NR 116.07(3)(b)1.1. The techniques to determine skew under par. (a) 1. may not be used if data from a gaging station in the watershed is not available or is available for a period of less than 10 years. In other cases, the technique to determine skew in par. (a) 1. shall be modified as follows:

NR 116.07(3)(b)2.2. If the difference in the drainage area at the study site and the drainage area at a gaging station on the same watershed is less than or equal to 50%, the regional flood discharge at the study site shall be determined by transferring the calculated regional flood discharge at the gage by using Bulletin #17B techniques to the study site using a drainage area ratio taken to the “n” power, from page 12 of “Techniques for Estimating Magnitude and Frequency of Floods for Wisconsin Streams”, U.S.G.S., Open File Report 80-1214, March 1981.

NR 116.07(3)(b)3.3. If the difference in the drainage area at the study site and the drainage area at a gaging station in the watershed is more than 50%, or if there is no gaging station in the watershed, at least 2 of the techniques described in par. (a) 2. to 7. shall be used to determine a weighted value of the regional flood discharge.

NR 116.07(3)(b)6.6. In all cases where dams or reservoirs, floodplain development or land use upstream have significantly altered the storage capacity or runoff characteristics of the watershed so as to affect the validity of any of the techniques listed in par. (a), the synthetic hydrograph technique in par. (a) 3. or the Technical Release No. 55 in par. (a) 4. shall be used for the determination of the regional flood flow discharge.

NR 116.07(3)(b)7.7. In rapidly urbanizing watersheds, the municipality shall require that computations for regional flood flow discharges reflect increased runoff from all projected future development. These computations shall be made using one of the following techniques:

NR 116.07(3)(b)7.b.b. A mathematical model shall be developed to determine the effects of all projected future development in the watershed on the regional flood flow discharge. Local units of government shall project what percentage of watershed development may occur under existing land use or subdivision ordinances and regional flood discharges shall be based upon that data. Where there are no existing land use or subdivision ordinances which control or regulate future development, total projected development shall be assumed to occupy 70% of the watershed. Where watersheds contain more than one municipality, agreements between those municipalities may be necessary to restrict future watershed development. In order to insure that future regional flood flows do not exceed the regional flood flow discharges used in local regulations, changes in existing land use or subdivision ordinances which may allow an increase or decrease in the projected development in the watershed shall be reflected in regional flood flow discharge values.

NR 116.07(4)(a)(a) Reconnaissance. The study contractor is responsible for the collection of all existing data with regard to flooding in the study area. This shall include a literature search of all published reports in the study area and adjacent communities and an information search to obtain all unpublished information on flooding in the immediate and adjacent areas from federal, state and local units of government. This information shall include specific information on past flooding in the area, drainage structures such as bridges and culverts that affect flooding in the area, available topographic maps, available community maps, photos of past flood events and general flooding problems within the community. The study contractor will coordinate the collection of all available data and published reports with the department. A field reconnaissance shall be made by the responsible engineer to determine hydraulic conditions of the study area including type and number of structures, locations of cross sections and other parameters including roughness values which are necessary for the hydraulic analysis.

NR 116.07(4)(b)(b) Base data. Cross sections to be used for the hydraulic analysis may be obtained by one of several methods, including surveying or aerial photography. New or previously surveyed cross sections or topographic information obtained from aerial photographs may be used independently or in combination as the base data to be used in hydraulic analysis. The elevation datum of all of the information to be used in the hydraulic model shall be verified. All information used shall be referenced directly to NGVD unless the elevation datum is otherwise approved by the department.

NR 116.07(4)(c)(c) Methodology. Flood profiles shall be calculated by the standard step method, using the Corps of Engineers HEC-2 computer model. Other methods may be used with prior department approval provided that any computer models submitted to the department for review are in a form acceptable for entry into the department’s floodplain data repository.

NR 116.07(4)(d)(d) Floodway determination. The hydraulic floodway lines shall be determined from the limits of effective flow based on the calculated regional flood water surface profile. Transitions shall take into account obstructions to flow such as road approach grades, bridges or natural restrictions. General guidelines for transitions may be found in “HEC-2, Water Surface Profiles-Users Manual, appendix IV, Application of HEC-2 Bridge Routine” published by the Hydrologic Engineering Center, Davis, California. All areas of the floodplain including overbank areas that can be assumed to convey flood waters shall be included in the hydraulic floodway.

NR 116.07(4)(e)(e) Previous floodplain studies. If differences exist between a study previously approved by the department and the contractor’s calculated hydraulic floodways or flood profiles, the study contractor shall document justification and obtain departmental approval for these differences. Where the contractor’s study differs from existing flood profiles or hydraulic floodways for adjacent communities, verification of the differences will be necessary for department approval of the hydraulic analysis.

NR 116.07(4)(f)(f) Calculation of the regional flood profile. The regional flood profile and changes to that profile caused by development in the floodplain, as determined by the hydraulic model, shall be calculated to the nearest 0.01 foot.

NR 116.07(4)(g)(g) Adequacy of the hydraulic model. The following factors shall be considered by the department to determine the adequacy of the hydraulic model and the regional flood profile. Upon written request by the department the study contractor shall submit written justification for the following factors: