A DNR issued general permit No. 2 will continue to be necessary prior to the beginning of any construction activity that disturbs more than one or more acres, or which is part of a larger project that disturbs one or more acres in total. The submittal of a site development project will require a descriptive narrative of the site, a site plan, and other supporting documents. They are provided as a minimum guide and are not to be construed as the specific information to be supplied on every project drainage report, and other information may be required. Pre-settlement, pre-development, and post-development conditions for any given site will require analysis unique to that area.
(A) A project report narrative shall include the following items.
(1) A cover sheet with project name and location, name of firm or agency preparing the report, professional engineer’s signed and sealed certification, and table of contents. Number each page of the report.
(2) The nature of the construction activity (for example, roadway construction, utility construction, single-family residential construction, and the like), an estimate of the total area of the project site, and the total anticipated impervious area.
(3) Watershed size for each drainage area (both on-site and off-site) to determine how much of the area to be developed is affected by other drainage flowing through the site and to design appropriately sized storm sewer, culverts, and drainage channels.
(4) Describe pre-settlement/pre-development land use, topography, drainage patterns (including overland conveyance of the 100-year storm event), and natural and human-made features. Describe pre-development ground coverage, soil type, and physical properties, such as hydrologic soil group and infiltration. Pre-settlement ground coverage should be assumed as open prairie in any non-wooded area.
(5) Describe post-developed land use and proposed grading, change in percent of impervious area, and change in drainage patterns. If an existing drainage way is filled, the runoff otherwise stored by the drainage way will be mitigated with stormwater detention, in addition to the post-development runoff.
(6) Describe contributing off-site drainage patterns, land use, and stormwater conveyance. Identify undeveloped contributing areas with development potential, and list assumptions about future development runoff contributed to the site.
(7) Discussion of soils located on site and their suitability for infiltration. If infiltration meets the exclusion criteria, state why.
(8) Describe the features that will be installed to control rate of runoff, pollutants in stormwater, and infiltration in the post-development condition.
(9) Describe the maintenance and repair plan for all stormwater BMPs including detailed maintenance and repair procedures to ensure their continued efficient function. These plans will identify the parts or components of a stormwater BMP that need to be maintained and the equipment and skills or training necessary.
(10) Indicate what permits have been applied for and received, including, but not limited to, IDNR notice of intent, flood permit for sites affecting FEMA FIRM Zone A or Zone AE, ACOE Section 401 and/or 404 permits for work in waterways or wetlands.
(B) Pre-settlement and pre-development runoff analysis, including:
(1) Describe overall watershed area and relationship between other watersheds or sub-areas. Include a pre-settlement/pre-development watershed map in the report appendix.
(2) The typical method used to predict runoff and peak discharge is the NRCS TR-55 method. Other methods may be used only with the preapproval of the City Engineer. If other methods are used, describe the method and provide documentation of correspondence with the City Engineer regarding the method.
(3) Describe the method used to calculate the time of concentration. Describe runoff paths and travel times through sub-areas. Show and label the runoff paths on the pre-settlement/pre-development watershed map. Pre-settlement runoff paths should be assumed to match predevelopment with respect to location and slope.
(4) List runoff coefficients or curve numbers applied to the drainage areas. The RCNs listed in Table 3a shall be used in all hydrologic calculations for pre-settlement conditions required for determining Qp limits (two-, ten-, and 25-year events). These RCNs have been selected with the intent to mimic hydrologic conditions that existed in the state prior to settlement and farming. The RCNs listed in Table 3b shall be used in all hydrologic calculations for pre-development conditions required for determining Qf limits (100-year event). These RCNs have been selected with the intent to reflect hydrologic conditions that exist after settlement and farming. If a geotechnical study of the site was used to determine HSG, provide boring logs and locations in the appendix of the report. If a soil survey was used to determine HSG, cite it in the references.
Cover Type | Curve Numbers for Hydrologic Soil Group | |||
A | B | C | D | |
Non-Wooded Areas1 | 30 | 58 | 71 | 78 |
Wooded Areas2 | 30 | 55 | 70 | 77 |
Notes to Table 1. RCNs are equivalent to the TR-55 listed values for meadow in good condition with the intent to mimic pre-settlement conditions. Apply to any pre-development condition area which is not wooded. 2. RCNs are equivalent to the TR-55 listed values for woods in good condition. | ||||
Cover Type | Curve Numbers for Hydrologic Soil Group | |||
Cover Type | Curve Numbers for Hydrologic Soil Group | |||
A | B | C | D | |
Continuous Cropped Agricultural Use1 | 55 | 69 | 78 | 82 |
Continuous Mature Wooded Area2 | 30 | 55 | 70 | 77 |
Continuous Meadow Area3 | 30 | 58 | 71 | 78 |
Lawn and Pasture Areas4 | 39 | 61 | 74 | 80 |
Impervious Area5 | 98 | 98 | 98 | 98 |
Notes to Table 1. RCNs are equivalent to mid-range TR-55 listed values for row crops and legume crops (excluding crops mown for hay) were conservation practices are used. 2. RCNs are equivalent to the TR-55 listed values for woods in good condition. 3. RCNs are equivalent to the TR-55 listed values for meadow (ungrazed and unmowed) in good condition. 4. Areas include residential lawns, golf courses, cemeteries, grazed or mowed farm pasture, and other areas with short grass. 5. Areas include roofs, sidewalks, paved streets and roads (excluding the right-of-way), curbs, paved parking lots, and driveways. | ||||
(5) The typical precipitation model and rainfall duration used for the design storm is the NRCS Type II 24-hour distribution. Total 24-hour rainfall amounts for given frequency shall reflect data from NOAA Atlas 14, Volume 8 as displayed in Table 4 below. Other methods (such as a user-defined model based on collected precipitation data) or durations (such as a critical duration analysis) may be used only with the preapproval of the City Engineer. If other methods are used, describe the method and provide documentation of correspondence with the engineer regarding the method.
24-hour Rainfall Depth for Various Storm Events | |
Return Period | Rainfall Depth (in) |
(6) Provide summary table(s) of model results including drainage area, time of concentration, frequency, peak discharge, and accumulative routed flows at critical points within the development (if any) and at development boundaries. At a minimum, flow rates for pre-settlement two-, ten-, and 25-year storms and pre-development 100-year storm must be provided.
(C) Post-development runoff analysis, including:
(1) Describe overall watershed area and sub-areas. Discuss if the post-development drainage area differs from the pre-development drainage area. Include a post development watershed map in the report appendix.
(2) The method used to predict runoff and peak discharge will be the same as used in the pre-development analysis, except for variables changed to account for the developed conditions.
(3) The time of concentration method used will be the same as used in the pre-development analysis. Describe change in times of concentration due to development (for example, change in drainage patterns). Show and label the runoff paths on the post-development watershed map.
(4) List runoff coefficients or curve numbers applied to the drainage areas. The RCNs listed in Table 3b above shall be used in all hydrologic calculations for post-development conditions. Include an analysis of the proposed increase in impervious area and describe the change in runoff volume due to development. Directly connected impervious area (roads, front of houses, and the like) shall be separated out from the pervious area and unconnected impervious areas (backs of houses, and the like) in the determination of a weighted RCN. Provide a summary of the total directly connected impervious area and the total unconnected impervious area for each sub-watershed/catchment.
(5) The precipitation model storm event, total rainfall, and total storm duration will be the same as used for the pre-development model.
(6) Provide summary table(s) of model results including drainage area, time of concentration, frequency, peak discharge, and accumulative routed flows at critical points within the development (if any) and at development boundaries. At a minimum, flow rates for one-, two-, ten-, 25-, and 100-year storms must be provided.
(7) Summary of post-development runoff shall include the following:
(a) Description of BMP including water quality practices (methodology);
(b) Provide table(s) including drainage area, time of concentration, frequency, duration, and peak discharge. Summarize in narrative form the change in hydrologic conditions due to the development; and
(c) Post-developed discharge should take into account any upstream offsite detention basins and undeveloped offsite areas assumed to be developed in the future with stormwater detention.
(D) Rate control structure and conveyance system analysis, including:
(1) Describe any detention basin locations by discussing existing topography and relationship to basin grading. Determine if rock deposits will affect construction and if a high water table precludes basin storage. Floodplain locations should be avoided.
(2) The detention basin size in final design should be based upon actual hydrograph routing for the design storms controlled by the basin. Note the TR-55 approximate method of sizing detention basins (TR-55 Chapter 6) may result in storage errors of 25%, and should not be used in final design.
(3) The top of any dike used in forming a detention basin should be a minimum of one foot above the 100-year storage elevation. Large detention basin design may require IDNR approval (see Iowa Administrative Code Title V, Chapter 70).
(4) Discuss the basin outlet design in terms of performance during low and high flows and downstream impact. Note that a single-stage outlet (for example, one culvert pipe) may not be appropriate because of its inability to detain post-developed runoff from storms less than the ten-year interval. A more desirable outlet has two or more stages (such as a riser structure with an orifice) which will serve to detain runoff for low-flow events of a two-year storm, and pass greater storm events via the riser outlet.
(5) Design the spillway for high flows using weir and/or spillway design methods. The steady-state open channel flow equation is not intended for use in spillway design.
(6) Describe methods to protect the basin during overtopping flow.
(7) Describe channel protection/lining and velocity dissipation at outlets.
(8) Provide a summary table of the respective volumes and discharge rates for the project area after routing though rate control structures, and a comparison to the calculated allowable release rate from the site for the two-, ten-, 25-, and 100-year storm events.
(E) A site map (or maps) including the following items.
(1) (a) A preliminary plat (with pre- and post-development topography) may be used to show the proposed development. Minimum scale of one inch equals 500 feet or larger to ensure legibility should be used for all drainage areas. (Drawings no larger than 24 inches by 36 inches should be inserted in eight and one-half-inch by 11-inch sleeves in the back of the bound report). The plat is to show street layout and/or building location on a contour interval not to exceed two feet.
(b) The map must show on- and off-site conditions. Label flow patterns used to determine times of concentration. Drainage plans (preliminary plat or topography map) must extend a minimum of 250 feet from the edge of the proposed preliminary plat boundary, or a distance specified by jurisdiction.
(c) The limits of swale and ditch easements should be established based upon the required design frequency. This includes 100-year overflow easements from stormwater controlled structures.
(2) Identify areas of the site located within the floodway or floodplain boundaries as delineated on flood insurance rate maps, or as determined by other engineering analysis. Identify wetland areas on the site, as delineated by the National Wetlands Inventory, or as determined by a specific wetland study.
(3) Soil map or geotechnical information.
(4) Location and elevations of jurisdictional benchmarks. All elevations should be on jurisdictional datum.
(5) Proposed property lines (if known).
(6) Existing drainage facilities and structures, including existing roadside ditches, drainage ways, gutter flow directions, culverts, and the like. All pertinent information such as size, shape, slope location, 100-year flood elevation, and floodway fringe line (where applicable), should also be included to facilitate review and approval of drainage plans.
(7) Proposed drainage facilities and structures, including storm sewers and open drainage ways, right-of-way and easement width requirements, 100-year overland flow easement, proposed inlets, manholes, culverts, erosion and sediment control, water quality (pollution) control, infiltration basins, energy dissipation devices, and other appurtenances.
(8) Cross sections and profiles of road ditches designed to carry storm flows and to ensure non-erosive velocities.
(9) Proposed outfall point(s) for runoff from the study area.
(10) The 100-year flood elevation and major storm floodway fringe (where applicable) are to be shown on the plans, report drawings, and plats (preliminary and final). In addition, the report should demonstrate that the stormwater system has adequate capacity to handle a 100-year storm event, or provisions are made for overland flow.
(11) Show the critical minimum lowest opening elevation of buildings for protection from major and minor storm runoff.
(F) Supporting computations. Computations may be done by hand or with computer software (P8, WinSLAMM, or other DNR approved models). If software was used, attach computer-generated reports and output and underline and label results, such as the peak discharge. Provided computations must include:
(1) Runoff coefficients and curve numbers for each sub-watershed;
(2) Total impervious area (square feet and percentage of total drainage area);
(3) Times of concentration for each sub-watershed;
(4) Storm sewer design summaries;
(5) Peak runoff calculations (show results in tabular format and pre- and post-developed hydrographs);
(6) Detention basin design (show tabular stage-storage-discharge results and inflow/outflow hydrographs. Include both extended-detention and rate control results);
(7) Water quality volume calculations;
(8) Infiltration basin design calculations;
(9) Open channel flow calculations. For ditches that drain areas over two square miles in urban areas and over ten square miles in rural areas, design may require the Iowa Department of Natural Resources (IDNR) approval (see Iowa Administrative Code Chapter 567.71);
(10) Culvert design calculations or nomographs. For culverts that drain areas over two square miles in urban areas and over ten square miles in rural areas, design may require the Iowa Department of Natural Resources (IDNR) approval (see Iowa Administrative Code Chapter 567.71); and
(11) Erosion protection design.
(G) Before the recording of final plat and issuance of a permit, or as condition of approval of final plat and issuance of a permit, the Commission and city shall require the following guarantees to ensure that the stormwater BMPs are installed by the permit holder as required by the approved stormwater management final plan.
(1) The amount of the performance guarantee shall be the total estimated construction cost of the stormwater BMPs approved under the permit, plus 10%. The performance guarantee shall be posted by the subdivider or subdivider’s contractor under § 157.024 with the City Council guaranteeing the improvements will be constructed and shall remain in full force and effect until final acceptance of the improvements by the city. Final acceptance of the plat will not constitute final acceptance by the city of any improvements to be constructed. Improvements will be accepted only after their construction has been completed, and no public funds will be expended in the subdivision until such improvements have been completed and accepted by the City Council resolution.
(2) The performance guarantee shall contain forfeiture provisions for failure to complete work specified in the stormwater management final plan.
(3) The performance guarantee shall be released in full only upon submission of “as built plans” of all stormwater BMPs specified in the stormwater management final plan and written certification by a professional engineer that the stormwater BMPs have been installed in accordance with the approved stormwater management final plan and other applicable provisions of this chapter. The city will make a final inspection of stormwater BMPs to ensure compliance with the approved stormwater management final plan and the provisions of this chapter. Provisions for a partial pro-rata release of the performance guarantee based on the completion of various development stages can be made at the discretion of the city.
(H) The city shall also require the submittal of a maintenance performance guarantee prior to issuance of a permit in order to ensure that the stormwater BMPs are maintained in an effective state for a minimum of two years.
(Prior Code, § 6-21-10)