REASON: The department is proposing to adopt a new version of Department Circular DEQ-8 (Circular), which provides design standards for subdivision storm water drainage facilities.  The existing Circular was last updated in 2018, and the proposed changes are necessary to update the department storm water regulations to make them more clear. A copy of the proposed Department Circular DEQ-8 (2024) may be viewed at the department's website using the following path: https://deq.mt.gov/public/lawsandregs-public.  Copies may also be obtained by contacting Hannah Monday at (406) 444-1801 or [email protected]. 

 

4. The proposed changes to the Circular are as follows:

Chapter 1: Introduction This chapter includes an applicability statement explaining the role and purpose of subdivision storm drainage review.  The proposed changes more specifically state that the Circular ensures proper drainage ways in subdivisions reviewed under the Sanitation Act rather than the current overbroad language that the Circular ensures proper drainage ways in subdivisions in Montana.  The proposed change also modifies the phrase "a reviewing authority (other than the Department)" to "an authority other than the Department" to clarify that the department is the "reviewing authority" for the Circular and that other "authorities," such as cities, may have their own more stringent requirements.  The department also proposes to change the last sentence of the existing applicability provision to clarify that the plans reviewed and approved under the Sanitation Act do not necessarily cover all the required permits needed to address storm water such as MS4 or construction storm water permitting.

            The proposed changes move the provisions currently found in Subchapter 2.7 regarding Illustrations, Spreadsheets and Examples, and Subchapter 2.6 regarding Deviations, to Subchapter 1.2 and Subchapter 1.3, respectively.  These changes are reasonably necessary to inform regulated entities at the outset of the Circular that the images, pictures, examples, and spreadsheets found in the Circular are illustrative only and that deviations from the requirements of the Circular may be obtained.  Proposed Subchapter 1.2 does not change existing language.  Proposed Subchapter 1.3 contains minor changes in language to maintain consistency with other circulars.  Proposed Subchapter 1.3 also deletes the unnecessary repetition of the provisions of ARM 17.36.601.  Reference to that rule is sufficient to incorporate its provisions into the Circular.

            A reference to ARM 17.36.101 is proposed to alert regulated entities that additional terms used in the Circular are defined in the administrative rules.  Definitions of "Building Location Area," "Limiting Layer," "Stabilized," and "Storm Water Facility" are proposed to clarify these terms' use in the Circular.  The definition of "Storm Water Facility" was added at the request of stakeholders.

 

Chapter 2: Submission of Plans This chapter outlines the documents that must be submitted in an application for storm drainage plan review, including a report, drawings, construction documents, and an operation and maintenance plan.  A proposed change to Subchapter 2.1 requires the submission of only one copy of drawings and specifications.  It is unnecessary to submit four copies at the time of application as there are often changes made before plans are finalized.  The proposed change is consistent with the procedure required for lot layout submittals described in ARM 17.36.104 and is intended to avoid wasting paper. 

Existing Subchapter 2.2 sets forth design report requirements for all applications, with an additional provision setting forth design report requirements specific to Standard Plans.  The proposed changes separate the design report requirements.  Requirements for Simplified Plans are set forth in Subchapter 2.2.1, and the requirements for Standards Plans are set forth in Subchapter 2.2.2.  This reorganization is reasonably necessary to make it easier for applicants to determine the information required in the specific type of plan submitted.  The report requirements are not substantively changed.

The department is proposing to relocate the offsite basin report requirement set forth in existing Subchapter 3.9.2.2 to proposed Subchapter 2.2.2.G.  The department is also proposing to add Subchapter 2.2.2H to alert applicants that submission of additional documents may be required by other provisions of the Circular.  These proposed changes are meant to facilitate the submission of complete applications.  No substantive changes are intended.

            In regard to Subchapter 2.3, the department proposes to add language indicating that storm water facilities must be shown on the lot layout pursuant to ARM 17.36.104.  This is not a new requirement but is proposed to facilitate submission of complete applications.

            The proposed changes to Subchapter 2.3 also add language requiring storm water facilities designed by a professional engineer to have separate stamped design drawings if design details and the professional engineer's seal and signature are not legibly shown on the lot layout.  This proposed change is consistent with the previous rule update in April 2023 that removed the expiring three-year approval requirements for storm water facility designed by a professional engineer.  Prior to this change, a storm water approval for PE designed storm water facilities that had expired (storm water facility was not built within three years) would need to re-submit an application to obtain a new approval.  The intent of the separate design drawings at that time was to avoid the need for a full COSA rewrite to reapprove the storm water facilities as they would be the sole change.  Now that the expiring approval is removed, the need for separate storm water drawings is not always necessary.  However, for detailed storm water facility designs that cannot be clearly shown on the lot layout, separate drawings will still be required as outlined in this subchapter.

The proposed changes also require the submission of one copy of the storm design drawing rather than the four copies required under the existing Circular. The reasonable necessity is the same as for the similarly proposed change to Subchapter 2.1

The department is proposing to set forth the requirements for storm water drainage drawings in proposed Subchapter 2.3.1 General Layout.  The requirements are the same as the requirements set forth in existing Subchapter 2.3, with some exceptions.  The department is proposing to delete existing Subchapter 2.3.1.L because it is redundant to proposed Subchapter 2.3 requiring applications to show existing and proposed storm water facilities on lot layout documents.  The department is proposing to delete existing Subchapter 2.3.1.M because it is redundant to proposed Subchapter 4, where it specifies which conveyance structures specifically need profile sheets. The department is proposing to add Subchapter 2.3.1.H to be consistent with ARM 17.36.104, which requires existing building and driveway locations to be shown.  The proposed addition of Subchapter 2.3.1.I enables the department to obtain information on the amount of impervious surface on a lot layout.  This information is necessary for the department to determine whether exemptions or PE design requirements apply.  This information is also used for sizing purposes and to provide guidance to future property owners as to how much impervious area was used for design of storm water facilities.  For instance, if the retention facility was sized and approved for 1,000 square feet of impervious surface, but only 500 square feet was currently built, they could build another 500 square feet of impervious surface without violating the current COSA approval.

            In Subchapter 2.3.2, the department is proposing to require an applicant to submit detailed plans for proposed storm water facilities when required by the Circular or requested by the department. 

            The department proposes to change the title of Subchapter 2.4 to "Technical Specifications" to be consistent with the language of proposed Subchapter 2.  The department also proposes to require the submission of only one copy when the technical specifications are not clearly identified on the design drawings or lot layouts.  The reasonable necessity is the same as for the similar proposed change to Subchapter 2.1.

            The proposed change to Subchapter 2.5 allows the specifications required for an operation and maintenance plan to be set forth on the lot layout.  This change reduces the submission of unnecessary documents if the operation and maintenance information can be shown on the lot layout.

Finally, the proposed changes to Subchapter 2.5 add a reference to ARM 17.36.122 to provide applicants the requirements necessary to create an acceptable easement for operation and maintenance of a storm drainage facility.

            As discussed in Chapter 1, the department is proposing to move the "Deviations" and "Illustrations, Spreadsheets and Examples" from Chapter 2 to Chapter 1.

            Proposed Subchapter 2.6 describes the requirements for computer models used in the submission of plans.  Computer-aided modeling is commonly used for complex storm water designs.  The proposed changes to the Circular move most of the requirements for computer modeling from Appendix B to Subchapter 2.6 to make it easier for an applicant to locate the computer modeling requirements.

            The department proposes to delete the list of computer models set forth in existing Appendix B.3 because the list is not complete and is not intended to limit the computer models that may be used.  The required computer modeling standards set forth in the existing Appendix B.3, Sections A and C, that discuss time of concentration is proposed to be moved to Subchapter 3.7.5, which specifically addresses time of concentration.  The required computer modeling standard set forth in Appendix B.3, Section B, that addresses rainfall intensity is proposed to be moved to Subchapter 3.6, which specifically addresses rainfall.  The relocation of these computer modeling requirements is reasonably necessary to allow an applicant to more easily identify applicable computer modeling requirements.

 

Chapter 3: Design Criteria

            Subchapter 3.1 General This subchapter sets forth general requirements of storm water designs submitted in an application for storm drainage review.  The department is proposing to delete the second and third sentences of existing Subchapter 3.1 because the sentences have been proposed in other parts of the proposed Circular.  The proposed additional text reiterates for clarity that storm drainage designs may qualify for either submission of a Simplified or Standard Plan and must include an initial storm water facility.

Subchapter 3.2 Simplified Plan The department is proposing to delete the requirement that lots qualifying for a Simplified Plan have a slope of three percent or less.  The requirement disqualifies many projects that would otherwise meet the requirements of a Simplified Plan.

When adopting the existing language in 2018, the department based the three percent maximum slope requirement on maximum erosive velocities for graded loam or graded silt used in Open Channel Hydraulics by Richard H. French.  During scoping meetings, stakeholders stated it was their experience that a three percent slope requirement is too conservative.  In response, the department considered scenarios with driveways or homes located on sites with slopes upwards of ten percent grade, and whether that parameter alone should preclude an applicant from submitting a Simplified Plan.  Lots maintain native vegetation if they are not actively landscaped since the lot must be less than 25 percent impervious.  The erosive velocities used in 2018 assumed bare earth, and overestimate erosion potential where there is still vegetation present.  Additionally, a requirement has been added to the Simplified Plan requiring all disturbed areas to be stabilized for erosion control until vegetation has been re-established.  The department agrees that lots having slopes greater than three percent may qualify for a Simplified Plan where native vegetation has been retained or where disturbed areas are stabilized pending re-establishment of vegetation.

            The department is proposing that the total number of lots that can qualify for a Simplified Plan is five or fewer.  This restriction is necessary to align the design criteria for a Simplified Plan with proposed Subchapter 2.2.A, which states that a storm water plan must be prepared by a professional engineer if the application proposes six or more lots.  Proposed subdivisions with six or more lots generally include roads requiring shared storm water facilities, drainage plans, storm water routing, and professional engineer design; therefore, due to the complexity of the storm water facilities and calculations a Standard Plan is required.

The department is proposing to slightly reword and move the language in Subchapter 3.2 of the existing Circular to proposed Subchapter 3.2.D.  This provision requires an increase in storm water runoff to be retained on the lot where it is generated.  No substantive change is intended.

The department proposes to exclude subdivisions that cross drainageways with a contributing area of more than five acres in proposed Subchapter 3.2.E.  The department determined a minimum 12-inch culvert requirement at roadway drainage crossings was not sufficient for drainage areas larger than five acres.  These drainages typically require more complex hydrologic and hydraulic calculations to ensure adequate conveyance of storm water runoff while maintaining adequate drainage ways and historic drainage patterns. These complex calculations would not be adequately addressed in a Simplified Plan and should be submitted as part of a Standard Plan.

            The department proposes to add a new method for determining the minimum retention pond size under a Simplified Plan.  The minimum size of a retention pond must be 250 cubic feet of volume per every 1,000 square feet of impervious area, with the minimum size of the retention facility being 750 cubic feet.  This methodology is consistent with that presented in Senate Bill 285 (which became in part changes to ARM 17.36.310).  The previously available Simplified Plan spreadsheet in Appendix F will still be accepted and must be used if a retention pond volume less than that required by the new methodology is proposed. This proposed change would simplify the application process for applicants, simplify the review process, and ultimately reduce review times. 

            The department proposes to require retention facilities to be designed in accordance with Chapter 5, corresponding with language in existing Subchapter 5.2 stating that retention facilities may be used in Standard and Simplified Plans.  No substantive change is proposed.

The department proposes to add a provision requiring a Simplified Plan to include a minimum 12-inch diameter culvert for roadside ditches and small onsite drainage crossings.  The department considered requiring a minimum 12-inch diameter culvert for the crossing of all driveways and presented this requirement at the Joint Engineer's Conference in November 2021.  A stakeholder provided feedback and an example that not all their projects would require such a culvert.  Therefore, the department added the language where "roadside ditches or small drainages are crossed."

The department proposes to exclude construction of roadways or driveways that cross drainages with a contributing basin of more than five acres.  These drainage crossings typically have flowrates that exceed the flow capacity of a 12-inch culvert and require more complex hydrologic and hydraulic calculations.

 

Subchapter 3.3 Standard Plan The department proposes to delete the reference to Appendix B because the acceptable hydrologic methods in existing Appendix B are moved to Subchapter 3.7 in the proposed Circular.  In addition, the department proposes new language to more clearly require a Standard Plan to either provide the post-development runoff flow rate from a proposed subdivision will not exceed the pre-development runoff or that any increase in the volume of runoff will be detained, retained, or infiltrated on the proposed subdivision.  No substantive changes are intended.

The department proposes to move existing Subchapter 3.3.A and 3.3.C into the body of the text.  In regard to Subchapter 3.3.A, the department proposes to delete "to an adjoining property" because proposed new language expressly requires increased runoff on the proposed subdivision to be "detained, retained, or infiltrated." The department proposes to delete Subchapter 3.3.B because it is redundant to restrictions set forth in Chapter 4.  No substantive changes are proposed.

The department proposes to add a requirement that the applicant must submit at the request of the department calculations supporting a narrative demonstration that buildings and drainfields will not be inundated during a 100-year storm event.  This allows the department to have sufficient reliable information to conduct the storm drainage review.  This requirement is being transferred from Subchapter 3.9.2.2 of the existing Circular.

 

            Subchapter 3.4 Initial Storm Water Facility When the existing Circular was adopted, the department did not intend to allow landscaping or the initial abstraction in the SCS (Soil Conservation Service) method to be considered in achieving the initial storm water facility volume.  The initial storm water facility is strictly designed for impervious surface runoff flows only, without any regard for surfaces that may absorb some runoff, therefore reducing the required volume. Because the existing Circular is silent on the matter and does not specifically state that runoff volumes cannot be reduced to the initial storm water facility due to this runoff absorption, some applicants believe these methods are allowed.  Therefore, the department is proposing to add specific language stating that these methods are not allowed.

            In addition, the department is proposing that a Standard Plan may allow best management practices to capture the initial storm water facility volume. The department was contemplating integrating BMPs into the Circular and received feedback from a stakeholder in 2021 requesting this option.  The department reviewed the Montana Post-Construction Storm Water BMP Design Guidance Manual (BMP Manual) to assess how to integrate it with storm drainage reviews and identify which BMPs meet storm drainage design criteria.  The BMPs must be designed in accordance with guidance provided in the BMP Manual and remove 80 percent of total suspended solids. This proposed change is consistent with the department's review of plans submitted by Municipal Separate Storm Sewer Systems throughout Montana.

 

Subchapter 3.5 Pre- and Post-Development Conditions By deleting the reference to ARM 17.36.112 and addressing requirements for areas with and without existing certificate of subdivision approval in separate paragraphs, the proposed changes more clearly state how pre-development runoff is to be calculated for the respective areas.  For areas without an existing certificate of subdivision approval, the pre-development runoff must be based on natural/undeveloped conditions and may include highways and roadways that existed prior to the application for subdivision review.  For areas with an existing certificate of subdivision approval, the pre-development runoff must be based on the previously approved use of the parcel.

The proposed changes to this subchapter also provide a method of estimating post-development conditions when the extent and location of post-development areas are not known.  The existing Circular provides little guidance as to what needs to be submitted to the department for approval under these circumstances.  The inclusion of this change and the criteria used to estimate the post-development conditions was developed by the department in discussion with stakeholders.

 

Subchapter 3.6 Rainfall In the identification of acceptable sources of rainfall data used in storm water drainage designs, the department is proposing to delete the rainfall data currently set forth in Appendix A and add a reference to the Montana Department of Transportation (MDT) Hydraulics Manual, Chapter 9, Appendix B-2022.  MDT's manual provides rainfall data that was developed through frequency analysis of rainfall depths and intensities for various storms specific to the state of Montana using data from the National Oceanic and Atmospheric Association National Climatic Data Center.  Adding this reference allows state agencies to be consistent in conducting rainfall analysis.  

            The department is also proposing to add requirements for determining rainfall intensity when calculating peak runoff rates.  The existing Circular allows use of an IDF curve for determining rainfall intensity in Subchapter 3.6.  The proposed changes add a reference to the MDT Hydraulics Manual to provide applicants with a readily available source of rainfall information that may be used in calculating an Intensity-Duration-Frequency curve.  Adding this reference allows state agencies to be consistent in conducting rainfall analysis. 

The proposed changes also add language indicating that the time of concentration must be determined in accordance with Subchapter 3.7.5, which states the minimum time of concentration is five minutes.  The proposed language is consistent with Appendix B.1.1.B of the existing Circular.  No substantive change is intended.

           

Subchapter 3.7 Acceptable Methods Subchapter 3.7 presents hydraulic and hydrologic methods that may be used to perform storm water design and analysis in applications for storm drainage review. 

 

Subchapter 3.7.1 Rational Method The department proposes to relocate the requirements for using the Rational Method set forth in Appendix B.1.1 of the existing Circular to Subchapter 3.7.1 of the proposed Circular.  Relocating the requirements to the main body of the Circular makes it easier for applicants to locate the requirements.

The Rational Method can be used to calculate both the peak runoff rate from a storm and the volume of runoff from a storm.  Accordingly, the proposed provisions of Subchapter 3.7.1 distinguish the equations that must be used when the Rational Method is used to calculate peak runoff and volume of runoff.

In regard to using the Rational Method to determine peak runoff, the department proposes to require intensity to be calculated at the time of concentration of the drainage area using an Intensity-Duration-Frequency (IDF) curve.  The existing Circular allows rainfall intensity to be determined either by an IDF curve or the tables set forth in Appendix A.  Use of the tables in Appendix A, however, may overestimate runoff.  By requiring rainfall intensity to be calculated under the Rational Method using an IDF curve, runoff values are more accurately calculated.  Calculating the time of concentration of the drainage basin is currently required for developing an IDF curve for the Rational Method in Appendix B.1.1.B of the existing Circular.

            The department also proposes to direct applicants to IDF curves developed by the Montana Department of Transportation.  This change is consistent with proposed changes to Subchapter 3.6 (Rainfall).  The department proposes to move the language regarding the minimum time of concentration to Subchapter 3.7.5, which describes how the time of concentration is applied.  The proposed changes also add language requiring time of concentration under the Rational Method to be calculated in accordance with the technique presented in proposed Subchapter 3.7.5.

 

Subchapter 3.7.2 Modified Rational Method The department proposes to relocate the requirements for using the Modified Rational Method set forth in Appendix B.1.1 of the existing Circular to Subchapter 3.7.2 of the proposed Circular.  Relocating the requirements to the main body of the Circular makes it easier for applicants to locate the requirements.

            Appendix B.1.1 of the existing Circular presents a technique to calculate a storm runoff volume using the modified rational method.  Appendices F and G of the existing Circular contain sample spreadsheets for use in calculating volume and flow using the rational and modified rational method.  These spreadsheets were set up to use the rational method with the time of concentration and the 24-hour storm event.  The modified rational method is not presented in either of these existing spreadsheets.  The only place in the existing Circular that uses the modified rational method is Appendix L.5 to create a hydrograph to size a detention facility. The existing Circular provides a brief description of the modified rational method.  The department proposes to provide a more detailed description of the modified rational method, along with figures to avoid confusion regarding application of the method.  The existing Circular requires a minimum storm event duration of 60 minutes, with a peak flow rate determined at the time of concentration for application of the modified rational method.  This was presented to provide a more straightforward and approachable use with readily available one-hour rainfall depths.  Through administrating this standard and discussions with stakeholders, the department has found that the presentation of the modified rational method in the current version is confusing and inconsistent with typical application of the method across the industry.  The stakeholders requested the department update the presentation of the modified rational method in the Circular to be more consistent with the industry-accepted application of the method.  The proposed method is adapted from the State of Virginia Storm Water Management Handbook, a commonly referenced industry standard for application of the modified rational method. 

 

Subchapter 3.7.3 Urban Hydrology for Small Watersheds Technical Release 55 (TR-55) or SCS Curve Number Method The department proposes to relocate the requirements for using the SCS Curve Number Method set forth in Appendix B.1.2 of the existing Circular to Subchapter 3.7.3 of the proposed Circular. Relocating the requirements to the main body of the Circular makes it easier for applicants to locate the requirements.

The department is proposing to make minor changes in language to make the discussion of the SCS Curve Number Method consistent with the application of the technique presented in the USDA Technical Release-55 (TR-55) Urban Hydrology for Small Watersheds.  Appendix B.1.2.D is proposed to be deleted because it is inconsistent with the use of the SCS Curve Number Method.  The TR-55 Bulletin presents the method using the 24-hour rainfall depth to calculate the depth of storm water runoff.  The time of concentration is still required for the TR-55 to calculate the peak runoff rate. 

The department proposes to add Subchapter 3.7.3.D in the proposed Circular to provide requirements for using the SCS Curve Number Method when the initial abstraction is greater than 50% of the precipitation storm depth.  The existing Circular does not clearly address how to handle initial abstractions over 50% and applicants' use of the method has been inconsistent and at times incorrect.  The proposed additional language directs applicants to use the method consistent with the TR-55 Bulletin.

The department also proposes to add a table of typical curve number values that can be used when applying the SCS Curve Number Method.  Language is proposed to allow alternative curve numbers with appropriate documentation of the number's applicability to the site. Including a table of curve numbers is reasonably necessary to increase the consistency of application of this method by applicants. The values in the table are consistent with those presented in the TR-55 Bulletin.

 

Subchapter 3.7.4 Storage-Indication Routing The department proposes to relocate the requirements for Storage Indication Routing set forth in Appendix B.1.3 of the existing Circular to Subchapter 3.7.4 of the proposed Circular.  Relocating the requirements to the main body of the Circular makes it easier for applicants to locate the requirements.

The department proposes to change the wording of the first sentence of the existing Appendix B.1.3. to indicate that storage-indication routing is used to calculate detention or infiltration facility volumes rather than to "analyze storage detention practices," which is unclear in the existing Circular.  This modification more accurately describes the use of Storage-Indication Routing.  Additional language is proposed after the routing equation to require inflow values and outflow values used in the equation to be based on specified information.  These proposed changes are needed to improve consistency in the application of this method by applicants.  The department proposes to require the maximum time step used in the routing method to be less than the time of concentration.  Use of these shorter time steps in this method will result in more accurate results as they better reflect the specific basin characteristics.

 

Subchapter 3.7.5 Time of Concentration The department proposes to relocate the TR-55 equations used to calculate the Time of Concentration for a drainage basin set forth in Appendix B.1.3 of the existing Circular to Subchapter 3.7.5.  The provisions regarding sheet flow in existing Appendix B.1.3 are relocated to proposed Subchapter 3.7.5.1, and the provisions regarding shallow flow in existing Appendix B.1.3 are relocated to proposed Subchapter 3.7.5.2.  Relocating the requirements to the main body of the Circular makes it easier for applicants to locate the requirements.  No substantive changes are proposed.

            The department is proposing to delete Subchapter 3.7 Acceptable Methods, Subchapter 3.8 Storm Water Volume, and Subchapter 3.9 Peak flow that are set forth in the existing Circular.  These subchapters are duplicative of Subchapter 3.2 Simplified Plan, Subchapter 3.3 Standard Plan, and Subchapter 3.4 Initial Storm Water Facility set forth in the proposed Circular.

 

Chapter 4 Conveyance Structures This chapter outlines the methods used in standard engineering practices to determine the capacity flow rate of the three most common types of conveyance structures (open channels, pipes, and culverts).

 

Subchapter 4.1 General Standards for conveyance structures are addressed in Chapter 4 The first paragraph of the existing Chapter 4 defines conveyance structures, provides peak flow design requirements, and directs applicants to where they can find examples of the calculations used to determine conveyance structure capacity.  The definition of a conveyance structure is provided in Subchapter 1.2.4, and specific examples of the main three conveyance structures are retained (Open Channel, Storm Sewer, and Culverts).

The department has added language to clarify that "drainfields" usually mean "soil absorption systems."

The department has also added specific requirements for conveyance structures during a ten-year storm event to clarify what is meant by "not overtopping."  Stakeholders noted that driveways that only serve a small population (under three single-family homes) should be allowed to have minor driveway overtopping since the risk to the general public is small.  Two inches of water is considered to be small enough as to not provide a significant hazard for an automobile or emergency vehicles.  Driveways that serve three or more single-family homes are considered to have enough of a population where the risks increase, as there are greater chances of these inundated roadways being crossed by traffic or emergency vehicles.  Not allowing any over-topping of culvert crossings of roadways is a current requirement and does not change.  Two inches of flow depth at cross-pans or valley pans at roadway intersections are allowed to overtop by two inches since it is a small area of water crossing.  Parking lots are allowed a flow depth of up to two inches since they are not roadways, and flow depth and spread from roadside ditches is allowed as long as there is a dry ten-foot-wide travel lane at the crown of the road to allow for emergency vehicle traffic.  As noted above, two inches is considered to be small enough to not pose a significant hazard to vehicles, yet allow some flexibility in the design of storm water drainage facilities. 

 

Subchapter 4.2 Open Channel The department proposes to require designs to include the slope or profile of a typical reach of an open channel.  Inclusion of this variable is reasonably necessary for an applicant to calculate the flow rate in an open channel.

 

Subchapter 4.3 Storm Sewer The proposed changes restate in narrative form the storm water design requirements set forth in Subchapter 4.3.A and C of the existing Circular.  Subchapter 4.3.D of the existing Circular is proposed to be deleted as this is not a feasible design for storm water management. The proposed changes add an additional requirement that applicants calculate the capacities of storm sewer inlets (drop inlets, grates, etc.).  The department needs this information to determine whether the inlet is adequately sized so that the storm drainage system functions properly.  Existing Subchapter 4.3.A erroneously refers to "volume" rather than "velocity" calculations.  The proposed narrative corrects this error.  The department is proposing to delete the design requirement set forth in Subchapter 4.3.B of the existing Circular because it is redundant to information required in proposed Subchapter 2.4.2.B.

The proposed changes relocate "Table 1 Minimum Grades to Ensure 3 fps, for Full flow (ft/ft)" that is located in Appendix E.3 of the existing Circular to Subchapter 4.3 of the proposed Circular. This proposed change is to facilitate ease of reference by an applicant.

 

Subchapter 4.4 Culverts The proposed changes restate in narrative form the storm water design requirements set forth in Subchapter 4.4.A through C and make minor changes in language to more clearly state the requirements.  The proposed changes also add a reference to "Hydraulic Design of Highway Culverts," Third Edition, Publication No. FHWA-HIF-12-026, April 2012, Federal Highway Administration in Subchapter 4.4.A.  Culvert design projects can be complex, and this nationally accepted publication provides extensive background information on this subject.  The proposed changes also add a minimum culvert size of 12 inches.  This requirement is reasonably necessary because use of smaller diameter pipe is likely to result in clogging.  Subchapter 4.4.B of the existing Circular requires culvert designs to include "runoff water elevations."  The proposed narrative changes this reference to "headwater and tailwater" elevations to accurately identify the required design element.

 

Chapter 5: Retention Facilities Retention and detention pond standards are addressed in Chapter 5 of the existing Circular.  The department is proposing to address the design of retention ponds in Chapter 5 and the design for detention ponds in Chapter 6 of proposed Circular.  While there are some similarities in retention and detention ponds, their differences are substantial.  The department aims to reduce confusion by separating these two facilities into different chapters.

 

Subchapter 5.1 General The department proposes to move the retention facility design requirements from existing Subchapter 5.1 General to the more appropriate proposed Subchapter 5.2 Designs. Proposed additional text clarifies how retention facilities are used for storm water management.  The minimum design storm requirements for Standard Plans or Simplified Plans found in existing Subchapter 5.1.A and the location requirements of facilities found in existing Subchapter 5.1.B are provided in narrative form for clarity.  No substantive change is intended.  The department proposes deleting existing Subchapters 5.1.C and 5.1.D because their requirements are in the proposed Subchapter 5.2 Designs.  The requirement for 3-to-1 slopes has also been moved to proposed Subchapter 5.2.

            The proposed changes also clarify that when infiltration is used to reduce storage volume, the retention facility is considered an infiltration facility. This clarification is needed to ensure the appropriate design criteria apply based on the function of the facility.

The proposed changes require the installation of check dams when driveway ditches are proposed as retention facilities.  If a driveway ditch is proposed as a retention swale and installed on a slope, the storm water runoff may flow out of the base rather than be retained in the ditch. Requiring check dams to be installed under these circumstances is reasonably necessary to ensure retention of the storm water.

 

Subchapter 5.2 Designs The department realizes that the existing Circular provides minimal design requirements for the construction and location of a retention pond.  The proposed addition of the criteria in proposed Subchapter 5.2 ensures that retention ponds are constructed and located appropriately, and to allow ease of reference for those applicants proposing to use these facilities in their design.

            In general, "H" has been added to indicate horizontal and "V" to indicate vertical when clarifying side slopes used in the retention facility design.  Units of rise over run are often utilized in engineering practices and this addition will alleviate potential confusion.

The department proposes to create a minimum distance requirement between the base of the storm water facility and a limiting layer.  Some distance between the base of the storm water facility and a limiting layer is needed to allow infiltration into the soil which provides some treatment.  Without a separation between the base of the storm water facility and the seasonally high groundwater, storm water may flow directly into groundwater and could lead to contamination.  The two-foot minimum requirement aligns with a similar requirement for an extended detention basin in Subsection 5.7-2 of the Montana Post-Construction Storm Water BMP Design Guidance Manual, 2017 (BMP Manual).

The department proposes different design requirements for retention facilities equal to or less than four feet and greater than four feet deep.  Maintenance of a retention pond deeper than four feet is difficult without gentle side slopes (4H:1V) and requires fencing to prevent public access to the facility.  This distinction is consistent with requirements for an extended detention basin in Subsection 5.7-4 of the BMP Manual.

The department proposes to include language for consideration of an emergency outlet for retention structures.  Without an emergency outlet and proper location of the overflow area, damage could occur to the retention facility and unintended flooding on neighboring properties could occur for storm events that could not be fully retained.

The department also proposes additional text expanding on the design requirements of existing Subchapter 5.1.C.  The additional explanation of the design requirements is reasonably necessary to facilitate submission of complete applications for storm drainage review.  

 

Chapter 6 Detention Facilities The department realizes there was minimal design criteria and guidance included in the existing Circular DEQ-8 regarding the minimum storage volume and the release flow rate calculations for detention ponds.  Including requirements for minimum storage volumes and release flow calculations is reasonably necessary to facilitate submission of complete applications for storm drainage review.

 

Subchapter 6.1 General The department is proposing to delete the reference to Appendix L because it does not contain methods used in the proposed Circular.  The department is proposing to require that calculating the storm event at the time of concentration to be consistent with the acceptable methods of calculating runoff set forth in proposed Subchapter 3.7. The department is proposing to require detention facilities to be located where runoff will naturally accumulate to be consistent with existing Subchapter 5.1.B.  The department proposes to delete the requirement that outlets from detention facilities must provide a stabilized transition to the receiving area because that requirement is set forth in proposed Subchapter 6.2.  The department proposes changes in the narrative description of outlet structures for clarity.  No substantive change is intended.  Finally, the department is proposing to delete references to Appendices D and N because the information in these appendices is widely available in hydraulic textbooks. 

 

Subchapter 6.2 Designs The department realizes that the existing Circular provides minimal design requirements for the construction and location of a detention pond.  The proposed addition of the criteria in proposed Subchapter 6.2 ensures that detention ponds are constructed and located appropriately, and to allow ease of reference for those applicants proposing to use these facilities in their design.

The department proposes to add "H" to indicate horizontal and "V" to indicate vertical when describing side slopes used in the retention facility design.  Units of rise over run are often utilized in engineering practices and this addition alleviates any potential confusion.  The department proposes to require a minimum distance between the base of the storm water facility and seasonally high groundwater or bedrock layer.  Some distance is needed to allow infiltration into the soil, which provides some treatment.  Without a separation between the base of the storm water facility and the seasonally high groundwater, storm water may flow directly into groundwater and may cause contamination.  The department proposes to allow wet detention basins when they are designed in compliance with the Montana Post-Construction Storm Water BMP Design Guidance Manual.  This allows the option of locating storm water facilities in areas of high groundwater while still protecting groundwater.  The department proposes different design requirements for detention facilities equal to or less than four feet and greater than four feet deep.  Maintenance of a detention pond deeper than four feet is difficult without gentle side slopes (4H:1V) and requires fencing to prevent public access to the facility.  This distinction is consistent with requirements for an extended detention basin in Subsection 5.7-4 of the BMP Manual.  The department may require an emergency outlet to be designed to convey the 100-year peak flow to prevent erosion of berms and outflow areas in cases where damage could occur to the detention facility and unintended flooding on neighboring properties could occur. 

The department also proposes additional text expanding on the design requirements of existing Subchapter 5.1.C.  The additional explanation of the design requirements is reasonably necessary to facilitate submission of complete applications for storm drainage review. 

 

Subchapter 6.3 Calculations In proposed Subchapter 6.3, the department lists the calculations that applicants must submit for a detention facility.  Submission of these calculations is reasonably necessary for the department to determine whether the detention facility has an adequate storage volume.

 

Chapter 7 Infiltration Facilities Because the proposed Circular splits the discussion of retention and detention facilities into separate chapters, the provisions addressing infiltration facilities that are set forth in Chapter 6 of the existing Circular are now presented in Chapter 7 of the proposed Circular.  Chapter 7 addresses different methods for treatment of storm water, including vegetated filter strips, vegetated swales, screens, oil/water separators, proprietary spinners/swirl chambers, and drain inlet inserts, designed to prevent pollutant-containing storm water from discharging into state waters or to preserve functionality of the facilities.

 

Subchapter 7.1 General The department is proposing to delete the first sentence of existing Subchapter 7.1 because it is redundant to a subsequent new provision requiring infiltration facilities to be sized to maintain pre-development runoff volumes in accordance with Subchapter 3.3.  The department is proposing to add "chambers" and "basins and trenches" to provide an updated list of subsurface and surface features that may be used as an infiltration facility.  The department is proposing to delete lawns and landscaping as infiltration facilities and the provision addressing infiltration facilities that rely on vegetation.  The use of lawns and landscaping in hydrologic models such as the Rational Method or SCS CN result in a decrease in the calculated storm water volume and peak flows to a degree that can cause the infiltration facilities to be undersized and increase the likelihood of overflow.  The department is proposing to delete the reference to Appendix O because the department is proposing to delete that appendix.

The department proposes to eliminate the use of infiltration facilities in a Simplified Plan.  The complex calculations and requirements for these facilities are not consistent with the intent of a Simplified Plan.

Finally, the department proposes to require infiltration facilities to be located where runoff naturally accumulates.  This requirement is reasonably necessary to align infiltration facility requirements with that of retention and detention facilities.

 

Subchapter 7.2 Design The department proposes to require infiltration facilities to be designed based on soil texture and the infiltration rates presented in Appendix A.  The proposed language precludes the construction of infiltration facilities where soil textures are sandy clay loam or finer.  These soil textures have smaller pore spaces, slower percolation rates, and are more likely to clog, precluding their effectiveness in infiltrating storm water runoff.  For example, the Iowa Storm Water Management Manual and Maryland's Method for Designing Infiltration Structures Appendix D.13 state that soils with clay content exceeding 40 percent are not suitable for using infiltration practices. 

As explained in proposed Subchapter 7.1, the department is removing lawns and landscaping as infiltration facilities. The department is proposing to require a two-foot minimum distance between the base of the storm water facility and the seasonally high groundwater or bedrock layer.  Some distance is needed to allow infiltration into the soil, which provides some treatment.  Without a separation between the base of the storm water facility and the seasonally high groundwater, storm water may flow directly into groundwater and may cause contamination.  The two-foot minimum requirement aligns with a similar requirement for an extended detention basin in Subsection 5.7-2 of the Montana Post-Construction Storm Water BMP Design Guidance Manual, 2017.

 

Subchapter 7.3 Calculations The existing Circular does not clearly describe how the required storage volume of an infiltration facility is to be calculated.  The department is proposing that the required storage volume of an infiltration facility be calculated according to the criteria set forth in Subchapter 7.3 of the proposed Circular.  Consideration of these criteria is reasonably necessary to ensure that infiltration facilities are adequately sized. 

            The department proposes to require infiltration facilities to have an emergency outlet designed to convey the 100-year peak flow to prevent erosion of berms and outflow areas.  Without an emergency outlet and consideration as to where the overflow area would be located, damage could occur to the retention berms and outflow areas and neighboring properties could be inundated. 

 

Subchapter 7.4 Test Pit Requirements Appendix C.1 of the existing Circular contains minimal requirements for the location of test pits used to determine infiltration rates. The department is proposing to replace the test pit requirements in the existing Circular with Subchapter 7.4 of the proposed Circular.  Moving the requirements into the body of the text avoids confusion and makes the requirements more readily available to applicants.   

Proposed Subchapter 7.4 discusses the number and location of test pits required to determine infiltration rates. The department proposes to allow a reduced number of test pits for multiple infiltration facilities when consistent soils can be shown across the project area.  The language was requested by stakeholders to avoid the cost of obtaining extraneous soils information for larger projects with multiple infiltration facilities such as parking lots.  These new requirements are reasonably necessary to ensure that adequate representative samples of the soil profile are obtained for determining the infiltration rate of an infiltration facility. 

Proposed Subchapter 7.4 also identifies the information that must be included in a test pit log.  The information was selected to be consistent with the requirements presented in Circular DEQ-4 for drainfield test pits.  The detailed test pit requirements will ensure consistent quality of submittal information.

 

Chapter 8 Pre-Treatment Facilities Because the proposed Circular splits the discussion of retention and detention facilities into separate chapters, the provisions addressing pre-treatment facilities that are set forth in Chapter 7 of the existing Circular are now presented in Chapter 8 of the proposed Circular.  Proposed Chapter 8 addresses different methods for treatment of storm water, including vegetated filter strips, vegetated swales, screens, oil/water separators, proprietary spinners/swirl chambers, and drain inlets inserts.  Furthermore, Chapter 8 of the proposed Circular carries over the provisions of existing Subchapter 6.2 requiring a pretreatment facility where trash and sediment are likely to impact the facility.  Chapter 8 requires a pretreatment facility for infiltration facilities because sedimentation and trash in storm water is very detrimental to the function of an infiltration facility. Chapter 8 recommends a pre-treatment facility for detention and retention facilities to reduce maintenance requirements. 

            The allowed increase in 50 percent infiltration rates through the use of pre-treatment facilities will now be reflected in the infiltration rate Table 1.  Since the proposed infiltration rates have already been adjusted for the pre-treatment facilities, the language allowing the increase has been removed.

 

Appendices

 

Appendix A – The proposed Circular Appendix A will now be the infiltration testing procedures.

The department is proposing to delete Appendix A of the existing Circular which provides 24-hour rainfall depths across the state.  In the proposed changes to Subchapter 3.6, the department has identified NOAA Atlas 2 and Chapter 9, Appendix B-2022 of the Montana Department of Transportation Hydraulics Manual as acceptable sources for rainfall information.

The department is proposing to move the infiltration testing procedures set forth in existing Appendix C to Appendix A of the proposed Circular.  The department proposes to increase the allowable design infiltration rates in Table 1 of proposed Appendix A by 50 percent with the use of sediment reducing pre-treatment facilities in accordance with Chapter 8.  This is not a change from what is currently allowed.

Clarification has been added to change "infiltration systems" to "infiltration facilities" for greater clarity and to match terms used throughout the Circular.  "Infiltrative area" has been added to clarify what area is being described.

Appendix B – The proposed Circular Appendix B will now be the Simplified Plan spreadsheet.

The department proposes to move the provisions of existing Appendix B presenting acceptable hydrologic methods that can be used in sizing storm water facilities to Subchapter 3.8 of the proposed Circular. The department also proposes to delete the provisions of existing Appendix B that present detention pond outlet structure equations and the Chezy-Manning formula because they are typically only required for complex standard plan designs and are widely available in hydrologic textbooks.

The department is proposing to relocate the Simplified Plan spreadsheet set forth in Appendix F of the existing Circular to Appendix B of the proposed Circular.

Appendix C – The proposed Circular Appendix C will now be the Standard Plan spreadsheet.  The department is proposing to move the infiltration testing procedures set forth in existing Appendix C to Appendix A of the proposed Circular.  The department is proposing to relocate the standard plan spreadsheet set forth in Appendix G of the existing Circular to Appendix C of the proposed Circular.

Additionally, the department proposes to move test pit requirements for infiltration facilities into Chapter 7.  The department proposes to increase the allowable design infiltration rates in Table 2 of proposed Appendix A by 50 percent with the use of sediment reducing pre-treatment facilities in accordance with Chapter 8.

Appendix D – The department is proposing a new Appendix D in the proposed Circular to set forth an example of a Simplified Plan reflecting proposed changes.

The department is proposing to delete existing Appendix D because this appendix included simple hydraulic equations widely available in storm water textbooks.

Appendix E - The proposed Circular Appendix E will now be the example of the Standard Plan.  The department is proposing a new Appendix E in the Circular to set forth an example of a Standard Plan reflecting proposed changes in the proposed Circular.  The existing Appendix E was deleted because this appendix included simple hydraulic equations widely available in storm water textbooks.

Appendix F – This appendix is removed from the proposed Circular.  The department is proposing to relocate the Simplified Plan spreadsheet set forth in Appendix F of the existing Circular to Appendix B of the proposed Circular.

Appendix G – This appendix is removed from the proposed Circular.  The department is proposing to relocate the standard plan spreadsheet set forth in Appendix G of the existing Circular to Appendix C of the proposed Circular.

Appendix H – This appendix is removed from the proposed Circular.  The department is proposing to delete Appendix H because the example set forth in Appendix H has been moved to Appendices D and E of the proposed Circular.

Appendix I – This appendix is removed from the proposed Circular.  The department is proposing to relocate the simplified plan example set forth in Appendix I of the existing Circular to Appendix D of the proposed Circular.

Appendix J – This appendix is removed from the proposed Circular.  The department is proposing to relocate the standard plan retention facility example set forth in Appendix J of the existing Circular to Appendix E of the proposed Circular.

Appendix K – This appendix is removed from the proposed Circular.  The department is proposing to delete existing Appendix K because it does not use methods used in the proposed Circular.

Appendix L – This appendix is removed from the proposed Circular.  The department is proposing to delete existing Appendix L because it does not use methods used in the proposed Circular.

Appendix M – This appendix is removed from the proposed Circular.  The department is proposing to delete existing Appendix M because it does not use methods used in the proposed Circular.

Appendix N – This appendix is removed from the proposed Circular.  The department is proposing to delete existing Appendix N because the exhibit set forth is widely available in hydraulic textbooks.

Appendix O – This appendix is removed from the proposed Circular.  The department is proposing to delete existing Appendix O because the examples set forth are not necessary for the design of storm drainage facilities.

Appendix P – This appendix is removed from the proposed Circular.  The department is proposing to move the references located in existing Appendix P to Appendix F of the new Circular.  No changes are made.