In 1935, Congress passed Public Law 46 (PL-46) to protect land from flood damages. This effort was funded to protect structures in the floodplain, reduce sediment transport into the watershed due to turn of the century agricultural practices, and reduce large discharges into the watershed following a wildfire. At that time, agricultural land use and fire protection was much different than it is today.
The inventory of DC dams has exceeded their 50-year design life spans and are antiquated. Additionally, contemporary safety requirements make operations and maintenance financially untenable for the DC tax base without significant funding. In an effort to mitigate and manage the financial liability to Douglas County, DC Engineering is actively applying a conversion plan for the structures. View current dam projects.
Douglas County has since developed and adopted criteria requiring much more stringent adherence to preventative planning and public safety measures regarding the flood flows and the impacted areas. Cutting edge engineering analysis, exemplary governmental support of water quality, and predevelopment discharge rates all serve to achieve the same original purpose as the 1960’s dams. Additionally, fire protection districts and agricultural practices have evolved significantly in the time since the 1950’s planning which has greatly reduced the anticipated sediment loading and wildfire hazards.
The dams do not meet current criteria for watershed protection; they provide some protection but not everything required.
Douglas county has thoroughly studied and examined the purpose and need for the watershed scale structures (jurisdictional, large dams) vs a more contemporary smaller-scale based network of structures (ponds, non-jurisdictional structures) and has continued to support the criteria and method used by the vast majority of governments across the county. Small scale water quality structures are built, operated, and maintained by the associated development or user, tailor-made to the purpose and need. This is congruous with the guidance published by the Mile High Flood District, FEMA, EPA, CDPHE, USACE, and all other watershed management related entities.
Since the current best practice and criteria that call for a network of small and affordable structures like ponds and related achieve the same and better than the large scale, higher risk, much more costly single structure; Douglas County will carry the current criteria across the entire county. This places the dams into a category of performance-based redundancy or excess infrastructure.
While the dams don’t achieve criteria defined water quality, they do offer some varying degrees of flood flow attenuation. That’s the equivalent of ‘shock absorbers’ for the watershed with regard to flood flow rates. This is unnatural, non-native though and the vast majority of watershed management has moved well away from dams and artificially attenuated flood flows. In recent decades, significant steps have been made to return native, cyclic flows to channels which increase wetlands, riparian inundation, and thus all the natural cycles within the channel for flora and fauna. There was no NEPA process and analysis when these dams were built so the negative, artificial impacts to the riverine systems were not considered. Currently, the dams starve riparian and similar channel elevations of cyclic flows which does reduce a wide variety of naturally occurring ecological systems.
They do attenuate infrequent flood flows to varying degrees. They changed the natural floodplain and flood flows which many developers have in a few locations in the county have leveraged to build structures appreciating that change. Consequently, FEMA has accepted floodplain models and base flood elevations that reflect the attenuation. This creates a significant cost burden to the County in that those that appreciate the structure’s effects are not financially carrying the cost for the operation and maintenance of the structure.
A lot! As a very recent example at just one location, Douglas County pursued an NRCS grant to modernize and bring Baldwin Gulch dam up to a current dam safety standard in order to reduce the hazard classification of the structure. Originally, this grant obligated the County to pay over $1,000,000 towards a project that was approximately $4,000,000 to complete. Even before the design and construction work could begin, all the pre-project federal requirements had to be met which required $750,000 of analysis, principally NEPA. During initial technical design, federal requirements added $500,000 to the County’s share; final design wasn’t close and additional changes were anticipated driving total cost to the county towards $2,000,000. The County was also liable for all construction change expenses. This was the traditional, federal project practice for modernizing the dam and pool area structures. At no fault or cost to the County, the grant was abandoned in the interest of a more efficient, cost-effective solution that would also reclassify the structure as non-jurisdictional to avoid future and sustained cost. This County solution also respected the desire of the adjacent homeowners which were much less pleased with the federal design.
Prior to the federal grant endeavor, Douglas County had commissioned additional engineering studies to comply with state and federal requirements for a structure like that one which totaled approximately $250,000. In addition to the mandatory technical analysis, on going O&M of the structure was recurring annually.
Ever-increasing dam safety criteria. When these were built in the early 1960s, they were sound structures and deemed very safe. By today’s dam safety criteria, there’s a lot of room for improvement. In the case of Baldwin Gulch dam, engineers would have to show that the structure could safely pass the 1,000,000 year event! That’s several times more than what the County and FEMA regulate as the 100-year storm event and flows. Additionally, federal funding brings with it a very expensive and wide range of additional requirements that are not county requirements.
There are only two high-hazard dams in Douglas County, FP-P1 and FP-B1, both of which are in the process to mitigate the hazard. A high hazard classification means that if it failed during its design safety criteria required storm event (1,000,000 year storm), there may be harm to people or structures. Even an inch of water against a barn triggers the classification and requirements. The risk of harm to people or property if one failed varies a lot based on the location and structure, as well as what event causes the failure. The vast majority of DC dams do not currently pose a risk, particularly when considering the 100-year or even 500-year event. A lot of expensive engineering confirmed that fact very recently at Baldwin Gulch.
Sustained operation as-is until such time as pro-action is necessitated thru regulatory compliance, development or infrastructure encroachment, or conversion to a non-jurisdictional structure. Where FEMA has modeled a structure’s attenuation (effects) in the regulatory 100-year floodplain, the same effect will be provided following the structure’s conversion where practicable or decommissioning through other means. Douglas County will continue to work with and support the current best practices, criteria, and watershed management strategy widely implemented to date while also reducing untenable cost liabilities due to creeping dam safety criteria requirements.
Douglas County will continue to coordinate with property owners who have one of these structures and associated easement, as well as all the applicable permitting entities like the USACE, Colorado State Dam Safety, as well as DC’s own stringent criteria and requirements.
Modification to a non-jurisdictional structure classification. Typically this means reducing the height of the structure by building a new, lower spillway. This is done with some earth moving equipment and takes only a week or two generally. The results enable the structure to safely convey the 100-year storm in keeping with DC, FEMA, MHFD, and USACE requirements. This is generally less than $80,000 for a small structure. Where we must also provide attenuation, there will be reshaping work in the flood pool area and costs much more than a simple crest reduction, hence our needing to stay ahead of the development growth into inundation zones and flood modeling.
Once the structure is no longer jurisdictional, meaning the extremely prohibitive (expensive) dam safety regulations do not apply, these structures could be used for a wide variety of watershed functions. Some functions include regional water quality facilities for lower flows, or possibly specific species habitat improvement. In the event that development or other watershed-scale work necessitates the need for flood attenuation, a converted facility may be reconstructed but to the contemporary safety standard and funded, operated, and maintained entirely by the appreciating entity with the County remaining as the owner to the structure. This reconstruction is often much cheaper than working through the process with an existing structure.