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5 - Handout - Technical Memorandum - Boulder Stormwater Strategic PlanLT~(~ I ONE COMPANY 1 1J.~ MeMy So[wtionr` Technical Memorandum To: Bob Har6erg, City of Boulder Fran: Rich Thomton, Jerry Kenny- HDR Project: Boulder Slormwater Strategic Plan CC: File Date: February 22, 2007 Job No: 34640 RE: South Boulder Creek Lower Urbanized Storm Center - 700 year Flood Impacts Introduction The purpose of this analysis was to develop an order-of-magnitude understanding of the potential flood impacts within the urbanized subbasins tributary to South Boulder Creek during a 100-year design storm event centered over the southeast portion of the city. This analysis is a follow-up to the observations noted in the Hydrologic Impacts of Downstream Storm Centers memorandum (HDR, February 24, 2005) which identified the potential for flood hazards resulting from local, urban subbasin runoff during an extreme storm event. Analvsis and Results The analysis consisted of an evaluation of the basin hydrologic and hydraulic response to a 100-year rain event using the XPSWMM model developed for the Boulder Stormwater Strategic Plan (SSP). The XPSWMM model developed for the SSP was developed for the analysis of the city's collector system and as a result includes a more detailed assessment of subbasin boundaries and local/collector conveyance elements. As such, there are minor differences in the subbasin boundaries delineated for the South Boulder Creek (SBC) analysis and the SSP analysis. The South Boulder Creek Lower Urbanized Storm Center -100 year Flood Impacts analysis approach and results are summarized in the following sections. Hydrology The rainfall depth and temporal distribution associated with a thunderstorm cell centered over the South Boulder Creek C2 subbasin was entered into the XPSWMM model. Runoff hydrographs were generated for the applicable subcatchments and then routed through the collector storm drain and open channel systems. The resulting 100-year runoff peak simulated in XPSWWM was approximately 1200 cfs. This runoff peak is slightly lower than the 1650 cfs predicted with the MikeFlood model used in the SBC analysis. The slightly lower peak from XPSWMM is attributed to runoff being routed through the collector storm drain and open channel system thus attenuating the hydrograph peak. ~ Flood Risk Assessment The XPSWMM model was developed for the collector system and used to evaluate the basin response for relatively minor rairifall events (up to a 5-year design storm). As a result, it does not provide an adequate, standalone tool to assess potential flood risk and flood hazard damage. However, the XPSWMM model can be used in conjunction with the XP-2D module to assess the combination of piped conveyance and surface flow conveyance. The XP-2D model was developed to route surface flow that exceeds the capacity of the existing storm drain system. In addition, where flow depths are greater than the open channel top of bank, overland flow is also routed via the 2D model. Therefore, the potential flood risk was assessed at this order-of-magnitude level using the XPSWMM/XP-2D model. A simplified dataset was used for the XP-2D model development. Data for the XP-2D model consisted of a digital elevation model (DEM) and an average surface roughness. Data such as ineffective flow areas (e.g. buildings) were not used due to the large urban area the analysis covers. In addHion, muliiple roughness factors accounting for different land use types and land cover characteristics were not used, again, as a result of the large urban area the analysis covere. The 2D analysis was limited to the C2 sub@asin boundary. The City's (DEM) was sampled at 10-foot grid intervals and used as the DEM within the XP-2D model. The grid cell size within the XP-2D model was limited to 65-feet due to the large analysis area and model license requirements. As this analysis is intended to provide a general estimate of flood risk, the relative{y farge cell size was deemed appropriate. An average surface roughness of 0.030 was assigned based on the area consisting mainly of pavement, lawns, open space, and buildings. Analysis results indicate shallow surface flow for a majority of the study area with depths equal to or less than 1-foot as shown on Figure 1. There are locations where depths exceed 2-feet. However, these locations are isolated in areas where the DEM identifies localized depressions in the ground surface. These high inundation depths may be attributed to the accuracy of the DEM or 2D grid size. Flood hazard risk can be expressed by the product of overland flow velocity and depth. Typical definition of low hazard areas is if the velocity-depth product is less than 4. Flood hazard maps were produced with the XP-2D (Figure 2) and generally indicate low flood hazard risk across a majority of the study area with risk values of less than 1. However, there are localized areas where the risk value approaches 4. Speciflcally, there is a depressed area modeled in the DEM southwest of the intersection of Foothills and US 34 and another location approximately 800- feet southwest on the Viele Channel. Given the assumptions used to develop tfiis model, it is anticipated that a 100-year thunderstorm event centered over the lower subbasins of South Boulder Creek would produce surface flooding on average less than 1.5-foot in depth. This 100-year inundation depth within a collector system meets the City's level of service defined within the Design and Construction Standards. 2- Depth ~ , ~ 4.00 3 56 I 3.11 ' ,~'~ . --- - ' - 2.67 2.22 . - ' ~ It i.~e ~ - -~_._ 1 ~___ 1.33 _ ..... . _- _ 1 - - 0.89 1 _ ~J ~`~s~ H ' 0.44 ~ ' 1 ~' - ,'= 0.00 - ~- ~ - - ti , , ~ , i I '~ ' ~ I- -- _~._~ \ ~ , .., - __ C ~gure 1- Approximate Inundation Depths, 100-year Design Storm Hazard t I 4.00 ~ 3 56 i ~ I - - .- ; . 2.67 222 ; . . ~ ._ ~ ~ ~ ~ ~ { ~. r ~ , 178 33 1 i 1.. _ ! ._.. ~ - ~ - . 0.89 O.dd `~ ~ ~~; `~~ ~ - f~ : 0.00 ---~ - - ~ R - - -- ~.. ., _ ~ • I I~ ~ . _ , ,.r~ ~ - - _- ~ ~..~ '~ i ' ~ _ , - _._ _ _ _ . ~! 'I _ ` ' ~^ ' ~ , . . ;c •. ~ }! ~ ~~ ~1 - ,r;: _ ~. " - .fr.J _.^'.. I I4"_ , Y ~ ~ ' ~; ~'~ . - ~~ ~ igure 2- Approximate Flood Hazard, 100-year Design Storm -3- waAa.~a~~ u.a .i ~...y.._.. I ~ ~ ~~~= f ~~ ~.~' / ~.r.e o~ ~ ak ~ r - ~ ,~f~ ~° , ~~ ~ ~: ~~~~ ~. _~. ,~~ ~ _ ~ ,, , i ~ . .. \ N \~ ~ ' C3 ~ ~ ~~ 0 1000 Leaend Modeled network - Bear Carryon Creek - Elmers Twomile Creek I Kings Guich i South Boultler Creek /~/ Pipes - Bluebell Canyon Creek - Fourmile Canyon Creek i Lower Boulder Creek ~ Twomile Canyon Creek ' CanaU Ditch _ pry Creek _ Goose Creek Middle 8oulder Creek - Viele Channel /~/ Major drainageway - pry Creek No. 2 - Gregory Canyon Creek , _ Skunk Creek _ Wonderland Creek Q SBC SubBasin Figurr 3- South Boidder Creek and Stor~rn~ater Str~ttegc P(nn Subbas:ns Property Affeetetl by AE antl X Floodplain Delineation AE City Zoning Zone Zone Acres Structures RM-1 Resitlential-Medium 1 21.38 37 RR-1 Residential-Rurall 2720 38 RE Residential-Estate 54.64 160 RH-5 Residential-Hi h 5 33.52 18 RM-2 Residential-Medium 2 31.95 50 RL-1 Res~denGal-Low 1 46.77 34 RH-4 Residential-Hi h 4 7.54 3 P Public 194.29 11 E Enclave 24.96 32 IM Industrial-Manufactunn 5.86 3 RL-2 Residentlal-LOw 2 74.45 169 BC-2 Business-Communi 2 2.08 1 BC-1 Business-Communi 1 27.97 23 IS-2 Industrial-Service 2 3.65 0 BT-t Business-Transitional i 321 3 IG Indusirial-General 114.46 70 Tolal 674 652 X City Zoning Zone Zone Acres SVUCtures RM-1 Residential-Medium 1 423 10 RR-1 Residential-Rurall 0.87 B RE Residential-Estate 23.71 99 RH-5 Residential-Hi h 5 13.67 19 IM Indusirial-Manufaclurin 28.35 31 RM-2 Residential-Medium 2 16.96 74 RL-1 Residential-Low 1 39.16 156 RH-4 Residential-Hi h 4 1.47 6 P Public 29.99 14 E Enclave 2.59 6 RL-2 Residential-LOw 2 95.72 479 BC-2 Business-Communi 2 0.28 0 BC-1 Business-Communi 1 568 17 IS-2 Industrial-Service 2 1.30 0 BT-1 Business-Trensitional i 8.03 14 IG Industrial-General 172.91 68 Total 385 1,001 AE County Zoning Zone Zone Acres Struetures A A riculWral 491.11 48 F Forest 0.52 0 C Commercial 3.59 10 LI Li ht Intlustrial 20.82 24 MH ManuNactured Home 6.11 ~ 80 GI Generellndusfrial 59.55 9 XBO Boulder 4.68 4 RR Rural Residential 646.80 257 ED Economic Develo ment 34.56 1 ER Estate Residential 16.42 24 T Transitional 0 95 1 SR Suburban Residential 25.81 25 B Business 1.64 2 Total 1,313 485 X Counry Zoning 2one 2one A~res SVUCtures A A ricultural 61.98 5 F Foresf 0 32 0 C Commercial 0.00 0 LI Li htlndustrial 1.56 3 MH Manufadured Home 4.23 37 GI Generellndustrial 71.14 1 XBO Boulder 2 05 2 RR Rural Residential 103 56 724 ED Economic Develo ment 5.83 0 ER Estale Residenlial 10.82 27 C Commercial 0.73 1 SR Suburban Residential 26.39 26 B Business 172 10 Total 230 236 Grand Total 1.987 1,137 615 1,237 22 February 2007 PREP Summary Statement on the South Boulder Creek Flood Mapping Project Climatology, Hydrology and Hydraulics Reports dated February 6, 2007. The Peer Review Evaluation Panel (PREP) has provided a technical review of the Climatology, Hydrology and Hydraulics Reports dated February 6, 2007 and are of the opinion thatthe South Boulder Creek Floodplain Mapping Study meets, and in notable ways exceeds, the accepted standards of engineering practice for floodplain mapping and delineation. The effort is commendable in taking advantage of modem data from weather radars and advanced state-of-the-art computer modeling of rainfall, flood runoff and two- dimensional floodplain hydraulics. The customized Design Storm for the 100-year return interval and related rainfall scenarios are considered to be reasonably representative of the flood producing climatology for the Boulder area. The hydrologic modeling defines a range of possible flood scenarios which could occur and the 2-D hydrodynamic modeling provides a high level of detail in defining risks of flooding in east Boulder from South Boulder Creek. We believe that the reports, supporting data and computer models provide a scientific information base for deliberations on South Boulder Creek flood plain management by the Ciry of Boulder and by potentially impacted residents. This opinion is provided at a point in the study where the climatology, hydrology and hydraulics work is essentially complete subject to some work to refine details of flood risks and to respond to regulatory agency policies. The PREP does have some ongoing concerns with these reports that are provided in an itemized list of detailed comments being provided to the City. Lynn Johnson, PhD, PE Doug Laiho, PE Gordon McCurry, PhD Conveyance Zone Determination Draf~ dated February 26, 2007 The Conveyance Zone identifies the area of the floodplain that should be preserved to assure that flood flows are conveyed with minimal impact to adjacent properties. It reflects the loss of floodplain storage and flood carrying capacity due to encroachment into the flood fringe. Development activities and allowable uses within the conveyance zone are generally restricted to reduce the hazard within this part of the floodplain. The criteria defining the conveyance zone differ slighfly between the City and the County. The City allows encroachment into the floodplain only to the extent that it increases flood water elevations by 6 inches or less. The County allows encroachments that result in water surface changes of up to one foot. They hydraulic model used to simulate the South Boulder Creek floodplain is both fully dynamic and two dimensional. That is, it represents the movement of water in a way that accounts for backwater affects, floodplain storage and hydrograph variations as well as allowing water to move in rivo dimensions. This is in marked contrast to a wnventional steady state approach that assumes concurrent peak flows, dces not account for any potential floodplain storage but dces consider backwater and only allows water to move linearly down the channel. This fundamental difference in modeling approach significanfly alters the approach to simulating the conVeyance zone. A conventional evaluation of the conveyance zone encroaches into the floodplain from each side in a way that equally reduces the conveyance azeas lost in the overbanks. Encroachment continues until the water surface rise has reached a maximum depth of six inches within the City or one foot in the County. Because the flow rate is constant, there is no consideration of floodplain storage and flow moves in only one dimension, Hvs is a relatively straight forward process. In the fully dynamic, two dimensional representation, each encroachment has several direct impacts on the water surface. First, the encroachment imposes a restriction on the potential flow path and direcflon. Rather than merely constricting the channel, it may actually block a primary flow path. Secondly, the encroachment may reduce the available floodplain storage thereby actually increasing the amount of water that remains in the channel. Finally, as the floodplain is encroached, water may be forced down new or different flow paths further exacerbating the amount of flow that may have traveled down a previously identified flow path. These ramifications, in combination, compound the impacts of encroachment. These resulting complications make the automation of the encroachment analysis exceedingly difficult. It was concluded that the South Boulder Creek two dimensional analysis areas were unsuitable for automated conveyance zone determination. Fortunately, the upper reaches of the South Boulder Creek model were represented using a fully dynamic model but using one dimensional flow. While the impacts of reductions in floodplain storage aze still seen, the relatively confined channel doesn t show much storage. As such, the simulation was able to follow the equal conveyance encroachment approach used in conventional models. The floodplain boundary was encroached until a maximum change of one foot was identified corresponding to the standards imposed by the County in this area. MIKE 11 (the one dimensional engine in MIKE FLOOD) has an automated routine to establish these limits. After the automated limits were defined, some manual smoothing and interpolation was required to assure that the one foot rise was not exceeded at any locations along the channel. Along the two dimension reaches of the simulation, a more iterative approach was taken. Prior to initiating any sunulations, a fundamental question needed to be answered: should the conveyance zone be limited to the main channel areas or should it include the west valley as well. After considerable deliberation, it was concluded that, if possible, the conveyance zone should be lunited to the main channel. This was the historic flow path and should be favored over a disconnected alternative flow path. It was also noted that flows didn t enter the west valley until approximately the 50-year flood and that a second conveyance zone would formalize this unanticipated flow path. Because of the two dimensional nature of the sunulation, no automated process for conveyance zone delineation existed. It was necessary to define reasonable limits of encroachment manually and test the assumption. To simulate the conveyance boundary, the model bathymehy was physically changed to elevate the ground at the limits of encroachment. This precluded water from flowing outside these boundaries. Because some of the fributary inflows were introduced into the hydraulic model along the western fringe of the floodplain, it was aLso necessary to redefine the location and character of inflows. These were added to the two dimensional grid elements at the appropriate locations or into channel elements that remained in the model. After evaluaflon, if the results did not comply with the target water surface rise, refinements could be made and the sunulation repeated. As a starting point, the team engaged in discussions and attempted to define a reasonable boundary based on judgement. The boundaries of the conveyance zone were encroached considerably in the area near US 36 and in the reaches downstream to Arapahoe. The conveyance zone was lazgely confined to the area immediately adjacent to the main channel. The resulting sunulation showed major violations of the allowable water surface rise. Upon review, several contributors to the exceedance were identified. First, the loss of floodplain storage and the elimination of the overflow capacity of the west valley resulted in significant increases in flow rates creating a fundamental rise in water surface. The encroachments along roadways that were being overtopped also created major backwater impacts that coniributed to exceedance. The encroachment also limited the ability of flow to move freely across the floodplain and therefore several alternative flow paths were eliminated forcing more water down the path along the main channel. In combination, this simulation was deemed to have an unacceptable impact on water surface elevations. The team recognized the difficulty of simulating a reasonable limit of encroachment was going to be difficult using only the professional judgement of the team and their collective experience. It was concluded that the next iteration should be based on a mathematical encroachment and was set as being those areas where the floodplain depth was six inches. It was hoped that this would eliminate those fringe areas where there was little active conveyance and that the resulting conveyance zone would comply with the prescribed water surface rise. Unfortunately, this analysis also exceeded the allowable rise considerably. Afber review, it was concluded that two primary criteria resulted in the exceedance: the necessary increase in head at roadway overtopping due to the encroachment into the overflow areas and the loss of the floodplain storage resulting from the elimination of those ueas less than six inches in depth. In combinaflon with the introduction of the west valley flow, this presented an unacceptable rise and would require further refinement. It was concluded that one additional simulation would be undertaken in the hopes of finding a conveyance zone that met the established criteria but restricted flows from entering the west valley. Two major refinements were introduced in response to the previous results. First, it was acknowledged that storage along South Boulder Creek was an important characteristic of the floodplain and needed to be retained wherever possible. Two major storage areas had been eliminabed from the eazlier conveyance zone sunulations: the inflows into Baseline Reservoir were blocked and the storage near the interchange of US 36 and South Boulder Road was not incorporated into the model. The conveyance zone boundary was adjusted to preserve both these important storage areas. The second major adjustment was to broaden the limits of the conveyance zone between US 36 and Arapahoe to allow water to move freely between flow paths across the main valley floor. This would allow the increase flows that were introduced from the west valley to find the most efficient flow path and minimize the water surface rise. The results of this configuration were determined to be within the prescribed allowable water surface increases of both the City and the County. While the rise at a specific pixel within the model may have slighfly exceeded the allowable rise, discussions with FEMA indicated that an approach that looked at the entire cross secflon in aggregate was acceptable. This process involved defining cross sections across the conveyance zone and establishing a computed conveyance zone water surface elevation across the entire cross section. This elevation was then compazed to the base flood elevation and, if found to be within the acceptable limits of rise, defined the regulatory conveyance zone water surface elevation. The attached figure defines the proposed regulatory conveyance zone. Proposed Conveyance Zone 22 February 2007 PREP Detailed Review Comments South Boulder Creek Flood Mapping Project Climatology, Hydrology and Hydraulics Reports dated 2/6/07 The Peer Review Evaluation Panel (PREP) review of the current documents describing the Climatology/Hydrology and Hydraulics documents was accomplished in accordance with the following: • The PREP "Mission StatemenY' dated February 28, 2004 • Editorial, typographic, grammar and format are not addressed except to the extent that significa~t issues are involved • Model input parameters, computations, etc are frequently outside of our view and at a level of detail we can not review but for which we depend on the consultanYs team QA/QC process to function properly • The length of the documents and limited time made available to us to review them did not permit a full re-read of these documents. • PREP comments on the Climatology dated December 5, 2005 • PREP comments on the Climatology and Hydrology dated August 3, 2004 • PREP comments on the Hydraulics dated December 8, 2006 In view of the many in-progress documents and reviews that have been conducted over the last three years, it is possible that some of the following comments have been adequately addressed at a location that is not readily identifiable or remains to be addressed in one of the documents. Please let us know where such items have been or will be addressed or why they will ~ot be. Climatology / Hydrology Report 1. Page 29, Section 2. Study team has provided an improved descriptioo of the updated rainfall climatology (note concerns below). Use of the 100-year 6-hour rainfall corresponds to the thunderstorm characteristics as determined through analysis'of the expanded number of rainfall events. Also, using the thunderstorm avoids complexities of a general storm flood runoff upstream of Gross Reservoir. 2. Page 34, Section 23. The use of an expanded radar-rainfall storm data set estabiishes a rational basis for characterizing the storm climatology for the SBC. However, assumptions on the design storm areal "footprinP' and its stationary placement are not well explained. 3. Page 34. It remains unclear how the large increase in volume is due to changing from a moving storm to a stationary storm. It would appear that the more volume is largely due to many of the isohyets covering more area. As requested in Comment 8 of our 12/5/OS review memo, please describe in enough physical and numerical detail so that the explanation is supported. 4. Pages 34-36 There is little comparison of the old and new storms in section 23, leading us to question the section title or the missing comparison-related text. 5. Page 37, Section 2.4. The expanded number of storm events and statistical reductions is considered to be an adequate basis for the design storm development. 6. Page 37, Section 2.4. The third criteria on page 31 for selecting past thunderstorms used for this analysis limits the storms to those with less than 5 inches. This has the potential to bias the storm duration and volume on the low side. More explanation and justification to limiting the storm population should be provided. 7. Page 44, Section 2.4. Study team description of the selection of a stationary storm placement rather than a moving system remains somewhat vague (D-37). The reasons seem to be driven by the regulatory agency review comments and difficulties with modeling a moving storm. (Text states that PREP agreed to this but we don'Y recall that.). Explanation based on storm movement data is lacking. 8. Page 58, Section 6.2. The hydrology study approach to compare three independent measures of flood frequency and magnitude - 1) standard flood frequency analysis of the Eldorado gage flows, 2) paleo-hydrologic assessment of flood peaks and locations, 3) rainfall-runoff modeling - is considered valid in general and represents an advanced state-of-the-art for these kind of studies. While there are some shortcomiogs, difficulties and peculiarities associated with each method, the aggregate outcomes are confirmatory and yield flood peak and volume estimates with moderate variability. 9. Page 61, Figure 34. The text needs to discuss the terms `weighted skew 0.6' and `station skew 1.2' in terms of their meaning and significance. The dashed blue and green lines on this figure need to be labeled or identified in the key. The text needs a brief discussion on how the 90% confidence limits were developed. 10. Pages 63-65. A paleoflood investigation is a representation of what has happened in the past and not necessarily "flows likely to be generated". Part of the value of a paleoflood investigation is that it can provide information on floods that have occurred outside of the period of gaged record or at locations with no gage; this point is missed entirely. There is still no clear indication of specifically how this information was used (e.g. a comparative reference to the paleoflood estimated peak 100-year flow and what meaning this has) and specifics are missing on how this work helped define the spatial distributiou of large storms. Appendix F is an i~adequate representation of the work that has been accomplished. We recognize that having a better product may be outside of consultant team controi and suggest noting that this section will be updated with the U.S.G.S. report when it is available. 11. Pages 79-80, Section 6.7.12. Calibration of the MIKE FLOOD model to the selected historical events was limited by a lack of data for significant flood events. However, the correspondence beriveen recorded and simulated floods is considered to be only marginally acceptable based on the simulated results underestimati~g observed flow volumes by 18.4, 14.6 and 33.1 percent compared to the study team's acceptable threshold of 15% for the calibration storm events, as shown in Table 19. This likely leads to an under-prediction of simulated flow volumes using the Design Storm. The Hydraulics repoR should acknowledge this potential under-prediction of flows and the City should consider this in its upcoming flood risk assessment and mitigation activities. 12. Page 80, Section 6.7.13. It appears that the sensitivity analyses reported are for a previous version of the model that had much lower flows. If that is the case, then the text needs to clearly note that the sensitivity results are not for the version of the model being used in the study. 13. Page 82, Section 6.7.14. The final paragraph of this sensitivity analysis on Antecedent Moisture Content inconectly states the results of the `typical' values used and those of the General Storm. The lower 95% confidence limit on the FFA flows at Eldorado Springs are 3040 cfs while the sensitivity result for the general storm is 2770 cfs, based on information presented in Tables 16 (FFA flows) and 20 (Sensitivity results). The text should be revised accordingly. 14. Page 82. 080304 comment #32 and to some extent #34 - The description of the operation of Gross Reservoir in the previous and current narzative is inconectly represented. The phrase "....Gross could be at spillway crest levels any time during the year...." may be true, but is misleading. In fact, Gross does not fill every year and is rarely full during the time of the year when thunderstorms or general storms typically occur. Furthermore, Gross may have a"limited impact" on major floods originating from the new design thunderstorm, but is has and will continue to have a significant impact on more frequent floods produced by general storms (e.g. the 1969 event). This is one reason why South Boulder Creek has rarely been out of bank in the last 37 years, why the mean annual peak flow in a gage analysis of the full period of record is so low and even why the consultant team has chosen not to use the period of gage record after 1954. Please change this wording to represent the facts and statistics. The current wording does not provide a correct ` justification" of a modeling assumption. It would be better, simpler and more easily accepted to describe the final decision on the reservoir level as being a well established policy that the reservoir can not be used as a flood control reservoir in the model as it was not constructed nor is it operated to provide that function. I5. Page 83, Section 6.7.16. The text notes that significant localized trib~tary flooding is possible under the scenario of downstream storm locations. The City needs to consider this possibiliry in its flood mitigation efforts and the text should note this concern. 16. Page 84, Section 6.8. A number of flood scenarios have been examined including the selected "best estimate", the `bld" design storm, the transposed storms for the Big Thompson and FoR Collins floods, and the comparison to the Taggert Study. Results of these simulations yield a range of possible flood flows and inundation areas for the South Boulder Creek flood plain. It seems that generally reasonable and rational choices of these factors have been made by the study team, and that the overall study represents an advanced state-of-the-art of professional practice for this type of endeavor. 17. Page 85, Section 6.9. Definition of a°single" number which characterizes flood risk is hard to pin down. There is inherent uncertainty in the complexity of meteorological, hydrological and hydraulic processes; and in mountainous areas, these factors can vary within short distances depending on topography, soils, and land use. Lack of assessment of the uncertainty in the various components that result in the predicted flows is a major shortcoming of the report. It is understood that some effort is being directed to this topic which is highly encouraged. 18. Page 85, Section 6.9.1. The text needs to be revised to note that the simulated General Storm falls below the 95% confidence limits of the FFA flows at the Eldorado Gage, and discuss the implications of this underestimate. 19. Pages 87-88. While it is stated that the thunderstorm controls the 100-year flood, it is also stated that the general storm controls floods occuning more frequently than the 25-year event. Is this because of the higher antecedent moisture conditions that develop during a general storm? It needs to be made clear how this will be reflected in the hydrology that is proposed to be the new basis for regulation? 20. Page 89, Section 7.0. The wording describing the PREP involvement implies that all our comments are incorporated into the work. This statement must be changed. While our opinions and recommendations have in most cases been incorporated into the project, not all of them have been. We understand that the City together with the consultant team has the final say on how comments are addressed. A simple statement in the text that our opinions have been incorporated where judged applicable would be sufficient. 21. Page 90, Section 7.0. The concluding sentence appears to refer to the incorrect table of simulated flows (10 vs 24) and should be corrected. 22. Appendix D. This is an improvement over previous drafts in providing mote clear explanations of the development of the design storm, flood frequency and related topics. 23. Appendix G. The MIKE-FLOOD model is evidently an acceptable model to FEMA, although documentation of FEMA acceptance has yet to be provided and should be included as an appendix and/or posted on the City's project web page. Inclusion of the MIKE User Guide as an appendix in the Climatology/Hydrology report does not seem to be necessary and adds to the overall large volume of the report; it also could be included separately on the City's project web page. 24. Appendix J. The QA/QC Appendix J is brief and barely adequate in describing procedures for minimizing errors for the modeling study. However, through the PREP's participation in a number of weekly teleconferences and various meetings with the study team we acknowledge that the study team has employed state-of-the- art methods for data collection, a~atysis, computer modeling and, more recently, error checking. 25. Appendix L. The Storm Center discussion addresses the impacts of placeming a design storm at other locations which result in local storm flooding greater than what would be expected from the chosen design storm (which emphasizes main stem flooding). The assessment is considered reasonable. It is understood that flooding on the main stem of South Boulder Creek is the primary focus of the floodplain study. 26. Appendix M. The comparison of the MIKE FLOOD model with the Taggert swdy modeling establishes some basis for comparison between so-called "standard" methods and the MIKE FLOOD modeling effort. That effort mainly highlights the differences that can arise given the basic assumptions of rainfall magnitude and areal distribution. The SBC study effort is considered to be better founded on actual rainfall climatology and basin conditions than the Taggert study, and therefore the results obtained are considered more realistic of the flood threats to be expected. 4 Hydraulics Report 1. Page 1. It is considered that the South Boulder Creek study, particularly the 2-D hydraulics modeling, exceeds the standard of practice typically applied for floodplain studies. . 2. Page 22 and Appendix E. The sensitivity analyses are informative. Differences between inundation areas for the range of roughness values aze difficult to discern from the high resolution maps provided. However, lack of assessment of uncertainty bounds is a major shortcoming of the report. It is understood that some effort is being directed to this topic which is highly encouraged. 3. Page 26. The specific values of the smaller volume and the resulting discharges should be mentioned. 4. Page 8 and Appendix C. Dry Ditch No. 2 was selected for inclusion as a 1-D model component because its capacity was judged to be greater than 1000 cfs. In the model runs does this channel carry 1000 cfs ar more? How does Dry Ditch No. 2 get through the US36/S. Boulder Road intersection? It is not listed on the table of blockages (pg GS). 5. Page 8. The location and significance of water flowing out of the 2-D grid (such as into Baseline Reservoir) remains inadequate{y addressed. 6. Page 8.Locations of drainage channel features identified in text are not shown on accompanying figures. Where is Viele Channel? An overview map of drainage features with notations of all features mentioned in the text should be provided 7. Page J-6, roughness values in figure do not match values in text. 8. Page J-9. The figure on page J-9 appears to be out of place. Also there is no link to High Hazard information of web site. 9. Appendix A. The report does not cite the link to FEMA testifying that the MIKE models are fully acceptable; this needs to be included. 10. Appendices C, D and E. Appendices clarify a number of questions on hydraulic model parameter selections and rationales for these. The Appendix A- MIKE FLOOD Documentation - is long (~800 pages) and should not be part of the report placement on the City's project web site would be adequate. 1 I. Appendices F and I. The appendices F(Floodplain Zone Definitions and Island Determination) and I(Flood Insurance and Regulatory Implications) are informative additions to the SBC study reports, and provide valuable guidance to citizens in understanding the nature of flood plain regulations that may apply to their properties. 12. Appendix D. The calibration/ground-truthing effort using aerial photos of historic flooding and citizen recollections is understandably limited in accuracy. However, the effort is perhaps the best that can be done and does provide some guidance in assessing the hydraulic modeling results. 13. Appendix G. (Floodway/Conveyance Zone) describes the procedures being developed to define the conveyance zone. It is understood that there are policy directives from FEMA which dictate these procedures. However, the assumptions requiring all flow volume to be funneled through the conveyance zone negates the influences of flood plain storage (occurring mostly in reserved open space) which greatly reduces the channelized flood volumes. Thus the conveyance zone will be required to be large in comparison with modeled flood flows associated with the regulatory flood plain delineation which is based on the 2-D hydraulic model. 14. Appendix I. It needs to be made clear that there are islands, whether they are shown or not, and how they ue to be handled from a regulation standpoint. Members of the impacted public may feel that the study may be limited in value if they need to appeal or process a LOMA. 15. Appendix I. The extensive use of this Zone X and the variety of flooding conditions that can result in mapping an azea as Zone X are well described, however, the precise manner in which this zone will be regulated should be more completely described. 16. Appendix J. Although brief, Appendix J on hydraulic modeling QA/QC procedures is a valuable addition to the study and describes the types of interactions (e.g., field visits) between the consultant and city staff in explainiug and selecting hydraulic modeling parameters.