9-12-13 WATERWAYS, DRAINAGES AND FLOOD HAZARD AREAS

Special attention shall be given to ensure that development is setback sufficiently from waterways, drainages, and flood hazard areas to prevent erosion, flooding and damage to development and the waterways per state and federal standards. Such features should be constructed to give a natural "streambed" appearance to the waterway or drainage area by making small meanders, placing rocks of various sizes (pebbles, cobble, rocks, and boulders) in the streambed and banks, and planting clumps of trees and shrubs along the outside edge. A. Design Storm: 1. As each site is unique for elevation and time of concentration, the following website can be used for determining the precipitation intensity, currently found at: http://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.htm 2. The use will input the latitude and longitude for the particular site and choose "precipitation intensity" from the drop-down menu on screen (the user will verify the elevation is within acceptable limits for the project area). 3. If a storm distribution is being utilized for the stormwater design, a standard SCS type II distribution should be used. This distribution shows approximately fifty to seventyfive percent (50 -75%) of total rainfall to occur in a brief period (approximately two (2) hours), which is typical of the intense short duration storms experienced in the town area. A. Detention Ponds: On site detention ponds are to be sized for the one hundred (100) year twenty-four (24) hour storm. C. Stormwater Conveyance: Stormwater conveyance pipes are to be sized based on a ten (10) year 24-hour storm. 1. Pipes will be sized using a rainfall intensity determined by the time of concentration for the applicable drainage basin with a minimum pipe size of fifteen inches (15”). 2. The time of concentration will be estimated using the soil conservation service technical release 55 (SCS TR-55) method. SCS TR-55 method uses three (3) distinct runoff patterns in a watershed: a) sheet flow; b) shallow concentrated flow; and c) channel flow. Sheet flow occurs in the upper reaches of a watershed and persists for a maximum of three hundred feet (300'). The minimum time of concentration to be used for design shall be five (5) minutes. After flowing in sheets, water then typically becomes less sheet-like and more concentrated. Following shallow concentrated flow, water typically collects in natural or manmade channels (U.S. soil conservation service, 1986). D. Other Permits: Below is a list of other permits that may be required for development activity in the Town. This list is for informational purposes only and may not include all necessary permits, depending on the project location: (amd. ord. 24-014, 10-8-24) 1. Utah pollutant discharge elimination system (UPDES) general permit for storm water discharges associated with construction activities. This permit is required for any land disturbance of one acre or greater. The permit requires submittal of a notice of intent (NOI) to the Utah Division of Water Quality (DWQ) with appropriate fee, preparation of a stormwater pollution prevention plan (SWPPP) and erosion control plan. Additional information regarding this permit can be found at: http://www.waterquality.utah.gov/UPDES/stormwaterco 2. Utah stream alteration permit: This permit is required for any construction activity occurring along a creek or stream. The permit requires submittal of a stream alteration permit to the Division of Water Rights (DWR) with applicable design drawings and calculations. Additional information regarding this permit can be found at: https://waterrights.utah.gov/strmalt/default.asp 3. U.S. army corps of engineers (USACE) wetland permits. Potential wetlands need to be assessed, and jurisdiction needs to be determined and approved by the USACE. Possible requirements are a wetland boundary delineation, drawings, meetings with the USACE and permits. (Ord. 08-016, 8-12-2008)