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<br />3J <br /> <br />37 <br /> <br />Channels: Shown on Figures 5-1 through 5-5 are tbe conceptual channel details <br />developed for the Dirty Woman and Crystal Creek drainageways. Sections representing a 100- <br />year channelized. a lO-yearlow l10w channelized,and selective improvernent concepts vary in <br />feasibility depending upon reach. The floodplain preservation/do nothing alternative has also <br />been evaluated. Taking into account the evaluation parameters presented earlier, the selective <br />improvement and floodplain preservation concepts are feasible within most reaches of Dirty <br />Woman and Crystal Creeks. The limited encroachments of residential or commercial struCtures <br />intalhe l()(}.yearfloodplainzones is the primary reason for tile high feasibility of these concepls. <br />In all likelihood the floodplains will remain unencroached in the fulUre. A channelization <br />concept is practical within Reach A of Lake Fork Dirty Woman Creek, and portions of Crystal <br />Creek reaches A and B. Lake Fork Dirty Woman Oeek Reach A is severely eroded downstream <br />of the Lake Woodmoor spillway and should be stabilized by means of a low flow channel and <br />grade controls. Crystal Oeek Reaches A and B have portions of the creek which are alrr;ady <br />channelired and pass through areas ofresidcntial andcommercia]]and uses. <br />Various materials exist for use as bank linings ranging from natural vegetation 10 <br />concrete linings. As a drninageway is narrowed, the use of natural vegetation as II means of bank <br />stabilization becomes mOll:: limited because of high velocities, and hatdcr linings such a~ riprap <br />or conCll::te mUSt be inrroduced in otder to withstand such high velocities. The majority of the <br />Crystal Creek and Diny Woman drainageways are unenaoached except at roadway crossings. <br />rn many locations the drainageway inverts are well vegetated with wilIows and grasses which are <br />capable of withstanding erosion. Since there is a rela!ively minorincrcasein pcakdischllrgesfor <br />a lOO-year flood event, and link evidence of a tOO-year event of recent occurrence. the base <br />flow and higher frequency storm flows (i.e., the 2-year of 5-year frequencies), would appear to <br />present the greatest impact upon channel and bank stability. Riprap (exposed or buried) has been <br />used only in short segments of the drainageways up to now. The starulard treatment has been to <br />leave the drainages natura!. Grasslined low flow areas have been usedina large portion of the <br />upper reaches which pass through residential arollS. Concrete linings have been used in only 1I <br />very shun segment of Crystal Creek in the vicinity of Colorado Hi!(hway 105 and the Denver <br />and Rio Grande Railroad crossings. Intennittent segments of IOO-year or 100year channel lining <br />could be implemented as part of a selective improvement, <br />Dedicated drJinage right-of-ways are almost non-existent along Dirt}, Woman and <br />Crystal creek!;, For the most port this has not been a detrimental fa,tor since the lOO-year <br />floodplains have been left unencroached and the need for channel maintenance is minimal. tn a <br />channelization concept. the lack of dediealedright-of,ways is a negative factor since long-term <br />maintenance access (0 the channel improvements is limited to roadway crossings or utility <br />easements. This is the situation for a majority of Dirty Woman creek within the Woodmoor <br /> <br />area. It appears that the propeny owners along the drainageways have managed the vegetation <br />where the creek is mild and open, and leflthe drainages alone where tbey are steep and <br />narrower. Since operations and maintenance aspects of the drainages is a key con<:ern of the <br />propeny owners and the County. channel concepts whieh limit the need for a,tive maintenance <br />would be desirablc. <br /> <br />Detention: The need for OlIsile or regional detention within the Dirty Woman and <br />Crystal Creek basins to control flood flows can not be justified based upon peak discharge <br />rediJction atone. The hydrology lInalysis revealed that a relatively small increase in peak <br />discharges is anticipated in a fully developed scenario. Numerous small to moderately sired <br />storage areas e;o;ist along the drainageways wbich in combination with the wide floodplains have <br />provided flow attenuation. Many oflhese storage areas have developed sensitive habitats. <br />Though the natural storage was nOI accounted for in the hydrologic modelling, if the incidental <br />storage areas are left in place they will provide cffective peak flow allenuation and water quality <br />benefits for the basin(s). Some modification of the storage/discbarge conditions may occur at <br />roadways where aculven upgrade or replacement is needed. <br /> <br />The Town of Monument has an onsite detention criteria for all new resilk:ntial and <br />commercial developments. Onsile detention within these basins wouW have little impact upon <br />reducing peak flows discharging to Dirty Woman or Crystal creeks. The use of onaire detention <br />will however have a po~;live in'pact upon omite erosion and water quality control if properly <br />impitmtnted. <br /> <br />Closed conduits: The use of closed conduits as a means of handling storrnwater nmofT <br />was not considered feasible along any of the main rea<:hes of Dirty Woman or Crystal creeks. <br />There are areas within the Town of Monument thal will require that storm sewers be ,ons!rUeted <br />to handle runoff from commercial siles and 10 solve localized dr:ti.nage prohlems in the <br />downtown area of the Town. The locmion, size and cost of such systems will be defined in the <br />preliminary design phase of the study, <br /> <br />Evaluation ofConc~pls <br />Presented on Tahles 5-1 through 5-3 arc qualitative and <.juantitative comparisons of the <br />floodpl:ti.n preservation (do nothing). chlll1l1elization and selective improvement concepts. Each <br />alternative has been compared with respect to flood hazard, habitat loss, operations and <br />mailllenance, open spa(:e/aesthetk~ and waler quality. Each alternate's relative impact upon the <br />drainage planning pllrameters has been assessed, The arcal impact upon the habitat resources <br />