Cherry Creek Basin Paleoflood

Paleoflood Hydrology of Cherry Creek Basin Colorado USA

Final Report

Executive Summary

This report documents the paleoflood hydrology study of the Cherry Creek basin conducted for the City of Denver. Paleoflood hydrology involves the study of past flood events that occurred over hundreds or thousands of years ago, which provides critical data for flood risk assessments and floodplain management.

Study Purpose

The purpose of this study is to provide insight into extreme flood events in the Cherry Creek basin by using paleoflood techniques to assess the frequency and magnitude of large, rare floods.

Methodology

• Data Collection: Geological and geomorphological evidence were collected from the Cherry Creek basin to identify past flood events.

• Analysis: The data were analyzed to determine flood magnitudes, return periods, and related hydrologic factors.

Findings

• The study identified several significant paleoflood events that have occurred in the Cherry Creek basin over the last 2,000 years.

• The estimated magnitudes of these floods exceed the largest flood in the recorded history of the basin.

• These findings suggest that the Cherry Creek basin is susceptible to extreme flood events, which may exceed those considered in the current floodplain management practices.

Recommendations

Based on the study’s findings, it is recommended that the City of Denver consider the following:

1. Floodplain Management Updates: Reevaluate and potentially revise the floodplain management practices to account for the possibility of larger floods than previously considered.

2. Risk Communication: Enhance communication with the public about flood risks, especially concerning the potential for rare, extreme floods.

3. Further Research: Conduct additional research to refine the estimates of paleoflood magnitudes and frequencies in the Cherry Creek basin.

Conclusion

The paleoflood hydrology study of the Cherry Creek basin provides valuable insights into the basin’s flood history and potential flood risks. This information is crucial for informed decision-making in flood risk management and for ensuring the safety and resilience of the community in the face of future flood events.

Introduction

Flood risk management is a critical concern for urban areas, especially for cities like Denver, which are situated near large water bodies. The Cherry Creek basin, which runs through the heart of Denver, has been subject to various flood events in recorded history. However, understanding the potential for extreme flood events that could surpass these historical floods requires a study of paleofloods.

Paleoflood hydrology focuses on understanding past floods that occurred before the era of systematic hydrological records. By studying geological evidence, such as sediment deposits and geomorphic features, scientists can infer the magnitude and frequency of these past floods. This information is invaluable for assessing flood risks that may not be apparent from the historical record alone.

This report details the findings of the paleoflood hydrology study conducted on the Cherry Creek basin. The study was commissioned by the City of Denver’s Stormwater Enterprise to support ongoing flood risk management efforts.

Methodology

The methodology for the paleoflood hydrology study involved several key steps:

1. Site Selection: Sites along the Cherry Creek basin were selected based on their potential to preserve evidence of past flood events. These sites included natural and anthropogenic features such as terraces, alluvial fans, and floodplain deposits.

2. Field Investigations: Fieldwork was conducted to collect data from selected sites. This included mapping geomorphic features, collecting sediment samples, and identifying stratigraphic evidence of past floods.

3. Laboratory Analysis: Collected samples were analyzed in the laboratory to determine the characteristics of the sediment deposits. This included grain size analysis, radiocarbon dating, and other techniques to establish the timing and magnitude of past floods.

4. Data Analysis: The laboratory results were combined with hydrologic modeling to estimate the magnitude and frequency of the identified paleofloods. This involved comparing the stratigraphic data with modern hydrological data to assess the potential impact of similar floods in the present-day context.

5. Reporting: The findings were compiled into this report, which includes recommendations for flood risk management based on the paleoflood data.

Study Area Description

The Cherry Creek basin is located in the eastern part of the Rocky Mountains, extending through the city of Denver, Colorado. The basin covers an area of approximately 386 square miles and is characterized by a semi-arid climate, with most precipitation occurring during the summer months in the form of thunderstorms.

The creek itself originates in the high plains and flows in a northeasterly direction, eventually joining the South Platte River in downtown Denver. The basin includes a variety of land uses, ranging from agricultural areas in the upper reaches to dense urban development in the lower reaches near Denver.

The basin’s topography is generally gentle, with elevations ranging from about 5,200 feet at the mouth to over 7,000 feet in the upper reaches. The geomorphology of the basin reflects a history of fluvial processes, with well-developed floodplains, terraces, and alluvial fans.

Geological Setting

The Cherry Creek basin is underlain by a complex sequence of sedimentary rocks, which have been extensively modified by fluvial processes over geologic time. The bedrock consists primarily of Cretaceous sandstones and shales, which have been eroded and redeposited as alluvial sediments in the basin.

These alluvial deposits form the primary aquifers in the basin and are also the main source of the floodplain sediments that were the focus of this study. The sediments are typically fine-grained, with a mixture of sand, silt, and clay, reflecting the low-energy depositional environment of the floodplain.

In addition to the alluvial deposits, the basin contains a number of geomorphic features that are indicative of past flood events, such as cutbanks, levees, and terraces. These features were targeted for detailed study in the paleoflood investigation.

Paleoflood Evidence

The paleoflood hydrology study identified several lines of evidence for past flood events in the Cherry Creek basin. This evidence was derived from both geomorphic features and sedimentary deposits observed at multiple sites along the creek.

Geomorphic Features

• Floodplains and Terraces: The presence of well-developed floodplains and terraces along Cherry Creek indicates repeated episodes of flooding over geologic time. These features were mapped and analyzed to identify the extent and frequency of past floods.

• Levees and Cutbanks: Natural levees and cutbanks provide evidence of channel migration and overbank flooding. These features were used to infer the magnitude of past flood events and to estimate floodplain inundation areas.

Sedimentary Deposits

• Stratigraphy: The stratigraphic record at selected sites revealed distinct layers of sediment that correspond to past flood events. These layers were characterized by differences in grain size, organic content, and sediment structure.

• Radiocarbon Dating: Radiocarbon dating of organic material found within the flood deposits provided age estimates for the paleoflood events. This allowed for the construction of a timeline of major floods in the basin over the past 2,000 years.

Paleoflood Magnitudes

Based on the analysis of geomorphic features and sedimentary deposits, the study estimated the magnitudes of the identified paleofloods. The estimates were derived from hydraulic modeling that incorporated the physical characteristics of the floodplain and the stratigraphic data.

The paleoflood magnitudes were found to range from approximately 10,000 cubic feet per second (cfs) to over 50,000 cfs. These estimates suggest that the Cherry Creek basin has experienced floods significantly larger than those recorded in the historical period, highlighting the potential for extreme flood events.

Flood Frequency Analysis

In addition to estimating the magnitudes of the paleofloods, the study conducted a flood frequency analysis to assess the return periods of these events. The analysis was based on a combination of paleoflood data and modern hydrologic records.

The results indicated that the return periods for the largest paleofloods ranged from 500 to over 1,000 years. This suggests that while these extreme floods are rare, they are still a significant risk that should be considered in floodplain management.

Implications for Floodplain Management

The findings of the paleoflood hydrology study have important implications for floodplain management in the Cherry Creek basin. The identification of extreme flood events that exceed the magnitude of recorded floods suggests that current floodplain management practices may need to be reevaluated to ensure they adequately account for the risk of such events.

Flood Risk Assessment

One of the primary implications of the study is the need for an updated flood risk assessment that incorporates the paleoflood data. The existing flood risk models are based primarily on historical records, which may not fully capture the potential for extreme floods. By integrating the paleoflood data into these models, more accurate predictions of flood risk can be developed, which will support better decision-making in floodplain management.

Floodplain Mapping

The paleoflood data also suggests that the extents of the 100-year and 500-year floodplains, as currently mapped, may underestimate the areas at risk of flooding. The evidence of larger paleofloods implies that these floodplains could extend further than currently depicted. Updated floodplain maps that reflect the potential for larger floods would provide a more accurate representation of the areas at risk, allowing for better land-use planning and zoning.

Infrastructure Design

The study’s findings have implications for the design of infrastructure within the Cherry Creek basin. Existing infrastructure, such as bridges, culverts, and stormwater systems, may not be adequately designed to handle the larger floods suggested by the paleoflood data. As a result, it may be necessary to reevaluate and possibly redesign these structures to ensure they can withstand extreme flood events.

Emergency Preparedness and Response

Given the potential for larger and more destructive floods than previously considered, there may be a need to enhance emergency preparedness and response plans. This could include updating evacuation plans, improving flood warning systems, and increasing public awareness of flood risks. By preparing for the possibility of extreme floods, the City of Denver can reduce the potential impacts on life and property.

Recommendations

Based on the results of the paleoflood hydrology study, the following recommendations are made:

1. Update Flood Risk Models: Incorporate the paleoflood data into the existing flood risk models to improve the accuracy of flood predictions.

2. Reevaluate Floodplain Maps: Update the 100-year and 500-year floodplain maps to reflect the potential for larger floods based on the paleoflood evidence.

3. Review Infrastructure Resilience: Assess the resilience of critical infrastructure in the Cherry Creek basin and consider redesigning structures to handle larger floods.

4. Enhance Public Communication: Increase efforts to communicate flood risks to the public, particularly the potential for rare but extreme floods.

5. Conduct Further Research: Continue research on paleoflood hydrology in the Cherry Creek basin and other areas to refine estimates of flood magnitudes and frequencies.

Conclusions

The paleoflood hydrology study of the Cherry Creek basin has provided valuable insights into the basin’s flood history and potential flood risks. The identification of significant paleoflood events highlights the need for a reevaluation of current floodplain management practices. By incorporating the paleoflood data into flood risk assessments, mapping, infrastructure design, and emergency preparedness, the City of Denver can enhance its resilience to future flood events and protect its citizens and property from the impacts of extreme floods.