Table 4-14 Jefferson County Dam Failures and Incidents
| Date | Dam Name | Waterway | Nearest Town | Dam Hazard Potential | Event | Failure? |
|---|---|---|---|---|---|---|
| 1952 | Clear Lake1 | Clear Creek | Georgetown | Significant | Inflow flood-hydrologic event | Yes |
| 1974 | Oberon Lake No. 1 | Ralston Creek | Arvada | Significant | Inflow flood-hydrologic event | Yes |
| February 1979 | Maple Grove | Lena Gulch | Lakewood, Wheat Ridge | High | Vandalism | Yes |
| January 1993 | Standley Lake | Big Dry Creek | Westminster | High | Reservoir-Wind Waves | No |
| April 1998 | Fairmount | Clear Creek | Wheat Ridge | High | Reservoir Incident | No |
| June 5, 2013 | Montgomery2 | Middle Fork S. Platte | Unincorporated County | High | Seepage/Internal Erosion | No |
| Sept. 12, 2013 | Chase Gulch2 | S. Platte | Golden | High | Seepage/Internal Erosion | No |
| Sept. 12, 2013 | Leyden | Chase Gulch | Arvada | High | Hydrologic/flooding | No |
| Sept. 13, 2013 | Tucker Lake – South Dam | S. Platte River | Arvada | High | Hydrologic/flooding – High Reservoir Level | No |
| May 22, 2015 | Strontia Springs | S. Platte River | Littleton | High | High Reservoir Level | No |
| June 16, 2015 | Eleven Mile Canyon2 | S. Platte | Unincorporated County | High | Hydrologic/flooding – High Reservoir Level | No |
| June 17, 2015 | Cheesman | S. Platte River | Unincorporated County | High | Hydrologic/flooding – High Reservoir Level | No |
| Date | Dam Name | Waterway | Nearest Town | Dam Hazard Potential | Event | Failure? |
|---|---|---|---|---|---|---|
| June 22, 2017 | Jefferson Lake2 | Jefferson Creek | Unincorporated County | High | Seepage/Internal Erosion – Excessive/increased Seepage | No |
Source: National Performance of Dams database, Stanford University and Association of State Dam Safety Officials Dam Incident Database 1This dam is located in Clear Creek County, but the dam failure affected the City of Golden in Jefferson County
2 These dams are located outside of Jefferson County but have the potential to impact the County.
2013 Flooding Event: In September 2013, Jefferson County and the entire Front Range experienced heavy rainfall over an eight-day period from the 11th to the 18th. The rainfall caused many dam spillways to flow in Jefferson County and the surrounding area. The dam spillway overflows mitigated structural damage to the dam but was cause for concern for some downstream communities not used to seeing spillways full of water. There was also concern that spillway flows and outlet discharges could cause flooding downstream. Per a CBS Denver report, residents living near Leyden Dam in Arvada were voluntarily evacuated on September 12th, 2013. While there was no fear of the dam failure, concern was centered around excess runoff from the spillway creating dangerous flooding on roadways. The event caused damage to Indiana Street that caused the road to be closed for several weeks for repairs.
According to the Urban Drainage and Flood Control District “A September to Remember” document the flooding exposed an 18-inch water main encased in a 36-inch concreate pipe, overtopped the upstream embankment of the Croke Canal, and resulted in shallow flooding of several homes and businesses along Leyden Creek. The document also suggests that dam improvements in 2001 likely averted a catastrophic dam failure, which would have caused severe property damage and likely cost lives.
Ralston Reservoir is owned by Denver Water and is a water supply reservoir on Ralston Creek west of Arvada. Because it has no flood storage it released water over its emergency spillway on September 12, 2013, causing significant erosion on a steep hillside near Highway 93. The spillway discharge added to the downstream watershed contribution, causing substantial channel and erosion damage before reaching Arvada/Blunn Reservoir.
For the most part, communities in Jefferson County had seen substantial investment in dam improvements prior to the 2013 floods, which paid off when the storm and its impacts arrived. Pat Dougherty, Arvada City Engineer was quoted in “A September to Remember” as saying “the story is that there is no story, because the story is what we did over the years to prevent flood damages.” Bear Creek Reservoir was constructed to protect Lakewood and Denver from flooding. A significant amount of water was impounded during 2013 and 2015 flood events. While this caused some damage to the City of Lakewood’s park facilities it likely prevented flood damage to residents and businesses downstream.
Non-jurisdictional dams or impoundments did not fare so well. These are low hazard dams that are not inspected by the State Engineer. At least two of these structures breached, both located west of Highway 93 near Leyden. One of these created severe erosion that was visible from the highway.
Probability of Future Occurrences
Section titled “Probability of Future Occurrences”There have been 13 dam incidents in Jefferson County since 1890. The methodology for calculating the probability of future occurrences is described in Section This formula evaluates that the probability of a dam failure occurring in any given year is 6%. This corresponds to a probability of future occurrences rating of occasional.
Magnitude and Severity
Section titled “Magnitude and Severity”Information from the event of record is used to calculate a magnitude and severity rating for comparison with other hazards, and to assist in assessing the overall impact of the hazard on the planning area. In some cases, the event of record represents an anticipated worst-case scenario, and in others, it is a reflection of common occurrence. There is no event of record for Jefferson County with a sufficiently detailed profile that allows for a specific discussion on the severity and magnitude of such an event.
However, the rating systems utilized in dam classification is a useful measurement for assessing the
potential magnitude and severity of a dam failure. In addition, all high-hazard dams in Colorado are required to have Emergency Action Plans (EAPs) that include predicted inundation maps for dam failure scenarios. These tools allow planners to measure the estimated worst-case or event-of-record occurrences for a dam failure.
Water released by a failed dam generates tremendous energy and can cause a flood that is catastrophic to life and property located in the inundation area (downstream). The largest four dams in terms of maximum storage are the Cheesman (79,064), Standley Lake (43,344), Chatfield (26,600), and the Ralston Dams (10,749). Chatfield and Cheesman Dams both are located on the South Platte River and a failure to either could be catastrophic for Jefferson County. A failure of the Chatfield Dam would impact the City of Littleton but would also affect unincorporated areas of the County. Unincorporated areas of the County, specifically Deckers, would also be impacted if Cheesman Dam were to fail. Failure of the Standley Lake Dam would affect Westminster and the Ralston Dam would have the greatest impact on the City of Arvada if it were to fail.
Police stations, fire stations, or health care facilities are located directly in the inundation areas, they would be indirectly impacted by the event, which would not only overwhelm local emergency response capabilities (who would be entirely consumed in the evacuation process and require additional assistance from neighboring counties to assist in both the evacuation and routine calls), but hinder response activities through the direct impacts on roads, bridges and railways.
Potential injuries caused by a failure are considered numerous and severe, and the high-hazard rating placed on the dam indicates that human fatalities are anticipated during a failure. The medical response of the County would be severely impacted or overwhelmed, though nearby jurisdictions are anticipated to help. However, the dam break would also impact Denver, Adams, and Weld Counties directly, which would stretch support resources even thinner. Based on these factors, the magnitude severity ratings for dam failure are considered critical and perhaps even catastrophic.
Climate Change Considerations
Section titled “Climate Change Considerations”The potential for climate change to affect the likelihood of dam failure has been incorporated into the 2020 Rules and Regulations for Dam Safety and Dam Construction. The climate-change related Rule is based on a state-of-the-practice regional extreme precipitation study completed in 2018 (DWR, 2018). This study determined a very high likelihood of temperature increases, resulting in increased moisture availability to extreme storms. As such, an atmospheric moisture factor of 7% is required to be added to estimates of extreme rainfall for spillway design.
Vulnerability Analysis
Section titled “Vulnerability Analysis”The impacts of a dam failure to existing development in Jefferson County could be catastrophic. Specific inundation maps and risk information are included in the dam-specific emergency action plans housed the Jefferson County Office of Emergency Management. The estimated impacts to property within the County and its municipalities from a dam failure are discussed in this vulnerability analysis. However, dam failures would potentially result in a much greater loss of life and more extensive destruction to property and infrastructure due to the potential speed of onset; greater depth, extent, and velocity of flooding; and the wider damage areas caused by the ability of dam failures to flood areas outside of mapped floodplains. For reference, high hazard dams threaten lives and property, significant hazard dams threaten property only.
In general, communities located below a dam and along a waterway are likely to be exposed to the impacts of a dam failure. The reservoirs located in the foothills and Rocky Mountains have the greatest potential impacts; this includes reservoirs located in the planning area, and reservoirs that may be located outside and upstream of the planning area but could still have impacts in Jefferson County. The dams within the planning area include the large reservoirs of Arvada, Ralston, and Standley Lake. Bear Creek Dam is primarily a flood control dam. Antero, Chatfield, Cheesman, Eleven Mile, Strontia Springs, Marston Lake, and Spinney Lake are mostly outside of the planning area on the South Platte River. The South Platte River is also the southeast border of Jefferson County. Impacts in the South Platter River Canyon could be severe if any of these dams failed, but fortunately most of this area is sparsely developed. The impacts of any of these dam failures would be great in the Denver Metropolitan Area, but
this would mostly be outside of Jefferson County. Jefferson County’s first responders would likely be heavily involved in mutual aid assistance should an event occur.
The portions of the planning area exposed to significant impacts by a dam failure are numerous. Within the planning area (the County limits) there are 30 high hazard and 11 significant hazard dams. The jurisdictions and the number of dams upstream of them are listed in and dam locations are shown in the maps in the hazard profile earlier in this section. The table notes the first jurisdiction to be impacted by dams. Note that the dams that threaten communities such as Golden in the Clear Creek watershed may also impact Wheat Ridge or other parts of the unincorporated areas.
There are numerous dams outside the county limits whose failure could have impacts inside the county. An analysis of all the watersheds that drain into Jefferson County revealed that there is one high hazard dam and one significant hazard dam whose failure could have impacts in unincorporated Jefferson County.
Table 4-15 Summary of High and Significant Hazard Dams Inside Jefferson County
Section titled “Table 4-15 Summary of High and Significant Hazard Dams Inside Jefferson County”| First Downstream Area At-Risk | # of High Hazard Dams Upstream | # of Significant Hazard Dams Upstream |
|---|---|---|
| Arvada | 8 | 1 |
| Bow Mar | 0 | 1 |
| Golden | 1 | 0 |
| Lakewood | 7 | 5 |
| Littleton | 2 | 2 |
| Morrison | 2 | 0 |
| Pleasant View | 1 | 0 |
| Unincorporated Jefferson County | 4 | 2 |
| Westminster | 4 | 0 |
| Wheat Ridge | 1 | 0 |
| Total | 30 | 11 |
Source: Jefferson County, National Inventory of Dams, NHD
General Property
Section titled “General Property”Losses from a dam failure vary based on the dam, cause of failure, warning time for impacted communities, and time of day. Potential property loss estimates are in the billions, along with multiple anticipated deaths and injuries. Inundation maps that identify anticipated flooded areas (which may not coincide with known floodplains) are produced for all high hazard dams and are contained in the Emergency Action Plan (EAP) required for each dam. However, the information contained in those plans is considered sensitive and is not widely distributed. Therefore, structures and potential loss estimates in the county are based on approximate estimates for some of the dams present countywide and are provided in and
The total properties at risk and their improvements were found by counting the number of parcels intersecting with the dam inundation extents available and summing those improvement values.
Table 4-16 Dam Inundation Risk to Properties and Population by Jurisdiction
Section titled “Table 4-16 Dam Inundation Risk to Properties and Population by Jurisdiction”| Jurisdiction | Improved Parcels | Building Count | Improved Value | Content Value | Total Value | Population |
|---|---|---|---|---|---|---|
| Arvada | 6,921 | 7,427 | $2,576,108,097 | $1,563,274,479 | $4,139,382,576 | 16,194 |
| Jurisdiction | Improved Parcels | Building Count | Improved Value | Content Value | Total Value | Population |
|---|---|---|---|---|---|---|
| Golden | 522 | 572 | $338,765,969 | $237,380,701 | $576,146,670 | 883 |
| Lakewood | 5,120 | 5,473 | $1,595,152,222 | $834,074,380 | $2,429,226,602 | 11,461 |
| Morrison | 6 | 6 | $1,851,531 | $1,031,533 | $2,883,064 | 10 |
| Wheat Ridge | 2,373 | 3,072 | $786,308,504 | $523,827,235 | $1,310,135,739 | 4,418 |
| Unincorporated | 3,083 | 3,283 | $1,246,893,571 | $779,693,472 | $2,026,587,043 | 7,387 |
| Total | 18,025 | 19,833 | $6,545,079,894 | $3,939,281,799 | $10,484,361,693 | 40,354 |
Source: Jefferson County Assessor, National Inventory of Dams, NHD
Based on the above results, Arvada has over 6,000 parcels potentially exposed to dam inundation hazards, followed by Lakewood (5,120 parcels exposed), Wheat Ridge (2,373 parcels exposed) and the unincorporated areas of the county (3,083 parcels). Further analysis shows Wheat Ridge has the greatest total percentage (21%) of parcels at risk to inundation, followed by Arvada (16%) and Lakewood (10%), refer to
summarizes parcels at risk by property type and jurisdiction. The table below indicates that Residential properties are at highest risk based on their total counts and total values, followed by Commercial, Exempt, Industrial and Mixed Use parcels. The estimated total value exposed to the available dam inundation layers amount to over $10 billion based on the available data, which again may be limited in detail and extent.
Table 4-17 Dam Inundation Effects on Parcels – Estimates by Parcel Type
Section titled “Table 4-17 Dam Inundation Effects on Parcels – Estimates by Parcel Type”| Jurisdiction | Property Type | Improved Parcels | Building Parcels | Improved Value | Content Value | Total Value | % of Parcels at Risk |
|---|---|---|---|---|---|---|---|
| Arvada | Agriculture | 3 | 3 | $265,224 | $265,224 | $530,448 | |
| Arvada | Commercial | 174 | 238 | $191,228,879 | $191,228,879 | $382,457,758 | |
| Arvada | Exempt | 23 | 26 | $35,076,293 | $35,076,293 | $70,152,586 | |
| Arvada | Industrial | 135 | 174 | $140,155,569 | $210,233,354 | $350,388,923 | |
| Arvada | Mixed Use | 56 | 75 | $43,559,327 | $43,559,327 | $87,118,654 | |
| Arvada | Residential | 6,530 | 6,911 | $2,165,822,805 | $1,082,911,403 | $3,248,734,208 | |
| Arvada | Total | 6,921 | 7,427 | $2,576,108,097 | $1,563,274,479 | $4,139,382,576 | 16% |
| Golden | Commercial | 65 | 92 | $68,831,101 | $68,831,101 | $137,662,202 | |
| Golden | Exempt | 12 | 13 | $42,696,376 | $42,696,376 | $85,392,752 | |
| Golden | Industrial | 3 | 3 | $3,441,445 | $5,162,168 | $8,603,613 | |
| Golden | Mixed Use | 48 | 54 | $17,585,066 | $17,585,066 | $35,170,132 | |
| Golden | Residential | 394 | 410 | $206,211,981 | $103,105,991 | $309,317,972 | |
| Golden | Total | 522 | 572 | $338,765,969 | $237,380,701 | $576,146,670 | 9% |
| Lakewood | Agriculture | 1 | 1 | $46,378 | $46,378 | $92,756 | |
| Lakewood | Commercial | 56 | 149 | $32,534,002 | $32,534,002 | $65,068,004 | |
| Lakewood | Exempt | 8 | 13 | $32,249,867 | $32,249,867 | $64,499,734 | |
| Lakewood | Industrial | 1 | 3 | $1,087,099 | $1,630,649 | $2,717,748 | |
| Lakewood | Mixed Use | 5 | 6 | $5,992,092 | $5,992,092 | $11,984,184 | |
| Lakewood | Residential | 5,049 | 5,301 | $1,523,242,784 | $761,621,392 | $2,284,864,176 | |
| Lakewood | Total | 5,120 | 5,473 | $1,595,152,222 | $834,074,380 | $2,429,226,602 | 10% |
| Morrison | Commercial | 1 | 1 | $211,534 | $211,534 | $423,068 | |
| Morrison | Residential | 5 | 5 | $1,639,997 | $819,999 | $2,459,996 |
| Jurisdiction | Property Type | Improved Parcels | Building Parcels | Improved Value | Content Value | Total Value | % of Parcels at Risk |
|---|---|---|---|---|---|---|---|
| Total | 6 | 6 | $1,851,531 | $1,031,533 | $2,883,064 | 4% | |
| Wheat Ridge | Agriculture | 1 | 1 | $11,380 | $11,380 | $22,760 | |
| Wheat Ridge | Commercial | 107 | 159 | $90,654,979 | $90,654,979 | $181,309,958 | |
| Wheat Ridge | Exempt | 18 | 22 | $8,220,916 | $8,220,916 | $16,441,832 | |
| Wheat Ridge | Industrial | 150 | 197 | $78,103,977 | $117,155,966 | $195,259,943 | |
| Wheat Ridge | Mixed Use | 13 | 14 | $6,250,736 | $6,250,736 | $12,501,472 | |
| Wheat Ridge | Residential | 2,084 | 2,679 | $603,066,516 | $301,533,258 | $904,599,774 | |
| Wheat Ridge | Total | 2,373 | 3,072 | $786,308,504 | $523,827,235 | $1,310,135,739 | 21% |
| Unincorporated | Agriculture | 2 | 2 | $249,352 | $249,352 | $498,704 | |
| Unincorporated | Commercial | 54 | 79 | $162,511,303 | $162,511,303 | $325,022,606 | |
| Unincorporated | Exempt | 14 | 17 | $19,518,307 | $19,518,307 | $39,036,614 | |
| Unincorporated | Industrial | 88 | 171 | $61,105,441 | $91,658,162 | $152,763,603 | |
| Unincorporated | Mixed Use | 28 | 30 | $8,003,529 | $8,003,529 | $16,007,058 | |
| Unincorporated | Residential | 2,897 | 2,984 | $995,505,639 | $497,752,820 | $1,493,258,459 | |
| Unincorporated | Total | 3,083 | 3,283 | $1,246,893,571 | $779,693,472 | $2,026,587,043 | 4% |
| Grand Total | Grand Total | 18,025 | 19,833 | $6,545,079,894 | $3,939,281,799 | $10,484,361,693 | 9% |
Source: Jefferson County Assessor, National Inventory of Dams, NHD
Each dam owner is responsible for having an EAP and inundation map for their facility. These documents are regularly updated and shared with Jefferson County Emergency Management and other governmental entities that have a direct role in emergency response. Emergency Management and response entities use the EAPs and inundation maps when developing response plans. Questions should be directed to the Emergency Management Department or the facility owner.
People
Section titled “People”Persons located downstream of a dam are at risk of a dam failure, though the level of risk can be tempered by topography, amount of water or material in the reservoir/dam/structure, and time of day of the breach. Injuries and fatalities can occur from debris, drowning, or release of sludge or other hazardous material. People in the inundation area may need to be evacuated, cared for, and possibly permanently relocated. Impacts could include hundreds of evacuations and possibly casualties, depending on the dam involved. Specific population impacts are noted in total people at risk were calculated by multiplying the average number of persons per household in Jefferson County based on Census estimates times the number of properties where the dam inundation extents were available. An estimated total of 40,354 people could be at risk countywide based on the rough estimation used, though again it is unlikely that all the parcels or properties found to overlap with dam inundation extents will be populated by the total persons estimated or actually affected by a dam failure event simultaneously. This estimate does not account for non-resident or visitor population.
Low head dams pose a risk to even the most experienced recreational users of rivers due to the difficultly to detect the dams when approaching from upstream and risk of becoming trapped in the low head dams recirculating currents. According to the Colorado Department of Natural Resources, Dam Safety Division, in recent years Colorado has experienced 1 fatality annually and there have been a total of 13 fatal incidents recorded since 1986 (Zimmer 2019). The Dam Safety Division, Low Head Dam Inventory Final Report (October 2019), notes an increase of low head dam incidents in the state directly correlated to increased recreational water usage by out-of-state tourists, new residents, and long-term residents (Zimmer 2019). As the population increases in Colorado and in Jefferson County there is the potential for increased fatalities from low head dams.
Critical Facilities and Infrastructure
Section titled “Critical Facilities and Infrastructure”A total dam failure can cause catastrophic impacts to areas downstream of the water body, including critical infrastructure. Any critical asset located under the dam in an inundation area would be susceptible to the impacts of a dam failure. Of particular risk would be roads and bridges that could be vulnerable to washouts, further complicating response and recovery by cutting off impacted areas. Based on the critical facility inventory considered in the updating of this plan and intersected with the dam inundation extents available, 316 critical facilities were found to be at risk. These at-risk facilities are listed in the tables below by jurisdiction and critical facility classification as based on the FEMA Lifeline categories (FEMA Community Lifelines, 2019).
Table 4-18 Dam Inundation Effects on Critical Facilities – Estimates by FEMA Lifeline
Section titled “Table 4-18 Dam Inundation Effects on Critical Facilities – Estimates by FEMA Lifeline”| FEMA Lifeline | Critical Facility Type | Count |
|---|---|---|
| Communications | Land Mobile Private Towers | 62 |
| Communications | Microwave Service Towers | 21 |
| Communications | Paging Transmission | 1 |
| Communications | Total | 84 |
| Energy | Electric Substation | 3 |
| Energy | Power Plant | 3 |
| Energy | Total | 6 |
| Food, Water, Shelter | Wastewater Plant | 2 |
| Food, Water, Shelter | Water Facility | 2 |
| Food, Water, Shelter | Total | 4 |
| Hazardous Material | RMP Facility | 1 |
| Hazardous Material | Tier II | 30 |
| Hazardous Material | Total | 31 |
| Health and Medical | Nursing Home | 15 |
| Health and Medical | Total | 15 |
| Safety and Security | EOC | 2 |
| Safety and Security | Fire Station | 5 |
| Safety and Security | Government Facility | 4 |
| Safety and Security | Law Enforcement | 3 |
| Safety and Security | School | 16 |
| Safety and Security | Total | 30 |
| Transportation | Bridge | 146 |
| Transportation | Total | 146 |
| Grand Total | Grand Total | 316 |
Source: National Inventory of Dams, HIFLD, CERC
Economy
Section titled “Economy”Extensive and long-lasting economic impacts could result from a major dam failure or inundation event, including the long-term loss of water in a reservoir, which may be critical for potable water needs, agriculture, or local wildlife. A major dam failure and loss of water from a key structure could bring about direct business and industry damages and potential indirect disruption of the local economy, and potentially affect important transportation routes enabling business and tourism into the county.
Historical, Cultural, and Natural Resources
Section titled “Historical, Cultural, and Natural Resources”Dam or reservoir failure effects on the environment would be similar to those caused by flooding from other causes. Water could erode stream channels and topsoil and cover the environment with debris. For the most part the environment is resilient and would be able to rebound from whatever damages occurred, though this process could take years. However, historic and cultural resources could be affected just as housing or critical infrastructures would, were a dam to fail and cause downstream inundation that could further erode surfaces or cause scouring of structural foundations.
Future Development
Section titled “Future Development”An analysis of the Year Built field in County Assessor’s Office data shows that from 2015 through 2020, 784 new structures have been built in dam inundation areas. While not a large number compared to the total structures at risk described above, it does show that new development is continuing in areas potentially at risk of dam related flooding.
It is important that the County and municipalities keep the dam failure hazard in mind when permitting new development, particularly downstream of the high and significant hazard dams present in the County. New residential development is occurring in western Arvada in the vicinity of Indiana and County Road 19, west of Standley Lake and below Welton reservoir. This development increases the number of properties, population, and infrastructure vulnerable to a dam failure, and may even change the ratings of upstream dams.
There are currently 72 low hazard dams within the County boundaries. These could become significant or high hazard dams if development occurs below them and the consequences of failure increase. Regular inspection and monitoring of dams, exercising and updating of EAPs, and rapid response to problems when detected at dams are ways to mitigate the potential impacts of these rare but potentially catastrophic events.
Overall Hazard Significance
Section titled “Overall Hazard Significance”Dam failures in Jefferson County have a large potential impact on the planning area. The geographic extent of the hazard is considered significant. The probability of future occurrences is considered occasional and the magnitude/severity for the event of record is critical or even catastrophic. The HMPC considers the hazard to have a medium overall impact rating on the County. This corresponds to the available data drawn from known occurrences; however, the potential record of event equates to an overall impact rating of high.
An event that would cause all dams in the planning area to fail is extremely unlikely. However, events which may impact the structural integrity of dams, such as earthquakes, may also be region-wide and therefore it is important to assess the planning-area wide impact of all dams, not just incident-specific occurrences. Furthermore, the failure of any high-hazard dam in the planning area is considered an event of critical magnitude and severity, and therefore, despite having a more limited geographic extent, is still a significant planning consideration.