Drainage Report

Excerpts from the
Hidden Lake Drainage Study

Preliminary Engineering Assessment
Prepared by:
HRP Associates, Inc.
167 New Britain Avenue
Plainville, CT 06062
June 18, 2002

The Hidden Lake Drainage Study was commissioned after the 2002 annual meeting as part of the continued program to maintain the quality of the lake and in accordance with the Ecological Survey conducted in 1995. The Association is now in the second round of four-year septic tank pumping, and recently completed the weed removal project in the West Cove. Road drainage improvements are the next phase in our long-range plan to insure our lake is healthy for years to come.

Area 1

This area includes the eastern shoreline of the lake, and properties on and accessed by East Shore Drive. We noted several forms of drainage control in place, however they are not all hydraulically connected. Most of these individual systems do not provide any benefit to directing storm water directly to the lake or provide any form of sediment protection or removal. Therefore, the pipes were often clogged by obstructions such as silt and leaves, or collapsed. We observed various types of piping installed at spot locations, including concrete pipe, corrugated plastic, metal, and PVC. Recent roadway improvements hinder sheet flow, thereby trapping storm water in some specific locations along East Shore Drive. Several drainage swales along this section were eroded and the bottom and sides were unprotected.

Area 2

This area includes the southwestern shoreline along Shore Drive and areas west of the masonry dam. Several drainage structures were noted during the field visit. They appear to be operational and functioning as a hydraulically connected system. In the vicinity where the pipes discharge to the lake, we observed open drainage channels which were well defined, but no protected from erosion.

Area 3

This area includes the northwest shoreline along West Shore Drive from Hidden Lake Road to the beach. It was noted that this area is generally much higher in elevation than the lake. This change in elevation allows storm water to drain away from individual property owners. Once again, we observed that the drainage swales into the lake were unprotected and showed signs of erosion. The general sizes of the swales indicate that and abundant volume of water discharges during a storm event.

Area 4

This area includes the northerly shoreline and properties situated to the east of West Shore Drive. Portions of this area also are generally higher in elevation than the lake. At the end of West Shore Extension, at a substantially lower elevated area, there is evidence of an inadequate drainage system. The prior storm event created or contributed to an eroded path from a natural swale. Evidence of debris collected on the roadway indicates that the cross culvert at the end of this roadway is probably under capacity or clogged, and that storm water flows over the roadway to get to the lower elevations. There appears to be a drainage system in place at the intersection of West Shore Drive and West Shore Extension, however, silt and leaves have clogged the system making it inoperable. 

Recommendations

General

Many of the drainage problems observed around the lake result from the lack of engineered collection systems, and clogging of pipes and trenches from the migration of silt. In general, we recommend that the lake association develop and implement a regular maintenance program that includes the periodic cleaning of pip inlets and catch basins. Historically, silt migration has been a significant concern at Hidden Lake, and therefore erosion control will be an important factor for controlling runoff.

In our opinion, Areas 1 and 4 have the most immediate need for improvements. Attached are example details of suggested structural devices, which could be used to impede erosion and decrease the amount of silt flowing into the lake. However, it is important that structures, piping, and erosion control measures be designed as a system, and sized to accommodate the calculated flows. Some surveying will also be required to define site grades and pitch drainage systems appropriately. We recommend that any future drainage systems which are constructed in the vicinity of the lake should be designed by a licensed civil engineer.

Area 1

In Area 1, problems are occurring from individual pipe systems that may be clogged or of inadequate size. Also, discharge to the lake is occurring at several different drainage swales, which promotes silt migration at multiple locations. HRP recommends that an engineered system be designed to replace the existing system of non-connected pipes. The new system could be situated along East Shore Drive, and route storm water to one or more catch basins and discharge points. Discharge locations could be lined with riprap to prevent scour. In order to decrease the discharge of sediments, catch basins with three-foot sumps and/or a sedimentation chamber could be designed. Also, installation of a subsurface drain with a protected perforated pipe is suggested to eliminate the ponding along the roadway.

Area 2

At this location there is an existing drainage system, and flooding only at certain isolated locations. HRP recommends that this drainage system be cleaned to remove any sediments and leaves and to improve all discharge locations by placing properly sized riprap to prevent erosion. Although it is not an urgent area for improvement, future plans should include replacing the drainage structures with three-foot sump catch basins and/or the use of a sedimentation chamber prior to discharge into the lake.

Area 3

In Area 3, we did not observe any specific flooding, but did note the condition of drainage swales. HRP recommends that an engineering review of the drainage swales be performed in the future. This would include a calculation of the runoff volume so that a properly designed riprap scour hole or stilling basin could be constructed, to slow down the velocity of the runoff and decrease erosion.

Area 4

Many of the problems we observed in this area relate to the cleaning and management of the existing drainage system. Several pipes were observed with partial or complete clogging by silt or vegetation. Therefore, we recommend that a condition survey be performed and the pipes first cleaned out. Release points of any culverts should be protected with properly sized riprap to prevent scour. The cross culvert at the end of West Shore Extension should be analyzed and sized correctly to prevent overtopping and erosion of the shoreline. Drainage calculations should be performed and the existing system augmented where necessary with additional piping.

SUMMARY

In summary, most of the localized flooding and erosion problems around Hidden Lake are due to inadequately sized or clogged pipe collection systems, and from the lack of collection systems in certain areas. Some suggested improvements include:

• A regular program of cleaning the existing collection systems

• Replacing certain individual pipes with hydraulically connected systems, including reducing the number of discharge points to the lake

• Better control of silt migration, including catch basins with sumps, sedimentation chambers, using collection pipes with geotextile fabric protection, and erosion control measures around outlet structures.

• Engineering design where necessary, to size system components for adequate capacity.

Estimated Costs for Drainage Improvements:

Area 1

Area 2

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Photos of Road Drainage Walkaround