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<br />k lot z 4~. ass 0 cia t e s <br /> <br />channels not on the developed tract are not increased, and 2) the cost of implementing the <br />mitigation is borne by the developer (in some manner) of the site. <br /> <br />Regional mitigation provides for a facility, almost invariably an off-line surface detention <br />pond, to be constructed in the watershed where runoff control is needed, with the design <br />of the detention pond being such that mitigation is achieved for a number of sites <br />simultaneously. This mitigation is accomplished by two effects: 1) reduction of flows <br />downstream of the detention site because during a storm event some flow is diverted <br />from a channel into the pond, and 2) reduction of downstream flows lowers the tailwater <br />effects on upstream water levels, resulting in lower maximum water levels at upstream <br />points for similar discharges, which in net effect is as if runoff is reduced from upstream <br />sources. Because of this latter reduction, the runoff from an upstream site does not, at <br />least conceptually, have to be mitigated; the mitigation is provided by the regional <br />detention pond. <br /> <br />In estimating needed detention, currently undeveloped land areas were assumed to <br />become fully developed with dense residential lots of a typical size of 0.25 acre. The <br />increased flows and runoff volume caused by the increase in development for the future <br />conditions would be mitigated with the construction of regional detention ponds. Also, <br />the construction of regional detention ponds assumes that the receiving streams have the <br />needed improvements to efficiently convey the flows out of and into the receiving <br />streams. <br /> <br />Several factors affect the operation of a regional detention facility and its effectiveness: <br />1) the runoff from an upstream development site must get to the regional detention site; if <br />the runoff is to reach the detention site, then the channel must be sufficiently large that <br />the increased runoff from the site does not exceed channel capacity (or, from an <br />alternative perspective, the channel must be modified to allow extra flow because there <br />will be more flow than would occur for existing conditions); and 2) off-line detention has <br /> <br />4-10 <br /> <br />Klotz Associates Project No. 0127.008.000 <br />January 2009 <br /> <br />La Porte Citywide Drainage Study <br />City of La Porte <br />