The City of Hitchcock is located in Galveston County, bordering Galveston West Bay. A sizeable portion of the city is located within a FEMA floodplain. About 2/3 of the city population lives within the 100-yr floodplain (Zone AE) and Coastal High Hazard (Zone VE) area, while over 90% reside within the 500-yr. floodplain (Zone X-shaded). During Hurricane Harvey in 2017, floodwaters damaged numerous areas around the city including historic buildings and public facilities. More than 100 structures were determined to be substantially damaged. To become more resilient, the city must focus where residents are at risk and include these considerations into land use decisions and practices.
Managing storm waters and drainage are critical to the city’s long-term goals. Due to the nature of watershed hydraulics and the widespread floodplain coverage within the city boundaries, it shall be the policy of the City to maintain zero net increase in stormwater runoff rates and to assure no negative impacts attributable to new development and redevelopment that increases impervious cover by at least 5%. Although it is the city’s long-term goal to construct and maintain facilities that will contain 100-year storm flows within drainage rights-of-way, it is recognized that further impacts to the existing system cannot be tolerated.
Therefore, the current city policy is based upon an “on-site detention” storm water management policy. Strict adherence to this policy will ensure that existing run-off rates will not increase as a by-product of development, and therefore no off-site impacts will occur. It is further recognized that impacts to other landowners and jurisdictions outside the city’s boundaries are unacceptable and the city believes that this policy will effectively eliminate any such out-of-city impacts.
It is the goal of the city to support channel improvements of the waterways within the city. These include Highland Bayou, Highland Bayou Diversionary Canal, Basford Bayou, and Halls Bayou. In addition, the city recognizes the primary authority of Galveston County Drainage District # 1 and Galveston County Drainage District #2 for the areas in their jurisdictions. The authority of Galveston County controls development and maintenance of the Highland Bayou Diversionary Canal. Texas Department of Transportation (TxDOT) controls drainage activities along state highways (TX 6, FM 519, FM 2004) in the city.
Individual developers are expected to provide infrastructure to meet the city’s stated objective of zero net increase in runoff rates and no negative impacts. Practically, this means that developers will provide adequate on-site detention volume to off-set increased runoff rates and must provide compensating storage volume for all fill placed in the floodplain.
In addition to the normal, Type “A” drainage requirements to front of properties that apply to most of the city, there are exceptions that are designed to enhance drainage plans. The Harborwalk development is a waterfront community, and those residential properties are all located on canals connected to Galveston West Bay. As such, all storm water drainage in this sub-division is directed to the rear of the property and into the canals. No drainage is permitted to run toward the streets. Canals and bulkheads in the Harborwalk development are maintained by the Flamingo Isles Municipal Utility District.
This policy also establishes minimum right-of-way requirements for certain ditches, channels, and bayous within the city’s jurisdiction. These minimum right-of-way requirements are based upon past engineering studies and generally allow for conveyance of the projected 100-year flow in a ditch section that can be readily accessed and maintained by the city. City ordinance prohibits construction and earthwork within city drainage easements to assure adequate storm water flow and access for maintenance. Property owners are expected to maintain roadside ditches on their property.
PURPOSE OF THE DRAINAGE CRITERIA MANUAL
The primary purpose of this drainage criteria manual is to establish standard principles and practices for the analysis, design, and construction of primary drainage systems in the City of Hitchcock. Storm water management is an essential component of community infrastructure and serves to provide both increased convenience and protection of life and property. A properly designed drainage system will detain and/or carry away runoff from more frequent rainfall events while allowing the movement of vehicles to homes and businesses. Such a system will also detain and/or drain storm water from an infrequent “extreme rainfall event” so that habitable structures are not damaged, and major streets are passable to public safety vehicles. Providing the City of Hitchcock with an effective storm water management system that allows sustainable community growth is a continuing challenge. It involves setting minimum standards, planning for future detention basins and drainage channels, working with private development interests, coordinating with governmental agencies, and maintaining the efficiency of the existing system.
Recognizing that storm water system development should be guided by adopted policies and criteria, the City of Hitchcock planning process is aimed at setting consistent standards responsive to the needs of property developers and design engineers and compliant with federal and state regulations. This Drainage Criteria Manual applies to all areas within the City of Hitchcock boundaries. However, if the project falls within the jurisdiction of the Galveston County Drainage District #1 or #2, the submittals related to the project must be sent to them. The County Drainage District will then forward a copy of the approval letter to the City of Hitchcock. For projects within the City of Hitchcock that are outside the boundaries of the defined Drainage Districts identified herein, the Galveston County Drainage District #1 Criteria Manual shall be utilized for drainage system design.
In the context of this Drainage Policy, an impact is defined as a change in the response of a watershed to a storm event. The most common impacts are changes in the volume of runoff, changes in the rate of runoff, and changes in flooding depths. Impacts may be adverse or beneficial. Adverse impacts are those which increase the potential for flooding damages. Beneficial impacts, on the other hand, reduce the potential for flood damage. The term zero impact is normally defined as the absence of adverse impact. The City of Hitchcock maintains a strict zero impact policy in all watersheds located wholly or partially within the incorporated boundaries of the city. This means that neither increases in upstream flood levels nor in downstream flow rates are allowed in areas where there is the potential for flooding damages from storms with recurrence. Adverse impacts associated with new development must be identified and mitigated. Acceptable mitigation measures may include storm water detention, creation of new flood plain storage, channel improvements, and improvements to channel structures. A “zero impact” policy will be enforced by the City of Hitchcock. No adverse impacts on downstream peak flow rates or upstream flood levels will be allowed. No net loss in existing flood plain storage will be allowed.
STORM WATER DETENTION
Storm water detention refers to the temporary storage of storm runoff in ponds or other storage facilities. The provision of this temporary storage allows storm runoff to be discharged to a receiving stream at a lower rate, thereby protecting downstream areas from increased flooding damages associated with increased flow rates and higher flood levels. The City of Hitchcock recognizes the value of storm water detention in reducing the potential for flood damages and allows the use of detention facilities in addition to adding conveyance capacity for mitigating impacts associated with new development and drainage improvements.
FLOOD PLAIN STORAGE
Flood plain storage is defined for the purposes of this manual as the space below the FEMA 100-year flood levels (BFE). This space is available for the temporary storage of flood waters during extreme storm events. Preservation of this space is extremely important because flood plain storage serves to reduce downstream peak flow rates. The City of Hitchcock prohibits reductions in existing flood plain storage volumes within the Special Flood Hazard Areas (Zones AE and VE) of the city. Practically. This means that net addition of fill dirt is not an acceptable method of structure elevation in the SFHA.
THE NATIONAL FLOOD INSURANCE PROGRAM
The City of Hitchcock is a participant in the National Flood Insurance Program (NFIP) and is committed to participation in the Community Rating System (CRS) for the benefit of our residents. This program provides federally subsidized flood insurance to those cities and counties which elect to participate. The program is administered by the Federal Emergency Management Agency (FEMA), which is headquartered in Washington, D.C. Flood insurance data for participating cities and counties is published by FEMA in two formats: bound flood insurance studies, which describe the results of flooding studies completed for significant streams, and Flood Insurance Rate Maps (FIRMs), which provide data on 100-year flood levels, illustrate the boundaries of the floodway, 100-year flood plain, and 500-year flood plain, and designate flood hazard zones for insurance purposes.