02-18-08 Drainage and Flooding Committee Minutes

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2 WHAT IS NOT THE CAUSE There has been some concern over the completion of Phase I of the Pond to Park project within the detention pond servicing the F airmont Park East subdivision. \Vhen the City of La Porte reviewed the design and supervised the construction for the pond, we ensured that no fill was added. In fact, when the previously non-functioning pond was cleaned, re-graded and deepened at the outfall end adjacent to Driftwood, the volume of the pond was increased. The pond during the recent storms was not at capacity and while the recently completed railing and walkway were under water, the pond exhibited plenty of "freeboard" (level of the pond not under water). The downstream ditch that serves as the "outfall" for the pond was moving at a rapid pace but clearly swollen as downstream water accumulated. As a side note, that ditch was recently maintained and re-graded by the Harris County Flood Control District. As water is combined with other sources and bayous downstream, however, the pressure generated by the volume will cause the upstream sources to back up. At some point, given the total accumulation of water during the many storms over the past few weeks, the water will have no place to go. CONTRIBUTING FACTORS Based upon the observations of the Planning Department and the actions ofthe Public Works Department, here are some contributing factors that the City Staff are exploring: );. A Public Works crew removed grass and minor debris from the sewer system. Debris such as this causes friction and a loss of velocity of the water en route to the detention pond. ,., It was noted that the inlets at key intersections had whirlpool-like activity as water attempted to enter the system. Indications were that the storm sewer still had available capacity for storage of water but given the short duration/high intensity of the rainfall, the water had little chance to enter the inlets. Public Works crews have already replaced some solid manhole covers with grated covers at key inlet locations which will allow more water to enter the system in less time. );. Public Works has inspected critical locations and specifically the junction box located between the detention pond and the outfall to the downstream ditch. Engineering will analyze the size of the box in comparison with the system design. Upsizing of the box, if applicable, may be helpful. );. Staff noted some turbulence at the outfall of the detention pond as water was trying to enter the downstream ditch. Once water recedes, the Engineering Division will assess the outfall pipes to insure the absence of any blockage and/or any re-sizing that may be applicable. Keep in mind that the pond is metered to only release the amount of water at a rate that is able to be accommodated by the receiving ditch. In this way, just as upstream developments are regulated to prevent draining water from impacting Fairmont Park East, the design and outfall rate of your detention pond is relegated to a rate that will avoid any downstream flooding. );. Engineering is considering a modification to the curb cut on Driftwood that was made during the construction of the improvements to the detention facility to allow ponding water on Driftwood to sheet- flow into the pond. The cut may have to be altered to avoid the discharge of water onto Driftwood. I .--.... Given what we observed over the past few weeks, we understand your concerns about the drainage within Fairmont Park East. The City Staffwill continue to analyze the storm sewer system within your subdivision. As information becomes available, we will keep you informed of any additional modifications that mal' be needed and what steps the city is able to take to improve the conditions. Cc: John Joerns Steve Gillett Rodney Slaton Department File