5-21......... Devers.......... TX.............. 30°0 94°35
2[min] [min]
Aug. 4-5, 1943............... 42 OR 3-30......... Nr. Glenville... WV.............. 38°5 80°50
6[min] [min]
June 10-13, 1944............. 43 MR 6-15......... Nr. Stanton..... NB.............. 41°5 97°03
2[min] [min]
Aug. 12-15, 1946............. 44 MR 7-2A......... Cole Camp....... MO.............. 38°4 93°13
0[min] [min]
Aug. 12-16, 1946............. 45 MR 7-2B......... Nr. Collinsville IL.............. 38°4 89°59
0[min] [min]
Sept. 26-27, 1946............ 46 GM 5-24......... Nr. San Antonio. TX.............. 29°2 98°29
0[min] [min]
June 23-24, 1948............. 47 ................ Nr. Del Rio..... TX.............. 29°2 100°3
2[min] 7[min]
Sept. 3-7, 1950.............. 48 SA 5-8.......... Yankeetown...... FL.............. 29°0 82°42
3[min] [min]
June 23-28, 1954............. 49 SW 3-22......... Vic Pierce...... TX.............. 30°2 101°2
2[min] 3[min]
Aug. 17-20, 1955............. 50 NA 2-22A........ Westfield....... MA.............. 42°0 72°45
7[min] [min]
May 15-16, 1957.............. 51 ................ Hennessey....... OK.............. 36°0 97°56
2[min] [min]
June 14-15, 1957............. 52 ................ Nr. E. St. Louis IL.............. 38°3 90°24
7[min] [min]
June 23-24, 1963............. 53 ................ David City...... NB.............. 41°1 97°05
4[min] [min]
June 13-20, 1965............. 54 ................ Holly........... CO.............. 37°4 102°2
3[min] 3[min]
June 24, 1966................ 55 ................ Glenullin....... ND.............. 47°2 101°1
1[min] 9[min]
Aug. 12-13, 1966............. 56 ................ Nr. Greely...... NB.............. 41°3 98°32
3[min] [min]
Sept. 19-24, 1967............ 57 SW 3-24......... Falfurrias...... TX.............. 27°1 98°12
6[min] [min]
July 16-17, 1968............. 58 ................ Waterloo........ IA.............. 42°3 92°19
0[min] [min]
July 4-5, 1969............... 59 ................ Nr. Wooster..... OH.............. 40°5 82°00
0[min] [min]
Aug. 19-20, 1969............. 60 NA 2-3.......... Nr. Tyro........ VA.............. 37°4 79°00
9[min] [min]
June 9, 1972................. 61 ................ Rapid City...... SD.............. 44°1 103°3
2[min] 1[min]
June 19-23, 1972............. 62 ................ Zerbe........... PA.............. 40°3 76°31
7[min] [min]
July 21-22, 1972............. 63 ................ Nr. Cushing..... MN.............. 46°1 94°30
0[min] [min]
Sept. 10-12, 1972............ 64 ................ Harlan.......... IA.............. 41°4 95°15
3[min] [min]
Oct. 10-11, 1973............. 65 ................ Enid............ OK.............. 36°2 97°52
5[min] [min]
----------------------------------------------------------------------------------------------------------------
Table 2_Storms With Rainfall >=50% of PMP, U.S. West of Continental Divide (for 10 mi \2\ 6 Hours or 1,000 mi\2\
for One Duration Between 6 and 72 Hours)
----------------------------------------------------------------------------------------------------------------
Storm center Duration
Index ----------------------------------------- for
Storm date No. Latitude Longitude 1,000
Town State mi\2\
----------------------------------------------------------------------------------------------------------------
Aug. 11, 1890................... 1 Palmetto........... NV................ 37°2 117°4 ........
7[min] 2[min]
Aug. 12, 1891................... 2 Campo.............. CA................ 32°3 116°2 ........
6[min] 8[min]
Aug. 28, 1898................... 3 Ft. Mohave......... AZ................ 35°0 114°3 ........
3[min] 6[min]
Oct. 4-6, 1911.................. 4 Gladstone.......... CO................ 37°5 107°3 ........
3[min] 9[min]
Dec. 29, 1913-Jan. 3, 1914...... 5 ................... CA................ 39°5 121°2 ........
5[min] 5[min]
Feb. 17-22, 1914................ 6 Colby Ranch........ CA................ 34°1 118°0 ........
8[min] 7[min]
Feb. 20-25, 1917................ 7 ................... CA................ 37°3 119°3 ........
5[min] 6[min]
Sept. 13, 1918.................. 8 Red Bluff.......... CA................ 40°1 122°1 ........
0[min] 4[min]
Feb. 26-Mar 4, 1938............. 9 ................... CA................ 34°1 117°1 ........
4[min] 1[min]
Mar. 30-Apr. 2, 1931............ 10 ................... ID................ 46°3 114°5 24
0[min] 0[min]
Feb. 26, 1932................... 11 Big Four........... WA................ 48°0 121°3 ........
5[min] 0[min]
Nov. 21, 1933................... 12 Tatoosh Is......... WA................ 48°2 124°4 ........
3[min] 4[min]
Jan. 20-25, 1935................ 13 ................... WA................ 47°3 123°3 6
0[min] 0[min]
Jan. 20-25, 1935................ 14 ................... WA................ 47°0 122°0 72
0[min] 0[min]
Feb. 4-8, 1937.................. 15 Cyamaca Dam........ CA................ 33°0 116°3 ........
0[min] 5[min]
Dec. 9-12, 1937................. 16 ................... CA................ 38°5 122°4 ........
1[min] 3[min]
Feb. 27-Mar. 4, 1938............ 17 ................... AZ................ 34°5 111°4 12
7[min] 4[min]
Jan. 19-24, 1943................ 18 ................... CA................ 37°3 119°2 18
5[min] 5[min]
Jan. 19-24, 1943................ 19 Hoegee's Camp...... CA................ 34°1 118°0 ........
3[min] 2[min]
Jan. 30-Feb. 3, 1945............ 20 ................... CA................ 37°3 119°3 ........
5[min] 0[min]
Dec. 27, 1945................... 21 Mt. Tamalpias...... CA................ 37°5 122°3 ........
4[min] 4[min]
Nov. 13-21, 1950................ 22 ................... CA................ 36°3 118°3 24
0[min] 0[min]
Aug. 25-30, 1951................ 23 ................... AZ................ 34°0 112°2 72
7[min] 1[min]
July 19, 1955................... 24 Chiatovich Flat.... CA................ 37°4 118°1 ........
4[min] 5[min]
Aug. 16, 1958................... 25 Morgan............. UT................ 41°0 111°3 ........
3[min] 8[min]
Sept. 18, 1959.................. 26 Newton............. CA................ 40°2 122°1 ........
2[min] 2[min]
June 7-8, 1964.................. 27 Nyack Ck........... MT................ 48°3 113°3 12
0[min] 8[min]
Sept. 3-7, 1970................. 28 ................... UT................ 37°3 109°0 6
8[min] 4[min]
Sept. 3-7, 1970................. 29 ................... AZ................ 33°4 110°5 6
9[min] 6[min]
June 7, 1972.................... 30 Bakersfield........ CA................ 35°2 119°0 ........
5[min] 3[min]
Dec. 9-12, 1937................. 31 ................... CA................ 39°4 121°3 48
5[min] 0[min]
----------------------------------------------------------------------------------------------------------------
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Appendix D to §222.6—Recommended Guidelines for Safety Inspection of Dams
Department of the Army—Office of the Chief of Engineers
Preface
The recommended guidelines for the safety inspection of dams were prepared to outline principal factors to be weighed in the determination of existing or potential hazards and to define the scope of activities to be undertaken in the safety inspection of dams. The establishment of rigid criteria or standards is not intended. Safety must be evaluated in the light of peculiarities and local conditions at a particular dam and in recognition of the many factors involved, some of which may not be precisely known. This can only be done by competent, experienced engineering judgment, which the guidelines are intended to supplement and not supplant. The guidelines are intended to be flexible, and the proper flexibility must be achieved through the employment of experienced engineering personnel.
Conditions found during the investigation which do not meet guideline recommendations should be assessed by the investigator as to their import from the standpoint of the involved degree of risk. Many deviations will not compromise project safety and the investigator is expected to identify them in this manner if that is the case. Others will involve various degrees of risk, the proper evaluation of which will afford a basis for priority of subsequent attention and possible remedial action.
The guidelines present procedures for investigating and evaluating existing conditions for the purpose of identifying deficiencies and hazardous conditions. The two phases of investigation outlined in the guidelines are expected to accomplish only this and do not encompass in scope the engineering which will be required to perform the design studies for corrective modification work.
It is recognized that some States may have established or will adopt inspection criteria incongruous in some respects with these guidelines. In such instances assessments of project safety should recognize the State's requirements as well as guideline recommendations.
The guidelines were developed with the help of several Federal agencies and many State agencies, professional engineering organizations, and private engineers. In reviewing two drafts of the guidelines they have contributed many helpful suggestions. Their contributions are deeply appreciated and have made it possible to evolve a document representing a consensus of the engineering fraternity. As experience is gained with use of the guidelines, suggestions for future revisions will be generated. All such suggestions should be directed to the Chief of Engineers, U.S. Army, DAEN-CWE-D, Washington, D.C. 20314.
Recommended Guidelines for Safety Inspection of Dams
Table of Contents
Preface
Chapter 1—Introduction
Para.
1.1 Purpose.
1.2 Applicability.
1.3 Authority.
Chapter 2—General Requirements
2.1 Classification of dams.
2.1.1 Size.
2.1.2 Hazard potential.
2.2 Selection of dams to be investigated.
2.3 Technical investigations.
2.4 Qualifications of investigators.
2.5 Reports.
Chapter 3—Phase I Investigation
3.1 Purpose.
3.2 Scope.
3.3 Engineering data.
3.4 Field inspections.
3.5 Evaluation of hydraulic and hydrologic features.
3.5.1 Design data.
3.5.2 Experience data.
3.6 Evaluation of structural stability.
3.6.1 Design and construction data.
3.6.2 Operating records.
3.6.3 Post contruction changes.
3.6.4 Seismic stability.
Chapter 4—Phase II Investigation
4.1 Purpose.
4.2 Scope.
4.3 Hydraulic and hydrologic analysis.
4.3.1 Maximum water surface based on SDF peak inflow.
4.3.1.1 Peak for 100-year flood.
4.3.1.2 Peak for PMF or fraction thereof.
4.3.2 Maximum water surface based on SDF hydrograph.
4.3.3 Acceptable procedures.
4.3.4 Freeboard allowances.
4.4 Stability investigations.
4.4.1 Foundation and material investigations.
4.4.2 Stability assessment.
4.4.2.1 Seismic stability.
4.4.2.2 Clay shale foundation.
4.4.3 Embankment dams.
4.4.3.1 Liquefaction.
4.4.3.2 Shear failure.
4.4.3.3 Loading conditions.
4.4.3.4 Safety factors.
4.4.3.5 Seepage failure.
4.4.3.6 Seepage analyses.
4.4.4 Concrete dams and appurtenant structures.
4.4.4.1 Requirements for stability.
4.4.4.2 Loads.
4.4.4.3 Stresses.
4.4.4.4 Overturning.
4.4.4.5 Sliding.
4.4.4.5.1 Sliding resistance.
4.4.4.5.2 Downstream resistance.
4.4.4.5.3 Safety factor.
Chapter 5—Reports
5.1 General.
5.2 Preparation of report.
5.2.1 Phase I reports.
5.2.2 Phase II reports.
Tables
Table
1 Size classification.
2 Hazard potential classification.
3 Hydrologic evaluation guidelines.
4 Factors of safety (embankment dams).
Figures
Fig.
1 Seismic zone map of contiguous States.
2 Seismic zone map of California, Nevada and Arizona.
3 Seismic zone map of Alaska.
4 Seismic zone map of Hawaii.
5 Design envelope for Case I (Table 4).
6 Design envelope for Cases II and III (Table 4).
Appendixes
Appendix I to App. D—Engineering data
Appendix II to App. D—Inspection items
Appendix III to App. D—Pub. L. 92–367
Chapter 1—Introduction
1.1. Purpose. This document provides recommended guidelines for the inspection and evaluation of dams to determine if they constitute hazards to human life or property.
1.2. Applicability. The procedures and guidelines outlined in this document apply to the inspection and evaluation of all dams as defined in the National Dam Inspection Act, Public Law 92–367. Included in this program are all artificial barriers together with appurtenant works which impound or divert water and which (1) are twenty-five feet or more in height or (2) have an impounding capacity of fifty acre-feet or more. Not included are barriers which are six feet or less in height, regardless of storage capacity, or barriers which have a storage capacity at maximum water storage elevation of fifteen acre-feet or less regardless of height.
1.3. Authority. The Dam Inspection Act, Public Law 92–367 (Appendix III), authorized the Secretary of the Army, through the Corps of Engineers, to initiate a program of safety inspection of dams throughout the United States. The Chief of Engineers issues these guidelines pursuant to that authority.
Chapter 2—General Requirements
2.1. Classification of dams. Dams should be classified in accordance with size and hazard potential in order to formulate a priority basis for selecting dams to be included in the inspection program and also to provide compatibility between guideline requirements and involved risks. When possible the initial classifications should be based upon information listed in the National Inventory of Dams with respect to size, impoundment capacity and hazard potential. It may be necessary to reclassify dams when additional information becomes available.
2.1.1. Size. The classification for size based on the height of the dam and storage capacity should be in accordance with Table 1. The height of the dam is established with respect to the maximum storage potential measured from the natural bed of the stream or watercourse at the downstream toe of the barrier, or if it is not across a stream or watercourse, the height from the lowest elevation of the outside limit of the barrier, to the maximum water storage elevation. For the purpose of determining project size, the maximum storage elevation may be considered equal to the top of dam elevation. Size classification may be determined by either storage or height, whichever gives the larger size category.
Table 1_Size Classification
------------------------------------------------------------------------
Impoundment
Category ---------------------------------------
Storage (ac-ft) Height (ft)
------------------------------------------------------------------------
Small........................... <1,000 and >=50 <40 and >=25.
Intermediate.................... >=1,000 and >=40 and <100.
<50,000.
Large........................... >=50,000.......... >=100.
------------------------------------------------------------------------
2.1.2. Hazard Potential. The classification for potential hazards should be in accordance with Table 2. The hazards pertain to potential loss of human life or property damage in the area downstream of the dam in event of failure or misoperation of the dam or appurtenant facilities. Dams conforming to criteria for the low hazard potential category generally will be located in rural or agricultural areas where failure may damage farm buildings, limited agricultural land, or township and country roads. Significant hazard potential category structures will be those located in predominantly rural or agricultural areas where failure may damage isolated homes, secondary highways or minor railroads or cause interruption of use or service of relatively important public utilities. Dams in the high hazard potential category will be those located where failure may cause serious damage to homes, extensive agricultural, industrial and commercial facilities, important public utilities, main highways, or railroads.
Table 2_Hazard Potential Classification
----------------------------------------------------------------------------------------------------------------
Loss of life (extent of Economic loss (extent of
Category development) development)
----------------------------------------------------------------------------------------------------------------
Low.................................... None expected (No permanent Minimal (Undeveloped to occasional
structures for human habitation). structures or agriculture).
Significant............................ Few (No urban developments and no Appreciable (Notable agriculture,
more than a small number of industry or structures).
inhabitable structures).
High................................... More than few...................... Excessive (Extensive community,
industry or agriculture).
----------------------------------------------------------------------------------------------------------------
2.2. Selection of dams to be investigated. The selection of dams to be investigated should be based upon an assessment of existing developments in flood hazard areas. Those dams possessing a hazard potential classified high or significant as indicated in Table 2 should be given first and second priorities, respectively, in the inspection program. Inspection priorities within each category may be developed from a consideration of factors such as size classification and age of the dam, the population size in the downstream flood area, and potential developments anticipated in flood hazard areas.
2.3. Technical Investigations. A detailed, systematic, technical inspection and evaluation should be made of each dam selected for investigation in which the hydraulic and hydrologic capabilities, structural stability and operational adequacy of project features are analyzed and evaluated to determine if the dam constitutes a danger to human life or property. The investigation should vary in scope and completeness depending upon the availability and suitability of engineering data, the validity of design assumptions and analyses and the condition of the dam. The minimum investigation will be designated Phase I, and an in-depth investigation designated Phase II should be made where deemed necessary. Phase I investigations should consist of a visual inspection of the dam, abutments and critical appurtenant structures, and a review of readily available engineering data. It is not intended to perform costly explorations or analyses during Phase I. Phase II investigations should consist of all additional engineering investigations and analyses found necessary by results of the Phase I investigation.
2.4. Qualifications of investigators. The technical investigations should be conducted under the direction of licensed professional engineers experienced in the investigation, design, construction and operation of dams, applying the disciplines of hydrologic, hydraulic, soils and structural engineering and engineering geology. All field inspections should be conducted by qualified engineers, engineering geologists and other specialists, including experts on mechanical and electrical operation of gates and controls, knowledgeable in the investigation, design, construction and operation of dams.
Chapter 3—Phase I Investigation
3.1. Purpose. The primary purpose of the Phase I investigation program is to identify expeditiously those dams which may pose hazards to human life or property.
3.2. Scope. The Phase I investigation will develop an assessment of the general condition with respect to safety of the project based upon available data and a visual inspection, determine any need for emergency measures and conclude if additional studies, investigation and analyses are necessary and warranted. A review will be made of pertinent existing and available engineering data relative to the design, construction and operation of the dam and appurtenant structures, including electrical and mechanical operating equipment and measurements from inspection and performance instruments and devices; and a detailed systematic visual inspection will be performed of those features relating to the stability and operational adequacy of the project. Based upon findings of the review of engineering data and the visual inspection, an evaluation will be made of the general condition of the dam, including where possible the assessment of the hydraulic and hydrologic capabilities and the structural stability.
3.3. Engineering data. To the extent feasible the engineering data listed in Appendix I relating to the design, construction and operation of the dam and appurtenant structures, should be collected from existing records and reviewed to aid in evaluating the adequacy of hydraulic and hydrologic capabilities and stability of the dam. Where the necessary engineering data are unavailable, inadequate or invalid, a listing should be made of those specific additional data deemed necessary by the engineer in charge of the investigation and included in the Phase I report.
3.4. Field inspections. The field inspection of the dam, appurtenant stuctures, reservoir area, and downstream channel in the vicinity of the dam should be conducted in a systematic manner to minimize the possibility of any significant feature being overlooked. A detailed checklist should be developed and followed for each dam inspected to document the examination of each significant structural and hydraulic feature including electrical and mechanical equipment for operation of the control facilities that affect the safety of the dam.
3.4.1. Particular attention should be given to detecting evidence of leakage, erosion, seepage, slope instability, undue settlement, displacement, tilting, cracking, deterioration, and improper functioning of drains and relief wells. The adequacy and quality of maintenance and operating procedures as they pertain to the safety of the dam and operation of the control facilities should also be assessed.
3.4.2. Photographs and drawings should be used freely to record conditions in order to minimize descriptions.
3.4.3. The field inspection should include appropriate features and items, including but not limited to those listed in Appendix II, which may influence the safety of the dam or indicate potential hazards to human life or property.
3.5. Evaluation of hydraulic and hydrologic Features.
3.5.1. Design data. Original hydraulic and hydrologic design assumptions obtained from the project records should be assessed to determine their acceptability in evaluating the safety of the dam. All constraints on water control such as blocked entrances, restrictions on operation of spillway and outlet gates, inadequate energy dissipators or restrictive channel conditions, significant reduction in reservoir capacity by sediment deposits and other factors should be considered in evaluating the validity of discharge ratings, storage capacity, hydrographs, routings and regulation plans. The discharge capacity and/or storage capacity should be capable of safely handling the recommended spillway design flood for the size and hazard potential classification of the dam as indicated in Table 3. The hydraulic and hydrologic determinations for design as obtained from project records will be acceptable if conventional techniques similar to the procedures outlined in paragraph 4.3. were used in obtaining the data. When the project design flood actually used exceeds the recommended spillway design flood, from Table 3, the project design flood will be acceptable in evaluating the safety of the dam.
Table 3_Hydrologic Evaluation Guidelines
[Recommended spillway design floods]
------------------------------------------------------------------------
Spillway design
Hazard Size flood (SDF) \1\
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Low............................. Small............. 50 to 100-yr
frequency.
Intermediate...... 100-yr to \1/2\
PMF.
Large............. \1/2\ PMF to PMF.
Significant..................... Small............. 100-yr to \1/2\
PMF.
Intermediate...... \1/2\ PMF to PMF.
Large............. PMF.
High............................ Small............. \1/2\ PMF to PMF.
Intermediate...... PMF.
Large............. PMF.
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\1\ The recommended design floods in this column represent the magnitude
of the spillway design flood (SDF), which is intended to represent the
largest flood that need be considered in the evaluation of a given
project, regardless of whether a spillway is provided; i.e., a given
project should be capable of safely passing the appropriate SDF. Where
a range of SDF is indicated, the magnitude that most closely relates
to the involved risk should be selected.
1000-yr=100-Year Exceedence Interval. The flood magnitude expected to be exceeded, on the average, of once in 100 years. It may also be expressed as an exceedence frequency with a one-percent chance of being exceeded in any given year.
PMF=Probable Maximum Flood. The flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the region. The PMF is derived from probable maximum precipitation (PMP), which information is generally available from the National Weather Service, NOAA. Most Federal agencies apply reduction factors to the PMP when appropriate. Reductions may be applied because rainfall isohyetals are unlikely to conform to the exact shape of the drainage basin and/or the storm is not likely to center exactly over the drainage basin. In some cases local topography will cause changes from the generalized PMP values, therefore it may be advisable to contact Federal construction agencies to obtain the prevailing practice in specific areas.
3.5.2. Experience data. In some cases where design data are lacking, an evaluation of overtopping potential may be based on watershed characteristics and rainfall and reservoir records. An estimate of the probable maximum flood may also be developed from a conservative, generalized comparison of the drainage area size and the magnitude of recently adopted probable maximum floods for damsites in comparable hydrologic regions. Where the review of such experience data indicates that the recommended spillway design flood would not cause overtopping additional hydraulic and hydrologic determinations will be unnecessary.
3.6. Evaluation of structural stability. The Phase I evaluations of structural adequacy of project features are expected to be based principally on existing conditions as revealed by the visual inspection, together with available design and construction information and records of performance. The objectives are to determine the existence of conditions which are hazardous, or which with time might develop into safety hazards, and to formulate recommendations pertaining to the need for any additional studies, investigations, or analyses. The results of this phase of the inspection must rely very substantially upon the experience and judgment of the inspecting engineer.
3.6.1. Design and construction data. The principal design assumptions and analyses obtained from the project records should be assessed. Original design and construction records should be used judiciously, recognizing the restricted applicability of such data as material strengths and permeabilities, geological factors and construction descriptions. Original stability studies and analyses should be acceptable if conventional techniques and procedures similar to those outlined in paragraph 4.4 were employed, provided that review of operational and performance data confirm that the original design assumptions were adequately conservative. The need for such analyses where either none exist or the originals are incomplete or unsatisfactory will be determined by the inspecting engineer based upon other factors such as condition of structures, prior maximum loadings and the hazard degree of the project. Design assumptions and analyses should include all applicable loads including earthquake and indicate the structure's capability to resist overturning, sliding and overstressing with adequate factors of safety. In general seepage and stability analyses comparable to the requirements of paragraph 4.4 shouldbe on record for all dams in the high hazard category and large dams in the significant hazard category. This requirement for other dams will be subject to the opinion of the inspecting engineer. (continued)