Dam Safety

A DNR dam construction permit may be required to construct a dam, modify an existing dam, drawdown the water level, or remove a dam.

The thresholds for when a Flood Plain Permit from this department is required are outlined in 567 Iowa Administrative Code Chapter 71.3 and are listed below. The thresholds are primarily based on both dam height and water storage volumes. The height of a dam is defined as the vertical distance from the top of the dam to the lowest elevation at the downstream toe of the dam, typically the streambed.

Any dam with a height of at least 25 feet and a storage volume of at least 15 acre-ft

OR

Any dam with a storage at the top of dam of at least 50 acre-ft and a height of at least 6 feet.

OR

Any high hazard dam (see dam classifications).

Modifications to existing dams also require permitting:

Modification or alteration of any dam or appurtenant structure beyond the scope of ordinary maintenance or repair, or any change in operating procedures, if the dimensions or effects of the dam exceed the applicable thresholds above. Changes in the spillway height or dimensions of the dam or spillway are examples of modifications for which approval is required.

Estimating storage volumes: 

An acre-foot of water is equal to a one foot depth of water over 1 acre, or also equal to 43560 cubic feet.  The volume of a pond can be estimated by using the following formula:

Normal Pond Storage in acre-feet = (Surface area in acres at normal pool elevation)  X   (max depth in feet at normal pool elevation)   X 0.4

Top of Dam Pond Storage in acre-feet = (Surface area in acres at top of dam elevation)  X   (max depth in feet at top of dam elevation)   X 0.4

Example:    A 4 acre pond is proposed with a 30 foot tall dam, the normal water level is expected to be 5 feet below the top of dam, if water rose to a top of dam elevation it would cover 6 acres (areas can typically be estimated by looked at topographic maps.   We would estimate normal storage at 4 acres x 25 feet x 0.4 = 40 acre-feet.  The flooded pool at top of dam is expected to be 6 acres, we would estimate top of dam storage to be 6 acres x 30 feet x 0.4 = 72 acre-feet.  This pond would need a permit. 

To apply for a dam construction permit visit our PERMT Online tool.

If a proposed dam or repairs to an existing dam requires a permit, it must be designed by an engineer.  The engineer will have to provide signed and sealed construction plans and calculations to the Department showing that the proposed dam meets all Department criteria outlined in 567 Iowa Administrative Code Chapters 73.  Iowa DNR Technical Bulletin No. 16 is referenced by rule and submitted dam designs must meet the technical criteria outlined in this document. 

Dams should generally be designed in accordance with national published guidelines and design references such as those published by the USDA Natural Resources Conservation Service, US Bureau of Reclamation, US Army Corps of Engineers, Federal Emergency Management Agency, Federal Energy Regulatory Commission, and the National Weather Service.  The following list provides some common references for use in the design of dams. 

• Earth Dams and Reservoirs TR-60, USDA Natural Resources Conservation Service
  
• Conservation Practice Standard: Pond, Code 378, USDA Natural Resources Conservation Service
• Design of Small Dams, US Bureau of Reclamation
• Technical Manual: Conduits through Embankment Dams, FEMA
• Filters for Embankment Dams, FEMA
• NOAA Atlas 14 Precipitation Frequency Atlas of the United States
• Selected and Accommodating Inflow Design Floods for Dams, FEMA P-94, FEMA
• Hydrometeorological Report No. 51, Probable Maximum Precipitation Estimates, United States East of the 105th Meridian, NOAA

Other useful references are available from the Association of State Dam Safety Officials

All existing and proposed dams reviewed by the department shall be assigned a hazard potential. Anticipated future land and impoundment use shall be considered in the determination of hazard potential. The hazard potential shall be determined using the following criteria:

Low hazard. A dam shall be classified as “low hazard” if failure of the dam would result in no probable loss of human life, low economic losses, and low public damages.

Significant hazard. A dam shall be classified as “significant hazard” if failure of the dam would result in no probable loss of human life but may damage residential structures or industrial, commercial, or public buildings; may negatively impact important public utilities or moderately traveled roads or railroads; or may result in significant economic losses or significant public damages.

High hazard. A dam shall be classified as “high hazard” if located in an area where failure would result in probable loss of human life.

Consideration of changes affecting hazard potential. In locating the site of a dam and in obtaining easements and rights-of-way, the applicant shall consider the impacts to the hazard potential of a dam from anticipated changes in land use downstream or adjacent to the impoundment, the operation of the dam, and the potential liability of the dam owner.

Changes in hazard potential. Any future changes in downstream land use, development, impoundment use, or critical hydraulic structures shall require a reevaluation of the hazard potential of the dam. If the hazard potential of the dam changes, the dam shall be required to meet all applicable criteria for that hazard potential. This may require additional increases in spillway capacity for the dam. The owner and any other persons responsible for the construction and operation of the dam shall assume all risks for future costs to upgrade a dam in the event there is a change in hazard potential.

Dam owners are responsible for the safety of their dams, this responsibility includes keeping the dam well maintained and repairing deterioration.  Please review the following resources that explain dam owner responsibilities:

A great resource is http://www.DamOwner.org

Iowa DNR Dam Maintenance Manual ASDSO Dam Owner Liability Brochure ASDSO Resources for Dam Operations and Maintenance Dam Owner Emergency Intervention Toolbox Outlet Works Rehabilitation Guide for Small Dams

Being prepared in the event of an emergency is one of the most important responsibilities of a dam owner. The Federal Dam Safety Guidelines and the National Dam Safety Program Act, passed by Congress in 1996, and reauthorized in 2006, both consider a well-planned and coordinated EAP to be an essential responsibility of the owner.

It has long been established that having an EAP reduces the potential for loss of life downstream of dams. The dam owner can be held responsible and liable for loss of life caused by failure of the dam. Compliance with government or professional standards does not necessarily absolve an owner from liability, but it does establish a standard of care to be used by owners.

The EAP accomplishes three important objectives:

Identifies the area below the dam that would be flooded from a failure,

Establishes lines of communication for the dam owner and emergency response personnel, and

Provides for warnings and evacuations to be conducted by police, fire, and rescue teams.

As the dam owner, you are responsible for developing and maintaining the EAP and for updating it on annual basis. The importance of meeting with your local emergency management agencies at least once a year cannot be overstated. This meeting ensures that everyone understands the EAP, including pre-planned emergency procedures and inundation maps. Once completed, the EAP should be submitted to this office for review and approval.

Emergency Action Planning Resources from the Association of State Dam Safety Officials