CARRIAGE HILLS NO. 2 DAM (DAMID 040110) SEPTEMBER 2013 DAM FAILURE FORENSIC INVESTIGATION REPORT Division of Water Resources, Dam Safety Branch

1. REFERENCES 1. Dam Failure Investigation Guideline, ASDSO, Dec. 8, 2011. 2. Exceedance Probability Analysis for the Colorado Flood Event, Sept. 9‐16, 2013, NOAA NDSC, Sept. 17, 2013 3. Colorado Front Range Flood of 2013: Peak Flow Estimates at Selected Mountain Stream Locations, NRCS, Dec. 2013. 4. Public Communications Guide, Colorado Rains and Flood, 2013, USBR, Updated October 21, 2013. 5. Peak Discharge Indirect Measurements of the September 11‐13, 2013 flood for Selected Streams along the Colorado Front Range, Technical Memorandum No. 86‐6833000‐2014‐24, USBR, June 2014. 6. Hydrologic Evaluation of the Big Thompson Watershed, Post September 2013 Flood Event, Colorado Department of Transportation, prepared by Jacobs, August 2014. 7. Fish Creek Hydrology Report, prepared by Matrix Design Group, August 2014.

2. SCOPE & PURPOSE The purpose of this report is to document the failure of the Carriage Hills No. 2 Dam, which occurred during the September 2013 floods, with respect to the following: possible causes of the failure, possible damages from the breach floodwave, and lessons that can be learned to help avoid future dam failures. The scope of the study included review of State Engineer’s Office (SEO) construction & dam safety inspection files, post‐failure field inspection and surveys of the dam breach and stream channel high water marks, review of processed rain data, review of previous studies, associated analysis and discussion, and report documentation.

The SEO Dam Safety Branch (DSB) assigns dams to an individual Dam Safety Engineer (DSE) in a region, who is then primarily responsible for inspecting his/her assigned dams and enforcing state dam safety regulations there. In order to provide an independent review in accordance with Reference 1, this dam failure forensic study was performed by a member of DSB staff not previously associated with the inspection or regulation of the Carriage Hills No. 2 Dam.

3. BACKGROUND The Carriage Hills No. 2 Dam (DAMID 040110) is classified by the DSB as a Low Hazard, Minor Size dam. The DSB assigns hazard classification based on a “sunny day” failure scenario, absent flooding. The dam reportedly overtopped and failed during the September 2013 widespread catastrophic flooding in Northern Colorado. The storm was an unusual, late season event whereby warm moisture and upslope winds allowed up to 16‐17 inches of rain to fall at elevations as high as 10,000 ft. The rain event did not have unusually high rain intensity, but had a long duration and high volume. It caused extensive flooding throughout Northern Colorado.

The Carriage Hills No. 1 & No. 2 Dams are owned by the Town of Estes Park. They are located in the Carriage Hills subdivision approximately 2‐1/4 miles southeast of downtown Estes Park. The dams lie in series on an

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 1 February 27, 2015 unnamed tributary, approximately 400‐ft upstream of it confluence with Fish Creek. Carriage Hills No. 2 is the downstream of the two, and it lies about ½ mile east of (St. Vrain Avenue). Figures 1, 2 and 3 show a vicinity map, location map, and 2011 aerial image around the dam.

Figure 1. Carriage Hills No. 2 Dam vicinity map.

Figure 2: Location map for Carriage Hills No. 2 Dam.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 2 February 27, 2015 Carriage Hills No. 2 Dam

Figure 3: 2011 NAIP areal image showing the Carriage Hills No.2 Dam and surroundings.

The DSB was notified by phone on the morning of Sept. 12, 2013, by the Town of Estes Park water manager that the Carriage Hills dams were overtopping by flood flows. The Dam Safety Engineer (DSE) assigned to the dam made a follow‐up call to the Town’s public works director and understood the Town was taking emergency action to evacuate downstream residents along Fish Creek. The DSB subsequently lost all communication with Estes Park for the duration of the flood and for some time thereafter. A DSE made it to Estes Park on Sept. 19, 2013, and confirmed that the Carriage Hills No. 2 (lower) Dam had failed. Carriage Hills No. 1 (upper) Dam sustained damage from overtopping but did not fail.

4. FIELD INVESTIGATIONS: The DSB performed the following field investigations of the Carriage Hills No. 2 Dam failure and surrounding flooding:

Sept. 19, 2013: Just under a week after the dam failure, the DSB visited the site as part of on‐going flood emergency response. The main purpose of this visit was to document conditions at the breach and the dam site for forensic purposes and to ensure the dam was in a safe condition.

Sept. 26, 2013: As our immediate emergency response effort slowed a crew of two DSEs returned to the site to perform a detailed survey of the dam and talk to eyewitnesses. Table 1 shows results from our level survey of the dam, breach, and spillway. Figure 4 shows a profile of our survey. Photos 1‐14 show conditions encountered during the investigation. We made the following visual classifications of soils in the right breach side slope (see Photo 5 of the soil profile at the breach): 0’‐1.4’: organic soil 1.4’‐2.4’: light‐colored sandy clay, very stiff 2.4’‐2.6’: clayey sand with gravel 2.6’‐ 6.0’ (bottom of breach at sample location): dark colored sandy, gravely clay with organics Bag samples were collected from the embankment, but no lab testing was performed as part of this study.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 3 February 27, 2015 Table 1: Sept. 26, 2013 level survey notes for Carriage Hills No. 2 Dam breach.

Figure 4: Plot of survey points along Carriage Hills No. 2 Dam embankment, breach and spillway.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 4 February 27, 2015

Photo 1: Looking upstream through the Carriage Hills No. 2 Dam breach. Note Photo 2: Right breach side slope, looking upstream from the downstream toe. plastic liner. Material in foreground appeared to be weathered rock.

Photo 3: Dam breach (red arrow) viewed from the Carriage Hills No. 2 Photo 4: Looking right across the upstream slope. Red arrow shows the breach reservoir. location.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 5 February 2015

Photo 5: Carriage Hills No. 2 Dam embankment soil profile on right side of the Photo 6: Left breach side slope, looking left from the downstream toe. breach.

Photo 7: Looking right across the dam from the left abutment. Red arrow Photo 8: Head cut erosion damage on the downstream slope of the Carriage shows the breach location. Hills No. 2 Dam, located left of the dam breach.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 6 February 2015

Photo 9: Looking downstream through the spillway (red arrow). Photo 10: Erosion damage at Scott Ave. caused by Carriage Hills No. 2 Dam spillway flows.

Photo 11: Bent grass along the spillway return flowpath below the dam. Photo 12: Sand, gravel, and rock debris deposited below the dam where spillway return flows spread out.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 7 February 2015

Photo 13: Rock weir (red arrow) and willows in spillway approach. Photo 14: Spillway approach viewed from the left side of the reservoir showing thick vegetation.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 8 February 2015

Nov. 14‐15, 2013: Two DSEs returned to survey the upstream and downstream stream channels and associated high water lines, to survey flood damage, and to meet with more eyewitnesses.

5. EYEWITNESS ACCOUNTS DSB staff attempted to interview neighboring residents after the Carriage Hills No. 2 Dam failure. This effort was hindered because many people in the surrounding neighborhood were either evacuated or had left town after the flood. Table 2 summarizes eyewitness accounts obtained by the DSB.

Table 2: Eyewitness Accounts obtained by the DSB Name/address/phone Date of number interview Eyewitness Account Henry Rische / 1160 9/26/2013 Mr. Rische reported water was flowing over the top of both Carriage Hills dams by Lakeshore Dr. / 970‐ daybreak on Thursday, 9/12/13. He reported a 2nd hand account that his neighbor 586‐5591 heard a roaring sound early in the morning on Friday 9/13/13. Photos 15‐18 were taken by Mr. Rische’s daughter who had left town by the time of the interview. The photos are believed to have been taken on Thursday morning, 9/12/13. Anonymous woman 9/26/2013 Reported the lower Carriage Hills dam apparently broke overnight of 9/12‐13. The residing at 1100 Scott dam was gone on the morning of Friday, 9/13/13, when she woke up. Ave. Bob Harvey/ 11/14/2013 Mr. Harvey reported there were significant flood inflows to Carriage Hills No. 1 Pinewood Drive from a side tributary (unnamed) east of Larkspur Ave, east of Hwy 7 and entering (upstream of Hwy 7) / over Carriage Drive. Also there was ~6‐in deep flow overtopping the paved bike 970‐576‐5576 path located ~300‐ft downstream of Hwy 7 (survey Cross‐Section 1) on the north tributary. He corroborated that the maximum high water level on the CO HWY 7 5’x7’ box culvert was probably at 3/4 height of the culvert. He said the culvert was not submerged and the road did not overtop. He reported significant flows coming off Prospect Mountain on the north side of north trib through the subdivision where he lives (Pinewood Drive). Charles Bonza / 11/15/2013 Mr. Bonza reports at 9/12/13, 2200 hrs Fish Creek water level was at the bottom 1551 Fish Creek Road of his snowplow, immediately west of his driveway, when he looked outside. At (corner of Country 9/12/13, 2220 hrs Fish Creek surged and the water level rose approximately to the Club Road) top of his snowplow (measured 2.1‐ft vertical). He believes this surge was caused by the Carriage Hills No. 2 dam failure. The floodwave reportedly receded in approximately 30 minutes (2250 hrs). Fish Creek level was reportedly steady before and after the surge.

[DSEs surveyed the Fish Creek channel at the reported location of the snowplow during the flood]. Bonza videos / 9/12‐ Mr. Bonza provided us with 11 videos of Fish Creek flooding from Thursday 1551 Fish Creek Road 13/2013 9/12/13 and the morning of Friday, 9/13/2013. We made the following notes from our review of the videos and based on Mr. Bonza’s comments in the videos:

9/12/2013, 03:00 hrs – Mr. Bonza reports he was still able to walk across Fish Creek to help his neighbor move things out of the flood (00365.MTS)

9/12/2013, 14:46 hrs [corresponds to a lull in rain according to AWA rain data, which is supported by the video] – Fish Creek flow had risen such that Mr. Bonza presumably could not walk across. Country Club Road was still intact but was shown overtopping by Fish Creek flood flows. Mr. Bonza notes that water had earlier pooled in his driveway and around the bottom of his snowplow (00365.mts)

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 9 February 27, 2015 9/13/2013, 05:50 hrs – Country Club Road is gone (washed out) at the Fish Creek crossing. Mr. Bonza said “No more road…not unexpected. In fact, it took too long and as a result it flooded our basement before it broke”. He said the flood water came up about 9‐inches in their basement and nearly covered the top of the snowplow (00365.MTS)

9/13/2013, 09:23 hrs – 00375.MTS video shows flood damage in basement at 1551 Country Club Road (Bonza house) from overnight flooding, 9/12‐13/2013. Water damage to kitchen, laundry room, tool room, bedroom, bathroom, storage area, piano, etc. High water line reported as 3 inches above sill on the glass doors, and about 1 foot above the basement slab. Video shows inside the bathtub and Mr. Bonza reports “it doesn’t look like we’ve had too much water coming back up backwards there” [backflow through sewer]. Don Cheley / Cheley 9/19/2013 On 9/19/2013 Mr. Cheley gave an SEO Dam Safety Engineer a tour of extensive Colorado Camps, 3960 (site visit) flood damage on the Camp Cheley property, located along Fish Creek and an Fish Creek Road / 303‐ and unnamed tributary of Fish Creek that drains the north side of the Twin Sisters 377‐3616 2/13/2015 Peaks between The Crags and Pierson Park. Damage observed by the DSE included (phone call) washout of the camp’s main access road at Fish Creek, head cutting erosion at the road crossing the unnamed tributary of Fish Creek, failure of a small irrigation diversion dam and pipeline located west of Sanborn Lake, damage to structures including large boulders and debris deposited inside of a building along the unnamed tributary, and severe channel erosion and debris deposition.

In order to verify the timing of Fish Creek peak flows we contacted Mr. Cheley by phone in early Feb. 2015. We conducted a phone interview with Mr. Cheley on 2/13/2015. As would be expected he still had a clear recollection of the flood events. Regarding the timing of flood peaks, he recalled that the rain and runoff began to get heavy in the early morning hours of 9/12/13 (Thursday). At that point, he regarded the rain as a good soaking rain. That morning the camp staff worked to close ditches and other routine tasks to prevent flood damage. They worked in rain gear throughout the morning but generally felt like things were under control. By the afternoon on 9/12/13 that changed and the flood began to do serious damage. Flood flows overwhelmed the camp’s access road culvert and the road overtopped and washed out. Mr. Cheley reported that the flooding broke loose that night sometime after midnight (9/13/13) with the highest flood flows and large amounts of debris. He reports hearing the pounding of debris and boulders moving down the creek overnight. This flood peak and debris is reportedly when one of the camp’s buildings along the unnamed tributary of Fish Creek was filled with boulders and sediment.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 10 February 27, 2015

Spillway return flow

Scott Ave CH2 Dam crest

To CH2 dam To Fish Cr.

Photo 15: Overtopping flood at Carriage Hills No. 2 (lower) dam. Photos Photo 16: Spillway return flow joining overtopping flows en route to Fish 15‐18 are believed to be from the morning of Sept. 12, 2013. Creek.

Photo 17: Total flow below CH2 Dam crossing Lakeshore Drive, at Photo 18: Overtopping and spillway flow at Carriage Hills No. 1 Dam confluence with Fish Creek. (upper). Carriage Hills No. 2 reservoir is in the foreground.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 11 February 2015 6. FILE REVIEW: CONSTRUCTION & FILE HISTORY The State Engineer approved construction plans for the Carriage Hills No. 1 and No. 2 dams on Jan. 5, 1967 (Construction File C‐1195). SEO files contain no documentation of construction and the drawings are not marked as‐built. The C‐1195 plans (see Appendix A) show the Carriage Hills No. 2 Dam as a zoned earth embankment with 2H:1V upstream and downstream slopes and a crest width of 10‐ft (Figure 5). The upstream slope is shown protected with 12‐inch thick riprap. An upstream embankment zone of “selected impervious fill” is shown, followed by a 10 MIL polyethylene membrane, and then a downstream zone of “clay and disintegrated granite”. The dam is shown founded on bedrock. No foundation treatment is shown. A 24‐inch diameter 12 gauge corrugated metal pipe (CMP) outlet conduit is shown with an upstream slide gate. The spillway was to have a 10‐ft bottom width excavated into bedrock with vertical slopes, and 5 feet of freeboard below the dam crest. The spillway capacity is given as 377 cfs.

Figure 5: C‐1195 plans maximum section of the Carriage Hills No. 2 Dam.

The first record of performance in the SEO files is from a 1983 dam inspection performed by a water commissioner. He notes trees and brush growing on the dam. In 1985 the water commissioner noted “no outlet found” and in 1986 the DSB Engineer’s Inspection Report (EIR) stated that willows were obstructing spillway flow. The Town of Estes Park took ownership of the dams in 1991. In July 1991 the DSB EIR required the Town to remove obstructions from the spillway, provide a minimum of 3‐ft of freeboard, and repair or replace the outlet works. The Town subsequently requested they be allowed to operate pumps for emergency drawdown purposes in lieu of replacing the outlet works, which the DSB agreed to with conditions.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 12 February 27, 2015 In November and December 1991, the DSB performed a dam breach study using NWS Breach 87. Four dam failure cases were investigated: (1) piping failure of the upper dam only, (2) piping failure of the lower dam only, (3) simultaneous piping failure of both dams, and (4) overtopping failure of the lower dam caused by a piping failure of the upper dam. Peak breach flows for cases (2) and (4), considered most relevant here, were 1162 cfs and 5409 cfs. No formal review of hazard classification was documented, but the Hazard is denoted as “3” in the analysis, where 3 in the previous 1‐3 hazard rating system corresponds to the current Low Hazard rating.

The 2002 DSB EIR includes a photo of a 4‐ft diameter hole filled with water at the downstream toe of the dam, which was assumed to be related to the abandoned outlet. The 2002 EIR required the Town to restore the spillway to the size shown on the C‐1195 construction plans or else hire an engineer to do an alternative design. The 2008 DSB EIR reported that work was done to the spillway to restore it to the design width of 10‐ft as well as adding riprap protection to the approach and downstream channel. The 2008 EIR cited the old CMP outlet as a potential problem if it was not properly abandoned and required plans & specifications for outlet rehabilitation.

A tabulated log of the SEO File is provided in Appendix A.

7. ANALYSIS 7.1 Rainfall Analysis: Applied Weather Associates, Inc. provided the SEO DSB with quality‐controlled spatial and temporal rain data for the September 2013 storm processed using their SPAS program.

7.1.1 Spatial Rain Data: Figure 6 maps rain totals for various subasins above the Carriage Hills dams and on Fish Creek. Subbasins 1‐3 are for Fish Creek downstream of the Carriage Hills dams. Subbasins 4, 5, 6 & 8 drain directly to the Carriage Hills dams. Subbasin 9 corresponds to the USBR’s Marys Lake. Marys Lake did not fill and reportedly did not discharge into the Carriage Hills drainage area during the Sept. 2013 flood. Finally Subbasin 7 is for the mainstem Fish Creek upstream of the Carriage Hills dams. Figure 7 maps Subbasins 4, 5, 6 & 8 (those that directly affected the Carriage Hills dams) on the USGS topographic map.

Figure 6 shows the spatial distribution of rainfall. Maximum rain totals of 16‐17 inches occurred at the south end of the Fish Creek basin (SB7) on the upslopes of the Twin Sisters peaks. It is worth noting that large rain amounts fell as high as 10,000 ft elevation, in contrast to the often cited belief that rain events are not significant drivers of flooding above 7,500 ft along Colorado’s Front Range. Lower rain totals of 9‐10 inches fell at lower elevations (SB 1 & 2). The spatial average total rainfall for the Carriage Hills subbasins (SB 4, 5, 6 & 8) was 10.7 inches. Table 3 shows total rain amounts by depth (inches) and volume (acre‐feet) for each subbasin and for the aggregate.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 13 February 27, 2015

Figure 6. Spatial distribution of total rain depths over the Fish Creek drainage basin during the Sept. 2013 storm event (rainfall analysis by Applied Weather Associates Inc).

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 14 February 27, 2015

Figure 7: Subbasins (SB 4, 5, 6 & 8) draining to the Carriage Hills dams delineated on the USGS topographic map.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 15 February 27, 2015

Table 3: September 8‐17, 2013, Rain Totals (depth and volume) Subbasin Area (sq. mi.) Total Rain Depth Total Rain Volume (inch) (acre‐feet) 1 0.80 9.6 411 2 1.55 9.6 792 3 0.24 10.1 128 4 0.12 10.4 66 5 0.91 10.4 505 6 0.11 10.7 66 7 10.23 13.9 7,593 8 1.49 11.1 882 9 0.16 10.4 87 Total 15.60 12.7 10,560

7.1.2 Temporal Rain Data: AWA reported rain time series in Coordinated Universal Time (UTC); the DSB subsequently converted all times to Mountain Daylight Time (MDT), local time at the time of the flood, for this study. According to the AWA data, adjusted to MDT, rain began on the afternoon of Sept. 8, 2013 (Sunday). The first inch of rain slowly accumulated on the Fish Creek basin by Sept. 11, 2013 (Wednesday) at 00:50 hrs. The second inch of rain had fallen by Sept. 11, 21:10 hrs. The largest amount of rain (approximately 9.3 inches averaged over the entire basin) fell between Sept. 11, 21:10 hrs and Sept. 13, 00:00 hrs (see Figure 8). A small rain event followed on Sept. 15th.

9/12/13, 22:00‐9/13/13 00:00

9/12/13, 06:00‐14:00

9/11/13, 21:10 ‐ 9/12/13 04:00

Figure 8: Temporal mass curves corresponding to selected Fish Creek subbasins

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 16 February 27, 2015 7.1.3 Rainfall Intensity: Figure 9 shows show three distinct spikes in rain intensity that occurred within the largest portion of the storm (Sept. 11, 21:10 – Sept. 13, 00:00). These spikes generally correspond to spikes in observed flooding on Fish Creek, storage at Lake Estes, and possibly to the failure of the Carriage Hills No. 2 (see Section 8). Table 4 shows maximum rain intensity during each of the three rain spikes, averaged for the Carriage Hills subbasins (SB 4, 5, 6 & 8) and for mainstem Fish Creek (SB 7).

9/12/13, 06:00‐14:00

9/11/13, 21:10 ‐ 9/12/13 04:00 9/12/13, 22:00 – 9/13/13 00:00

Figure 9: Incremental 5‐minute rain intensity for selected Fish Creek subbasins

Table 4: Sept. 2013 rain intensities and total rain during the three time periods of highest intensity Carriage Hills dams Subbasins (SB 4, 5, 6 & 8) Mainstem Fish Creek (SB 7) Equivalent Equivalent Max. 5‐ Hourly Total Rain Max. 5‐ Hourly Total Rain minute Intensity during Interval minute Intensity during Rain (in) (in/hr) (in) Rain (in) (in/hr) Interval (in) 9/11/13 21:10 – 0.105 1.26 3.30 0.120 1.44 4.56 9/12/13 04:00 9/12/13 06:00 – 0.046 0.56 2.08 0.107 1.28 3.10 9/12/13 14:00 9/12/13 22:00 – 0.075 0.89 0.75 0.099 1.19 0.88 9/13/13 00:00

The intensity data show the Sept. 2013 storm was not extreme in that sense. The maximum hourly intensity has less than a 1‐year average recurrence interval (ARI) by NOAA Atlas 14. However the storm was a long, high volume event: the 48‐hour rain total for SB 5 & 8 (main Carriage Hills subbasins) had more than a 1000‐YR ARI. Figure 10 plots rain totals for SB 5 & SB8 at various durations, using the maximum total for each duration, along

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 17 February 27, 2015 with NOAA Atlas 14 frequency estimates for each corresponding duration. Figure 10 shows the Sept. 2013 maximum 15‐minute and 1‐hour rains were well below the NOAA Atlas 14 100‐YR estimates. For the 6‐hour duration the Sept. 2013 rain was approximately equal to the Atlas 14 100‐YR estimate. The maximum Sept. 2013 24‐hour rainfall plots between the Atlas 14 500‐YR and 1000‐YR frequency estimates. The maximum Sept. 2013 48‐hour rainfall exceeded the Atlas 14 1000‐YR estimate. NOAA published the Sept. 2013 storm data for the Justice Center rain gage in Boulder, Colorado (see Reference 2) and found a similar trend in ARIs.

Figure 10: Plot of Sept 8‐17, 2013, maximum rainfall in the Carriage Hills dams basin for various durations compared to corresponding precipitation frequency estimates from NOAA Atlas 14 (NA 14). AEP is Annual Exceedance Probability or 1/ARI.

7.2 Indirect Estimates of Peak Streamflows: On November 14‐15, 2013, DSB staff returned to Estes Park to survey the stream channel and hydraulic structures above the Carriage Hills dams. Two tributaries come together above the dams; neither are named on the USGS topographic map, but for the purpose of this study we call them CH North Trib (subbasin SB 5) and CH South Trib (SB 8). They combine into CH Main (SB 4 & SB 6) approximately 1500 ft upstream of the upper dam. During our site visit we surveyed a 5‐ft x 7‐ft box culvert under Colorado Highway 7 (St. Vrain Avenue), a bike path crossing with 3‐15 inch HDPE culverts (Cross‐section 1), the Carriage Drive crossing with 2x24 inch culverts (Cross‐section 2), and a channel section on CH Main about 1000‐ft upstream of the upper dam (Cross‐section 3). Figure 11 shows the location of the sections.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 18 February 27, 2015

Figure 11. Location of survey cross‐sections made above Carriage Hills dams for indirect flow measurements.

7.2.1 Colorado Highway 7 Culvert: The culvert is a concrete box with a 5‐ft span x 7‐ft rise. The length was measured at 114‐ft and the slope surveyed at 3.0%. We surveyed a high water line at the upstream end at 5.09‐ ft above the culvert invert. A neighbor reported the highest water level during the flood at the culvert was about ¾ of the culvert height (see Section 5), which agrees well with our measured high water line (73% of culvert height). The resident confirmed that the culvert was not submerged and Highway 7 did not overtop during the flood. We used Federal Highway Administration’s HY8 Culvert Analysis Program to estimate the peak culvert flow corresponding to the surveyed maximum headwater: Peak flow=175 cfs at the surveyed high water mark elev. 110.9 (assumed survey datum). A sketch of the culvert and summary results from HY8 are shown in Figures 12 & 13 below.

Figure 12: Cross‐section and profile of surveyed 5’x7’ box culvert at Colorado Highway 7 crossing over the Carriage Hills North Tributary.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 19 February 27, 2015

Figure 13: FHWA HY8 summary output for Colorado Highway 7 5’x7’ box culvert.

7.2.2 CROSS‐SECTION 1, Bike path and three 15‐inch culverts: Cross section 1 is at the bike path crossing over the CH North Trib, immediately downstream of CO HWY 7. We surveyed the bike path profile for overtopping analysis and the three 15‐inch HDPE culverts through the embankment. We surveyed a high water mark at approximately 0.9 ft above the low point in the bike path. This gives reasonable agreement with a resident’s account of about ½‐ft of water flowing over the bike path during the flood (see Section 5). Again using FHWA’s HY8 we calculated: Peak flow=151 cfs at the surveyed high water elevation 99.24 (assumed survey datum). A sketch of the culverts and summary results from HY8 are shown in Figures 14 & 15.

Figure 14: Surveyed bike path and three 15‐inch diameter HDPE culverts. Surveyed elevations for the bike path profile are not shown.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 20 February 27, 2015

Figure 15: FHWA HY8 summary results for Cross‐Section 1, bike path crossing over the CH North Trib.

7.2.3 Cross‐Sections 2 & 3: We performed redundant measurements on the North and South Carriage Hills tributaries and on the main channel, knowing it can be difficult to get good estimates using indirect methods. After significant analysis we decided to discard our indirect flow measurements at Cross‐sections 2 and 3. After numerous checks we felt our Cross‐section 2 estimate was too low and our Cross‐section 3 estimate was unreasonably high. In the end our reasons for discarding these measurements were: we were uncertain in determining the high water line associated with overtopping flows at Carriage Drive (Cross‐section 2), and on further analysis Cross‐section 3 was determined to have non‐uniform flow in a series of drops and pools, which prevented us from estimating discharge using Critical Depth or Normal Depth methods.

7.2.4 Estimated Inflow to Carriage Hills No. 2 Reservoir: Of our various indirect flow measurements we feel most confident in the 175 cfs estimate at the Highway 7 culvert for the CH North Trib because: (1) we have eyewitness corroboration of the high water line, and (2) it is a standard hydraulic structure for hydraulic analysis purposes. Also we obtained reasonable agreement from the less certain estimate at Cross‐section 1 (151 cfs), ~14% error. Therefore we decided to estimate the peak flow into the Carriage Hills reservoirs by scaling up the unit discharge from the CH North Trib (0.91 sq. mi.) to the total basin area (2.63 sq. mi.):

Estimate of Peak Inflow to Carriage Hills No. 2 Reservoir = (175 cfs/0.91 sq.mi.) x 2.63 sq.mi. = 506 cfs

Photo 19 provides a visual of the flood flows into the Carriage Hills reservoirs. Photos show that both dams were overtopping. Reportedly Photo 19 was taken on Thursday morning, Sept. 12th, which was after the longest, most intense period of rain occurred in the basin, but not necessarily the peak streamflow. Hydrologic modeling (Jacobs/CDOT, August 2014) suggests stream flow on Fish Creek peaked at around 14:00 hrs on Sept. 12th.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 21 February 27, 2015

Photo 19: Flood photo from Lakeshore Drive resident, reportedly taken on the morning of Thursday, Sept. 12, 2013, showing inflows to Carriage Hills No. 1 Dam (dam overtopping in foreground).

Matrix Design Group (August 2014) estimated a natural (i.e. absent the Carriage Hills No. 2 dam breach flood) peak flow during the Sept. 2013 flood of 2,400 cfs over the 15.9 sq.mi. drainage area for Fish Creek. The resulting yield is approximately 152 cfs/sq.mi. Our estimate for the Sept. 2013 peak flow into the Carriage Hills No. 2 reservoir yields 192 cfs/sq. mi. We believe our estimate is reasonable but probably on the high side.

7.3 Spillway Discharge Rating: The existing spillway at the time of the Sept. 2013 flood did not match the spillway on the approved C‐1195 construction plans. The plans showed a spillway with a 10‐foot bottom width and 5 feet of freeboard; the estimated discharge capacity on the plans was 377 cfs. The existing spillway control section, as surveyed on Sept. 26, 2013, was found to have a 4‐foot bottom width and approximately 3.4 feet of freeboard to the low point of the dam. The spillway was also found overgrown with brush. Figure 16 shows a cross‐section of the spillway drawn from our Sept. 26th survey.

We modeled the existing spillway in USACE’s HEC‐RAS open channel hydraulics program to estimate the spillway discharge capacity at the time of the flood. We used a roughness coefficient of 0.1 to account for brush in the spillway. We used the following four cross‐sections:

River Section (RS) 0: Downstream end of spillway control section and dam crest RS 6.5: Upstream end of spillway control section and dam crest RS 20: Spillway approach section, with rock weir about 1‐ft higher than the spillway crest RS 40: Reservoir section to determine the full static head for spillway discharge rating

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 22 February 27, 2015

Figure 16: Surveyed cross‐section of the Carriage Hills No. 2 Dam’s emergency spillway, after the Sept. 2013 flood.

Table 5 shows the HEC‐RAS stage (W.S. Elev) vs. discharge (Q Total) relationship for the Carriage Hills No. 2 spillway. HEC‐RAS calculates a spillway capacity of 213 cfs at Reservoir stage 7644.74, which corresponds to the surveyed low point of the dam crest.

Table 5: HEC‐RAS Stage‐Discharge relationship for the Carriage Hills No. 2 Dam’s emergency spillway

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 23 February 27, 2015 7.4 Dam Breach Analysis: This section documents the breach analysis we performed to estimate empirical breach dimensions, compare them to the observed breach dimensions, and to estimate the peak breach discharge and hydrograph resulting from the dam failure. Dam and reservoir properties were estimated using results from our Sept 26, 2013, breach survey and other analyses described below.

7.4.1 Reservoir capacity: SEO files do not have a reservoir capacity curve for the Carriage Hills No. 2 Dam. For this study, we estimated the volume by delineating the surface area in Google Earth at key levels: bottom of the breach, normal water level (spillway crest), and maximum water level (dam crest) (see Figure 17). We used elevations from our dam breach survey performed on Sept. 26, 2013, and estimated reservoir volume using the average end area method. Table 6 documents our reservoir capacity estimate of 12.7 acre‐feet at the low point of the dam crest. We note that we obtained a similar reservoir volume to that estimated in the 1991 SEO breach analysis, which used a normal pool volume of 15 AF.

Figure 17: Delineation of Carriage Hills No. 2 Dam reservoir surface areas corresponding to the bottom of the dam breach, normal water level, and the maximum water level (Google Earth, 10/22/13 aerial image).

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 24 February 27, 2015

Table 6: Carriage Hills No. 2 Dam, Estimated Reservoir Stage‐Capacity Table

7.4.2 Dam Embankment Geometry: For purposes of the breach analysis the Carriage Hills No. 2 Dam embankment geometry was determined from our Sept 26, 2013, breach survey. Figure 18 shows the surveyed embankment cross‐section.

Figure 18: Carriage Hills No. 2 Dam cross‐section, as surveyed on Sept. 26, 2013.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 25 February 27, 2015

7.4.3 Dam Breach Analysis : Figure 19 shows the surveyed dam breach from our Sept 26, 2013, survey.

Figure 19: Carriage Hills No. 2 Dam breach section, as surveyed on Sept. 26, 2013

We modeled the dam breach based on the observed breach dimensions shown in Figure 19 using the DSB’s Froehlich Breach Parameter Estimation spreadsheet for an overtopping failure. Table 2 of the DSB’s Guidelines for Breach Analysis recommends the Froehlich method for an overtopping failure of a dam of this size (Minor Size embankment, Storage Intensity < 5). From Observed Breach Height of water over the breach (Hw) 9.4 ft Volume of water above bottom of breach (Vw) 12.1 AF Reservoir Surface Area (As) 1.9 Ac Height of Breach (Hb) 9.4 ft Average Breach Side Slope (Zb) 1.5

The Froehlich method computes the following breach characteristics:

Average Breach Width (Bavg) 26.0 ft Bottom Width of Breach (Bb) 11.9 ft Breach Formation Time (Tf) 0.24 hr Predicted Peak Flow (Qp) 801 cfs

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 26 February 27, 2015 In this case the Froehlich method predicts breach dimensions almost identical to the surveyed breach bottom width and average width as shown in Figure 19. Parameter checks suggest that a full breach would be expected to form and the estimated erosion rates are considered reasonable for the Froehlich method breach formation time of 0.24 hr.

In order to estimate the Carriage Hills No. 2 dam breach hydrograph we modeled the overtopping breach in USACE’s HEC‐HMS hydrologic modeling program, using the surveyed breach dimensions and the breach formation time from the Froehlich Method. We used a base inflow of 213 cfs, corresponding to our estimated emergency spillway capacity, to represent the natural flood at the time of the dam failure. The actual inflow at the time of dam failure unknown, but we are primarily interested in the peak and duration of the breach hydrograph above the natural flood. Figure 20 shows HEC‐HMS results for the Carriage Hills No. 2 dam breach. The estimated peak is 677 cfs above the assumed base flow and the breach hydrograph duration is approximately 36 minutes. The volume under the breach hydrograph above the base flood is approximately 10 acre‐feet; the model shows the 12.1 acre‐feet reservoir volume would not have fully drained during the dam failure due to reservoir inflows.

Figure 20: HEC‐HMS results for the Carriage Hills No. 2 overtopping dam breach model

7.5 Fish Creek Flooding: This section documents our review of other studies and our analysis of Fish Creek flooding for the purpose of trying to determine the timing of the Carriage Hills No. 2 Dam breach floodwave and to help determine possible downstream damage that resulted. As a qualifier, it was very difficult to reconstruct the Sept. 2013 flood events on Fish Creek. For one, the channel was virtually destroyed by erosion and crossings were washed out. Much of the flooding occurred at night and some residents had already evacuated, so we found few eyewitness accounts or photo documentation of the worst flooding. Last but not least, there were no CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 27 February 27, 2015 operating gages on Fish Creek at the time of the flood ‐‐ the previous Division of Water Resources (DWR) stream gage on Fish Creek had been discontinued as of October 2012 when the sponsor agreement was terminated.

7.5.1 Review of Other Studies on the Sept. 2013 Fish Creek Flood: The following studies provided important information for our investigation of the Carriage Hills No. 2 Dam failure flood.

7.5.1.1 Colorado Front Range Flood of 2013: Peak Flow Estimates at Selected Mountain Stream Locations, USDA NRCS, December 2013: The NRCS published some of the first peak flow estimates following the Sept. 2013 floods. They used a critical depth method at surveyed cross‐sections and surveyed high water marks. They analyzed and averaged results at three cross‐sections on Fish Creek, located approximately 1000‐ft upstream of Lake Estes. They originally computed a peak flow of 6,900 cfs on Fish Creek, with reported accuracy of +/‐15% for the critical depth method. This results in 442 cfs/sq. mi., one of the highest yields reported. According to Matrix Design Group’s Fish Creek Hydrology Report (Reference 7) the NRCS later revised their peak estimate for Fish Creek down to 4,800 cfs after better representing backwater effects from Lake Estes.

7.5.1.2 DRAFT Public Communications Guide, Colorado Rains and Flood 2013, USBR, updated October 21, 2013: The USBR documented its flood response at Olympus Dam/Lake Estes in this document. Lake Estes is located about 1.6 miles downstream of Carriage Hills No. 2 Dam at the confluence of Fish Creek and the . The DSB used the USBR’s timeline of events to compile Table 7 below, showing Olympus Dam/Lake Estes estimated inflows and outflows.

Table 7

7.5.1.3 Hydrologic Evaluation of the Big Thompson Watershed, Post September 2013 Flood Event, Colorado Department of Transportation, prepared by Jacobs, August 2014: Jacobs created a calibrated hydrological model of the 10‐day Sept. 2013 flood for the Big Thompson River basin. The purpose of their study was to model frequency storms for revising flood frequency estimates throughout the basin. The model was created in HEC‐HMS and was driven by rain data from Applied Weather Associates. Jacobs obtained estimated Olympus Dam discharge and Lake Estes Storage hydrographs from the USBR. Olympus Dam releases were reportedly CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 28 February 27, 2015 estimated by the USBR using reservoir stage data along with measured outlet gate openings throughout the storm. Using the storage and outflow hydrographs, Jacobs was able to estimate the Lake Estes inflow hydrograph for the Sept. 2013 flood. This combined inflow hydrograph does not differentiate between various sources of inflow to Lake Estes, so all subbasins, including Fish Creek, were calibrated uniformly. Caution needs to be exercised in using model results from individual subbasins (e.g. Fish Creek) since they were not individually calibrated. Jacobs reports that they calibrated the model to best‐fit basin‐wide peaks, so as not to overly rely on individual observed or estimated peak flows. They cited Robert Jarrett’s work and a USGS report (Godt et. all, 2013) in concluding that surge flows from sudden failures of debris dams, embankment dams, road embankments, etc. all played a major role in some peak flows during the Sept. 2013 flood. Their HEC‐HMS model is solely a rainfall‐runoff model and is not intended to reproduce dam breaks or surge flows.

The Jacobs model shows Fish Creek peaking around 2000 cfs on Sept. 12, 2013 at 1400 hrs (MDT), which was after the storm’s second large spike in rain intensity (Figure 21). The model shows a second lower peak of 1900 cfs on Fish Creek on Sept. 13, 2013, around 0200 hrs, which is after the third and final large spike in rain intensity. The Jacobs model also shows that large inflows to Lake Estes (peak inflow of 5280 cfs) in the early morning of Sept. 13th could have been explained by rain‐driven runoff on the mainstem Big Thompson (model Reach R330, peak 3500 cfs) (see Figure 22). DWR’s Big Thompson Above Lake Estes (BTABESCO) stream gage gave a peak flow of around 3000 cfs on Sept. 13th, between 0000 and 0600 hrs.

Figure 21: CDOT/Jacobs HEC‐HMS model Sept. 2013 Fish Creek hydrograph (model subbasin BT05) in Universal Coordinated Time (UTC).

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 29 February 27, 2015

Figure 22: CDOT/Jacobs HEC‐HMS model Sept. 2013 Big Thompson River hydrograph above Lake Estes (model reach R330) represents total inflow to Lake Estes except for Fish Creek (BT05) and ungaged side inflows (BT06, 1.123 sq. mi.) (UTC time).

7.5.1.4 Fish Creek Hydrology Report, Matrix Design Group, August 2014: Matrix Design Group provided a valuable contribution towards estimating the peak flow in Fish Creek both above and below the Carriage Hills No. 2 Dam. They used 38 surveyed high water marks along Fish Creek and a post‐flood HEC‐RAS model to best‐ fit water surface profiles corresponding to the maximum flood. Matrix Design Group was primarily concerned with the natural (i.e. absent dam breach) flood flow on Fish Creek for the purpose of updating flood frequency estimates. Matrix estimated a range of Fish Creek peak flows from 800‐2000 cfs upsteam of the Carriage Hills dams, and 2000‐4800 cfs downstream of the dams. Matrix scaled their best estimate upstream of the dam to the total basin size and thereby provided a best estimate natural (i.e. without dam breach) peak of 2400 cfs at the mouth of Fish Creek. This Sept. 2013 peak estimate is more than double their recommended revised 100‐YR flow of 990 cfs. Figure 23 is taken from the Matrix August 2014 report and compares Fish Creek peak flow estimates from various sources. Matrix Design Group’s 2400 cfs estimate removes the effect of the Carriage Hills dam breach by scaling the peak from estimated Fish Creek flows above the dam. The CDOT estimates are from Jacobs’ HEC‐HMS model, which solely accounts for rainfall‐driven runoff and not for the dam breach (the cumulative 10‐day model estimate of 1994 cfs is more relevant to our study). The NRCS peak flow estimates are indirect flow estimates on Fish Creek downstream of the Carriage Hills dams and so should account for the Carriage Hills No. 2 dam breach flood. Matrix Design Group reports that the NRCS revised their original estimate of 6900 cfs down to 4800 cfs after better representing backwater effects from Lake Estes.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 30 February 27, 2015

Figure 23: Various estimates for Sept. 2013 peak flow on Fish Creek (source: Matrix Design Group, August 2014)

7.5.2 Fish Creek Mass‐Balance Hydrograph: Jacobs (August 2014) used a mass balance on Lake Estes to estimate an aggregate inflow hydrograph. We repeated the same analysis using an estimated Olympus Dam discharge hydrograph provided to us directly by the USBR and using Lake Estes storage data from the Division of Water Resources (DWR). The USBR’s Olympus Dam discharge hydrograph was estimated using observed gate openings and gaged pool elevations (USBR e‐mail, 1/28/15). We calculated similar inflow values to Jacobs, although different timing. All times in our study are in MDT; we do not know the source of the discrepancy with the Jacobs model data, which we converted from UTC. We used Equation 1 to perform our mass balance and estimate inflows to Lake Estes during the flood event:

I = ∆S + O [1]

Where, ∆S is change in Lake Estes storage over a given 15 min. time interval O is the estimated Olympus Dam discharge provided by the USBR, averaged over a given 15 min. interval I is the estimated TOTAL Inflow to Lake Estes for the given 15 min. interval

Figure 24 shows the Lakes Estes Storage, Olympus Dam estimated discharge, and Lake Estes estimated inflow hydrographs for Sept. 12‐14, 2013, which corresponds to the main portion of the flood event. Point values of Olympus Dam/Lake Estes inflow and outflows from Reference 4 and Table 7 are plotted for verification.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 31 February 27, 2015

Figure 24: Olympus Dam/Lake Estes storage, outflow, and estimated inflow hydrographs. “REVISED” Lake Estes inflow refers to the SEO recalculation after Jacobs (August 2014)

DWR provided a straight‐line diagram of the Colorado Big Thompson system including Lake Estes (Figure 25). The main inflows to Lake Estes are the Big Thompson River (8), Fish Creek (9), and diversions from Marys Lake (5). Reference 4 indicates that all west slope imports were shut down early on the morning of Sept. 12, 2013. Therefore, the Big Thompson and Fish Creek were the main inflows to Lake Estes during the flood.

Figure 25: Straight line diagram of the Colorado‐Big Thompson system including Lake Estes, Big Thompson River, and Fish Creek.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 32 February 27, 2015 DWR maintains a stream gage on the Big Thompson above Lake Estes (BTABESCO). Stream flows reportedly did not bypass the gage during the flood, stage data appear to be correct, and in general the gage appeared to function properly, thereby providing a continuous record of the Big Thompson flood flows into Lake Estes (personal communication R. Stroud, DWR, and V. Lee, USBR). In order to better understand flooding in the Fish Creek basin surrounding the Carriage Hills No. 2 dam failure, we subtracted the Big Thompson River hydrograph from our Estimated Lake Estes Total Inflow hydrograph to estimate a Fish Creek hydrograph. We refer to this hydrograph as the Fish Creek Mass Balance hydrograph, although in reality it would include other ungaged minor inflows to Lake Estes. Figure 26 plots our various Lake Estes inflow hydrographs. The Jacobs (August 2014) HEC‐HMS model Fish Creek hydrograph is included for comparison.

Figure 26: Estimated Lake Estes hydrographs including mass balance and modeled Fish Creek hydrographs.

Our Fish Creek Mass Balance hydrograph needs to be used with the following caveats: (1) The USBR reports that conveyance through the culverts under US Route 36 was compromised during the flood due to debris. US 36 divides the Fish Creek Arm of Lake Estes from main, gaged body of the reservoir. Therefore, we should not assume the water surface elevation in the Fish Creek Arm was the same as that reported by the Lake Estes gage during the flood (USBR e‐mail, 1/28/15). This discrepancy would likely result in Fish Creek flows being under‐represented in Lake Estes storage data and underestimated and/or time‐delayed in our mass balance hydrograph. This issue may explain the large difference between the HEC‐HMS model Fish Creek hydrograph (Jacobs, August 2014) and our mass‐balance hydrograph during the period of Sept. 12, 0000 hrs – 2000 hrs (see Figure 26). (2) Based on USBR HEC‐RAS modeling of peak flood flows on the Big Thompson River, there is evidence that the BTABESCO discharge rating significantly underestimated the peak flood flow. DWR gage data show a

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 33 February 27, 2015 peak flow of 2980 cfs occurred on Sept. 13, 2013, at 05:30 hrs, whereas the USBR estimated a peak of 4500 cfs (USBR, June 2014). DWR confirmed that the upper end of the gage’s discharge rating is extrapolated and has not been verified by measurement. This discrepancy could result in underestimation of the Big Thompson flood hydrograph, thereby resulting in overestimation of Fish Creek flows in our mass balance hydrograph.

8. DISCUSSION 8.1. Possible Causes of Dam Failure 8.1.1 Overtopping Erosion: This is the most obvious cause of the Carriage Hills No. 2 Dam failure. Eyewitnesses have indicated water was flowing over both Carriage Hills dams on Thursday morning, Sept. 12th, which appears to have been before peak streamflows occurred in the Fish Creek basin. Photos, reportedly from the morning of Sept. 12th, show significant overtopping of the upper dam and at least minor overtopping of the lower dam. Residents report the lower dam failed sometime Thursday night or Friday morning. We estimated the peak reservoir inflow at around 506 cfs and the spillway capacity at 213 cfs. We observed headcutting damage to the Carriage Hills No. 2 Dam’s downstream slope both at the right abutment and to the left of the breach. Our survey indicates the dam crest was low at the location of the breach (see Figure 4), which would have concentrated overtopping flows.

On the other hand, we note that the Carriage Hills No. 1 Dam did not fail despite significant overtopping. Based on visual inspection we estimated its spillway had a smaller capacity than the Carriage Hills No. 2 spillway, and so the upper dam would have had higher overtopping flows. The upper dam experienced scour erosion on the downstream slope as a result of overtopping. The dams were of similar construction and materials. One significant difference is that the crest of the Carriage Hills No. 1 Dam appeared to be level and overtopping flows were distributed fairly evenly along the embankment (see Photos 15‐18).

8.1.2 Internal Erosion Along Outlet Conduit: An alternative explanation for the failure could be that seepage increased along the abandoned corrugated metal pipe (CMP) outlet conduit due to sustained high reservoir head, resulting in internal erosion of the embankment and eventually formation of a full breach. The C‐1195 construction plans show a 12 gage CMP conduit. There is no direct evidence that the outlet works exists, but reports as far back as the 1980’s make reference to the outlet being abandoned or buried. The 2002 EIR shows a photo of hole at the downstream toe, which was assumed to correspond to the outlet outfall. We have no record that the pipe was properly abandoned, so we can assume that it was not. The pipe was likely corroded and may have failed, leaving voids and paths for embankment seepage. The impervious soil zone in the dam was thin and appears to have relied on the 10 MIL polyethylene liner under it. If the 40+ year old liner had defects around the outlet conduit, it could have allowed a short seepage path from the reservoir to the downstream zone of “clay and disintegrated granite”. The observed sandy clay embankment soils are considered to be erodible based on visual evidence of surface erosion.

One concern with this explanation is that the surveyed breach bottom elevation of 7635.30 (Table 1, Figure 4) is 8.3 feet higher than the outlet invert elevation of 7627 ft shown on the C‐1195 construction plans. The C‐1195 plans are not “as‐built”, and so may not accurately reflect finished elevations. Nevertheless, the breach bottom

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 34 February 27, 2015 was estimated to be about 4 feet above the bottom of the reservoir, so if the outlet works existed it was set above the bottom. We saw no evidence of the outlet works during our inspection of the breach, so it either did not exist or it was washed downstream by the breach flood.

8.1.3 Combined Overtopping and Internal Erosion: Reference 1 recommends considering a combination of possible failure mechanisms. In the case of the Carriage Hills No. 2 Dam failure, it seems likely that the low area of the dam crest at the breach location corresponded to the outlet conduit location. Crest settlement may have occurred in the past due to failure of the CMP conduit. The low area likely caused concentration of overtopping flows, which may have accelerated the process of scour and headcutting of the embankment at this location. As the headcut formed, it would shorten the seepage path along the outlet conduit and increased the potential for internal erosion. In this manner overtopping erosion at the low area of the dam crest and internal erosion along the conduit may have combined to cause the failure of the Carriage Hills No.2 Dam.

8.1.4 Poor Maintenance & Repair: The Carriage Hills No. 2 Dam’s spillway was inadequate to pass flows from the Sept. 2013 flooding. The spillway as surveyed on Sept 26, 2013, did not conform to the approved C‐1195 construction plans. We estimated the capacity of the existing spillway to be 213 cfs. The capacity on the C‐1195 plans was reported as 377 cfs. Based on our file review, the SEO cited problems with willows obstructing the spillway and inadequate spillway size as far back the 1980’s. In 1991 the DSB required the Town to remove obstructions from the spillway and provide a minimum of 3 feet of freeboard. In 2002, the DSB required the Town to restore the spillway to the conditions shown on the C‐1195 plans or hire an engineer to provide an alternative design, which was not done. It appears the Town did little to maintain or repair the Carriage Hills dams. In 2008 the DSB cited the safety problems associated with the improperly abandoned CMP outlet conduit and required the owner to prepare plans and specification to rehabilitate the outlet works. To our knowledge the owner made no progress on repairing the outlet works prior to the failure. The low area of the dam crest was likely associated with the abandoned outlet conduit and appears to have contributed to the dam failure.

8.2 Emergency Response: There was miscommunication when the DSB understood from Town staff that residents along Fish Creek were being evacuated on Thursday morning, Sept. 12, 2013, as the dams were overtopping. In reality the Town verified afterwards (e‐mail, Feb. 27, 2015) that a notification of minor flooding was issued at 03:43 hrs on Sept. 12, 2013, but a full evacuation notice did not occur until 18:12 hrs on Sept. 13, 2013, after the dam failure. It also appears no emergency action was taken to save the dams after they began overtopping. The photos taken on the morning of Sept. 12th suggest that an excavator or backhoe may have been able to access the spillways and do emergency clearing and excavation to safely pass the floods. However, the Town’s staff were likely overwhelmed by the catastrophic nature of the historic flood.

8.3 Possible Downstream Damage Resulting from the Carriage Hills No. 2 Dam Failure Mr. Bonza, homeowner at 1551 Fish Creek Road, provided an eyewitness account and videos of Fish Creek flooding on Sept. 12‐13, 2013 (see Section 5 for details). His house is located along Fish Creek immediately upstream of Country Club Road and approximately 2700 ft downstream of the Carriage Hills No. 2 Dam. Based on his account, Fish Creek at this location experienced its maximum flood stage around 22:20 hrs on Sept. 12th when it flooded his walkout basement with about a 1 foot depth of water. Mr. Bonza believes this surge in Fish

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 35 February 27, 2015 Creek was caused by the Carriage Hills No. 2 Dam failure. He reports the surge receded after about 30 minutes. Country Club Road was reportedly still intact at the time of the surge, but washed out sometime later that night.

The Jacobs (August 2014) hydrology model suggests that rain‐driven (i.e. natural) flow on Fish Creek peaked at 2000 cfs around 14:00 hrs on Sept. 12th (Figure 26) in response to the second spike in rainfall, with a slightly lower rain‐driven peak of 1900 cfs around 02:00 hrs on Sept. 13th in response to the third rainfall spike and after the surge flow reported by Mr. Bonza. The Jacobs model does not account for dam breaks or other surge flows. Videos and photos confirm that significant flooding occurred during the daylight hours of Sept. 12th (Photo 20). Mr. Bonza’s videos show that flood waters had already been up to his driveway by 14:46 hrs on Sept. 12th, but had not yet flooded his house until later that night. The Carriage Hills No. 2 Dam breach flood could explain the timing of the surge. We estimated a dam breach hydrograph duration of 36 minutes, corresponding well to Mr. Bonza’s account of the duration of the surge.

Photo 20. Source: Remembering September 2013, Images of a Community’s Determination, Estes Park Trail Gazette, Sept. 2014.

Next we attempted to create a hydraulic model of the reported 2‐ft stage increase during the surge to see if the incremental rise could be explained by our estimated peak dam breach flow of 677–801 cfs. Our model, based on our survey of the post‐flood Fish Creek channel, required much larger flows to arrive at the reported stage increase. The problem is that Country Club Road was reportedly still intact at the time of the surge and likely created backwater hydraulic conditions at the Bonza house. We reviewed the pre‐flood FEMA HEC‐2 Fish Creek hydraulic model, but it did not include the Country Club Road crossing. After the extensive flood damage we

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 36 February 27, 2015 have no way to recreate the Country Club Road crossing in our model and tie the pre‐failure road crest elevations to the elevation of Bonza house.

As for the overall magnitude of the reported surge at 22:20 hrs, the Jacobs HEC‐HMS model shows a natural flood flow of 1471 cfs at 22:30 hrs, and our mass balance hydrograph gives a flow of around 1335 cfs at 22:30 hrs (22:30 hrs gives some allowance for travel time to Lake Estes where both hydrographs apply). We estimated a peak dam breach flow of 677 cfs to 801 cfs above the base flood, so assuming the surge at 22:20 hrs was caused by the Carriage Hills No. 2 Dam failure this would give a range of 2012 cfs to 2272 cfs for the surge peak flow. A surge peak of 2012‐2272 cfs is not significantly more than the modeled natural peak flow of 1994 cfs on Fish Creek around 14:00 hrs, and so would not appear to explain the observed peak flood stage at Mr. Bonza’s walkout basement.

Both the HEC‐HMS hydrograph and mass‐balance hydrograph have several potential sources of inaccuracy, including that the HEC‐HMS model was not calibrated specifically to the Fish Creek subbasin and that the Fish Creek Arm of Lakes Estes may have been isolated from the gaged portion of the reservoir during the flood. Therefore for more information about the magnitude of flows on Fish Creek we turn to the Matrix Design Group (August 2014) and NRCS (Dec. 2013) studies. Matrix Design Group’s high water mark analysis on Fish Creek suggests a peak flow range of 2000‐4800 cfs below the Carriage Hills dams. The NRCS estimated a peak flow of 4800 cfs below the dams.

One possible explanation for the observed peak stage at Mr. Bonza’s house at 22:20 hrs is that the overnight natural flood was significantly larger than the flood earlier that afternoon, contrary to the HEC‐HMS hydrograph (recalling that the HEC‐HMS model was not specifically calibrated to Fish Creek). There is considerable evidence to suggest that the overnight flood was larger: the USBR’s Olympus Dam discharge hydrograph (Figure 24), the Lake Estes inflow hydrograph (Figure 24), the Big Thompson Above Lake Estes gage hydrograph (Figure 26), and our Fish Creek mass‐balance hydrograph (Figure 26). Further, the USBR reported that massive amounts of debris entered the Fish Creek Arm of the reservoir overnight on Sept. 12th‐13th (e‐mail, Feb. 5, 2015, see Photo 21), Country Club Road washed out overnight, and the Matrix Design Group and NRCS peak flow estimates suggest higher flows than those that occurred during the day. The dam breach flood on top of higher natural flood flows could explain the peak flood stage at Mr. Bonza’s house around 22:20 hrs on Sept. 12th. However, based on the timing of rain and the runoff hydrographs, we are not convinced that the Carriage Hills No. 2 dam breach and observed surge contributed to the peak flow on Fish Creek. Mr. Cheley’s eyewitness account (see Section 5) indicates that natural flooding on Fish Creek, around 2 miles upstream of the Carriage Hills dams, peaked sometime after midnight on Sept. 13th and broke loose with large amounts of boulders and debris (see Photo 22). Country Club Road reportedly washed out sometime after the Bonza’s basement flooded, and the road failure would have lowered the channel grade and water surface profile. Timing of the rain‐driven runoff indicates the peak flow likely occurred several hours after the peak stage at the Bonza house, and possibly after Country Club Road washed out.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 37 February 27, 2015

Photo 21: Debris in Fish Creek Arm of Lake Estes at 06:44 hrs, Sept. 13, 2013 (source: USBR)

Photo 22: Flood debris inside of a building at Camp Cheley, located on an unnamed tributary of Fish Creek north of the Twin Sisters Peaks and around 2 miles south (upstream) of the Carriage Hills dams. This flood damage reportedly occurred sometime after midnight on Sept. 13th (SEO Dam Safety Branch photo).

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 38 February 27, 2015 9. CONCLUSIONS The Carriage Hills No. 2 Dam failed during the catastrophic September 2013 flood event. Both the Carriage Hills No. 1 and No. 2 Dam spillways were inadequate to pass inflows and both dams overtopped. Failure of the No. 2 dam may have been caused by a combination of overtopping erosion and internal erosion along an old CMP conduit. A low area of the crest around the assumed outlet location likely contributed to the failure by concentrating overtopping flows. Historically the Dam Safety Branch cited spillway obstruction by willows and inadequate spillway size and freeboard as safety problems. In 2002 the DSB required the owner to restore the spillway to the size on the approved C‐1195 construction plans. The DSB also cited the improperly abandoned CMP outlet conduit as a safety problem and required the owner to rehabilitate the outlet works. Neither of these required repairs was apparently done.

Clearly the Sept. 2013 flood was extreme, especially for high elevations above 7500 ft (Carriage Hills No. 2 Dam crest elevation was around 7645 ft). Our analysis shows the 48‐hr rainfall for the Carriage Hills basin had less than a 1/1000 Annual Exceedance Probability based on NOAA Atlas 14. We cannot say for sure if the No. 2 dam would have survived had it been properly maintained and repaired, but it certainly would have stood a better chance. The causes of the dam failure can be attributed to poor maintenance and repair of the dam as well as the infrequent, large magnitude of the flood.

It seems likely that the dam began to fail on Sept. 12, 2013, around 22:00 hours with a failure time of approximately 14 minutes, causing the surge flow observed ½ mile downstream by Mr. Bonza at around 22:20 hrs. The timing corresponds to the third rain spike of the storm, which may have dealt the final blow to the dam. We estimate the floodwave duration was approximately 36 minutes, and it briefly added between 677‐ 801 cfs on top of the natural flood flow in Fish Creek. The dam breach floodwave likely contributed to the peak flood stage observed at the Bonza house around 22:20 hrs on Sept. 12th while Country Club Road was still intact; however, we are not convinced that dam breach caused the peak discharge on Fish Creek, due to the timing of runoff and channel erosion associated with subsequent failure of Country Club Road. High water mark estimates, Lake Estes hydrographs, the timing of debris entering the Fish Creek Arm of Lake Estes, and the eyewitness account of peak flows and debris higher up on Fish Creek after midnight on Sept. 13th all suggest that a larger natural flood peak occurred overnight than what can be explained by the dam breach hydrograph.

Although it seems likely that the dam failure contributed to the observed surge at 22:20 hrs, we have to acknowledge the large uncertainty associated with the Sept. 2013 Fish Creek flood event despite our efforts to synthesize the best available information. A number of other factors could account for the surge and peak stage observed at the Bonza house: (1) the surge and peak stage could have resulted solely from rain‐driven runoff from the third and final spike in rain intensity; the timing and peak flows may not be reflected in the Jacobs’ HEC‐HMS model and our mass‐balance hydrograph due to the potential sources of error described above, or (2) the surge and peak stage could have been caused by the failure of any number of debris dams, upstream road crossings, landslide debris, etc. as explained in the Jacobs (August 2014) report.

If the dam failure flood contributed to the peak flood stage at the Bonza house, it may have caused incremental damage at other low‐lying structures along Fish Creek, particularly upstream of Country Club Road. The flood

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 39 February 27, 2015 damage at the Bonza house appears to be consistent with the DSB’s definition of Low Hazard, i.e., less than 2 feet flood depth and a depth x velocity product less than seven. Low Hazard classification is not meant to trivialize the injury to an individual property owner.

Due to the dam breach hydrograph’s short duration, the floodwave would have rapidly attenuated. It also would have been fully contained within Lake Estes due to its small volume. The Carriage Hills No. 2 Dam breach floodwave most likely did not affect the operation of Lake Estes. The floodwave volume of 10 acre‐feet is negligible compared to both the peak Lake Estes storage volume (over 2800 acre‐feet) and to the event’s total rainfall volume of 10,560 acre‐feet on the Fish Creek basin. Even a conservative estimate of runoff conversion shows the volume of the dam failure was minor in the scope of the overall flooding. We do not believe the dam breach floodwave made any significant contribution to channel erosion, road damage, utility damage or other widespread devastation along Fish Creek. Likewise we do not believe the breach floodwave caused the massive inflow of debris to Lake Estes: the debris along with the Big Thompson River and Lake Estes hydrographs, and the eyewitness account of upstream flooding point to a large natural flood peak along with large amounts of debris on Fish Creek after midnight on Sept. 13, 2013.

It was noted that the Carriage Hills No. 1 (upper) Dam also overtopped by flood flows and experienced erosion damage. It also suffered from most of the same maintenance deficiencies as the No. 2 Dam. If the upper dam had failed it likely would have caused a cascading failure of the lower dam. A 1991 DSB breach study estimated a peak breach flow of 1162 cfs from a piping failure of the lower dam only, but a peak of 5409 cfs from a cascading failure of both dams. The latter floodwave would likely have caused significant additional damage on top of the natural flooding along Fish Creek.

Finally the Carriage Hills No. 2 Dam should not be rebuilt without thorough engineering design and analysis. The hazard potential associated with failure of both the No. 1 & 2 Dams, including a cascading failure scenario, needs to be carefully studied. Both dams would need to meet safe design standards appropriate for their hazard classification, in accordance with the State of Colorado’s Rules and Regulations for Dam Safety and Dam Construction. Safe standards would include hydrologically adequate spillways, proper abandonment of old outlet works, level dams crests, and appropriate emergency action planning.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 40 February 27, 2015

APPENDIX A

FILE HISTORY & CONSTRUCTION PLANS

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 41 February 27, 2015 DATE DESCRIPTION 1967 SEO plan approval, C-1195 CARRIAGE HILLS RESERVOIRS. CH 2 dam shows 2H:1V upstream and downstream slopes, 10-ft crest width. The embankment is zoned with an upstream impervious zone, followed by a 10 MIL Polyethylene membrane, and a clay & disintegrated granite downstream zone. The plans show a 24” dia. 12 ga. Corrugated metal pipe (CMP) outlet conduit with an upstream slide gate and trash rack (outlet capacity = ~35 cfs). The spillway was to be 10-ft wide with vertical side slopes through rock and then laid back at some unspecified slope, with 5-ft of freeboard. Spillway capacity was listed as 377 cfs. [Plans are not marked as-constructed]. 1983 1st record of SEO inspection. Water Commissioner (WC) inspection. Capacity estimate: 5AF, Surface Area: 1 Ac, Height: 18 ft. Notes trees & brush on the embankment. 1984 WC inspection. Reservoir full, 1 cfs of spillway flow 1985 WC inspection. Flow obstructed by willows in spillway. “No outlet found” 1986 Engineer’s Inspection Report (EIR). Willows obscure embankment. Willows obstructing spillway flow & there is flow along downstream toe of dam 1988 WC inspection. Seepage flow “adjacent to outlet”, “outlet obstructed by overgrowth and dirt”. 7/24/91 Town of Estes Park annexed land and took ownership of the CH dams. 7/31/91 EIR (Mark Haynes): Rated Conditionally Satisfactory due to spillway and outlet condition. Required Actions: Remove obstructions from spillway and provide minimum 3-ft of freeboard, locate outlet and make operable or Replace. Notes no change in downstream hazard, and hydrology not rated pending evaluation [No record that hydrology study was every performed] 8/21/91 Town of EP requested that they be allowed to use pumps for emergency drawdown in lieu of new outlet works. 9/3/91 Response to SEO allowing use of pumps with conditions: reliable power source, access to dam maintained, accessible during emergency, 4 cfs min. capacity 11/12/91 Dam Breach Analysis by SEO using NWS Breach 87. Four cases were investigated: (1) piping failure of CH1 (u/s) only, (2) piping failure of CH2 only, (3) simultaneous piping failure of both dams, and (4) overtopping failure of CH2 dam caused by a piping failure of CH1 dam. Peak breach flows for cases (2) and (4)[considered most relevant] were 1162 cfs and 5409 cfs. No formal review of hazard classification was performed, but the Hazard is denoted as 3 in the analysis, where 3 in the previous 1-3 hazard rating system corresponds to Low Hazard. 6/14/96 EIR (Jim Dubler). Spillway return flows along the toe of the dam and obscures seepage inspection. Outlet was not located and suggests that outlet may not exist. Cites confusion on the Town’s part about their pump procedure for emergencies. Required Actions: 3-ft min. freeboard. 10/22/02 EIR (Jim Dubler). Crest: Right end appears low. Outlet could not be located, but describes (with a photo) a 4-ft dia. pool at the downstream toe that presumably was the outlet outfall. Required Actions: Restore the spillway to the C-1195 design conditions (10-ft wide, 5-ft freeboard) or hire an engineer to do an alternative study/design. 11/7/02 Letter from Town of Estes Park to SEO documenting available pumps for emergency drawdown requirement. 10/1/08 Outlet can’t be found but presumed to be at wet area at downstream toe. No record that outlet was properly abandoned and C-1195 plans show conduit is CMP. States recent work was done on the spillway, reportedly restored to the design width of 10ft, as well as riprap protection added in the approach and downstream channel. Spillway flow routed along the downstream toe, seepage noted in “vicinity of outlet”. Required Actions: Plans & specs for outlet rehabilitation, Monitor seepage around the downstream end of the conduit. Conditional Satisfactory Rating & Conditional Full Storage based on the requirement for outlet rehab.

CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 42 February 27, 2015