2 Colo. Code Regs. § 402-1-7

Current through Register Vol. 48, No. 1, January 10, 2025

Rule 2 CCR 402-1-7 - Design Requirements

7.1 This Rule applies to design of new dams and alteration, modification, repair, or enlargement of existing dams. In the case of existing dams, only the pertinent sections will apply.

7.2Inflow Design Flood (IDF) for Spillway Sizing.

7.2.1Prescriptive Method. Table 7.1 provides rainfall requirements for the Inflow Design Flood (IDF) based on Hydrologic Hazard. The spillway must safely route a flood generated by Critical1 Rainfall shown in Table 7.1.

Table 7.1:Prescriptive IDF Requirements

Hydrologic Hazard	Critical1 Rainfall
Extreme	Probable Maximum Precipitation (PMP)
High	0.01% AEP
Significant	0.1% AEP
Low	1% AEP

¹Critical refers to the controlling storm duration, spatial pattern, temporal distribution and other storm variables that result in the highest maximum reservoir water surface elevation during reservoir routing.

7.2.2Consequence Estimation. Consequence estimation for Hydrologic Hazard may be determined based on total flood depth associated with an overtopping (or other plausible hydrologic failure mode) dam failure flood or based on the incremental consequences between such dam failure flood and that caused by the spillway base flood immediately prior to dam failure. The spillway size is acceptable when it meets or exceeds the IDF requirements of Table 7.1 for a given Hydrological Hazard category.

7.2.3Allowable Rainfall Estimates for developing the IDF.

7.2.3.1Probable Maximum Precipitation (PMP). The Probable Maximum Flood (PMF) shall be developed using the most current PMP estimates approved by the State Engineer.

7.2.3.2Precipitation Frequency Estimates. Frequency-based IDFs shall be developed using the most current precipitation frequency estimates approved by the State Engineer.

7.2.3.3Site-Specific Extreme Precipitation Studies (SSEPS). SSEPS may be used to determine the appropriate site-specific extreme storm precipitation (PMP or precipitation frequency estimates) for the determination of the IDF. The SSEPS must be approved by the State Engineer prior to acceptance.

7.2.4Atmospheric Moisture Factor . All rainfall depth estimates calculated by means acceptable to the State Engineer shall be multiplied by a factor of 1.07 prior to calculating runoff to account for expected increases in temperature and associated increases in atmospheric moisture availability over the 50-year period 2020 to 2070.

7.2.5Flood Frequency Analysis. Using systematic records, historical flood information, and paleoflood and botanical information, flood frequency analysis may be used to determine a required frequency flood for spillway sizing purposes. Flood frequency analysis shall follow applicable, current, published guidelines and procedures, such as Guidelines For Determining Flood Flow Frequency (ACWI Bulletin 17C, USGS, 2018).

7.2.6Hydrologic Basin Response Requirements. Rainfall-runoff modeling used to develop an IDF shall consider basin size, elevation of the basin, various soil permeabilities, various vegetative covers, and other factors related to the routing of the storm event. Snowmelt conditions and rain-on-snow events shall be considered as base flow when appropriate.

7.3Geological and Geotechnical Investigations.

7.3.1 Geological and geotechnical engineering investigations shall be conducted under the supervision of an Engineer or Geologist experienced in geotechnical or geological engineering for dams.

7.3.2Geological Site Characterization. A geological assessment of the dam and reservoir site is required for all dams classified as High or Significant Hazard. The geological assessment shall include, at a minimum:

A. Regional geologic setting;

B. Local and site geology;

C. Geologic suitability of the dam foundation, reservoir rim stability, and reservoir area leakage;

D. Regional and site seismicity;

E. All other potential geological hazards affecting the project; and

F. A site-specific geologic map based upon published records and field observations. The geologic mapping shall cover the reservoir area, dam, abutments, and the locations of all appurtenant structures.

7.3.3Subsurface Investigation Plans. A subsurface investigation plan must be approved by the State Engineer prior to mobilization for all proposed subsurface investigations. The plan shall include the following:

A. Objective(s) of the investigation and descriptions of the specific Potential Failure Modes being addressed in the investigation;

B. Names and qualifications of the investigation team including lead geotechnical or geological engineer, field engineers, and geologists;

C. Figures and description of the existing conditions;

D. Drilling, test pits, and other in-situ testing procedures; and

E. Contingency plans.

7.3.3.1 Drilling methods in all dams and dam foundations shall be chosen to minimize the risk of hydraulic fracturing or otherwise damaging the strata or formations being drilled. Drilling on or within 200 feet of existing dams is prohibited unless approved by the State Engineer.

7.3.4Subsurface Geotechnical Investigations. Subsurface investigations shall be conducted for all new dams and for all modifications to existing dams where appropriate. The subsurface investigation shall include a characterization of the geotechnical and geologic foundation conditions as follows. More extensive investigation and reporting may be required, depending on project-specific needs.

7.3.4.1High and Significant Hazard Dams. Subsurface geotechnical investigations for High and Significant Hazard dams shall require the following, at a minimum:

A. Drilling at least three borings along the dam centerline to a depth 1.5 times the height of the dam and at least 10 feet into competent bedrock;

B. Additional borings or test pits within or near the dam footprint, as required;

C. Logs of borings and test pits;

D. Standard Penetration Testing;

E. Field density tests, as appropriate;

F. Field classification of rock and soil;

G. Measurement of the water level in each drill hole; and

H. In-situ permeability tests.

7.3.4.2Low Hazard and NPH Dams. Subsurface geotechnical investigations for Low Hazard and NPH dams shall require the following, at a minimum:

A. Drilling at least three borings along the dam centerline to a depth 1.5 times the height of the dam or at least 10 feet into bedrock;

B. Field classification of rock and soil;

C. Logs of borings and test pits; and

D. Standard Penetration Testing.

7.3.4.3Spillways, Outlet Works, and Appurtenant Structures. Subsurface geotechnical investigations for spillways, outlet works and appurtenant structures shall include the following, at a minimum:

A. An evaluation of the site's suitability to accommodate the spillway or structure;

B. Field classification of soils along the alignment of the spillway or under the structure;

C. A profile of soils along the spillway channel extending to a depth of at least five feet below the bottom of the spillway;

D. Density or bearing capacity of foundation soils beneath structures except for riprapped or unlined sections of the spillway channel;

E. Erodibility of unlined natural spillway channels; and

F. For structures founded on rock, a geologic description of the foundation rock including a description of the bedding and jointing patterns.

7.3.4.4Underground Construction. Where tunneling or other underground construction is anticipated, subsurface investigation depths, orientations, methods, and testing shall be tailored to the geologic setting and details of the underground construction anticipated at each site, as recommended by a Geologist.

7.3.4.5Borrow Sources.

7.3.4.5.1 Subsurface geotechnical investigations for borrow sources shall include an evaluation of the availability, suitability, and quantity of all earth and rock materials proposed for construction of the dam as designed. Determination of the adequacy of borrow sources shall be based on field and laboratory testing.

7.3.4.5.2 Borrow areas shall be located so they do not negatively impact the dam stability or foundation seepage. Borrow areas shall be located at least 200 feet outside the dam footprint, unless an analysis approved by the State Engineer indicates a lesser distance is acceptable.

7.3.5Laboratory Testing. Laboratory testing of all proposed native and imported construction materials, and foundation and abutment materials, shall be performed to provide engineering justification for the selected design criteria.

7.3.5.1High and Significant Hazard Dams. Laboratory testing for High and Significant Hazard earth and rockfill embankment dams shall include the following tests, at a minimum:

A. Classification of all soil and rock materials based on standard index tests, including hydrometer tests as necessary for clay soils;

B. Directly measured shear strengths of all materials using test methods appropriate for the material tested;

C. Residual strength of high plasticity soils or weak rock;

D. Compressibility of all soil and rock materials;

E. Consolidation and expansion characteristics of all soil and rock materials;

F. Permeability of all in-situ and placed materials;

G. Moisture/density relationships of all materials to be compacted;

H. Potential dispersiveness and erodibility of all soils;

I. Solubility of all rock materials;

J. Density, quality, soundness, and durability of all rock materials; and

K. Corrosion potential.

7.3.5.2Low Hazard and NPH Dams. Laboratory testing for Low Hazard and No Public Hazard earth and rockfill embankment dams shall include the following tests, at a minimum:

A. Classification of all soil and rock materials based on standard index tests, and

B. Moisture/density relationships of all materials to be compacted.

7.4Embankment Dam Design.

7.4.1Foundation and Abutment Design. The dam foundation and abutments shall be analyzed and design criteria selected to meet the following requirements:

7.4.1.1 Unsuitable materials shall be removed from the dam foundation and abutments, unless appropriate analyses demonstrate the unsuitable material can be adequately treated so it will not adversely affect the safety and performance of the dam. Unsuitable materials include, but are not limited to liquefiable, dispersive, organic, expansive, and collapsible soils; slaking shales; soluble rock; clay seams in rock; and poor-quality rock.

7.4.1.2 The dam foundation geometry shall be designed to prevent the creation of low stress zones in the embankment that could cause differential settlement and cracking of the dam.

7.4.1.3 The foundation shall be treated as required to prevent deformation or instability of the dam caused by foundation movement as a result of heave, swell, rebound, settlement, or collapse.

7.4.1.4Seepage Control and Foundation Drainage Design Criteria.

7.4.1.4.1 The design shall include quantification of the anticipated seepage beneath and around the dam. Seepage through the abutments and foundation shall be minimized through adequate treatment. Foundation and abutment seepage shall be controlled through filtered exits to prevent piping and internal erosion.

7.4.1.4.2 Foundation drainage design shall be provided to reduce or control uplift pressures that would affect the stability of the dam. The efficiency of the drainage system to reduce uplift pressures under the dam shall be based upon the geology of the dam foundation. The ability to maintain the drainage system to meet the requirements assumed for the design of the dam shall be addressed.

7.4.2Embankment Design Requirements. The dam embankment shall be analyzed and designed to meet the following requirements:

7.4.2.1Crest Design.

7.4.2.1.1 The crest width shall be equal to the jurisdictional height of the dam in feet divided by 5, plus 10 feet. The maximum crest width required shall be 25 feet.

7.4.2.1.2 The crest shall have a camber sufficient to maintain the design freeboard, based on the anticipated magnitude of crest settlement. The anticipated magnitude of crest settlement shall be based on engineering analyses. In no case shall the camber be less than 0.5 feet.

7.4.2.1.3 The crest design shall include details to protect impervious cores from desiccation or frost penetration.

7.4.2.1.4 The crest shall be provided with adequate cross slopes to the upstream edge to prevent ponding and facilitate drainage.

7.4.2.1.5 Roads located on the dam crest shall have appropriate surfacing to provide a stable base that resists rutting and provides adequate traction for safety in wet conditions.

7.4.2.2Freeboard Design. Freeboard for earth and rockfill embankment dams shall be designed in accordance with Freeboard (Design Standard No. 13, Chapter 6, Reclamation, 2012), except as follows:

7.4.2.2.1 The minimum normal freeboard shall be the greater of 3 feet or the wave setup and runup generated by a sustained 100 mile per hour wind.

7.4.2.2.2 The minimum residual freeboard shall be the greater of 1 foot or a 10 percent AEP wind generated setup and runup.

7.4.2.3Embankment Zoning. Shells, cores, filters, and drains for embankment dams shall be designed using industry standards consistent with the current state of the practice.

7.4.2.3.1 All dam embankments shall be protected against internal erosion and piping with suitable filters and drains.

7.4.2.3.2 Shells shall be designed to support the core/impermeable barrier. Transition zones shall be provided as necessary to prevent migration of core material.

7.4.2.4Seepage and Internal Drainage Design. Evaluation of steady state seepage and internal drainage conditions shall be performed. The seepage and internal drainage design shall include, but not be limited to, the following:

7.4.2.4.1 Steady state seepage shall be analyzed using numerical modeling. All modeling input parameters shall be justified and clearly documented.

7.4.2.4.2 All seepage exit points shall be filter protected.

7.4.2.4.3 Drains shall collect and convey seepage to designated monitoring points.

7.4.2.4.4 The filter compatibility of the drain, transition zone, and embankment materials shall be evaluated utilizing current state of the practice methodologies. Granular filter materials must be self-healing and free of cementitious properties.

7.4.2.4.5 Drains shall consist of slotted or perforated pipe surrounded with filter-compatible free draining gravel. The gravel shall be filter-compatible with the surrounding filter material.

7.4.2.4.6 Pipes to collect and safely route seepage flows from internal filters and drains shall:

A. Be no smaller than 6 inches in diameter;

B. Accommodate internal inspection of the entire drain system;

C. Be designed to flow with a water depth no greater than 1/4 of the diameter of the pipe for the estimated seepage flows;

D. Be provided with cleanouts and access points for internal camera inspection, cleaning, and repair;

E. Be comprised of material that is non-corrodible and non-collapsible for the estimated overlying earth pressures and anticipated settlement or ground movement associated with dam construction;

F. Discharge freely into locations where flow rates can be measured, such as galleries, manholes, vaults, or headwalls;

G. Project beyond vertical walls to facilitate discharge measurement;

H. Be inspected after a maximum of 3 to 5 feet of fill placed over pipe, and again after remaining fill has been placed;

I. Be equipped with rodent screens in locations where the discharge ends of the pipes are accessible to animals; and

J. Be designed with multiple discharge points in order to isolate seepage to various sections of the dam and foundation.

7.4.2.4.7 All penetrations through embankments shall be filter protected against concentrated leakage along the conduit.

7.4.2.5Embankment Stability.

7.4.2.5.1High and Significant Hazard Dams. Stability analyses shall be performed for all High and Significant Hazard dams to demonstrate that embankments are stable during construction and under all conditions of reservoir operation.

7.4.2.5.1.1 Analyses shall represent the critical cross section(s). Where appropriate, the analyses shall consider non-circular or wedge-shaped failure surfaces, as well as circular failure surfaces.

7.4.2.5.1.2 Loading conditions selected for evaluation shall be based on the full range of conditions anticipated before, during, and after construction. Soil strength parameters, pore pressure characteristics, and target minimum factors of safety for the required loading conditions shall be selected in accordance with the principles provided in Static Stability Analysis, (Design Standards No. 13, Chapter 4, Reclamation, 2011) or Slope Stability, (EM 1110-2-1902, U.S. Army Corps of Engineers, 2003).

7.4.2.5.2Low Hazard and NPH Dams. Low Hazard or NPH dams shall be designed with upstream slopes no steeper than 3H:1V and downstream slopes no steeper than 2H:1V unless it can be demonstrated that steeper slopes will be stable.

7.4.2.6Settlement and Consolidation. All dams shall be analyzed and designed to prevent deformation or instability caused by movement as a result of settlement, consolidation, or collapse.

7.4.2.7Cracking. All dams shall be analyzed and designed to prevent the formation of cracks due to differential settlement or creation of low stress zones that could lead to hydraulic fracturing.

7.4.2.8Upstream Slope Erosion Protection. Embankments shall be protected against external erosion. Slope protection for wave action is required to be provided on the entire upstream slope of the dam, unless lesser coverage is justified based on engineering analysis and reservoir operational criteria.

7.4.2.8.1Rock Riprap. Rock riprap shall be well-graded, durable, sized to withstand design wave action, and shall be placed on a well-graded pervious sand and/or gravel bedding or acceptable geotextile fabric that is filter compatible with the underlying embankment zone.

7.4.2.8.2Soil Cement. Soil cement slope protection design and construction specifications shall be based on the principles provided in Soil Cement Slope Protection (Design Standards No. 13, Chapter 17, Reclamation, 2013).

7.4.2.9Downstream slope erosion protection. The downstream slope of earth embankment dams shall be provided with a well maintained vegetative cover to prevent surface erosion.

7.4.2.10Geosynthetics. The use of geosynthetics shall be evaluated by the State Engineer on a case-by-case basis. Geosynthetics will not be accepted where failure of the geosynthetic could jeopardize the safety of the dam. Geosynthetic materials shall be used in accordance with the manufacturers' recommendations and intended use for each product.

7.4.3Material Placement and Compaction Requirements. Material placement and compaction shall meet the minimum requirements:

7.4.3.1 Minimum compacted density for embankment materials shall be 95 percent of maximum dry density for ASTM D698 (Standard Proctor).

7.4.3.2 Impervious zones with clay fines shall be placed at close to optimum moisture content to prevent overcompacted, brittle zones.

7.4.3.3 The density for cohesionless filter and drain materials shall range between 65-and 75-percent relative density as determined by ASTM D4253 and D4254, or other method(s) approved by the State Engineer.

7.4.3.4 Construction of filters and drains shall be based on placement procedures developed through a test fill program to verify acceptable density and avoid excessive particle breakdown.

7.4.3.5 Filter and drain zones shall be constructed with sufficient thickness to prevent contamination or loss of continuity that would adversely impact the performance of these features.

7.5Concrete Dam Design Requirements.

7.5.1 For all concrete dams, the following design considerations shall be addressed and documented in the Design Report:

7.5.1.1 The crest of the dam shall have a width of not less than 5 feet.

7.5.1.2 If the crest of the dam is designed to function as the emergency spillway, it shall not be overtopped by floods more frequent than the one percent AEP flood.

7.5.1.3 Emergency spillway discharge for flows up to the inflow design flood shall not cause excessive downstream erosion of the abutments and foundation.

7.5.1.4 The design shall include provisions for installation, maintenance, and monitoring of drainage features.

7.5.1.5 A concrete mix design containing proposed aggregate properties, source of aggregate, concrete properties, and proposed cementitious contents shall be provided.

7.5.1.6 Specifications shall include provisions for placing concrete under cold weather, hot weather, and rain.

7.5.2Arch Dams. Concrete arch dams shall be designed in accordance with principles provided in Arch Dam Design (EM 1110-2-2201, U.S. Army Corps of Engineers,1994), Design Criteria for Concrete Arch and Gravity Dams (Engineering Monograph No. 19, U.S. Bureau of Reclamation, 1977), Design of Arch Dams (U.S. Bureau of Reclamation, 1977), or Arch Dams (Chapter 11, Federal Energy Regulatory Commission, 2018).

7.5.3Gravity Dams. Concrete gravity dams shall be designed in accordance with the following Rules and Gravity Dam Design (EM 1110-2-2200, U.S. Army Corps of Engineers, 1995), Design Criteria for Concrete Arch and Gravity Dams (Engineering Monograph No. 19, U.S. Bureau of Reclamation, 1977), Design of Gravity Dams (U.S. Bureau of Reclamation, 1976), or Gravity Dams (Chapter 3, Federal Energy Regulatory Commission, 2016) with the following additions:

7.5.3.1 When the design relies on the reduction of uplift pressures from dam and foundation drains, the effectiveness of the drains shall be verified and monitored for the life of the dam via the installation of piezometers.

7.5.3.2 If the seismic loading scenario shows a crack may form along the base of the dam or the foundation may sustain damage, a post-earthquake analysis will be required to show that the dam and foundation can withstand the usual and unusual loading conditions in their "damaged" state.

7.5.3.3 Dams in excess of fifty feet in height shall include a drainage gallery.

7.5.4Roller Compacted Concrete (RCC) Dams. Roller compacted dams shall be designed in accordance with the following Rules and Roller-Compacted Concrete (EM 1110-2-2006, U.S. Army Corps of Engineers, 2000) or Roller-Compacted Concrete (U.S. Bureau of Reclamation, 2017) with the following additions:

7.5.4.1 The dam design shall include adequate control of cracking in the upstream facing system and concrete mass caused by thermal shrinkage of the concrete. Crack control provisions shall include controlling excessive heat of hydration by use of fly ash and limiting in-place concrete temperature.

7.5.4.2 Adequate cold joint treatment shall be provided in the specifications to prevent formation of unbonded lift joints that could become potential paths for seepage.

7.5.4.3 Design dimensions shall be able to be constructed with conventional earthwork equipment, particularly between the upstream face of the dam and the drainage gallery, and within the chimney section.

7.5.4.4 RCC shall be protected with conventional facing concrete, or equivalent protection.

7.5.4.5Material Placement. The construction of RCC dams shall meet the following requirements:

7.5.4.5.1 An RCC test section shall be constructed outside the dam footprint at least twenty one (21) days before production placement of the RCC. The final mix design and method of construction shall be approved by the State Engineer prior to production placement.

7.5.4.5.2 The Engineer shall provide full-time observation by qualified field staff during RCC test section and production placement.

7.5.4.5.3 Locations of all cold joints shall be documented.

7.5.4.5.4 Representative RCC cores shall be taken from the completed dam to verify design strengths. RCC cores shall be 6-inch diameter.

7.5.4.5.5 The dam shall be allowed to reach design strength before initial filling of the reservoir.

7.6Seismic Design Requirements. Seismic stability shall be evaluated for all concrete dams and High and Significant Hazard embankment dams. The level of analysis required shall be commensurate with the known and anticipated site conditions and the level of effort given to developing input parameters. In general, analyses should start at a screening level and progress to more detailed analyses only when necessary. Seismic stability analyses shall be based on the principles provided in Earthquake Analyses and Design of Dams (FEMA-65, Federal Guidelines for Dam Safety, FEMA, 2005), Best Practices Chapter II-3 (Reclamation and U.S. Army Corps of Engineers, 2015), Seismic Analysis and Design (Design Standards No. 13 Chapter 13, Reclamation, 2015), Earthquake Design and Evaluation for Civil Works Projects (Engineering Regulation 1110-2-1806, U.S. Army Corps of Engineers, 2016), or Earthquake Design and Evaluation of Concrete Hydraulic Structures (Engineering Manual 1110-2-6053, U.S. Army Corps of Engineers, 2007).

7.6.1Seismic Hazard Analysis. The seismic hazards, consisting of the design earthquakes and associated ground motions, shall be determined. The seismic hazards shall be justified with due consideration to the hazard classification of the structure, regional and site-specific seismic hazard considerations, and the designated operational function of the dam.

7.6.2Dynamic Response Analysis. Analyses to predict the structural response to seismic loading are required except as described in Rule 7.6.2.1 . All seismic analyses shall be evaluated assuming loading and pore pressure conditions expected immediately prior to the earthquake. Acceptable methods for predicting structural response to seismic loading include, but are not limited to, post-earthquake stability, embankment deformation, and probabilistic analyses. Pseudostatic analyses are not an acceptable means of predicting structural response to seismic loading.

7.6.2.1 Dynamic Response Analyses are not required for embankment dams meeting all of the following conditions. The potential for embankment cracking (transverse or longitudinal), damage to appurtenant features (e.g., outlet-works tunnels), and overtopping due to seiche waves as the result of seismic activity are not addressed by these exceptions and shall be considered separately.

A. The dam and foundation materials are not subject to liquefaction and do not include sensitive clays;

B. The dam is reliably compacted to at least 95 percent of the laboratory maximum dry density, or to a relative density greater than 65 percent;

C. The slopes of the dam are 2.5H:1V or flatter, and/or the phreatic line is well below the downstream face of the embankment;

D. The peak ground acceleration (PGA) at the base of the embankment is less than or equal to 0.35g at 0.01% AEP;

E. The static stability factor of safety for all potential failure surfaces involving loss of crest elevation (i.e., slides other than shallow surficial slides) are greater than 1.5 under loading and pore-pressure conditions expected immediately prior to the earthquake;

F. The minimum freeboard is at least 3 to 5 percent of the embankment height and never less than 3 feet; and

G. There are no appurtenant features that would be harmed by small movements of the embankment, or that could create potential for internal erosion or other potential failure modes.

7.7Instrumentation and Monitoring Requirements.

7.7.1 The Owner shall submit a plan for installation of all new instrumentation and flow measurement devices for review and approval.

7.7.2Instrumentation Plan. An instrumentation plan is required and shall meet the following requirements:

7.7.2.1 All instrumentation shall be properly identified in the field to correspond to the identification of the instrumentation in the long-term monitoring plan required in Rule 13.4.

7.7.2.2 Gage rods shall be installed at all dams to accurately measure reservoir levels. The zero mark of the gage shall be aligned vertically with the invert elevation of the entrance to the outlet. The gage rod shall be located in an easily accessible location and clearly marked in feet and tenths of feet, and extend to within one foot of the crest of the dam. If the Division Engineer so requires, the gage shall be marked in hundredths of a foot.

7.7.2.3High and Significant Hazard Dams. High and Significant Hazard dams shall have the following minimum instrumentation:

7.7.2.3.1 Monuments that allow measurement of the horizontal and vertical movements of the dam, installed in accordance with industry standards and in a manner acceptable to the State Engineer. Monuments shall be located with such spacing as deemed appropriate by the Engineer and approved by the State Engineer. Control or benchmark monuments shall be placed off the dam on natural ground in areas not subject to movement.

7.7.2.3.2 Weirs, flumes, or other measuring devices to provide for monitoring of seepage through the embankment or foundation, installed in a manner acceptable to the State Engineer. Positive drainage away from all seepage monitoring devices shall be provided to prevent the device from becoming submerged.

7.7.2.3.3 Station markers at least every 100 feet along the crest of the dam.

7.7.2.3.4 Piezometers to allow monitoring of the phreatic surface within the dam or uplift pressures within the foundation, installed in accordance with industry standards and in a manner acceptable to the State Engineer. A Subsurface Investigation Plan shall be submitted for approval by the State Engineer pursuant to Rule 7.3.3 prior to new piezometer construction.

7.7.2.3.5 Where drainage galleries are provided for concrete dams, seepage measuring devices shall be provided at the appropriate locations and be accessible for making the necessary readings.

7.7.2.4Low Hazard Dams. Low Hazard dams shall have weirs, flumes or other measuring devices to provide for monitoring and measurement of seepage through the embankment or foundation, installed in a manner acceptable to the State Engineer.

7.8Spillway and Outlet Works Design Requirements.

7.8.1Spillway Design. All spillways shall be designed and constructed in a manner acceptable to the State Engineer and to meet the following criteria:

7.8.1.1 The starting water surface elevation when routing the IDF shall be the emergency spillway crest unless a lower starting water surface can be justified.

7.8.1.2 The spillway shall safely route the IDF back to the natural channel or drainage way that would exist if the dam were not built. The Owner shall possess title to the property, a right-of-way, or easement from the high water line in the reservoir to the natural channel, including the stilling basin downstream.

7.8.1.3 Log booms or other methods approved by the State Engineer shall be installed in the spillway approach where logs and other debris may block spillway flow or damage the spillway structure.

7.8.1.4 Pipe emergency spillways are not acceptable.

7.8.1.5 The design report shall include discharge tables (in cubic feet per second) for all spillways showing the discharge for each foot of head between the crest of the spillways and dam. The equation(s) used for determining the discharge shall also be included. Crest elevations of all spillways and the dam shall be clearly noted on the tables.

7.8.1.6Overtopping Protection Design. Overtopping protection for existing embankment dams may be used to safely route the IDF only where no other alternatives are feasible. The design of overtopping protection shall be based on the principles provided in Overtopping Protection for Dams (P-1015, FEMA, 2014).

7.8.1.6.1 Soil-cement shall not be used for embankment overtopping protection.

7.8.2Outlet Works Design. All outlet systems shall be designed and installed in a manner acceptable to the State Engineer and shall meet the following criteria:

7.8.2.1 Outlets shall be capable of releasing the top five feet of the reservoir capacity in five (5) days. Final outlet size should reflect consideration of seasonal reservoir inflows and consequences of releases or dam failure. The outlet shall be capable of releasing the entire reservoir in a reasonable period of time. In addition, outlets shall be capable of passing inflow to the reservoir with a minimum of ten feet of head, in order to meet the demands of downstream senior water rights and the Owner's release requirements. The minimum size required for outlet conduits and controls is 12 inches.

7.8.2.2 All outlets connected to a pipeline shall have a bypass valve near the dam that will meet the capacity criteria as defined in Rule 7.8.2.1.

7.8.2.3 Outlet conduits for all dams, except for dams with ungated outlets, shall have a guard gate installed at the upstream end of the conduit.

7.8.2.4 Intake structures for outlet works shall have trash racks.

7.8.2.5 The Design Report shall include an outlet discharge table (in cubic feet per second) showing the discharge for each foot of head between the invert of the intake structure and the crest of the dam. The equation(s) used for determining the discharge shall also be included. Elevations of all outlets and spillways shall be clearly noted on the table.

7.9Reservoir and Site Requirements.

7.9.1 The area to be submerged by the new or enlarged reservoir shall be cleared of trees and debris.

7.9.2 The dam crest and appurtenant structures shall be accessible by equipment and vehicles for emergency operations and maintenance.

7.9.3 The Owner shall demonstrate ownership or recorded easement for the following:

7.9.3.1 Footprint of the dam, appurtenant structures, and permanent access for a minimum distance of 50 feet or the height of the dam, whichever is greater, extending downstream from the toe of the dam.

7.9.3.2 Spillway discharge channels meeting the requirements of Rule 7.8.1.2.

7.9.3.3 All areas inundated by the reservoir and IDF surcharge.

7.9.4 Pipelines, utility lines, or any other construction that penetrates through the dam, abutment areas below the dam crest elevation, or that are within a distance of 50 feet or the height of the dam, whichever is greater, from either toe of the dam shall not be allowed without prior written approval by the State Engineer.

2 CCR 402-1-7

42 CR 23, December 10, 2019, effective 1/1/2020