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TOWARD COPING WITH FUTURE STORMS Greater Denver's history of intense rain- storms and destructive flooding extends back into pioneer days and even into Indian legend, (See Follansbee and Sawyer, 1948, for an ex- cellent general reference, and Matthai, 1969.) Floods undoubtedly will occur in the future just as they have in the past. In fact, the South Platte was out of its banks again on May 24, 1973, and flash-flooding tributaries on June 12, 1973, took three lives. As urbanization spreads over the Colorado Piedmont, the frequency and intensity of urban flooding are likely to increase. Measures can be taken, nevertheless, to fore- stall problems, and the initial cost of prevention in the areas of new construction can be more than recovered in the long-term savings from minimized storm damage, In older areas, where the problem is more complex, rectification is more difficult and much more costly, Geologic processes triggered by the May 5-6 storm were intensified in places where the natu- ral regimen had been altered by man. Inasmuch as such alterations are inevitable in any rapidly growing urban area, careful planning to ensure compatability with natural processes is essen- tial if destructive side effects of heavy storms are to be minimized. Some common land-development practices in the Greater Denver area that tended to accel- erate geologic processes and aggravate the in- tensity of storm effects were (1) channeling drainage into concentrated systems that are incapable of accommodating peak storm runoffs, (2) inhibiting infiltration of moisture by reduc- ing permeability and speeding runoff, (3) block- ing or constricting natural drainages by dikes, bridges, culverts, and landfills, and (4) over- steepening natural slopes by undercutting or by filling at angles steeper than those that commonly persist in nature, particularly along rights-of-way for highways but also in housing developments in hilly areas. The effects of some of these practices seem to be self-evident, and to recapitulate them may be to restate obvious generalities, but there is some value in viewing diverse land-development practices in a per- spective that relates one consequence to another. All these practices tend to compound the overall effects of a storm by reinforcing one another at the critical time of maximum storm intensity, Channeling drainage into systems that are un- able to accommodate peak runoffs, for example, is a common consequence of inhibited infiltra- tion, which, in turn, often results from the multitudinous land modifications associated with urbanization. Another lesson that should now be clear is that gully bypasses, such as flumes, conduits, or siphons, are needed to prevent unwanted storm drainage from entering and overwhelm- ing canals and ditches. Inhibited infiltration, particularly because of impervious surfaces, such as roofs, driveways, sidewalks, streets and especially parking lots, increased storm runoff enormously, Artificial retention systems can be designed, on the other hand, to reduce runoff rates to acceptable levels. Large flat-topped roofs can be designed to pro- vide temporary storage for slow release or evaporation. Downspouts from houses and com- mercial buildings can be directed onto lawns or other pervious areas where infiltration is possi- ble and desirable. In some places, runoff can be recharged directly to the ground water by means of French drains, dry wells, or similar infiltration systems. Infiltration obviously should be avoided in potentially unstable slope areas where landsliding might be initiated or in areas where moisture might gain access to high swell- shrink clays beneath footings or poured slabs. In some places ponds can be designed to catch and hold excess runoff, When suitably land- scaped, such ponds are assets that increase the value of adjoining real estate. Heavy earthmoving equipment has made pos- sible the economic development of large tracts of land. Such construction practices in the Greater Denver area, however, commonly denude large acreages of land for extended periods of time prior to construction and, during heavy rain- storms, greatly increase the vulnerability of the land to sheet wash, gullying, scour, mudflowage, and sedimentation (fig, 19), These costly storm effects can be reduced if, following the lead of the highway builders, provisions are made for rapid, even temporary, revegetation with fast- growing grasses and herbs to reestablish pro- tective ground covers, reduce erosion, and retard runoff. Better yet, careful analysis of the land- scape at the planning stage can often minimize the initial alteration of the natural land surface 18