I. - INTRODUCTION

Purpose

This DRAINAGE CRITERIA MANUAL (the "Manual") provides design guidance for use by developers and engineers in preparation of drainage plans for development within the City of Dickinson (the "City"). It establishes rules and regulations that must be consistently followed and will be enforced throughout the City's jurisdiction including the portions of the Dickinson Bayou watershed lying within the City's boundaries. The design methods presented in this manual are intended to provide guidance for determination of runoff rates; methods of storm water collection, conveyance, and detention; and design standards for facilities (ditches, ponds, detention basins, etc.).

Methods of design and analysis other than those included in this Manual may be considered in certain cases where there may be inherent problems with the traditional methods. However, any deviation from this Manual will require consideration and acceptance by the City before approval will be granted for any work based on these alternatives.

Policy

Due to the nature of the watershed hydraulics within the City's boundaries and the prevalent existence of flood plains that exceed the banks of the creeks, it shall be the policy of the City to maintain zero net increase in storm water runoff rates and to insure no negative impacts attributable to new development and redevelopment that increases impervious cover by at least five percent (5%). Although it is the City's long-term goal to construct and maintain facilities (i.e., channels and regional detention 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 insure that existing runoff rates will not increase as a by-product of development, and therefore no off-site impacts will be developed. It is further recognized that impacts to other land owners and jurisdictions outside of the City's boundaries are unacceptable, and the City believes that this policy will effectively eliminate any such out-of-district impacts.

It is the goal of the City to support channel improvements of the major Creeks within its jurisdiction in general conformance with the adopted regional Master Plan. The City will also support and cooperate with the other governmental entities including the City of League City, City of Santa Fe, City of Texas City, Galveston County, Galveston County Consolidated Drainage District, Galveston County Drainage District No. 1, Galveston County Drainage District No. 2 and Brazoria County Conservation and Reclamation District No. 3 to improve local drainage.

Individual developers must provide infrastructure required to meet the City's stated objective of zero net increase in runoff rates and no negative impacts. Practically, this will mean 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. The City will require separate "off-line" detention facilities, and no "in-line" detention scenarios will be approved. Additionally, development in the delineated floodway will be restricted by the City.

This Manual 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 readily be accessed and maintained by the City.

Jurisdiction

The area within the City of Dickinson boundary is drained entirely by the Dickinson Bayou. Subdivisions and other developments in the area that are not located directly on this major drainage artery generally drain by man-improved or man-made ditches and storm sewers which convey the rainfall runoff to the major drainage artery. The City of Dickinson is responsible for the maintenance of those drainage ways that drain City road rights-of-way (within rights of way and identified in the Master Plan) within its City Limits.

Watersheds

The City's service area covers approximately 12 square miles and lies in north central Galveston County. Historically, this service area was primarily rural agricultural, but recent growth in the area is changing the land use to urban development at an increasing rate. The City's boundaries include a portion of one major watershed: Dickinson Bayou.

Dickinson Bayou

Dickinson Bayou watershed covers the entire City service area. Dickinson Bayou drains surface water from Alvin, Friendswood, League City, Santa Fe, Dickinson and unincorporated Galveston County as it flows from its upstream end in Brazoria County to its outfall in Galveston Bay (approximately 22 miles in length).

The topography of the Dickinson Bayou basin is relatively flat, soils are typically low permeable clays, and land use includes agricultural, some light industrial/commercial and residential development. The main channel of Dickinson Bayou is generally unimproved, and while some individual land owners have cleared underbrush along the banks, there has been no major effort by any jurisdictional entity to realign, widen or otherwise change the natural channel.

Definitions

Administrative Official: The individual charged with the administration and enforcement of this manual, or their duly authorized representative

City: City of Dickinson

Conduit: Any open channel or closed pipe system for conveying flowing water.

Developer: An individual or entity that makes improvements to real property for the purpose of reselling that property as a course of business, or for personal, non-residential use, and subdivision development greater than two adjoining lots shall be considered a Developer by the City. An individual engaging in construction of a single homestead to be used by that individual and his/her immediate family, including all reasonable associated improvements (i.e., driveways, garages, personal storage buildings and swimming pools) or infill development where public infrastructure including roadways and utilities exist adjacent to the parcel for lots less than one-half (1/2) acre in size shall not be considered a Developer by the City. Additionally, an individual who is subdividing a lot without improvements and only for subdivision purposes shall not be considered a Developer by the City.

Development: The term includes New Development, Redevelopment and In-fill Development.

1.

In-fill Development: Development of open tracts of land in areas where the storm drainage infrastructure is already in place and takes advantage of the existing infrastructure as a drainage outlet.

2.

New Development: Development of open tracts of land in areas where the storm drainage infrastructure has not been constructed and a drainage outlet must be extended to a conduit system under the jurisdiction of the City and/or Galveston County.

3.

Redevelopment: A change in land use that alters the impervious cover from one type of Development to either the same type or another type, and takes advantage of the existing storm drainage infrastructure in place as a drainage outlet.

Drainage Plan: All Developers shall provide the City with a drainage plan prepared by a Professional Engineer licensed to practice in the State of Texas showing the overall approach for the collection, conveyance and storm water detention required by the City's "Drainage Criteria Manual" to assure a "no impact" development. Individuals constructing a homestead for their personal use shall be required to provide a grading and/or drainage plan that will eliminate negative impacts to the adjacent properties but that may not need to be prepared by a licensed engineer as determined by the City Council or the Administrative Official.

Floodplain Administrator: The individual charged with enforcing the City's floodplain management ordinance.

Infiltration Trenches: Trenches or basins that temporarily detain a design water quality volume while allowing infiltration to occur over a prescribed period of time.

Green Roof: A roof that is partially or completely covered with vegetation and growing medium with a drainage and irrigation system.

Hard Roof: Horizontal roof surfaces used to attenuate peak runoff associated with rainfall and effectively detain flow resulting from smaller rain events.

Porous Pavement: Permeable surface course that allows for infiltration of storm water runoff into a permeable layer of uniformly graded stone bed.