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II II 11 11 il 11 II 11 il II 11 :1 II :1 11 II 11 11 II Lower Big Dry Creek Hydrologic Study . City of Broomfield: "Construction that will impair surface or subsurface drainage will not be accepted., The City reserves the right to issue and enforce more stringent criteria should adverse <conditions exist. Designs varying from the criteria will require written approval of the variance by the City Engineer prior to final acceptance of the plans." . City of Arvada: "If the City Engineer determines that a longer detention time is in the City's best interest, he may require a flow rate less than half the historic peak flow rate (QH/2) before blVO (2) times the historic concentration time (2 x Tc) is reached." 5.2 General Legell Principles Governing Drainage Planning While this study does not provide a legal opinion with regarding the drainage concerns along Big Dry Creek, the Urban Storm Drainage Criteria Manual, Volume 1, Chapter 2 provides an overview of legal responsibilities with regard to drainage. Additionally, in recent drainage planning studies, UDFCD has sought a general legal opinion regarding the proposed drainage improvements that is sometimes included in the drainage report. In general, the basic guiding drainage law principles, as summarized in an excerpt from a letter by Edward Krisor, Esq.(1998) contained in one of th(: Big Dry Creek area master plans (Kiowa 1999) is as follows: Natural drainage conditions may be altered by an upper (dominant) owner provided the water is not sent down in a manner or quantity to do more harm than formerly to the lower (servient) owner, Hankins v. Borland, 163 Colo. 575, 431 P.2d 1007 (1967). Colorado follows the "rule of reasonableness " as related to drainage matters and in each drainage situation, the court will look at the relationship of the parties and at who is doing what to whom in order to determine "what is reasonable under the circumstances." Prior to the Hankins Case, the Colorado Supreme Court ruled in Ambrosio v. Perl-Mack, 143 Colo. 49, 317 P.2d 803 (1960), that downstream owners had to accept additional runoff from an upstream developer. As the engineering technology has become more precise in drainage matters, it is possible to determine from the actual development and from land use decisions that will determine future development what the additional (more than natural) runoff will be because landowners make their land impermeable. The Colorado Legislature in 1973 recognized this and adopted Sec. 30-28- 113(4)(b) which requires developers in unincorporated areas to detain greater than historical flows. As a matter of drainage practice, cities as well as counties and the Urban Drainage District work to identify with precision "who is doing 971-179.092 June 2005 Wright Water Engineers, Inc. Page 30