Phase A - Alternatives Analysis Report - Urban Drainage and Flood ...

PHASE A – ALTERNATIVES ANALYSIS REPORTMajor Drainageway Planning Update for Hidden Lake-Bates Lake5 ALTERNATIVE DEVELOPMENTS5.1 Alternative Development ProcessAlternatives were identified to mitigate problems within the Hidden Lake and Bates Lake watersheds,including stormwater detention, stormwater conveyance and water quality. In general, the drainagewayplanning evaluated a variety of alternatives, including regional detention, major underground conduits, roadculvert crossings, open channels, regional water quality ponds, concrete forebays for detention facilities,removing directly connected impervious surfaces and incorporating water quality into detention facilities.The general approach taken for the alternatives development was to minimize the extent of planned facilitiesto only those deemed necessary. In general, major crossing improvements and other facilities were designedfor the 100-year event. In some instances the County or other local jurisdictions indicated that a lesser levelof protection, such as the 50-year event, was acceptable for drainage improvements. In those cases the lowerfrequency was considered in the alternatives analysis.Proposed alternatives were discussed with the project stakeholders at several progress meetings throughoutthe planning process. A public meeting was held Wednesday June 25 th 2008 to present the findings of thehydrology and hydraulics studies, and the proposed drainage improvement alternatives.5.2 Alternative CategoriesBased on the problem areas identified, four alternative groups were considered to be viable for developmentof potential improvements in each reach. These alternative groupings are described below. For the purposesof this report, the alternatives assume that all local entities will continue to provide floodplain regulation,and will require on-site detention for new development within the drainage basin.A total of nine different types of improvement alternatives were initially considered for each reach in theHidden Lake and Bates Lake watersheds. Costs were estimated for the alternatives that generally includedthe cost associated with the drainageway improvement, traffic control, utility relocation, land acquisition,operation and maintenance over a 50-year period as well as contingencies, engineering, and constructionmanagement costs. A general description of the alternative categories considered includes the following:Alternative 1: No Drainageway Improvements – Maintain Existing ConfigurationThis alternative will maintain the existing channels, pipes, and crossing structures. This “do nothing”alternative is considered applicable in areas where no structures are inundated in the 100-year floodplain orif no improvements are warranted because the hazards or damages associated with flooding are low. Thisoption does consider changes to general routine, restorative and rehabilitation maintenance of the existingchannel and facilities. This alternative does not solve existing flood hazard problems.Alternative 2: Regional DetentionThis alternative will either construct regional detention facilities to help attenuate peak flows or modifyexisting detention facilities to make them regional and maintenance eligible by the Urban Drainage & FloodControl District. Agreements would be needed between private owners, the local sponsor, and the District5-1for a flowage and maintenance easement on the facility. This alternative includes creating additional storagefor flood reduction. The storage of floodwater within the system can have a dramatic impact on the peakflood discharge if volumes for detention are large enough, and the facility is placed in the proper locationwithin the drainage basin.Alternative 3: Improved ConveyanceThis alternative replaces undersized storm drain pipes, boxes, channels, and culvert crossings with ones thatwill convey enough runoff to minimize flooding damages in a 100-year storm event. This alternative alsoincludes constructing new channels, pipes, boxes, and culvert crossings where previously no improvementsexisted.Major Underground Conduits - These would include large diameter pipes or box culverts that would beinstalled to convey the flood flows in areas where a channel could not be constructed due to right-of waylimitations.Culvert Improvements - This alternative includes culvert additions or culvert upsizing improvements toincrease flow capacity under roadways. This alternative is selected when streets are being overtoppedcausing a loss of function for vehicle traffic or isolated improvements are warranted.Open Channel Improvements – This alternative includes new or increasing the size of open channels tosafely convey storm runoff through the watershed. This alternative is selected when there is enough room toconstruct an open channel.Alternative 4: Water QualityThis alternative improves the stormwater quality within the study area. This will be accomplished byconstructing stormwater quality detention facilities either in the bottom of existing detention facilities,constructing new stormwater quality detention facilities, or directly connecting open channels to perviousareas allowing runoff to be filtered by vegetation and the soil before it reaches water bodies or groundwateraquifers.Regional Water Quality Ponds – This alternative includes constructing inline or offline detention basins totreat storm runoff and settle out sediments and heavy metals. This alternative is selected when there are noother water quality features along the main stem and where there is enough area to construct a regionalwater quality pond.Water Quality Forebay – This alternative includes constructing forebays at the entrances to existingdetention facilities. These facilities trap trash and allow sediments to settle out of stormwater before enteringa regional detention facility. Since the forebay is concrete, it is easy to get in with a tractor and scoop outsediment and trash. This alternative is selected when there is not enough space or when it is not costeffective to construct a regional water quality pond.Removing Directly Connected Impervious Surfaces – This alternative removes concrete low flow channelsfrom open channels to allow storm runoff to infiltrate into the groundwater aquifer and allow vegetation totreat storm runoff. The concrete low flow channels will be replaced with buried soil riprap which could be