County of Santa Cruz

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Rio Del Mar Flats Stormwater Drainage Improvements Project

Area of Study

Project Overview

Drainage District Background

This project is a component of a larger project effort to construct and then operate a stormwater pump station in the Rio Del Mar Flats community. The new pump station is yet to be built, but the County has obtained construction funds, design, and permits. Additional funds for permanent operation and maintenance of the facility are still needed, and their obtainment needs to be secured prior to the expenditure of grant funds awarded for facility construction.

The Santa Cruz County Flood Control & Water Conservation District - Zone 6 was formed in 1986 and the need for fixing pre-existing drainage problems within the Rio Del Mar Flats was then identified. Unfortunately, Zone 6 does not receive a recurring annual tax base assessment but obtains funds only from a one-time development fee on net new impervious surfacing. With Zone 6 largely built out, the available declining fees are insufficient to fund construction or maintenance of a significant sized new project, such as the pump station intended to reduce the extensive rainfall ponding in this neighborhood. The County does not anticipate being able to successfully acquire at this time, a voter-approved tax assessment for the entirety of Zone 6. Thus, property owners within a smaller region more focused on those most benefiting from the stormwater pump station are being approached for the formation of a Special Financing District in this project.

Introduction

The Rio Del Mar Flats neighborhood in the City of Aptos has a long history of repetitive flood events since development began in the area in the 1920s. The Rio Del Mar Flats neighborhood has approximately 400 residential units, one hotel, and several small commercial buildings. The vast majority of the structures are pre-FIRM structures.

The most frequent flooding problem in Rio Del Mar Flats is ponding during rainfall-runoff events. The ponding occurs because the existing gravity flow stormwater drainage system has inadequate conveyance, in large part because of the very gentle slopes in this neighborhood which result in low flow velocities. The limited conveyance is also exacerbated by frequent impairment of conveyance from sediment and vegetative debris accumulation in the pipes. Stormwater drainage ponding flooding in the Rio Del Mar Flats neighborhood is concentrated along Aptos Beach Drive, the adjacent side streets, and the Esplanade, which are the lowest areas in the Flats neighborhood.

Street flooding in Rio Del Mar Flats is very frequent, typically occurring several times per year. More significant damage, with ponding depths sufficient to result in building damages, occurs in local rainfall events beginning with the 2-year event.

The proposed mitigation project addresses this very frequent stormwater drainage ponding flooding problem. The proposed stormwater drainage improvement project is designed to provide the capacity to handle 10-year rainfall-runoff events, plus an additional margin of capacity.

The Rio Del Mar Flats neighborhood is also subject to overbank flooding from AptosCreek and stormwater surge flooding from the Pacific Ocean. River overbank flooding or stormwater surge flooding occurs with return periods greater than 10 years. Mitigation measures for these longer-return period flood events will be much more expensive and will have significant permitting issues. Mitigation measures for these flooding events are not included in the current proposed mitigation project.

The proposed project addresses and solves the most frequent highly-repetitive flooding problem which is ponding in the lowest part of the Rio Del Mar Flats neighborhood during local rainfall events.

Proposed Mitigation

The proposed mitigation project has two main elements, which will substantially reduce future flood damages and losses from rainfall-runoff events in Rio Del Mar Flats:

  • Construct a stormwater pump station, which will substantially increase the conveyance capacity of the stormwater drainage system; and
  • Construct a gravity flow pipe to intercept runoff flowing down the steep section of Rio Del Mar Flats at an elevation well above the Flats. This project element reduces runoff volume into the flats and complements the flooding reductions achievable by the pump station alone.

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Demographics and Key Facts for the Area of Study

The demographics, key facts, infographics, and data shown below have been obtained from ESRI's ArcGIS Community Analyst. CivicMic makes no claims, no representations, and no warranties, express or implied, concerning the validity (express or implied), the reliability or the accuracy of the demographics, key facts, infographics, and data shown below, including the implied validity of any uses of such data.

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